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Problem with Pierce using thin crystals?
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! |
#2
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Problem with Pierce using thin crystals?
"lw1ecp" wrote in message
... 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! You might wish to consult the chapter on crystal oscillators in "Vacuum Tube Oscillators" by William A. Edson. In particular you should read 9.9 Power Output and Crystal Dissipation in the Pierce Oscillator. You might also wish to read the section on the Miller oscillator. Don't let the "vacuum tube" title scare you away as the book is really good at fundamentals. Do a Google search for this book. At one time, someone had graciously posted copy in Adobe format. 73, Dr. Barry L. Ornitz |
#3
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Problem with Pierce using thin crystals?
Barry, many thanks for suggesting this book!. Don't worry, I have a
great esteem towards vacuum tubes and their underlying time-defying principles (after all, the book is just 4 years older than myself...). Power applied to the crystal by a tube oscillator is surely higher than in an equivalent solid state counterpart, but 50's xtals were also bigger (e. g. FT-243 format) and with metal contact plates that I guess contributed to dissipate heat as a side effect. Honestly, it adresses the crystal dissipation from a frequency stability viewpoint, not quite my present concern, but I have found interesting material in different chapters for other projects at home, such as induction heating, and analysis of intermittent behavior. And now I admit I didn't understand the principle behind the Miller oscillator. Ok, I will do some more experimenting and web search, and will eventually post the results. |
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