US2383359A

US2383359A - Frequency modulation receiver

Info

Publication number: US2383359A
Application number: US478705A
Authority: US
Inventors: Ziegler Mare
Original assignee: Hartford National Bank and Trust Co
Current assignee: Hartford National Bank and Trust Co
Priority date: 1942-11-03
Filing date: 1943-03-10
Publication date: 1945-08-21
Anticipated expiration: 1962-08-21
Also published as: US2406309A; DE821047C; FR957645A; CH264774A

Description

Aug. 21,1945.

M. ZIEGLER FREQUENCY MODULATION RECEIVER Filed March 10 1943 2 Sheets-Sheet 1 STABiLISED REACTANCE L. P. l6 OSCILLATOR] TUBE FILTER /l2 /l4 /l8 '/ZO '/Z2v 24 F- WIDE amp COMPENSATED A. F MIXER a. F. FREQUENCY AMPLIFIER AMPUHER COUNTER AMPI IF|ER 3 .2 5 Z 'l6d 2a 0 STABLSED FRE' REACTANCE QUENCY MODULAT- E o OSCILLATOR E 50 /|4 1w 2o 22 24 NARROW comrsusmnl E MIXER BAND I.F FREQUENCY 1 AMPLIFIER AMPLIFIER comma AMPL R MARC ZIEGLER INVENTOR,

ATTORNEY Patented Aug. 21, 1945 UNITED STATES PATENTEOFFICE.

2,383,359 FREQUENCY MODULATION RECEIVER Marc Ziegler, Buenos Aires, Argentina, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application March 10, 1943, Serial No. 478,705

20 Claims.

This invention relates to' thereception of frequency modulated waves, and more particularly to methods, apparatus and circuits of a more simple and ellicient nature than those heretofore used for a like purpose.

- Hitherto the type of detecting device most generally used for deriving the intelligence impressed on the received wave, has been ofthe type known as a discriminator, which device converts the frequency modulated received wave into an ampli-' tude modulated wave prior to detection. Certain difficulties characterize this type of frequency modulation detection, as for example the adjustment of the discriminator" is rather critical and. requires much care and skill in order to avoid dis-' tortion. Also with the known types of receivers itis necessary to use a wide band intermediate frequencyamplifier and a consequent low intermediate frequency gain per stage. Furthermore, in certain designs, use has'to'be made of limiter stages to remove from the incoming signal all amplitude modulation which may be present. The common limiter deforms the signals by cutting out everything above a certain amplitude, so that following every limiter stage a tuned circuit must'be interposed for restoring the sinusoidal character of the signal. Again, ordinary limiter stages demand a considerable signal voltage level before satisfactory limiting action is achieved. Hence the receiver must operate with a very corrsiderable reserve of amplification, with the-consequence that the well-known difficulties of high gain amplifiers are introduced. Furthermore, the loading imposed by the grid current of the limiter tube and other tubes affects the selectiv ity of the receiver in the presence of large signals.

To overcome some of these difficulties it has been proposed to use, as the detecting element,- a frequency counter of the resistance capacity type, whereby the necessity for conversion from" fre'- quency to amplitude modulation is avoided; As the output of such a frequency counter, as is well known, does not depend on the amplitude of the applied signal, the necessity of using limiterstages is thereby avoided. Such an arrangement does not, however, overcome the'dimcultiesin volved when using awide band intermediate froquency amplifier because in such an arrangementthe frequency excursion of the intermediate fre-' quency signal is, in absolute value, equal to that ofthe signal-received. Furthermore special care must be taken 'toensure a proper inherent constancy of frequency of the local oscillator. g

In my prior U, S. patent application Ser. l fo. 451,186; filed-July 16, i942, I have described a method of reception for frequency modulated wavesv based on phase excursion shrinkage giving a corresponding reduction or shrinkage of the frequency excursion and I have pointed out the fundamental advantages of such 'an operation as including'an amplitude nonresponsive detection of the'frequency modulation of the wave and enabling the use or a narrow band intermediate frequency amplifier in the receiver.

