I built two versions of a Colpitts VCO using red LEDs as varicaps.
Version one had too much load capacitance and the center frequency was
a few kHz below nominal. However, it had a fairly wide tuning range as
the LED was varied about 10p it would tune around 1kHz. A second version
had exactly the right base load capacitance (it oscillated within a few
Hz of 25MHz on powerup), but the tuning range using almost an identical
setup is much narrower.

Is this strong attraction to the natural Crystal frequency normal?

--
Ben Jackson AD7GD

http://www.ben.com/

Ben Jackson wrote:
I built two versions of a Colpitts VCO using red LEDs as varicaps.
Version one had too much load capacitance and the center frequency was
a few kHz below nominal. However, it had a fairly wide tuning range as
the LED was varied about 10p it would tune around 1kHz. A second version
had exactly the right base load capacitance (it oscillated within a few
Hz of 25MHz on powerup), but the tuning range using almost an identical
setup is much narrower.

Is this strong attraction to the natural Crystal frequency normal?

If you're using a load capacitance you're not using the crystal's
natural resonance frequency -- that occurs at the series resonant frequency.

Were you using the same crystal for both circuits? The same LED?

I would try this out with a trimmer and parallel cap that mirrored the
expected range of the varicap, _then_ I'd add the extra variable of an
unknown varicap.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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On 2006-07-25, Tim Wescott wrote:
If you're using a load capacitance you're not using the crystal's
natural resonance frequency -- that occurs at the series resonant frequency.

Point taken. It's a crystal cut for a parallel resonance with a specified
load capacitance (18p, I think).

Were you using the same crystal for both circuits? The same LED?

Not physically the same parts, but identical parts purchased at the
same time.

I would try this out with a trimmer and parallel cap that mirrored the
expected range of the varicap, _then_ I'd add the extra variable of an
unknown varicap.

That's an obvious idea I had totally overlooked!

--
Ben Jackson AD7GD

http://www.ben.com/