Path: utzoo!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!hplabs!otter!tgg
From: tgg@otter.hpl.hp.com (Tom Gardner)
Newsgroups: sci.electronics
Subject: Re: economical to modify a scanner radio ?
Message-ID: <1770004@otter.hpl.hp.com>
Date: 15 Nov 88 14:13:25 GMT
References: <2030@spdcc.COM>
Organization: Hewlett-Packard Laboratories, Bristol, UK.
Lines: 20

There is another reason why AM may be preferred to FM in life-critical 
applications, which is best illustrated with a graph of baseband S/N ratio
plotted against RF signal strength.  

When the RF signal strength is adequate, FM will give a better S/N ratio. One
point to FM. 

However, as the RF signal strength is reduced, there comes a point at which
the FM S/N ratio starts to fall very sharply, and the baseband signal very
rapidly becomes unintelligable. This knee effect is very undesirable as
communication may suddenly fail with very little warning (assuming that the 
pilot is not keeping an eye on the signal strength meter). Minus one point to
FM.

AM communication not only does not have this knee, but it is also intelligable
with a lower RF signal strength, and hence is usable over longer distances
and in more marginal conditions. Plus one point to AM.

Please do not interpret this note as contradicting other notes explaining
why AM was adopted in preference to FM.