"Joel Koltner"
"Randy Day"
Yes, absolutely. Their accuracy is usually in the ballpark of some
thousands of PPM (e.g., 2500-10000 PPM), whereas typically quartz crystals
for microcontrollers are 25-100PPM -- two orders of magnitude better --,
although if you're trying to build, e.g., accurate RF oscillators, you can
get quartz crystals as good as 2.5 or even 1PPM.
Actually, once you're in the "handful of PPM" ballpark for accuracy, what
kills you is temperature drift,
** The "tempco" spec for an average crystal is +/- 50ppm, but be aware that
this is for the whole operating temperature range, while the accuracy spec
( typically +/-20ppm) is given at ONE temperature, usually 25C.
This causes lotsa confusion for novices, as the operating temp range can be
70 to 100 degrees C wide.
If you divide the tempco spec by the number of degrees in the operating
range - you come up with a number of about +/-1ppm for the drift per
degree C. In fact, the number may be rather better in the temp range
around 25 C. Plus of course, it is a simple matter to use a trimmer
capacitor adjust the frequency of a crystal oscillator to be "spot on" at a
particular temperature.
So, using only a standard crystal and with an indoor temp that stays within
the range of say 20 to 25 C - one can easily have a crystal oscillator
with an accuracy of a few ppm.
My 1GHz bench frequency counter suffered from an annoying " warm up" drift
of about 25ppm ( due to internal heat coming from the AC tranny and a 5 volt
regulator) until I figured out a simple fix - I just added a 40mm DC fan
blowing outside air DIRECTLY onto the crystal.
Warm up drift is now gone.
...... Phil