US2075503A - Reception of frequency modulated waves - Google Patents

Reception of frequency modulated waves Download PDF

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Publication number
US2075503A
US2075503A US70929A US7092936A US2075503A US 2075503 A US2075503 A US 2075503A US 70929 A US70929 A US 70929A US 7092936 A US7092936 A US 7092936A US 2075503 A US2075503 A US 2075503A
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United States
Prior art keywords
frequency
waves
signal
modulation
noise
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Expired - Lifetime
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US70929A
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English (en)
Inventor
Joseph G Chaffee
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AT&T Corp
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Bell Telephone Laboratories Inc
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Publication date
Priority to NL49750D priority Critical patent/NL49750C/xx
Application filed by Bell Telephone Laboratories Inc filed Critical Bell Telephone Laboratories Inc
Priority to US70929A priority patent/US2075503A/en
Priority to GB2728/37A priority patent/GB489636A/en
Priority to FR822127D priority patent/FR822127A/fr
Application granted granted Critical
Publication of US2075503A publication Critical patent/US2075503A/en
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03D—DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER
    • H03D3/00—Demodulation of angle-, frequency- or phase- modulated oscillations
    • H03D3/001—Details of arrangements applicable to more than one type of frequency demodulator
    • H03D3/003—Arrangements for reducing frequency deviation, e.g. by negative frequency feedback
    • H03D3/004—Arrangements for reducing frequency deviation, e.g. by negative frequency feedback wherein the demodulated signal is used for controlling an oscillator, e.g. the local oscillator
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
    • H03J7/00—Automatic frequency control; Automatic scanning over a band of frequencies
    • H03J7/02—Automatic frequency control
    • H03J7/04—Automatic frequency control where the frequency control is accomplished by varying the electrical characteristics of a non-mechanically adjustable element or where the nature of the frequency controlling element is not significant

