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Re: [Discuss-gnuradio] pll_refout_cc - finding optimum alpha & beta ??

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] pll_refout_cc - finding optimum alpha & beta ??
Date: Thu, 16 Mar 2006 12:35:42 -0800
User-agent: Mutt/1.5.9i

On Thu, Mar 16, 2006 at 03:04:31PM -0500, Charles Swiger wrote:
> I'm looking for clues on the transfer function for gr.pll_refout_cc()
> as to how to find optimum values for alpha & beta. 
> Using experimental methods: http://webpages.charter.net/cswiger/pll/
> ( script requires special versions of gr_pll_refout_cc.h,i )
> Interesting how, as the input signal is reduced in amplitude and
> disappears DOWN into the grass, the output signal remains constant
> and the grass comes UP to swamp it! Neat.
> Interesting tutorials: http://www.complextoreal.com/
> --Chuck

Chuck, I could have this totally wrong (the code definitely needs some
comments), but it's a second order control loop.  We're adjusting both the
estimates of the phase and the derivative of the phase (commonly
called frequency ;)) 

I believe that setting

  alpha = some value
  beta = 0.25 * alpha * alpha

will result in a critically damped control loop.
Try some experiments that maintain this relationship.


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