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Author Topic: Zetron 38A and MSR2000  (Read 7835 times)
N0PQK
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« on: August 09, 2013, 07:38:09 PM »

I have a Motorola MSR2000 connected to a Zetron 38A tone panel, every time someone unkeys (with PL) I hear a squelch tail. It is driving our ham users crazy, and although I don't mind it, the others would be grateful for a way to eliminate this.

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NJ1K
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« Reply #1 on: August 13, 2013, 08:24:04 AM »

Although I haven't ever had a Motorola repeater, I have an ICOM as well as built several repeaters from scratch.  I also have worked on and managed several GE repeaters.

I am assuming you are talking about the squelch "bark" after an unkey.

Most commercial transceivers have a feature called "STE" (squelch tale eliminator) that does two things.  First, when the user unkeys, it sends out a "reverse burst" of the PL tone, which helps to close the PL decoder in the repeater.  Secondly, it keeps the carrier on for a several milliseconds after the PL the drops so that the repeater PL tone decoder drops audio throughput during the "full quieting" period, thus no "squelch bark".

Most ham transceivers do not have the STE feature.  Of course the squelch bark is aggravated if the repeater squelch is set too low or set wide open.

I fought with this for years on most ham repeaters I managed.  I finally found a solution that works quite well, although it's a little pricey.  Here's how I figured it out.

I found that all PL decoders require a minimum amount of time to decode a PL tone, as well as a "fade time" at which the PL squelch stays open for several milliseconds to allow for fading signals from mobiles.  Each different brand of PL decoder has it's own timing personality and many are inconsistent.   So, here’s what I ended up with:

I used two SCOM digital audio delay modules (DADM).  Each DADM has an audio gate on its output that is externally controllable.  I pinned out the audio gate control lines on one DADM and on the other DADM I defeated the gate altogether making it wide open.  Then I connected the DADMs with their audio paths in series with the one audio gate between the two.  Connect the input of the first DADM to your discriminator, the output of that DADM into the input of the second DADM, then the output of the second DADM on to your controller.  Connect the audio gate control line to the PL decoder control output. 

Now, the first DADM is recording discriminator audio all the time, the audio gate between the boards keeps this audio from going into the receiver.  When a valid signal with PL is received, the first board is recording it, but this audio doesn’t get passed on to the second board until the PL is decoded.  By that time, all recorded audio in the first DADM should be full quieting.  If you set the timing right on the first DADM, the first bit of audio that sometimes is missed (due to a slow-keying fast-talking op) is now heard.  If you set this timing too long, you will get a squelch bark on key-up.  If set too short, you will miss the first bit of audio.

When the op un-keys, during the fade time of your PL decoder, any recorded audio on the second DADM continues to be passed.  If you set the timing of the second board right, your audio gate will close before any squelch noise makes it through to the transmitter. 

It takes some fiddling with the two board timings to get it right, but I have found that once it’s correct for my PL decoder decode and fade times, I no longer need the carrier squelch at all.  It’s set wide open and my effective range increased a little but because the PL decoder is more sensitive than the carrier squelch. 

One more observation is that some ham transceivers have very fast TX to RX transition times.  This can result in their hearing their own last word or bit of a word when they unkey.  Fine tuning of the DADM timings can eliminate most of this but a few radios will still have this issue. 

It’s a bit of a project and a little cumbersome to get it all working correctly, but it was well worth it for me.  Of course I already had the DADM boards in my junk box.

Hope that helps.

Tom NJ1K
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