[sdiy] VCO tracking w/o counter

[Magnus Danielson]

From: Ian Fritz <ijfritz at earthlink.net>
Subject: [sdiy] VCO tracking w/o counter
Date: Sun, 28 May 2006 07:49:11 -0600
Message-ID: <6.2.1.2.2.20060528073907.03226a80 at mail.earthlink.net>

> 
> >On 2006-05-26 20:47 -0500, Paul Schreiber wrote:
> >
> > > You *cannot* tune a 1V/Oct input (VCO, etc) with a scope. You
> > > need a frequency counter.
> 
> I've done it without a counter, as follows:
> 
> 1.  Mix a 440Hz pulse (from a xtal-oscillator timebase) with the VCO signal 
> and feed it to a scope.
> 
> 2.  By looking for stable beating patterns, set the VCO to various 
> octaves:  110Hz, 220Hz, 440Hz, 880Hz, etc.
> 
> 3. Measure the control voltage required to get the VCO to each of these 
> frequencies.
> 
> 4. Adjust tracking controls as usual to get 1 V/Oct.
> 
> :-)

You cannot tune a 1V/Oct input (VCO, etc) sufficiently good with a scope
alone. But with a few handy extras, it make a wonderfull tool to tune it!

Having a very good reference, you can tune up old style atomic references
with a scope. The real trick is that the scopes timebase is really not the
time reference, rather just a relative time which doesn't really contribute
to the errors in the end. Trigger with the reference and display the
waveform of what you want to be calibrated. You quickly learn which way to
turn the trimmer to cause the speed of beating to go down and eventually you
pass a truningpoint when the pattern changes direction, then you move on
back a little and eventually it just slowly goes here and there but on the
average is stable. Now you are in tune.

Cheers,
Magnus