[sdiy] Upper frequency limit for VCO

[Scott Gravenhorst]

Peter Grenader <peter at buzzclick-music.com> wrote:
>What about tracking across the entire band?  Freq doublers I've found have
>limited response.  Getting one to maintain tracking, and at very least
>waveshape purity across the entire spectrum is dicey, no?

Agreed.  I think it's not something that can be made perfect, rather best you
can.  And I doubt that you can simply keep tacking on more doublers without
problems (offset and noise to say the least).  I don't see why frequency
tracking would be an issue at all (the circuits I've seen are simple opamp
based DC level-shifter-and-subtractor types), but harmonic content would be,
say if the switching point for a waveshaper were not accurately set, the
waveform would become asymmetrical and would deviate from the original VCO's
spectral content pattern.  I think this can be trimmed.

But those are the alternatives, start from the top, divide down and be stuck
with rectangular waveforms or start at the bottom and frequency double hoping
for the best with decent doubler design and adjustment.

I've seen a schematic that does successive approximation to make tri from a
square input.  How fast it is, I don't know.  And it's somewhat complex.  But
as I discovered with PLLs, the often loathed frequency matching slew time can
give interesting character to a sound while it's changing frequency.

>
>Scott Gravenhorst wrote:
>
>> 
>> If the idea is to replicate the original VCO waveform in all of the divided
>> outputs, wouldn't it be easier to use frequency doubling waveshapers and
>> start at the bottom instead of starting at the high end and using dividers?
>> I've seen frequency doubling circuits for tri and saw inputs.  AFAIK,
>> divider outputs are rectangular which doesn't lend well to waveshaping back
>> to saw or tri.  However, if you _want_ rectangular, that's easy to divide
>> and you could also investigate Walsh functions.
>> 
>> 
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>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.