US2462759A

US2462759A - Apparatus for receiving frequencymodulated waves

Info

Publication number: US2462759A
Application number: US528908A
Authority: US
Inventors: Claudius T Mccoy
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1942-06-13
Filing date: 1944-03-31
Publication date: 1949-02-22
Anticipated expiration: 1966-02-22

Description

Feb. 22, 1949. c. T. M coY 2,462,759

APPARATUS FOR RECEIVING FREQUENCY MODULATED WAVES Filed March 51, 1944 Patented Feb. 22, 1949 APPARATUS FOR RECEIVING FREQUENCY- MODULA'I'ED WAVES Claudius T. McCoy, Narberth, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Continuation of application Serial No. 446,954,

June 13, 1942.

1944, Serial No. 528,908

17 Claims. 1

This invention relates to frequency modulation receivers, and more particularly to detection apparatus for frequency modulation receivers.

This application is a continuation of application Serial No. 446,954, filed June 13, 1942, now abandoned.

In the usual frequency modulation receiver, the received frequency-modulated signal is converted, by means of selective circuits, into an amplitude-modulated signal, following which the amplitude-modulated signal is detected in a conventional amplitude-modulation-detector or rectifying device.

A material improvement over the ordinary method of detection of frequency-modulated signals is shown and described in U. S. Patent No. 2,332,540, issued October 26, 1943, to Charles Travis. That patent describes a detector in which locally generated oscillations are frequency-modulated in substantial accordance with the modulation of the received signal, and in which the received signal and the locally generated oscillations are supplied to a device which generates an intelligence signal which is proportional to a function (e. g. the cosine) of the phase angle between said received signal and said oscillations.

It has been found that the detector described in the above-named patent may, under certain conditions of operation, require rather complete shielding between the tank circuit of the oscillator and certain circuits of the carrier frequency amplifier. Without adequate shielding, the oscillator may tend to lock in with the received signal, thus giving rise to the production of spurious signals in the output circuit of the detector. This undesired lock-in effect is evidently due to the presence of stray couplings, capacitive or inductive, between the carrier frequency amplifier (I. F. or R. F.) and the oscillator. According to the present invention, this difliculty is eliminated by operating the controlled oscillator at a sub-harmonic of the signal frequency, and by combining, in the phase detector circuit, the intermediate frequency signal and the corresponding harmonic frequency signal of the oscillator, operation proceeding thence substantially as described in the above-named Travis patent. This mode of operation eliminates the tendency to- Ward oscillator lock-in, and improves the frequency control characteristics of the control device.

It has also been found that at operating frequencies in the order of a megacycle or of several megacycles, the maximum frequency devi- This application March 31,

ation possible with conventional frequency control circuits is substantially independent of the operating frequency, the deviation being more or less fixed by practical circuit limitations. This is in contrast to what might be expected, namely, that the percentage frequency deviation would be constant, the absolute frequency deviation increasing with operating frequency. The result of this condition is that the effective control sensitivity, the ratio of maximum deviation to operating frequency, decreases with increased operating frequencies. In the neighborhood of four megacycles, a frequency commonly employed in the intermediate frequency stages of frequency modulation receivers, the control tube sensitivity has often been substantially less than that required for optimum performance. By operating the local oscillator at half the intermediate frequency, in accordance with the present invention, the effective control tube sensitivity is approximately doubled, since the absolute frequency deviation for the second harmonic signal of the oscillator is double that of the oscillators fundamental frequency signal. Similarly, by operating the local oscillator, at one-third the intermediate frequency, the effective control sensitivity may be approximately tripled. The advantages of this increased control sensitivity are improved noise elimination, better detection linearity, decreased intermediate frequency input signal requirements, and the simplification of the control tube circuits.

It is one object of the present invention to eliminate the deleterious effects, on the controlled oscillator, of R. F. or I. F. signal energy, and thereby to make unnecessary the provision of extensive shielding means or devices.

It is another object of the invention to use the frequency control tubes more efficiently and over a smaller portion of their characteristic curve than in the past, and thereby to reduce distortion.

It is another object of the invention to permit the operation of the oscillator on a lower frequency than the signal frequency, thereby permitting improved oscillator operation and efficiency.

Other objects and features of this invention will be apparent to those skilled in the art from the following description and by reference to the accompanying drawings, in which:

Fig. 1 is a block diagram illustrating the method and apparatus of the present invention; and

Fig. 2 is a schematic diagram illustrating the invention as applied to a superheterodyne receiver.

