US2163707A - Wide band modulation - Google Patents

Description

Jung 27, 1939. cf SCHAFFSTEIN WIDE BAND MODULATION Filed Feb. 27, 1937 MUM/LA 7'50 WAVE AMPl/F/ER AMPL I FIER SOURCE OF MODl/M TING POTENTIAL OUTPUT I NVEN TOR SCHAFFSTf/N M744 ATTORNEY FREQUENCY 000 ooooodo a SOURCE OF I MODULAT/NG' POTENTIAL V Patented June 27, 1939 UNITED STATES PATENT OFFICE WIDE BAND MODULATION tion of Germany ApplicationFebruary 27, 1937, Serial No. 128,045

' In Germany February 27, 1936 4 Claims.

This application concerns an arrangement for amplitudes or phase modulation of radio frequency oscillations by means of a wide band of I signals.

This invention is concerned with a novel modulator circuit organization adapted to handle very 7 wide frequency bands.

In the modulation of television transmitters it is known in practice to be difficult to transmit the Wide frequency band extending from 25 cycles all the way up to 10 cycles. In the usual grid voltage or plate voltage modulationit is necessary for full utilization (fullv swing of plate current from zero to saturation) -of the power tube that there should be provided invariably comparatively high audio frequency voltages. Because of inevitable grid plate capacitance of the power tube in the audio frequency amplifier, the said high audio frequency plate potential reacts upon the grid circuit of the audio frequency power tube. Inasmuch as this reaction by way of a capacity is a function of the frequency, it follows that the frequency characteristic or response of the audio frequency amplifier is seriously impaired. Another eifect of such reaction is that the audio frequency phase positionof the comparatively high modulation frequencies is altered and this is especially undesirablein television work.

In practice itis also impossible to prevent these audio frequency actions by using a screen grid tube in the audio frequency power stage, for the reason that screen grid tubes possess a very high internal resistance which is higher for high frequencies than the capacitive reactance in parallel relation to the power tube, say. to the grid cathode path of the modulation tube following the power tube. In other words, theaudio frequency current is capacitively shorted with the result that frequency dependence prevails as before.

Also, in other modulation circuit organization, it can be demonstrated that the transfer of very broad frequency bands is associated with" very serious difiiculties. However, these difficulties are surmountable in a simple manner by an arrangement as hereinafter to be disclosed.

Quite some time ago the so-called absorption scheme was disclosed in the art of modulation circuits, though this method has been little used in practical radio work because of the fact that its modulation characteristic is non-linear. For this reason this method of modulation by absorption has been practically discarded in modern practice. The arrangement is predicated upon the fact that a tube subject to audio frequency control constitutes a variable damping resistance in parallel relation to an oscillation circuit. In order that both halves of the radio frequency current may be damped it has also been suggested in the prior art to connect in opposition two tubes controlled in phase opposition.

Now, according to the present invention such an absorption modulation scheme is used for the modulation of transmitters, especially televisors working with very wide frequency bands. Be cause of the audio frequency short circuit of the plate filament path by the circuit to be acted upon, it is impossible for any high plate alternating potentials being formed, with the consequence that the above mentioned reaction upon the grid circuit is unable to occur. Hence, the circuit organization here disclosed is completely independent of the frequency and is especially well suited for handling broad frequency bands. In the transmission of television impulses, the problem of rectilinearity of the modulation characteristic is not so serious, with the result that such drawbacks in the scheme as inhere in broadcast work will be absent in the use of the television transmitter. In describing my invention in detail reference will be made. to the attached drawing wherein, p

Figures 1, 2, and 3 illustrate three embodiments of my wide frequency band modulator system.

One arrangement of this invention is illustrated by way of example in Figure 1. Referring to the same, I denotes the radio frequency oscillator which includes in its circuit an inductance L in coupling relation with the push-pull inductance 2, 2a. 3 and 3a denote the condensers contained in the oscillatory energy circuits. In parallel relation to the inductance are the anode to cathode impedances of the modulator tubes 5 and 5a which are acted upon in phase by the source of modulating potential 6. To the plates H3 and I2 of the two modulator tubes is applied, if desired, a plate potential from a source 8 by the aid of which. together with the proper choice of the grid biasing voltage supplied by source I, the shape of the modulation characteristic may be influenced. The source 8 may in some cases be dispensed with so that the cathodes of the tubes 5 and 5a are directly connected. by way ofthe radio frequency choke-9 and coil ;2.and 2a with the plates I0 and 12. l is a suitably chosen source of grid biasing voltage. .The modulated radio frequency, moreover, is fed by way of coupling condensers to the grids of a pair of amplifier tubes 4 and 4a. The amplifier tubes 4 and 411 have their input electrodes connected in push pull by the circuit comprising inductances 2, 2a, and 9. Grid bias may be supplied to the grids of tubes 4 and 4a in any conventional manner, such as, for ex ample, by grid leak resistances.

