[sdiy] dspic VCDO

[ryan williams]

thanks,

the 30MHz crystal was actually a mistake. i didn't realize it was out
of range. but it isn't as far out of range as you thought. this dspic
has a HS xtal mode which handles 10-25MHz with PLL. it is actually
xtal/2 with PLL * 8. now the 15MHz xtal version of my crystal isn't in
stock anywhere so I haven't replaced it.

yes, now that its done and i see the power is much higher than i
thought it would be. I didn't consider this much because i've never
used a uC at these speeds. i'll have to look at the 33F series, but a
quick glance at shows me that i'd need a new PCB layout to try one
out. and some redesign since it runs at 3.3V. I'm actually considering
now to use the NXP LPC ARM chips. that is what i use at work and it
should get the current draw down to 30-40mA.

if/when I make another one, I want to have waveform banks with VC
control over waveform. I have at least proven the concept to myself:
good 1V/Oct with audio rate CV.

On Sat, Aug 16, 2008 at 5:57 AM, Csaba Zvekan <czvekan at gmail.com> wrote:
> Hi Ryan,
>
> Great work on your VCDO . I was wondering why you used a 30MHz crystal in
> your design with PLL ? I thought that 4-10MHz was the maximum clock allowed
> .  Do you really  need the 4x PLL to get the 120MHz?
> I don't think I need to tell you that the slower you clock the less current
> it's being consumed.
> But also have you considered the dsPIC33Fxxxx series with nanowatt
> technology ?
>
> Csaba
>
> On Aug 15, 2008, at 8:26 PM, ryan williams wrote:
>
>> hi all,
>>
>> i built a new module a VCDO programmed in a dspic w/16bit ADC. exp
>> conversion is done with a lookup table. 24bit tuning word and a 32bit
>> phase accumulator. it does FM, sync, and triangle output. the normal
>> waveforms are provided with analog waveshapers. i'm especially happy
>> with the sine shaper which uses the OTA with feedback method. it
>> tracks 1v/oct very closely. i also added a linear fine frequency
>> offset added after the expo conversion to set constant beating but
>> i've only built one of these so far. aliasing is not an issue since i
>> sample at 500KHz and have a 2nd order (100khz fc) at the output.
>> really the only issue i have is that it is a power hog. it uses 120mA
>> off the +15v  supply. most of that is the dspic since it runs at
>> 120MHz.
>>
>> all the documents and a few sound tests are posted on my website.
>> http://www.homebuilthardware.com/index.php/projects/dspicvcdo/
>>
>> -Ryan
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