A study of the effects of baseband carrier modulation on subcarrier performance in a phase modulation communication system

The problem of deterioration of subcarrier performance by baseband modulation of the carrier by low-frequency information in a phase modulation communication system is investigated. The dependency of this deterioration on the relationship between several received signal and receiving system parameters are defined. The ground receiver designed for support of the Apollo missions is assumed for analysis of the deterioration. The steady-state amplitude and phase responses of the carrier tracking portion of the receiver are computed for baseband modulation by a sinusoid and a periodic square wave. It is shown that the steady-state response is dependent on the ratio of the frequency of the baseband modulation to the natural frequency of the loop, on the loop damping factor, and on the peak deviation of the carrier due to the baseband modulation. The performance of the wideband phase detection process is determined. One of the sinusoidal subcarriers is isolated and the ratios of signal-to-noise plus interference, signal- to-noise, and signal-to-interference in the subcarrier pre-detection bandwidth are determined for baseband modulation by a sinusoid and a periodic square wave. It is shown that the amount of interference is dependent on the steady-state phase error of the loop and on the bandwidth of the subcarrier predetection filter. The results of subcarrier performance tests are compared to performance predictions generated using the results of the analysis of the carrier tracking loop and wideband phase detection process. It is shown that if the amplitude and phase response of the loop are known the subcarrier performance can be predicted very accurately. It is also shown that the subcarrier performance can be predicted with limited accuracy when the baseband modulation is a nonreturn to zero pseudo-random bit stream.