US2354510A

US2354510A - Automatic frequency control

Info

Publication number: US2354510A
Application number: US451292A
Authority: US
Inventors: Earp Charles William
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1941-07-11
Filing date: 1942-07-17
Publication date: 1944-07-25
Anticipated expiration: 1961-07-25

Description

July 25, 1944. c. w. EAR 2,354,510

AUTOMATIC FREQUENCY CONTROL Filed July 17, 1942 OSCILLATOR 0 1 cm L saaaa'winm l VOLTA5 C V F V FREQUENCY com-non.

F1 fig. Z. F f) FILTER -r MODULATOR MODULA on MI DELAY nnwonx rnsouzucv DISCRIMINATOR cow ring. P 1 F OSCILLATOR comnov. VOLTAGE CV man 1 ,3JNTERMEDIATE rnzquzncy 3 rasouzucv AMPLIFIER mxsn AMPLIFIER DETE TOR H1 H7 I [F4 DET MODULATOR MODULATOR F1 MI- fl f) v nun nun ' DISGRIMINATOR I mrj V DISCRIMINATOVR F O R g v PC .6 //VI. /Wfl/P OSCILLATOR 1W I CONTROL RECTIFIER (9% C. W Efl/FP Patented July 25, 1944 UNITED STATES I {PATENT 2,354,510 AUTOMATIC FREQUENCY CONTROL.

Charles William EarpQLondori, England, assignor to International Standard Electric Corporation, NewYork, N. Y.

Application July 17, 1942, Serial No. 451,292 In Great Britain July 11, 1941 7 Claims. (012 250-40) whichit is set, the value of sa d direct current voltage determining the rate of changeofire-,5 quency of said oscillator and bringing the fre ,quency thereof to such a value as to maintain said, Q intermediate frequency constant,

In carrying out the invention theoscillator 1 such that it remains stable in frequency at any 71 frequency to which it is set and Within the band,

width of the control circuits but 'is -readi1y iis -v l narrower aspect of the invention,

can oscillate on any of a wide range of frequencies comprises-an oscillator and a frequency dis-- criminator or countingarrangement adapted to produce a direct current voltage in the output thereof depending'upon the frequency of the in put fed to the said discriminator or-counting arrangement, and means for feeding said output I voltage to said oscillator to -maintainthe frequency generated by the latter.

According to another narrower aspect of the invention, a circuit arrangement for the stabilisation of an oscillator at any frequency to which it is set comprises .a frequency discriminator'or counting arrangement fed from the oscillator and adapted to produce a direct current voltage in turbed in frequency by applicationof-an addi: tional control voltage from an ext'ernaleource;

The invention will be better understood fromthe. following description. takenin conjunction ,lfi with the accompanying drawing in which Fig. 1 shows in block;schematic a ifrequency-j control arrangement accordingtothe invention applied to an oscillation generator; V a, Fig. 2 shows in block schematic the. invention applied to another and preferredformof oscillation generator; and l 1 Fig. 3 shows in block schematic the invention incorporated in a superheterodyne' receiyen: Referring to Fig. 1, the oscillator O .feeds.a;

; ;.frequency discriminator D, which provides -an--in-.

the output thereof depending upon the frequency of the input and means for feeding said output direct current voltage to said oscillator in such manner that the value of said output voltage maintains the frequency of said oscillator, the said oscillator and discriminator being so chosen that their characteristics showing the relationship :between frequency and controlling voltage or frequency and output voltage respectively, are substantially linear,- and have equal slopes, in the vicinity of the required stabilised frequencies.

,The' invention is particularly suitable for application to theoscillator of a superheterodyne radio receiver to supply the frequency changes of the oscillator where the received signal drifts in frequency so as to maintain the resulting intermediate frequency substantially constant.

