Message from discussion Where is the noise when I tune to a FM station.
From: b...@netcom.com (Benjamin P. Carter)
Subject: Re: Where is the noise when I tune to a FM station.
References: <email@example.com> <firstname.lastname@example.org>
>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.