This image is of a hard audio limiter used to limit the amount of deviation in an exciter.
After the "knee" the compression ratio is infinite to 1 (hard limiter.)  Smaller amounts of compression ratio can be achieved by inserting resistance between the output of the rectifier, and the DC input of the fet (gate.)  This circuit can be used ahead of the processing in a standard exciter to add compression, or hard limiting.
The "R" and "G" are red and green led's.   The op amp can be a 5532, LF-353, or TLO82.
With the use of the high slew rate op amps, this circuit can be used to about 19 Kilohertz as shown by the plotter image below.
The low frequency cut-off is approximately 35 Hertz.
This circuit was designed by Kevin K. Custer  W3KKC  1-9-1986
Images courtesy of Matt Krick, K3MK
 

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