Abstract:
A narrow-band digital frequency-modulation (FM) limiter-discriminator (LD) receiver with two independent detectors that combine to remove most of the bit errors caused by FM-clicks in an encoded channel. The output of the LD circuit is presented to a sample-and-hold (SH) detector and to an integrate and dump (ID) detector. Because the SH and ID detector outputs are offset in time by one-half bit and they are not entirely correlated, an error in one does not necessarily imply an error in the other. Using convolutional coding and Viterbi decoding, with threshold-compensation of the ID detector output and threshold- or envelope-compensation of the SH detector output, averaging the two compensated detector signals improves the receiver bit error rate (BER) performance by more than 3 dB over the soft-decision thresholded ID detector alone, which until now was believed to be optimum in the art.