MOTM

I just finished 2 MOTM300 VCOs last night and spent some time playing 
around with the calibration.  I started at A440 using a very accurate 
frequency counter.  If I get the trimmer set exactly right for the 
octave interval between 440 and 880, the next octave which should be 
at 1760 is invariably a little flat at about 1758 with another octave 
up flatter still at around 3495 instead of 3520.  Going down from 440 
the octaves are very close.  If I adjust over several octaves I can 
get the 1760 and 3520 closer, but the 880 octave ends up a little 
sharp at about 882 and the 220 octave a little flat at 219 with 440 
right on.
This holds true for both oscillators, but one of them is a little 
worse on the high end than the other. 
I remember from my old Moog days that those oscillators tended to go 
off track as well in the upper range.  It seems to me that they had a 
trimmer for high end linearity which had limited effectiveness. 
Questions:
1. Am I being too picky?  
2. Should I just worry about the lower octaves and not worry too much 
about the 1760 and 3520, 7040 octaves?
3. Is it possible to improve the high end linearity by playing around 
with the value of R50 or is that asking for trouble?
Thanks to all for the help in advance.

> I just finished 2 MOTM300 VCOs last night and spent some time playing
> around with the calibration.  I started at A440 using a very accurate
> frequency counter.  If I get the trimmer set exactly right for the
> octave interval between 440 and 880, the next octave which should be
> at 1760 is invariably a little flat at about 1758 with another octave
> up flatter still at around 3495 instead of 3520.  Going down from 440
> the octaves are very close.  If I adjust over several octaves I can
> get the 1760 and 3520 closer, but the 880 octave ends up a little
> sharp at about 882 and the 220 octave a little flat at 219 with 440
> right on.

Ah, the age-old problem :)

The way I do it at the factory is set the VCOs for the best tracking where the *ear* most
sensitive. That is the range from 100Hz to 800Hz.

I use a 5 octave keyboard (starts at 'C') and set the 'C's for:

50Hz
100Hz
200Hz
400Hz
800Hz
1600Hz

I go back-and-forth between 100 and 800Hz, setting them to within 0.1Hz. At 800Hz, that is an
error of 0.01%. That's 10 TIMES better than any other VCO :) My freq counter is a Philips PM6666
with accuracy to 0.00001Hz, has oven-stabilized time-base.

The other intermediate frequencies (200, 400) are then well within 0.1% as well. Yes, you will
see errors "creep up" beyond 800Hz. But the *ear* (as opposed to the freq counter) is not nearly
as sensitive.

You can "warp" the response by using smaller values, like 2M instead of 2M2. If you are off say
more than 15Hz at 1600Hz, you may find lowering R50 will get you closer.

The reason HFT is not adjustable is that is a source of drift.

Also, what CV source is *driving* the VCO? It had *better* be within 50uV (yes, 50uV) if you want
errors less than 0.1% across the range. That requires a 6 1/2 digit DVM to calibrate :)

Heck, 5Hz off at 3520Hz is still <0.2% error. My Roland D-50 is about that, and it's DIGITAL.

Paul S.

I learned the hard way not to use a DCO synth as a tuning tool reference.
Yes, maybe they don't drift, but they are full of "compromises" in exact
frequency. At least my Juno 60 is.  Of course, that never bothers my ears.

----- Original Message -----
From: Paul Schreiber <synth1@...>
Heck, 5Hz off at 3520Hz is still <0.2% error.
My Roland D-50 is about that, and it's DIGITAL.