Thanks all for your advice, hints, tips and links. Lots to read , do and some hardware to check. I don’t have a frequency generator so I’ll have to go another route.
Oh. One last Q. Has anyone tried repairing the « spring » wire electric connections on large quartz plates. In one large unit I have they had corroded and dropped the plate, luckily no damage. I have done one, but I have no Idea what the original wire composition was so have certainly induced some stray capacitance/resistance. It is possible that it was a filter rather than a frequency source as it was not in a vacuum. Have a good one. > Le 4 juin 2016 à 18:49, Bernd Neubig <bneu...@t-online.de> a écrit : > > > Tim Shoppa wrote: >> The Pierce logic-gate-biased-active oscillator is pretty reliable to start >> and will oscillate somewhere with most crystals from kHz to MHz. >> As you found out, it will often come up on one of many overtones. >> To reduce chance of coming at an overtone, a series resistor from logic gate >> output to the crystal is often enough. If not, a RC low-pass will cut down >> even further (although of course adding phase shift.) > > This is certainly the easiest and fastest way for a go/no-go test and to find > the approximate resonance frequency. > In the attached circuit diagram make CX1 and CX2 about 10 pF and RGK several > MegOhms. > The inverter gate should be preferably an unbuffered HCMOS or other fast > inverter. > For crystals in the MHz range you can replace RV by a short, for kHz crystals > make it a few kOhms. If testing small watch crystals @ 32768 kHz or around, > RV should be 100 kOhm at least. RV reduces the crystal drive level (RF > current) to an acceptable level to avoid overloading or even damaging of the > crystal. For low frequency crystals the RV-CX2 lowpass also avoids start-up > at the overtone. > It is recommended to add a second inverter gate at the output to isolate your > oscilloscope or counter input from the oscillator stage. Add some >330 ohm in > series to the output of the 2nd inverter, if you connect a coaxial cable. > Then terminate the coax at the oscilloscope or frequency counter end with 50 > Ohms, so the square wave form will be roughly maintained. > > In this circuit the crystal will not operate at its series resonance, but at > a load resonance with load capacitance of something between 8 pF and 10 pF > (depending on the inverter input and output capacitance plus the stray > capacitances of your test fixture). > If you want to operate the oscillator at a (low) overtone, such as 3rd (or > maybe 5th), you must add a series combination of 10 nF plus an inductor in > parallel to CX2. The 10 nF is to avoid DC short-circuiting of the output. The > inductor together with CX2 must have a resonance frequency mid between > fundamental mode and 3rd overtone (not at one of them). So the tuned circuit > acts like a capacitor at the 3rd OT and is inductive at fundamental mode > (thus the phase condition for oscillation is not fulfilled at the fundamental > mode) > Have fun > > Bernd > DK1AG > <Pierce_Logic_Gate.gif>_______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. "The power of accurate observation is commonly called cynicism by those who have not got it. » George Bernard Shaw _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
