WO2025149293A1

WO2025149293A1 - Radar system using a digitally controllable oscillator for radar signal transmission

Info

Publication number: WO2025149293A1
Application number: PCT/EP2024/086242
Authority: WO
Inventors: Nikolaj ANDERSEN; Sumit Bagga
Original assignee: Vitalthings AS
Current assignee: Vitalthings AS
Priority date: 2024-01-08
Filing date: 2024-12-13
Publication date: 2025-07-17
Anticipated expiration: 2026-07-08
Also published as: GB202400260D0; TW202532878A

Abstract

A radar system including: a digitally controllable oscillator (DCO) for generating a DCO output signal with a frequency based on a control input; a transmitter for transmitting one or more signals using the DCO output signal; a memory; and a controller. A transmit process performed by the radar system includes: the DCO generating a DCO output signal with a frequency based on a control input; the transmitter transmitting a first transmit signal using the DCO output signal; and the controller loading data from the memory and using said data to provide the control input to the DCO.

Description

RADAR SYSTEM USING A DIGITALLY CONTROLLABLE OSCILLATOR FOR RADAR SIGNAL TRANSMISSION

Technical Field

The present disclosure relates to radar systems. In particular, the present disclosure relates to radar systems that use a digitally controllable oscillator to generate a DCO output that can be used for radar signal transmission.

Background

Radar systems may be arranged to transmit a radar signal (e.g. in the case of an impulse radar, a set of one or more pulses) at a certain centre frequency. In applications, such as ultrawideband applications, it may be important to ensure that the frequency band of the transmitted pulses is within an allowable frequency range, e.g. to ensure radio-regulatory compliance or coexistence with other users of the wireless spectrum.

Typically, radar systems use a reference oscillator, such as a crystal oscillator, to provide a reference signal that is used in the generation of the transmitted radar signal. Monostatic radar systems may also use the reference signal for receiving a signal (e.g. a reflection of the transmitted signal). Such reference oscillators may provide a reference signal having a very accurate, known, reference frequency. However, reference oscillators, such as crystal oscillators, consume a relatively large amount of power and have a relatively long start-up time before the amplitude and/or frequency of the signal generated by the reference oscillator is sufficiently stable at the reference frequency. In applications which require low power and/or fast start-ups, it is desirable to reduce the start-up time of the radar system and/or to reduce the power consumption of the radar system.

Summary

According to a first aspect, the invention provides a radar system comprising: a digitally controllable oscillator; a transmitter; a memory; and a controller; wherein the radar system is arranged to perform a transmit process, wherein: the digitally controllable oscillator is arranged to generate a DCO output signal with a frequency based on a control input; the transmitter is arranged to transmit a first transmit signal using the DCO output signal; and the controller is arranged to load data from the memory and use said data to provide the control input to the digitally controllable oscillator.

This arrangement results in a significant power saving as transmission can be accomplished without using an external reference oscillator, such as a crystal oscillator, to initiate the DCO. Thus, the invention recognises that once a control word for the DCO has been established for a given set of conditions, that control word can be stored and reused without having to re-calibrate the DCO with a reference oscillator. This saves the time required to achieve stability in the reference oscillator as well as the power required to run that reference oscillator during the stabilising period. Thus, faster start-up and lower power operation can be achieved.

The transmitter may be arranged to transmit the first transmit signal for a first period of time. For instance, the radar system may be an impulse radar system, and the first transmit signal may be a first set of pulses.

This invention may be particularly relevant for impulse radar systems, as there can often be a relatively long period of time between pulse transmissions in impulse radar systems during which the DCO can be powered down, thus saving power. However, while this time period is long relative to the time taken to transmit a pulse, it may still be short relative to the timescale on which environmental conditions such as temperature, DCO supply voltage, etc. affect the operation of the DCO.

The radar system may be an impulse radar system. For instance, the radar system may be an ultra-wideband impulse radar system. For example, the transmitter may be arranged to transmit a signal with a frequency bandwidth falling within a designated ultra-wideband frequency range. Current regulations specify for example that the designated ultra-wideband frequency range is 3.1 GHz to 10.6 GHz (for the US) or 6.0 GHz to 8.5 GHz (for Europe) or similar for other countries. However, this is subject to change as regulations are changed.

The DCO output may be used as a clock signal by the transmitter, or may be used to generate a clock signal for the transmitter. The clock signal may be used for either or both of carrier wave generation and envelope generation for the transmitted signal(s).

By way of example, in an impulse radar system the pulse(s) generated by the impulse radar system may have a pulse shape that is substantially the same shape as a Gaussian-like function (e.g. a Gaussian function, a Kaiser window, a Hermite polynomial function or the like). Such pulse shapes enable efficient use of bandwidth such that a high amount of power can be transmitted with each pulse, while still achieving radio-regulatory compliance. A Gaussian-like pulse shape also has low sidelobes, expressed through a beneficial zero-Doppler ambiguity function, which may also be preferable for radar applications. Other pulse shapes are also possible with different benefits for different implementations.

The digitally controllable oscillator (DCO) could be a voltage controlled oscillator (VCO), a numerically controlled oscillator (NCO) or the like.

In some embodiments, the data loaded from the memory could comprise a function defining a certain relationship between certain inputs and control word values, i.e. such that when provided with certain inputs, such as temperature, supply voltage to the DCO, usage data or the like, the function produces a suitable control word. However, in some embodiments, the data loaded from the memory is numerical data. Loading numerical data from the memory is particularly fast and efficient both in terms of the amount of memory required and the speed with which a suitable control word can be produced.

Depending on implementation, the numerical data loaded from memory may require some additional processing (e.g. scaling, compensating, interpolating, extrapolating) to produce the final control word. However, in some embodiments, the data loaded from the memory comprises the control input. Loading the control word and supplying it directly to the DCO is fast and highly efficient in terms of speed and processing resources.

In some embodiments, when the radar system performs the transmit process: the controller is arranged to obtain a temperature measurement and/or DCO supply voltage measurement; and the controller is arranged to adjust the control input based on the temperature measurement and/or the DCO supply voltage measurement before it is provided to the digitally controllable oscillator. As previously mentioned, temperature variation and/or variation of the supply voltage may occur slowly relative to the time between signal transmissions. The obtained temperature and/or supply voltage measurements need not be measured immediately before the transmit process. For instance, temperature may be measured periodically and the controller may be arranged to obtain the most recent measurement. Hence, it will be understood that if more than one transmit process occurs between supply voltage/temperature measurements, the same supply voltage/temperature measurement may be obtained by the controller for each transmit process.

In many applications, the supply voltage to the DCO will be controlled to a high degree of accuracy, as the DCO output may vary if the supply voltage to the DCO is varied. However, drift can occur over time. Thus, by obtaining a measurement of the supply voltage to the DCO and using this to adjust the control input for the DCO, it may be possible to produce a more accurate DCO output and/or enable less accurate control of the supply voltage, thereby enabling more freedom of design of the radar system.

The frequency of the DCO output may fluctuate with temperature or supply voltage. In particular, it may vary with temperature or supply voltage significantly more than the output of a crystal oscillator or other reference oscillator. For instance, in some implementations a DCO output may be expected to vary by up to 1000 times more than the output of a crystal oscillator within a typical operating range of ~-40 degrees Celsius to 85 degrees Celsius. Therefore, even relatively small variations in ambient temperature may result in changes to the DCO output frequency that would potentially breach the regulatory spectrum requirements. If the temperature of the radar system is within a certain range (e.g. within 0.1 degrees, 1 degree, 5 degrees, 10 degrees, etc. depending on the application) of a temperature for which the control word is known to be valid (e.g. a calibration temperature), then it may be possible to make a minor adjustment to the control word to compensate for the temperature variation without having to recalibrate for the new temperature with a reference oscillator. For example, the DCO control word may be increased or decreased based on the temperature measurement (e.g. based on the difference between the measured temperature and a temperature for which the loaded DCO control word was known to be valid). A similar approach could be used for supply voltage, e.g. in addition to or instead of temperature. For instance, if the supply voltage is within a certain range (e.g. within 1%, 2%, 5%, 10%, etc. depending on the application) of a supply voltage for which a control input is known to be valid, the control input may be increased or decreased based on a measured supply voltage (e.g. based on the difference between the measured supply voltage and a supply voltage for which the loaded DCO control word was known to be valid). Depending on the architecture of the oscillator core of the DCO, a variation of the supply voltage by a few percent could result in a significant change in oscillation frequency when using the same control input.

