Towards Optimum Demodulation of Bandwidth- Limited and Low SNR Square-Wave Subcarrier Signals

Y. FeriaCommunications Systems Research SectionW. HurdRadio Frequency and Microwave Subsystems SectionThe optimum phase detector is presented for tracking square-wave subcarriersthat have been bandwidth limited to a finite number of harmonics. The phase detec-tor is optimum in the sense that the loop signal-to-noise ratio (SNR) is maximizedand, hence, the rms phase tracking error is minimized. The optimum phase detectoris easy to implement and achieves substantial improvement. Also presented are theoptimum weights to combine the signals demodulated from each of the harmonics.The optimum weighting provides SNR improvement of 0.1 to 0.15 dB when thesubcarrier loop SNR is low (15 dB) and the number of harmonics is high (8 to 16).