Crystal Tester Schematic?

[tobrew]

Does anybody have schematics or a link to a 72mhz crystal tester? I searched the forum archives but only found dead links. I know crystals are really cheap but I think it would be cool to have a tester. Plus, I'm always looking for an excuse to build a circuit. :D

[planophore]

I can't point you to a specfic link at the moment but if you do a quick Google search on crystal tester of xtal tester you will get lots of hits. Xtal is a common short form used in the radio industry and ham radio. You find something suitable.

Don't forget the reciever crystals (and transmit crystals) we use in our RC equipment don't oscillate on 72mhz. They generally work at 1/3 of 72 - the receiver or transmitter uses the third (or could be 5th or 7th) harmonic of the fundmental crystal frequency to produce the desired frequency.

good luck and have fun,

cheers, Graham in Embrun near Ottawa Canada

[Mr.RC-CAM]

What type of test did you need it to do? Frankly, a low cost and effective test would be to install the xtal in a Rx and perform a range test. Perhaps toss in some vibration during the test too.

Otherwise, I can't imagine you would learn much about the health of the xtal on the bench unless you had a way to measure the drive level and frequency. Besides the Osc circuit, that means an O-scope and Freq counter (or suitable equiv) at a minimum.

Some Osc test circuits can be found in this doc: http://www.northcountryradio.com/PDFs/column007.pdf

RC-CAM

[jeffs555]

Like RC-Cam said, the best test of a crystal is going to be in circuit in the receiver.

Graham is right about the fundamental frequency of the crystal not being 72mhz, but to be more precise, the crystal actually operates on an overtone and not a harmonic. An overtone is a little different than a harmonic, in that a harmonic is precisely 3 or 5 or 7 times the fundamental, and the overtone is just a little less than that. You can't just measure the fundamental frequency and multiply by 3. In a proper circuit, overtone crystals do actually oscillate at 72mhz, but they need a tuned circuit to force them to oscillate at the overtone rather than the fundamental.

[tobrew]

Thanks for the info guys. I'm fairly new at RC and didn't know about the overtone/harmonic shift. I had already sort of figured the best way to test was in circuit but was just curious. I guess I will buy a few spare crystals to have around for test purposes.

[vintage1]

Two things can go wrong with crystals.

(i) they stop working altogether, or intermittently

(ii) They go off tune.

Inboth case the best thing to do is plug tehm into an old receiver of the type you want to use them in, and check the oscillator freuency witha freq. counter. With luck there is enough leaking down the antenna to actually pick up on.

Then tap the Xtal to see if the numbers jump :D