Definitions

  • This invention relates to reception of frequency modulated waves and more particularly to methods and circuits for reducing noiseand disturbances introduced during reception of frequency modulated waves.
  • An object of the invention is to receive frequency modulated waves and to derive from them the signal or other control waves by which they were originally modulated,v and at the same time lo to effect a substantial reduction in the noise currents or other disturbances occurring in the course of the reception process.
  • a further object of the invention is to reduce the extent to which the wave form of the signaling wave is distorted in the process of reception and detection.
  • the well-known superheterodyne method for reception of frequency modulated waves is employed in which the received waves are combined with locally produced oscillations to yield intermediate frequency waves which, after amplification, are converted from frequency modulated to amplitude modulated waves and thereafter demodulated in the ordinary manner.
  • the resulting audio frequency signals correspond to the frequency modulations of the'incoming waves but are accompanied by disturbing ⁇ audible waves. These may arise from tube noise generated with- 30 in the receiving system, from various extraneous sources, and froml distortions resulting from nonlinearity in the characteristic of the receiving system.
  • an opposing elecquency output of the receiver so as to produce a frequency modulation of the local oscillationsl which beat or combine with the received waves.
  • the feedback component is so predetermined as to phase and so adjusted as to amplitude that the distortion components are effectively reduced leaving an audio frequency output current which more faithfully represents the modulations of the received waves.
  • the local oscillator frequency varies in such manner as to yproduce the noise counteracting electromotive force in the final audio output circuit, it will, during signaling periods, also undergo variations in frequency corresponding to the desired audio frequency signal waves with which the received waves were frequency modulated. This is for the reason that the feedback path is subjected not only to the noise components but also to the desired signal components of the output circuit. It follows that the frequency modulation of the oscillator also produces a signal counteracting electromotive force in the output circuit so that along with the reduction of the 'noise current there is a corresponding reduction of the desired signal. This l may be overcome by increasing the effective modulation of the wave sent out at the remote transmitting station.
  • this result may be attained without increasing the transmitting station output power by increasing the depth of modulation at the transmitterA by which is meant the absolute change in frequency that occurs at the peaks of the modulating waves. Since the 'change in frequency occasioned by the modulation process is a function of the amplitude of the modulating Waves, it is only necessary to increase the amplitude of the modulating waves to increase the depth of modulation. Accordingly, with increased depth of modulation at the transmitter, the loss of signal strength occasioned at the audio output terminals of the receiver by the 50 feedback frequency modulation operation may be compensated. Compensation can also be effected by additional signal frequency amplification .following the detector, but the net 'result as far as 55 unwanted effects are concerned will be different as will be explained subsequently.
  • Fig. 1 shows an embodiment of the invention in a radio receiver circuit for receiving frequency modulated waves
  • Fig. 2 shows a modification of the feedback circuit of Fig. 1.
  • variable tuning condenser 4 constitutes an input circuit broadly tuned to the desired incoming waves.
  • incoming circuit is associated with a push-pull high frequency detector or combining device 5 comprising two electron discharge devices having a grid polarizing path extending from a central point in inductance 3 by way of conductor 6, coupling coil 1, grid bias source 8 and shunting condenser 8 to earth at III.
  • a beating oscillator I I illustrated in the drawing as of the Barkhausen type is provided with a tuning circuit including variable tuning condenser I2 and coil I3 coupled to coil 1 so as to impress local beating oscillations in like phase on the grids of the electron discharge tubes of combining device 5.
  • Intermediate frequency oscillations produced by device 5 are impressed by transformer I4 on the grid circuit of push-pull intermediate frequency amplifier I5.
  • the transformer windings are preferably self-tuned, or, in other words, are so designed that with their inherent self and connected shunt capacities I6 and with sufficient coupling between the windings they are very broadly tuned to the intermediate frequency band.
  • Intermediate frequency amplifier I5 increases the amplitude of the intermediate frequency waves and is coupled by a transformer I1 to two-stage intermediate frequency converter I8.
  • 'I'he transformer I1 is similar to transformer
  • Tubes I9 and 2 I l are each provided with a tuned output circuit 23, 24 tuned to a frequency considerably higher than the intermediate frequency resulting from interaction of oscillations of the local beating oscillator I I with received waves of the normal or unmodulated carrier frequency so that the normal intermediate frequency will fall at approximately'a midpoint of the most linear portion of the ascending side of the over-all amplitude response-frequency characteristic of circuits 23 and 24.
  • Output circuits 25, 26 of tubes 28 and 22 are eachtuned to a frequency correspondingly lower than the normal intermediate frequency which will therefore fall at approximately the midpoint of the descending side of the linear por- 70 tion of the over-all amplitude response-frequency characteristic of circuits 25 and 26.
  • the circuits 23, 24 and 25, 26 each include a damping resistance element 21 to make their freqency response characteristics somewhat more linear than 75 they would otherwise be.
