40 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



frequency can be set anywhere within the bandwidth of the travehng- 

 wave ampUfier by tuning the klystron oscillator. 



The pulse center frequency is shifted from that of the klystron os- 

 cillator frequency by this helix gating process. An over-simphfied but 

 helpful explanation of this effect can be obtained by considering that 

 the microwave signal voltage on the helix causes a bunching of the elec- 

 tron stream. This^ bunching has the same periodicity as the microwave 

 signal voltage when the dc helix potential is held constant. However, 

 since the helix voltage is continuously increased in the positive direction 

 during the time of the first pulse, the average velocity of the last bunches 

 of electrons becomes higher than that of the earlier bunches in the pulse, 

 because the later electrons come along at the time of higher positive 

 helix voltage. This tends to shorten the total length of the series of 

 bunches, resulting in a shorter w^avelength at the output end of the 

 helix and therefore a higher output microwave frequency. On the second 

 pulse, obtained when the helix voltage returns toward zero, the process 

 is reversed, the bunching is stretched out, and the frequency is de- 

 creased. This second pulse is, however, gated out in this arrangement 

 by the beam-forming electrode pulsing voltage. The result for this 

 particular tube and pulse length is an effective output frequency ap- 

 proximately 150 mc higher than the oscillator frequency, but this figure 

 is not constant over the range of pulse frequencies available within the 

 amplifier bandwidth. 



OSCILLATOR AND 



CATHODE FOLLOWERS 



100 KC 



KLYSTRON 



OSCILLATOR 



9000 MC 



BEAM FORMING 



ELECTRODE 



PULSER 



HELIX 

 PULSER 



^ 



PULSED 

 TW TUBE 



MILLI/aSEC 

 9000 MC 

 PULSES 



SYNC SIGNAL TO 

 INDICATOR SCOPE 



Fig. 3 — Block diugram of the double-gated traveling wave tube millimicro- 

 second pulse generator. 