Referring first to Fig. 1, there is shown in block form a radio receiver embodying the present invention. This receiver may comprise, by way of example, a selective amplifying system i, a phase detector 2, a controlled oscillator 3, an oscillator frequency control device an audio amplifier 5, and a sound translating device .6. In the above-mentioned Travis patent, the oscillator is arranged to operate at a frequency substantially the same as the frequency of the incoming signal. As previously explained, unless care is taken to decouple the oscillator from this incoming signal, the oscillator may receive .sufiicient energy from the signal to cause the oscillator to become locked in with the incoming signal, thereby enabling the signal to exercise di:

rect control over the oscillator frequency. Operation of the oscillator on a sub-harmonic frequency, according to the present invention, greatly reduces this tendency since the oscillator circuit is not tuned to the signal frequency, and, consequently, signal voltages of ordinary magnitude will not deleteriously affect the oscillator. With sub-harmonic operation, it is, therefore, not necessary to shield the oscillator from the preceding signal circuits.

In Fig. 2 there is shown a schematic wiring diagram of the detector circuit as applied partlcularly to a superheterodyne receiver. In this embodiment of the invention the phase detector comprises resonant circuit 9 which is tuned to the receiver's intermediate frequency, the diodes 1 and 8, and the load circuit I'0-l l. The resonant circuit 9 comprises the secondary winding of the intermediate frequency transformer 12 and the tuning condenser I3. The frequency modulated intermediate frequency signal from the output of the preceding intermediate frequency amplifier (not shown) is supplied to the diodes in push-pull relation through transformer 12, and a signal from the oscillator tank circuit Hi is also supplied to the diodes, but in like phase, through the coupling condenser l5. The oscillator comprises the plate coil It, the grid coil ll, which is tuned by the adjustable condenser i8, and the vacuum tube i9 which is shown in this instance as a triode. In the above-mentioned Travis circuit, the oscillator is tuned to the intermediate frequency, and the frequency control circuit serves to establish a phase quadrature relation between the unmodulated I. F. signal and the signal supplied from the oscillator, and .to loosely synchronize the oscillator with the I. F. signal. According to the .present invention, however, the oscillator is tuned to a sub-harmonic of the intermediate frequency, and the frequency control circuit serves to establish a su'b stantially phase quadrature relation between the intermediate frequency signal and that harmonic of the oscillator which corresponds in'frequency to the intermediate frequency. Thus if the oscillator is adjusted to operate at the nth subharmonic of the intermediate frequency, the frequency control circuit will function to produce a substantially phase quadrature relation between the nth harmonic signal of the oscillator and the intermediate frequency signal.

The control circuit 29 comprises the frequency control tube 2.5, the

resistors

22, 23 and 24 land the by-pass condenser 25. In this particular circuit, the tube 21 is caused to appear as a capacitive reactance whose magnitude is varied by the control signal. The variable capacitive admittance Yo of the tube 2| is determined approximately by the formula Yc=Y (1+A) where Y is the total grid-plate admittance and A is the gain of the tube. Since the gain A varies in accordance with the control voltage applied-to the grid, the effective reactance of the tube is varied by the control voltage derived from the load resistors l8 and I! of the phase detector, by way of the conductor 27 and the tank coil ii.

.In operation, as the frequency-modulated I. F. signal varies from the mean or center frequency, there is produced in the output of the phase detector a signal whose magnitude is proportional to the cosine of the phase angle between the two input signals, 1. e. between the intermediate frequency signal and the corresponding harmonic frequency signal of the oscillator ii). The control device, responding to the detector output signal, acts to oppose the departure from the phase quadrature relation of the two input signals in a manner described in the aforementioned Travis patent. As pointed out in the Travis patent, the sensitivity of the control deviceis preferably-adjusted in such a manner that as the I. F. signal is swung during modulation from one extreme end of the band to the other end thereof, the phase angle between the signals may depart from quadrature by not substantially more than :90 degrees, and preferably, :during normal operation, by considerably less than 90 degrees, say by not more than :30 degrees as a maximum.

Since, in the particular embodiment illustrated i ,Fig. 2, the low potential end 33 of the tank coil I? is connected to the high potential terminal 34 of the diode load impedance iii-H, it is not possible to connect the low potential end 33 of the coil H directly to ground and. to the low potential terminal 35 of tuning condenser i=8. Accordingly the oscillators resonant tank circult Ii, It may be completed by providing a suitableR. F. by-pass circuit 28 connected between the low potential terminals 33 and 35 of elements 41 and 18. In this connection it will be seen that, with respect to the oscillator tank coil 1 the eifective capacitance of the frequency control circuit 28 is in series with the impedance of path 28. The higher this impedance, the less will be the oscillator frequency deviation which it is possible to obtain from a given change of capacitance in the control circuit 251. Hence, path 28 should have as low an impedance as possible at the fundamental oscillator frequency. The path 28 may therefore advantageously comprise a series resonant circuit, including inductance 2s and capacitance 3B, tuned to the oscillator frequency.

