Maximum tolerable local oscillator noise in a 12-GHz f.m. satellite television receiver

When considering the design of receivers for direct broadcasting of f.m. television from satellites at 12 GHz, a common requirement in all variations is for a local oscillator which supplies several milliwatts of power at approximately 11.7 GHz. A specification is derived for the maximum allowable noise on the local oscillator when the device used is an avalanche diode oscillator or a Gunn effect oscillator. Spectra are derived for these two cases for two margins of safety. These results are than applied to two measured spectra from different Gunn effect oscillators and it is shown how the measured spectra compare with the calculated specification. Practical tests using a coaxial varactor-tuned Gunn oscillator indicate that there is no degradation in picture quality when using such devices. In fact they suggest that the signal/noise criteria chosen for the local oscillator are rather more conservative than is necessary.