[time-nuts] How far can I push a crystal?

Rick Karlquist richard at karlquist.com
Fri Jan 18 02:21:22 UTC 2013


Ed Breya wrote:
> Maybe I should clarify what I meant by pushing the crystal frequency. I
> meant only using various topologies and electronic components in the
> associated circuitry, that would detune it from its natural resonance
> far enough to reach the new frequency, and still have it be sort of a
> narrow-bandwidth crystal oscillator - not doing any mechanical changes
> to the crystal element itself.
>
> Since the ceramic resonators seem to work well, and can be pushed (or
> pulled?) fairly far away by proper selection of the associated component
> values, I was wondering how far quartz crystals can reasonably go. I

Since you asked:

You can get something like a range of 0.1% by resonating out the
holder capacitance with a shunt inductor.  You then put this
assembly in series with an inductor and varactor.  If you want to
get into the lunatic fringe, you use a high Q inductor wound on
Fair-Rite 61 or 67.  Now you can seriously pull the crystal below
its resonant frequency.  How far you can go depends on the Q of
the inductor.  I am not sure if you can also pull it above,
but even if you could, there are spurious resonances up there that
could get you.  The lunatic fringe might get you .2 or .3%, still
not .6%.  You'll definitely take a hit in temperature stability
and phase noise with high pulling.  If you don't have experience
with VCXO's, you will find the circuit design quite challenging.

It wasn't clear if you needed a 10.05944 MHz VCXO, or just a
source at that frequency.  There a lots of one chip synthesizers
that could generate that frequency as I'm sure you know.

Rick Karlquist N6RK




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