[sdiy] VCO Tuning goals

[cheater cheater]

Hey guys, some interesting posts! And some questions from me if you
would be so kind to enlighten me..

Ian, what are 'second order errors'? And for that matter, what are
'first order errors'? Is there a strict
mathematical/physical/electrical definition of that?

David, I suggest a footage select for the 0V frequency so that it can
be set where the oscillator should be performing the best. Maybe if
you want to only use very high pitches, set the footage very short,
and you're OK. Or if you want it to perform best at the bass range,
set it there. Generally, a lot of sounds are not going to happen
across the whole range of the keyboard, and will only be played in one
or two octaves.

With regards to the HF trim: isn't the diode a critical component?
What can happen when the diode is less than perfect?

Wouldn't it be possible to have multiple HF trims, say - two or three
- by using precision voltage comparators? Maybe one trimmer per
octave?

David, with regards to the integrator reset 'problem': are you
referring to the error introduced by the comparator being off by some
percentage, say A? In that case the output frequency will be 1/A f'
where f is the frequency we're trying to get. Which in turn means that
a simple offset voltage fed into the linear voltage input in the EC
would be good enough I guess?
And if so, then it's a problem only in the (quite probable) situation
when you want to run multiple oscillators from the same EC.

Then there could be the consideration of the switching lag in the
comparator, which I think might introduce a linear frequency shift and
then a small exponential freq shift if the switch speed depends on the
voltage across the comparator.. but that can't be bothersome until
we're in HF range, right? Definitely not in the audible range... I
think.. but that's just guessing, not backed up with any real
knowledge or experience =)

Thanks
D.

On Fri, Oct 2, 2009 at 9:52 AM, Ian Fritz <ijfritz at comcast.net> wrote:
>
>> My question was really meant to be more to the general
>> "best" way to approach tuning to get the most musically useful VCO.
>
> First, we have to understand that if the method used for high-freq tracking
> compensation could provide exact compensation, then it wouldn't matter what
> procedure was used to dial it in.  However the compensation scheme generally
> doesn't have the correct mathematical form for exact compensation, so there
> will be second order errors.  The nature of these will depend on the
> particular design.
>
> There are many ways to set up a VCO tracking, and you will probably get lots
> of ideas here.  My method (as described on my dial-a-tempco page) is as
> follows.  I have a little box with a pushbutton that puts out either zero or
> 1.0000V.  I connect this to the V/Oct input of the VCO.  Starting near the
> low end, I set the VCO to ~100 Hz and adjust the V/Oct trimmer for a correct
> octave (using the box).  Then I go to a higher frequency and set the HF
> Track to get a correct octave there.  Then back to readjust the100 Hz
> octave, then up to the HF octave again, etc, until both octaves are correct.
>  Then I measure all the octaves from 25 Hz up to 20 kHz and look at the
> deviation in tuning at each point.
>
> The tricky part is deciding what to use for the upper frequency.  This you
> might have to experiment with.  2 kHz or 5 kHz are reasonable places to
> start.
>
> Musically, what is objectionable about mistracking between VCOs is the beats
> they make against each other.  A 1 Hz beat frequency is (of course) 1%
> mistuning at 100 Hz and 0.1% mistuning at 1 kHz.  So tracking is most
> critical at the top end of the musically useful pitch range.  So there is a
> good argument for getting the best HF tracking at the 1kHz / 2kHz octave or
> the next one and forgetting about errors at higher frequencies.  OTOH if the
> oscillator is so good that it only requires a small amount of compensation,
> then it works fine to adjust the HF Track at a much higher point.
>
> HTH
>
>  Ian
>
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