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>


> There are some nice (inexpensive(<$100)) DSP eval boards around which have
> audio codecs on them.  I don't know if the sample rate would be high enough
> though... If you want to do a PLL at the operating frequency, you'd
> probably need to sample at, say, 100 times the fundamental frequency: 100
> kHz times 100 is 10MHz sample rate. 

That would be the case IF you'd want to do the PLL _and_ waveform
generator fully in software...

An DSP certainly would be elegant, but, many uC's have some sort or other
of integrated square wave generators, t.ex. integrated PWM (in PICs and
AVRs). Most have _autonomous_ counters (PIC, 8051, AVR?), where you just
load in some 16bit value once, which specifies the freq. Then you can go
on executing other code - no uC supervision necessary. And no darn fast
DSP. :o)

> On the other hand, if you didn't need to actually control the waveform, but
> could closed loop control some parameter of the waveform generator (say,
> the period, phase, amplitude?), then, your control loop only would need an
> update rate on the order of the fundamental frequency (say, you wanted to
> update 2 times per cycle... I suspect that spark growth occurs on this sort
> of time scale).  

Hmm... Actually just a frequency comparator between the driver frequency
and the actual TC base current frequency would be sufficient. Then a
comparator with a bit of hysteresis could check if the freqs are "too
much" apart, cause an interrupt on the uC and give it the info whether the
drive freq is "too low" or "too high" and let the uC take an educated
guess on what the corrected drive frequency should be. After adjustment,
the uC can do other tasks, or just take a nap... ;o) until it is
interrupted by the frequency adjustment request again.

At least, that's like I was planning to program this thing.

(well, now finally my keyboard input works, and interruptor, and 50% duty
output... maybe it's in for testing soon...)

cheers,
 - Jan

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