[sdiy] VCDO using PIC16C771

[Tim Parkhurst]

On 2/21/07, jean-pierre.desrochers at ville.quebec.qc.ca
<jean-pierre.desrochers at ville.quebec.qc.ca> wrote:
>
> Maybe this has been already found
> on some listing but I thought it would be
> useful for anybody to share..
> To get rid of any temp problem on my new VCO design
> I'll use a Microchip PIC16C771 to control an Exar XR2206
> VCO's frequency.. The XR2206 has a VERY clean sine output
> and other waveform already available.
> (snip! )

> This is an embrio idea but I'll give it a try..
> J-P Desrochers
> Quebec City
>

Sounds like a neat idea. I proposed the same thing about a year ago
and got lots of "Why bother? If you've got a micro, why not just make
a fully digital oscillator?" However, I think you've hit on the main
advantage: you still have nice, analog waveforms. You can still use
analog waveshapers, you get ZERO aliasing distortion, and the whole
thing should still be inexpensive yet very stable. One drawback might
be that you will have a limit on the highest modulation frequency the
processor can handle (things like audio freqency FM might be limited,
depending on the maximum update rate of the A/D inputs).

The only enhancement I can think of might be to use a PLL to raise the
frequency of the 'reference' square wave fed back to the processor. A
higher reference frequency will mean that the micro can determine the
VCO's frequency that much faster, which makes for a quicker
compensation loop. Still, this may not be necessary.

Also, I would say that this will give you a Digitally Compensated VCO
(DCVCO?), as opposed to a Voltage Controlled Digital Osciallator, as
the actual oscillator core will be analog rather than digital.

Still, I think it sounds like a very cool enhancement for analog VCOs.
If done properly, it is something that could be added to practically
any VCO.


Tim (not done properly... more like over easy) Servo
-- 
"Imagination is more important than knowledge." - Albert Einstein