1062 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



of the deviation oscillator. The length of circuit to the movable short is so 

 chosen (about 35 inches) to provide the optimum rate of change of reactance 

 with frequency. At optimum adjustment, the reactive component of the 

 load pulls the frequency of the generator by just the amount necessary to 

 straighten out the deviation curve. The deviation sensitivity is at the same 

 time increased about 25%, which reduces the required video driving voltage. 



A portion of the output signal of the deviation oscillator is fed through a 

 directional coupler to a crystal microwave converter where it is mixed with 

 a 4210-megacycle signal from another klystron to produce a 70-megacycle 

 FM signal. The microwave output from the deviation oscillator is about 50 

 milHwatts, and after losses in the directional coupler and converter about 

 one milliwatt of 70-megacycle FM output is available. This signal is 

 amplified in a broad-band limiter-amplifier for application directly to the 

 radio transmitter or indirectly through appropriate switching circuits. 



2. Clamper and AFC Circuit 



For television transmission the voltage suppHed to the repeller of the 

 deviation oscillator is clamped to a predetermined negative value during 

 each synchronizing pulse in a conventional manner. This clamping action 

 enables the transmission of video signal components down to direct current. 

 For message telephone transmission the clamping circuit is disabled. 



The automatic frequency control circuit used to control the frequency of 

 the beat oscillator provides a high gain and stable AFC without a d-c ampli- 

 fier. As shown in Fig. 16, a portion of the 70-megacycle output signal is 

 diverted and after passing through a gated amplifier is appHed to a dis- 

 criminator. The discriminator network is of conventional design and the 

 detector elements are germanium diodes. The direct-current output voltages 

 are applied to the grids of two triodes acting as a pulse modulator. The 

 anodes of these triodes are supplied with a high level positive pulse used for 

 gating from a blocking oscillator associated with the clamper circuit. 

 This oscillator is free running for message signals but is triggered by the 

 synchronizing pulses when video signals are being transmitted. The un- 

 balance voltage on the triode grids controls the amplitude and polarity of 

 the pulse produced by this modulator. After two stages of a-c. amplification 

 this error signal is combined with a second high level pulse from the same 

 blocking oscillator source in a phase detecting circuit, and, after integration, 

 the d-c. output of this detector is used for AFC. With television operation 

 the gated amplifier operates only during synchronizing pulses, and the 

 discriminator is adjusted for an output frequency of 74 megacycles. With 

 multi-channel message operation, the gated amplifier is operated as a 

 straight-through amplifier, and the discriminator is adjusted to hold an 

 average output frequency of 70 megacycles. 



