Subcarrier Placement in a PCM-FM-FM/FM Modulation Scheme

Abstract

In a PCM-FM-FM/FM modulation scheme, one pulse code modulated (PCM) signal is added to a series of FM subcarrier modulated signals, and the sum is modulated on one FM carrier frequency. After the signal is carrier demodulated at the receiver and the signals modulating the subcarriers are individually filtered and demodulated, the information carried by the subcarrier frequencies may be distorted or lost due to interference power of the PCM signal that is passed by the subcarrier signals’ bandpass filters. The effect of the interference power may be reduced when the subcarrier frequencies are chosen to coincide with the zero crossing frequencies of the PCM signal. It will be shown that this choice results in a lower interference power than when the subcarrier frequencies come between the zero crossings. The PCM signal used in this study is of polar nonreturn to zero format.