Re: Software PLL (SPLL)

"Noway2" <no_spam_me2@hotmail.com> wrote in message
news:1139263012.009629.210960@z14g2000cwz.googlegroups.com...
> Tim,
>
> That was a very nice explanation.  I have always considered software
> PLLs to be a bit of an enigma, though a future project goal of mine is
> going to require one.  In my case, the reference and the incoming
> signal will both be aprox 50 or 60hz sine waves.  For the PFD, I was
> planning on multiplying them and filtering the result which would be
> used to drive the NCO.
>


You need to watch the stability - ie phase margin. Make sure you sample at
about ten times the unity gain cross-over freq.  Your unity gain freq could
be around 12Hz - this should filter out the 2f at 120Hz. You may also need a
phase-lead compensator depending on the number of integrators you want (ie
for just the VCO you have one integrator - but you can add a phase-lag to
increase the slope). You need a Bode Plot in your head or on paper.If you
use a low-pass filter this will introduce phase lag and the amount depends
on the order of teh filter. The more you filter, the less 2f carrier
feedthrough but the more instability.


Tam