The M6709 / M7716 is a complete bi-level or switched hysteresis squelch circuit on a chip.  It is found only in the Micor radio on the Audio & Squelch board. The schematic shows the M6709 as a complete squelch circuit. Note that the circuit could be added to any model radio. Its "Open Emitter" output from the shunt switches can be interfaced to nearly any repeater controller or other circuitry.

Review of the chip and supporting circuitry:

How it works:
The M6709 IC contains two amplifier band pass filter sections. These reject the speech audio and pass only the high frequency discriminator noise. The amplifier filter is followed by a rectifier and two amplitude detectors.

If the received quality of the input signal provides at least 20 db quieting, (above 1 uV on properly working receiver), the IC's logic circuit gives immediate audio shut-off, and no squelch tail.

If the signal is below 20 db quieting, (below 1 uV, or having some noise), there is a squelch tail of 150 milliseconds,  long enough so there is no audio chopping under "flutter" conditions, and since the received signal level must be low enough for the long turn-off delay to occur and the squelch tail audio level is comparable to that of the received signal, the squelch tail is not annoying.

Simply, the Micor bi-level squelch works better than anything else on the air.  Not having a squelch tail on your repeater sounds great, plus you can tell how strong your signal is into the repeater.  If there is no squelch tail, you are full quieting; if there is a tail, you are noisy. A very handy feature!

Adding the Micor squelch is a nice addition to any repeater receiver and naturally, when a need shows up, somebody fills it for a price. Link-Comm makes a conversion board called the "RLC-MOT" that will add a Micor squelch into any receiver that provides raw squelch noise. The board is much smaller than the Micor audio squelch board. One example of its application is in the Motorola Mitrek.   This circuit works great and makes the Mitrek squelch action a lot more pleasing and eliminates the squelch tail and setting problem the stock Mitrek creates. It also fits right inside the Mitrek housing... from the outside you can't tell that the Mitrek has been converted, except by the sound of a nice crisp squelch.

A quote from an email:

Open Emitter outputs?;  Let me explain. We all agree that this chip is the sweetest thing since putting icing on a cake. These outputs are just as slick as the rest of the chip. You may note somewhere in the manual that these outputs are called shunt switches. That isn't just some Motorola buzz name, it means something special. What those outputs want to see is AC at a level of less than 1 VPP or so and no DC component. Circuit wise, the ideal condition would be to connect a resistor from the output of the chip to ground. Then connect two  capacitors to the output of the chip, one going to the output and one toward the input. Finally connect a resistor from the input cap to the audio input to drop the signal across when the chip output goes to ground.

Now for a description of the shunt switch. The output transistors are wired with the emitter going to the output pin of the chip and the collector grounded. This provides a transistor with a gain of one. The drive for that transistor pulls the base high, forward biasing the base collector junction which causes the emitter to go low and kiss the collector tied to ground.  Why you ask? If the transistor were hooked up the conventional way, the audio would turn on and off with a click or pop in the audio.  The squelch chip configuration causes a soft switching action with no annoying click or pop.

Since the Emitter is present at the chip output, the voltage at that point must never exceed 4 VDC as the base/emitter junction would break over with disastrous results.

The circuits referenced below don't violate this specification so all is just fine.

Bob Swoger, Motorola Micor Design Engineer.

More information on the Link-Comm Micor Squelch board is available here on the Link Communications index. Look for the RLC-MOT. It's even been put in a Sensicon-A tube-based receiver from the 1950s.

From an email to repeater-builder from Eric Lemmon WB6FLY in 2005:
>
> Several articles I have recently read about the MICOR squelch refer to
> the M7716 bi-level squelch IC in the past tense, lamenting its
> nonavailability. To paraphrase Mark Twain, the reports of the M7716's
> demise is greatly exaggerated.
>
> The famous M7716 squelch IC, AKA the M6709 or SC6709, is now a current
> Motorola replacement part. Ask for part number 5183977M16, at just
> $17.85 each.
>
> 73, Eric Lemmon WB6FLY

Replicas of this circuit/chip have been designed and are available.  They do not have the famous Motorola Micor chip, instead other circuitry has been designed to replicate its operation. In no particular order:

• http://www.repeater-builder.com/projects/super-squelch/supersquelch4schematic.gif
• http://www.repeater-builder.com/projects/super-squelch/supersquelch3schematic2.gif
• http://www.repeater-builder.com/projects/super-squelch/supersquelch3power.gif
• http://www.nhrc.net/nhrc-squelch/
• http://www.catauto.com/sq1000.html

From: "Jeff DePolo WN3A" 
Date: Fri, 8 Nov 2002 11:21:53 -0500
Subject: RE: Micor squelch problem

> Instead of the usual "click" when squelching from a strong signal, I 
> get a short noise burst. I am using the RLC micor squelch board and 
> an RLC 4 controller. The repeater is a GE Mastr II high band factory 
> repeater. The squelch board is driving the RLC 4 directly using logic 
> high true. Audio to the RLC-MOT is sourced from the line feeding the 
> high side of the volume and squelch pots. Anyone seen this before?
>
> Anyone know which caps on the Micor squelch chip effect the time
> constants?

First of all, watch the level of the audio. If the audio is too hot 
going into the Motorola squelch chip it may think that the signal is 
always below the fast-squelch cutoff point, i.e. it's being fooled 
into always being in the long squelch mode.

Second, on a regular Micor audio/squelch board, there is 180K and an 
electrolytic cap (3.3uF comes to mind) in parallel from pin 12 on the 
chip to ground. This R/C combination determines the "long squelch" 
timing. Reducing either the cap or resistor will shorten the long 
squelch time (I often change the 180K to 100K for a not-quite-so-excessive 
long squelch).

Third, there is a capacitor that hangs off the "channel activity" 
line (not the receiver unsquelched line).   This cap affects the 
transition rate of the following squelch circuitry.   If you 
intentionally pull down this line with a resistor such that it 
never exceeds the fast squelch limit point (which, if memory serves, 
is 4.7VDC) it will force the chip to always be in long squelch mode. 
I forget what value of shunt R will hold the chip in long squelch 
mode; maybe around 100K?   You may also see a note in the IMTS 
station Micor manual about an extra resistor being in place for IMTS 
stations - those stations were hardwired for long-squelch operation 
via the insertion of this resistor.

In voting applications, I usually set up all of the Micor receivers 
for hardwired long squelch, reduce the long squelch hold time per 
the above, and then remove the squelch crash with a single analog delay 
board AFTER the voter. This way all of the system input receivers have 
the same squelch hold time and squelch "chopping" is eliminated.

(signed) Jeff DePolo WN3A

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Top section Copyright © April 2 2000 by Kevin K. Custer W3KKC.
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This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors.   All Rights Reserved, including that of paper and web publication elsewhere.