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>

Hi,

I'm trying to do a simple phase locked loop to keep the SSTC driver &
sec always in tune, schematic is 
 http://www.hut.fi/~jwagner/tesla/SSTC/PLL-schem.gif
for the PLL and the mains side is the half bridge as in
 http://www.hut.fi/~jwagner/tesla/SSTC/half-bridge.htm 
and half-wave rectifier mains as input.

But, well, something seems to be wrong with it... Has anyone else been
doing something like this? PLL coil?

If I got it right then in this circuit the frequency lock range ==
frequency tracking range? At least, testing with a square wave input
suggested this is the case... I was careful to make the upper freq vs
lowest freq (R1 and R2) ratio 1.8 so it shouldn't lock on to TC sec
harmonics (with a ratio >2 this could happen??).

The drive signal reacts just nicely to moving a hand towards the coil.
But, there's a changing phase shift in the signal, from about 40
towards 60 deg, so while the frequency is correct, the phase just
isn't. => mosfet switching losses go up and streamer output goes down.
In addition to this, slowly increasing the bridge input voltage keeps
everything in tune, but then most of the time if I try to draw an arc
to a screwdriver it just makes *zip*, one short spark, and then it is
totally out of tune and mains must be powered down and then up again.
Just to get a second small and unimpressive *zip*.

On occaison it gives out some wimpy 5cm streamers, though. But that's
about 1/6th of the output possible with manual tune... :o(

Any ideas whats going on here?

every help would be welcome!
thanks, 
 - Jan

PS: A small current sense transformer senses the current into/out of
the primary coil (half-bridge). Specs: 1 pri turn, 100 sec turns, 820
ohm resistor, coupled via 10nF to CD4046 input incl two back to back
6.2V Zeners. Then I'm using phase comparator II in the CD4046, which
should keep a 0deg phase shift between the sensed current and the
output drive signal, over the entire frequency range. Loop filter: 1nF
and 1.2kOhm


--
*************************************************
 high voltage at http://www.hut.fi/~jwagner/tesla