US2239776A

US2239776A - Balanced modulator circuit

Info

Publication number: US2239776A
Application number: US259546A
Authority: US
Inventors: Robert B J Brunn
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1939-03-03
Filing date: 1939-03-03
Publication date: 1941-04-29
Anticipated expiration: 1958-04-29

Description

April 29, 1941. R. B.' J. BRUNN D MoDgLAer-KEUIT Filed March s. 1939 M" Y BALANCE 2 n@ ww Anw L im. m m. Mwah@ INVENTOR ROBERT B. J. BRUNN Elx/Mm ATTORNEY Patented Apr. 29, 1941 Robert B. J. Brunn, New .York, N. Y., assignor to Hazeltine Corporation,v a corporation of Dela- Ware Appucauun March 3, 1939, serial No. 259,546 5 Claims. (Cl. 179-1715) This invention relates generally to balanced modulator circuits and particularly to balanced modulator circuits from which it is rdesired to derive onlyr heterodyne-frequency signals of two signal-input sources. While the invention is of general application, it is of particular utility in television signal-translating apparatus.

Balanced or push-pull modulators have been utilized in prior art arrangements to derive heterodyne-frequency signals from two signal-input sources. In push-pull modulators of-the prior art, it has been possible to balance the arrangement from one of the sets of input terminals to the output terminals, whereby signals of thefrequency of the input to said one of said sets of input terminals do not appear at the output terminals ofthe modulator. If only the heterodynefrequency signals are desired, however, it is necessary to provide some other means for eliminating signals of the frequency of the other input signal from the output of the modulator. Selective circuits or lters have commonly been utilized for this purpose. However, in order that the desired signals of heterodyne frequencies may be selected in this manner, it is necessary that they differ substantially in frequencyfrom the signals of input frequency which are to be rejected by the selective circuit. This relationship is sometimes difficult or impossible to procure. Furthermore, the selector circuits required in such arrangements add to the cost and compleXity of the modulator and associatedcircuits.

It is, therefore, an object of the invention to provide a balanced modulator circuit which is not subject to the above-mentioned disadvantages Y of the arrangements of the prior art.

It is a further object of the invention to provide a balanced modulator in which only signals of heterodyne frequencies of two signal inputs thereto appear at the output terminals thereof.

It is still another object of the invention to provide a modulator circuit in which signals of the frequencies of each of vtwo input sourcesare balanced with respect to the output circuit of the modulator.

In accordance with the invention, a balanced modulator comprises two sets of input terminals adapted to have signals rapplied thereto, a set of output terminals, and means for developing signals of heterodyne frequencies across the output 'terminals and balancing out at the output terminals the signals applied to both of the sets of input terminals. The modulator includes two pairs of vacuum tubes each tube having input and output sets of electrodes together with meansforV coupling the output electrodes to the output terminals, the output electrodes of one pair of tubes being coupled to the output terminals with a polarity opposite to those of the other pair.

The modulator also includes means for connecting one of the sets of input terminals in pushpull relation to input electrodes of both of the pairs of tubes, the push-pull connections to the pairs of tubes being of like polarity with respect to corresponding tubes 'of each pair. The modulator additionally includes means for connecting the other of the sets of input terminals in pushpull relation to input-electrodes of both of the pairs of tubes,` the push-pull connections of such other set of input terminals .to the pairs of `tubes being of Vopposite polarities with respect to corresponding tubes of each pair. Further, in accordance with a preferred embodiment of the invention, the modulator is. incorporated in a modulated--carriersignal transmitter including an output circuit, aw-source or high-frequency oscillations, and a source of modulated-carrier frequency signals. In this embodiment of the invention, one of the sets of input terminals of the modulator is coupled to the source of high-frequency oscillations,the other of the sets of input terminals is coupled tothe source of modulatedcarrier signals, and the output terminals of the modulator .arercoupled to the antenna circuit.

. Preferably, the input terminals, the output terminals, andthe above-referred-to pairs of vacuum tubes have a common neutral which is grounded.

