[sdiy] Roland DCOs

[jpdesroc at oricom.ca]

Here is my next DCO (or DVCO..?) project.. hope this could give some
ideas..
 
I want to build the DVCO around a 20mhz Microchip PIC.
I'm gonna read an external DC control voltage (1v/oct) using
the PIC inner 12 bits ADC, use the ADC read value to read a lookup
table 
period value that will generate a narrow pulse (16bits inner counter)
and discharge a continually charging cap
at the needed time period.
This will give a saw wave shape. Ok.
But the amplitude wil vary according to the wave period time.
So to correct that I'll charge the capacitor with a controlable
current source that will keep the amplitude the same for
all the DVCO range. A simple formula will calculate
the correct current to charge the cap with according to the
instantaneous
frequency period. Doing so I get a clean (no alliasing) very
temp/freq stable saw waveshape
of several octave range. The only thing that could vary with
temperature
is the wave amplitude but very little. I can do a wave shaping
further on
for all other wave shapes.
I hope I'm clear enough here.
Thanks for your time.
JP