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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