In some embodiments, the radar system is arranged to record device usage data; and when the radar system performs the transmit process, the controller is arranged to adjust the control input based on the device usage data before it is provided to the digitally controllable oscillator. Device usage data could include any data that is indicative of the usage of the DCO, e.g. the total accumulated runtime of the DCO, the number of times the radar system or the DCO has been started up, the number of transmitted signals of the radar system or the like. This may be useful to account for aging of the DCO, as the DCO output for a certain control input may change with usage, e.g. as components degrade. The DCO output may change differently, (e.g. more quickly) if used at high temperature or high supply voltage, thus the radar system may be arranged to record device usage data in combination with the temperature and/or supply voltage at which it has been used, or to store a usage value or an aging value based on the combination of the usage and temperature/supply voltage. It will be appreciated that any combination of aging data, temperature data and supply voltage data may be used to adjust the control input.

Other approaches may be used to measure aging of the DCO to provide device usage data, e.g. in combination with or as an alternative to those mentioned above. For instance, the radar system could comprise a second DCO, arranged to be operated infrequently, and thus age slowly, such that it can be used as a reference to determine the effect of aging of the first DCO by comparing the DCO outputs of the first DCO and the second DCO.

A similar approach may also/alternatively be used wherein the radar system comprises a first component that ages when the DCO ages, e.g. when it is used, and a second component, which may be used as a reference to determine the effect of aging of the first component. Usage/aging data of the DCO may be determined based on the effect of aging of the first component. This may be more size and/or cost efficient than including a second DCO. In some embodiments, the memory has a look up table stored therein in which each of a plurality of control inputs is stored in association with a value of one or more look up variables. For example, a certain control input may be stored in association with a certain temperature, DCO supply voltage, and/or device usage for which it is suitable. Measurements of temperature, DCO supply voltage, and/or device usage may then be used to look up an appropriate control input in the look up table.

The data loaded from memory may include a first control input which is suitable for a first set of operating conditions, such as DCO supply voltage, temperature and/or usage, and a second control input which is suitable for a second set of operating conditions, such as DCO supply voltage, temperature and/or usage. In such an example, the controller may use the data loaded from memory to provide the control input to the DCO by interpolating between (or extrapolating from) the control inputs, e.g. for obtained or measured operating conditions, such as DCO supply voltage, temperature and/or usage.

In some embodiments, when the radar system performs the transmit process, the controller is arranged to obtain one or more look up variables. For example, look-up variables may include DCO supply voltage, temperature, device usage or the like.

In some embodiments, the data loaded from the memory comprises the control input corresponding to the one or more obtained look up variables. Thus, a suitable control input for the current operating conditions may be obtained from a look up table.

In some embodiments, the data loaded from the memory comprises a first control input corresponding to a first set of one or more look up variables and a second control input corresponding to a second set of one or more look up variables; and the control input is obtained by interpolating between the first control input and the second control input based on the obtained one or more look up variables. In other embodiments the control input may be obtained by extrapolating from the first control input and the second control input based on the obtained one or more look up variables.

In some embodiments, at least one look up variable is temperature.

In some embodiments, at least one look up variable is DCO supply voltage. In some embodiments, at least one look up variable relates to device usage. In some embodiments, the memory is a persistent memory, e.g. the data stored on the memory may be retained even if the radar system is powered off. For example, the memory may be a volatile memory and the radar system could comprise a battery arranged to power the volatile memory when the radar system (or at least the transmitter) is turned off. In some embodiments, the memory is a non-volatile memory. Such a non-volatile memory may retain the data stored therein even without power, such that a battery or the like is not necessary for the memory to retain the data stored therein when the radar system is turned off.

In some embodiments, the controller is arranged to obtain the control input from the data loaded from the memory, without using feedback from the DCO output signal. For example, the control input obtained from the data loaded from memory may be passed to the DCO directly and the DCO simply starts operating and is used without any check that its output is suitable. This enables fast start-up of the DCO, and hence the radar system, and also provides power saving, but it relies on trust and/or confidence that the loaded data from memory will result in DCO operation and consequent transmission without breaching the spectrum mask, interfering with other systems, or the like. Such an approach may also rely on trust that the DCO will operate as expected, e.g. it will not fail to turn on and/or that conditions (e.g. temperature) have not altered significantly such that a previously valid control input is no longer valid.

In some embodiments, when the radar system performs the transmit process: the digitally controllable oscillator is arranged to turn off so as to stop generating the DCO output signal after the transmitter transmits the first transmit signal.

Thus, power may be saved by turning off the DCO when it is not required.

In some embodiments, when the radar system performs the transmit process: after turning off, the digitally controllable oscillator is arranged to turn on so as to resume generating the DCO output signal; and the transmitter is arranged to transmit a second transmit signal using the DCO output signal; wherein the same control input is used for the digitally controllable oscillator to generate the first transmit signal and the second transmit signal.

Using the same control input for the DCO for the second transmit signal as is used for the first transmit signal is highly efficient and requires minimal operations (e.g. only reloading the control input). By way of example, for fastest start up for the second transmit signal, no new temperature or usage data is obtained. Instead, as no data has changed, exactly the same control input is obtained and is supplied directly to the DCO with minimal processing. This process again relies on trust and/or confidence that environmental conditions have not changed significantly. However, this is a reasonable assumption in many radar operations, such as impulse radar applications.

In impulse radar applications, it is often desirable to transmit one set of pulses, comprising one or more pulses, for a single measurement (several pulses often being used so as to allow for integration of the received signal for improved signal to noise ratio). This first set of pulses may be referred to as a frame. The impulse radar may transmit several frames per second, i.e. several sets of pulses per second, but may be idle in between those sets. A typical range of frame rates for an ultrawideband impulse radar system may be between one frame per second (or less than one frame per second) and several thousand frames per second (depending on the application), however it is possible to operate ultrawideband impulse radar systems outside of this range. Thus, the time between pulse sets may be only a fraction of a second, during which time the ambient conditions are very unlikely to change. However, being able to shut down the DCO during the time period between pulses provides a huge power saving. Not having to start up a reference oscillator to restart the DCO provides a further power saving. Even at lower frame rates, e.g. sets of pulses separated by a few seconds, this technique may still be usable. The time period for which a control input can be deemed valid may depend on operating circumstances and may be programmable or selectable by the user. It may be noted that the power saving from shutting down the DCO between frames becomes more valuable as the time between frames becomes longer. The savings from a fast start up only apply on a per frame basis and thus contribute less absolute power saving in applications having low frame rates. However, such applications are often highly power-constrained, so the technique is still very valuable.

In some embodiments, the radar system comprises a receiver arranged to sample a receive signal using the DCO output signal to look for reflections of the first transmit signal. A monostatic radar system, i.e. comprising both a transmitter and a receiver, that uses the DCO output signal for both the transmit and receive process is more tolerant of small drifts in the DCO output frequency (i.e. small enough that there is no risk of regulatory non-compliance, interference with other co-existing systems, or the like). When compared with a bistatic radar system or communications system (where the receiver is part of a separate system to the transmitter), the monostatic system can tolerate such drift because it affects both transmitter and receiver equally. Thus, in a monostatic radar system, it becomes viable to reuse a control input for longer provided the drift is within a certain tolerance. On the other hand, a bistatic radar system (or communications system) needs to maintain its reference frequency highly accurately so as to match its frequency with another, separate system. Thus, the bistatic system would need to either recalibrate its oscillator, e.g. a controllable oscillator such as a DCO, continually with a stable reference oscillator, such as a crystal oscillator (which may be difficult in practice) or use a stable reference oscillator in place of a controllable oscillator to limit frequency mismatch between transmitter and receiver.

In some embodiments, the data stored in the memory may be loaded from a database. For example, every radar system may have the same data stored in the memory from which the controller obtains the control input for the DCO.

