US1871741A - Constant beat frequency system - Google Patents

Description

ca.` RoDwlN 1,871,741 CONSTANT BEAT FREQUENCY SYSTEM Filed Jan. so. 1929 2 .sheets-sheet 1 Aug. 16, 1932.

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lwoe/ntoz GEORGE Rouwm @wha aumwj//MM Patented Aug. 16, 1932 UNITED STATES PATENT oFFic-E GEORGE RODWIN, Ol' NEW YORK, N. Y., ASSIGNOB. T0 RADIO CORPORATION OF AMERICA, A CORPORATION OF DELAWARE CONSTANT BEAT FREQUENCY SYSTEM Application med January 80, 1829. Serial No. 838,089.

This invention relates to a system for controlling the local oscillator in a'heterodyne circuit, and more particularly to a system for maintaining the locally generated frequency 5, in a superheterodyne circuit at a definite fixed frequency relation with the frequency of the incoming signal.

It is often necessary, in reception of radio signals, to maintain a constant intermediate 1Q frequency in a superhetrodine receiver. This is tors, bot

andreceiver is equal to the intermediate frequency desired.

A small change in crystal frequency, when using short waves, may, therefore, result in a considerable change in intermediate frequency. By means of the system described so and disclosed herein, it is possible to obtaina constant beat frequency at the receiver even when small variations of frequency are occurring in the transmitter or receiver, or both, and the said system has been found to be of special efficacy.

It is one of the main objects of m present invention, then, to provide a system or maintaining a beat frequency constant by utilizing variations in the said frequency to control the action of the local oscillator.

Another important object of the invention is to provide a method of maintaining the intermediate frequency constant in a su erheterodyne circuit by transforming eat frequency variations into potential variations, aud utilizing the said potential variations to control the action of the local frequency source in that direction which tends to maintain the intermediate frequency.

And still another object of the inventionv -well known manner.

is to provide an intermediate'frequency control in a superheterodyne receiving circuit, which control is adapted to maintain the intermediate frequency constant regardless of varia-tions in signal frequency or local frequency, the control including a frequency differentiating circuit adapted to transform beat frequency variations 1nto potential variations, means to amplify the potential variations, and means to utilize a portion of the amplified potential to control the source of local frequency in that direction which tends tov maintain .the intermediate frequency constant.

The novel features which' believe to be characteristic of my invention are set forth in particularity in the appended claims, the

invention itself, however, as to both its o rganization and method of operation will best be understood b reference to the following description ta en in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

The principle and certain applications of this invention are illustrated 1n the accom panying drawings, in which: Fig. 1 is a schematic diagram showing the superheterodyne' receiver embodying a circuit for controlling the locally generated intermediate frequency;

Fig. 2 is a graphic representation of the action of the control circuit shown in Fig. l;

Fig. 3 is a modified form of the control circuit shown in Fig. 1;

Figs. 4, 5, and 6 are detailed views of three modified types of frequency differentiating circuits.

Referring more particularly to Fig. 1, there is shown a conventional superheterodyne receiver circuit, embodying two stages of radio frequency amplification coupled to a lconventional frequency changer, the latter being coupled to a local oscillator in the In its usual form, this frequency changer is a circuit employing an ordinary three electrode tube biased to operate on the lower curve portion of its plate Current-grid voltage characteristic. Both the incoming signal frequency and the locally generated frequency are applied to the grid of this tube. T he difference frequency, or intermediate frequency, is taken out of the plate circuit thereof.

As is well known, the intermediate frequency is amplified by an intermediate frequency amplifier, the latter being coupled t0 a. detector, which in turn is connected to utilization means of any type suchas telephones, a loud speaker, and the like. The intermediate frequency amplifier is connected to a conventional limiting tube arrangement, and the output from the limiter is passed into a circuit connected to the local oscillator as shown. The circuit between the limiter and the local oscillator comprises two main elements, hereinafter termed rel spectively, the direct current amplifier tube, and the control tube. y

. The voltage from the intermediate frequency amplifier is fed into the limiter, and the output from the latter is passed into the subsequent frequency differentiating circuit connected thereto. The constants of capacity C1, resistor R1, and the frequency dfferentiating circuit, are such as to give a voltage across resistor R1 whichis variable with impressed frequency. Plate voltage for the control tube T2 is taken through the same resistance R3 as the plate supply to the local oscillator T3. The constants of the oscillator circuit are such that lits generated frequency varies inversely `with the applied direct plate voltage.

