Quote:
Originally Posted by sentinel040
Consequently I come cap in hand to the great repository of knowledge to see if anyone can help, OK I admit I am creeping, but when needs must the devil drives!  |
Lummee, how do you follow that?
I don't know those sets at all, but I assume they have a straight-forward single-loop PLL synthesiser, done with dividers and PSD on a chip?
Often the PSD drives a 'charge pump' which applies width modulated pulses of current to a passive CR network to ground. The filtered DC component of the voltage biases the varactor diode in the VCO.
If these guesses are reasonable, then the tuning voltage is a very good point to look to see how a loop is performing. The synthesiser will hold a VCO on pretty much exactly the right frequency right up to the point where VCO tuning errors mean that the varactor bias reaches the limits of the voltage that the charge pump can supply.
These phase detectors, implemented as a few flip-flops and gates are phase-frequency detectors, so if they can't get phase lock, they at least steer the tuning voltage as far as it can go in the right direction. By monitoring the tuning voltage and doing some finger poking in the RF parts of the VCO, and setting the full range of frequencies the synth can do, you can get an idea of how much voltage swing the charge pump can really give.
Varactor diodes go nasty and lose Q at their low voltage end, but in radios with limited supply voltages available, then there may be no other option than to let it go low at one end. Low is less than 1v of bias. At the high end they might be able to get within 0.7v of the power supply rail voltage feeding the synth chip.
Once you've guessed these voltages, tune the synth across its range and see how well the tuning voltage fits between the goal posts. Seeing it hit an end stop as the thing loses lock gives you a confirmed kill of one of the limits! A bit of twiddling will be needed if it starts getting too close at either end.
It's likely that the same synth runs on the channel freq on transmit, mut offset by the IF freq for receive. If you're lucky the designer put in a switched capacitor in the VCO to help with the tuning range needed for this offset. You still need to check tuning voltage across the full band in both transmit and receive modes.
If you can ID the synth chip, then a data sheet and applications note can give a lot of help in the absence of a manual for the radio.
All finest guesswork. Contents liable to settle!
David