[sdiy] Square to Sine

[David G Dixon]

The "sine wave" being generated by that 3RC circuit is not very good, with 4
or 5% THD at best.  A sine wave from a good sine VCO would be less than 1%,
and a self-oscillating filter sine can be 0.1%.
 
Plus, this circuit only works at one frequency.  It would have to be
redesigned for other frequencies.
 
I just designed a circuit that works OK.  You simply take the square wave
through a linear VCA (like a linearized 2164) and into a Miller integrator.
The integrator must be slightly "leaky" to keep the resulting triangle wave
centred about 0V -- maybe a 1nF cap and a 1M to 3.3M resistor across the
feedback -- the bigger the resistor, the straighter the sides of the
triangle wave, but the slower the response.  Then, the gain of the linear
VCA must be proportional to the frequency of the square wave.  This can be
achieved with something like an LM331, or a simple charge pump circuit will
do (a couple caps, a diode, a transistor, a resistor).  The problem is that
the "homemade" F2V converter might not have a very good response time, so
the triangle wave's amplitude will be all over the map if the frequency
changes suddenly.
 
I'm still working out some quirks, but I'll report back when I have
something worth building.
 


  _____  

From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Elaine
Klopke
Sent: Tuesday, April 04, 2017 3:33 AM
To: synth-diy at synth-diy.org
Subject: [sdiy] Square to Sine


Does this actually work? 
http://www.learningaboutelectronics.com/Articles/Square-to-sine-wave-convert
er-circuit.php

I would assume that being 3 RC networks in series means quite the drop in
volume.

Also, in a related article they show two networks in series converting the
square wave to a triangle. 

All of the converters being dependent on the frequency of the input wave
being roughly the same as what the RC networks are tuned to. 

Thanks guys!

Sent from my iPhone

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