[sdiy] square to sine

Scott Gravenhorst music.maker at gte.net
Tue Oct 9 17:25:44 CEST 2012


nvawter at media.mit.edu wrote:
>Quoting Tom Wiltshire <tom at electricdruid.net>:
>
>> Hi Dave,
>>
>>> I'm looking for a good way to convert square to sine.
>
>I'm not sure about the details, but I would also consider using a PLL.
>You could use the VCO output of the PLL to drive a sine oscillator.
>That's 100% analog.  Although iiuc, PLLs have a tracking tag, but I don't
>know how severe it is, or how severely your signals will be changing.
>
>Also, does anyone have experiencing tracking square waves with PLLs?  How well
>do they handle it?  To improve the edges, perhaps you could also  
>estimate frequency through counting zero-crossings, drive the Fc of an  
>LPF with that.

I've done work with a PLL tracking a rectangular audio signal for musical purpose.  IMO, it
is musically useful, the delay in tracking is small enough as to be negligible in many if
not most applications (however, we don't really know yet what Dave wants to do with this).
 My particular system involves frequency multiplication (by 12 in my case) which
exacerbates the tracking delay - yet I did not hear portamento unless I set the loop filter
to do that specifically, so this is mitigated by proper loop filter values.  But in fact,
there is a delay, however small, so it depends on whether Dave's application will tolerate
it or not.  The use of a PLL will, however, require a fairly good voltage controlled sine
oscillator to attain the 0.5% THD he desires.

-- ScottG
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