Re: Phase Margin Question for Phase Locked Loops

radio913_at_aol.com
Date: 01/04/05 

Date: 4 Jan 2005 13:53:52 -0800

radio913@aol.com wrote:
> Tim Wescott wrote:
> > >
> > What were you using for a filter? In general a low-pass filter
will
> > have negative phase shift. I find it hard to believe that a
> practical
> > PLL loop filter would _add_ phase margin -- you're generally
engaged
> in
> > a battle between reducing phase detection noise and keeping a
stable
> loop.
> >

That other ascii diagram didn't turn out,
so let's just say it's a single-ended input
inverting integrator op-amp, your garden variety
active pll filter, with input R1=66k, feedback
R2=33k, and feedback C1=4.7uF ("+" input grounded).

We get -90 from the feedback capacitor, but then
+180 from the inversion, so it's a positive phase shift
overall:

This is a simplified version,
but if you put it in your simulator,
you should get a positive phase shift
 at around 10Hz.
 
 
 Slick

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