[sdiy] VCO that goes SHARP in highest octaves??

[Neil Harper]

On 7/14/22 19:02, Ian Fritz wrote:
> It would be interesting to compare the two separately, if you are so inclined.
>
> Ian

I had a chance today to do exactly this!

(cents)	C4	C5	C6	C7	C8
BEFORE	0	0	0	+3	+11
IAN	0	0	-1	-3	-5
PHIL	0	-1	-1	0	+2

each one required a slight adjustment to the scaling calibration.

Ian's solution (1/2 integrator cap, 1/2 reference voltage) returned the
VCO to the expected operation matching the TH version. I'm used to using
the HF-TRIM to null out this amount of error.

Phil's solution (4.7k at 137000's Iabc input) flattened the dip as well.
The results here might not be perfect, because I lost the original
integrator cap after modifying for Ian's solution and could only replace
it with a generic/noname 1000pF cap with no claims of being NPO/COG. So
maybe that could account for the +2 cent rise at C8?

Anyways, sorry for the bad science. (0805 components and a speckled
epoxy floor do not mix)

I gave Phill's solution another try on another oscillator:

(cents)	C4	C5	C6	C7	C8
BEFORE	0	0	0	+2	+8
PHIL	0	0	-1	-1	-2


This time, with the original NPO/COG cap in place, there was no rise at
C8 - although the initial rise was less.

Anyways, I'm really happy with both solutions - thank you Ian Fritz and
Phil Macphail for identifying the problem and coming up with solutions.


--
/// Neil Harper
/// Every Wave is New Until it Breaks