US2373616A - Frequency modulation detector - Google Patents

Description

April 10,1945. 6. c. SZIKLAI FREQUENCY MODULATION DETECTOR Filed pc. 15, 1942 SUPER-REGENERATIVE DETECTORS on om T U C Am 6 TO SOURCE of F. M. WAVES zizmkom 51.50 05:4 $8232 45;

INVENTOR GEORGE C. SZIKLAI BY ATTORNEY I Patented Apr. 10, 1945 STATES PATENT OFFICE I a 2,373,616 f g FREQUENCY MonULA'rIoNDE'rEoma George G. Szikl'ai, Princeton, N. .J., assignor to I Radio Corporation of America, a corporationo'f U Delaware Application December 15, 1942, strains. 469,089

Claims. (01. 250 -27) My present invention relates to demodulators of angular velocity-modulated carrier waves, and more especially to a highly sensitive'balanced detector which is essentiallynon-sensitive to. amplitude changes of the modulated carrier energy.

In'his U. S. Patent No. 2,273,090, granted February 17, 1942, M. G. Crosby has disclosed and 1 claimed limiter networks for receivers of angular velocity-modulated carrier wave energy. The limiters disclosed in that patent are amplifier circuits utilizing super-regeneration to effect sub stantially complete elimination of amplitude modulation effects of the received carrier energy. Since the functioning of th super-regeneration limiters of the said Crosby patent is fullydescribed therein, it issufiicient'for this application to point out that the, limiter action is generally due to the. following: The modulated signal which isto'be limited is applied'to a. super-re-.

generative. amplifier, and the radio frequency. en-

, ergy built ;up by the super-regenerative effect is employed as the limited signal. The radio frequency oscillations built up are not detected withv in the super-regenerative tube, but are passed on as radio frequency energy. This energy consists of pulses of oscillations which appear once percycle of the quench frequency. The pulses arepassed through a following selective circuit to be smoothedout so that the'quench frequency does not appear in the output energy fed to a subsequent discriminator-rectifier.

vide. a pair of super reg'enerative detectors with output circuits in push-mull relation, and whose input circuits are tuned apart in excess of the -maximum frequency deviation of received FM waves. I

Yet other objects of my invention are toil nprove andrendersmore efiicientdetectorsof FM waves, and: more especially to provideFM detectors which are highly sensitive,- selective and economical to manufacture andassemble.

The novel features which I believe tobe'characteristic of my invention are set forth with particularity inthe appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken inconnection with the drawing in which I have indicateddiagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing: fig. 1 showsa circuit embodying the inven tion,

Now, I have discovered that an additional function can be derived from the super-regenerative limiter'circuit of thejCrosby type. In additionto securing elimination of amplitude changes of the received carrier, the same circuit, with a simple and economical change, can be made to act as a highly sensitive and selective, demodu- 1ator. i if v Accordingly, it is one of the essential objects of my present invention to provide a balanced demodulator, insensitive-to amplitude variations of thereceivedicarrier, which comprises a pair of supere-regeneration detectors having the outputcircuits thereof in push pull w relation, and

whose. input circuits are oppositely mistuned rel'-- ative-tothe center, or midband, frequency of the appliedwaves." 1

I Another object of my present invention is to provide; aLp'air'Qf super-regenerative detectors of Fig. 2 shows the detection characteristic of the detector circuit. i

Referring, now, to the accompanying'drawing,

the demodulation network of Fig. 1 may be employed in any desired type of receiver of angular velocity modulated carrier -waves. angular velocity modulated carrier wave is generic in its scope, since it isequally applicable to FM, or phase modulated, carrier waveienergy. The receiver may be of the superheterodyne type, and the demodulator will then'function-asqthe second detector. On the contrary, there could be used anflantenna. circuit capable of passing the entire i2to 50 megacycle (mc.)' band. In such case, an aperiodic amplifierwould couple the antenna to the primary circuit of :the demodulator input transformer. The invention isinot restricted to'the assigned FM band of-4'2-50 mcz, since the circuit is operative at higher frequencies.

