Thermal effects in modulated IMPATT oscillators

The modulation sensitivity and the inherent a.m. percentage of IMPATT oscillators with internal current modulation were measured as a function of the modulation frequency in the range 10Hz-1 MHz. Although constant above 100kHz, both quantities were found to vary with the modulation frequency below 100kHz. Assuming thermal effects as the origin of these variations, the possible relative change of the modulation sensitivity was calculated to be about 20%, taking into account the temperature dependence of the ionisation coefficient and of the drift velocity. With Q factors ranging between 20 and 150, the modulation sensitivity varied between 3 and 0.3 MHz/mA, and the ratio of a.m. percentage and modulating current varied between 7 and 10% per mA.