Since it is desired to obtain a voltage variation across the resistance R1 which is directly proportional to the intermediate fre-` quency variation only, and is net affected by amplitude variation, the conventlonal l1m 1t ingk arrangement is disposed between the intermediate frequency amplifier and the cir-v,

reached, whereby the limiter output is of constant amplitude, and can only varyin frequency. The limiter is conventionally shown to simplify the description, and since it is readily apparent to those skilled in the art that the particular type and arrangement used is unimportant, since any welllmown device adaptedto perform thedesired limiting function can be used.

In other words,l as the platevoltage of oscillator tube T3 goes up, the locally generatedv frequency f2 goes down. It may be shown, both experimentally and mathematically, that the generated frequency of an oscillator may be varied in a small degree by a change in the applied plate voltage. As the plate volta ge is increased, either an increase or decrease of frequency may occur, depending 65 upon the circuit and tube characteristic. Reference to the December, 1928, issue of the Proceedings of the Institute of Radio Engineers, page 1719, will reveal a description of this phenomenon. The case described therein is a particular one, and considerable variations are obtainable.

I have chosen the circuit, in which the generated frequency decreases when an' increase in plate voltage over the range of voltages is employed. For the circuit action to be car- 'I5 ried through it is, therefore, necessary that the local oscillator frequency be higher than the frequency of the incoming signal. For example, assume that the desired intermediate frequency is 25,000 cycles and the in coming frequency f1 is 12,500,000 cycles, then the local frequency f2 would be 12,525,000 cycles. If, however, a local oscillator was employed in which the generated frequency increased With an increase in plate voltage, the local oscillator would have to be operated at' a lower frequency than the incoming signal frequency.

If f2 should tend to increase, the intermediate frequency would obviously increase, and due to the lessening of impedance offered by capacity C1 to the increasing frequency, the alternating voltage across resistor R1 will increase. T1, the direct current amplilier tube, is normally operated at the lower bend of its plate currentvgrid voltage characteristic. -Accordingly, the plate current of tube T1 will rise as the voltage across resistor Rl increases.

The negative bias onthe grid of T2, the control tube, will increase, and the plate current of this tube will, therefore, decrease. There will now be less voltage drop across resistor Rf, and the plate voltage of the local oscillator T3 will increase causing the generatedvfrequency of oscillator T3 to decrease. The adjustment of resistorR2 is effected by the vslider S, the latter being connected to the grid of 4tube T2. The slider S is so positioned jon-resistor R2 that the result of de- 110 crease in frequency of oscillator T3, due to the increase in plate voltage of the latter, is just suflicient tocompensate for-the rise in frequency f2 which initiated the increase in the plate voltage of oscillator T3. 115

It is seen, therefore, that the intermediate frequency will be maintained constant. The purpose of capacity C3 is to slow down the action of the circuit. If capacity G3 were absent, the action would be almost instantane- 120 ous, and the locally generated frequency 55 T1, Will be lowered.

ment of resistance It2 is made such that the frequency. The abscissae--intermediate frequency-are regarded as the cause of the action, and, the Various ordinates-frequency, voltage and current-the result. Thus, an attempted rise in intermediate frequency,due to the change in either the locally generated frequency f2 or the incoming frequency f1 will result in a decrease in the locally generated frequency.

It is to be noted that the rise in intermediate frequency, according to the curve shown in Fig. 2, results in an increase in voltage across resistance R1; increase in the bias of control tube T2; decrease of plate current of tube T2; increase of plate voltage of oscillater T3, and finally decrease of the locally generated frequency.

Another method of accomplishingthe same result, which o'ii'ers some advantages over the arrangement shown in Fig. 1, is disclosed in Fig. 3. For the sake of simplicity and brevity, the main members of the superheterodyne receiver are omitted, and only the control circuit is shown. Tube T1 is connected tothe limiter as in Fig. l, the tube being connected through a resistance R2 to a control tube T2, as shown in Fig. 1.

