Two pilot frequency control for communication systems

Abstract From the Canadian patent

Apparatus for synchronizing the frequency of locally generated pilot signals with those of remotely generated pilot signals where frequency errors are introduced during transmission. A pair of demodulated pilot signals F1'and F2' are filtered and mixed to derive a third signal having a frequency equal to F2'-F1'. The difference frequency has no frequency error component so the third signal is used in a phase locked loop to regenerate a reference frequency (fr) signal. Two methods are disclosed to extract a control signal which is proportional to the error frequency. In one embodiment one of the received pilots (F1) is divided in frequency by 1/k1 and then subtracted from fr to yield 1/k1 …