WO2003039125A1

WO2003039125A1 - Method and device for low-pass filtering

Info

Publication number: WO2003039125A1
Application number: PCT/EP2002/011220
Authority: WO
Inventors: Peter Caldera; Herbert Zojer
Original assignee: Infineon Technologies Ag
Priority date: 2001-10-31
Filing date: 2002-10-07
Publication date: 2003-05-08
Also published as: DE10153740B4; DE10153740A1

Abstract

In telecommunication networks it is particularly necessary to separate signals of various frequency regions from each other, particularly by means of low-pass filters. In said low-pass filter a signal is tapped from a tapping point in a telecommunication line (2, 3), particularly digitally low-pass filtered and injected into the telecommunication line (2,3) after inversion by means of a transformer. As a result of the inversion carried out the injected signal is subtracted from the original signal. As the low-frequency components are removed in the high-pass filtering, no change of the original signal occurs in this frequency band. The high-frequency signal components are cancelled out however after inversion. The transformer (1) can preferably be of a low-ohmic embodiment, such that the impedance of the telecommunication line (2, 3) is little altered. A low-pass filter which operates by said method is particularly used for the separation of telephone signals and data signals with the service provider.

Description

description

Method and device for low-pass filtering

The present invention relates to a method and a

Device for low-pass filtering a signal transmitted over a line.

Low-pass filtering can be used to dampen or block higher-frequency components of a signal. This is used in particular in communications technology in order to be able to transmit different signals simultaneously via one line. Different signals are transmitted in different frequency ranges, the different signals all having to be separated on the transmitter and receiver side with filters.

At present, telephone signals and data signals are increasingly being transmitted simultaneously via telecommunications networks. In this way, it is possible for end users to make calls at the same time and to be connected to the Internet, for example. In this case, for example, the xDSL standard (Digital Subscriber Line Standard) follows an approach in which the telephone signals, as has been known for a long time in the prior art, are transmitted analogously and in a higher frequency band

Data signals are transmitted. The phone signals must be limited to a certain frequency band so that the phone signals and the data signals do not interfere with each other. Conversely, the data signals must be kept away from the devices for transmitting the telephone signals in order not to cause interference in them. A feature of the conventional analog transmission of telephone signals is that the transmission line must have a very low resistance, since a current flows through the line which is used to operate the conventional telephone devices. For the separation of the data signals and the telephone signals, certain filters or splitters are known which feed the data signals into a conventional telephone line and also couple them out again, whereby it must be ensured that the data signals do not reach devices for processing the telephone signals. These are, in particular, a telephone device on the subscriber side and the devices of the telephone service provider for transmitting and receiving the telephone signals on the distributor side.

As is known, such splitters are made up of analog elements. They either consist of passive components or contain active elements such as operational amplifiers. All of these solutions are specially tailored to the impedance of the telecommunications line and the method used to transmit the telephone and data signals. Programming can only be done to a limited extent, e.g. B. by using relays.

The present invention is based on the object of providing a method and a device for low-pass filtering of signals transmitted via a telecommunication line, with which a broad range of applications can be achieved largely independently of the impedance requirements of the telecommunication line and of the method used for the transmission.

According to the invention, this object is achieved by a method having the features of claim 1 and an apparatus having the features of claim 14. The subclaims each define preferred and advantageous embodiments of the present invention.

According to the invention, a signal is tapped on the telecommunication line on which the low-pass filtering signal is transmitted, this is high-pass filtered and inverted as compensation signals via a transformer Telecommunications line coupled. Due to the previous inversion, the injected signal is subtracted from the original signal on the telecommunication line. Since the low-frequency components of the tapped signal were removed during high-pass filtering, there is no change in the original signal on the telecommunications line in this frequency band. However, the high-frequency signal components cancel each other out through this inversion. This type of filtering has the particular advantage that the electrical properties of the telecommunications line are only influenced by the transformer. This makes it possible to change the impedance of the telecommunication line only very slightly by using a transformer with a low number of turns or a low inductance. Since only inverted higher-frequency signal components have to be coupled in via the transformer, a transformer with fewer turns can also be prevented. This method is particularly suitable for very low-resistance telecommunication lines. In this case, a transformer with a first low-resistance winding can be used, which is looped into the telecommunications line. The inverted signal is coupled in via a second winding inductively coupled to the first winding. Due to the low resistance of the first winding, the impedance of the telecommunications line is changed only very slightly or not, so that this type of low-pass filtering can also be used in different countries with different impedance requirements.