In some embodiments, the radar system is arranged to perform a calibration process to determine the data to be loaded from the memory, wherein: the controller is arranged to provide one or more candidate control inputs to the digitally controllable oscillator; the digitally controllable oscillator is arranged to generate a corresponding one or more candidate output signals; the radar system is arranged to compare the one or more candidate output signals with a reference signal; the radar system is arranged to select a candidate control input which best matches the reference signal; and the controller is arranged to store data associated with the selected control input in the memory.

The calibration process provides a mechanism by which the radar system can determine the data to be stored in the memory, and from which the control input for the DCO can later be obtained. The calibration process allows the DCO output to be matched to a reference frequency in much the same way as has traditionally been done for every DCO startup. However, in this case the calibration process additionally stores the resulting control input in the memory so that it can be reused subsequently without going through the calibration process. As the calibration process is performed on the device (as opposed to storing data on the memory that is not specific to the device), the data stored on the memory accounts for device-specific factors and thus accommodates variation between devices, e.g. resulting from the variation of components between devices. The calibration process can be used as required, e.g. as often as required to reset the DCO and avoid regulatory non-compliance. The calibration process may be used only when necessary to save power.

There may be a desired relationship between the reference frequency and the frequency of the DCO output. By way of example, the desired relationship may be that the DCO output frequency is the same as the reference frequency. Alternatively (and more usually), the desired relationship may be that the frequency of the DCO output is a certain multiple of the reference frequency. Thus, the control input which best matches the reference frequency may be the control input which produces a DCO output with a frequency that most closely corresponds to the desired relationship with the reference frequency. By way of example, in some embodiments the reference signal frequency has an order of magnitude of 10s of MHz, while the desired DCO output frequency is in the range of several 100s of MHz to a few GHz, with some known (integer or non-integer) ratio between the two frequencies.

In some embodiments, the radar system comprises a phase locked loop; wherein the phase locked loop is arranged to compare the one or more candidate output signals with the reference signal and to adjust the candidate control input based on the comparison.

A phase locked loop may enable the controller to determine a suitable control input for the DCO in a simple and straightforward manner. For example, a PLL can be implemented largely in hardware such that very little software/firmware is necessary to program the controller to determine a suitable control input for the DCO.

In some embodiments, to compare the frequency of the one or more candidate output signals with the reference signal, the controller is arranged to: count a number of cycles for each of the candidate output signals in the given period of time; and compare the counts to a target number of cycles for the given time period.

For example, the controller may be arranged to determine a given period of time using the reference signal, e.g. by counting a number of cycles of the reference signal corresponding to the given period of time. The given period of time may be defined as a certain number of cycles of the reference signal.

In some embodiments, when the radar system performs the calibration process, the controller is arranged to obtain a temperature measurement and/or a DCO supply voltage measurement and/or a device usage data measurement; and wherein the data associated with the selected control input stored in the memory comprises an association between the selected control input and the temperature and/or the DCO supply voltage and/or the device usage data.

Storing control inputs in association with the temperature and/or DCO supply voltage for which they are valid essentially allows a look up table to be built up as several temperatures and/or DCO supply voltages are covered. This may be done sporadically as new temperatures and/or DCO supply voltages are encountered or it may be part of a more formal calibration process.

In some embodiments the calibration process may be carried out for a plurality of temperatures. For example, the calibration process may be an iterative process performed at a plurality of temperatures. The calibration process may be performed as part of the production process, and may comprise adjusting the temperature between iterations. In this manner, it may be possible to calibrate the radar system for a range of temperatures in production and thus the radar system may not require a reference oscillator at all. For example, if the relationship between temperature and control input is stable over time, then a sufficiently detailed look up table can provide an appropriate control input for any measured temperature. As noted above, interpolation and/or extrapolation may be used as necessary.

In some embodiments the calibration process may be carried out for a plurality of DCO supply voltages, for example the calibration process could be carried out for a plurality of DCO supply voltages at a plurality of temperatures. For example, the calibration process may be an iterative process performed at a plurality of DCO supply voltages. The calibration process may be performed as part of the production process, and may comprise adjusting the DCO supply voltage between iterations. In this manner, it may be possible to calibrate the radar system for a range of DCO supply voltages, optionally in combination with a plurality of temperatures, in production and thus the radar system may not require a reference oscillator at all. As noted above, interpolation and/or extrapolation may be used as necessary. In some embodiments, the radar system is arranged to perform the transmit process at least twice without performing the calibration process between the transmit processes. For example, the radar system may be arranged to only perform the calibration process in production or in a dedicated calibration session, or it may be arranged to undergo calibration periodically (e.g. every N transmissions or every N times that the DCO is powered off). In other examples the calibration process may be carried out only when a temperature change of a threshold magnitude is detected (or more generally when any relevant operating condition has changed by a threshold amount). In yet further examples, the calibration process may only be performed if the radar system encounters an operating temperature and/or DCO supply voltage for which it does not have data stored in the memory from which it can obtain a suitable control input. This may occur, for example, if the radar system encounters an unusually high or low temperature or an unusually high or low DCO supply voltage for which it has not yet (or not recently) been calibrated. In some examples, the calibration process may be a periodic process, e.g. performed after a certain number of transmit processes and/or after a certain amount of time has passed. Periodically performing the calibration process may be useful to account for aging of the radar, e.g. aging of the DCO, and thus can accommodate the associated changes to the frequency of the DCO output.

In some embodiments, the radar system is arranged to receive the reference signal from an external reference oscillator. As previously mentioned, the radar system may be calibrated using a reference oscillator without necessarily comprising the reference oscillator. For example, the radar system may be calibrated as part of its production process using an external reference oscillator. This may result in a cheaper and/or smaller radar system, which may be particularly convenient for certain applications, such as portable or wearable devices comprising the radar system. The radar system may be connectable to an external reference oscillator periodically for calibration I recalibration.

In some embodiments, the radar system comprises a reference oscillator arranged to generate the reference signal. As previously mentioned, if the radar system comprises the reference oscillator, periodic calibration may be possible, e.g. to account for aging or to identify a suitable control input for a certain operating temperature or DCO supply voltage. Having a reference oscillator as part of the system also allows functionality such as performing on-demand calibration, e.g. for newly encountered conditions as discussed above. In some embodiments, the reference oscillator is a crystal oscillator.

In some embodiments, the transmit process comprises operating the radar system without generating the reference signal.

In some embodiments, when the radar system performs the transmit process: the controller is arranged to obtain a temperature measurement and/or DCO supply voltage and/or a device usage data measurement; and the controller is arranged to obtain the control input for the digitally controllable oscillator based on the loaded data and the temperature measurement and/or DCO supply voltage and/or device usage data measurement.

Purely by way of example, the stored data (i.e. the data that is loaded) may define a relationship between temperature and/or DCO supply voltage and control input. For example, the stored data may be coefficients of a known relationship function. Thus, loading the data and combining with the measured temperature and/or DCO supply voltage can allow a rapid determination of a suitable control input based on the stored data. As noted above, the stored data can of course be updated periodically following calibration.

In some embodiments, the radar system is arranged to perform a/the calibration process if a control input cannot be obtained for the obtained temperature measurement and/or DCO supply voltage and/or the device usage data measurement.

In some embodiments, the radar system is battery powered. For example, a portable or wearable device may comprise the radar system. Thus, it may be particularly important that the radar system is power efficient so as to prolong battery life.

The techniques described above are applicable to a number of different radar systems. For example, the radar system may be a frequency-modulated continuous wave (FMCW) radar, an impulse radar or any other type of monostatic radar system. In some embodiments, the radar system is an impulse radar system and the first transmit signal is a first set of one or more pulses. By way of example, the impulse radar system may be an ultra-wideband impulse radar system.

According to a second aspect, the invention provides a method of operating a radar system comprising: performing a transmit process, the transmit process comprising: loading data from a memory and using said data to provide a control input to a digitally controllable oscillator; using the digitally controllable oscillator to generate a DCO output signal with a frequency based on the control input; and transmitting a first transmit signal using the DCO output signal.

It will be appreciated that the method according to the second aspect of the invention may be carried out by any of the embodiments of the radar system according to the first aspect. Thus, optional features of the first aspect of the invention may apply equally to the second aspect and vice-versa. In particular (and not by way of limitation):

In some embodiments, the data loaded from the memory is numerical data.

