I want to build a crystal tester. Will this work? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: November 29, 2019, 03:18:21 pm
content: The tester should allow me to test if a crystal works, and have a rough idea about its frequency. I wonder if this will work: build a simple oscillator out of one of the two gates in 74LVC2GU04, use the other gate as a buffer, then output the signal to an BNC connector. If I want to check a crystal, I can plug the crystal in, power the board from USB, and hook a BNC cable to my oscilloscope.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: November 29, 2019, 05:20:53 pm
content: I dont see any reason why this shouldn't work I mean, I think this is pretty much the only way of making a crystal tester.On ebay you can get tiny frequency counter modules very cheap. If you built one of those into your crystal tester you wouldn't even need the PC oscilloscope!

author: tszaboo
author_rank: Super Contributor
timestamp: November 29, 2019, 05:30:01 pm
content: And then the frequency changes with the parasitic and the different built-in capacitors.

author: edavid
author_rank: Super Contributor
timestamp: November 29, 2019, 07:18:29 pm
content: It depends on the frequency range you want to cover. It's hard to make a single oscillator that will start reliably with crystals over the whole AT cut range of say 100kHz-30MHz. If you can limit it to the more commonly encountered crystals of say 2MHz-24MHz, that will probably work.Tuning fork crystals will need a separate circuit, otherwise you are likely to damage them with excessive power.

author: rhb
author_rank: Super Contributor
timestamp: November 29, 2019, 07:34:44 pm
content: It may be more elaborate than you want, but Chris Trask's crystal impedance tester is worth building.http://home.earthlink.net/~christrask/Bottom of the page under "The Solderwick Chronicles"Crystal Impedance Test SetYou should at a minimum read the article.Another option is to use a nanoVNA to measure the S21 response. I built a couple of versions of Trask's set, but never got around to interfacing it to an MCU to automate the measurements as I had intended. I think the nanoVNA sort of makes that concept obsolete. Subsequent to my work with Trask's design I bought an HP 8560A and 8753B/85046A before the nanoVNA burst on the scene.A well made Pierce oscillator done dead bug style using short leads ought to work over a pretty wide range, though as noted, watch out that you don't overdrive the crystal.Have Fun!Reg

author: james_s
author_rank: Super Contributor
timestamp: November 29, 2019, 07:38:36 pm
content: I have one of these:https://www.ebay.com/itm/DIY-Digital-LED-1Hz-50MHz-Crystal-Oscillator-Frequency-Counter-Meter-Tester/252957151186?It's not the most amazing thing ever but it does work fairly well and the cost is very little. It has been a handy gadget to have on my bench.

author: edavid
author_rank: Super Contributor
timestamp: November 29, 2019, 07:52:45 pm
content: Quote from: james_s on November 29, 2019, 07:38:36 pmThat uses a single transistor oscillator... I found that with the supplied capacitors (which might vary by seller?), it didn't start with crystals below about 4MHz. Of course you could tweak that, but then maybe it would stop working with high frequency crystals.It definitely doesn't work with tuning fork crystals.

author: DaJMasta
author_rank: Super Contributor
timestamp: November 29, 2019, 09:10:14 pm
content: Do you have a signal generator?A crystal is a very tight bandpass filter, so if you sweep one input with a signal, the maximum output on the other pin signal level appears at the crystal's resonant frequency. I would stick to lower power levels because, as mentioned, normal crystals are not rated for much power, but you should be able to tell accurately where its frequency lies just by monitoring output power/voltage provided you terminate the line after the crystal.

author: james_s
author_rank: Super Contributor
timestamp: November 30, 2019, 04:21:31 am
content: Quote from: edavid on November 29, 2019, 07:52:45 pmI'm not actually sure that I've tried it with anything under 3.58MHz, mine worked fine with that. Shouldn't be too hard to add multiple ranges or as cheap as it is, build a few of them to cover other frequencies. Or just build an improved one yourself based on the basic concept.

author: rhb
author_rank: Super Contributor
timestamp: December 03, 2019, 01:59:02 am
content: A single transistor Pierce oscillator with provision to adjust the xtal drive should handle anything for a basic functional test. Add a voltage follower buffer followed by an ebay log detector.A nanoVNA will show you as much detail about the serial and parallel resonances as you could ever wish for under $50.