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Subject: Re: vco preparation

From: "Dave Bradley" <daveb@...
Date: 1999-03-25

Scope? You don't need no steenkin' scope (or freq counter)<g>!

There are lots of good reasons to buy a scope, and I plan on doing so also,
but it's not really necessary for setting the tracking on a VCO. It will be
more useful to verify waveform amplitude and purity, DC offsets, etc.
Believe it or not, the most sensitive instrument for tuning and scaling is
the human ear.

I've calibrated dozens of VCOs over the years, all by ear. Use the same
CV->Midi convertor or V/Oct keyboard you plan to normally use to calibrate
it. If you are driving it from more than one source, you have to make sure
the sources track identically first, but that's another story.

The easiest thing to do is to take 2 oscillators, unhook 1 from pitch
control, hook the other through its 1V/Oct jack, play a note, and tune them
in strict unison (no beats) around 100 Hz or so. Now play 1 octave higher
and tweak the scaling depending on whether the interval is sharp or flat of
a true octave. Now play the lower note and tune them back together again,
and repeat. When you get that sounding in tune, play 2 octaves apart, then
increase the interval. Eventually you get to where they will not beat when
they are many octaves apart. Then you set the scaling on the first
oscillator the same way, except that you can allow them both to track the
keyboard voltage now.

It used to be that many oscillator designs had a separate adjustment for
tracking when you get into high frequencies. If so, it gets more complicated
since adjusting the hi freq tracking can affect the low freq tracking
somewhat. I'm not sure what current state of the art is.

Paul, does your design use a separate high frequency tracking adjustment?

Caveat 1 - you need a decent sense of pitch to do this, as in being able to
tell if one pitch is sharp or flat relative to another. If a tracking
oscillator plays beat free octaves with a fixed pitch oscillator, you're

Caveat 2 - Paul will probably have his own preferred procedure, and will
tell you to ignore all of this.

Dave Bradley
Principal Software Engineer
Engineering Animation, Inc.

> -----Original Message-----
> From: Gur Milstein [mailto:gur-m@...]
> Sent: Thursday, March 25, 1999 4:32 PM
> To:
> Subject: [motm] vco preparation
> From: Gur Milstein <gur-m@...>
> hi list.
> well its about time for me to prepare for the vco.
> i need to buy a scope but my question is how the tune caliberation process
> is gone be,is the scope is the best way to tune your vco,
> or do i need to go for a freq counter ?
> allso can you guys give some tips on what is importent when going
> to buy a second hand scope,and what is a good price for 50Mhz ,
> and a good manufecture name..... ?
> its gone be hard for me to find where
> i leave the old TEK scope as Paul once suggest.
> thanx
> Gur Milstein
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