  • the tubes I9 and 20 are eachtuned to a frequency correspondingly lower than the normal intermediate frequency which will therefore fall at approximately the midpoint of the descending side of the linear por- 70 tion of the over-all amplitude response-frequency characteristic of circuits 25 and 26.
  • the circuits 23, 24 and 25, 26 each include a damping resistance element 21 to make their freqency response characteristics somewhat more linear than 75 they would otherwise be.
  • the intermediate frequency waves are further amplified and at the same time their frequency modulations are converted into amplitude variations by virtueof the sloping characteristics of circuits 23, 24, 25, and 26. Since the slopes of the characteristics of circuits 23 and 24 are opposite to those of the characteristics of circuits 25 and 26 when viewed at the intermediate carrier frequency, the envelope of the modified wave impressed upon the detector tube 28 will vary in opposite sense to that of the wave applied to the other detector 29. These envelope variations correspond to the modulating voltage applied to the transmitter. tubes 28 and 29 there will result voice frequency currents in the plate circuits of these tubes which will be of opposite phase. Hence output transformers 38 and 3
  • resistances 34 and 35 Connected in series in the space current paths of detectors 28 and 29 are resistances 34 and 35, each shunted by a large by-pass condenser 38. If, as a resultant of a slow drift of the intermediate frequency from normal frequency, the space current rises in resistance 34 in consequence of the increasing amplitude of intermediate frequency waves applied to the upper detector 28 and simultaneously falls in resistance 35, the potential between points 31 and 38 slowly drifts from its normal zero value to some denite magnitude which is a function of the frequency drift.
  • the frequency of the oscillations produced by source may be made to slowly drift in the same manner and to practically the same extent as the frequency of the incoming wave (and hence of the intermediate frequency) varies. It follows that so long as the frequency drift of the incoming wave and of the local oscillations are equal and in the same direction, that is, both ini easing or both decreasing, the resultant central intermediate frequency which is the numerical difference of the incoming wave frequency and the locally generated oscillation frequency will remain of substantially constant value. It will, therefore, be apparent that slow drifts in the frequency of the incoming oscillations or of the -local beating oscillations will be automatically compensated by a correction of the frequency of the beating oscillator such as to bring the central intermediate frequency practically back to its normal value.
  • Reversing switch 46 is thrown to such a position that the signal frequency and noise impulse electromotive forces transmitted to potentiometer resistance 48 will tend to increase the frequency of the beating oscillator when the frequency of the incoming wave increases in response to sigv nal modulations at the transmitter, and decrease it when the frequency of the incoming waves decreases in response to transmitter signal modulations.
  • Reversing switch 45 may be opened to cut off the feedback path during intervals in 50 which the Barkhausen oscillator is being initially adjusted.
  • the effect of the degenerative feedback is to reduce audio frequency noises and distortions in the output current which have been introduced in the receiving process and thus to impart to the receiver as a whole more nearly the characteristics of a distortionless or linear circuit in so far as the modulations of the received waves are concerned.
  • this result is secured at the cost of a considerable amount of amplification or gain as it is commonly designated.
  • Compensation for this loss may be effected in two ways: (l) a voice frequency amplier as indicated at 32-by dotted lines may be inserted between output transformers 30, 3
  • the modulating voltage at the transmitter is reduced to the extent of, say, l0 decibels.
  • the output voltage of fundamental frequency will likewise be reduced to the extent of 60 10 decibels.
  • the second harmonic of the modulating frequency generated within the receiver will be reduced to the extent of 20 decibels.
  • the feedback action through the generation of second harmonic counteracting electromotive forces as previously described, will further reduce the output distortion product an additional decibels, resulting in a net reduction of 30 decibels. There is thus obtained an improvement in signal to second harmonic distortion amounting to decibels. Since the noise output has been reduced to the same extent as the fundamental signal, no improvement in their ratio will be realized.
  • the feedback action will produce counteracting electromotive forces which will tend to reduce the net distortion output to the extent of 10 decibels. While this result is usually obtained in the case of second harmonic distortion, higher order effects often make it impossible to realize the full improvement in the case of the higher harmonics.
  • the Barkhausen oscillator Il is of conventional type with a high positive electromotive force grid bias source 5
  • a plate bias source 49 which may be either positive or negative, but in any event is of a low electromotive force, radio frequency choke coils 52 and a variable high grid circuit resistance 53 for adjusting the grid bias potential.
  • Barkhausen oscillators readily adapt themselves to frequency modulation, the invention is in no way limited to sources of this type but may utilize any type of oscillator in which the desired frequency modulation may be obtained. It is desirable that the frequency modulation of the transmitting station and of the local oscillator be linear with respect to modulating signal amplitudes. It is also advantageous to effect frequency modulation of the beating oscillator without unduly large amplitude modulations since the less the distortions which the feedback must correct the more satisfactory will be the final result.