The method according to the invention is particularly suitable for

Application on telecommunications lines over which the conventional telephone service is to be transmitted. In this case, the telecommunication line has a very low resistance and flows a current that is used by a connected telephone to operate it. This current flow causes a lot with conventional filters high effort, which is avoided in the inventive method.

An essential feature of the present invention is that the low-pass filtering occurs only in the direction of transmission from the tapping point at which the signals coupled in after high-pass filtering and inversion are tapped to the transformer. A different transfer function results in the other transfer direction.

In order to achieve a low-pass filter function for the other transmission direction as well, a second tapping point can be provided on the side of the transformer opposite a first tapping point, at which a signal is also tapped, high-pass filtered and, after inversion, coupled into the telecommunications line by means of the transformer. In this way, a symmetrical filter can be achieved which has a low-pass filter effect in both transmission directions. In principle, two separate transformers can also be used, with the coupling of the two signals using a single transformer representing less effort.

At the tapping points, the signals can be tapped via capacitors or a transformer. A digital filter is particularly suitable for low-pass filtering, since it is particularly easy to program and its filtering effect can be changed. In this case, the signal must be digitized at the tap with an analog / digital converter and fed to the digital filter. At the end of the digital signal processing chain, which can also include inversion, the signal must be converted back into an analog signal by means of a digital / analog converter. All digital signal processing operations can advantageously be carried out in a digital signal processor. In the case of a two-sided filter with two tapping points, the filter functions and counter if necessary, the inversion function for both signal sections can also be carried out by a single processor. To acquire the signals tapped at two tapping points, either two analog / digital converters can be provided, or an analog / digital converter with an analog multiplexer or with two inputs can be used.

The signal tapped at a tap can advantageously also be used to obtain data transmitted on the telecommunications line. For this purpose, the tapped signal can be demodulated or fed directly to a decision maker who extracts the data from the signal.

This is particularly useful in the case where telephone signals with data transmission signals are to be transmitted together over one line. In this case, the unnecessary, higher-frequency components that could come into conflict with the data signals must be removed from the telephone signal. For this purpose, on the side of the transformer on which only the telephone signal is to be present, a signal is tapped, high-pass filtered and inverted according to the invention - Coupled back in via the transformer. On the other side of the transformer, the telephone signal together with the data signal are on the telecommunications line. In order to prevent the data signal present on the other side of the transformer from spreading into the section of the telecommunications line with the telephone signals, a signal is also tapped on the mixed signal side, on which telephone signals and data signals are present, and after

Inverted high-pass filtering coupled in via the transformer. The signal tapped on the mixed signal side is simultaneously demodulated, so that the tapped signal is used both for obtaining the data and for filtering the data signals. In this application it can be provided that the signal is tapped on the telephone signal side using an inexpensive capacitor and on the mixed signal side using a transformer. The tap on the mixed signal side by means of a transformer has the advantage that data transformers on the mixed signal side can also be fed into the telecommunication line via this transformer. The data fed in can advantageously also be taken into account in the filter circuit in order to block the data signals coupled in at the tapping point on the mixed signal side on the transformer. In this way, both the data signals arriving on the mixed signal side and the signals fed in there can be blocked on the transformer and prevented from spreading on the telephone signal side.

In an advantageous development, it is conceivable to use a single transformer both for feeding in the compensation signals required for filtering and for feeding in or tapping data.

A control circuit can be provided, in particular to compensate for non-linearities of the transformer. In this circuit, the signal tapped from the telecommunications line at a tapping point is compared with a control signal tapped at the other side of the transformer. The signal obtained at the tap corresponds to the signal on the telecommunication line before filtering, whereas the control signal corresponds to the filtered signal. When comparing these two signals or forming a difference, ideally the signal components that should be removed by means of low-pass filtering or that are the result of high-pass filtering must result. If the desired filter effect does not appear, this can be done by comparing the difference between the signal at the tap and the control signal and the high pass filter obtained at the tap. th signal can be recognized. This information can be used to optimize the filter effect and in particular to compensate for non-linearities of the transformer. If a signal processor is used for signal processing, the control loop can also be implemented digitally in the signal processor.

