Re: phase locked loop SSTC

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

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

Malcolm wrote:
> If you apply one signal to the "D" input and the other to the
> clock ... An XOR gate fed from the same signals gives the degree
> of difference

Ah, I see what you mean,  but the trouble is, John has rectified
and smoothed the feedback signal, so this won't work.

John Tomacic wrote:
> I always operate my coil just below resonance or at resonance, so
> an increase in current amplitude would mean that the comparator
> would drive the VCO input voltage lower, lowering the frequency to
> compensate for the increase in current 

Thanks, I'm clear on that now.  That sounds a little delicate - if
the resonance of the coil ever fell below the VCO frequency...
I wondered if you were using some sort of control logic, eg
 1. Tune the VCO a tad in some direction
 2. Secondary current higher?   If not, change direction.
 3. goto 1.

Jan Florian Wagner wrote:
> ... phase locked loop seems to work if and only if the upper
> freq of the VCO is set to the unloaded reso freq of the secondary
> and the lowest freq is maybe 50kHz below this (300kHz coil).

How far above Fres can you set the upper freq before problems occur,
and what are those problems?  When you get things settled down, it 
would be interesting to see, by looking at the vco control voltage,
just how low the secondary Fres can dip during heavy arc loading of 
the secondary.

I've always built self-oscillating sstc, ie direct RF feedback from
the secondary, but I'm a bit reluctant to use this method at higher
power.  Hence, I'm keen to learn about how others are using PLLs.
Does anyone have any good links?

> not a "plug'n'play"-for-any-secondary thing like I had wished...

That would be tricky, for impedance matching as well as frequency
control.  I think there'd have to be some preset constraint on the
vco range to avoid it latching on to an unwanted mode of secondary
resonance.

Regards,
--
Paul Nicholson,
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