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UMARS Projects Past Balloon95 Squelch

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Balloon95
Squelch Circuit

SQUELCH CIRCUIT FOR THEE MFJ DATA RADIO

The Motorola VHF Receive chip that is used in the MFJ Data Radio has a Received Signal Strength Indicator (RSSI) output on pin 12. MFJ has thoughtfully put a test pin on the board from this pin. Using this pin and TP1 (I believe that's what it is; I'm going from memory) which is the audio out from the discriminator, I have built a simple audio squelch circuit. The schematic is as follows:


              +5V
               |                +5V
               \15K              |
               /fixed or         |       O To TP1 (audio out)
               \variable         \       |
               /                 /1K0    |
               |    2 |\         \       |
               |------|+ \       /    | /C
               |      |    \_____|____|/   2N3904 (or equiv.)
      RSSI     |    3 |    /7        B|\
     O----------------|- /            | \E
               |      |/ LM311           |
               \                         |
               /2K0                      |
               \                        GND
               /                          
               |                         
              GND              +5V in on Pin 8
                               GND to Pins 1 and 4

The circuit is a simple comparator: when the voltage at pin 3 (RSSI) rises above the preset voltage at Pin 2, the transistor turns off and audio is passed out of the radio. When the voltage falls below that of Pin 2, the transistor turns on, muting the audio.

The resistor values are not critical. I found that 0.6V was a suitable level for Pin 2. A variable resistor would allow you to adjust the squelch level.

A further addition to this would be to mute the audio while the radio is transmitting. This may or may not be a problem with your TNC but since the receiver is always operational, even during transmit, your TNC may be confused or APRS may not function correctly. I have not been able to investigate this possible problem as the radio is crystalled for 145.790 and there is no activity here in Winnipeg on this freq. A possible fix would be to use a diode from the audio test point to the PTT line.

I hope someone finds this useful. Send any questions and/or comments to me direct and not the sig.

Michael Markmann, VE4MJM
ve4mjm@draco.bison.mb.ca
VE4MJM@VE4KV.#WPG.MB.CAN.NA (AX.25)
Hobbies: Electronics, Amateur Radio, Computers, Model Trains, ...