US2262139A - Television transmitter modulator - Google Patents

Description

Patented Nov. 11, 1941 TELEVISION TRANSMITTER MoDULAToa Thomas L. Gottier, Westmont, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 21, 1939, Serial No. 285,666

(oli. 179-171) 7 Claims.

This invention relates to television transmitter modulators and, more particularly, to a modulator which covers a wide frequency band with increasing gain at the higher frequencies.

In television transmitters of the present day type, a modulation band of several million cycles is required to transmit pictures of the desired detail. The frequency band covered by a conventional modulator generally exhibits a gain characteristic which falls off at the higher fre-` quencies and, furthermore, the phase may be shifted by an undesirable amount. While the attenuation may be corrected at the higher frequencies, the correcting means often introduces more than the permissible phase shift with resulting` picture distortion.

It is one of the objects of the present invention to provide means for modulating a television transmitter. Another object is to provide means for modulating a carrier over a frequency band of several million cycles. Another object is to provide a television modulator with means for obtaining increasing gain as the frequency increases Without shifting the phase beyond permissible limits.

The invention will be described by referring to the accompanying drawing in which Figure 1 is a circuit diagram representing one embodiment of the invention; Figures 2 and 3 are circuit diagrams of modifications of the invention; Figures 4 and 5 are graphs used in describing the invention. In the several figures, similar reference numerals will be used to indicate similar elements.

In modulators of the character about to be described, thermionic tubes are employed. The input impedance of such tubes depends upon their output load. This is especially noticeable at the higher operating frequencies. Since the tubes have capacity between their input elements, the input reactance varies with frequency. Furthermore, the effective input resistance is not consta'nt with frequency if the input is coupled to the output load as is the usual case.

When the modulator covers a narrow frequency band, the effect of the input characteristic may be neglected; for a Wide band, the varying impedance has a detrimental effect. This effect cannot be neutralized by a shunt inducance because of the phase shift and the resonant characteristics of circuits including inductance and capacity, and the resistance component of the tube input. While resonant circuits may be broadened by shunting resistors, this solution is not practical if a broad band modulator is to CAD i less effective.

have any substantial gain and a uniform frequency characteristic.

One solution of the problem is represented in Fig. 1. A lter composed of a capacitor I and an inductor 3 is connected between the input 'terminals 5 and the input of a thermionic tube l. The output of the tube includes an impedance load 9 and terminals Il for an output connection. The effective and variable input reactance and resistance of the tube are represented respectively by the capacitor I3 and resistor I5. The characteristics fora given tube and given output load are represented by the curves R and X of Fig, 4. The relatively high input reactance X and resistance R at the lower frequencies and low input reactance and resistance at the higher frequencies correspond to an undesirable characteristic in a television modulator. The characteristic may be modified according to this invention by connecting one terminal of a resonant circuit (including a capacitive reactor Il and an inductive reactor I9) to the grid of the tube l and the other terminal of the resonant circuit Il, I9 through a resistor 2l to the cathode of the tube 1. The circuit Il, I9 is adjusted to resonate at or near the higher frequency end of the band.

The resulting operation of the modulator is as follows: At the lower frequencies, the circuit I1, I9 offers inductive reactance which is low compared to the resistance 2|, and therefore the resistance 2| terminates the lter and the input impedance of the tube may be neglected. As the .frequency of the applied currents is increased, lthe effective impedance of the resonant circuit I9 becomes higher and the resistance 2l becomes At the resonant frequency of the circuit I9, the impedance becomes so high that for practical purposes the input impedance of the tube becomes the termination for the filter I', 3. The resultant input impedance is represented by the curve Z of Fig. 5. The phase shift is not excessive, as shown by the lower curve of Fig. 5. It will be noted that the phase shift approaches 45 at the higher frequencies and corresponds to an elementary dot of a television picture being displaced about one-eighth of its diameter.

In the practical application of the invention, the resonant circuit Il, I9 of Fig. 1 may be replaced by a reactor or inductor 23, as shown in Fig. 2. 'I'he inductor may have a value which offers an inductive reactance throughout the frequency range, or it may be chosen so that its distributed capacity tunes the inductor within the range. It has been found advantageous t0 broad band television modulator.

- was made using three cascaded circuits according to the invention. The rst stage employed iive tubes of the type 807 which fed into an anode circuit load consisting of a 12 ,ah inductor. The tubes of the second stage were three type 831s. The correcting network connected to the grid of the second stage was an inductor of 12 ch and a series resistor of 450 ohms. The output load of the second stage was a 17 ,ah indicator. The third stage included a single type .848 tube having an output load including an inductor of 14 ,ch and an input correcting network including an inductor of 34 ,uh and a series resistor ofA '700 ohms. The final stage employed vtwo type 848 tubes having an input correcting network including a resonant circuit consisting of Aan inductor of 5.8 ch and a capacitor of 200 ,Lt/rf and a resistor of 240 ohms connected in series therewith. In this cascade arrangement, the output load inductors in each stage were the inductors of the filter networks corresponding to ythe filter I,3 of Figs. 1, 2 and 3. The function yof the lters is tobroaden the frequency re- ,.sponse.

Thus the invention has been described as a Filter networks are `used between the input terminals and the amplifier tubes. Since the input impedance of the amplifiers varies throughout the frequency band, a correcting network is connected to the tube input circuit so that the filter is terminated by a resistive network at the lower frequencies and by the tube input at the higher frequencies whereby a substantially uniform amplication is obtained over a several-million cycle frequency band with an increasing amplification at' the higher frequencies. It should be understood that the actual capacitors and resistors may be replaced by capacity and resistance inherent in the 4amplifier tubes.

I claim as my invention:

1. A modulator for television and the like including a therrnionic tube having an input and an output, said input including a grid and cathode electrode, said output including an anode and said cathode electrode, said grid and anode reactively coupling said input and output, an impedance load connected to said output so that Vsaid input has an impedance varying with the effective impedance of said output, a lter connected to said input, and a correcting network connected effectively across said tube input, said f network including a reactor and a resistor connected in series, said reactor having a characteristie so varying with frequency that said resistor has a negligible effect at the higher modulation frequencies and whereby said resistor becomes the effective termination of said filter at the lower modulation frequencies.

2. In a device of the character of claim 1, a capacitor connected across said resistor.

3. In a device of the character of claim 1, a reactor of opposite sign connected across said 4irst-mentioned reactor whereby the reactances of said reactors neutralized each other at a fre- .in series,` said reactor having an impedance characteristic so varying with frequency that said resistor has a negligible eiect on the input impedance of said tube at the higher modulation frequencies and whereby said resistor becomes the eiective input impedance of said `tube at the lower modulation frequencies.

5. In a device of the character of' claim 4, a capacitor connected across said resistor.

6. In a device of the character of claim 4, a

capacitor connected across said reactor whereby its reactance is neutralized at a frequency corresponding to the higher of the applied. modulation frequencies.

'7. A modulator for television and the like including a thermionic tube having an input and an output, said input including a grid and cathode, said output including an anode and said f cathode, said grid and anode being so relatively located as to form a reactive coupling between said input and output, a load connected to said output whereby through said grid-anode reactive coupling and said output load said input has reactive and resistive characteristics varying with frequency, and a correcting network connected across said input, said network including a resonant circuit and a resistor connected in series, the resonant frequency being so selected with respect to the applied modulation frequencies that said resonant circuit offers a high impedance at the higher modulation frequencies and at which resonant frequency said resistor has a negligible effect and whereby said resonant circuit offers a low impedance at the lower modulation frequencies and the said resistor becomes the controlling input impedance.

THOMAS L. GOTTIER.

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