US2489948A - Radio receiving system - Google Patents

Description

Nov. 29, 1949 5. .5514 2,489,948

RADIO RECEIVING SYSTEM Filed Aug. 14, 1944 2 Shets-Sheet 1 DAVID A. BELL Nov. 29, 1949 D; A. BELL 2,489,948

RADIO RECEIVING SYSTEM Filed Aug. 14, 1944 2 Sheets-Sheet 2 AMPLFR.

.I 32 :[wua/rvwt 7w 7 DAVID A. BELL Patented Nov. 29, 1949 UNITED STATES NT OFFICE RADIO RECEIVING SYSTEM ish company Application August 14, 1944, Serial No. 549,459 In Great Britain September 17, 1943 3 Claims.

This invention relates to apparatus for demodulating an input signal which consists of a succession of identical direct voltage pulses, which signal is angularly modulated (that is to say the frequency or phase of pulse repetition is modulated) in accordance with the intelligence to be conveyed.

The original received input signal, unless it already consists of a succession of identical direct voltage pulses, must be converted into such form. If, for example, it consists of a succession of short trains of radio-frequency oscillation, this conversion may be effected by an ordinary detector, together with a limiter if necessary.

The invention makes use of a known method of effecting demodulation of such an input signal, namely mixing with the input signal consisting of a succession of identical direct voltage pulses a saw-tooth oscillation having the same mean frequency as the signal in order to produce an output component which varies in accordance with the phase relationship of the si nal and the saw-tooth oscillation and therefore represents the modulation of the input signal.

The maximum depth of time or phase modulation, that is the maximum deviation of frequency or phase, that can be dealt with by the known method referred to is limited by the fact that it must be arranged that only one of the direct voltage pulses occurs during each cycle of the saw-tooth oscillation. It is known to be desirable to use a relatively high depth of modulation at least over the earlier part of a communication channel in order to obtain a favorable signal to noise ratio and the present invention has for its object to permit the depth of modula-. tion to be increased beyond what is possible with the known method referred to.

For this purpose the invention applies a novel form of negative feedback which while permitting the depth of modulation to be increased also has, in common with known forms of negative feedback, a beneficial effect upon the faithfulness of reproduction of the original modulation. The negative feedback is applied in the present invention to vary the instantaneous frequency or.

phase of the saw-tooth oscillations in accordance with the instantaneous frequency or phase of the input signal and to nearly but not quite the same extent. The effect of this, as with known forms of negative feedback, is to reduce the amplitude of the output signal, but the reduction in output signal can readily be compensated for by further amplification at modulation lation shall not vary to such a degree that during a cycle of the saw-tooth oscillation either more or less than one pulse shall occur. The frequencyor phase-following by the saw-tooth oscillation may, however, be so close that the modulation may have a depth of many cycles of the mean pulse repetition frequency.

In the accompanying drawing, Figures 1 and 3 are circuit diagrams of receivers for radio signals consisting of a succession of short trains of radiofrequency oscillation, the frequency of repetition of which is modulated. Figures 2a, 2b and 2c are diagrams to illustrate the operation of the receiver shown in Figure 1.

In the receiver shown in Figure l, the radio signal is picked up by the aerial 22 and selected in the radio-frequency tuned circuit comprising inductance l and condenser 2. The voltage developed across this tuned circuit is applied to the input grid of the pentode valve 3, which is arranged to operate as an anode-bend detector.

By this means each train of oscillations in the received signal produces a pulse of current tothe anode 4| of the valve 3, and these pulses of current in turn are converted into direct voltage pulses by the anode resistance 24. This voltage is applied via the lead 42, D. C. blocking condenser 5 and lead 43 to the mid-point of the,

secondary winding of transformer 21.

This transformer forms part of a balanced diode circuit which comprises the transformer I 27, diodes 25 and 26, diode load resistances 28 and 29, the latter being shunted by condensers 30 and 3!; and the output from the balanced diode circuit is delivered to the audio-frequency amplifier 33 through the transformer 32. Two voltages are applied to the balanced diode circuit, the first consisting of the pulses applied through the lead 43 from detector 3, as already described, and the second being a saw-tooth voltage oscillation applied through the primary of the transformer 21.

