Assuming the VCO is part of a PLL, the PLL will tend to "correct" the modulation, especially those low frequency (example CTCSS and DCS signalling ) components that fall within the loop filter. Most modern land mobile transmitters have modulation compensation where a small amount of modulation is fed to the Reference Oscillator in order to fool it into ignoring the "error' it would see due to the modulation of the VCO. So to answer your question, you should insert modulation where it can feed both the VCO and the Reference Oscillator Modulation Compensation circuitry other wise the result will be distorted.

The matters of preemphasis/deemphasis and channel BW modulation mask are another important set of topics and beg the question, why do you want to drive the VCO directly without employing the existing filtering?

Gurus,

What are the limitations with audio-driving a 50MHz FM Transmit VCO directly?

Sorry that's a really open-ended question.  I suppose it will vary per each chan_ driver as well, as well as the emphasis settings and so on.

So dealing with the issues of emphasis, compression, (audio)bandwidth limiting and so on - how much can be taken out of the radio and handled by the chan_ drivers, and how different are they each?

It was suggested elsewhere, that everything over 12KHz be not permitted anywhere near an FM modulator stage, but how much does that actually happen with the CM119 chipset?

I ask because on a transmit-only site, there hardly seems to be any point to having a "transceiver" there at all, when a clock generator, buffer, PA, and filtering, is a fairly simple unit.


Cheers,
Steve

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