The invention is explained in more detail below on the basis of preferred exemplary embodiments with reference to the accompanying drawings.

FIG. 1 shows the structure of a low-pass filter according to a first exemplary embodiment of the present invention,

FIG. 2 shows a symmetrical low-pass filter according to a second embodiment of the present invention,

FIG. 3 shows a two-sided low-pass filter for receiving data according to a third exemplary embodiment of the present invention,

FIG. 4 shows a low-pass filter with devices for transmitting and receiving data and a correction control according to a fourth exemplary embodiment of the present invention, and

FIG. 5 shows a low-pass filter with a device for sending data and a correction control according to a fifth exemplary embodiment of the present invention.

1 shows a circuit arrangement for low-pass filtering of a signal transmitted via a telecommunication line 2, 3. The telecommunications line consists of a telephone signal section 2, into which a provider of a telephone service feeds the telephone signal, and a transmission section 3, which corresponds to the line to the end user. The circuit arrangement shown in Figure 1 is intended for use on the service provider side. The signal fed into the telephone signal section 2 comprises, in addition to the low-frequency telephone useful signal, also higher-frequency interference components and is low-resistance. A not inconsiderable current flows via the telecommunication line 2, 3 and is used on the end user side to operate a telephone. The telephone signal section 2 is separated from the transmission section 3 by means of a transformer 1, the low-resistance secondary winding of which is looped into the telecommunication line 2, 3. The secondary winding has a low resistance and has a resistance of 0.3 to 0.4 ohms, for example. Two capacitors 4 for tapping a signal are connected to the telephone signal section 2. An analog / digital converter 6 is connected to the capacitors 4, the output of which converts with the input of a digital

Filters 7 is connected. In the digital filter 7, which is implemented by means of a digital signal processor, the signal tapped off by means of the capacitors 4 on the telephone signal section 2 is high-pass filtered. The desired spectral components of the telephone signals are removed from the signal, so that the higher-frequency interference signals to be removed are at the output of the digital filter 7. The output of the digital filter 7 is connected to a digital / analog converter 8, in which the signal is additionally inverted. The output of the digital / analog converter 8 is connected to a driver 9, which in turn is connected to a primary winding of the transformer 1.

By coupling the inverted interference signals, these are compensated in the telecommunication line 2, 3, so that only the desired spectral components of the telephone signals are present on the transmission section 3 of the telecommunication line 2, 3. However, this low-pass filtering only acts in the direction from the telephone signal section 2 to the transmission section 3. A low-pass filter according to a second embodiment is shown in FIG. In this embodiment, the low-pass filter effect is achieved in both transmission directions. In addition to the components of the low-pass filter according to the first embodiment, the low-pass filter according to FIG. 2 has a second pair of tapping capacitors 5, which are connected to a second analog / digital converter 10. The second analog / digital converter 10 is followed by a second high-pass filter 11, the output of which is connected to a second digital / analog converter 12. The second tapping capacitors 5 are connected to the transmission section of the telecommunication line. Thus, on both sides of the transformer 1, a signal is tapped from the telecommunications line, digitized, high-pass filtered, and coupled in in an inverted analog manner by means of the transformer 1. This symmetrical structure results in the same transmission characteristic in both directions of transmission or a low-pass filter effect is achieved in both directions of transmission. The two analog / digital converters 6, 10, the two high-pass filters 7, 11 and the two digital / analog converters 8, 12 are each to be understood as function blocks, which can also be combined depending on the desired circuit structure. In particular, the two high-pass filters 7, 11 can be implemented with a single signal processor.

In the low-pass filter shown in FIG. 2, different transmission characteristics can be achieved for the two high-pass filters 7, 11 by programming the two high-pass filters 7, 11 differently. If, for example, such a low-pass filter is used as a splitter in a telecommunications network to separate data signals and telephone signals, the filter can be designed in such a way that the undesired high-frequency components of the telephone signal are removed in the transmission direction from the telephone signal section 2 to the transmission section 3 and the data signals in the opposite direction be removed. In In this case, an additional device (not shown) must be used to ensure that the data signals are tapped at the transmission section 3 or are coupled in there.

