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The use of "AND Squelch" to eliminate those annoying squelch bursts
By Kevin Custer W3KKC
This concept is difficult for some to follow.
If you have trouble understanding what is written here, read it again or several times as it may come to you.
Some of you may have a problem with long squelch tails when PL (CTCSS) is being used in your repeater system . This page will explain to you what to do to eliminate long tails (squelch burst). This concept does not only apply to receivers that employ switched hysteresis squelch circuitry, as it will work on all.
There are two scenarios that give long squelch burst.
The first is in a receiver that employs switched hysteresis squelch circuitry, (long and short reaction time) like the Motorola Micor, GE Mastr II, Kendecom MR4 or in the circuitry of the SEITS squelch board. If a user is noisy, the squelch tail is longer so as not to cut or chop syllables in flutter. This action of squelch burst is desirable, and should not be eliminated.
The second scenario may exist when a person has employed the use of PL on their repeater receiver. There are a few different ways to to tell the controller or voter that a user is transmitting PL. A common way is to connect the PL sense lead directly to the controller or voters PL input. This may cause the controller to pass audio until the PL decoder stops sending logic to the controller. Some controllers interpret the PL signal differently than others. If your controller is set up for "and squelch" you are in luck, if not, I will tell you how to accomplish it anyway.
Most commercial receivers were never intended to be used on a system using PL and not employing reverse burst. Reverse burst is the process of reversing the phase of the PL tone for a short period of time after the user unkeys the mike. During reverse burst the transmitter actually keeps transmitting for a short period of time after the user unkeys the mike while the phase of the PL tone is is rotated 180 degrees, stopping the PL reed, in the receiver, in its tracks. This is how most commercial equipment eliminates the burst. Most ham equipment does not have the ability to transmit reverse burst. In commercial service where PL is used, the receivers squelch is sometimes controlled solely by the PL decoder, and the setting of the squelch control is irrelevant to when the receiver unsquelches. This is the case in an unmodified Micor, and this concept is used in some repeater controllers. This concept is referred to as "Or" Logic ..... or Or Gating.
The Solving Concept: " And Squelch"
Consider a user having no reverse burst while transmitting PL into a receiver equipped with a Motorola or other mechanical reed PL decoder, then the user unkeys, the reed will continue to vibrate (like a tuning fork) until it stops naturally, thus producing an annoying squelch tail noise burst because of the lagging logic signal that is being produced is holding the squelch open. This response time is also evident in other style decoders like the Comm Spec TS series decoders. This is where "And Squelch" comes in. Enabling "and squelch" allows the receivers regular noise squelch circuit to work in conjunction with the PL logic signal.
"And Squelch" is the function of using the receivers cos logic and the PL logic together by "and gating" them together, and then applying the resulting signal to the input of the controller, or voter. These two signals must not be slaved together in any other way as they must remain separate up to the and gate.
How it works:
When a user keys the mike, the cos will be applied to the and gate immediately, after the nominal recognition delay of the PL decoder, it then applies logic to the and gate and the gate opens.
(This is the neat part) When the user unkeys, the cos is dropped immediately and the gate closes, thus eliminating the PL response time from keeping the controllers audio from staying open and allowing the burst to pass. In this concept, it takes no longer for the controller to open, but does allow the controller to close right when the user unkeys. If the user is in flutter, you will still get the burst because of the slow response time of the regular squelch (desirable.)
If your controller is set up for "and squelch" all you need to do is to separate the cos, and PL logic signals and run them into the controller. If you are using a Micor receiver in a system not employing reverse burst and having an original Micor PL decode deck, you must cut the connection between the PL deck and the squelch shunt switches, Why? As an internal function of the unmodified receiver, the PL signal is fed to the shunt switches and overrides ("or" gates) the cos signal. The PL signal will force the cos (pin 8 on A&S board) high, thus sending a cos to the controller that is slaved from the PL deck forcing the controller to pass audio until the reed stops by itself. "And Squelch" is accomplished in a Micor by cutting jumper wire JU-204 between IC 202 pin 8, and P 201 pin 3, and using the cos and PL logic's independently or together with the use of an and gate.
If your controller is not set up for And squelch, simply "and gate" the cos and PL logic's together (assuming positive logic) and use the resulting logic for the PL input. Connect the cos as normal.
If your controller does not have a PL input at all, simply "and gate" the cos and PL logic's together (assuming positive logic) and use the resulting logic for the cos. You may want to have some form of substituting constant voltage to the PL input of the and gate to allow carrier operation. One can use a user function to accomplish this.
If I were building the SEITS bi-level circuit board, I would not manage PL the way they intended. I feel that changing part values in the PL decoder will produce decoder falsing. And Squelch is the simplest and the best way to approach eliminating squelch burst in systems using PL and not employing reverse burst, and it can be implemented on any system.
For the actual "And Gate" I have used diodes, and Cmos Nand gates like the 4011 and configuring them to "and" (invert the output.) You can create two "and gates" from a 4011 by using two of the sections as inverters (connecting the inputs together) and inverting the output of the Nand.
For more detailed information on the subject on the Micor in specific, Go to our page describing reverse burst.
Kevin Custer W3KKC
html date 7-25-1999