In some embodiments, the transmit process comprises: obtaining a temperature measurement and/or a DCO supply voltage measurement; and adjusting the control input based on the temperature measurement and/or DCO supply voltage measurement before it is provided to the digitally controllable oscillator.

In some embodiments, the transmit process comprises: adjusting the control input based on device usage data before it is provided to the digitally controllable oscillator.

In some embodiments, the data loaded from the memory comprises the control input. In other embodiments, the data loaded from the memory may not comprise the control input. For example, the data loaded from memory may include a first control input which is suitable for a first set of operating conditions, such as temperature, DCO supply voltage and/or usage, and a second control input which is suitable for a second set of operating conditions, such as temperature, DCO supply voltage, and/or usage. In such an example, the controller may use the data loaded from memory to provide the control input to the DCO by interpolating between the control inputs, e.g. for obtained operating conditions, such as temperature, DCO supply voltage, and/or usage.

In some embodiments, the memory has a look up table stored therein in which each of a plurality of control inputs is stored in association with a value of one or more look up variables.

For example, the transmit process may comprise obtaining one or more look up variables; and the data loaded from the memory may comprise one or more control inputs corresponding to the one or more obtained look up variables. For instance, two or more control inputs corresponding to the one or more look up variables may be interpolated so as to provide the control input for the DCO.

In some embodiments, at least one look up variable is temperature.

In some embodiments, at least one look up variable is DCO supply voltage.

In some embodiments, at least one look up variable relates to device usage.

In some embodiments, the memory is a non-volatile memory.

In some embodiments, the control input is obtained from the data loaded from the memory, without using feedback from the DCO output signal.

In some embodiments, the transmit process further comprises: turning off the digitally controllable oscillator so as to stop generating the DCO output signal; turning on the digitally controllable oscillator so as to resume generating the DCO output signal; and transmitting a second transmit signal using the DCO output signal; wherein the same control input is used for the digitally controllable oscillator to generate the first transmit signal and the second transmit signal.

In some embodiments, the method further comprises a receive process, the receive process comprising: sampling a receive signal using the DCO output signal to look for reflections of the first transmit signal.

In some embodiments, the method further comprises performing a calibration process to determine the data to be loaded from the memory, the calibration process comprising: providing one or more candidate control inputs to the digitally controllable oscillator to generate a corresponding one or more candidate output signals; comparing the one or more candidate output signals a reference signal; selecting a candidate control input which best matches the reference signal; and storing data associated with the selected control input in the memory.

In some embodiments, the calibration process comprises using a phase locked loop to compare the one or more candidate output signals with the reference signal and to adjust the candidate control input based on the comparison.

In some embodiments, the comparing step of the calibration process comprises: counting a number of cycles for each of the candidate output signals in the given period of time; and comparing the counts to a target number of cycles for the given time period.

For example, the comparing step may comprise determining the given period of time using the reference signal, e.g. by counting a certain number of cycles of the reference signal corresponding to the given time period.

In some embodiments, the calibration process comprises obtaining a temperature measurement and/or DCO supply voltage measurement and/or a device usage data measurement; wherein the data associated with the selected control input stored in the memory comprises an association between the selected control input and the temperature and/or DCO supply voltage and/or the device usage data.

In some embodiments, wherein the calibration process is carried out for a plurality of temperatures and/or DCO supply voltages. For example, the calibration process may be an iterative process performed at a plurality of temperatures and/or DCO supply voltages.

In some embodiments, the method comprises performing the transmit process at least twice without performing the calibration process between the transmit processes.

In some embodiments, the calibration process comprises receiving the reference signal from an external reference oscillator.

In some embodiments, the calibration process comprises generating the reference signal.

In some embodiments, the reference oscillator is a crystal oscillator.

In some embodiments, the transmit process comprises operating the radar without generating the reference signal.

In some embodiments, the transmit process further comprises obtaining a temperature measurement and/or DCO supply voltage measurement and/or a device usage data measurement; and wherein the control input for the digitally controllable oscillator is obtained based on the loaded data and the temperature measurement and/or DCO supply voltage measurement and/or the device usage data measurement.

In some embodiments, the method further comprises performing a/the calibration process if a control input cannot be obtained for the obtained temperature measurement and/or DCO supply voltage measurement and/or the device usage data measurement.

According to a third aspect, the invention provides a method of operating a battery powered radar system using the method of any of the examples of the second aspect of the invention.

Brief Description of Drawings

One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which:

Figure 1 shows the main parts of an impulse radar system;

Figure 2 shows a method of operating the impulse radar system of Figure 1 ;

Figure 3 shows another example of the main parts of an impulse radar system, which may be operated using the method of Figure 2;

Figure 4 shows example waveforms of the current draw and power consumption with respect to time when an impulse radar system, such as the impulse radar systems of Figures 1 and 3, use a reference oscillator to transmit and receive a set of pulses; and

Figure 5 shows an example waveform showing the power consumed with respect to time when another impulse radar system, such as the impulse radar systems of Figures 1 and 3, use a DCO to transmit and receive a set of pulses, e.g. without using a reference oscillator.

Detailed Description

Figure 1 shows the main parts of an impulse radar system 100, which may be an ultrawideband impulse radar system. The system 100 includes a memory 101 , a digitally controllable oscillator (DCO) 102, a transceiver 103, and a controller 104. In this example, the system 100 also includes a temperature sensor 105 and a reference oscillator 106.

The memory 101 in this example is a non-volatile memory such that data stored on the memory 101 is retained even when the radar system is powered off. It will be appreciated that in other examples, the memory 101 may be a volatile memory, which may not retain the data stored on it without power, and the impulse radar system 100 could include a power source, e.g. a battery, to provide power to the memory 101 when the impulse radar system 100 is turned off. The DCO 102 may be any suitable type of digitally controllable oscillator. In use, the DCO 102 receives a control input in the form of a digital control word (an N-bit digital number), and produces a DCO output in the form of a periodic waveform with a frequency determined by the received control input. For example, the control word may take a range of numerical values between 0 and 2^AN-1 and the DCO may output a waveform with a frequency between a lowest frequency (corresponding to the ‘0’ input) and a highest frequency (corresponding to the ‘2^AN- T input) with each digital input resulting in a different discrete frequency output.

In use, the radar transceiver 103 transmits electromagnetic signals and receives reflections of those transmitted signals. It will be appreciated that a separate transmitter and receiver are also possible. A separate transmitter and receiver that operate off the same clock can still form a monostatic radar system. The transceiver 103 in this example receives the DCO output signal as a clock and uses this to generate the transmitted signals. Parameters of the signal transmitted by the transceiver, e.g. a carrier frequency and/or envelope of the transmitted signal, may be based on the clock signal (i.e. based on the DCO output).

In order to comply with radio regulatory requirements, it may be necessary to control certain parameters of the transmitted signals. For example, the bandwidth and centre frequency of the transmitted pulses need to be controlled to ensure that the frequency spectrum of the transmitted pulse does not breach the spectrum regulations.

The controller 104 controls operation of the DCO 102, and may control other aspects of the operation of the impulse radar system 100. In this example, one specific function of the controller 104 includes a frequency counter 107. The frequency counter 107 may be used to count a number of cycles of a signal. In this example, the controller 104 is arranged to perform a search algorithm 108 for identifying a suitable control input for the DCO 102.

In this example, the radar system 100 includes several sensors 105. The sensors include a temperature sensor 105a, configured to measure a temperature associated with the impulse radar system 100, and a voltage sensor 105b, configured to measure the voltage supplied to the DCO 102 and an aging sensor 105c configured to measure the aging of the DCO 102. In this example, in use, the controller 104 obtains a temperature measurement associated with the impulse radar system 100 from the temperature sensor 105a and/or a measurement of the voltage supplied to the DCO 102 from the voltage sensor 105b and/or an aging measurement from the aging sensor 105c. While in this example, the impulse radar system 100 includes all three of the temperature sensor 105a and the voltage sensor 105b and the aging sensor 105c, in other examples any of these sensors could be an external sensor that supplies a measurement to the radar system 100. In other examples, only one of these sensors 105a, 105b, 105c may be used. Any combination of the sensors may be used.

