REPLEX OSCILLATORS ■ 611 



of the oscillator is operated at ground and the cathode of the oscillator 

 is pulsed negative with respect to this ground. The repeller voltage is 

 referenced from the cathode and this reference is maintained during the 

 j)ulse. Some frequency modulation occurs during the rise and fall of the 

 pulse because of the changing electron velocity. This can be reduced in 

 part by applying a part of the pulse in the repeller circuit proportioned 

 in such a way as to tend to maintain the drift time independent of the 

 cathode to resonator voltage. Satisfactory performance is achieved in 

 this way. 



As mentioned previously, the intelligence is conveyed by pulse position 

 modulation. The AN/TRC-6 system uses time division multiplex to provide 

 eight communication channels. The multiplexing is achieved by trans- 

 mitting a four micro-second marker pulse which provides a time reference 

 followed by eight one micro-second pulses. The time of each of the latter 

 pulses is independently varied in position with reference to the marker 

 pulse. 



The time interval from the marker to each pulse could be measured to 

 either the leading or trailing edge of the pulse. In Section XII it is shown 

 that the leading edge of the r.f. pulse will be subject to what is commonly 

 called "jitter" because of the random time of rise which will result if oscilla- 

 tion starts from shot or Johnson noise. Conceivably oscillation might be 

 started by shock excitation of the resonant circuit by the pulsed beam cur- 

 rent. However, in Appendix X it is shown that the initial excitation pro- 

 duced by shot noise in the beam exceeds that induced by the current tran- 

 sient by a factor of approximately 100. The trailing edge of the pulse will 

 not be subject to this form of jitter provided two conditions are met. First, 

 the pulse duration must be long enough so that oscillation builds up to full 

 amplitude during the pulse. Second, the receiver must have a sufficient 

 bandwidth so that the transient which occurs on reception of the leading 

 edge has fallen to a small value by the time the trailing edge is received. 

 Since these conditions were met in the AN/TR('-6 system, the trailing edge 

 of the pulse was used. 



In the latter stages of the development of the AN/TRC-6 system it was 

 decided to remove the restriction that the required tuning range should be 

 covered with a single transmitter tube. This made possible the achievement 

 of a design which would i)rovide an improved performance throughout the 

 band. In order to improve the circuit efficiency of the resonator, the new 

 designs were based on the oscillator structure which was employed in the 

 2K4.S. It has been pointed out previously that this makes possible a con- 

 siderably higher resonant impedance of the cavity, partly because of the 

 reduced gap capacitance and also because the smaller first and second grids 

 reduce the resonator losses. These effects were reflected in the higher 

 efficiency obtained in the 2K54 and 2K55. 



