In ham radio applications it may be advantageous to use the "and squelch" feature if using the original Motorola PL decoder board in your Micor receiver. Why? Because Motorola developed "PL" or private line for commercial users that ALL had the ability to transmit "reverse burst."
Reverse burst is usually generated on commercial 2-way transmitters to eliminate squelch tail burst noise in systems using PL.

How? The Motorola PL reed is very similar to a tuning fork that is excited in movement by coils that apply the exact resonant frequency to the fork making it vibrate, another set of coils sense that the fork (reed) is vibrating, thus working like a precise frequency sensitive transformer.
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, and transmitting just quiet for a little longer.

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.

Consider a user having no reverse burst while transmitting PL into a receiver equipped with a Motorola 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 the lagging logic signal that is being produced.  This is where "And Squelch" comes in.

Enabling "and squelch" allows the receivers regular noise squelch circuit to work as in any other receiver, but in conjunction with the PL logic signal. Thus a user, without reverse burst capability, transmitting PL will be muted as soon as the carrier is dropped, even though the PL reed is slowly coming to a stop.

A receiver set-up for "and squelch" uses the PL and the squelch control to control the unmuting of the receiver audio. If you are going to use an outboard controller to manage the functions of the repeater, and / or, if you want the squelch control to control squelch sensitivity, you will want to use "and squelch."

Since, in most cases, an external repeater controller is used to manage the functions of control, the radio can be run in carrier mode. The PL output is simply used to feed a logic signal to the controller and the controller manages the function of controlling the audio. Of course the controller must be able to do the "And Squelch" function, not the radio. The Micor receiver was never intended to be used on a system using PL and not employing reverse burst, so the receiver does not have enough circuitry in it to do this type of "and squelch" without adding additional circuitry, or by using an external controller that supports the "and squelch" function. Many controllers do the "and squelch" function, if yours does not, "and squelch" can be accomplished with a simple AND gate. The output of the AND gate will then control the squelch gating of the repeated audio.

In a Micor receiver being used 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 or the receiver will produce the squelch tail burst from both the external speaker and from the controllers repeat audio.

Why? As an internal function of the receiver, the PL signal is fed to the shunt switches and overrides the cos signal. The PL signal will force the cos on, 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. Obviously you want a PL signal and a COS signal that operates totally independently from one another when using an external controller, or when adding circuitry to the receiver to allow the use of "and squelch" in non reverse burst operation.

The "And Squelch" modification is also necessary when implementing an "AND gate" to create a composite squelch signal that requires both COS and PL to activate but will drop if either goes away. This is a nice feature when you have a controller that does not have a dedicated PL input. Putting the controller in PL then can be accomplished by using a logic output signal from the controller. The controller can be put into carrier access by replacing the PL signal with a constant voltage so the COS signal alone will activate the controller. A relay or an "OR" gate could be used on the PL input side of the "AND" gate to control carrier access.

7/3/1998 Kevin Custer W3KKC