One could also get FM by doing it like many old FM mobiles did, by having a separate FM oscillator that was then mixed up with the LO to the TX frequency.
In Mobira RC25 this was a 10.7MHz crystal vxo FM modulated with a varactor and doubled to 21.4MHz, the same as the RX IF, before my?s xing with the LO and entering the TX chain.
This was done due to the low tuning range of the synthethizer, typically only 1.4MHz.
So doing the 21.4MHz RX/TX difference with a single synthethizer was not possible. Ditto 4.6MHz commercial repeater split, for that a 13MHz modulator crystal was used. As when that got doubled to 26MHz the synth's tuning range was enough and being different from the IF, made fullduplex more possible.
I have also seen folks use a varactor to FM the reference crystal to a synth. But I wonder about the temperature stability of such builds.?
-Ismo, de OH2FTG
Ismo V??n?nen OH2FTG