In use, the controller 104 may receive a reference signal from a reference oscillator 106 to calibrate the impulse radar system 100. The reference oscillator 106 in this example is a part of the impulse radar system 100. In other examples the reference oscillator 106 could be an external oscillator which supplies the reference signal to the radar system 100. For example, the impulse radar system 100 may be calibrated using an external reference oscillator as part of its manufacturing process or may be connected to the external reference oscillator periodically for recalibration.

In this example, the reference oscillator 106 is a crystal oscillator. A crystal oscillator may produce a reference signal at a frequency that is known to a very high degree of accuracy and is stable. While in this example, a crystal oscillator is used as a reference oscillator 106, in other examples any oscillator which is able to generate a stable refence signal with a well-defined frequency may be used as a reference oscillator 106.

However, as will be discussed in more detail below, it will be appreciated that the reference oscillator 106, the frequency counter 107, the search algorithm 108 and the sensors 105 are not required for certain basic implementations of the radar system 100 according to embodiments of the invention.

The impulse radar system 100 may be a battery powered impulse radar system. For example, it may be suitable for use in a portable device, such as a laptop, mobile phone or the like. Applications for such an impulse radar system could include, for instance, presence detection, i.e. to detect the presence of a user.

Figure 2 shows a method 200 of operating the impulse radar system 100. The method 200 includes a transmit process 220. In this example, the method 200 also includes a calibration process 210. In other examples the calibration process 210 may be omitted.

In this example, the calibration process 210 is performed to determine data to store on the memory 101 of the impulse radar system 100, which is subsequently used to determine a control input for the DCO 102. In other examples, the data stored on the memory 101 may be loaded without calibration (e.g. provided from a database). Thus, calibration of the impulse radar system 100 may not be necessary. However, a calibration process 210 may be helpful, e.g. to account for any parameters that may vary between devices, e.g. as a result of process variations which result in component sizes or values that are specific to one device.

The calibration process 210 includes a first step 211 of the controller 104, providing a candidate control input to the DCO 102.

In a second step 212 the DCO 102 generates a candidate DCO output using the candidate control input.

In a third step 214 the controller 104 compares the candidate DCO output with a reference signal. The reference signal in this example is generated by the reference oscillator 106 as illustrated in optional step 213. However, in other examples the reference signal may be received from an external source, e.g. an external reference oscillator. An external reference oscillator may be part of the radar system (but a part that is not used as frequently as the rest of the system), or it may be external to the radar system such that it needs to be specifically connected for the calibration process (e.g. in a factory calibration). A crystal-based reference oscillator will always be an off-chip component, but may still be part of a radar system as a whole. Other reference oscillators may be on-chip (i.e. part of the silicon die), e.g. a CMOS-MEMS resonator. The impulse radar system 100 may be calibrated in production using an external reference oscillator which may not be necessary thereafter, thus enabling a cost and/or size reduction of the impulse radar system.

The third step 214 of the calibration process 210 includes comparing the candidate DCO output with the reference signal. This may include comparing the frequencies of the signals. For example, the third step 214 of the method may include the controller 104 using the frequency of the reference signal to determine a certain period of time. The period of time may correspond with a certain number of cycles of the reference signal, and the controller 104 could determine the time period by counting this number of cycles using the frequency counter 107. Thus, comparison of the reference frequency with the DCO output frequency may include the controller 104 counting the number of cycles of the DCO output signal using the frequency counter 107 and comparing the counted number of cycles of the DCO output signal with an expected number of cycles for the determined period of time. The first step 211 , second step 212 and third step 214 in this example may be repeated to iterate for several candidate control inputs. Iteration of the first 211 , second 212 and third 214 steps may include using feedback from the comparison performed in the third step 214 of the calibration process 210 to select the next candidate control input to provide to the DCO 102. For example, the controller 104 may be arranged to use a search algorithm 108 that uses feedback to select the next candidate control input for the DCO 102 (e.g. a binary search strategy). In other examples, the controller 104 may instead use a brute force search algorithm 108 to select candidate control inputs for the DCO 102, e.g. wherein the controller 104 selects every possible control input in turn for use as the candidate control input.

The calibration process 210 includes a fourth step 215 in which the controller 104, selects a candidate control input based on the comparison of the candidate control input(s) with the reference frequency. The selected candidate control input is the one that best matches the reference frequency.

In some examples, there may be a desired relationship between the reference frequency and the frequency of the DCO output. By way of example, the desired relationship may be that the DCO output frequency is the same as the reference frequency. Alternatively, the desired relationship may be that the frequency of the DCO output is a certain multiple of the reference frequency.

Once a candidate control input has been selected, a fifth step 217 of the calibration process 210 includes storing data associated with the selected candidate control input in the memory 101. The data associated with the selected candidate control input may include the selected candidate control input itself or, for example may be data from which the candidate control input can be derived. The stored data may additionally include other parameters associated with the current operating state for which the control input has been determined, for example, a temperature measurement, DCO supply voltage, and/or usage data.

Optionally, the calibration process 210 includes a step 216 in which the controller obtains a DCO supply voltage measurement and/or a temperature measurement, e.g. from the sensors 105. In examples that include this optional step 216, data relating to the obtained temperature measurement and/or DCO supply voltage may be stored in association with the data relating to the selected candidate control input in the memory 101. By way of example, the selected candidate control input, the temperature measurement and the DCO supply voltage may be stored together in a look-up table.

In some examples, the calibration process includes an optional step 218 of adjusting the operating conditions, such as temperature and/or DCO supply voltage, e.g. so as to iterate the calibration process 210 at different operating conditions. For instance, the impulse radar system 100 may be calibrated for a range of temperatures as part of its manufacturing process, e.g. to minimise, or possibly eliminate, the need for any calibration of the device thereafter. Instead, or in addition, the supply voltage may be varied (e.g. as part of the manufacturing process or at any time thereafter) and the impulse radar system 100 may be calibrated for a range of supply voltages. The impulse radar system 100 may even be calibrated for a range of DCO supply voltages at each temperature for a range of temperatures.

A calibration process for temperatures may be much easier to do in a production process, e.g. where temperature can be varied in a test setting. However, calibration for DCO supply voltage could be done in either a production test setting or later on the device (e.g. in the field).

Having a control input for a certain temperature and/or DCO supply voltage may be useful, as the frequency of the DCO output may vary with temperature and/or DCO supply voltage. Thus, for different operating conditions a different control input may be required to generate a DCO output with a desired frequency.

As previously mentioned, in some examples the impulse radar system 100 receives the reference signal from an external reference oscillator. This may allow the impulse radar system 100 to be smaller, cheaper and/or more power efficient. In these examples, it may be particularly useful for the calibration process 210 to be performed at a range of temperatures and/or DCO supply voltages, as the impulse radar system may not receive the reference signal when in use, and thus calibration in use may not be possible. However, even in examples where the impulse radar system 100 does include a reference oscillator 106, it may be more power efficient for the impulse radar system 100 to be calibrated for a range of operating conditions (either as part of its manufacturing process or during subsequent measurements) than it is to perform the calibration process 210 every time it is used. For example, if the temperature has not changed, or not changed significantly, it may be sufficient (from e.g. a regulatory compliance perspective) simply to load the same data as was used for the last operation, i.e. to use the same control input as was used in the last operation. An improvement can be made by taking a new temperature measurement and obtaining a new control input based on the new temperature measurement (this may be done every time the DCO starts up or it may be done less frequently). If conditions have changed significantly or if a certain length of time has elapsed since the last calibration then a full calibration may be performed against a reference frequency to ensure compliance.

The method 200 of operating the impulse radar system 100 includes a transmit process 220 by which the impulse radar system 100 transmits a series of pulses.

The transmit process 220 optionally includes a first step 221 of obtaining one or more variables. Variables may include, for example, a temperature measurement, DCO supply voltage or device usage information. By way of example, the first step may include the controller 104 obtaining a temperature measurement, e.g. from the temperature sensor 105a included in the sensors 105, and/or the controller 104 obtaining usage information, e.g. from the memory 101 or from aging sensor 105c. Usage information could include, for example, a representation of the estimated degradation due to the current age of the system. This may be estimated using, for example, accumulated runtime, accumulated runtime at temperature, a number of start-ups of the DCO 102, a number of pulses transmitted by the impulse radar system 100 or the like.

