US2441418A - Pulse generator and modulator system - Google Patents

Description

May 11,1948. w. D. HOUGHTON 2,441,418

PULSE GENERATOR AND MODULATOR SYSTEM Filed Sept. 20, 1945 2 Sheets-Sheet 2 NOOUIAT/ON uni/7:: r: 9 fig 4a =5 7;;

fif m V 157E415 I I l N V EN TOR. Y 1441: MM 22 flow/r70 lrroe/viy Patented May .11, 1948 PULSE GENERATOR AND MODULATOR SYSTEM William D. Houghton, Port Jefferson, N. Y., assignor to Radio Corporation of America, a, corporation of Delaware Application September 20, 1945, Serial No. 617,634-

12 Claims. 1

This invention relates to a pulse signaling system, and more particularly to a method of and apparatus for generating pulses of constant average frequency and varying their occurrence time within fixed limits.

An object of the present invention is to enable the phase or timing modulation of pulses of constant frequency at an audio rate within fixed limits regardless of the amplitude. of the applied modulation.

Another object is to provide a pulse generator and pulse timing modulation system suitable for use in a multiplex system in which a pulse is produced for each assigned period regardless of the magnitude of modulation.

A further object is to provide an improved pulse generator which is synchronized and controlled by a triangular wave generator.

A more detailed description of the invention follows, accompanied by a drawing wherein:

Figs. 1, 2 and 3 illustrate three different einbodiments of the invention, and

Figs. 4a to 4] are curves given in explanation of the operation of the invention. 7

Throughout the figures of the drawing, the same or like parts are represented by like reference characters. 7

Referring to Fig. l in more detail, there is shown a simplified form of pulse generator and modulator system of the invention-comprising a triode vacuum tube V1 normally biased to be non-conducting and having its anode connected to a source of positive polarizing potential +3 through one winding of a difierentiating transformer T, its grid G connected to the outputof a saw-tooth voltage generator, and its cathode K connected to the anode of a modulator tube V2 and also to ground via resistor R1. Any appreciable flow of current through tube V1 must be through tube V2 since resistor R1 is large in value. In shunt to resistor R1 there is provided a capacitor C1 which is large enough to maintain the bias required on tubevi but small enough so that it does not cause undue attenuation of the higher audio frequencies developed across tube V2 from the audio modulation, as described later. The modulator tube V2 comprises a triode vacuum tube to whose grid there isapplied the audio input modulation from terminal 2 over resistors R4 and R3. Resistor R3 is a limiting resistor in the grid circuit of tube V2 which prevents the grid from being driven positive above the zero voltage value. Resistor R1Iis a potentiometer which is used to controlthe amplitude of the audio modulation voltage applied to thegrid of V2. A- resistor R2 in the cathode circuit of tube V2 enables a control of the anode resistance of tube V2 and; hence a control of the value at which tube V1 conducts on the rise of the sawtooth.

In theoperation of the system of Fig. 1, tube V2 acts as a resistance in the cathode circuit of tube V1. The value of the resistance offered by tube V2, is a function of its grid-to-cathode volt- 10 age and is determined by thesetting of the tap on resistor R2.

- The saw-tooth voltage generator I supplies the grid; of tube: V1 with a saw-tooth wave of positive polarity having a shape similar 'to that shown in the graph of Fig, 4a. Tube V1 is normally biased to be non-conducting, and is arranged (due to; the setting, of R2) to conduct at about the middle ofthe applied saw-tooth voltage wave with no audio voltage present on terminal 2'. Ob-

viously, any variation in the resistance of tube V2 will result in a variation of the point on the saw tooth voltage wave at which tube V1 will conduct, inasmuch as the grid-to-cathode voltage of V1 is determined by the average current through V1 and the value of the resistance ofierecl by tube V2. g

When tube V1 conducts, current flows through differentiation transformer T resulting in a pulse of voltage appearing at output terminal 3 in circuit with the secondary windin of the transused in the following stages.

generated at terminal 8 a positive pulse when tube V1 starts to conduct and a negative pulse when tube V1 ceases conducting. Since the starting time of V1 is the variable, then the correspondingly produced pulse is the one which 'varies with the modulation. The $01111 line pulse in the curve of Fig. 4b shows the position of this output pulse relative to the applied saw-tooth wave, with no audio modulation on input terminal 2. The current cut-off to current limiting region of tube V1 is made to be small with respect to the amplitude of the applied saw-tooth wave because a sharp current change is desired in tube V1, and this region is moved over the slope of the saw-tooth wave with modulation.

