Paul Burridge wrote in message . ..
Hi,

I'm currently working on this VCXO that achieves frequency shift by
applying DC bias to two varactor diodes connected cathode to cathode
(bias applied to the junction between them). If I can't get enough
shift with the available bias voltage, is there any problem with just
putting another pair of the same diodes in parallel with the existing
ones?
This is a ceramic resonator oscillator, BTW, so will stand a lot more
'pulling' than a xtal would, so don't worry about that aspect of it.

So the right way to do this is to lower the _effective_ minimum
capacitance. You can do that by adding an inductor, to cancel out
capacitance. You can end up making the tuning range as wide as you
want, but at the expense of the crystal (ceramic resonator in your
case) being less of the overall frequency determination. In other
words, there comes a point where you'd be as well off to just do an LC
oscillator. But to double, say, the range, it's a good way to go.

I guess I re-discovered what was already well known, but a few years
ago I designed such a VCXO, and was amazed how linear the
freq-vs-controlvoltage curve was (a good thing for use in a PLL).
Don't know what range you're trying to achieve, but I had no trouble
getting a bit more than 0.1% (~20kHz at 14MHz) that way, with a
crystal.

Cheers,
Tom