. The Bell System technical journal . imi-lar manner to that used for gating out the undesired pulses in the re-generative pulse generator. However, it is not necessary to use anotheramplifier, as was required there, since the same tube can be used forthis purpose, as well as for producing the microwave pulses. Its beamforming electrode is biased negatively about 250 volts with respect tothe cathode, and then is pulsed to the normal operating potential forabout 50 millimicroseconds during the time of the first short pulse ob-tained by gating the helix. Thus, the beam forming electrode potentia
. The Bell System technical journal . imi-lar manner to that used for gating out the undesired pulses in the re-generative pulse generator. However, it is not necessary to use anotheramplifier, as was required there, since the same tube can be used forthis purpose, as well as for producing the microwave pulses. Its beamforming electrode is biased negatively about 250 volts with respect tothe cathode, and then is pulsed to the normal operating potential forabout 50 millimicroseconds during the time of the first short pulse ob-tained by gating the helix. Thus, the beam forming electrode potentialhas been returned to the cutoff value during the second helix pulse,which is therefore A block diagram of the resulting double-gated pulse generator isshown in Fig. 3. Comparison with Fig. 1 shows that it is simplerthan the regenerative pulse generator, and it has also proved moresatisfactory in operation. It can be used at any frequency where a sig-nal source and a traveling-wave amplifier are available, and the pulse. Fig. 2 — Envelopes of microwave pulses at the output of a traveling wave am-lifier with continuous wave input and helix gating. The gating voltage is higheror the lower traces. 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 buthelpful explanation of this effect can be obtained by considering thatthe microwave signal voltage on the helix causes a bunching of the elec-tron stream. This^ bunching has the same periodicity as the microwavesignal voltage when the dc helix potential is held constant. However,since the helix voltage is continuously increased in the positive directionduring the time of the first pulse, the average velocity of the last bunchesof electrons becomes higher than that of the ea
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