  • the circuit 43 which includes a low-pass lter 44 to guard against possible singing as a result of feedback for currents of high or intermediate frequency, operates to feed back slow changes in unidirectional potential differences between points 3l and 38 which are occasioned by slow drifts in the frequency of the remote transmitting or the local beating oscillator, and also operates to feed back audio frequency currents to such an extent as to partially counteract and wipe out noise and other audible disturbances, and to reduce signal distortion introduced at any point in the receiving circuit between the point at which the received waves are combined with the local oscillations and the points of connection of the sliders 39 and 40.
  • the audio frequency feedback energy is derived from the secondary windings of transformers 30 and 3
  • are traversed by the unidirectional current flowing in the output circuits of the detectors 28 and 29 and serve to set up between the points 6l and 62 differences of potential corresponding to the slow drifts which transpire in the effective intermediate frequency.
  • By-pass capacity elements 63 and 64 divert from the resistances 59 and 60, all variations of the signal frequency range and those of higher frequency. Slowly drifting potential variations are therefore impressed by way of path 65, reversing switch 66, unidirectional frequency control amplifier 61 and potentiometer 68 to the anode bias path of Barkhausen oscillator Il.
  • a by-pass condenser 69 diverts from potentiometer 68 all except low frequency drift effects of the order of a few cycles and which are, in general, considerably below audibility.
  • circuit of Fig. 2 enables entirely separate control of the amplitudes of the audio frequency and the slow drift effects and simplifies the design of the feedback paths since each may then be constructed with a view only to transfer of its individual effects.
  • the signal frequency feedback feature of the invention is not limited thereto but is equally applicable to any type of conversion circuit whether two or more paths or one path be employed and irrespective of whether electron discharge devices are'utilized or not.
  • the use of pushpull conversion circuits is to be preferred since they make it possible to secure a balance against even order harmonics arising from curvature of the conversion circuit characteristics, as well as a balancing out of the amplitude effect of noise originating ahead of the conversion stages during the non-signaling intervals.
  • the signal frequency feedback energy may also be derived from potentiometers connected across the primary windings of the output transformers as in Fig. 1.
  • signal waves as used in the specification and the appended claims is intended to be generic to waves used for communication, control systems, or any other teledynamio operation and that the term waves of the expression is intended to connote impulses of electromotive forces of any wave form and frequency whatever and regardless of their 'continuity or discontinuity.
  • noise is to be construed to include not only what is ordinarily known as tube noise but also noise producing influences arising at any point subsequent to the transmitter itself.
  • the method of receiving frequency modulated waves which comprises converting the received waves to amplitude modulated Waves, demodulating the amplitude modulated waves to reproduce the modulating signal waves and utilizing energy of the resulting signal waves to introduce a frequency modulation component in the received waves which will yield in the finally demodulated output a component which reduces noises and distortions introduced in the receiving process.
  • the method of signal transmission comprising imparting to unmodulated carrier waves a kfrequency modulation in accordance with the ⁇ signals of greater extent than that desirable in the usual frequency to amplitude conversion operation, transmitting the modulated waves over a transmitting medium to a remote reception point, combining the transmitted waves with beating oscillations similarly modulated but to a somewhat lesser degree with the reproduced signals and also with noise impulses generated in the reception process whereby the effects of noise and distortion in the reception process are substantially reduced.
  • A. frequency modulated wave receiver comprising means for converting received frequency modulated waves to-amplitude modulated waves and for demodulating the amplitude modulated waves to recover modulating signal waves, and means for combining with the received frequency modulated waves other waves frequency modulated by some of the recovered signal modulation energy ⁇ to counteract distortions and extraneous noises which are introduced in the receiver and which would otherwise appear as disturbances in the recovered signal wave output.
  • a receiving system for frequency modulated waves which comprises means for converting received frequency modulated waves to amplitude modulated waves, means for demodulating the amplitude modulated waves to reproduce signal waves by which the received waves were modulated, means for causing energy of the reproduced signal waves to generate a counter-electromotive force, and means for causing the counter-electromotive force to introduce a modulation into the received frequency modulated waves which will reduce noise and distortion currents which the receiving system tends to superimpose upon the reproduced signal waves.
  • a receiving system for frequency modulated waves comprising a source of locally generated beating oscillations, means for combining the beating oscillations with the received waves, means for deriving audio frequency signal waves from the resultant of the combination and means for frequency modulating the beating oscillations by the derived audio frequency signal wave energy.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Transmitters (AREA)
  • Noise Elimination (AREA)
US70929A 1936-03-26 1936-03-26 Reception of frequency modulated waves Expired - Lifetime US2075503A (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
NL49750D NL49750C (en:Method) 1936-03-26
US70929A US2075503A (en) 1936-03-26 1936-03-26 Reception of frequency modulated waves
GB2728/37A GB489636A (en) 1936-03-26 1937-01-29 Carrier wave transmission systems
FR822127D FR822127A (fr) 1936-03-26 1937-03-25 Systèmes récepteurs pour ondes porteuses modulées

