Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
By its very nature an AM signal is much more susceptible to noise than an FM signal. I've never heard the term threshold effect used in an AM system, and I believe that the SNR of an AM signal remains linear across all possible values. There is no hard threshold below which the performance drops. This explains why the FM radio signal tends to die at a certain point without fading into a static filled mess, where AM fades gracefully as you get out of range.