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,

On 27 Dec 2001, at 7:49, Tesla list wrote:

> Original poster: "Paul Nicholson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> 
> 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.
> 
> I wrote:
>  
> > 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?
> 
> Malcolm wrote:
> 
> > A digital approach can pull a "direction" out of the signal (a D FF
> > is all it takes).
> 
> Malcolm,
> 
> Well, I don't understand this. John doesn't seem to be giving any
> hints, maybe you can explain?  How can you tune the VCO with just an
> amplitude feedback and a comparator?

If you apply one signal to the "D" input and the other to the clock 
input, Q will be high or low depending on relative phasing of the two 
signals. I realize that because the frequencies are different, the 
phase will slide from lagging to leading (or vice-versa) but if the 
difference is not too great, the direction indication should last for 
long enough to enable locking. An XOR gate fed from the same signals 
gives the degree of difference.
      Kind of handwaving stuff from the digital realm but I thought 
it might be useful or applicable to the problem.

Regards,
Malcolm