Crystal Oscillator

There is an old saying, when you have one clock you know what time it is, but when you have two clocks you are never sure. The same concept applies to frequency counters. I currently have three: a really old optoelectronics Opto-7000 which I don't use much, a newer Mitronics MIC-1028, and my most recent acquisition, a used Tektronics 2236 'scope purchased at Dayton 2009, which has a very handy frequency counter built-in.

I was working on a recent project and found the Tektronics and the Mitronics counters were annoyingly out of agreement. How to check the calibration? So I built this 10 MHz crystal oscillator.

schematic of the crystal oscillator

There are a lot of crystal oscillator circuits if you search the internet, some of them amazingly complex. I wanted one that was simple, had no tuned circuits, and could be tweaked to get it on frequency. 

Crystal Oscillator
I use my shortwave receiver tuned to WWV at 10 MHz. I adjust C2 to zero-beat with WWV. This can most easily be heard when the tones are present, since mis-tuning makes the tones sound wierd. Once zero-beat, the frequency counter should read 10 MHz too. I found my Tektronics was spot on, and the 
Mitronics needed adjustment, which was easy to do since the calibration trimmer was accessible through a small hole in the front panel.

Since completing this project, I came across an interesting article on how to calibrate a counter (that has an internal 10MHz oscillator) without any extra gear (besides a receiver for WWV). Have a look at VE2ZAZ's blog. It doesn't work with my
Mitronics counter, though, so it must use a different reference oscillator frequency.

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