Crystal tester

[Edward Cain]

   I picked up a crystal tester kit off eBay (http://cgi.ebay.com/Electronic-Kit-Crystal-Tester-1715-/390269372382?pt=LH_DefaultDomain_0&hash=item5addde07de) the other day. I find that it seems to work fine with the test crystals supplied with the kit (approx 4.? MHz to 20 MHz).

   However, when I tested several 1 MHz crystals I have, they didn't oscillate (no green light).
While the seller says it's good for 1 MHz to 80 MHz, I'm wondering if the crystals are in fact bad or the tester is not capable at this frequency.

   My question is what determines the lower and upper freq. limits of a tester like this? Type of transistors, capacitance/resistance values?

   If needed, I can probably figure out how to scan and upload the schematic.

Thanks,
Ed

[KA0HCP]

I found the problem....

The kit is from "Nightfire Electronics" and you were doing your testing during the day! 

The description says it uses a Colpitts Oscillator, so that would be dependent on the capacitor value.   You could try paralleling a capacitor of equal value and see if it oscillates at a lower frequency.

It would help to see the schematic.

73, Bill

[Edward Cain]

Thanks Bill,

   I changed the capacitors from 30 pf/10 pf to 50 pf/30 pf and tested at night and it works just fine.

Thanks,
Ed

[k4kyv]

The most reliable device for testing a crystal is the oscillator you plan to use it in.

Some crystals will oscillate vigorously in some circuits but oscillate feebly or not at all in others.  If the oscillator circuits are not identical, even if the oscillation is robust, it almost certainly will be on a different frequency.  The difference may be anywhere from a few Hz to several kHz.

A pocket size xtal tester might be a handy thing to have at a hamfest, along with a good multimeter.