Regardless of the source of the FMwave en'- ergy,the latter is applied to the-primary'resonant circuit of. the, input transformer. The secondary resonant circuits, 3'and 4, are tunedto opposite sides ofthe frequency Fa of circuit I. The frequency F0, which is the center ormid-band frequency ofthe applied modulated waves, will be of the operating intermediate frequency (I. F.) value when the receiver is ofthe superheterodyrletype. The frequencies 'F1 and F may be spaced apart by'a frequency valueexceeding'the maximum frequency deviation .ofthe carrier. I In the assigned FMband the. maximum frequency The term" quency deviation is proportional to the modula-' tion amplitude, whereas the rat of deviation is a function. of the modulation frequencies per se. The electron discharge devices and 8 are shown as triodes. However, they may be of any other desired construction. For example, the triodes may be mounted within a common tube envelope for purposes of economy. The cathode 5' of triode 5 is connected to an intermediate point on the coil 3 of circuit 3. The control grid 1 of triode 5 is connected through condenser B to the high potential side of circuit 3. The opposite side of the circuit is grounded. The plate 9 of triode 5 is connected to one side of the primary winding of audio output transformer ID.

The plate ll of triode 6 is connected to the opposite end of the primary winding of transformer ID. The center point of the primary winding is connected to the positive terminal (+13) of the direct current energizing source. The cathode 6' of triode 6 is connected to a low potential tap on the coil 4 of input circuit 4. The grid l2 of triode 6 is connected through condenser l3 to the high potential side of input circuit 4, while the low potential side is at ground. Grid 1 is returned to ground through the grid leak resistor I 4, while grid leak resistor I5 connects grid I2 to ground.

The secondary winding of audio output transformer I0 is connected to the input electrodes of a following amplifier, and the latter may be followed by one or more additional audio stages terminated by a reproducer of any well known type. Each of the condenser-resistor networks 8-44 and |3-l5 is given ,atime constant such that super-regenerative action takes place in each of the detector stages.

It will be noted that each of the detector tubes is connected to function as a regenerative detector of the grid leak type. The time constant of each grid circuit is chosen so that superaudible quenching action takes place so as to provide the required super-regenerative effect. The relaxation rate (quenching period) may be chosen from 30,000 to 200,000 cycles per second. The smaller the quench frequency, the more total oscillation time and, therefore, more audio output is secured. It is not believed necessary, for the purposes of this application, to explain in detail the manner in which a self-quenching detector of the super-regenerative type operates. It is sufficient to point out that the regenerative action in each detector stage is permitted to build up to a point such that oscillation has commenced, the quenching action then preventing self-oscillation. This results in a highly sensitive typeof detector with high gain simultaneously secured. By mistuning the input circuits 3 and 4 to opposite sides of the center frequency Fe, discrimination is secured. The detection characteristic is shown in Fig. 2. The opposite peak frequencies F1 and F2 are located at substantially the peaks of the S- shaped curve. The characteristic is substantially linear between the peak frequencies. Frequency as abscissae is plotted against instantaneous audio output potential as ordinates. The curve shows the conversion of FM signals into audio voltage.

As stated previously, this circuit eliminates the need for a special amplitude limiter stage prior to the detector input transformer. The present demodulator network provides substantially complete elimination of amplitude modulation effects which may appear on the FM wave energy transmitted to the input transformer. The limiting function has been explained in the aforementioned Crosby patent, and reference is made to the said patent for a detailed explanation of the manner in which the limiting action is secured. Briefly, the super-regenerative limiting action is due to the limited grid voltage swing, which is provided by the grid current and cutoff points.

According to my present invention, demodulation is secured in addition to limiting. I have discovered that demodulation is readily secured from the limiting action provided in the aforesaid Crosby circuit, by means of an integrating action. It is to be understood that in the absence of received carrier energy, or where carrier energy is received which is not frequency modulated, the plate circuits of the detector tubes are balanced, and hence the output of the demodulator network will be zero. With varyin frequency of the received carrier energy, the output of the demodulator network will follow the characteristic shown in Fig. 2. Since due to grid detection the plate current in each detector tube is limited, the output will be essentially free from audio components due to amplitude modulation.

In a self-quenched super-regenerative circuit the duration of the oscillation in the grid circuit will depend on the external carrier. The larger the carrier amplitude, the shorter will be the oscillation period. The amplitude of this oscillating voltage will depend on the circuit P parameters only, and is practically independent of the external signal. The oscillating and relaxing voltage is rectified by the detector tube. and the output voltage is integrated by the re actance in the plate circuit. If, then, the signal is applied to a discriminator network, which develops two radio frequency voltages which are:

degrees out of phase, at two different points, and varying in magnitude according to the frequency of the carrier, there will be produced a modulation signal voltage across transformer Ill which is identical to the signal originally frequency modulating the carrier. Demodulation takes place due to the non-linear characteristic of the tubes which rectify the radio frequency current produced in the super-regenerative circuit. Limiting action is due to the inherent property of super-regenerative circuits, i. e.: that only the duration of the radio frequency oscillation changes, but not the amplitude thereof.