In this case, however, the control tube is coupled to thel local oscillator tube T3 through an amplifier tube T4. The in ut capacity of tube T4, which is acting as a` igh' frequency amplifier, is used to control the generated `frequenc of the local oscillator T3. The input capacity of tube T4 is shunted across the tuning condenser V of the vcircuit embodying T3. Theoretically, it can be' shown that the input capacity of the vacuum tube employed as an amplifier will vary with the impedance in its plate circuit.

The total resistance in theplate circuit of T4 is made up of resistance It4 and tube :T2v

in parallel. The ,input capacity of tube Tj,r can thus be varied by changing the plate impedance of tube Tg. The action ofthe remaining part of the' circuit is the same as thatv Using the same nomenclature, when the local frequency f2 increases the in- 0f Fig. i.

termediate frequency Will' rise, and thedrop across resistance R1 Will rise.

Thel negative bias on the grid of 'tube Tg will then increase negatively, and the iinpedance of tube T2 will rise. input capacity of tube T., will rise. and, therefore, the generated frequency of the oscillator As before, the adjustloivering of the generated frequency of oscillator of T., is just equal to the rise of frequency which initiated the action of the cor.- trol circuit. Itis to be observed that any kind of load impedance. may be substituted for resistance IL, such as an induetance or tuned circuit. By adjusting the con-'tants of the ei'rf'uits. the action may he varied over a considerable range. It is also to be noted As a result, the

that in lace of the frequency differentiating circuit 1, R1, shown in Figs. 1 and 3, numerous other arrangements may be used, y as shown in Figs. 4, 5, and G. In Fig. 4,"for eX- ample, capacity CL is replaced by a resistance R, and the resistance R1 is replaced by an inductance L. l

' When the frequency differentiating c'rcuits shown in Figs. 5 and 6 are employed in place of capacity C1 and resistance R1 of Fig. l, it is necessary to operate the local` oscillator at the frequency f2, lower than `the incoming signal frequency f1, since the available voltage on tube T1 will now decrease With an increase of intermediate frequency.

The embodiments illustrated above have been shown by vvay o-f preference only. The appended claims should be construed as broadly as permissible since many and various changes may be made Without departing from the vscope of the invention as defined in the broader claims.

Havingl thus described my invention, what I claim and desire to secure by Letters Paten t of the.l United States is:

1. A'niethod Aof maintaining a local oscillator circuit including an electron discharge tube in a. s'iiperhetrodyne receiving system at a definite, fixed frequency relation with the frequency ofthe incoming signal regardless of signal andlocalfrequency variations which consists in liniiting'a portion of the heat frequency energy lto remove amplitude variations therefrom transtorininfr the beat fre- .vv e. quencyvariations into potential Variations,

Y amplifying the potential variations, and iinpressing a predetermined` portion ofv the amplified potential upon the anode circuit of 2. A method of maintaining a constant intermedi-ate frequency in a superheterodyne receiving system which consists in collecting signal energy, generatingv local oscillations Withan electron discharge tube at a frequency different from the incoming signal energy bythe intermediate frequency Y to beeinployed, limiting apportion of the intermediat`e`-freqiiency energy to remove yamplitude variations, transforming intermediate frequency variations intov potential variations, amplifying potential variations caused by intermediate rfrequency variations, and alteringthe'potcnti al of the anode of the `tube ivith the amplified potential variations.

3. In the. art of heterodyne reception, the f steps of maintaining the beat frequency constant regardless of 'signal and/or local frequency variations which consist in limiting a portion of the beat i'iezpieniv energy to remove amplitude varie-.tions` therefrom, tran.- foiining the. beat frcqueni'jv variations into potential variations, iiiiiplif \iiig potential .'ariations caused bj.' beat frequency variatiOIlS` 21nd "oiiti'ollin'. with a I iedetermined portion of Sie ampliiied potential, the action of the source of local frequencY in such a manner as to maintain a certain efinite fixed frequency relation between the local frequency and signal frequency.

4. A method of controlling the frequency of an oscillator tube in a superheterodyne receiving system so as to maintain a predetermined fiXed relation `between the local frequency and the signal frequency which consists in limiting a portion of the beat frequency'energy to remove amplitude variations therefrom, transforming the beat frequency variations into potential variations, amplifying potential variations caused by variations in the difference frequency, biasing, with a portion of the amplified potential, the grid of a control tube whereby anode current fiow of said control tube is varied, and finally varying, with the varying anode currerg, the anode potential of the oscillator tu 5. In combination with a superheterodyne receiving circuit provided with an oscillator tube and an intermediate frequency am lifier, means coupled to said intermediate requency amplifier for limiting a portion of the intermediate frequency energy to remove amplitude variations, means for transforming said intermediate frequency variations into potential variations, an amplifying tube coupled to the last named means, a control tube coupledto the output circuit of said amplifying tube, the output circuit of said control tube being connected to the output circuit of the oscillator tube of said superheterodyne circuit for varying the anode potential of the oscillator tube.

