Re: phase locked loop SSTC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jan Florian Wagner by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jwagner-at-cc.hut.fi>


> Why is it that you're always a step of two ahead of me in your
> thinking?
;o)

> 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. 

The info I got out of my (few) PLL experiments said that it is not worth
the effort and grief. And it might even be an impossible task. Who
knows... Well, anyone is free to prove the contrary, of course... :o)

> 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.

Sounds familiar... ;o) I first had considered a serial DAC and some uC
with integrated DAC, to do exactly that. But the uC would've been way too
slow. Or then, >$200.

Also for the frequencies involved, any real-time uP or a DSP aren't
exactly cheap... 

But maybe a laptop PC with >300MHz and fast parallel port could stand
this task. And ok there probably are cheap data acquisition cards that can
do better than 500kHz sampling as well. If you are running plain old
Microsoft DOS (yes, it still does have its uses! :) on the laptop, then
you could have a real-time (maybe <10us response time?) system.

There are also PC programmable FIR filter boards ("download your own
filter") that might be helpful for a feedback-based, self-resonant driving
scheme. When you swap the TC secondary, just download a new filter to the
board memory and rock'n'roll! :o)
And because the filter function is just a simple rectangle, a small uC
would be sufficient for filter function assembly and download supervision.
No big PC or handheld Palm/iPaq necessary.

> 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.

Yes, the frequencies are the same. But, the duty cycles aren't! 

The uC output is 50% always.

But if the TC sec is driven at a freq somewhere around resonant, the
signal derived from the base current (amplfied,clipped=>square wave) will
not be 50%. The way the digital PLL ICs phase comparators work, the
non-50% signal will cause a different VCO input voltage than a 50%-signal
would.
At least this is what I found when I was fiddling around with the PLL ICs
and my never-in-tune PLL driver system... And that is the thing I'm trying
to use as an advantage in this uC controlled SSTC scheme.

If it actually works is still to be found out.

(The 89C52 + LCD + some ICs didn't cost much more than $20, so no prob if
it doesn't work. It might even be transformed into a full fledged home
automation system! ;o)

> 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! :)

hehe!! :o)
ROTFLMO!

Yeah, that of course would be something to hope for. An off the shelve 
always-in-tune SSTC driver certainly would have its benefits. Let's hope
you or I or someone else succeeds in doing something like that.


Well then happy coiling! And may god Thor be with you when pondering SSTC
driving problems! :)

cheers,
 - Jan

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