Two pilot frequency control for communication systems

Abstract

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 times the error frequency df. By integrating this signal it becomes usable as an AFC signal to control a local oscillator. In a second embodiment a signal having a frequency equal to the sum of the received pilot signals (F1'+F2') is mixed with a second signal having a frequency equal to (F1+F2+fr), and the difference signal is extracted. The difference signal contains the error frequency component and the reference frequency component. The reference frequency is cancelled out leaving the error frequency signal. By integrating the error frequency signal, it becomes usable as an AFC signal to control a local oscillator.