[sdiy] VCO Accuracy

[harrybissell at wowway.com]

Great minds think alike (lol)

I use this method. I have a digital divider (CD4040)that allows
12 octaves of division. There is an adjustable clock oscillator
which I set with a guitar tuner to a precise reference. Then I use
a selectable voltage source (1V / step) to drive the unknown VCO
and zero beat it using lissajous patterns on a scope. Its a little
hard (sometimes) to interpret, so I use my ears as a sanity check.

The test oscillator only has to be accurate short term. A better way
would be a crystal controlled clock.

 Better still would be some individual LPFs on each divider output to get sine
waves.  Lissajous patterns are squirrley with sine waves, with a square
and a ramp they are really nasty.

Once you make up some simple test jigs like these, its pretty straightforward
to cal any VCO.  OTOH its sad to find that many VCO don't work very well
anyway...

H^) harry



On Mon, 31 Dec 2007 00:05:03 -0500, John Mahoney wrote
> At 11:32 PM 12/30/2007, Grant Richter wrote:
> 
> >The most accurate piece of test equipment you have, is probably your
> >ears.
> 
> Certainly the most important ones. :-)
> 
> Here's a little brainstorming:
> 
> A divide-by-four (sub-suboctave) circuit could provide a way to tune 
> the low and high ends of a 4 octave range against a reference tone.
> 
> How about using a digital keyboard as a tuning reference? Tune the 
> VCO until the beating (of VCO vs DCO) is minimized.
> 
> Peterson Strobotuners have like 0.1 cent accuracy, so there's 
> another potential tool.
> 
> None of the above would necessarily help one to calibrate a 
> MIDI-to-CV converter, though. Well, I said it was the storm of a 
> little brain. ;-)
> --
> john
> 
> (OMG, it's now the last day of the year, here!)
> 
> -- 
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Harry Bissell & Nora Abdullah 4eva