In another prior U. S. patent application Ser. N 0. 464,380; filed November 3, 1942, in the names of Marc Ziegler and M. J. Kobilsky, a method is described for stabilizing the frequency of an oscillator by means or a resistance capa'city type of frequency counter on the input of which is impressed the output of a mixer wherein the oscillator output is mixed with that of a pilot oscillator, the compensated output of the frequency counter being fed back to the oscillator so as to r'naintain the frequency of the oscillations thereof substantially constant. The said application likewise provides for compensation of the frequency counter output in a variety of ways in order to offset the effect which variations of battery voltage might have on the stabilization frequency.

In co-p'ending U. S. application Ser. No. 477,990., filed March 4, I943, I have furthermore described and claimed a novel type of frequency counter, as wellas its application for frequency stabilization and to frequency modulated transmission. l

' I have now found that novel and hitherto unsuspected advantages and simplifications may be introduced into'a frequency modulated receiver having a detector of the frequency counter type by the introduction of a frequency stabilization link as described in the above mentioned prior application Ser. No. 464,380,]filed November 3, I942, by'the'simultaneous' application of a shrinkage of the frequency excursion and the subsequent use'of antirrow-band intermediate frequency amplifieras taught in my application Ser. No. 451,186, filed July 16, 1942., using preferably the inductance type o'ffrequency counter discriminator described in my application Ser. No. 477,990, filed March 4, 1943.

Very briefly the operation may be described as follows. In the above mentioned prior application Ser. No; 464,380; filed November 3; 1942,

it is explained that the basic principle involved is to measure the frequency difference between a stable pilot and an oscillator andto use the resulting; magnitude for automatically making the frequency diiierence as nearly as possible nearly in the same way as that of the pilot so that the frequency difference is practically equal to said pre-determined value.

If in a frequency stabilizing link of the characteristics described above, a. stage for amplifying an incoming frequency modulated wave is substituted for the said pilot, and if the said frequency counter is capable of reproducing the highest modulation frequency impressed on-the incoming wave, the said frequency stabilizing link together with the input amplifier stage and a suitable audio amplifier stage will constitute a receiver for frequency modulated waves wherein the output of the counter reproduces the intelligence and wherein the fluctuations of the frequency difference between signal Wave and local oscillator wave, that is to say of the intermediate frequency wave, are reduced; in other words wherein we obtain an intermediate frequency wave of. shrunk excursion. In accordance with the general feedback theory the resultant fluctuations of the intermediate frequency wave will be A times less than in the case of reception with a local oscillator of constant frequency. The maximum realizable value of A, the feedback factor, is determined by the product of the frequency counter sensitivity in volts per kc. and the sensitivity in kc. per volt of the frequency control means, and can easily attain several hundreds, even several thousands, and, accor-ding to the'teachings of my above-mentioned prior U. S. application Ser. No. 451,186, filed July 16, 1942, I purposely apply a considerable excursion shrinkage action in order to reduce the excursion considerably and, in general, to a value substantially smaller than the highestm'odulation frequency. In that case, the spectrum width is determined by the highestv modulation frequency and I am enabled to use a narrow band intermediate frequency amplifier with all its attendant advantages. The bandwidth of the intermediate frequency amplifier may even be somewhat lower than twice the highest modulation frequency. It can be shown that this will not affect the reproduction of the intelligence.

In U. S. Patents Nos. 2,075,503 and 2,272,401 to J. G. Chaffee it is proposed to use negative feedback from the audio-stage to the local oscillator in order to reduce noise and distortion. The teaching of these two patents is however restricted to the common discriminator type of receiver in which such negative feedback acts more ed, as disclosed, does not permit of reducing the noise to signal ratio as far as desired when receiving a signal of given excursion, so that the solution pro-posed Iby Chaffee is to increase the frequency excursion of the modulation to which the transmission is subjected. Hence the advantages of the feedback as to noise reduction are achieved there in the first instance by affecting the transmission and the prime purpose of the excursion reduction is apparently to make possible the increase of the excursion of the transmission.

