[sdiy] Idea - Triangle wave DCO core

[Neil Johnson]

Hi Tom,

> It occurred to me today that you could fairly easily build a triangle core DCO. This wouldn't use a reset pulse, but would instead use a bipolar DAC.
>
> You'd simply output a charging CV from a DAC to an integrator. After half a cycle has been timed by the uP, it switches the DAC charging CV to the opposite polarity. And so on. Instant triangle wave at known frequency.
>
> Has anyone ever done/seen anything like this?
>
> Presumably once you'd got the basic version working, you could tweak the rise/fall CV calculation to get variable waveforms between ramp(ish) and triangle. Aside from that, I can't see any massive advantages beyond simplicity.
>
> What do you think?

You'd need some sort of servo mechanism around the integrator to deal
with any DC drift, otherwise you'll end up hitting one of the rails
sooner or later as DC terms get integrated over time.

A sawtooth is easy as you always reset to a known starting position.
With the scheme you propose there is no known starting position -- all
you're doing is changing direction at known points in time, but you
have some control over how far each segment goes.  The integrator will
integrate all the current flowing into it, which as well as the output
of the DAC will also include any op-amp offsets, noise, etc.  You're
also relying on the DAC to output EXACTLY the same magnitude of
current for both signs.  Unlikely in reality.

Analogue triangle cores don't suffer this as the timing and direction
is handled by level-sensitive comparators.  The handle you turn to set
frequency is solely the integrator current.  Since the amplitude is
set by the comparators then any DC offsets simply disappear.

Cheers,
Neil
-- 
Modules and more: http://www.cesyg.com
Homepage: http://www.njohnson.co.uk