The audio output of the detector is derived from the load resistors l6 and H, and is preferably filtered so as to remove the I. F. signal. This .may be accomplished by means of an RC filter circuit consisting of the series resistor 31 and the shunt condenser 32.

For purposes of explanation, it may be assumed that the intermediate frequency amplifier is tuned to 4.3 m. c., this being an intermediate frequency commonly used in frequency-modulation receivers. The oscillator circuit is tuned to a sub-harmonic of 4.3 megacycles, and since con ventional oscillators generate non-sinusoidal waves which are particularly rich in even har monies, especially the second, it is convenient to tune the oscillator so that its second harmonic corresponds to the intermediate frequency. Thus, with a 4.3 m. cintermediate frequency, the oscillator frequency may be approximately 2.15 megacycles.

Whereas in the aforementioned Travis patent, the oscillator operates at the frequency of the intermediate frequency amplifier, as has been previously explained, and may become locked in with the output of the intermediate frequency amplifier, in the present invention the oscillator operates at a sub-harmonic of the intermediate frequency circuit. However, since the oscillator signal is strong in second harmonic content and the circuit 9 is resonant to the second harmonic, this harmonic will combine with the I. F. frequency signal to provide detector action of the type described in the aforementioned Travis patent. It is to be noted, of course, that since the circuit 9 is not tuned to the oscillator frequency, the sub-harmonic frequency of the oscillator will neither cause detector action nor will it deleteriously affect the action of the detector. Conversely, induced intermediate frequency voltages are effectively by-passed in the oscillator circuit and will not affect the frequency of oscillation thereof.

In the illustrated embodiment, balanced diode detection is used in combination with a single tube oscillator circuit and a single-sided control network, and the audio output is applied to a single-sided audio amplifier. It is to be understood, of course, that the circuit may be modified in any desired manner. For example, the above-mentioned Travis patent illustrates possible variations of the circuit employing a combined detector and oscillator tube or a balanced control circuit. Furthermore, the invention is not limited to a superheterodyne receiver, but is applicable to any frequency modulation receiver, such as a tuned R. F. receiver.

I claim:

1. A system for the detection of frequencymodulated carrier waves, comprising a source of signal oscillations whose frequency varies in accordance with the frequency variations of the transmitted signal, means for generating local oscillations whose mean frequency is substantially equal to a sub-harmonic of the mean frequency of said signal oscillations, a frequency control device connected to said generating means for loosely synchronizing a harmonic of the local oscillations with the signal oscillations and for establishing a predetermined mean phase angle therebetween, sa d phase angle varying in accordance with the deviations of the signal oscillations from their mean frequency, and means responsive to both said signal oscillations and said harmonic oscillations for producing an intelligence signal which is proportional to a function of the departure of the phase angle from said predetermined angle.

2. A system for the detection of frequencymodulated carrier waves, comprising a source of signal oscillations whose frequency varies in accordance with the frequency variations of the transmitted signal, a local oscillator having a tank circuit adjusted to resonate at substantially a predetermined sub-harmonic of the mean frequency of said signal oscillations, a frequency control device connected to said tank circuit to enable the resonant frequency of said circuit to be varied about said sub-harmonic frequency, a vacuum tube circuit functioning as a phase detector, means for supplying signals from said source to said phase detector. means for supplying a harmonic signal from said local oscillator to said phase detector, an output circuit for said phase detector for deriving therefrom a voltage which is proportional to a function of the phase angle between the signals supplied to said detector, means for applying said voltage to said frequency control device thereby to vary the fre quency of said local oscillator in accordance with the output voltage of said phase detector, and means for additionally utilizing the voltage derived from said phase detector as the audio output of the system.

3. A system for the detection of frequencymoclulated carrier waves, comprising a source of signal oscillations whose frequency varies in accordance with the frequency variations of the transmitted signal, a local oscillator having a tank circuit adjusted to resonate at substantially a predetermined sub-harmonic of the mean frequency of said signal oscillations, a frequency control device connected to said tank circuit to enable the resonant frequency of said circuit to be varied about said sub-harmonic frequency, a vacuum tube circuit functioning as a phase detector, means for supplying signals from said source to said phase detector, means for supplying a harmonic signal from said local oscillator to said phase detector, an output circuit for said phase detector for deriving therefrom a. voltage which is proportional to a function of the phase angle between the signals supplied to said detector, and means for applying said voltage to said frequency control device thereby to vary the frequency of said local oscillator in accordance with the output voltage of said phase detector.