According to a feature of the invention therefore, in a superheterodyne receiver arrangements are provided for eliminating the effect due to frequency drift of the received waves. These arrangements comprise a frequency discriminator or counting arrangement fed from the output of the intermediate frequency stage and adapted to produce a direct current voltage in the output thereof depending upon the input frequency, and means for feeding said output current voltage to said oscillator stabilised at any frequency to put voltage to the frequency control tube FV. 1 in turn coupled back to the oscillatoreO}. The'e fre- 1. quency control tube operates on well known prin-"g ciples, and may depend, forexample,-.-on the Now, it is arranged that for 'agiven displacement in frequency-of the-oscillator O, the discriminator sensitivity is; such that-its-l change of output voltage is exactly that, which is required to produce that same change "i'n'fre quency by application of the voltage to 'the fre=- quency control tube. If, now, the discriminator characteristic (input frequency versus output voltage) exactly matches the frequency control tube-oscillator characteristic, .(inputv-voltage" to versus output frequency of 0*) the oscillator.

does not tend to drift from 'an'y'frequency to which it has been set. t

A small additional frequency control :voltage, applied from some external source represented at CV will now cause the oscillator frequency to;

increase or decrease according to the sense of.

p in the path Ml to M2.

the frequency applied to the discriminator.

nected directly to a modulator M2, and via a I phase controlling thermionic tube PC to modulator Ml, to which a frequency band of mean frequency f is also passed as hereafter explained. Filter Fl selects from the output of MI a band of frequencies, of mean frequency (Ff) (or F+f) which is passed to M2.

control tube PC so as to shift the frequency in the opposite direction.

If, now, the D. C. voltage from the discriminator D, as applied to the control tube PC, is supplemented by an additional D. C. frequency control voltage from an external source CV, the frequency of the oscillations speeds up or slows down, according to the sense of the control voltage, until the control voltageis removed.

The frequency for controlling the oscillation circuit may be obtained from the output of either filter. Likewise the output of either filter may The function of the delay network DL'is to v comprise a voltage generated in response to the provide a linear phase shift of 0 radians per A 'cy cles change in the frequency f. If the freproduced byjthe delay network .DL will be ad'- vanced 'jby afradians. Similarly anyjother fre quencyijchange between +,A ','and A willproduce a corresponding phase'shift between +0 and -0.

If the amplification around the circuit is sufficiently great, oscillations are set up, and these oscillations must be 'of'mean frequency ,f, in order to pass throughv the filter F2, and also of frequency Fif to pass through the filter Fl. The delay network is designed to give a linear distortion over a frequency band of ,f- -A. If a certain'frequencyj is produced when the frequency F, applied to the modulator Ml, has a definite'phase relationship to the output of the oscillator 0' applied tothe modulator M2, and if a change in phase relationship occurs, the correct phase relationship around the loop has now been upsetand' the frequencyf will rise or fall to accommodate itself to the new conditions.

For instance, ifthe phase of the frequency'F injectd'jfiiito"th modulatorMl is advanced by the 'phasebontrol tube'PC 0 radians with respect'to tlie sani'freq'uencyF from the oscillator 0 'directly injected into the modulator M2, oscillations can only be maintained in the loop if the'frequency ,f is increased to f+A and the frequency Fif-is increased to F:(f+A). Oscillations at these new frequencies arequite stable. FilterFZ selects from the output of M2 the frequency band of 'mean frequency j, which is passed to Ml. At some point in the loop path MI'M2MI is inserted a delay network DL or alternativelythe required delay may be produced in 'any'of the components of the circuit, for instance,-thc filters.

The circuit as described up to this point producesoscillation'of frequency f in the path M2 to Ml,and'oscillation of frequency F-f or F+,f Now, as explained above and in the said specification of application 460,578, a phase shift produced by the phase controltube PC produces a corresponding'change in the frequency The frequency f is now fed from as constant of the circuit between the discriminafilter..F2- to' a discriminator D, or frequency counter, the ouput voltage of which is applied to the phase controltube PC and exactly matches the phase'control voltage necessary to maintain With the discriminator connected in this manner the frequency fwill be held constant, since any change thereof will produce a voltage in the output'of the discriminator D which will affect the ceiver.

be used for any desired purpose, for example, as

the local oscillation of a superheterodyne re- As shown, the filter Fl is shown with an outputcircuit for this purpose.