The saw-toothoscillation is generated bytriode valve 35 which 'operates in known manner as a blocking oscillator. The frequency of the sawtooth oscillations is dependent upon the potential applied to the grid 44 through grid -leak resistance 33. Current from the anode 46 of the valve 35 flows through coil 41, which -'couplesto=-the grid coil 48 for the purpose'of maintaining oscillations, and through resistor 36 which is shunted by condenser 31. The anode current of the blocking oscillator consists of 'short pulses, and the time-constant of the network 36,. 31, is such that the time of discharge of condenser 31 through resistor 36 isconsiderably greater than its time of charging through the .anode-to-cathode pathof triodei3 5. The"pulse.output .normally produced'by the blocking oscillator is thus converted into an"oscillation"of saw-tooth (but not linear saw-tooth) "form. "The condenser 37 must,'

of course, be considerably discharged'between successive pulses "from the blocking oscillator; the time-constant ofthenetwork'36, 3'5 should therefore be of'theorder' of half the mean pulseleads 39, 50, is developedacross the anode loads 1 of the diodes, comprising resistors 28, 29, with parallel condensers 36;3 I. Thesefcondensers. are designed'to by-pass components having frequencies of the order of the pulse-repetition frequency, so that it is the modulation-frequency component of the resultant which appears as a voltage across the anode loads. "Thejunction of "theresistors 28 and ZQ is at earth potential (and is connected to the cathode of valve 3'by lead 53) and therefore the modulation -frequency signal is taken through audio-frequencytransformer 32 "before amplification in' amplifier :33 and reproduction in loud speaker 34.

The modulation "frequency signal developed across resistors 28"and29 is also'app'lied throughleadsfil and52 to the catho'de45 and to the end of leak resistance 38"to'controlthefrequency of the blocking oscillator. In this .case the direct component of'the signal is required'to "be conserved, and thereforea separate high tension battery B2 isj'provi'de'd tosupp'ly this oscillator. The conservation of the direct component is necessary so that themean frequency -of the oscillator shall coincideiwith the mean repetitionfrequency of the pulse signal.

The blocking .oscillator comprising triode 35 is designed to oscillate at approximately the mean repetitionfrequency of the piilses 3 in the signal to be received andfits frequenc ,of osc -illation is highly dependent on the voltage applied between the cathode of triode 35 and the end of grid leak 38. The modulation frequency component of the voltage developed across resistors 28 and 29, being so applied, is therefore able to control the frequency of the saw-tooth oscillation output which is developed across resistor 35, which is thence applied through D. C. stopping condenser 39 to the primary of transformer 21. The degree of this control is such that the frequency or phase of the saw-tooth oscillation is modulatediin'accordance with, and to nearly the same depth as, the frequency or "phase modulation of the signal developed across resistor 24.

the circuit of each of the diodes 25 and 26,

:the-saw-toot-h voltage injected through transformer 2'! and the pulse voltage applied through -"condenser"5-=will be added. The resultingwaveform of voltage applied in the circuit of diode 25 is shown in Figure 2a. The three successive .:cycles in thisfigurerepresent different phases of the direct'voltage pulse signal with respect to thesaw-tooth voltage. Diode 25 becomes con ducting only when its cathode is negative, so the only parts .of the resultingwave-form which are effective-to vproduce current through the load 28--39 are those represented by the shaded areas Figure 12a.

-Figure 2b similarly represents the voltages applied in the circuit ofdiode '26, the pulses in this figure having the same sense as those in Figure 2a, while. the saw-tooth has opposite sense.

The curve shown in-Figure 2c is obtained by subtracting the shaded-area of Figure 2b from the-shaded area of Figure'Za. lhe resultant outputvoltage from the balanced diode circuit is the modulation-frequency component of the resultant of this-subtraction. It will be seen from inspection of this figure that the contribution from the saw-tooth input, over a whole cycle, is equally positive "and negative, and therefore averages zero; but'the contribution due tothe applied pulses'ma'y be positive, zero or negative,

accordingto the'relative phase of the pulses and thersaw-tooth' wave.

It will be seen that because of thecontrol of the frequency of oscillation ofthe blocking o5- cillator by'the modulation frequency voltage fed thereto through leads 5% and 52, the depth of modulation of the signal received by the circuit of Figure 1, that is to say the frequency or phase deviation of the direct voltage pulses, may be '1 .made much greater than would be possible in the absence of such control of frequency, having re gard to the fact that only one pulse must be allowed to occur during each cycle of the sawtooth oscillation. 'The overall signal to noise ratio can, therefore, be increased as compared with systems in which no such frequency control is provided. In addition the control behaves much in the same way as the well-known forms of negative feedback in amplifier circuits and leads to an improvement in the faithfulness of reproduction of the original modulation.

The multiplicative nature of the circuit will also be seen by consideration of Figure 20. It will be evident that the amplitude of the output will decrease if the amplitude of either the pulses or the saw-tooth be decreased, and will..fall to .zero if either of these be reducedtozero.

