Ok, we have a terminology problem then. AGC (Automatic Gain Control) is found in
AM receivers to control the gain of the RF amp stage and one or two IF amplifier
stages.

FM receivers almost never use any gain control (some designs that have to work
under very high signal strength conditions may use a simple AGC to keep the
impedance constant at the IF filter), depending instead on the IF limiter(s) to
produce a constant output. They may have an RSS (Relative Signal Strength)
output to drive S-meters. It is usually derived from current changes in the
limiter/last IF stages as the signal input rises. Any signal that falls within
the IF filter passband can make this rise, be it spark/arcing noise from car
ignition, broadband noise from computers, or spill over from nearby tranmitters.

In FM voting systems, this RSS may not be the best signal to use since any of
these sorts of interference may cause this level to jump up without opening the
squelch. The squelch circuit senses the audio noise level at the detector ouput
(though some old designs may use the lack of audio noise on the last limiter
RSS) and is a much better indicator of signal quality. If you can dig out of the
receiver schematic where this noise detector is and use it instead you'll have
far fewer false votes to a jammed receiver.

Now, to your original question, Excel and graphing. I'm not sure exactly what
you're looking for. Simple instructions on how to produce a graph for print-out?
Or something a bit more involved like curve fitting to get a smooth curve
instead of straight lines through the data points? Excel ought to have examples
of the former in the 'help' tab. For curve smoothing, look for 'statistical
analysis' in the help also. My search on google.com for 'Excel "polynomial
regression" ' yielded several good sources of more information.

73's

On 27 May 2007 22:10:47 -0700, Karl wrote:

Dave,

I guess I didn't explain myslef. Acually all FM receivers can and may
use the the IF to generat a DC
voltage depending on the strength of the signal coming down the IF.
This voltage can be used to drive
an "S" meter or other measuring device. If you calibrate this voltage
by plotting on a graph for the Y axis
with a sig gen in the into for the X axis you can plot an AGC curve.
Real handy for determing RSLs.
We do it in microwave work all the time.

Regards,
Karl