[Milsurplus] Crystal tester

[Hubert Miller]

Thanks for the reminder – I thought about this too, since it's a staple of breadboard experimenter circuits. Maybe one of the unused gates could be driven by a diode, and this gate driving an LED to indicate output.
LEDs actually have a pretty good range of brightness/  current. I have a dipmeter circuit that's really minimal, one FET with an LED in the drain, and it indicates.
One thing I am interested in, besides the crystal's activity, is how to determine whether it's 3rd overtone or actual frequency cut. Yes, I know the 'typical numbers' already. Just curiosity, no big deal. I used my
dipmeter recently to determine that a 14 MHz rock I have is fundamental, while a 21 MHz ( of course ) is 3rd overtone. Yes, those are predictable results, but I wanted to know for sure that the 14 MHz rock
wasn’t really a relabeled 7 MHz or 3rd overtone. Since crystals will resonate at odd harmonics, I don't yet figure how to find this for sure. Except maybe that for non – overtone cuts, the F/ 3 or F/5 dipmeter
response will be a lot less than at F, since harmonic power of the dipmeter is less. Also, the overtones are not at a perfect 3x or 5x the actual cut freq, but that takes a freq counter.

Remember the passive 'calibrator' circuit in the TBY and BC-222/322 ? That's minimalist perfection.
-Hue

>From: milsurplus-bounces at mailman.qth.net <milsurplus-bounces at mailman.qth.net> On Behalf Of Al Klase
Sent: Saturday, August 10, 2019 9:49 AM
To: milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] Crystal tester

Gang,

Crystal testers have been of interest to me for a long time.  There are a lot of published circuits out there, and there have been some actual products over the years.  Most of these work just fine for crystal in the MHz range, but don't work well with things like 100KHz calibrator xtals or military FT-241's that are the the 300-500KHz range.

Most modern devices implement crystal oscillators by biasing a CMOS logic gate/inverter into analog operation by connection a 1-2 meg resistor from input to out put, and connecting the crystal like so:

[cid:image002.jpg at 01D54F71.A78FB710]
This Texas Instruments app note dopes out the whole matter:  http://www.ti.com/lit/an/szza043/szza043.pdf<https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.ti.com%2Flit%2Fan%2Fszza043%2Fszza043.pdf&data=02%7C01%7C%7C10e38f7ba5f443ac4d4f08d71db2a0f3%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C637010525346621420&sdata=VPy9naRU%2BS9pATe6q%2BACOUBogsWHJ7I78NZGxN%2BAtxE%3D&reserved=0>

Basically, all one has to do is change the value of Rs to accommodate different frequencies.  E.g. 50K ohms for 100 KHz rocks.  I started in on this a while back and had encouraging results with a 100K pot for Rs.  (can't find my breadboard. :-()  I intend to add some sort of analog meter to the output of the oscillator gate to indicate activity, and buffer the output with the other devices in the IC to and output connector and possibly a $10 counter module from E-bay.

Too many projects!
Al




-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/milsurplus/attachments/20190810/7fb029e4/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image002.jpg
Type: image/jpeg
Size: 7451 bytes
Desc: image002.jpg
URL: <http://mailman.qth.net/pipermail/milsurplus/attachments/20190810/7fb029e4/attachment-0001.jpg>