Performance of the DSA's subcarrier demodulation digital loop
Abstract
The subcarrier demodulation digital loop is part of the baseband assembly. The subcarrier demodulator is a fourthorder Costastype loop corresponding to a type 2 analog loop in terms of steady state response. The expected value and the variance of the error signal are determined as functions of the input SNR. A Nyquist sampling rate of the input signal is assumed. From the integrodifference equations a mixed s/z domain block diagram is obtained. From the loop's transfer function a set of gains for the loop filter is obtained. Also, a set of state equations is presented for future reference. Finally, the noiseequivalent bandwidths are calculated for normalized computation times of 0, 0.25 and 0.5. The subcarrier demodulator analyzed tracks a parabolic phase input with finite steady state error. Since at each update instant the loop gains are adjusted to compensate for the variations in SNR of the input signal, the noiseequivalent bandwidth is maintained constant.
 Publication:

In its The Telecommun. and Data Acquisition Rept. p 180194 (SEE N8519224 1032
 Pub Date:
 February 1985
 Bibcode:
 1985tdar.nasa..180S
 Keywords:

 Carrier Waves;
 Demodulators;
 Signal Processing;
 Signal To Noise Ratios;
 Differential Equations;
 Equations Of State;
 Errors;
 Feedback Control;
 Integral Equations;
 Transfer Functions;
 Communications and Radar