Motorola MICOR Squelch Operation and Theory

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MICOR® Squelch
An explanation as to why the
MICOR squelch works so well
By Kevin K. Custer W3KKC

Concept:
The MICOR was Motorola's first all solid state trunk mount mobile. It was also the high water mark for squelch circuit development. Since the use of PL and DPL had not become universal yet, Motorola put a great deal of effort into the MICOR's noise squelch circuit. The heart of the squelch is a special IC designed for that particular job, the SC6709 or M6709 (also marked M7716 on some later models). This circuit is also used in the MICOR Station to control audio to the local speaker, but not to control the repeater's keying or through audio muting.

The M6709 / M7716 is a complete bi-level or switched hysteresis squelch circuit on a chip. It is found in the MICOR radio on the Audio and Squelch board, and the same chip and/or even the same audio and squelch board is also used in a link receiver chassis that goes with some paging systems and in the Spectra TAC receiver audio control module. The invention 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:

http://www.repeater-builder.com/micor/pix/micor-bilevel.gif

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. Audio Test Solutions 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. In addition, Masters Communications makes a similar product for less money and offers several options. 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.

Links to circuits and boards that use the MICOR M6709 squelch chip:
The MS-25 squelch board.
The SC-50 squelch and CTCSS board.
Audio Test Solutions (the old Link-Comm) RLC-MOT squelch board.

Replicas of this circuit/chip have been designed and are available from various sources. 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/micor/super-squelch/supersquelch4schematic.gif

http://www.repeater-builder.com/micor/super-squelch/supersquelch3schematic.gif

http://www.repeater-builder.com/micor/super-squelch/supersquelch3power.gif

http://www.nhrc.net/nhrc-squelch/

http://www.catauto.com/ SQ1000 manual

Some comments on the MICOR squelch
By Jeff DePolo, WN3A

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 22K? 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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