Ultimate limitation in the coherence of millimetre wave oscillators due to propagation phase noise

The concepts of propagation phase and fm noise are presented comparing oscillator noise with phase fluctuations introduced by the propagation medium. Numerical values of the spectral density and variance are presented. The experimental apparatus used to measure the noise using Italsat's 40 GHz beacon is surveyed and experimental values of the excess propagation spectral noise observed are presented. The ultimate effect of the propagation phase noise is analysed for its effects in (i) coherence time, (ii) carrier acquisition systems (PLL) and (iii) the effect on MQAM constellations. The possible implications for 20/30 GHz deep space tracking and telecommand are illustrated. The conclusions point towards two orders of magnitude of improvement in on board oscillator short-term stability as worth pursuing.