Measuring modulation noise from a high-power c.w. klystron amplifier

The noise spectrum of a high-power klystron amplifier shows that, as well as the r.f. shot noise, the power output contains modulation sidebands caused by variations in gain associated with low-frequency fluctuations. Modulation noise can be caused by any low-frequency fluctuations in the system, e.g. power supply ripple, microphony, or spurious beam oscillations. Apparatus for investigating these effects includes a panoramic spectrum analyser for observing and correlating the r.f. noise associated with the carrier and the video noise fluctuations. A new method of noise detection is described which uses a coaxial diode at high power level instead of the conventional low-power crystal detector. Results show that the coaxial diode has some advantages over the crystal mixer for measuring a.m. noise from a high-power source. The noise phenomena described include effects due to relaxation oscillations in the electron beam, and the variation of noise with cathode temperature.