Alternatively, or in addition, the effect of aging on the DCO 102 may be determined by comparing the DCO output with a DCO output of a second DCO, which is similar to the first DCO 102 and which is used only as a reference to determine the effect of aging of the first DCO 102. Thus, the second DCO does not age significantly, e.g. when compared with the aging of the first DCO 102. The second DCO may therefore be used as the aging sensor 105c.

Another approach may involve providing a first component which ages at the same rate as the DCO 102 and a second, similar, component which is used as a reference for the first component and which does not age significantly, e.g. when compared with the aging of the first component. Comparison of the first and second components may be used to estimate the aging of the DCO 102. The second component may therefore be used as the aging sensor 105c.

The transmit process 220 includes a second step 222 in which the controller 104 loads data from the memory 101. This step 222 may include selecting which data to load from the memory 101 based on the variables obtained in the first step 221. For example, the memory may have a stored control input associated with a certain temperature, DCO supply voltage and/or usage and the step of loading data from the memory 101 may comprise loading appropriate data from the memory for the obtained temperature, DCO supply voltage and/or usage. By way of example, the data stored in the memory 101 may include a look-up table that provides a certain control input for certain variables, such as temperature, DCO supply voltage and/or usage. The method 200 in this example includes performing the calibration process 210 if there is no data stored in the memory 101 associated with the obtained variable(s). However, if there is data stored in the memory 101 associated with the obtained variable(s) the calibration process 210 may be omitted. Alternatively, stored data may be interpolated or extrapolated to produce a suitable control input.

The transmit process 220 includes a third step 223 in which the controller 104 provides a control input to the DCO 102 using the data loaded from memory 101. By way of example, the data could include the control input to provide to the DCO 102. Alternatively, the data may be data from which the control input can be derived.

In certain examples, the third step 223 may include the controller, adjusting the control input before providing it to the DCO 102. For instance, the control input may be adjusted based on one or more of the variables obtained in the first step 221 , such as usage, temperature or the like.

As previously mentioned, at different operating conditions a different control input may be required to generate a DCO output with a desired frequency. Thus, obtaining temperature and/or DCO supply voltage information prior to obtaining a control input for the DCO 102 may be useful so that an appropriate control input can be used for the obtained operating conditions. For example, the control input obtained may be adjusted based on an obtained temperature measurement. Alternatively, obtaining the control input may include using the obtained temperature measurement, e.g. by using the obtained temperature measurement as a look-up variable to determine a suitable control input in a look-up table.

Aging may affect the frequency of the DCO output. Thus, obtaining data indicative of the age of the DCO 102 may enable an appropriate control input for the DCO’s age to be used. For example, the control input obtained, e.g. for a certain temperature, may be adjusted based on the age of the DCO 102. Alternatively, obtaining the control input may include using data associated with the DCO’s age to obtain a suitable control input, e.g. by using data associated with the DCO’s age to look-up a suitable control input in a look-up table.

The transmit process 220 includes a fourth step 224 wherein the DCO 102 generates a DCO output using the control input provided by the third step 223. A frequency of the DCO output is based on the control input.

The transmit process 220 includes a fifth step 225 wherein the transceiver 103 transmits a first set of pulses using the DCO output. By way of example, the transceiver 103 may receive the DCO output directly, or it may receive a signal that has been generated using the DCO output (e.g. via a frequency multiplier). The transmitter 103 may use the received signal as a clock to generate pulses which have the same centre frequency as the received signal.

In this example, the method 200 includes an optional sixth step 226 of sampling a receive signal using the DCO output to look for reflections of the set of pulses transmitted in the fifth step 225. More specifically, in this method 200, the DCO output is used as a clock signal by the transceiver 103 to sample a received signal which may include a reflection of the transmitted pulses, e.g. if they reflect off an object. Thus, in this example the transmit process 220 is a transmit and receive process. In other examples, the transmitted pulses may be received by another system. However, using the impulse radar system 100 to both transmit and receive the set of one or more pulses enables the DCO output to be used for both the transmit process and the receive process. For a monostatic radar system, it is beneficial for the receive process to be coherent with the transmit process such that good signal to noise ratio is achieved when sampling a received signal for a reflection of the transmitted pulses and/or to ensure that any information encoded in the phase of the received signal can be recovered accurately and is not lost. It will be appreciated that in other examples, coherency may be sacrificed for simplicity of design and reduced cost. Where the transmitter and receiver are clocked from a common source (e.g. from the same DCO) any drift in the DCO will affect both the transmitter and receiver equally. Thus, the monostatic radar system is more resilient to DCO drift which means it can benefit particularly from the power savings of the process described here as it is not necessary to calibrate the DCO output as often, so long as regulatory compliance is achieved.

By way of example, in certain ultrawideband applications, although a frequency drift of 10 MHz would be tolerable for ensuring radio-regulatory compliance, it would likely lead to a complete loss of coherence in a bistatic radar system with separately driven transmitter and receiver. Therefore, in certain ultrawideband applications, it is desirable to keep the clock signals used for the transmit and receive processes to within 100 ppm, preferably within 10 ppm and more preferably still, less than 1 ppm. For example, a frequency drift of less than 1 MHz (at an 8 GHz carrier frequency), and preferably a drift of less than 0.1 MHz. Purely by way of example, the IEEE 802.15.4z wireless communication standard requires less than +/- 20 ppm carrier frequency offset (CFO) from the nominal frequency in both transmit (TX) and receive (RX). During packet reception of the reflected signal, the CFO is often tracked and compensated down to the sub-1 ppm range.

In this example, the transmit (and receive) process 220 includes an optional seventh step 227 of turning off the DCO 102. This step may occur, for example, after the fifth step 225 of transmitting a first set of pulses or after the sixth step 226 of sampling a receive signal using the DCO output. Turning off the DCO 102 when not in use results in a power saving, which may be particularly beneficial in battery powered radar systems to extend battery life, reduce battery size and/or cost or the like. However, in other applications power saving may also be important, e.g. to save operating costs etc.

In this example, the transmit (and receive) process 220 includes an optional eighth step 228 of turning the DCO 102 back on using the same control input as was used to generate the DCO output in the fourth step 224. For example, the eighth step 228 may not require the first 221, second 222 and/or third 223 steps of the transmit process 220 to be repeated to determine the control input used in the eighth step 228. Instead, the eighth step may include the controller 104 providing the same control input as was used in the third step 223 to the DCO 102 without further processing.

In this example, the transmit (and receive) process 220 includes an optional ninth step 229 of transmitting a second set of one or more pulses using the DCO output provided by the eighth step 228. There may be a corresponding sampling step, such as the sixth step 226, for the second set of pulses. There may be a corresponding step for turning off the DCO 102, such as the seventh step 227, for the second set of pulses.

As discussed above, in certain applications, such as those requiring fast start-up of the DCO 102 and/or low power consumption, it is particularly advantageous to skip the steps of determining the control input between sets of pulses as these take time and consume power. Hence, the impulse radar system 100 may be configured to only perform the steps associated with determining which control input to provide to the DCO 102 periodically. By way of example, the impulse radar system 100 may be configured to only perform the steps associated with determining which control input to provide to the DCO 102 once every second set of pulses, once every third set of pulses, etc.

Alternatively, or in addition, the impulse radar system 100 may be configured to only perform the steps associated with determining which control input to provide to the DCO 102 once every second, once every several seconds, once a minute, once an hour, etc. The time period for repeating the steps associated with determining which control input to provide to the DCO 102 may depend on the application for which the impulse radar system 100 is intended to be used and the likelihood of changes to relevant variables, such as temperature, DCO supply voltage, or the like.

Alternatively, or in addition, the impulse radar system 100 may be configured only to perform the steps associated with determining a control input for the DCO 102 when one or more environmental conditions has changed by a threshold amount (e.g. temperature has changed more than two degrees or supply voltage to the DCO has changed by more than 5%).