The circuit organization shown in Figure l insures in addition, a very special advantage, namely, a neutralization of the modulation tubes for radio frequency. As has already been pointed out inductances 2 and 2a make it impossible for any audio frequency voltage to arise at the plates I!) and i2 of tubes 5 and 5a so that audio frequency reaction upon the grids is avoided. However, at the plates acts a radio frequency voltage which, in the case of a non-neutralized circuit scheme, comes to react upon the grids, thereby resulting in distortions. However, in the push-pull organization here indicated a'neutraL- izing bridge is formed as shown in Figure 2. In this bridge the ratio C1/C2 equals Cos/Cos. C1 is the capacity of, say, the upper side of the system to ground, while C2 is the capacity of, say, the lower side of the system toground. CGA represents the grid to anode capacity of each of tubes 5 and 5a, and, if these tubes have equal capacities, C1 equals C2. Equal capacities, however, are not essential, since a balance is all that is required. The designations in Figure 2 are the same as in Figure 1. It will be readily seen that between points B and C no radio frequency voltage will arise.

Audio frequency damping may also be effected in an additional circuit rather than in the oscillator circuit proper provided that the said additional circuit is in coupling relation with the oscillatory circuit of the transmitter.

Instead of audio frequency damping, recourse could be had also to audio frequency changes of tuning in an oscillator circuit as shown in Figure 3, where the same denotations are used as in Figure 1. In this scheme only a part of the oscillation circuit coils 2, 2a, is more or less shortcircuited by the tubes 5, 5a respectively, with the result that the tuning of the circuit is altered. In this circuit organization the oscillations are subject to phase modulation. Any undesirable additional amplitude modulation may be avoided by including that portion of the circuit indicated in Figure 3 in the plate circuit of a tube oscillating in the presence of over-voltage. Ihe output will then exhibit perfect amplitude stability. The leads marked output may be connected,-to modulated wave amplifier tubes such as ti and la of Figure 1.

I claim:

1. In a system for impressing a wide band of signalling potentials on wave energy of carrier wave frequency, a modulated wave amplifier comprising a pair of tubes having input electrodes and having output electrodes from which amplified modulated wave energy may be taken, a reactance of low impedance to potentials of modulation frequency connecting said input electrodes in push pull relation, a pair of electron discharge devices each having a plurality of electrodes including one which serves to control the impedance thereof, means connecting a pair of the electrodes in each device in series and connecting the series connection in shunt to a portion of said reactance, said reactance forming a short circuit of the impedances of said devices for potentials of modulation potential frequency, means for applying modulating potentials of like phase to said one electrode in each device to control at modulation potential frequency the impedance of said devices, and means for impressing wave energy of carrier wave frequency on said reactance.

2. In a system for impressing a wide band of signalling potentials on wave energy of carrier wave frequency, a modulated wave amplifier comprising a pair of tubes having input electrodes and having output electrodes from which modulated wave energy may be taken, an inductive reactance of low impedance to potentials of modulation frequency connecting said input electrodes in push pull relation, a pair of electron discharge devices each having an anode, a cathode and a control grid, means connecting the anodes of said devices to spaced points on said inductive reactance and the cathodes of said device it a third point on said inductive reactance, means for applying modulating potentials in phase between said control grids and cathodes of said devices to control the impedance thereof at modulation potential frequency, and means for impressing wave energy of carrier wave frequency on said inductive reactance.

3. In a system for impressing a wide band of signalling potentials on wave energy of carrier wave frequency, a modulated wave amplifier of the push-pull tube type having input electrodes on which modulated wave energy is to be impressed, a reactance of low impedance to potentials of modulation frequency connected in pushpull relation between the input electrodes of the tubes of said modulated wave amplifier, said reactance being of high impedance to energy of said carrier wave frequency, an electron discharge system comprising a pair of electron discharge devices each having corresponding electrodes, means connecting corresponding electrodes of said discharge devices in series and connecting said series connection in shunt to a portion of said reactance, means for impressing modulating potentials in phase on corresponding electrodes in said discharge devices, and means for causing wave energy to be modulated to flow in on said reactance, the capacity between the corresponding electrodes of said devices and the capacities of said reactances and connections comprising a bridge circuit balanced with respect to the push pull input of said amplifier.

4. In a system for impressing a wide band of signalling potentials on wave energy of carrier wave frequency, a utilization circuit having three input electrodes, one of which is of substantial fixed potential relative to the other two, an inductive reactance, leads connecting a portion of said inductive reactance between said other two electrodes, a second reactance connecting a point on said portion to said terminal of substantially fixed potential, a pair of electron discharge devices each having an anode. a cathode and a control grid, connections between the anodes of said discharge devices and different points on said inductive reactance, a source of modulating potentials, means connecting one terminal of said source to said control grids and the other terminal of said source to the cathodes of said devices, means connecting the cathodes of said devices to said point of substantially fixed potential, and means for impressing wave energy to be modulated on said inductive reactance.

' GOSWIN SCI-IAFFSTEIN.

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