US2192967A - Radio transmitter - Google Patents

Description

March l2, 19:0. LEV/Ns '2,192,967

RADIO TRANSMITTER Filed Nov. 17, 1936 Gtfomeg Patented Mu. 12, i940 UNITI-:D sTATr-:s

ausser RADIO 'Imm r.num lining,v odin, N. J., mmm u naal Corporationot America, a corporation of Deleware Application November 17, 193e', No. 111.183 v a am. (ci. ive-111.5)

My invention relatesito improvements in radio transmitters, and more specicallyto a method of and means for efliciently modulating a radio frequency carrier over a wide band of frequencies.

i with the signal or modulation current.

while the neising system of medication is en-V tirely satisfactory over a limited range of modulation frequencies, the constant current reactor.

becomes very inefficient when currents having a frequency range from a few cycles per second to several hundreds of thousands cycles per second are applied. The modulation frequencies employed in high delity television transmission often cover a frequency range of a million or more cycles per second, and when Vsuch modulation frequencies are applied in the VHeising modulation system, the losses become excessive. The present invention contemplates a modulation system capable of elciently handling large amounts ofrpower at modulation frequencies of the order of a million or more cycles per second.

An object of my invention is to provide improvide means for modulating a radioffrequency carrier by signal currents covering `a wide range of frequencies.

Another object is to provide improved means for applying signal currents to modulate an auxiliary carrier, rectifying the modulated auxiliary carrier currents, filtering the carrier currents,

and applying the rectified currents, which are Vpulsating at signal modulation frequencies, to the anode circuit of a modulator which is coupled to the main carrier frequency generator.

A further object is to provide a radio transmitter with means for modulatingthe auxiliary carrier, means for attenuating -currents of one side band. and forV passing currents of the auxiliary carrier and the other side bond, the latter currents, after being rectified and after lf tering the auxiliary carrier, becoming the source of anode currents of a modulator which is coupled to the main carrier frequency generator.

A still further object is toprovlde improved means for modulating a carrier current, radi?" ating the modulated carrier, and suppressing the carrier during periods of no modulation..

My invention may be best understood by lreference to the accompanying drawing, in which 5 Figure 1 is a schematic circuit diagramr of one embodiment of my invention, and e Figure 2 is a circuit diagram illustrating a ,modification of the circuit of Fig. 1.

Referring to Fig. 1, a. source of signal-fre- 10 quency current is representedby reference numeral I-. 'Ihe signal frequency source I is connected to the resistance coupled amplifier 3, whose output circuit includes an anode current source 5'and a resistor 1. A connection is made l5 from a .point intermediate the ends of the' resistor 'I to a tuned circuit 9 which is connected to the anodes of a pair of push-pull modulator tubes II, I3 which are preferably biased for Class C operation. The input circuit of the modulator ,20 tubes Il, I3 is connected to a source I5 of carrier frequency currents. The currents from source `I5 will hereinafter be called the auxiliary carrierto distinguish from the normal carrier which will be described below. By Way of ex- 2o ample, the auxiliary carrier may be operated at a frequency of seven megacycles.

'I'he tuned circuit 9 in the anode circuits of the push-pull tubes Il, I3 is suitably coupled to a second tuned circuit I1. Both tuned circuits 80 l, I1 are resonant to the frequency of the auxiliary carrier, and the mutual coupling M is adjusted to give the, desired characteristics to thereby pass the modulated auxiliary carrierA currents without attenuating the signal frequen- 35 cy components. A' point I9 on the tuned circuit I'I is grounded. The terminals of the tuned circuit Il are connected to a full wave rectifier 2l.

The output of therectifier 2| includes a filter which comprises an inductor 23 and a capacitor 40 25. The lter 23, 25 is adjusted to remove the auxiliary carrier ripple and preferably has a time constantof the order of the period of the aux- Claas `Cv operation and are, connected to a gener- A4;

ator'35 of carrier frequency currents. The timed anode circuit 29 is suitably coupled by mutual inductance M, or the like, to anantenna system 31.