6. In combination with signal energy collecting means, a source of local oscillations of a frequency different from the frequency of the collected energy by a difference frequency to be detected, amplifying means for said difference frequency, means coupled to said difference frequency amplifier for limiting a portion of the difference frequency energy to remove amplitude variations, means for transforming said-difference frequenc variations into potential variations a secon amplifying means coupled to said last named means, a control tube connected to a predetermined portion of the output circuit of said second amplifying means, the output circuit of said control tube being connected to the output circuit of said local source.

7. 4In the art of heterodyne reception, the steps of maintaining the beat frequency constant which consist in heterodyning incoming waves with a local frequency source, transforming beat frequency variations into potential variations after limiting a portion of said beat frequency energy to remove amplitude variations, amplifying the potential variations, and controlling, with a predetermined portion of the amplified potential, the action of the local frequency source in that direction which tends to maintain the beat frequency. l

8. In the art of heterodyne reception, the steps of maintaining the beat frequency constant which consist in heterodyning incoming Waves with a local frequency source, limiting a portion of the beat frequency energy to remove amplitude Variations therefrom, thereafter transforming beat frequency variations into potential variations, amplifying the potential variations, and controlling, with a predetermined portion of the amplified potential, the source of local frequency so that its frequency is Varied in that direction which tends to maintain the beat frequency constant.

9. In a radio receiving system, a receiving circuit, means including an electron discharge tube foi` heterodyning incoming waves, means for converting beat frequency variations into potential variations substantially free of amplitude variations, and means including an additional electron discharge tube responsive to said potential variations for varying the anode potential of the first tube on a change in frequency of either the incoming waves or the heterodyning means.

l0. The method of receiving high frequency wave signals which consists in heterodyning the signals with a circuit including an electron discharge tube, transforming a variation in the beat current so produced, due.

to a change in the frequency of thewaves, into potential vari'ations free of amplitude variations, varying the potential of the anode of the tube with said potential variations, and

causing such alteration in potential to vary the frequency of said circuit.

11. The. method of receiving high frequency wave signals which consists in applying ay heterodyne thereto from a local oscillator tube to produce a beat frequency, converting a change lin the beat frequency to a potential change free of amplitude change, and altering the anode potential of the oscillator tube with said potential change.

12. In combination in a high frequency signalling system an oscillatory circuit including an electron discharge tube, means including a second electron discharge tube and associated circuits responsive to changes in the oscillator frequency and adapted to convert frequency variations into potential variations substantially free ofamplitude variations, and a control tube connected between said frequency responsive means and the oscillatory circuit whereby said potential variations are caused to vary the potential of the control electrode of said control tube which controls the frequency of the oscillatory circuit.

13. In combination with a high frequency receiving system, means including an electron discharge tube for heterodyning incoming waves, means including an additional elecmarinai tron discharge tube res onsive to beat frequency variations and at apted to convert the latter into amplified potential variations substantially free of amplitude variations, and

' intermediate frequency, a source of local oscillations comprising an electron discharge tube provided with a tunable input circuit coupled to said changer circuit and an output circuit,'means for converting intermediate frequency variations into potential variations substantially free of amplitude variations, a control tube coupled to said converting means, the anode circuit of said control tube being connected to the anode circuit of Said oscillator tube.

15. In a superheterodyne receiver, a source ofy amplified signal energy, a frequency changer circuit adapted to deliver energy at an intermediate frequency, a source of local os nations comprising an electron discharge tube provided with a tunable input circuit coupled to said changer circuit and an output circuit, means for converting intermediate frequency variations into potential variations substantially free of amplitude variations, a control tube having its input circuit coupled to said converting means, an amplifier adjustably connected between said converting means and control tube, the anode circuit of said control tube being connected to the anode circuit of said oscillator tube.

GEORGE RODWN.

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