View Single Post
Old 16th Feb 2018, 1:21 pm   #37
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: Quartz crystal accuracy over time

Thanks. What is really neat about the old HP method with the HP8405A VVM is that you can sit there with a box of random crystals and tap in the design frequency of the crystal to the sig gen and then test it very accurately for resonance with/without a 30pF loading cap.

I'm using a shorting bar to speed it up even more and it literally takes a few seconds to do the whole test. eg type in 48.000MHz to the sig gen for a 48MHz series resonant overtone crystal. I think that the 30pF loading cap is not needed in this test if the crystal is marked as series resonant.

Then press the shorting bar and calibrate for zero phase (this also means the counter shows 48.000000MHz because the sig gen has 0V DC at the FM input)

Then release the shorting bar and with the crystal in place simply read the series resonant frequency on the counter. I've got a bag of old 48MHz OT xtals from Golledge here and they are dated Nov 2007. There are still 7 in the bag but the first one out of the bag read 47.999793MHz in the above test.

So just over 200Hz low. Not bad at 48MHz after 10 years! That's within about 4ppm.
__________________
Regards, Jeremy G0HZU
G0HZU_JMR is offline