[sdiy] Cheap frequency counter for oscillator calibration

[Oren Leavitt]

In Electronotes (EN#75 p5), the write up for the ENS 76 VCO options, 
Bernie Hutchins suggests a scale calibration technique using a 1.0000 V 
reference, a DPDT switch, and two flip-flop dividers.

The FF dividers are connected to the output of the VCO being calibrated 
so that you have a divide by 2 and a divide by 4 square wave tone available.
One pole of the DPDT switch selects between a 0.000 V and a 1.000 V 
reference and sends this to 1V/oct CV input of the VCO being calibrated.
The 2nd pole of DPDT switch selects between the f/2 (when 1st pole is at 
0.000 v) and the f/4 (when 1st pole is at 1.000 v) output from the 
dividers and sends this to an amp and speaker.

The goal is that when the VCO scale is at exactly 1/Voct, there will be 
no change heard in the square wave pitch from the speaker as you flip 
the DPDT switch between 0V f/2 and the 1V f/4 positions.

I have used this "by ear" technique before. It works well and is quick 
to do.
Set the VCO to a low freq - say 200Hz, adjust scale trim so that you 
hear no change in pitch when you toggle the switch.
Do the same at several octaves higher and check HF tracking trim if 
applicable.

This could even be automated using an electronic DPDT switch being 
toggled at 1Hz or so.

As for "cheap", price = the cost of a DPDT switch, 2 flip-flops, and a 
1.000V reference.

- Oren