On Wed, 16 Sep 2009 15:15:45 -0700, Barry Gershenfeld wrote:

> Still no help, I'm afraid...

All constructive comments welcome :)


> I am surprised that LF crystals are all that different from the ones in the
> MHz.  Although they get quite large (physically) at these low frequencies.

I think that the biggest thing is the higher Rs of the crystal; it's about
16K for my 60KHz piece of quartz.  The capacitances used in the filtering
glue circuitry also seem a great deal smaller (10s of pF as opposed to 100s
of pF), which would make stray capacitances all the more significant.


> You could also lower
> the Q (probably throwing away some of the other good characteristics that
> made you choose the crystal in the first place).

Hans Summers (www.hanssummers.com) has a good bit on altering crystal
characteristics.  He opens the case and dopes the quartz with a marker pen; a
tad extreme for what I'm after :)


> And then of course, I
> could do the traditional PICList thing and tell you to do it a different
> way--which at these frequencies, I would certainly consider.  I suspect this
> isn't the first thing you've tried.

Far from the first thing I've tried :)  I did try some active filters,
but the high Q just isn't there without tight tolerance and low drift
components.  And there's also the much larger component count.  The main
reason for the crystal is that it's pretty successful in filtering out the
61.44KHz hash from my PC monitors to stop the amplifier stages from being
overwhelmed.  After the crystal I'm just attempting to get away with simple
RC low-pass filters before passing it to an ADC for further DSP processing.
Seems to work well apart from the transition times :(

Cheers,

Pete Restall
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