Minimizing Incidental Frequency Modulation in Amplitude-Modulated UHF Oscillators

It is possible to minimize incidental fm in amplitudemodulated uhf oscillators. The frequency changes at uhf that occur over an amplitude-modulated cycle stem, primarily, from the variation of the transit time and space charge within the oscillator tube. By appropriate selection of parameters of the feedback and cathode lines a compensation is obtained for changes in cathode-to-grid transit time. The exact parameters can be approximately calculated from an analysis of the oscillator circuit. However, a final experimental adjustment is usually necessary. Cathode or grid modulation is better than plate modulation for low incidental fm. The frequency changes caused by variations of grid-to-plate transit time and space charge tend to cancel for cathode of grid modulation; whereas for plate modulation they add. By use of cathode modulation and the previously mentioned compensation, the frequency changes in a 400-mc oscillator were less than 5 kc for a cathode-current variation of 9 to 13 ma.