The low-pass filter shown in FIG. 3 according to a third embodiment is used to remove undesired high-frequency interference signals from a telephone signal in one transmission direction and to tap data signals in accordance with the DSL standard in the other transmission direction and to remove them in the telecommunications line 2, 3. For this purpose, a transformer 1 with its low-resistance secondary winding is looped in again between the telephone signal section and the transmission section 3. As in the previously described exemplary embodiments, a signal is tapped off from the telephone signal section 2 by means of two tapping capacitors 4. The tapped signal is high-pass filtered and inverted in a signal processing chain with an analog / digital converter 6, a digital high pass 7 and a digital / analog converter 8 as described above. Finally, the output signal of the digital / analog converter 8 is fed into the primary winding of the transformer 1 via a driver 9. This structure corresponds to the low-pass filter according to the first embodiment and effects low-pass filtering in the transmission direction from the telephone signal section 2 to the transmission section 3 in order to remove the undesired higher-frequency interference signals of the telephone signal.

A DSL transformer 13 is connected to the transmission section 3 and is used for tapping data. The output of the DSL transformer 13 is connected to an amplifier 14, the output of which is connected to an analog / digital converter 15. At the output of the analog / digital converter 15, the data signal tapped by the transmission section 3 is present as a DSL signal. The DSL signal 16 is fed to the high pass 7 in the same way as the signal tapped at the telephone signal section 2. This means that the trans- formators 1, the high-pass filtered DSL signal 16 is also coupled inverted. In this way, the data signals are blocked by means of the transformer 1 and cannot spread into the telephone signal section 2.

In the low-pass filter shown in FIG. 4, a device for coupling in data is additionally provided. In principle, this low-pass filter has the same components as the low-pass filter according to the first embodiment in FIG. 1. As in the exemplary embodiments described above, a low-pass filter effect in the transmission direction from the telephone signal section 2 to the transmission section 3 is achieved with the aid of the signal tapped at the telephone signal section 2 by means of the capacitors 4, which is then coupled in again in a high-pass filter and inverted by means of the transformer 1. to keep undesired higher-frequency interference components of the telephone signal away from the transmission section 3.

In addition, in this exemplary embodiment, however, a correction amplifier 18 is provided, which receives as input signals, on the one hand, the signal picked up by the capacitors 4 on the telephone signal section 2 and, on the other hand, a control signal picked up on the transmission section 3 via second capacitors 5. The correction amplifier 18 forms the

Difference between these two signals, which is used to control the filter effect. Ideally, the signal difference formed by the correction amplifier 18 corresponds to the output signal of the high-pass filter 7. If, particularly due to non-linearities of the transformer 1, the higher-frequency interference signals of the telephone signal have not been completely removed by the signals fed in by means of the transformer 1, the signal difference formed by the correction amplifier 18 differs from the output of the high pass filter 7. Depending on the signal difference formed and the output signal of the high-pass filter 7, the correction amplifier 18 generates a correction signal which is connected to the connection path between the digital / analog converter 8 and the driver 9. The correction signal is formed in such a way that the signal difference corresponds as closely as possible to the output signal of the high-pass filter 7 or that the desired low-pass filter effect is established.

In addition, a DSL signal 17 is applied to the input of the high-pass filter 7. This DSL signal 17 is provided for transmission via the transmission section 3 and is to be coupled in by means of the transformer 1. Since this signal lies in a frequency band above the frequency band of the telephone signals, it passes through the high-pass filter 7 and, after inversion, is coupled into the telecommunication line 2, 3 by means of the driver 9 and the transformer 1. In this embodiment, the separate DSL transformer for coupling the DSL signal is advantageously omitted. In a further development of the embodiment shown in FIG. 4, it is conceivable to tap and output DSL data arriving on the transmission section 3 likewise by means of the second capacitors 5 which are present anyway.

FIG. 5 shows a low-pass filter according to a fifth embodiment, in which DSL data 17 are to be transmitted via the transmission section 3 in addition to the low-pass filtering with control function. As in the previous exemplary embodiments, the low-pass filter is designed by means of the signal processing chain comprising an analog / digital converter 6, high-pass filter 7, inverting digital / analog converter 8, driver 9 and transformer 1, the input of the analog / digital converter 6 using two tapping capacitors 4 is connected to the telephone signal section 2. In addition, as in the fourth embodiment, a correction amplifier 18 is provided which, as in the fourth embodiment, Example described the function of the low-pass filter optimized.