The audio modulation voltage applied to the grid of tube V2 varies the resistance of tube V2 and the bias on tube V1, hence varying the critical value or time at which tube V1 starts to conduct over the range of the applied saw-tooth wave. When the starting point of current flow in tube V1 is varied by the modulation, the occurrence time of the resulting output pulse in transformer T is also varied with respect to the leading and trailing edges of the applied saw-tooth Wave. When audio voltages are applied to the grid of tube V2 of sufficient magnitude to cut off the flow of current in tube V2, then the resistance in the cathode circuit of tube V1 becomes the resistance of R1. The value of R1 is such that with the flow of current in tube V2 cut off, the tube V1 will conduct at the peak of the applied saw-tooth wave. This feature is important because it assures an output pulse from. the system for each applied saw-tooth wave in the event tube V2 is cut off by excessive modulation.

As mentioned above, resistor R3 is a grid limiting resistor. Hence the resistance of modulator tube V2 cannot go below a minimum value regardless of the amplitude of the audio modulation voltage applied to the grid of this tube. This minimum value is such that tube V1 will conduct at the leading edge of the saw-tooth wave. Consequently, regardless of the amplitude of the applied audio modulation, the position of the output pulse at output terminal 3 always falls within the time interval or duration of the applied sawtooth wave. This is shown by the dotted line pulses in Fig. 4b which illustrate the limiting positions of the output pulses with extreme positive and negative modulation voltages.

A more detailed description is as follows: Tube V1 is normally made to conduct at the middle of the linear rise time of the applied sawtooth wave. After V1 startsto conduct, it continues to conduct for the remaining portion of the linear rise time.

When tube V1 conducts, a' charge is stored in condenser C1. This charge leaks off through the impedance of tube V2 and R1 during the time interval during which V1 is non-conducting. Since R1 is large compared to the impedance offered by tube V2, its effect may be neglected as long astube V2 is conducting. If the value of C1 is very large it willbe seen that it will require a number of cycles of the sawtooth wave to reach stability, that is, when the charge stored in C1 during each sawtooth is exactly balanced by the decrease in charge during the time interval when V1 is cut-off. Also, when the voltage on the grid of tube V2 is changed it will require a number of cycles for a new equilibrium condition to be obtained. For example, when the bias on tube V1 is zero and C1 is large, it will require a number of cycles of the applied sawtooth wave before the charge stored in C1 is balanced by the decrease in charge provided by tube V2. If after this equilibrium condition is reached and the bias on tube V2 is set at a value where tube V1 is conducting at the middle of the linear rise portion of the applied sawtooth wave, the voltage on the grid of tube V2 is made negative the charge leaked off by tube V2 will not be as great as the charge stored when tube'V1 conducts; hence the voltage across C1 will rise and tube V1 will conduct for a smaller portion of the applied sawtooth. After a number of cycles, a new equilibrium will be established with tube V1 starting to conduct at a higher or later point on the sawtooth, thus producing a pulse in the output winding of transformer T at a later time than when the bias on tube V2 was zero. Likewise, if the bias on tube V2 is made positive, the charge leaked ofi by tube V2 will be greater than that stored when tube V1 conducts and hence tube V1 starts to conduct at a lower point on the sawtooth. After a number of cycles, a new equilibrium point is reached where tube V1 is conducting for a greater portion of the applied sawtooth than when the bias on tube V2 was zero.

It will thus be seen that if condenser C1 were very large it would require a large number of cycles of the sawtooth wave before equilibrium would be reached. It is common practice to make the pulse carrier (the sawtooth frequency) two or three times the highest audio frequency and it is therefore necessary to make condenser C1 of such a value that equilibrium may be established within this time. C1 must have a minimum such that with the type of tube used for V2, a bias will be developed suflicient to cause tube V1 to conduct at the middle of the linear rise time.

Fig. 2 shows modification of the system of Fig. 1. In Fig. 2, audio modulation voltage from input terminal 2 is first applied to the grid of a cathode follower or cathode amplifier tube V3 whose cathode is connected to the cathode of tube V2. Both tubes 'V2and V2 use the same common cathode resistor R2. Any variation in current through vacuum tube V2 in response to audio modulation from input terminal 2 causes a variation of the cathode voltage of tube V2. Because the grid voltage of tube V2 is fixed, this variation in cathode voltage changes the average anode resistance of tube V2 and hence the cathode bias of tube V1.

