Re: phase locked loop SSTC

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Paul, Jan,

On 20 Dec 2001, at 6:55, Tesla list wrote:

> Original poster: "Paul Nicholson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> 
> Jan Wagner wrote:
> 
> >  An arc to a screwdriver looks like a short capacitive
> > discharge, and that's it
> 
> So the PLL goes into some sort of lockup.  Have you tried
> reversing the output terminals of the current transformer?
> 
> What happens to the vco frequency after the discharge,
> does the vco run off to one end of its frequency range to
> sit there with its arms folded?
> 
> I'd have thought you'd get better tracking of Fres with the
> current sensor in the secondary base lead.  Tried that?
> 
> > the current drawn at the primary coil doesn't look like a
> > sine wave. Instead it reminds more of a square wave at the
> > driver frequency, except it has a large ripple
> 
> What's the tuning range of the VCO?  You don't need a very
> clean waveform from which to extract the feedback signal,
> but it helps if the main component of it is a sine wave at
> the desired frequency!  Are you using a tuned primary or
> just a coil?  If you tune the VCO manually, can you find a
> tuning point that gives you sinusoidal current?  Tune manually
> and then with a dual beam scope, compare phases of the PLL
> output with the current sensor output.  Are they within, say,
> 10% or 20% of a cycle?
> 
> I've never built a PLL driver for a coil - but I'm thinking
> of doing just that.  I'm wondering whether a very wide loop
> bandwidth would be required, in order to track the sudden Fres
> change occurring when an arc is struck? 
> Thoughts anyone?

I calculated the loop filter TC to be somwhere around a uS which is 
pretty fast. I then wondered if it might not be too fast. As the TC 
is reduced, the filter ceases to do an averaging job.
  
> John Tomacic wrote:
> 
> > use a current sense transformer in series with the secondary
> > driving the magnifier coil (placed in between the secondary
> > and ground).  The signal from the sense tx should be rectified,
> > filtered then fed to a comparator.  Since maximum current is
> > drawn through the coil at resonance, the frequency of the driver
> > can be adjusted by the comparator through a vco to always keep
> > the coil in resonance.
> 
> John, can you help me understand just how the vco is tuned with
> this arrangement?  I don't see how, with only amplitude info
> from the sensor, the vco can 'know' which direction to tune, ie up
> or down?

A digital approach can pull a "direction" out of the signal (a D FF 
is all it takes).

Regards,
malcolm