Viii. Multipath Transmission

Theoretical investigation of the use of feedback around the limiter for improving the capture conditions at the input to the limiter has led to the formulation of a simple theory that unfolds some remarkable properties hitherto unreported. Contrary to the claim of R. M. Wilmotte (1), who made the first (and apparently only) around-the-limiter feedback proposal that exists in published work, this new theory shows that the use of inverse feedback around the limiter does not produce any improvement in the capture ratio of an FM receiver; indeed, it only brings about a deterioration of the capture conditions by decreasing the amplitude difference between the two competing signals delivered by the i-f amplifier; also, under most feedback conditions, it increases the effective limiting threshold seen from the i-f amplifier by decreasing the resultant signal amplitude seen by the limiter, thus causing the limiting conditions to deteriorate. The new theory also shows that improvement in the capture conditions is attained only through the employment of positive feedback, a discovery that we feel holds promise for improved FM reception under conditions of multipath, cochannel, and impulsive interferences, as well as random noise. On the basis of feedback bandwidth, we recognize two types of feedback: wideband feedback and narrow-band (band-limited) feedback. With wideband feedback, essentially all of the significant spectral components (at the output of the limiter) that result from the perfect amplitude limiting of the resultant of two signals of different amplitudes and frequencies delivered by the i-f amplifier and are centered about the frequency of the stronger signal (this part of the total spectrum is henceforth called the primary spectrum) are fed back from the output of the limiter to its input. With band-limited feedback, only a portion of the significant primary spectrum is fed back. The feedback bandwidth can be defined by the bandwidth of the limiter from whose output the feedback voltage is taken, by the bandwidth of the amplifier in the feedback path, or by both. This division on the basis of feedback bandwidth enables reaching some interesting conclusions that are summarized in the following theorems.