Thus, in normal operation, once the transmit process 220 has been performed, another transmit process 220 may be performed without necessarily performing the calibration process 210. Avoiding calibration each time the impulse radar system 100 is required to transmit one or more sets of pulses avoids any requirement to use the reference oscillator 106. The reference oscillator can often take a long time to become stable and thus consumes a lot of power before it can be used.

Figure 3 shows another impulse radar system 300. The impulse radar system 300 includes a DCO 302, a memory 301 and a transceiver 303. These elements of the impulse radar system 300 may be similar to their counterparts in the impulse radar system 100 shown in Figure 1. The impulse radar system 300 of Figure 3 also includes sensors 305, which include a temperature sensor, a voltage sensor and an aging sensor similar to those included in the impulse radar system 100 shown in Figure 1. The impulse radar system 300 also includes a reference oscillator 306, again, this is similar to its counterpart 106 in the impulse radar system 100 of Figure 1.

The impulse radar system 300 also includes a controller 304 and a phase locked loop (PLL) 308, which in this example is an all-digital phase locked loop (ADPLL). In other examples, the phase locked loop 308 may be at least partially analogue. The PLL 308 includes a comparator 307 and the DCO 302. The comparator 307 compares the output of the DCO 302 with the output of the reference oscillator 306. The PLL 308 adjusts the control input of the DCO 302 based on the comparison of the DCO output and the signal produced by the reference oscillator 306.

Operation of the impulse radar system 300 of Figure 3 may be largely similar, e.g. substantially the same as, operation of the impulse radar system 100 of Figure 1. For example, the impulse radar system 300 may be operated by performing the method 200 of Figure 2. However, instead of the controller 304 using the frequency counter and search algorithm to perform the fourth step 214 of the calibration process 210 of Figure 2, the controller 304 uses the phase-locked loop 308 to compare the candidate DCO output with a reference signal, and adjust the control input accordingly to provide a new candidate control input for the first step 211 of the calibration process. This process is repeated until a control input is found which produces a DCO output which matches the reference frequency, e.g. having a frequency that is a certain (integer or non-integer) multiple of the reference frequency. The phase locked loop 308 can then be disabled, meaning that the DCO 302 continues in a free-running mode with the most recent candidate control input. The phase locked loop 308 compares the DCO output with the reference signal. The frequency of the DCO output is adjusted, by changing the control input, until the DCO output signal has the desired frequency relationship, and in this case also a desired phase relationship, with the reference signal.

Figure 4 shows example waveforms of the current draw and power consumption with respect to time when an impulse radar system uses a reference oscillator, which in this example is a crystal oscillator, in combination with a DCO to transmit and receive a set of pulses. In this example, the reference oscillator is always used as a reference for the DCO and it is therefore started up each time the impulse radar system transmits and receives sets of pulses. The DCO output is used to transmit the set of pulses. During a first time period 1 , the impulse radar system performs a series of start-up operations as it is turned on. These include turning on a radar system controller, and initializing memory.

During a second time period 2, local voltage supply regulators (Low Drop Out, or LDO regulators) are enabled.

During a third time period 3, a series of configuration operations are performed, including turning on the reference oscillator (in this case a crystal oscillator) at the time indicated by 1a. During this period 3, the controller configures the radar (e.g. reads from memory and sets configuration registers accordingly).

During a fourth time period 4, the impulse radar system waits until the reference oscillator produces a stable signal. As can be seen from Figure 4, the period of time 4 spent waiting for the reference oscillator to produce a stable signal is very significant (around 1.7 ms).

During a fifth time period 5, the impulse radar system transmits, and optionally receives, a set of pulses using the DCO output. During this process, the radar subsystem is started, transmits and receives pulses and is turned off again.

During a sixth time period 6, the impulse radar system, including the reference oscillator, is turned off. The reference oscillator and radar subsystem are turned off at the time indicated by 1b once the transmit and receive process is completed. The remainder of the sixth period 6, may include copying of radar data from frame buffers to memory and subsequent signal processing.

During a seventh period 7, a companion or host processor reads out the sampled frame. After completion the system transitions to deep sleep.

Figure 5 shows an example waveform, which shows the power consumed with respect to time when an impulse radar system, such as the impulse radar systems 100; 300 of Figures 1 and 3, uses a DCO, such as the DCOs 102; 302 of Figures 1 and 3, to transmit a set of pulses without using a reference oscillator such as a crystal oscillator. This can be contrasted with the power consumption and timelines shown in Figure 4 which uses a crystal oscillator.

During a first time period T, the impulse radar system performs a series of start-up operations as it is turned on.

During a second time period 2’, local voltage supply regulators (LDOs) are enabled, however no local reference oscillator is started.

During a third time period 3’, a series of configuration operations are performed. Unlike the third period 3 shown in Figure 4, this does not include turning on a reference oscillator (as a reference oscillator is not used in the transmit process shown in Figure 5).

During a fourth time period 4’, the radar acquisition process is started. This includes loading the control input from memory, providing the control input to the DCO and starting the DCO running. It can be seen that this fourth period 4’ is much shorter (around 35 microseconds) than the corresponding period 4 of Figure 4 (around 1600 microseconds). This is because there is no reference oscillator and thus no need to wait for one to stabilise. Instead, the system moves rapidly to the fifth time period 5’. This enables the impulse radar system to turn on and transmit a set of pulses much more quickly than an impulse radar system that uses a reference oscillator as in Fig. 4 each time the impulse radar system is turned on. Additionally, as a reference oscillator consumes a relatively large amount of power, avoiding its use results in a significant power saving.

During a fifth time period 5’, the impulse radar system transmits and receives a set of pulses using a signal from the DCO, after which the DCO is turned off. It will be appreciated from these two Figures just how short the actual transmit period is compared to the start up period for the reference oscillator.

During a sixth time period 6’, radar data is copied from frame buffers to memory for subsequent signal processing.

During a seventh period 7’, a companion or host processor reads out the sampled frame. After completion the system transitions to deep sleep (with the DCO control word and/or other associated data stored and retained for future use).

Comparing Figures 4 and 5, it can be seen that a significant time saving (and corresponding power saving) may be achieved by obtaining a control input for the DCO from memory and starting it immediately rather than by waiting for the reference oscillator to turn on and produce a stable reference signal. In addition, when comparing Figures 4 and 5, it can be clearly seen that a significant power saving can be achieved by not using the reference oscillator. This is best exemplified when looking at the duration of the fourth time period 4 of Figure 4 (which is much longer than in the example shown in Figure 5) during which the impulse radar system is waiting for the reference oscillator to provide a stable reference signal.

It will be appreciated by those skilled in the art that this disclosure has been illustrated by describing one or more specific examples thereof, but is not limited to these examples; many variations and modifications are possible, within the scope of the accompanying claims.

Claims

1. A radar system comprising: a digitally controllable oscillator; a transmitter; a memory; and a controller; wherein the radar system is arranged to perform a transmit process, wherein: the digitally controllable oscillator is arranged to generate a DCO output signal with a frequency based on a control input; the transmitter is arranged to transmit a first transmit signal using the DCO output signal; and the controller is arranged to load data from the memory and use said data to provide the control input to the digitally controllable oscillator.

2. A radar system as claimed in claim 1 wherein the data loaded from the memory is numerical data.

3. A radar system as claimed in claim 1 or 2 wherein the data loaded from the memory comprises the control input.

4. A radar system as claimed in claim 1 , 2 or 3, wherein when the radar system performs the transmit process: the controller is arranged to obtain a temperature measurement; and the controller is arranged to adjust the control input based on the temperature measurement before it is provided to the digitally controllable oscillator.

5. A radar system as claimed in any preceding claim, wherein when the radar system performs the transmit process: the controller is arranged to obtain a DCO supply voltage measurement; and the controller is arranged to adjust the control input based on the DCO supply voltage measurement before it is provided to the digitally controllable oscillator.

6. A radar system as claimed in any preceding claim, wherein the controller is arranged to obtain device usage data; and wherein when the radar system performs the transmit process, the controller is arranged to adjust the control input based on the device usage data before it is provided to the digitally controllable oscillator.

7. A radar system as claimed in any preceding claim, wherein the memory has a look up table stored therein in which each of a plurality of control inputs is stored in association with a value of one or more look up variables; wherein when the radar system performs the transmit process, the controller is arranged to obtain one or more look up variables; and wherein the data loaded from the memory comprises the control input corresponding to the one or more obtained look up variables.

