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

[Robert McGwier]

Eric Blossom wrote:
> On Thu, Mar 16, 2006 at 05:49:34PM -0500, Charles Swiger wrote:
>   
> > On Thu, 2006-03-16 at 14:39 -0800, Eric Blossom wrote:
> >     
> > > On Thu, Mar 16, 2006 at 05:24:26PM -0500, Charles Swiger wrote:
> > >       
> > > > On Thu, 2006-03-16 at 12:35 -0800, Eric Blossom wrote:
> > > >
> > > > Here's the only place they're used:
> > > >
> > > >     d_freq = d_freq + d_beta * error;
> > > >         
New frequency  =  Old_frequency +   GAIN_FOR_FREQ * measured-phase-error


> > > >     d_phase = mod_2pi(d_phase + d_freq + d_alpha * error);
> > > >
> > > >         

phase is ALWAYS computed in an NCO by phase = phase + freq so this says that

phase_new  =  (phase_old + GAIN_FOR_PHASE*measured-phase-err)  + 
NewFrequency.  SO  Something is amiss.  You just turned your second 
order phase locked loop into a first order loop.  It is NOT a typo.





> > > > It's way over my head but is d_freq supposed to be in the d_phase
> > > > calculation, 2nd line? phase is mod_2pi but freq can be a very big
> > > > number, like mod_2pi(100000 + 1.572849). That is I'm USING very big
> > > > numbers for max_freq and min_freq - don't suppose they're normalized
> > > > somehow.
> > > >         
> > > OK.  I can see why that would be a problem.  mod_2pi is optimized for
> > > the expected "close in case" (symmetric around zero), thus the phase
> > > isn't *really* getting folded down to [-pi,pi].
> > >  
> > > Try changing mod_2pi to make the bounds check and then compute the
> > > modulus if it needs to using division, floor, multiplication and
> > > subtraction. It's not cheap, but it'll probably compute the right
> > > answer.
> > >
> > >       
> > Uh, I just mutated the "+ d_freq" term out, guessing a typo, and got a
> > very well behaved organism. Maybe suitable for synchronous AM carrier
> > detection.
> >     
>
>   



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