[sdiy] Cheap frequency counter for oscillator calibration

[Florian Anwander]

Hi Karl

> I'm looking to buy a cheap ($50-$200) frequency counter mainly to be used for calibrating the span and base tuning of analog oscillators - any suggestions? Is there a better way of doing this then just measuring the frequency?
To be honest: I would not recommend a frequency counter for spread 
tuning. In theory it would be ok, if you always tune the spread testing 
between the 0 Volt at the VCO-core and plus 1/2/... Volts. But in fact 
you will have always some kind of offset voltage at the output of the 
CV-mixer. This will cause the base frequency to be different with each 
change in the spread setting.
This means for each turn on the spread you have to re-adjust the tune.

I prefer the following method:

Use a wide range chromatic tuner with a analogue meter like a Boss 
TU-12H (in fact the TU12-H is the only today available tuner which I 
would recommend).
Connect the tuner to the output of the synth and listen to the synth 
through the headphone out.
Now send (multiple) octave jumps and adjust the spread trimmer and 
whatch the tuner: It does not matter WHICH note exactly is displayed (it 
may be a completely different notes at different settings of the spread 
trimmer), but you trim it until you hear a clean octave and the meters 
needle does noe longer move when the octave jump happens. The absolute 
pitch of those two notes does not matter at that moment.
When this is done I set the absolute pitch, and - if requires - the 
trimming of octave switches.

I recently tuned an OBXa (16VCOs !) in around 40 minutes with this method.

Flroian