Considerations regarding the use of negative feedback in a, phase modulation receiver are also divulged in U. S. Patent 2,197,518 of M. G. Crosby. I The purpose of that invention, however,

is especially to increase the handling capability or less as a substitute for the more usual limiter circuits. These disclosures point out that the negative feedback causes a reduction of thefrequency swing of the signal. As an example however, a reduction is cited to 40,000 cycles from an incoming frequency of 100,000 cycles, which is a reduction of only two and one-half times. The fact that moreover, the intermediate frequency amplifier described in said patents is of the wide band type clearly shows that nothing like the considerable excursion shrinkage employed in the present invention was in mind. Chaffees methof a phase modulation receiver with regard to the depth of degree of modulation, so as to allow relatively wide band modulation, though the detector proposed works correctly for excursions up to not much more than 1 radian only. The phase detector described therein is of a type which converts phase modulation into amplitude modulation, the amplitude modulation being then detected in the classical way. This receiver, without negative feedback would be essentially inadequate to handle wide bandfrequency modulated waves and therefore, since the first purpose of the feedback employed is to offset this restricting circumstance, it is essentially different from the idea underlying the present invention.

As a matter of fact, the type of receiver with which the present invention is concerned can, thanks to the propertiesof the counter type detecting element used, work perfectly well without any feedback, the chief purpose of incorporating the feedback being to enable a narrow band intermediate frequency amplifier to be used, and the consequent advantages obtained. As can be seen clearly my receiver is designed not onl with entirely different detecting instrumentalities than is the case in the methods heretofore proposed, but also along quite different lines of thought.

It should be understood that the essential feature of the present invention which distinguishes it from the prior art is the utilisation of the compensated frequency counter type detector. By this means I achieve several valuable results, even without the excursion shrinkage mentioned above. Since the frequency counter is inherently linear, any distortion due to detector characteristics is completely avoided; as the response of the counter is not dependent on the amplitude of the signal all the advantages f the frequency modulation with regard to noise reduction are obtained without the introduction of limiter stages and, by virtue of the stabilization, the receiver is no longer bound down, in communication practice and other applications to the use of crystal oscillators as was the case in the prior proposals to use frequency meters as detectors.

As discussed briefly above, the fact that I apply the-feedback to a receiver. including acompensated frequency counter entails a fundamental change in the operating conditions with respect to prior proposalsto employ negative feedback. If in said prior proposals the attempt were made to obtain'by feedback action, for the reception of a frequency modulated wave of given excursion, the maximumrelative noise reduction, so as to obtain results equivalent to those of a classical Armstrong receiver, it would be necessary to increase the feedback factor to a certain minimum value. As stated above, in my receiver, however, I

the voltage impulses of. like sign per given time period are counted to produce a potential proportional to the number of impulses impressed per given time period. By means of a compensating voltage derived from the potentiometerlSU, the said potential is compensated to zero at the central frequency of the intermediate frequency amplifier l8a.

The operation of the receiver of Fig. 3 will now be considered more in detail, but it should be noted that in the main the description applies also to the receiver of Fig. 2. For simplicity, suppose the local oscillator Ilia to be tuned exactly to the proper central frequency so that the difference between the oscillator central frequency and'that of the incomin signal is exactly the value Q==f (K) for which the I frequency counter 20 is compensated to zero, where K represented the constants of the compensated frequency meter. Since the average value of the signal excursion or difference between the instantaneous signal frequency and the central frequency of the incoming signal is zero, theoutput of the compensated frequency counter will, after passing through the low pass filter not give any controlling voltage unless the difference between signal and oscillator central frequencies becomes other than the value Q. In that event a controlling direct current tension is developed which tends to bring the central frequency difference back to the desired value.

The action of the alternating tension link 30a is to modulate the oscillator. From general feedback theory it can be shown that the fluctuations undergone by the local oscillator frequency will be A/(1+A) times the deviations of the signal frequency from its central frequency, and hence that the excursion of the intermediate frequency wave will be l/(l-l-A) times the excursion of the signal wave. In practice unity may be neglected against A, so that a shrinkage by l/A of the excursion of the intermediate frequency wave is obtained, and this shrinkage can readily be made large enough to enable narrow band intermediate frequency amplifiers to be used. The condition is, in fact that the shrunk excursion be small compared with the highest modulation frequency, so that the width of the intermediate frequency spectrum is uniquely determined by such highest frequency. It is not even necessary to design the intermediate frequency band pass filters sothat they shall transmit all the side bands with the same intensity, as unlike the case of an excursion which is large compared with the highest modulation frequency, distortion will not be caused by such a selective effect. If the output depended on the amplitude of the intermediate frequency output, the audio response would de pend on the intermediate frequency selectivity as in ordinary amplitude modulated receivers, but in my novel arrangement, both the-amplitude non-responsive frequency counter and the feedback action cancel this effect in all practical cases. Only for very small signals or for too low a preamplification might the highest modulation frequency not be reproduced with the original amplitude in the case of a very sharp intermediate frequency selectivity curve.

