[sdiy] tri-sin shaper and THD

[Ian Fritz]

Hi Ryan --

Nice work and a great writeup!

I agree that by just adding feedback you don't get a huge improvement in 
distortion.

For the shaper I recently did I varied both the input drive and the 
feedback level until I had the best sine I could get.  I didn't measure 
THD, but a spectral analysis showed all higher harmonics down more than 45 
dB from the fundamental.

Best regards,

   Ian


At 02:44 AM 10/16/05, ryan williams wrote:
>that discussion of tri-sin converters and the mention of using feedback 
>around the OTA caused me to write this note to show myself how it works. I 
>posted it the other day, but I added some THD calculations to it and also 
>another version with negative feedback.
>
>http://www.sdiy.org/destrukto/notes/tri-sin_converters_using_otas.pdf
>
>for anyone that knows about THD calculation. take a look at it and see if 
>that is corrent (pg.4). Or if thats to much work, just tell me if this 
>sounds right: I took the sum of the FFT of the converted sine and divided 
>by the FFT of a pure sine then subtract 1. expressed in percent.
>
>I ended up with 1.87% distortion with feedback and 3.52% without. but the 
>one with feedback looks much better (the sharp corner is almost completely 
>gone). Those were the best numbers I could get.
>
>-ryan