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>

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?

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?

Regards,
--
Paul Nicholson,
Manchester, UK
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