For a large value of the feedback factor, the frequency modulation imposed on the local oscillator is practically equal to the modulation of the incoming signal so that if the effect of the reactance tube 28 is arranged to be quite linear for the frequency variations to be imposed on the oscillator, the ouput voltage will be proportional to the modulation to be reproduced. It is known from the general feedback theory that the above holds even when the frequency counter and the audio frequency amplifier are not quite linear respectively in function of frequency and amplitude. Thus a considerable reduction in distortion is achieved.

Both by reason of the useof the frequency counter which is non-sensitive to amplitude and by the employment of feedback, my novel receiver is insensitive to amplitude modulation. Hence, insofar as noises may be considered as amplitude modulations superimposed on the de sired signal, I achieve all the advantages of noise reduction previously attainable only in receivers of highly complicated construction. This holds equally for external industrial or natural perturbations and, for internal fluctuations within the receiver insofar as they cause amplitude modulation.

To those skilled in the art it will be clear that the introduction of the negative feedback considered may have an influence'on the relative reproduction of different modulation frequencies. Taking into account the limitations imposed by the well known conditions of negative feedback stability, this circumstance may be utilized for shaping at will and very conveniently the response characteristic of the receiver audio signal against modulation frequency.

As I'have found that the output voltage does not depend on the strength of the signal re-' ceived, which is, of course, true only from a certain signal upwards, my novel receiverhas inherently automatic volume control, since the limit just referred to can be made as small as desired. When large signals are being received, some classical automatic volume control may conveniently be introduced in order to prevent overloading of the amplifier stages.

It Was explained above how satisfactory'noise reduction is obtained in my receiver insofar as amplitude modulation noise is concerned. With regard to undesired frequency modulation present in the signal the situation is of course different. Frequency modulation superposed on the signal before its entrance in the receiver can not of course be eliminated, and the same holds, in general, for frequency modulation created within the receiver by some undesired modulation tension, for example a hum voltage on the grid or the reactance tube or by some mechanical vibration causing for instance the tank capacity of the oscillator to vary periodically. The feedback action will reduce the frequency modulation obtained A times (A being the feedback factor), but, as it reduces the excursion of the desired modulation in the same ratio, no improvement is achieved in this way.

I have found however, that in the particular cases mentioned and similar cases, a considerable improvement can be obtained by not only tying the local oscillator to the frequency of the frequency modulated incoming wave but by stabilizing it, at the same time, with respect to a very stable local oscillation, as provided for example by a quartz oscillator.

- A feedback factor of the order of several hundreds, e. g., 600, can easily be obtained. The total stabilizing action obtainable might be successfully applied for stabilizing the central frequency of the local oscillator with respect to the central frequency of the incoming signal plus 1 or minus the pre-determined intermediate frequency. For "obtaining the advantages of the invention described above, it is not necessary or desirable however, to shrink the excursion 600 times. In communication practice, a shrinkage of 15 times for reducing the incoming excursion from kc. to :1 kc. is all that is necessary. A further reduction would proportionally but necessarily increase the audio frequency amplicatio'n required for conveniently reproducing this intelligence. Normally, an adequate 10 reduction of the feedback factor in the audio range will therefore be introduced, for instance by means of a potentiometer. It also is possible however, to counteract to'o eloseatyin g to the frequency fluctuations of .the incoming wave, by simultaneous stabilization with respect to 'a constant frequency. 1

