SYSTEM FOR PHASE LOCKING ON A VIRTUAL CARRIER

摘要

The present invention is directed to a phase lock system which can derive a demodulating carrier reference signal for use in a suppressed carrier quadrature, amplitude modulating digital data transmission system which does not require the transmission of a low level carrier or pilot tones. The system operates by multiplying the equalized received signal in each channel by the decoded n-level data signal of the other received channel, subtracting the products and dividing the result by the sum of the squares of the data signal levels to arrive at an error signal which is proportional to the phase error between the demodulating carrier reference signal and the suppressed transmitting carrier. The derived error signal can be made independent of data signal level values and the received signal levels even in the presence of severe intersymbol interference. The derived phase error signal is fed back to a variable oscillator providing the demodulating carrier reference signal to change the phase of the reference signal so as to reduce the error signal towards zero.