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Re: phase locked loop SSTC



Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

I wrote: 
> How is the PLL coming along?
Jan wrote:
> I tried some things, but it seemed all to be rather troublesome,
> gave up the "simple" PLL circuit idea and settled for a slightly
> more luxurious one...
> ...89C52 microcontroller...has three 16b timers...
> ...two PLL ICs with identical timing components
> ...One locks to the uC's 50% duty output signal, the other to
> the (overdriven) TC base current...
> ...gives info about whether both freqs are in-tune or lower
> or higher...VCO input voltages compared...

Jan,

Why is it that you're always a step of two ahead of me in your
thinking?  I wondered how you were getting on with the PLL because
I've more or less ground to a halt with mine.  I've not built one yet
because I can't even get it to work properly on paper.  I'm still
waiting to hear from anyone who's successfully driving a TC from a
PLL source phase locked to the coil base current. 

I'd just got to wondering whether it might be better to synthesise
the drive waveform, with a uP in the feedback loop to figure out, in
real time, the 'best drive',  bit like an engine management system
does for a car, except 10^3 times quicker. Thus turning the
difficulties into an embedded coding problem.

But I see a problem with your approach. Your two PLLs generate
voltages from the drive and feedback signals, which you
then compare. But the snag is, the two voltages relate to the
frequencies of the two locked signals, and these *are already the
same* since they come from the same source.  It's the relative phase
of the two that counts.  The controller must choose (search for?) a
drive freq which gives zero phase angle between coil base current and
drive voltage.

It seems to me that if the uP were fast enough, it could synthesise
the drive square wave, and read the phase of the base current by
sampling a single digital input.  The uP would look at the relative
phase, and adjust its output square wave period accordingly.

Hardware-wise, that's much simpler - no external PLLs, etc, the uP
does it all.

Your controller would then be a black box, with a single digital
input (base current sensor) and a single digital output (drive wave).
It's then just a matter of sorting out good algorithms to go in the
middle.  Software PLL, with some overseeing logic to sort things out
during arcing.

Maybe in years to come, we'll all be swapping the latest TC
superchips...(Man! You should try the new Wagner 2002e, that really
drives! :)
--
Paul Nicholson
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