In Fig. 4 is shown'schematically how this may be achieved. The common link is identical with that of Figure 2 and the feedback action by means of the conductor a is both forsub-audio and audio frequencies, and has, for instance, a feed-back factor of 600. The. additional link consisting of a. second frequency counter 32 connectedbetween the reactance tube 28 by aconductor 80b and an auxiliary mixer 34. to which a quartzstabilized oscillator 36 is connected, would stabilize, in the absence of the common link, the frequency of the common; oscillator withrespect to that of the quartz oscillator 36 -plus or minus the intermediate frequency, Thefeedback factor of the additional link is adjusted to say Now-it can be'shown that the resulting effect will be that the excursion shrink-. age of the incoming signal will be practically 15 times; as desired; the effectof mechanical vibrations on the local oscillator is reduced 40 times and the effect of undesired frequency; modulation caused by undesired alternating ten sions in the link- Iia.,-28, 3 2', 3 4, 36 is reduced 40 times also.- i v i 'What I1c1aim is: 1 l 1. q

1: The method of receiving-frequency modu lated waveswhich comprises mixing the received wave with frequency controllablelocal oscilla tions to obtain an intermediate. frequency wave, counting the 4 intermediate frequency; wave by creating equal electrical-voltage impulses: equal in number to the numberof periods of said intermediate frequency w'ave. non-r'esponsively to the amplitude thereof, to obtain an outputvoltage containing an audio. frequency, component re producing the intelligence,-.while opposing to said, output voltage a constantdirect current voltage. to obtain .a. compensatedrsultant voltage the mean value of which is zero. 'for a pre -determined valued; the central frequency. of the incoming intermediate frequenc wave, averaging the comperisated iesuuant voltage ever t me large in] comparison with the lowest modulation 'fr'ejquendy, so that theavera e r sultant voltage is probortidiiiu to the slew deviations tithe ntenmediate freq ency .f r 'ni said predetermined value, and applying said. averaged resultantvolt age to the source of frequency controllable lo; cal osc llations to stabilize the frequency thereof with respect to a desire-d value differing; from the central frequency of the incoming wave by saidpre 'deterihined valueof ti centric intermee af nwy 2. The method of 'ifeceivihg frequency modulated waves d'escribedih claim Lfwlir'eifi the electrical Voltage impulses afe ebtaiiiedaeios's an;

inductive edition of a circuit-mitoch-which-a current flows, said current being varied impulsively a like value for each half period of like sign of the intermediate frequency wave irrespective of the amplitude thereof.

3. A receiver for frequency modulated waves which comprises a source of local oscillations, means for varying the frequency 'of said oscillations in response to a voltage, means for com-.

bining the local oscillations with the received wave to obtain an intermediate frequency wave, frequency counter means for obtaining an output voltage determined by the frequency of said intermediate frequency Wave irrespectively of the amplitude thereof,- means for deriving from "said voltage an audio frequency component repvalue, and means for applying said slowly varying direct current potential to the frequency controlling means to maintain the frequency of the local oscillations substantially constant at a value differing from the central frequency of the incoming signal by said predetermined value of the intermediate frequency.

4. A receiver for frequency modulated waves according to claim 3 wherein the frequency counter means includes an inductance connected I to be impulsively charged for each period of the intermediate frequency wave non-responsively to the amplitude thereof.

5. The method of receiving frequency modu lated Waves which comprises mixing the received wave with frequency controllable local oscillations to obtain an intermediate frequency wave; counting the intermediate frequency wave nonrespon'sivcly of the amplitude thereof, to obtain an output tension; filtering train the output tension the audio frequency component reproducing the intelligence, applying said audio frequency component to the local oscillation to iiiodiilate the same so as to reduce the excursion of the intermediate frequency Wave to a. value substantially smaller than the highest modulation frequency, whereby the spectrum width is detere 'mined by the highest modulation frequency and distortion of the intelligence by selective amplification'is prevented. I

6. A receiver for frequency modulated waves comprising a source-cf local oscillations, a mixer for combining the received waves with the local oscillations to produce intermediate frequency waves, a frequency counter for detecting the frequency modulations of the intermediate free quency waves independently of the amplitude of the wave, means for filtering from the output of the frequency counter the audio frequency voltage represehting the intelligence and means for modulating the frequency of the local oscillations by said audio frequency voltage, whereby the excur'sioh of the intermediate frequency is reduced to a value substantially smaller than the highest modulation frequency.

