Measurement of the phase noise characteristics of an unlocked communications channel identifier

The authors review the technique of phase noise stability measurement using complex demodulation and the approach to an unlocked spread spectrum communications probe. This system has recently been developed to characterize a transhorizon communications channel at 11.6 GHz under both tropospheric scatter and anomalous propagation conditions. The receiver has a bandwidth of 31.25 MHz but has no carrier recovery and therefore no absolute phase reference. However, provided that the phase jitter and the frequency drift are small enough during the channel measurement period, it is possible to obtain an accurate channel transmittance estimate. The system has the capability of implementing time-domain signal averaging "on the fly", thus permitting high-speed channel identification under low SNR regimes. As the averaging period is increased (to improve the SNR), low phase jitter in the oscillators becomes imperative. The SNR improvement achieved is thus controlled by the maximum averaging period, which is a direct reflection of the stability of the oscillators