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#1
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Self-heating of crystal in inverter oscillator
Ben Jackson wrote:
Well, this might be the best advice I ever got from Usenet. I fought every parameter in the 'HC86 inverter oscillator and lost. The fact that it works great at 14.85MHz and the elegance of using up the extra gates lured me in. The whole thing is very voltage sensitive (around 1Hz/mV at the output of the gate, making it very sensitive to Rs as well). This might be due to the propagation parameters of the gate varying substantially with temperature and voltage. I built a Colpitts oscillator with a plain old 2N3904 and it hasn't moved by more than a few Hz in the hour it's been on. Thanks! Ben Jackson Hi Ben, I have been following this interesting thread. Thanks for posting the data. Could you do me a favor and post your 2N3904 schematic? I'd like to put it in my SPICE Crystal Analysis program and see if it can verify the performance you obtain. If you are interested, the paper is at http://tinyurl.com/qpcoz Regards, Mike Monett |
#2
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Self-heating of crystal in inverter oscillator
On 2006-07-04, Mike Monett wrote:
Ben Jackson wrote: I built a Colpitts oscillator with a plain old 2N3904 and it hasn't moved by more than a few Hz in the hour it's been on. Could you do me a favor and post your 2N3904 schematic? ___ +5V .--|33k|---+ | | | |/ .-o---o----| 2N3904 | | | | --- | | --- .--o---- _-_100p| | | |___| o---' .-. - | | | 150 25Mhz | --- | | | --- '-' | 100p| | o-----o------' | === GND The 33k/150 were chosen to get CMOS compatible output. The circuit will oscillate with much higher values for each. The two 100p might be slightly too high for the 18p crystal I'm using, but I didn't tune those carefully because I want to pull the crystal slightly, and in my actual circuit the grounded leg of the crystal is actually in series with a variable capacitance to tune the VCXO. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) -- Ben Jackson http://www.ben.com/ |
#3
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Self-heating of crystal in inverter oscillator
Ben Jackson wrote:
The 33k/150 were chosen to get CMOS compatible output. The circuit will oscillate with much higher values for each. The two 100p might be slightly too high for the 18p crystal I'm using, but I didn't tune those carefully because I want to pull the crystal slightly, and in my actual circuit the grounded leg of the crystal is actually in series with a variable capacitance to tune the VCXO. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) Ben Jackson Hi Ben, Thanks. What is the nominal setting of the cap in series with the crystal? 18pf or so? Do you have any info on the crystal, such as make and model? Or any idea what the series resistance might be? Would you like me to post the results to my web site? Regards, Mike Monett |
#4
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Self-heating of crystal in inverter oscillator
Mike Monett wrote:
Would you like me to post the results to my web site? To clarify - so you can see the waveforms? Regards, Mike Monett |
#5
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Self-heating of crystal in inverter oscillator
On 2006-07-04, Mike Monett wrote:
Ben Jackson wrote: The two 100p might be slightly too high for the 18p crystal I'm using, Hi Ben, Thanks. What is the nominal setting of the cap in series with the crystal? 18pf or so? The "varicap" is actually a red LED, which G0UPL characterized as about 30p down to about 20p over 0..5V: http://www.hanssummers.com/radio/varicap/index.htm Do you have any info on the crystal, such as make and model? It's a Digi-Key X231-ND, mfg p/n ECS-250-18-4-F. Digikey has the datasheet. I think the ESR was 40 ohms. Would you like me to post the results to my web site? Sure. -- Ben Jackson http://www.ben.com/ |
#6
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Self-heating of crystal in inverter oscillator
Ben Jackson wrote:
It's a Digi-Key X231-ND, mfg p/n ECS-250-18-4-F. Digikey has the datasheet. I think the ESR was 40 ohms. Would you like me to post the results to my web site? Sure. Hi Ben, I found the ECS spec at http://www.ecsxtal.com/search.html The numbers a Shunt capacitance C0 7pf drive level 0.5mW maximum max esr 30 ohms Using these values plus the standard beta value supplied with the SPICE model for the 2N3904, the oscillator runs pretty good. It doesn't clip, and puts out a good signal to drive CMOS. But the crystal dissipation is about 5.88 milliwatts, or about 11.7 times the max spec value. Since this is so far above spec, there's not much reason to go further and spend the time to post the waveforms. Regards, Mike Monett |
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