Fig. 3 illustrates a third modification of the invention in which a direct current voltage is impressed on the grids of tubes V1 and V2 by means of the bleeder circuit composed of resistors R5, R6 and R7. Here again, the audio modulation is applied to the cathode follower tube V3, but the modulation voltage is now inserted in the grid-to-cathode circuit of tube V2 across resistor R2; A degeneration resistor R8 in the cathode of V2 makes the anode resistance of tube V2 vary in a linear manner with applied audio voltage. The saw-tooth voltage from generator l is now applied to the grid of tube V1 through a condenser C2. The connections are such that there'is a direct currentvoltage' applied to the grid of tube V1 to ether with the saw-tooth wave. This raisesfthe average voltage on the grid of tube V1 and as a result raises the direct current anode voltage of tube V2. This allows tube V2 to work at a more suitable value of anode voltage. The amplitude of the applied saw-tooth voltage, however, remains the same in both conditions.

The maximum advance position in time of an output pulse-from the system of Fig. 3 due to negative modulating voltage is determined by the minimum anode resistance of tube V2 (maximum conduction condition) which occurs when tube V3 is cut off, and the actual value of this minimum anode resistance of tube V2 is set by the values of resistors Ba and R2. When a maximum positive modulating voltage is applied to tube V3, its current will cut 01? tube V2 and leave resistor R1 as the biasing resistor for tube V1. Thus the flow of current through resistor R1 when tube V2 is cut oif, automatically biases tube V1 so that it yields a useful pulse at the top position of any saw-tooth wave applied to the grid of tube V1.

The system; of the invention can be used in a pulse multiplex'communication system wherein different transmitting -channelsv have individual pulse generator and modulator circuits; like those shownii-n Figs. .1, .2 on3. :Whcn 'usedinamultiplea-system, the different pulse generator circuits are supplied with separate saw-tooth voltages, each displaced in time so that the start of one --saW-tooth fellows theideca of another saw-tooth limits of the pulse range when a channel is fully modulated. Only three channels are represented in Figs. through 4], although more channels .may be used. One suchpulse multiplex system utilizing the present invention is described in my copending application Serial No. 608,957, filed August 4, 1945. v

One advantage of using the invention in va pulse multiplex system lies in the assurance that a pulse will be generated in. each channel for each assigned period regardless of the presence of excessive modulation. This prevents .crossrno'dulation or cross-talk in the channels which may otherwise occur due to a change in power levels throughout the system if a pulse were not passed on the peaks ofmodulation by a channel. to

.The invention is not limited to th precise detailsshown in the drawingsince various changes can be made by the substitution of equivalent elements without departing from the spirit and scope of the invention. As an illustration, the

transformer T can bev replaced bya differentiating coil one terminal ,of which can be con- .nected to the anode of V1, and the other terminal .connected to the positive terminal of a source of uni'directionalpotential. In this case, the out- .put would be obtained from the anode of tube V1 via a, coupling condenser to .rem'ovethe direct current from the output. With such an arrangement, the pulse which is varied with modulation would .be negative, while the unmodulated pulse wouldbe positive. Alternatively, the transformer T could be replaced by a center-tappedlinductance coil one terminal of which can be connected to the anode of V1 while the other terminal is connected to +13. In this last case, the output pulse (pulse which is varied with modulation) would be positive and would be coupled to utilization circuits via a coupling capacity to remove direct current from the output.

If a variable width output pulse is desired, the transformer'T can bereplaced bya resistor of suitable value to provide a desired build-up and decay of the pulse. The variable width pulse generated in this event would be negative in polarity.

The invention can be used wherever there is need for a pulse generator controlled by a sawtooth wave, with or without the modulation feature. For example, the use of tubes V1 and Vi and associated circuit elements can be used in a radar system where the saw-tooth sweep wave for the cathode ray device is generated simultaneously with the outgoing wave. This sawtooth sweep wave can be applied to the, grid of tube V1. By calibrating resistor R2 in distance and adjustingRz manually, it is possible to produce amarker pulse-which can be made to occur at the same instant as the echo-wave is received.

For this particular application of the invention,

there'is no modulation applied to tube "V2.

