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

[Mr&MrsAccount]

The series Iabc resistor also makes the OTA resistance to any failures in the expo converter, as well as a slipped meter probe.  If you keep the resistor 
physically close to the OTA pin it limits the chances of a failure or accidental short circuit to the Iabc pin and one side of the series resistor. 
  
With no possible current limit, and the Iabc referenced to negative supply that is OTA murder waiting to happen... 
  
(but I never knew there was a better reason to include it.... :^) 
  
Harry 
  
   

-----Original Message-----

From: Ian <synth-diy at synth-diy.org>
To: Neil <metadata at gmx.com>; Phil <phil.macphail at liivatera.com>
Cc: synth-diy <synth-diy at synth-diy.org>
Date: Tuesday, 19 July 2022 12:54 PM EDT
Subject: Re: [sdiy] VCO that goes SHARP in highest octaves??

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

-- 
ijfritz.byethost4.com
https://www.youtube.com/user/frijitz001
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