The operation of the foregoing system is essentially as follows: The signal frequency currents, which may be derived from any source, such as a television scanning system, are amplified by the thermionic tube 3. The amplified currents vary the potential applied, through 'the tuned circuit 9, tothe anodes of the push-pull,

modulator II, I3 The auxiliary frequency carrier currents are impressed on the grid or input circuits of the modulators I I, I3, and subsequently transferred to the anode circuits of the modulators. Thus the currents of Yauxiliary carrier frequency are modulator by the signal frequency currents in' accordance Awith the conventional theory of anode circuit modulation.

The modulated auxiliaryc'arrier currents are transferred to the mutually coupled circuit I1, whereY the rectier 2| recties the modulated auxiliary currentsfand thereby produces pulsating unidirectional currents of twice the signal frequency. The pulsating currents are impressed on the lter 23, 25, which has a time constant of the proper characteristic to smooth lout the pulsating currents and deliver a steady current upon which is superimposed the original signal frequency Ycurrent.` These steady and varying currents become not only the source of anode current for the push-pull modulators 3|, 33, but also th'e modulation current for these tubes 3|, 33. The currents from the main carrier frequency generator are impressed on the input circuits of the modulator tubes 3l, 33.

The main carrier frequency currents are modulated by the rectied filtered currents, when both carrier and ltered currents are present in the anode circuits of the push-pull modulators 3l, 33. The thus modulated carrier frequency currents are transferred from the resonant anode circuit 29, through the mutual coupling M, to the antenna circuit 31. Since the anode current forpthe modulators 3l, 33 is-a steady current which is varied by the signal frequency currents. it will be apparent that amplitude of the main carrier currentkwill vary in proportion to the signal when signals are impressed on the source I in accordance with the foregoing description.

The above described method of modulation may be applied to a single side band transmitter, as indicated by the circuit diagram of Fig. 2. The signal frequency source 4I is connected to the input 'of an amplifier 43 which has an anode circuit comprising a source of anode cuirent and a'iesistorrv41. The outputA of the amplifier 43 is impressed on a resonant circuit 49, which is connected to the anodes of push-pull modulator tubes 5I, 53. The control grids of the modulator tubes are preferably biased for Class C operation 'and are connected tothe auxiliary carrier frequency generator 55.

The resonant circuit 49 is vmutually coupled to a second resonant circuit 51. The terminals of the second resonant circuit may be connected to the input of a band pass lter 59 through a balanced modulator 50, which may be omitted in accordance with a description which appears Ibelow. The output of the iilter 59 includes a third resonant circuit 6I which is mutually coupled to K av fourth resonant circuit 53. The fourth resonant circuit 63 is connected to a full- wave rectifier 65, 61. Y The output of the rectiiers 55, 61 is connected to a lter 89, I, which is in turn connectedtothe center tap of a push-pull transformer 13. The terminals of the push-pull transformer 13, which is preferably resonantto currents of the main carrier frequency, are connected to the anodes of a pair of push-pull thermionic modulators 15, 11. The input circuits of these tubes 15, 11 are preferably biased for Class C operation and are connected to the source.19 of the main carriencurrents.

The push-pull transformer 13 is mutually coupled M to a second resonant push-pull transformer 9|, which is connected to the input circuit of a power amplifier 83. The power amplifier is biased for Class B operation and comprised of suitable tubes 95, 81 which, if triodes, may be neutralized by capacitors 89, 9|. Screen grid tubes may be used in place of neutralized triodes 95, 81. 'I'he anode circuit of power amplifier 83 is comprised of a tuned circuit 93, which is mutually coupled M to an antenna circuit 95.

The `operation of the circuit of Fig. 2 is not greatly different from the circuit of Fig. 1. In the latter circuit both the auxiliary carrier and side band lcurrents are impressed on the rectiiier. In the present circuit (Fig. 2) the input to the balanced modulator includes two 7 megacycle voltages, one of which may be amplitude modulated. In the absence of modulation the two 7 megacycle voltages combine to produce a resultant 14 megacycle current which is not used since it is attenuated by the band pass filter 59. The relative amplitudes of the two 7 megacycle voltages are adjusted to reduce to a desired amplitude the 'I megacycle auxiliary carrier in the balanced modulator output.