'7. A receiver for frequency modulated waves according tdclaim' 7. wherein the frequency counter includes a self-inductance connected to beiinpu'lsively chargedfor each period of the intermediate ri quency wave independently of the amplitude of the wave.

8. The method of receiving frequency modulated waves which comprises mixing the received wave with frequency controllable local oscillations to obtain an intermediate frequency wave, counting the intermediate frequency wave by creating equal electrical voltage impulses, equal in number to the number of periods of the intermediate frequency wave, non-'responsively to the amplitude thereof, to obtain an output voltage, while opposing to said output voltage a constant direct current voltage to obtain a compensated resultant the mean value of which is zero for a pre-determined value of the central frequency of the intermediate frequency wave, averaging the compensated resultant over a time large compared with the intermediate frequency and substantially smaller than the shortest period of the modulation, so as to obtain an integrated direct current potential proportional to the instantaneous difference between the actual instantaneous intermediate frequency and said predetermined value which contains a sub-audio component and an audio frequency component representing the intelligence, applying said subaudio component and said audio frequency component to the source of local oscillations to stabilize the central frequency thereof with respect to the pre-selected value and to modulate said -local oscillations proportionally to said audiofrequency component whereby the frequency excursion of the intermediate frequency wave is reduced to a value substantially smaller than the highest modulation frequency of the received wave.

9. A receiver for frequency modulated waves comprising a source of local oscillations, voltage responsive means for varying the frequency of said oscillations, a mixer for combining the received waves with the oscillations from said source to produce an intermediate frequency wave, a frequency counter for measuring the frequency of the intermediate frequency Wave, irrespective of the amplitude thereof, means for combining a constant direct voltage with the output of the frequency counter' to obtain a compensated output voltage the mean value of which is zero for a pre-determined value of the central intermediate frequency, means for filtering out from the compensated output the components of intermediate frequency and harmonics thereof to retain a direct current voltage and an audio frequency component reproducing the intelligence, means for impressing said direct voltage on said frequency varying means to maintain the central frequency of the local oscillations substantially at a value differing from the central frequency of the incoming wave by the pre-determined intermediate frequency, and means for applying the audio frequency component to the frequency varying means for modulating the frequency of the local oscillations whereby the excursion of the intermediate frequency is reduced to a value substantially smaller than the highest modulation frequency of the received wave.

10. The method of receiving frequency modulated waves which comprises mixing the received wave with local oscillations to obtain an intermediate frequency wave, counting the intermediate frequency irrespective of the amplitude of the wave to obtain an audio frequency output voltage reproducing the intelligence, frequency modulating the local oscillations by said audio frequency output voltage to obtain an intermediate frequency wave of shrunk excursion, said method including the step of subjecting the interme-diate frequency wave of shrunk excursion to amplification .of bandwidth of any value lying between the excursion of the received signal and a bandwidth somewhat narrower than the spectrum of the signal of shrunk excursion.

11. In the method of receiving frequency modulated waves described in claim 8, the step of subjecting the intermediate frequency wave, the frequency of which has to be counted to a selective amplification of bandwidth of any value lying between the excursion of the received signal and -a bandwidth somewhat narrower than the spectrum of the signalof shrunk excursion.

12. A receiver for frequency modulated waves comprising a source of local oscillations, a mixer for combining the received waves with oscillations from said source to produce an intermediate frequency wave, a frequency counter for detecting the frequency modulations of the intermediate frequency wave irrespective of the amplitude thereof, means for modulating the frequency of the signal of said source by the component of the output of said frequency counter which represents the intelligence, so as to shrink the excursion of the intermediate frequency wave obtained, and intermediate frequency amplifier means interposed between the mixer and the frequency counter and having a bandwidth of a value lying between the excursion of the signal received and a bandwidth somewhat narrower than the spectrum of the signal of shrunk excursion.

13. A receiver for frequency modulated waves according to claim 9 in which the intermediate frequency wave obtained from the mixer is amplified before applying the same to the frequency counter by means of a selective intermediate frequency amplifier the bandwidth of which has a value lying between the excursion of the signal received and a bandwidth somewhat narrower than thespectrum of the signal of shrunk excursion.