The operation of a super-regenerative circuit differs somewhat from what of a simple detector circuit in the presence of signals of a varying frequency. In the case of the simple detector the output of the latter depends solely upon the voltage applied to the detector input, and not at all upon the frequency of this voltage. It is understood, of course, that the magnitude of this voltage will vary with frequency'if the voltage is obtained by circuit resonance, but this is not a characteristic of the detector itself. In the case of a super-regenerative receiver, if the signal voltage is of the same frequency at the oscillations generated by the detector circuit in the absence of signal voltage, the detector output will depend to a greater or less extent upon the magnitude of the signal voltage according to circuit constants employed and adjustment thereof. However, if the applied signal has a frequency substantially different from the natural oscillation frequency of the detector, the resulting voltlocally-generated oscillation, will depend upon the frequency of the signal.

Thus, a detected output is possible'from a super-regenerative detector in the presence of a variable frequency signal, even though the amplitude of this signal arriving at the input electrodes of the detector were perfectly constant. It is thus seen that the utilization of a super-,regen erative detector system gives a result which is not merely the improvement in the sensitivity characteristic of the super-regenerative detector, but brings in the possibility of a limiting action without the necessity for any separate limiting device. The push-pull arrangementis not required to insure this result, but has the advantage that it provides a still further balance against amplitude variation effects, especially when the instantaneous signal frequency is at, or near, its mean frequency value so that it differs substanamplitude, resistor-condenser means connected tially equally from the natural oscillation frequencies of the two detector systems.

p In place of the audio transformer I0, there may be employed resistance coupling. In that case, the resistors should be shunted with capacitors to bypass the radio frequency current and integrate the audio component. It will now be seen that I have provided a detector circuit for FM signalswhich isinherently non-sensitive for amplitude changes of the FM carrier. The'detector employs apair of balanced super-regenerative rectifiers working from a frequency discriminator which supplies radio frequency voltages 180 degrees out of phase. The rectifiers have a reactance in the output thereof to provide radio frequency by-passing and integration of the audio component.

While I have indicated and described a system for carrying my invention into effect, it will be devices, each device containing at least a cath- I ode, a control grid and an output electrode, a frequency discriminatory input network coupled to the control grid of each of said devices, a common audio output circuit coupling the output electrodes of said devices, means providing oscillation-producing regenerative feed-back between the output electrode and control grid of each of said devices, and separate means connected in circuit between said network and each of the control grids for providing quenching of oscillations thereby to provide super-regenerative action.

2. In acombined limiter-demodulator network for signals of variable frequency, an oscillation generator tube having input and output electrodes coupled in an oscillatory circuit, a second oscillation generator tube having input and output electrodes coupled in an oscillatory circuit, means for energizing the electrodes of both tubes to provide oscillations of substantially .-constant to the input electrode of each tube for interrupting said oscillations periodically, a common audio output circuit coupled to the output electrodes of said tubes, and a frequency discriminatory signal input network coupled to the input electrodes of said tubes.

3. A detector system for frequency modulated waves inherently non-sensitive for amplitude change of the carrier, comprising a pair of balanced self-quenching super-regenerative detectors of the grid leak type, a frequency discrimination wave input circuit coupledto the input electrode of each detector, a common audio output circuit connected to the output electrodes of the detectors, and a reactance in the audio output circuit to provide bypassing of radiofrequency resistor means connected to each of the control grids for providing quenching of oscillations thereby to provide super-regenerative action,

and means providing regenerative feedback between the output electrode and control grid of each of said devices.

5. In a combined limiter-demodulator network for signals of variable frequency, a first tube havinginput and output electrodes coupled in a radio frequency oscillatory circuit, a second tube having input and output electrodes coupled in a second radio frequency oscillatory circuit, means for energizing the electrodes of both tubes to provide oscillations of substantially constant amplitude, condenser-resistor means for interrupting,

said oscillations periodically to provide superregenerative action; a common audio output circuit coupled to the output electrodes of said tubes, and a commonfrequency discriminatory signal input network coupled'to the input electrodes of said tubes.

- GEORGE C. SZIKLAI. I

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