Re: phase locked loop SSTC

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

Original poster: "John Tomacic by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla_ownz_u-at-hotmail-dot-com>

Hi Paul,
Your question made me rethink the problem through and I realized that I made 
an assumption which may not always be true. 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.  However, if the 
VCO frequency is driven higher than the resonant frequency, the current 
signal will decrease, not increase, causing the lack of direction problem 
you mentioned.  In this situation the phase reversal associated with the 
transition from capacitive reactance to inductive reactance could be used as 
a direction signal (via the D FF Malcolm mentioned).  Sorry for the 
confusion, I guess I spoke too soon (not the first time, or last!)

John




>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: phase locked loop SSTC
>Date: Thu, 27 Dec 2001 07:49:18 -0700
>
>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?
>
>Regards,
>--
>Paul Nicholson,
>--
>
>
>




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