In addition, the low-pass filter shown in FIG. 5 has a DSL transformer 13, the secondary winding of which is looped into the transmission section 3 of the telecommunications line. The primary winding of the DSL transformer 13 is connected to a DSL driver 19, which is acted upon by the DSL signal 17 to be transmitted. The DSL signal 17 is thus coupled into the transmission section 3 after the transformer 1. In order to prevent the DSL signals coupled into the transmission section 3 from spreading into the telephone signal section 2, the DSL signal 17 is simultaneously switched to the input of the high-pass filter 7. This has the effect that an inverted signal corresponding to the DSL signal 17 is coupled in by means of the transformer 1, so that a low-pass filter effect is produced in the transmission direction from the transmission section 3 to the telephone signal section 2, which blocks the DSL signals 17. In this way, the higher-frequency interference signals of the telephone signal present in the telephone signal section 2 are prevented from spreading into the transmission section 3 and the DSL signals 17 coupled into the transmission section 3 are prevented from spreading into the telephone signal section 2.

Claims

claims

1. Method for low-pass filtering a signal transmitted via a telecommunication line (2, 3), in which method

- The signal is tapped at a first tap (4) of the telecommunications line (2, 3),

- the tapped signal is high-pass filtered, - the high-pass filtered signal is inverted and

- The inverted signal is coupled into the telecommunication line (2, 3) by means of a transformer (1).

2. The method according to claim 1, characterized in that the signal is tapped at a second tapping point (5) of the telecommunications line (2, 3), the transformer (1) between the two tapping points (4, 5) of the telecommunications line (2, 3 ) is arranged, the signal tapped at the second tap (5) is high-pass filtered, inverted and coupled into the telecommunications line (2, 3) by means of the transformer (1).

3. The method according to any one of the preceding claims, characterized in that the signal is tapped at a tap via at least one capacitor (4, 5).

4. The method according to any one of the preceding claims, characterized in that the signal is tapped at a tapping point via a tapping transformer (13).

5. The method according to any one of the preceding claims, characterized in that a tapped signal is digitized and digitally high-pass filtered.

6. The method according to any one of the preceding claims, characterized in that data transmitted from a tapped signal via the telecommunication line (2, 3) is obtained.

7. The method according to any one of the preceding claims, characterized in that a signal is tapped at a tapping point via a tapping transformer and data signals (17) are coupled into the telecommunication line (2, 3) via this tapping transformer.

8. The method as claimed in one of the preceding claims, characterized in that high-frequency components of a telephone service transmitted via the telecommunications line (2, 3) are removed by means of the low-pass filtering.

9. The method as claimed in one of the preceding claims, characterized in that data signals superimposed on a telephone service transmitted via the telecommunications line (2, 3) are removed by means of the low-pass filtering.

10. The method according to claim 8 and 9, characterized g e k e n e z e i c h n e t that in a one-way via the telecommunications line (2, 3) transmitted telephone service signal, the high-frequency, with the frequency range of a data transmission service in

Conflicting parts are removed and in the case of a signal mixture transmitted in the opposite direction from a telephone service signal and a data transmission signal, the data transmission signal is removed by low-pass filtering.

11. The method according to claim 8 or 9, characterized in that the data transmission service is an xDSL service.

12. The method according to any one of the preceding claims, characterized in that on both sides of the transformer (1) a signal is tapped on the telecommunications line (2, 3), the difference between the two tapped signals is formed, and on the basis of the difference and the high-pass filtered tapped signal the effect of low pass filtering is determined.

13. The method according to claim 12, characterized in that the signal coupled in by means of the transformer (1) is regulated in such a way that the determined filter effect comes as close as possible to the target filter effect of the low-pass filtering.

14. Device for low-pass filtering a signal transmitted via a telecommunication line (2, 3), with a signal tapping device (4, 5), a transformer (1) arranged in the telecommunication line (2, 3) and a signal processing device (β-9) is set up in such a way that it high-pass filters a signal picked up by the tapping device (4, 5) from the telecommunication line (2, 3), inverts the high-pass filtered signal and couples it into the telecommunication line (2, 3) by means of the transformer (1).

15. The device according to claim 12, characterized in that the device has devices for analog / digital conversion, devices for digital filtering (7, 11) and devices for digital / analog conversion (8, 12).

16. The apparatus according to claim 12 or 13, characterized in that the device for carrying out a method is set up according to one of claims 1 to 11.