The reduction of threshold by the use of frequency compression
A.J. Buxton, M.O. Felix · The proceedings of the Institution of Electrical Engineers. Part B, Radio and electronic engineering, including communication engineering · 1958
Any angle-modulated system suffers from a threshold when the instantaneous noise vector becomes equal to or larger than the carrier. This effect is a limiting factor in most f.m. scatter systems, since the signal to any one receiver will fall below threshold for a far from negligible part of the time.The paper shows that the threshold level depends only on the combined bandwidth of the r.f. and i.f. circuits, and is unaffected by the existence of the frequency-compression loop. Thus, such a receiver enables one to combine the large f.m. improvement of the wide deviation signal with the low threshold of the narrow i.f. band.