The-term ground used in the appended claims is deemed to include any point orsu-rface of zero potential for alternating current or direct current, and may be considered a point of reference potential. v

What is claimed is:

1. A pulse generator comprising firs and sec- 0nd vacuum tubes each having an anod acath- .ode, and a controlelectrode, a source of tri ngular waves coupled to'the control electrode of said first tube, a. differentiating circuit connected between the anode of said first tube and the positive terminal of a source :of unidirectional potential, a connection from the cathode of said first tube to the anode of said second tube, aresistor ,in the cathodecircuit :of said second tube, a connection between the .grid .ofsaid second tube ands i cathode resistor, and a resistor -shuntedby a capacitor connected-between the :anodeiof said second tube and that terminal of the cathode resistor farthest removed from the .calthod of said second tube, said second tube functioning as .a resistance in the cathode circuit of said first tube, the value of said capacitor :being such that said first tube is normally biased to the anode current cut-off condition, the efiectiveanode resistance of said second tu-be determiningthe point on the applied triangular wave :at which said first tube conducts, and an output terminal cou- -pled to said differentiating :circuit.

'2. A pulse generator comprising first-and second vacuum tubes each having .an anode, .a cathode, and a control electrode, a :source of triangular-waves coupled to the control electrode of said first tube, a differentiating transformer connected between the anode of said first tube and the positive terminal of a source of unidirectional potential, a connection from the cathode of said first tube to the anode of .said

second tube, a variable resistor in the cathode circuit of said :second'tube, a-connectionbetween the grid of saidsecond tube andsaid cathode resistor, and a--resistor connected between I the anode of said second tube and that terminal of the cathode resistor farthest removed from the cathode of said second tube, said second tube functioning as a resistance in :thecathode circult of said first tube, means-normally biasing said first tube to the anode current .cut-oii con- 'diticn, the effective anode resistance-of said second tube determining the point on the applied triangular wave at which said first tube "conductsjand an output terminal coupled to said differentiating transformer.

3. A pulse gene-rator comprising first and sec- -ond vacuu-mtubes each having "an anode, a

cathode, and a control electrode, asource of -.-ma1'anode resistance of said second tube being such and the circuit components having-such values and so arranged that said first tubeis normally biased to the anode current cut-oif condition in the absence of an applied triangular wave and said "firsttube conducts lat -a :point inmtermediate the ends of :the iorward slopeuof the coupled to said difierentiati'ng' circuits 1 j applied triangular wave} and; an output terminal 1. A pulse generator comprising first and second vacuum tubes each having an anode, a

cathode, and a control electrode, a source of tri- 1 angular waves coupled to the control electrode tube, a' resistor connected between'the cathode of 'said second tube and ground,-a connection from the control electrode of said'second'tube; to

a variable tap on said'last resistonfa resistor shunted by a condenser 'connectedlb'etweenfthe anode of said second tubezandground, the anode resistance of said second tube being such and said condenser having such valuethat said firsttube is normally .biased to cut-:ofi. in ,theabsence of an applied triangular wave, and an output terminal coupled to said differentiating circuit.

5. A pulse generator comprising first and',sec- 0nd vacuum tubes each "having an anode, a cathode, and a control electrode, a source of triangular waves coupled to the control electrode of said first tube, a difierentiating circuit connected between the anode of said'first tube and the positiveterminal of a source of unidirectional potential, a connection from the cathode of said first tube to the anode of said second tube, a resistor. connected between the cathode of said second tube and ground, a resistor connected between the control electrode of said second tube and ground, a resistor shunted by a condenser connected betweenthe anode of said second tube and ground, the normal anode resistance of said second tube being such and said condenser having such value that said first tube is normally biased to the anode current cut-off condition in the absence of an applied triangular wave and said first tube conducts at a point intermediate theends of the forward slope of the applied triangular wave, and means for varying the anode resistance of said second tube in accordance with a modulating voltage to thereby vary the current conducting point of said first tube.

6. 'A pulse generator comprising first and sec 1 0nd vacuum tubes each having an anode, a

cathode, and a control electrode, a source of triangular waves coupled to the control electrode of said first tube, a difierentiating circuit connected between the anode of said first tube and the positive terminal of a source of unidirectional potential, a connection from the cathode of said first tube to the anode of said second tube, a

resistor connected between the cathode of said second tube and ground, a potentiometer having one terminal connected to ground and another terminal connected to a source of modulating voltage, a connection including a grid limiting resistor from the grid of said second tube to a said-first tube, a difierentiating circuit connected between the anodeof said first tube and the positive-terminal of a source of unidirectional potenti'aha connection from the cathode of said first tube-'tothe anode of said second tube, a resistor connected between the cathode of said second tube and ground, a resistor shunted by a conde'nserconnected between the anode of said second tubeand ground, a resistance network connected between said positive terminal and ground, and connections from different points on saidresist'ance network to thecontrol electrodes of said first and second tubes, the value of said condenser and the normal anode resistance of said second tube being such that said first tube is normally biased to the anode current cut-off condition in the absence of an applied triangular wave and said first tube conducts at a point intermediate the ends ofthe forward slope of the applied triangular wave, and an output'terminal coupled to said differentiating circuit.