The saw-tooth waveeforms are shown in Figures 2a 2b and2c ,aslinear, butthe-reasoning is equally valid in the practical case where they are non-linear.

The receiver shown in Figure 3 is mainly similar to that shown in Figure 1, with the principal exception that a multiplicative mixing valve of the kind which may be generally described as the hexode type, is employed in place of the multiplicative mixing circuit of Figure 1. Such a valve has an output electrode and two input electrodes, and the output signal is the product of the two input signals. The valve actually employed is a pentagrid; this valve, as is well known, con tains, in addition to the necessary electrodes of a simple hexode, a further electrode which is ordinarily employed as an oscillator anode.

In Figure 3 the pentagrid valve Al has the signal consisting of a succession of direct voltage pulses, which is developed across resistor 24, applied to its first control grid 4a through condenser 5 by leads 5a and 5b; and has the saw-tooth voltage oscillation which is developed across resistor 36 applied to its second control grid 4d through lead 36a. Conventional self-biassing is provided by resistor 9 and condenser ill.

The output modulation frequency component of the resultant of the multiplicative mixing of the two input voltages may be obtained from either the anode 4 or from the screen. grids 4b, 40, 4c of valve 4. This component appears on these electrodes in opposite sense. In Figure 3 the component is shown as taken from the screen grids, the choice being made in view of the fact that the mean potential of the screen is lower than that of the anode. The screen current of the valve 4 develops its output potential across the resistance l9. Condenser 2i by-passes components having pulse-repetition frequency, and the output is applied through blocking condenser II to audio-frequency amplifier 33 and thence to loudspeaker 34.

The blocking oscillator of Fig. 3, including valve 35, operates in a manner similar to the blocking oscillator described with reference to Figure 1.

As in Figure 1, it is necessary to transmit the direct component of the output of the multiplicative mixer to the saw-tooth oscillator. The triode 85 of the blocking oscillator therefore has its grid connected through grid leak 38 and lead 38a to the screen grids of pentagrid 4, and has its cathode maintained at a substantial positive potential relative to earth by connection to a tapping on potentiometer i6 and I1, condenser l8 being provided to by-pass oscillations of pulserepetition frequency.

The output circuit comprising resistor 35 and condenser 2c is designed to convert the pulse output of the blocking oscillator to saw-tooth waveform in the manner described with reference to resistor 36 and condenser 3'! of Figure 1.

In order to increase the effectiveness of the received signal, a similar time-constant circuit is included in Figure 3 in the anode circuit of the derived from the received signal are similarly converted to saw-tooth wave-form. This timeconstant circuit consists of condenser 8 connected in parallel with resistor 24 in the anode circuit of detector valve 3.

I claim:

1. In a system for demodulating an input signal consisting of an angularly modulated succession of direct voltage pulses, the combination of a demodulator comprising a saw-tooth oscillation generator, a multiplicative mixer for mixing the output of said generator with said input signal and producing a modulation frequency component, output means deriving the modulation frequency component of said mixer, and means controlled by said component to produce angular modulation of the output of said generator in accordance with, but of a smaller magnitude than that of, said modulation frequency component.

2. A radio receiver for the reception or a signal consisting of a succession of pulses of radio-frequency oscillation, comprising means to convert said pulses into a similar succession of identical direct voltage pulses, a multiplicative mixer valve having two input electrodes and an output electrode, means to apply said direct voltage pulses to one of said input electrodes, a sawtooth oscillator having a connection for supplying a saw-tooth voltage to the other of said input electrodes, a modulation frequency filter connected to said output electrode, and means responsive to the modulation frequency current from said filter for Varying the frequency of said saw-tooth oscillator in accordance with, but of a smaller magnitude than that of, said modulation frequency.

3. A receiver for angularly modulated pulses comprising, in combination, a mixer to which said pulses are supplied, an oscillation generator for supplying saw-tooth waves to said mixer, said generator having a control grid, and the frequency of said saw-tooth waves being controlled by the potential applied to said grid, means for deriving from said mixer a modulation frequency component, and means for varying the potential of the grid of said oscillator in accordance with, but of a smaller magnitude than that of, said modulation frequency component.

DAVID ARTHUR BELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,896,780 Llewellyn Mar. 2, 1931 2,212,648 Poch Aug. 27, 1940 2,273,090 Crosby Feb. 7, 1942 2,391,776 Fredendall Dec. 25, 1945 OTHER REFERENCES Ser. No. 464,750, De France (A. P. C.) pub. June detector valve 3, so that the direct voltage pulses w 8, 1943. I

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