8. A radar system as claimed in claim 7, wherein at least one look up variable is temperature; and/or wherein at least one look up variable is DCO supply voltage; and/or wherein at least one look up variable relates to device usage.

9. A radar system as claimed in any preceding claim, wherein the memory is a non-volatile memory.

10. A radar system as claimed in any preceding claim, wherein the controller is arranged to obtain the control input from the data loaded from the memory, without using feedback from the DCO output signal.

11. A radar system as claimed in any preceding claim, wherein when the radar system performs the transmit process: the digitally controllable oscillator is arranged to turn off so as to stop generating the DCO output signal after the transmitter transmits the first transmit signal, and turn on so as to resume generating the DCO output signal; and the transmitter is arranged to transmit a second transmit signal using the DCO output signal; wherein the same control input is used for the digitally controllable oscillator to generate the first transmit signal and the second transmit signal.

12. A radar system as claimed in any preceding claim, comprising a receiver arranged to sample a receive signal using the DCO output signal to look for reflections of the first transmit signal.

13. A radar system as claimed in any preceding claim, wherein the radar system is arranged to perform a calibration process to determine the data to be loaded from the memory, wherein: the controller is arranged to provide one or more candidate control inputs to the digitally controllable oscillator; the digitally controllable oscillator is arranged to generate a corresponding one or more candidate output signals; the radar system is arranged to compare the one or more candidate output signals with a reference signal; the radar system is arranged to select a candidate control input which best matches the reference signal; and the controller is arranged to store data associated with the selected control input in the memory.

14. A radar system as claimed in claim 13, further comprising a phase locked loop; wherein the phase locked loop is arranged to: compare the one or more candidate output signals with the reference signal; adjust the candidate control input based on the comparison.

15. A radar system as claimed in claim 13, wherein to compare the one or more candidate output signals with the reference signal, the controller is arranged to: count a number of cycles for each of the candidate output signals in the given period of time; and compare the counts to a target number of cycles for the given time period.

16. A radar system as claimed in claim 13, 14 or 15, wherein when the radar system performs the calibration process, the controller is arranged to obtain a temperature measurement and/or a DCO supply voltage measurement and/or a device usage data measurement; and wherein the data associated with the selected control input stored in the memory comprises an association between the selected control input and the temperature and/or the DCO supply voltage and/or the device usage data.

17. A radar system as claimed in claim 16, wherein the calibration process is carried out for a plurality of temperatures and/or DCO supply voltages.

18. A radar system as claimed in any of claims 13 to 17 wherein the radar system is arranged to perform the transmit process at least twice without performing the calibration process between the transmit processes.

19. A radar system as claimed in any of claims 13 to 18, wherein the radar system is arranged to receive the reference signal from an external reference oscillator.

20. A radar system as claimed in any of claims 13 to 18, comprising a reference oscillator arranged to generate the reference signal.

21. A radar system as claimed in claim 19 or 20, wherein the reference oscillator is a crystal oscillator.

22. A radar system as claimed in any of claims 13 to 21 , wherein the transmit process comprises operating the radar system without generating the reference signal.

23. A radar system as claimed in any preceding claim, wherein when the radar system performs the transmit process: the controller is arranged to obtain a temperature measurement and/or a DCO supply voltage measurement and/or a device usage data measurement; and the controller is arranged to obtain the control input for the digitally controllable oscillator based on the loaded data and the temperature measurement and/or the DCO supply voltage measurement and/or the device usage data measurement.

24. A radar system as claimed in claim 23, wherein the radar system is arranged to perform a/the calibration process if a control input cannot be obtained for the obtained temperature measurement and/or the DCO supply voltage measurement and/or the device usage data measurement.

25. A radar system as claimed in any preceding claim, wherein the radar system is battery powered.

26. A radar system as claimed in any preceding claim wherein the radar system is an impulse radar system; optionally wherein the impulse radar system is an ultra-wideband impulse radar system.

27. A method of operating a radar system comprising: performing a transmit process, the transmit process comprising: loading data from a memory and using said data to provide a control input to a digitally controllable oscillator; using the digitally controllable oscillator to generate a DCO output signal with a frequency based on the control input; and transmitting a first transmit signal using the DCO output signal.

28. A method as claimed in claim 27, wherein the data loaded from the memory is numerical data.

29. A method as claimed in claim 27 or 28, wherein the transmit process comprises: obtaining a temperature measurement and/or a DCO supply voltage measurement and/or device usage data; and adjusting the control input based on the temperature measurement and/or DCO supply voltage measurement and/or device usage data before it is provided to the digitally controllable oscillator.

30. A method as claimed in in any one of claims 27 to 29 wherein the data loaded from the memory comprises the control input.

31. A method as claimed in claim 30, wherein the memory has a look up table stored therein in which each of a plurality of control inputs is stored in association with a value of one or more look up variables.

32. A method as claimed in claim 31, wherein at least one look up variable is temperature; and/or wherein at least one look up variable is DCO supply voltage; and/or wherein at least one look up variable relates to device usage.

33. A method as claimed in any one of claims 27 to 32, wherein the memory is a non-volatile memory.

34. A method as claimed in any one of claims 27 to 33, wherein the control input is obtained from the data loaded from the memory, without using feedback from the DCO output signal.

35. A method as claimed in any one of claims 27 to 34, wherein the transmit process further comprises: turning off the digitally controllable oscillator so as to stop generating the DCO output signal; turning on the digitally controllable oscillator so as to resume generating the DCO output signal; and transmitting a second transmit signal using the DCO output signal; wherein the same control input is used for the digitally controllable oscillator to generate the first transmit signal and the second transmit signal.

36. A method as claimed in any one of claims 27 to 35, further comprising a receive process, the receive process comprising: sampling a receive signal using the DCO output signal to look for reflections of the first transmit signal.

37. A method as claimed in any one of claims 27 to 36, further comprising performing a calibration process to determine the data to be loaded from the memory, the calibration process comprising: providing one or more candidate control inputs to the digitally controllable oscillator to generate a corresponding one or more candidate output signals; comparing the one or more candidate output signals with a reference signal; selecting a candidate control input which best matches the reference signal; and storing data associated with the selected control input in the memory.

38. A method as claimed in claim 37, wherein the calibration process comprises using a phase locked loop to compare the one or more candidate output signals with the reference signal and to adjust the candidate control input based on the comparison.

39. A method as claimed in claim 37, wherein the comparing step of the calibration process comprises: counting a number of cycles for each of the candidate output signals in the given period of time; and comparing the counts to a target number of cycles for the given time period.

40. A method as claimed in any one of claims 37 to 39, wherein the calibration process comprises obtaining a temperature measurement and/or a DCO supply voltage measurement and/or a device usage data measurement; and wherein the data associated with the selected control input stored in the memory comprises an association between the selected control input and the temperature and/or the DCO supply voltage and/or the device usage data.

41. A method as claimed in claim 40, wherein the calibration process is carried out for a plurality of temperatures and/or DCO supply voltages.

42. A method as claimed in any of claims 37 to 41 wherein the method comprises: performing the transmit process at least twice without performing the calibration process between the transmit processes.

43. A method as claimed in any of claims 37 to 42, wherein the calibration process comprises receiving the reference signal from an external reference oscillator.

44. A method as claimed in any of claims 37 to 43, wherein the calibration process comprises generating the reference signal.

45. A method as claimed in claim 43 or 44, wherein the reference oscillator is a crystal oscillator.

46. A method as claimed in any of claims 37 to 45, wherein the transmit process comprises operating the radar without generating the reference signal.

47. A method as claimed in any one of claims 27 to 46, wherein the transmit process further comprises obtaining a temperature measurement and/or a DCO supply voltage measurement and/or a device usage data measurement; and wherein the control input for the digitally controllable oscillator is obtained based on the loaded data and the temperature measurement and/or the DCO supply voltage measurement and/or the device usage data measurement.

48. A method as claimed in claim 47, further comprising performing a/the calibration process if a control input cannot be obtained for the obtained temperature measurement and/or DCO supply voltage measurement and/or the device usage data measurement.