[sdiy] 440Hz Reference Oscillator.

[René Schmitz]

Hej Magnus and all,

Magnus Danielson wrote:
> They all do, by definition, but the deviation is so small that you don't
> notice it.
> 
> It is all nice and dandy that the math goes together on your sound card, but
> there is more to it. Your crystal oscillator does not make EXACTLY 24576000
> cycles in a SI second, it only nominally does that. Most oscillators is of the
> standard variant giving some +/- 100 ppm of accuracy, or +/- 2457,6 Hz in your
> case. Now, that is only a 3-sigma value so it is only within that range for
> 99,7 % of all devices made, and BTW for such precission you don't see any
> temperature compensation or stabilisation. Changes in temperature will also
> cause the crystal to age and drift away to some new nominal frequency which it
> only reaches assymptotically. Long before that the temperature changes again.
> Add to that non-gaussian noise sources which contribute to the long term
> behaviour.
> 
> All oscillators does this, crystal or not.

Good point! And besides of initial accuracy, there is two variants of 
resonance on a crystal. The nominal value is for the more stable series 
resonance, while clock oscillators often employ the crystal in its 
parallel resonance. The difference between the two might be as high as a 
couple kHz. The crystal can be pulled in this range by the loading 
capacitances.

So a crystal for a tuning standard might still need calibration. (And a 
tuning standard to calibrate it against....)

> So, can you "trust" a sound-card based tuning? Well, if you verified the
> software, go ahead. It should work well enough.

Sometimes people use the internal timers of the computers for that... 
Well as if the time keeping crystals were any better than those on the 
soundcard. :-P

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159