For a`better understanding of the invention together with other and-further objects thereof, reference is `had to the following description taken in yconnection 'with the accompanying drawing, and its scope will be pointed out in the appended claims. V A

Fig. 1 of the drawing is a circuit diagram, partlyy schematic, of anV embodiment `of the invention VincorporatedV as amodulator in a 'modulated-carrier televisionsignal transmitter, while Fig. 2 is a circuit diagram of a'modulator similar A to that of Fig.- 1 utilizing vacuum tubes having a single control electrode. K v

`4Referring nowto Fig. l ofjthe drawing'and neglecting for the moment the operation of vthe modulator ycircuit of the invention presently to be described, there is illustrated Va circuitfdiagram of a modulated-carrier 'television signalv transmitter inwhch the conventional portions are indicated schematically' since, Y per se, vthey form no part'of the invention. 1 This'transmitter comprises t inone signal-translatingwchannel thereof, couquency combining amplifier I Whereinthey arev l further amplified, the two above-mentioned channels comprising common units in the remaining portions of the transmitter. The signal output of intermediate-frequency amplifier IS is coupled to input terminals |1 of the modulator circuit i3 of the invention, presently to be described. There is also coupled to the set of input terminals I9 of modulator circuit I8 aysource of locally-generated oscillationst. Connected to the output terminals 2| of modulator 18, in the order named, are a balanced transmission line 22, a radio-frequency amplier 23, a second balanced transmission line 24, and a double-

t antenna

25, 25.

Neglecting the momentY the portions of the system comprising the present invention, the operation of the circuit just described is well understood in the art and a detailed description thereof is believed to ber unnecessary., In brief, however, video-frequency signals, provided by source I3, are utilized to modulate a source of locally-.generated oscillations and amplified in oscillator-modulator and intermediate-frequency amplifier I. The intermediate-frequency output of unit l! is further, amplified and the carrier wave and la', single sideband thereof are selected in amplifier and s ingle-sideband selector |2. Audio-frequency signals, provided by source I3, are utilized to modulate a source ,of locally-generated oscillations in oscillator-modulator and intermediate-frequency amplifier I4 to provide an audio-modulated intermediate-frequency signal which is further amplified in amplifier-selector |5. The outputs of units I2 `and I5 are further amplied and combined in intermediatefrequency combining amplifier I6 and applied to modulator I3 wherein the heterodyne-frequency signals of the output of unit I6 and oscillator 20 yare provided at output terminals 2 I. The videoand audio-modulated carrier-frequency signals so provided are transmitted over balanced transmission line 22 to radio-frequency amplifier 23, wherein they are further amplified, and transmitted over balanced transmission line 24 to doublet

antenna

25, 25 for radiation in a manner well understood in the art.

Coming now to the portion of the system comprising the present invention, there is shown a modulator circuit including means for .developing signals of heterodyne frequencies across the output terminals 2|, 2| and balancing out atthe output terminals the signalsapplied to both of the sets of input terminals |1, |1 and I9, I9, this means comprising four vacuum Atubes 30, 3|, 32, and 3 3 each having two sets of input electrodes and one set of output electrodes. Means is provided for coupling the output electrodes of the pair of tubes and 3| with one polarity to output terminals 2| and the output

electrodes yof tubes

32 and 33 to output terminals 2| with the opposite polarity comprising 'an 'attenuator inluding a mid-'tapped inductance '31| variably inductively coupled to inductance 35. A set of input electrodes of the tubes of each of the pairs 30, 3| and 32, 33 is coupled to input terminals |1 with opposite polarity for balanced operation with respect to the output terminals 2|, and the remaining set of input electrodes of the tubes of one pair 3u, 3| is coupled to input terminals I9 with opposite polarity to the input electrodes of

tubes

32, 33 of the other pair, also for balanced operation with respect to output terminals 2|'. An attenuator comprising an inductance 36 variably inductively -coupled to a mid-tapped inductance 31 is provided for coupling input terminals I9 to one set of input electrodes of each of tubes 3-33, while `an attenuator comprising an inductance 33 variably inductively coupled to a mid-tapped inductance 39 is provided for coupling input terminals I1 to the others of the sets of input electrodes of tubes 30-33. 'Ihe mid-points of

inductances

34, 31, and 39 are grounded, the cathodes of

tubes

33 and 32 are grounded through a common source of grid bias 40, and the cathodes of tubes 3| and 33 are grounded through a common source of grid bias lil.V Thus, the

attenuator

36, 31 comprises balanced means Vincluding the common grounded neutral for yconnecting the set of input terminals |9, |9 in push-pull relation to one set of input electrodes of both of the pairs of tubes 3-33, inclusive, the push-pull connections to the pairs of tubes being of like polarity with respect to corresponding tubes of each pair. Likewise, the attenuator 33, 3S comprises balanced means including the common grounded neutral for connecting the set of input terminals I1, |1 in pushpull relation to the other set of input electrodes of both'of the pairs of tubes 30-33, inclusive, the push-pull connections of the latter set of input terminals to the pairs of tubes being of opposite polarities with respect to corresponding tubes of each pair.

