operation, a drop of beam voltage reduces the beam speed, increasing the phase delay between input and output signals. Using design values for the PPM tube, the total phase delay between input- and output-gap voltages is about 4400 degrees; a one percent drop in voltage increases this by one-half percent or 22 degrees. In the high-power tube the corresponding estimate is 19 degrees for a total phase delay of 3800 degrees. 42.6 NOISE, GAIN AND HARMONIC CHARACTERISTICS The separate cavities of the klystron give it an inherently narrow bandwidth, and hence advantages as a low-noise amplifier. Thermal noise originates in the low-level microwave oscillator and is amplified in the initial stages that precede the klystron. The combined noise and drive signals then modulate the beam, and the klystron acts as a filter by reducing spurious signals. This is in contrast to the amplitron, which itself generates noise and requires an additional output filter. The noise output of a klystron under saturated drive is calculated as follows. The gain-frequency characteristic is first obtained: a. A large-signal computation and the circuit theory for the input cavity give the ratio (output power/RF drive power) as the gain at the center frequency (see Appendix A). b. Small-signal space-charge-wave theory gives the gain at frequencies near the center frequency. c. When the power transfer characteristic is treated as linear for small modulations of the drive level, the gain of a noise signal at frequency f is approximately (saturation gain at ) + (small-signal gain at ) - (small-signal gain at ). d. Outside the band of cavity resonances the decrease of gain in a klystron with "n" cavities is given by:
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