14. A receiver for frequency modulated waves comprising an antenna, a radio frequency amplifier connected to the antenna, a beating oscillator, a mixer connected to the output of the radio frequency amplifier and the beating oscillator, a compensated frequency counter for conve rting the output of the mixer into audio frequency currents, an intermediate frequency amplifier interposed between the mixer and the frequency counter, the output of the frequency counterbeing connected to a signal translating device, and a central local frequency stabilizing link between the output of the frequency counter and the beating oscillator and including a low pass filter and a reactance tube.

15. A receiver for frequency modulated waves comprising an antenna, a radio frequency amplifier connected to the antenna, a local oscillator, a mixer connected to said radio frequency amplifier and to said local oscillator, a compensated frequency counter for converting the output of the mixer into audio frequency currents, a narrow band intermediate frequency amplifier interposed between the mixer and the frequency counter, the output of the frequency counter being connected to a signal translating device, and a stabilizing and frequency modulating link comprising a reactance tube directly connected between the output of the frequency counter and the beating oscillator.

16. A receiver for frequency modulated waves comprising an antenna, a radio frequency amplifier connected to the antenna, a beating oscillator, a mixer connected to the output of the radio a frequency modulating link directly connected between the output of the audio-frequency amplifier and said reactance tube.

17. In the reception of frequency modulated waves according to claim 8, the method of counter-acting the influence of a very strong frequency shrinkage action by stabilization of the local oscillation with respect to a constant frequency, whereby the influence of undesired internal frequency modulating effects is reduced without reducing the excursion of the signal to be received to more than a desired extent.

18. In a frequency modulation receiver according to claim 9, means for reducing an undesirably large excursion shrinkage to a desired value, said means comprising an arrangement for stabilizing the frequency of the local oscillations with respect to the frequency of a local oscillator of constant frequency, whereby the effects of internal frequency modulating actions are reduced relatively to the desired modulation.

19. A receiver for frequency modulated waves comprising a first source of local oscillations, a mixer for combining the received wave with the local oscillations from said first source to produce an intermediate frequency wave, a first frequency counter means for detecting the frequency modulations of the intermediate frequencywaves independently of the amplitude of the wave, means for filtering from the output of the frequency counter the audio-frequency voltage representing the intelligence, means for modulating the frequency of said local oscillations by said audiofrequency voltage whereby a large excursion shrinkage of the intermediate frequency wave is obtained, a second source of local oscillations of substantially constant frequency and means including a second mixer and a second frequency counter for applying to said first source a magnitude counter-acting the said frequency excursion shrinkage whereby to obtain a resultant excursion shrinkage of a smaller desired value and the effects of internal frequency modulation actions are reduced relatively to the desirable modulation.

20. 'A receiver for frequency modulated waves comprising an antenna, a radio frequency amplifier connnected to the antenna, a beating oscillator, a mixer connected to the output of the radio frequency amplifier and the beating oscillator, a compensated frequency counter for converting the output of the mixer into audio frequency current, an intermediate frequency amplifier interposed between the mixer and the frequency counter, the output of the frequency counter being connected to a signal translating device, a central local frequency control means including a reactance device between the output of the frequency counter and the beating oscillator, and a frequency shrinkage reducing link including a second local oscillator of substantially constant frequency, a second mixer connected between the second local oscillator and the beating oscillator and a second compensated frequency counter connected between the second mixer and the reactance device, whereby an undesirably large excursion shrinkage may be reduced to a desired value and the effects of internal frequency modulation actions are reduced relatively to the desired modulation.

MARC ZIEGLER.

7 CERTIFICATE OF CORRECTION. Patent No. 2,585,559. August 21, 191

MARC ZIEGLER.

It is hereby certified that error appears in the. printed specification of the above numbered patent requiring correction as follows: Page 5, first column, line 15 for "vaue" read --value-; page 5,;second column, line 71, claim 7, for the claim reference numeral "7" read -6-; page 7, 'first column, line 59, for "waves" read -wave--; and second column, line 25 for "current" read -currents--; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this l th day of December, A. 13.. 19MB Leslie Frazer (Seal) First Assistant Commissioner of Patents.