square wave to sinewave converter (link)

[gez]

Where you DO have problems with steppy sines, is using them as LFOs!

That's what I had in mind when I originally messed about with this approach (following RG's post in a thread about quad LFOs) , but I ended up running the thing in the audio range so that I could see better on the scope.  I came up with a far simpler way to do the quad thing so I'm not interested in using the stepped technique for that anymore.

However, as Ton mentioned, it would be cool for a little micro-synth with a graphic wave-shaping panel so I'll have a mess around with it some more sometime.  Having slept on it, I'm not too sure if I'd ever use it for a 'pure' sine wave in octave ciruits though as the parts count is a little excesive compared with what I'm doing at the moment (which works).

[Paul Perry (Frostwave)]

I've used a synth sequencer (the kind where you have sliders and step thru them to give  stepped voltage output) running at audio frequencies to give a variable waveform, and it is suprising how uninteresting it was.
OK, you can make the waveform whatever you like (within limits) but, because the harmonics are all locked together, it is pretty unexciting.
When you look at a 'real instrument' on a scope (especially a guitar), you notice the waveform is completely mobile as the harmonics change relative phase, which I suppose makes it less boring.
Those pictures in physics books of instrument waveforms have a lot to answer for!

[R.G.]

once you commit to a PLL, you might as well replace the VCO in the PLL with one that actually gives a sine anyway.

Yes, that works, but it does not give the ability to build up different waveforms with different harmonic content, and sine-out VCOs are not as easy to find with the wide range of the 4046 VCO - or at least they once were not. The MAX038 is pretty good.

OK, you can make the waveform whatever you like (within limits) but, because the harmonics are all locked together, it is pretty unexciting.

The neat thing about 4046's is that they're cheap. Use one 4046 for the base lock, generate the sine. Use a second 4046 and add a touch of noise or LFO signal to the feedback voltage to dither the base frequency and hence move the harmonics around. Add some harmonics locked, some dithered.