In either of the arrangements shown in Figs. 1 and 2 the source of control voltage CV may deviation of the intermediate frequency from normal in a superheterodyne receiver. Such a receiver incorporating the invention is shown in Fig. 3. In Fig. 3, HF. is the high frequency amplifier, FC the heterodyning stage, IFA the intermediate frequency amplifier and DET the low frequency detector stage. The oscillation generator is the same as that shown in Fig. 2 and like parts in both figures are given the same references. ated oscillation ,f at any frequency to which it is set is effected by the discriminator D and the voltage depending uponthe frequency drift of the received frequency is generated by discriminator Dl. These two voltages are applied in series to the phase control tube PC. The time tor DI and the frequency control tube PC is not fixed. In series with the automatic frequency control is introduced a four-element bridge rectifier R with capacity C across the line. A battery B is connected in series with the secondary winding of a transformer T, across the free corners of the bridge, in such sense that the four rectifier elements of the bridge are made nonconducting. In this condition, the time constant of RC circuit is infinity, or at least very arge.

A portion of the output from the intermediate frequency amplifier IFA is now introduced to the other or primary winding of transformer T, and, if it is sufficient, overcomes the bias on the rectifiers due to the battery, and passes rectified current through the battery, and through the rectifiers. In this condition,'the rectifier impedance may be reduced to a low value, the series impedance along the automatic frequency control line L is reduced, and the time constant of the line becomes short. This variable time constant feature forms the subject of my copending application Serial No. 449,293, filed July 1, 1942.

In the presence of a signal, the automatic control of frequency is operative and quick. If, now, the signal is interrupted, or quickly fades, the time constant of the RC circuit becomes very large, and the frequency control voltage across the condenser C is maintained.

The two discriminator voltages are connected in series-aiding before application to the variable time constant circuit R0. The filter F2 at frequency f is a band-pass filter covering the total range of frequency control required, and this filter itself provides the delay as stated in reference to Fig. 2.

Whilst a particular embodiment of the inven- The stabilisation of the locally generwill occur to those skilled in the art and which fall within the scope of the appended claims.

What is claimed is:

1. A frequency stabilized oscillation generating system comprising a closed oscillating circuit, an oscillator, an oscillator output circuit, means for shifting the phase of the oscillator output in said output circuit, means for introducing said phase shifted output into the closed circuit, the closed circuit oscillating frequency being varied in proportion to said phase shift, and means for varying said phase shift in proportion to variations in frequency in the closed circuit and thereby stabilizing said frequency.

2. A system as set forth in claim 1, in which said closed circuit includes a first modulating means for introducing said phase-shifted output into the circuit, filter means for selecting from the first modulating means output a frequency combining the oscillator frequency and another frequency to which the oscillating circuit is tuned, a second modulating means for combining the filter output and the oscillator output, and delay means including a filter for supplying said other frequency from the second modulating means to the first modulating means.

3. A system as set forth in claim 1, in which the means for varying the phase shift comprises a discriminator connected between the closed circuit and phase shifting means.

4. In a superheterodyne receiving system which includes a heterodyning stage followed by an intermediate frequency amplifier, a frequency stabilized oscillation generating system comprising a closed oscillating circuit, an oscillator, an oscillator output circuit, means for shifting the phase of the oscillator output in said output circuit, means for introducing said phase-shifted output into the closed circuit, the closed circuit frequency being varied in proportion to said phase shift, means for varying said phase shift in proportion to variations in frequency in the closed circuit and thereby stabilizing said frequency, and means for supplying oscillations from the closed circuit to the heterodyning stage.

5. A system as set forth in claim 4, including means for varying said phase shift in accordance with variations in the output frequency of the intermediate frequency amplifier and thereby compensating for said variations.

6. A system as set forth in claim 4, including discriminator means connected between the intermediate frequency amplifier output and the phase shifting means, arranged to correct variations in said amplifier output frequency by varying the closed circuit frequency through proportional changes in said phase shift.

'7. A system as set forth in claim 4, including discriminator means connected to said amplifier output and means for continuing the application of a regulating potential from the discriminator means to the phase shifting means after the signal producing said potential has failed, comprising a capacitance arranged in circuit between the discriminator means and the phase shifting means to transmit to the latter a regulating potential from the discriminator means, and means in circuit between the condenser and the discriminator means actuated by failure of said signal to block discharge of the condenser toward the discriminator means.

CHARLES WILLIAM EARP.