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