[sdiy] High frequency VCO designs

[Phil Macphail]

On 23 Sep 2014, at 16:08, "Needham, Alan" <Alan.Needham at centrica.com> wrote:

> Having played with a DDS (AD9850) I would have thought rather than clocking PIC from DDS (which still needs digital control), use the PIC to control the DDS, sine or square out, audio range not a problem.
> Mine did a single note as an organ tuner and the math allowed to less than 0.5 cent increments in pitch IIRC, but I can't see a huge problem with a bank of DDS chips controlled by a PIC, isn't vibrato just a rapid series of pitch changes?
> OR
> A PIC controlling a master DDS outputting something like 2MHz clocking a bank of audio rate DDS chips, each with a fixed division rate -  a TOG bank, only the master DDS need have the PIC send the 'vibrato' pitch sequence. TOG ratios and errors, equal or well temperament. Expensive but do-able!
> I suppose it depends on what the source and speed of vibrato is - voltage control?

Why not implement the DDS in the PIC? The PIC only has to have an interrupt, increment a register, and toggle an output when it overflows so you can set the interrupt rate to be pretty high. 
If that is a stretch for a PIC then a Cortex-M0 could certainly do this at a high enough rate (and it is significantly cheaper than an AD9850..).

> 
> 
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom Wiltshire
> Sent: 23 September 2014 13:04
> To: rburnett at richieburnett.co.uk
> Cc: synthdiy diy
> Subject: Re: [sdiy] High frequency VCO designs
> 
> 
> On 23 Sep 2014, at 11:40, rburnett at richieburnett.co.uk wrote:
> 
>> I'd vote DDS all the way, but a VCO it isn't unfortunately.
> 
> Fair enough. So long as it lets me apply a vibrato to the master clock, I don't think I care. I'm thinking about how you'd clone a 70's combo organ. So either you go with a TOG+dividers, or (better) 12 oscillators with dividers. Each note osc and its associated divider is a single PIC - giving the frequencies of the classic TOG. If you use separate crystals for each PIC, you can have the TOG sound without the locked-together pitches. If you use a master clock, you do lock the pitches together, but you can throw vibrato on the whole lot. Wider pitch modulation would be nice, but if a few semitones is as far as it goes, that's probably ok too.
> 
> I'll look into clocking a PIC from a DDS. Might throw up all sorts of things.
> 
> Tom
> 
> 
> 
> 
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