4. A system for the detection of frequencymodulated carrier waves, comprising a source of signal oscillations whose frequency varies in accordance with the frequency variations of the transmitted signal, a local oscillator having a tank circuit adjusted to resonate at substantially a predetermined sub-harnionic of the mean frequency of said signal oscillation, a frequency control tube connected to said tank circuit to enable the resonant frequency of said circuit to be varied about said sub-harmonic frequency, a vacuum tube circuit functioning as a phase detector, means for supplying signals from said source to said phase detector, means for supplying a harmonic signal from said local oscillator to said phase detector, an output circuit for said phase detector for deriving therefrom a voltage which is proportional to a function of the phase angle between the signals supplied to said detector, means for applying said voltage to control element of said frequency control tube thereby to vary the frequency of said local oscillator in accordance with the output voltage of said phase detector, and means for additionally utilizing the voltage derived from said phase detector as the audio output of the system.

5. A system for the detection of frequencymodulated carrier waves, comprising a source of signal oscillations whose frequency varies in accordance with the frequency variations of the transmitted signal, a local oscillator having a tank circuit adjusted to resonate at substantially a predetermined sub-harmonic of the mean frequency of said signal oscillations, a frequency control device connected to said tank circuit to enable the resonant frequency of said circuit to be varied about said sub-harmonic frequency, a pair of diodes connected to function as a phase detector, means for supplying signals from said source to the input circuits of said diodes in pushpull relation, means for supplying a harmonic signal from said local oscillator to the input circuits of said diodes, an output circuit connected to the output electrodes of said. diodes for deriving therefrom a voltage which is proportional to a function of the phase angle between. the signals supplied to the input circuits of said diodes, means for applying said voltage to said frequency control device thereby to vary the frequency of said local oscillator in accordance with the output voltage of said phase detector, and means for additionally utilizing the voltage derived from said phase detector as the audio output of the system.

6. In a frequency modulation receiver of the super-heterodyne type, an intermediate frequency amplifier, an oscillator adjusted to oscillate at a frequency which is approximately times the mean frequency supplied by the intermediate frequency amplifier, where n is a small integer other than one, a phase detector connected to the output circuits of the intermediate frequency amplifier and the oscillator, respectively, and adapted to produce an output signal proportional to a function of the phase angle between the oscillator nth harmonic signal and the intermediate frequency amplifier signal, and means for loosely sychronizing the nth harmonic of said oscillator with the output of said intermediate frequency amplifier and for establishing a predetermined mean phase relation between the said harmonic of the oscillator signal and intermediate frequency signal, the instantaneous phase relation varyin in accordance with the deviations of said intermediate frequency signal.

7. In a frequency modulation receiver of the superheterodyne type, converter means for changing the incoming frequency-modulated signals to frequency-modulated signals having a predetermined center frequency, a local oscillator having a tank circuit adjusted. to resonate at substantially a sub-harmonic of said center frequency, said oscillator being constructed and arranged to generate strong harmonics of its fundamental frequency, frequency control means connected to said tank circuit for varying the oscillator frequency, a phase detector circuit, means for supplying said phase detector circuit with harmonic signals from the oscillator, means for supplying the frequency-modulated signals of predetermined center frequency to the detector, means for supplying the output voltage of said phase detector to said frequency control means, and a circuit connected to said phase detector for deriving a signal therefrom.

8. In a frequency modulation receiver of the super-heterodyne type, converter means for changing the incoming frequency-modulated signals to frequency-modulated signals having a predetermined center frequency, a local oscillator having a tank circuit adjusted to resonate at substantially one-half said aforementioned center frequency, frequency control means connected to said tank circuit for varying the oscillator frequency below and above its normal frequency, a phase detector circuit, means for'supplying said phase detector circuit with. signals from said oscillator which are twice the fundamental frequency of the oscillator, means for supplying the frequency-modulated signals of predetermined center frequency to the detector, means for supplying the output voltage of said phase detector to said frequency control means, and a circuit connected to said phase detector for deriving a signal therefrom.

9. The method of" receiving a frequency-modulated carrier signal, which comprises deriving from said signal an oscillation varying in frequency in accordance with the variations of the received signal, generating a local signal having a mean frequency bearing an nth sub-harmonic relation to the mean frequency of the derived oscillation, Where n ma small integer other than one, establishing a predetermined mean phase relation between said derived oscillation and the nth harmonic oscillation of said local signal, deriving a control voltage from said derived oscillation and said nth harmonic oscillation, utilizing said control voltage to cause the phase relation of said oscillations to vary from said predetermined phase relation in accordance with the deviations of the derived oscillation from its mean frequency, and deriving an intelligence signal from said oscillations which is proportona-l to a functon of the departure of the phase angle from said predetermined phase relation.

