PHASE CORRECTION AND PHASE CANCELLING NETWORKS IN QPSK MODEMS

Abstract

At bit rates lower than 100Mbit/s, the Synchronous Oscillator (SO) [1-4] has substantial tracking band combined with steep skirt selectivity to satisfy all the requirements of a carrier recovery network without the need for a phase correction network. At higher bit rates however, there is a need for a phase correction or phase cancelling network, if the BER variations with respect to hard wired case must be confined to less than 0.2dB with IF offsets of ±30kHz. At bit rates higher than 100Mbit/s, the multiply by four process in a QPSK modem deteriorates the signal-to-noise ratio by more than 15 dB (18dB at Eb/No = 6.4 dB) and the synchronization signal-to-noise ratio at the input to the SO drops below 0dB (!5dB at Eb/No = 6.4dB). This reduction in the signal-to-noise ratio reduces the tracking band of the SO which in turn increases the phase shift per unit frequency offset.