When signal frequency modulation containing an alternating and a. varying direct current component is applied, two operations are First, the average amplitude of the 7 megacycle carrier which is applied to the balanced modulator input through transformer 51 takes a value which is determined by the direct modulation component, so that the amplitude of '1 megacycle current in the modulator output takes a corresponding value; and, second, modulation frequency components are superimposed on the carrier output of the modulator corresponding to the alternating components of the signal frequency.

The band pass filter 59 is adjusted to attenuate currents of one side band, and to pass carrier and the other side band currents. These currents are rectified, and tre ltered to remove, if necessary, any residual carrier ripple. The thus rectied and ltered currents comprise two components. There is a d-c component derived from the rectified and filtered 'I megacycle carrier, the amplitude of whichis proportional to the average carrier amplitude, and there is also a signal frequency component corresponding to the original signal frequency. The output currents are suillciently amplified to form the anode current source for the modulators 15. 11. The varying anode currents will transfer the main carrier currents in varying amplitudes, through the power amplier 83, to the antenna circuit 95 from which modulated waves will be radiated. During periods of no signal, the amplitude of the radiated carrier will be determined by the value of the bias applied to the output tubes by the battery connected to the mid-point of transformer 9|.

The purpose of the balanced modulator is to reduce the amplitudqof both the main and the auxiliary carrier during periods of zero modulation. 'I'his produces an appreciable saving in the transmitter power consumptionV during stand by periods. When the auxiliary carrier is so reducedv by the balanced modulator the radiated carrier is likewise reduced since only a carrier of reduced amplitudeand no side bands are impressed on the rectiiiers 65 and 61, and the radio frequency amplifier tubes- 15 and 11 are operated at reduced power.

In some installations, it may be desirable to l radiate a carrier of constant average amplitude by omitting the balanced modulator 60 or the like, `between the auxiliary modulators 53 and the rectifiers 65, 61. The nally modulated carriers may be transmitted by radiation, by concentric lines, or any suitable transmission system.v It should be understood that half-wave rectiers may be used in place of full-wave rectiers. The amount of ltering, between the rectiers of the auxiliary carrier wave and the main modulators, is determined by the permissible amount of ripple. In general, the time constant of the filter is approximately equal to the time of one cycle of the auxiliary carrier if a half-wave rectier is used, and to half the time of one cycle if a fullwave rectifier is employed.

While plate circuit modulation has been used with Class C modulators, by way of illustration, it will be'obvious to those skilled in the art that my invention may beapplied to grid modulation. If grid modulation isv contemplated, I prefer to bias the modulators for Class B modulation. The

"so-called Class B and Class C modulation or amplilication systems are well known to those skilled in the art and require no detailed descriptions.

I claim as my invention: 1. In combination, a source of signal frequency currents, a source of auxiliary carrier frequency currents, means for modulating the peak amplitudeofsaid auxiliary currents in accordance with variations of said signal frequency currents and for simultaneously modulating the average amplitude of said auxiliary frequencyA currents in proportion to the average amplitudeof said signal frequency currents, means including a balanced modulator coupled to the output of said first mentioned` modulating means for reducing the amplitude of said auxiliary carrier during periods of Zero modulation, means coupled to the output of said balanced modulator for producing a rectied current having a direct current component proportional to the average amplitude of said modulated auxiliary carrier and an alternating current component proportional to changes in the peak amplitude of said auxiliary carrier, a source of main carrier frequency currents, and means `for modulating said main carrier frequency cur-A rents in accordance with both components of said rectified current.

2. In combination, a source of signal frequency currents, a source of auxiliary carrier frequency currents, means for modulating the peak amplitude of said auxiliary currents in accordance with variations of said signal frequency currents and for simultaneously modulating the average amplitude of said auxiliary frequency currents in direct proportion to the average amplitude of said signal frequency currents, means including a v balanced modulator coupled to the output of said first mentioned modulating means for reducing the amplitude of said auxiliary carrier during periods of zero modulation, a rectiiier coupledA to the output of said balanced modulator, a filter network connected with said rectifier for smoothing out amplitude variations occurring at

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