PRC344 Scan Controller progress

Much of the technical design work for this has been completed, but the odd interesting challenge still shows itself! As part of the testing necessary to get a working system, yesterday I was looking at the LOCK and MUTE control lines within the radio.

The LOCK line, which originates from the synthesiser and indicates that the frequency is locked and steady, showed itself to be perfect for the job - a 3V logic 1 when locked. I have the PIC code now written such that unless a particular input pin, connected to the LOCK line, is high, the scan is inhibited. This means that the rate of scan is ultimately directly limited by the speed the synthesiser attains lock.

The MUTE line, on the other hand, proves to be a bit 'odd'.

When the receiver is muted fully, this is a +ve 5V line, but, when the radio is unmuted, i.e. there is a signal being received, then this line goes to -ve 4.4V! Worse than that though, is the condition in the presence of a poor mute or wideband noise, where the line seems to be able to jump around various positive values between 0-5V.

My thoughts at the moment are to use a simple resistive potential divider and a blocking schottky diode. This should give me a cleanish +ve or zero value for the PIC input pin.

All these extra parts will need to go on the controller PCB, since, as you can see in the photo below, there isnt any space near the connections to add any parts!

That pin there...

So the PIC code now is capable of sequencing the whole of the synthesisers 3600 frequency steps, plus the 18 HT tuning lines, and also of being inhibited by the LOCK line. Ive also checked that the outputs when set low will sink current not float, ensuring that the blocking diodes on the synthesiser control lines will result in clean control signals.

Control mock-up

There are just two controls to be implemented now in code - the MUTE control, and all its associated complex dwell times, and the Over-ride control, which will break the code out of the MUTE controls in the presence of a received signal, and continue the scan, if the radios CALL switch is activated.

Amongst all this, ive also found a moment to put another batch of my tropical wine on...

Tropic Thunder Brew