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From: ddd on 14 Mar 2010 16:08
Been some years since I commented on crystal problems, the ancient UK phone
line make it difficult and people seem to be getting more unplesant.
To work out the suitability and margin for oscillation I recommend plotting
the negative resistance of the circuit versus frequency. That is the
negative resistance that appears across the crystal. At the oscillation
frequency the negative resistance should exceed the crystal resistance by at
least a factor of 2.

The PI network oscillator can be shown to go from positive resistance up to
a peak negative resistance and then reduce over about 2 octaves. It is
possible to select an overtone by careful selection of values.My recent
discovery is that Switcher
CAD can be induced to plot the above curves!
Note that his is not exclusive to PI networks and I have a broadband two
transistor DC
coupled circuit that works from 1M to hundreds of megs without tuning and
selects the overtone purely by resistance , not reliabley though as
manufacturers of crystals are getting worse at making cheap consistent
product.
Currently Im rejecting 10% of bought in pagers as the crystal stops after a
few months.
This is due to the above and that crystals increase their "start up"
resistance with time if not properly manufactured.
Note crystal drive level is not the same as circuit activity and optimum
activity can be inconsistent with very low power consumption.
Nice to be back
Doug Dwyer






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