I reverse engineered the 6709 when I was in High school. It is basically a couple of op-amp sections configured as fairly narrow high frequency band pass filters, an audio to DC rectifier, window comparator and some switching logic to decide whether to be instant closure or switch to 150mS delay.
http://www.repeater-builder.com/projects/super-squelch/supersquelch4schematic.gif http://www.repeater-builder.com/micor/micor-bi-level-squelch-theory.html Modern "digital" squelch boards do all the same stuff, but the window comparator is replaced with a small micro controller that looks at an Analog voltage input to allow for easier programming of your set points.