In considering the operation of modulator circuit I8, it will be assumed that a signal is applied at terminals I9 to provide a radio-frequency voltage across inductance 31 of the instantaneous polarity shown and that modulated intermediatefrequency signals are applied at input Aterminals to provide a voltage across inductance 39 of the instantaneous polarity shown. It will be seen that a negative voltage from inductance 31 is applied to an input electrode of tube 3| and to an input,V electrode of tube 32 and, as the output electrodes of v.these two tubes are connected to output terminals '2| with opposite jpolarity, that substantially no `resultant signal Voltage of the frequency of the signal applied to terminals I9 is developed at the output terminals 2| by the two tubes. Similarly, a positivepotential from induotance 31 is applied 'to an input electrode of tube 30 and to an input electrode of tube 33 and, as these two tubesv are coupled to output terminals `2| with opposite polarity, substantially nofrersultant signal voltage of the lfrequency of the signal input to terminals I9 appears at the output terminals 2| under these conditions. Similarly,signals of the frequency of the input to terminals I1 are substantially balanced with respect to `ou'tput'terminals 2| and substantially do not appear at the output terminals '2|' of modulator I8. i

However, with respect to signals of heterodyne frequencies of the sources applied at input terminals I9 and input terminals |1, it Will ybe seen that vthe components developed by .the several tubesare additive with respect .tothe output terminals 2|. Thus, under the conditions assumed,

a negative potential is' applied from inductance 31 toV an input electrode of tube 3| and a negative potential is also appliedfrom inductance .39 to another input electrode of tube 3|. y Therefore, the sign of` the heterodyne-signal component is positive as indicatedadjacent the anode of tube 3l, Similarly, the sign of the heterodyne-signal output of tube 30 is positive for the reason that a positive potential, is appliedto one orf its input electrodes from inductance t 31 and"v a positive potential is applied to another input electrode from -inductance 39. Upon consideration, it will also be seen that, under the conditions assumed, the sign of the heterodyne signals provided by tubes 32`an`d 33 are negativeas indicated adjacent the anodes thereof. It i`s`, therefore, apparent that signals of heterodyne frequencies ofthe two signal sources applied, respectively,V at terminals l1 and .I9 are additive` inthe output circuit vrof modulator I8 and appear 2| ofthe modulator. v Y

It will be seen that the actionof thetransmitterillustrated is, withv respect to either'the video-frequency channel or the` audioffrequency channel, simply the reverse of that of a conventional superheterodyne receiver. Furthermore, it is apparent that, under these conditions, oscillator and radio-frequency amplifier 23 may be tunable and unicontrolled to provideya tunable transmitter for transmission at any selected carrier frequency within a predeterminedv range of frequencies. 1

While it is not'intended 'to limit the several units of the invention to operation at any particular frequency or within any particular frequency range, there follow two groups of recommended frequencies or frequency ranges for each of the uni'ts which are particularly suitable for lat the output terminals ing tubes of each pair, and means for connecting those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is,- therefore, aimed in the appended claimsv to coverall such changes and modiiications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A balanced modulator comprising, two sets of input terminals adapted to have signals ape plied thereto, a set of output terminals, and means for developing signals of heterodyne frequencies across said output terminals and balancing out at said output terminals the signals applied to both of said sets of input terminals, said means including two pairs of vacuum tubes each tube having input and output sets of electrodes, means for coupling said output electrodes to said output terminals, the output electrodes of one pair of said tubes being coupled to said output terminals with a polarity opposite to those ofthe other pair, means for connecting one of said sets of input terminals in push-pull relation to input electrodes of both of said pairs of tubes, the push-pull connections to said pairs of tubes being of like polarity with respect to correspondthe other of said sets of input terminals in pusha modulated-carrier television signal transmitter: Y

In Fig. 2 there is illustrated a modification of the circuit of Fig. 1 in which the modulator tubes -33, inclusive, comprise a single control electrode; that is, each tube has only one set of input electrodes. Similar circuit elements in the two iigures have identical reference numerals. In Fig. 2 the voltage supplied from transformer 36, 3l is applied directly to the control electrode of the vacuum tubes 30-33, inclusive, and secondary winding 3l has coupled between its mid-tap and ground a source of bias potential 5l). The input voltage supplied through transformer 38, 33 is connected in the cathode circuit of each of the tubes and a source of bias potential 5I is provided between the mid-tap of winding 39 and ground. The operation of the circuit of Fig. 2 is very similar to that of Fig. 1 and no further description thereof is deemed to be necessary.

