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

[Ian Fritz]

Neil --

Good work troubleshooting. You've given us some important results.

I'd say leave the series Iabc resistor in -- just good practice. If you 
can't get it to track well enough, then add the smaller cap and 
reference current.

Ian



On 7/18/2022 4:17 PM, Neil Harper wrote:
> 
> 
> 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

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