10. The method of receiving a frequency-modulated carrier signal, which comprises deriving from said signal an oscillation varying in frequency in accordance with the variations of the received signal, generating a local signal having a mean fundamental frequency bearing. an nth sub-harmonic relation to the mean frequency of the derived oscillation, where n is a small in.- teger other than one, establishing a predetermined mean phase relation betwecn said derived oscillation and the nth harmonic oscillation of said locally generated signal,v deriving a control voltage fromsa-id derived oscillation and said nth harmonic oscillation, utilizing said control voltage to loosely synchronize the said nth harmonic os-- cillation with the derived oscillation, said synchronizing being of such character that the mean phase relation of said oscillationsis substantially phase quadrature, but. varies from phase quadrature in accordance with the frequency modulation of the received signals, and deriving an intelligence signal from said oscillations which is pro portional to a function of the phase angle between said oscillations.

11. A system for the detection of frequencymodulated carrier waves, comprising a source of signal oscillations whose frequency varies in accordance with the frequency variations of the transmitted signal, means for generating a nonsinusoidal local oscillation whose fundamental frequency is approximately one-nth the mean frequency of said signal oscillations, a frequency control device connected to said generating means, means responsive to said signal oscillations and to the nth harmonic oscillation of said local oscillations for establishing a frequency control signal, and connections for applying said control signal to said frequency control device whereby to establish a mean frequency identity between said nth harmonic oscillation and said signal oscillations, n being a small integer other than one.

12. A system for the detection of frequencymodulated' carrier waves as claimed in claim 11, wherein n is the integer 2, and wherein the said control signal is utilized as the intelligence signal output of the detector.

13. A frequency modulation receiver comprising a source of signal. oscillations Whose frequency varies in accordance with the frequency variations of the transmitted signal, an. oscillator adjusted to oscillate at a. frequency which is approximately times the mean frequency of said signal oscillations, where n is a small integer other than one, means for loosely synchronizing thenth hare monic of said oscillator with said signal oscillations and for establishing a predetermined mean phase relation between the said nth harmonic signal of the oscillator and said signal oscillations, the instantaneous phase relation therebetween varying in accordance with the frequency deviations of said signal oscillations, and a signal-detecting means for detecting the frequency variations of said transmitted signal, said last-named means being coupled to said oscillator and responsive to a signal therefrom.

14. In a frequency modulation receiver, a source of frequency-modulated signal oscillations, an oscillator of controllable frequency for generating a signal whose fundamental frequency is one-nth the mean frequency of said signal oscillations, where n is a small integer other than one, and means operatively associated with said oscillator for varying its fundamental frequency and for loosely synchronizing the nth harmonic of said oscillator signal with the frequency-modulated signal from said source.

15. A radio receiver adapted to receive frequency-modulated signals, characterized by the provision of an oscillator of controllable frequency for generating a signal whose fundamental frequency is one-nth the mean frequency of the received signal, where n is a small integer other than one, and means operatively associated with said oscillator for varying its fundamental frequency and for loosely synchronizing the nth harmonic of said oscillator signal with the received signal.

16. In a frequency modulation receiver, a source of frequency-modulated signal oscillations, an oscillator of controllable frequency for generating a signal Whose fundamental frequency is one-nth the mean frequency of said signal oscillations, where n is a small integer other than one, means operatively associated with said oscillator for varying its fundamental frequency and for loosely synchronizing the nth harmonic of said oscillator signal with the frequency-modulated signal from said source, and means energized by said oscillator for producing a detected output signal.

1'7. In a, frequency modulation receiver, a source of frequency-modulated signal oscillations, an oscillator of controllable frequency for generating a signal whose fundamental frequency is one-nth the mean frequency of said signal oscillations, Where n is a small integer other than one, and means operatively associated with said oscillator for varying its fundamental frequency and for loosely synchronizing the nth harmonic of said oscillator signal with the frequency-modulated signal from said source, whereby the frequency deviations of the said oscillators fundamental signal are substantially one-nth the deviations of the oscillations from said source.

CLAUDIUS T. MCCOY.

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

UNITED STATES PATENTS Number Name Date 2,112,881 Crosby Apr. 5, 1938 2,280,525 Hunt Apr. 21, 1942 2,332,540 Travis Oct. 26, 1943 2,356,201 Beers Aug. 22, 1944 2,361,606 Crosby Oct. 31, 1944