Hi Didier, If you want to convert the Xtal to a VCO, you will have to adjust the parallel capacitance so that the crystal can get above the desired frequency, and then design the varactor circuit so that it can pull from there to below the desired frequency...which should be possible if the crystal was capable of being tuned on frequency.
The crystal might be high enough Q to run in a PLL with a very high division ratio, but I would still expect it to hiccup around about the period of the divider.... but as you say, it might not matter. The DDS works out to be such a simple solution, I think the minuscule offset is hardly worth getting worked up over. -Chuck shali...@gmail.com wrote:
"Basically, the higher the division ratio in a PLL synthesizer, which is what you are describing, the greater the phase noise." In that case, that may not be a problem. Since the oscillator is a crystal, phase noise should be low enough. One other issue is that most crystals only want to move in one direction (with the varactor pulling trick), so if it is on the wrong side of where you want it, that won't work. Otherwise, I have been thinking about that myself. Didier KO4BB
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