[sdiy] Cheap frequency counter for oscillator calibration

[David Brown]

I have only used a frequency counter if I can get the summing 
amplifier at 0 volts easily.  Otherwise you're just chasing your tail 
as both frequencies shift.  I always adjust the span by ear beating 
against another oscillator.  That way all my oscillators are in tune 
with each other.

I built a 440Hz reference oscillator that I tune all my VCOs 
to.  Since then I have built the JYETech LCD oscilloscope and the 
frequency mode is reasonably accurate.  I can tune them with that and 
it works quite well.  I know that's not what you were looking for but 
it was a nice inexpensive frequency counter with OK 
accuracy.  They've since added an FFT function.
http://modularsynthesis.com/modules/DJB-019/djb019.htm

Dave