8; A pulsegenerator'comprising first and second vacuum tubes each having an anode, a

cathode, and a control electrode, a source of triangular waves coupled to the control electrode of said first tube','a differentiating circuitconnected between the anode of said first tube andthe positive terminalof a source of unidirectional potentiaL'a connection from the cathode of said first tube to the anode of said second tube, a resistor connected between the cathode of said second tube and'ground, a resistor connected between the controlelectrode of-saidsecond tube and ground, a resistor shunted by a'condenser connected between the anode of said second tube and ground, the normal anode resistance of said second tube being such that said first tube is normally biased to the anode current cut-oil" condition in'the absence of an applied triangular wave and said first tube conducts at a point intermediate the ends of the forward slope of the applied triangular wave, meansfor varying the anode resistance of said second tube in accordance with an audio modulating Voltage to thereby vary the current conducting point of said first tube, said means including a third vacuum tube whose cathode is connected to the cathode of said second tube and whose control electrode is connected to a source of audioivoltage, and an output terminal coupled to said difierentiating circuit.

9, A pulse generator comprising first and second- 'vacuum tubes each having an anode, a cathode, and a control electrode, a source of sawtooth waves coupled to the control electrode of said first tube, a differentiating circuit connected between the anode of said first tube and the positive terminal of a source of unidirectionalpotential, a connection from the cathode of said first tube to the anode of said second tube, a resistor in the cathode circuit of said second tube, a contube determining the point on the applied sawtooth wave at which said first-tube conducts, means for varying the anode resistance of said second tube in accordance with an audio voltage to thereby vary the current conducting point of said first tube, said means including a cathode follower tube whose cathode is connected to the a connection from the cathode of said first tube to the anode of said second tube, a resistor connected between the cathode of aid second tube and ground, a connection from the control electrode of said second tube to a variable tap on said last resistor, a resistor shunted by a condenser connected between the anode of said second tube and ground, the value of said condenser and the anode resistance of said second tube being such that said first tube is normally biased to cut-oflf in the absence of an applied triangular wave, means for varying the anode resistance of said second tube in accordance with an audio voltage to thereby vary the current conducting point of said first tube, said means including a cathode follower tube whose cathode is connected to the cathode of said second tube and whose control electrode is connected to a source of audio voltage, and an output terminal coupled to said diiferentiating circuit.

11. A pulse generator comprising first and second vacuum tubes each having an anode, a cathode, and a control electrode, a source of triangular waves coupled to the control electrode of said first tube, an impedance network connected between the anode of said first tube and the positive terminal of a source of unidirectional potential, a connection from the cathode of said first tube to the anode of said second tube, a resistor in the cathode circuit of said second tube, a connection between the grid of said second tube and said cathode resistor, and a resistor shunted by a condenser connected between the anode of said second tube and that terminal of the cathode resistor farthest removed from the cathode of said second tube, said second tube functioning as a resistance in the cathode circuit of said first tube, the value of said condenser being such that said first tube is normally biased to the anode current cut-off condition, the effective anode resistance of said second tube determining the point on the applied triangular wave at which said first tube conducts, and an output terminal coupled to said impedance network.

12. A pulse generator comprising first and second vacuum tubes each having an anode, a cathode, and a control electrode, a source of triangular Waves coupled to the control electrode of said first tube, a differentiating circuit connectedibetween the anode of said first tube and the positive terminal of a source of unidirectional potential, a connection from the negative terminal ofgsaid source of unidirectional potential to a point of reference potential, a direct connection from the cathode of said first tube to the anode of said second tube, a resistor between the cathode of said second tube and said point of reference potential, a connection between the grid of said second tube and said cathode resistor, a resistor connected between the anode of said second tube and said point of reference potential, said second tube functioning as a resistance in theba-tliode circuit of said first tube, means for normall'gt biasing the first tube to the anode current cut-on condition, a source of audio modulating voltage cupled to said second tube, and an output terminal coupled to said differentiating circuit.

WILLIAM D. HOUGHTON.

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