Hi!.
I built a basic Pierce oscillator: emitter to gnd, about 33pF at B and
C, xtal from B to C, 180kohm from B to C. Feed thru choke. x10 scope
probe at collector.
With 3, 6 or 10MHz xtals, I sweep Vcc from 1.5V onwards, no problem.
With 24MHz, or 22MHz (actually a 110MHz 5th overtone, oscillating here
at its natural f), at first the amplitude increases gradually with
increasing Vcc, the negative peak being far from saturating the
transistor. Suddenly, at some Vcc the amplitude increases to near
twice Vcc and the transistor saturates at the negative peak. Going
backwards with Vcc, I also find hysteresis in this behavior.
I tried a different transistor model with same results.
Frequency shifts a mere 100Hz when jumping amplitude, so it doesn't
seem the xtal is falling into some spurious mode.
If using 1.8kohm to feed the collector, I get a nasty
superregeneration envelope.
I have read thin quartzs must be driven with lower levels to avoid
mechanical damage. But could it be that high levels produce an
instantaneous (not temperature related) lowering of the series
resistance? (which in turn causes more drive, hence more R lowering,
etc.).
Just in case, I will try a Butler oscillator (tuned at the natural
frequency, not an overtone as usual) because it imposes much lower
drive, but I will be glad to hear if anybody had a similar experience
with the Pierce.
Many thanks!