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Application Number Priority Date Filing Date Title
US70929A US2075503A (en) 1936-03-26 1936-03-26 Reception of frequency modulated waves

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US2075503A true US2075503A (en) 1937-03-30

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FR (1) FR822127A (en:Method)
GB (1) GB489636A (en:Method)
NL (1) NL49750C (en:Method)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE914866C (de) * 1940-08-01 1954-07-12 Lorenz C Ag Transponierungsempfaenger mit Frequenzgegenkupplung
US2684439A (en) * 1949-12-19 1954-07-20 Padevco Inc Frequency modulation receiver system for overlapping waves
US3001068A (en) * 1957-08-12 1961-09-19 Nippon Electric Co F.m. reception system of high sensitivity
US3053981A (en) * 1959-07-06 1962-09-11 Security First Nat Bank High-gain frequency modulation tuner
US3069625A (en) * 1958-03-20 1962-12-18 Nippon Electric Co Reception system of high sensitivity for frequency-or phase-modulated wave
DE1441150B1 (de) * 1961-04-25 1969-09-04 Western Electric Co Frequenzgegengekoppelter FM-UEberlagerungsempfaenger
US4293818A (en) * 1979-01-22 1981-10-06 International Telephone And Telegraph Corporation Frequency modulation threshold extension demodulator utilizing frequency compression feedback with frequency drift correction
US4991226A (en) * 1989-06-13 1991-02-05 Bongiorno James W FM detector with deviation manipulation
US5691666A (en) * 1995-06-07 1997-11-25 Owen; Joseph C. Full threshold FM deviation compression feedback demodulator and method

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1081938B (de) * 1955-10-29 1960-05-19 Int Standard Electric Corp Funkempfaenger zur Aufnahme von Traegerfrequenz- und Seitenbandkomponenten nach dem Korrelationsprinzip

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE914866C (de) * 1940-08-01 1954-07-12 Lorenz C Ag Transponierungsempfaenger mit Frequenzgegenkupplung
US2684439A (en) * 1949-12-19 1954-07-20 Padevco Inc Frequency modulation receiver system for overlapping waves
US3001068A (en) * 1957-08-12 1961-09-19 Nippon Electric Co F.m. reception system of high sensitivity
US3069625A (en) * 1958-03-20 1962-12-18 Nippon Electric Co Reception system of high sensitivity for frequency-or phase-modulated wave
US3053981A (en) * 1959-07-06 1962-09-11 Security First Nat Bank High-gain frequency modulation tuner
DE1441150B1 (de) * 1961-04-25 1969-09-04 Western Electric Co Frequenzgegengekoppelter FM-UEberlagerungsempfaenger
US4293818A (en) * 1979-01-22 1981-10-06 International Telephone And Telegraph Corporation Frequency modulation threshold extension demodulator utilizing frequency compression feedback with frequency drift correction
US4991226A (en) * 1989-06-13 1991-02-05 Bongiorno James W FM detector with deviation manipulation
US5691666A (en) * 1995-06-07 1997-11-25 Owen; Joseph C. Full threshold FM deviation compression feedback demodulator and method

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Publication number Publication date
GB489636A (en) 1938-07-29
NL49750C (en:Method)
FR822127A (fr) 1937-12-21

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