16 Apr
1999
16 Apr
'99
4:49 p.m.
>Generally the gauge is a reading of the AGC
(automatic gain control)
>level being applied by the radio's IF. There are many other ways to derive
>this signal, but this is the most common one.
How is this AGC level derived, and how is it compared
to the signal?
Nothing fancy - the AGC circuit is typically just a simple rectifier followed
by a RC time constant. The resulting level goes to your meter and is fed
back into the IF at a point where changing a bias affects the gain. If
there's a "skip over static" function in a digital tuner, it's
typically
done by comparing the AGC level with a fixed voltage (possibly settable
with a pot somewhere.)
There are fancier methods of detecting the presence of a signal if you
know something specific about that signal. For instance, if you know that
it's a FM stereo broadcast, you can look for the FM subcarrier.
--
Tim Shoppa Email: shoppa(a)trailing-edge.com
Trailing Edge Technology WWW: http://www.trailing-edge.com/
7328 Bradley Blvd Voice: 301-767-5917
Bethesda, MD, USA 20817 Fax: 301-767-5927