[sdiy] tri-sin shaper and THD

[Ian Fritz]

Hi Ryan --

>>Several months ago I built a Sin shaper for the aux VCO in my chaos 
>>box.  It incorporated the suggestion by JH to add negative feedback to 
>>suppress the points at the peaks of the usual OTA shaper design.  I 
>>adjusted both the amount of input drive and feedback empirically to 
>>obtain the best looking and sounding waveform.
>
>your results look pretty good. Thanks for posting that. I wonder if you 
>tried using more trimmers (feeback and offset)? Or was this just not needed?

My procedure was to put in a series of different feedback resistors and 
each time adjust the drive for the best sine wave. It didn't take long to 
converge on 100k as giving the best feedback level.  As you can see from 
the spectrum, the second harmonic is reasonably weak, so I didn't bother to 
trim the offset.

>>After Ryan's nice analysis of this method, I had another look at my 
>>circuit, measuring the direct and feedback inputs to the OTA.  I found 
>>that these inputs were almost exactly the same as Ryan found in his 
>>analysis: 0.26 V from the direct input and 0.26 * 0.47 = 0.12 V from the 
>>feedback.  Note that this is fairly heavy feedback (~ 32%).
>
>wow, that's really close to what I came up with. Closer than I would have 
>expected.

I think it's fair to say we both got the correct answer. :-)

One thing to keep in mind, is that in an actual VCO the tri will not always 
be exactly the same at all frequencies.  So there is a point of diminishing 
returns when it comes to make a sine shaper.

   Ian