While there have 'been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to pull relation to input electrodes of both of said pairs of tubes, the push-pull connections of said other set of input terminals to said pairs of tubesbeing of opposite polarities with respect to corresponding tubes of each pair.

2. 'A balanced modulator comprising, two sets of input terminals adapted to have signals applied thereto, a set of output terminals, and means for developing signals of heterodyne frequencies across said output terminals and balancing out at said output terminals the signals applied to both of said sets of input terminals, said means including two pairs of vacuum tubes, each tube having two sets of input electrodes and a set of output electrodes, means for coupling said output electrodes to said output terminals, the output electrodes of one pair of said tubes being coupled to said output terminals with a polarity opposite to those of the other pair, means for connecting one of said sets of input terminals in push-pull relation to one of said sets of input electrodes of both of said pairs of tubes, the push-pull connections to said pairs of tubes being of like polarity with respect to corresponding tubes of each pair, and means for connecting the other of said sets of input terminals in push-pull relation to the other of said sets of input electrodes of both of said pairs of tubes, the push-pull connections of said other set of input terminals to said pairs of tubes being of opposite polarities with respect to corresponding tubes of each pair.

3. A balanced modulator comprising, two sets of input terminals adapted to have signals applied thereto and having a common neutral, output terminals having the same common neutral, and means for developing signals of heterodyne frequencies across said output terminals and balancing out at said output terminals the signals applied to both of said sets of input terminals, said means including two pairs of vacuum tubes each tube having input and output sets of electrodes, means for coupling said output electrodes to said output terminals, the output electrodes of one pair of said tubes being coupled to said output terminals with a polarity opposite to those of the other pair. means comprising said common neutral for connecting one of said sets of input terminals in push-`pull relation to input electrodes of both of said pairs of tubes, the push-pull connections to said pairs of tubes being of like polarity With respect to corresponding tubes of each pair, and means comprising said common neutral for connecting the other of said sets of input terminals in push-pull relation to input electrodes of both of said pairs of tubes, the push-pull connections of said other set of input terminals to said pairs of tubes being of opposite polarities with respect to corresponding tubes of each pair.

4. In a modulated-carrier signal transmitter including an output circuit, a source of highfrequency oscillations and a source of modulated carrier-frequency signals, balanced modulator means for developing heterodyne frequencies across said output circuit and balancing out at said output circuit said modulated signals and ignals `of the frequency of said high-frequency source, said means comprising, two pairs of vacuum tubes each pair having input and output electrodes, means for coupling said output electrodes to said output circuit, the output electrodes of one pair of said tubes being coupled to said output circuit with a polarity opposite to those of the other pair, means for connecting said source of high-frequency oscillations in push-pull relation to input electrodes of both of said pairs of tubes, the push-pull connections to said pairs of tubes being of like polarity with respect to corresponding tubes of each-pair, and means for connecting said source of modulatedcarrier frequency signals in push-pull relation to input elee'ztodesl of both of said pairs of tubes, the push-pull connections' of said last-mentioned source to said pairs of tubes being ofV opposite polaritiesv with respect to corresponding tubes of each-pair'. A

5. In a carrier-frequency signal transmitter including a balanced output circuit, abalanced source ofl high-frequency oscillations, and a balanced source of modulating signals, balanced modulator means for developing heterodyne frequencies across said output circuit and balancing out atsaid output circuit said modulated signals and signals of the frequency of said high-frequency source, said means comprising two pairs of vacuum tubes each tube having input and output electrodes, balanced means for coupling said output electrodes to said output circuit, the output electrodes of one pair of said tubes being coupled to said` output circuit with a polarity opposite to those of the other pair, balanced means for connecting said source of high-frequency oscillations in push-pull relation to input electrodes of both of said pairs of tubes, the push-pullconnections to said pairs of tubes being of likeY polarity With respect to corresponding tubes of each pair, and balanced means for connecting said source of modulating signals in push-pull relation to input electrodes of both of said pairs of tubes, the push-pull connections of said source of modulating signals to said pairs of tubes being of opposite polarities with respect to corresponding tubes of each pair.

ROBERT B. J. BRUNN.