Speaking of Sine Shapers

[Haible Juergen]

Hi Terry,

thanks for this profound message.
I see there is more to consider than just the topology of the waveshaper.
The distortion figure you gave (or that article gave) is quite impressive
for
a single stage approach !

JH.


> -----Original Message-----
> From:	Terry Michaels [SMTP:104065.2340 at compuserve.com]
> Sent:	Thursday, November 30, 2000 2:42 PM
> To:	Haible Juergen; synth
> Subject:	RE: Speaking of Sine Shapers
> 
> Message text written by Haible Juergen
> >That's what I expected.
> I only wonder why anybody would bother to use an external feedback loop
> in combination with a *discrete* pair, where emitters are freely acessible
> to add local feedback. 
> 
> JH.<
> 
> Well, it's more complicated than that.  The article had a lot of
> mathmatical analysis and computer simulations, the authors found a certain
> magic combination that produced really low distortion, it required both
> emitter degeneration and feedback.  Either one  by itself is an
> improvement
> over a normal differential pair, though.  
> 
> In order to get extremely low sine distortion, you need to start with an
> extremely precise triangle wave.  This is not easy, for example, a 3080
> driving an integrator is not precise enough to get sine distortion much
> below about 0.25%.   Because the 3080 uses a different current mirror
> arrangement for the current source and current sink, it outputs slightly
> different current levels when sourcing or sinking current, resulting in a
> triangle wave that does not precisely have a 50/50 duty cycle.  A descrete
> version of the 3080 will have the same problem, unless it is carefully
> designed and built, with lots of trims, etc.   Getting it to work well
> over
> a wide frequency range is even harder.  
> 
> I built a signal generator for my bench using this technique, but even
> with
> the above design and adjustments, the best I could do was about 0.04%,
> which is far short of what the article said was possible.   
> 
> I would guess you won't get anything even close to that number for the
> real
> world example of a synthesizer VCO, especially when the triangle wave is
> derived from the sawtooth by the saturating transistor trick, or full wave
> rectification.
>  
> Another minor problem with deriving a sine wave from a triangle wave with
> the above technique is, the sine wave is derived from the highly amplified
> the output of a saturating differential transistor pair, this process adds
> a noticable amount of white noise and shot noise to the sine, compared to
> the triangle wave you start with.  I could clearly see it on a scope when
> I
> ran the sine wave through a high pass filter in tandem with a notch
> filter,
> which is the way I measured the residual distortion.
> 
> Terry Michaels