Need to delay COR/debounce PTT

[IMBEJames]

Hi all, I have a need to setup a crossband repeater (VHF/800) during a migration to 800MHz SN. The problem Im running into is that the old VHF repeater system's hangtime is causing the VHF side of the crossband repeater to assert PTT on the 800MHz side. The radios in the crossband setup end up keying each other due to the hangtime and things get caught in a vicious cycle.

What I need to do is debunce the PTT on the 800MHz side for up to 1 second so that the VHF system's hang time will not key the 800MHz radio and continue the cyclic keying.

Crossband setup consists of 2 XTL1500's. 800MHz COR is active low, but its VHF counterpart's COR is active high.

I tried eliminating the hangtime in the VHF fixed system. The comparator is a JPS SNV-12 and even with the hangtime set to 0, there is just enough tail to cause the cyclic keying.

Any ideas?

[wavetar]

You don't mention what you're using as the crossband interface controller. Some can do what you're asking.

Otherwise, a 555 timer running in monostable mode could buffer it the second or so you need easily enough.

[IMBEJames]

Thanks Todd... Sorry, forgot to mention, no controller. Direct cable connection. Used a transistor and a couple resistors to convert the active high COR from the VHF to active low to assert PTT on the 800MHz XTL.

Will the circuit you posted debounce the PTT on the 800 XTL and prevent it from being keyed by a short hangtime on the VHF system? Or will it buffer the COR from the VHF so that a SHORT pulse will still key the 800, just with a delay?

Out of curiosity though, what controllers will do what I am looking for? I have a few different types at my disposal.

[RFguy]

Years ago I had a similar issue. I ended up using a simple controller with an "anti ker-chunk" function.

I'm pretty sure it was an IDA EasyLink.
http://www.idaco.com/Rayel.pdf

It might be an inexpensive solution.

[Wowbagger]

Will the circuit you posted debounce the PTT on the 800 XTL and prevent it from being keyed by a short hangtime on the VHF system? Or will it buffer the COR from the VHF so that a SHORT pulse will still key the 800, just with a delay?

The circuit as shown will take a short pulse on the trigger and turn it into a longer pulse on the output. If you wanted to force the input trigger pulse to be of some duration >X, you could place a cap across the trigger line, and have the trigger pulse be applied through a resistor. The 555 will trigger when pin 2 is less than 1/3 of the supply voltage, so when you pull the cap down to ground through a resistor, the cap will discharge from Vcc toward ground (assuming the resistance the trigger is pulling through is much less than the pull up resistance).

[IMBEJames]

Sorry for the delayed response... Thanks for the input. I made a circuit like the one in the schematic, but added a PNP transistor to make it retriggerable. The chan activity (COR) from the 800 radio will hopefully activate the circuit, and the COR from the VHF will be inhibited for the pulse duration as it will be connected to the NC contacts on the relay. Should do what I need it to do.

Unfortunately though, I wont be able to test it for a little while. The 800MHz XTL went to the depot. Out of the box, the chan activity line only swings from 4.2V to 3.9V instead of the normal 5V to ground. My plan is to use a PNP transistor on that line to drive a relay to activate the monostable circuit, so that the voltage on the trigger pin of the 555 timer (at rest) doesn't smoke the chan activity line.