On Wed, 31 Mar 2010 21:59:19 +1100, "Phil Allison"
wrote:

The FM tuner might have an AGC section. This is required for direct
digital receivers that have an A/D converter in the front end of at
the IF frequency. The AGC is necessary to use the entire range of the
A/D. These usually have a limited dynamic (amplitude) range and are
easily overloaded. When amplified, this would cause a strong signal
to disappear as it runs out of head room.

** Massive pile of ******** !!!

See:
http://www.st.com/stonline/books/pdf/docs/8933.pdf
Note the AGC in the front end of the FM section (running a PIN diode).
The idea is to keep the output signal level within the maximum dynamic
range of the DSP A/D that follows this chip.

NXP also makes an AM/FM front end chip, the TEA5777 (which seems to
have disappeared from the NXP web pile.
http://www.kako.com/neta/2007-006/tea5777.pdf
See section 8.2.1 which proclaims:
An RF AGC circuit prevents the mixer and IF
filter from overdrive conditions.



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