From: b...@netcom.com (Benjamin P. Carter)
Subject: Re: Where is the noise when I tune to a FM station.
Date: 1998/09/26
Message-ID: <bpcEzwn8u.ALH@netcom.com>#1/1
X-Deja-AN: 395137227
Sender: b...@netcom15.netcom.com
References: <jhardis-2509982239330001@dialup09.wap.org> <6uimds$1s2$1@plutonium.compulink.co.uk>
Organization: ICGNetcom
Newsgroups: sci.physics.electromag,sci.physics

maes...@cix.co.uk writes:

>earlier radios have used
>AGC since the mid 20s. This turns down the gain of at least one stage of
>amplification (and usually mixing), thereby also reducing the noise
>which gets amplified, when a stronger signal arrives.

A traditional FM radio with vacuum tubes has the following stages:
optional RF amplifier, mixer, IF amplifier, limiter, discriminator, audio
amplifier. The limiter chops off the peaks of the (sinusoidal) IF signal. 
The output of the limiter approximates a square wave of constant
amplitude, regardless of the amplitude of the input IF signal.  The
discriminator measures the frequency of the square wave.  The output of 
the discriminator is a voltage proportional to the frequency deviation 
and independent of signal strength (even without AGC).  

I don't remember whether old FM radios included AGC.  It seems to me that
AGC would have been useful but not essential for a cheap FM radio.

-- 
    Ben Carter