[Top][All Lists]

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: Fri, 17 Mar 2006 10:16:42 -0800 User-agent: Mutt/1.5.9i

```On Fri, Mar 17, 2006 at 11:06:00AM -0500, Charles Swiger wrote:
> On Thu, 2006-03-16 at 19:55 -0500, Robert McGwier wrote:
>
> > 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
> > >>>
> > >>>
>
>
> Ok, I start to see - d_phase is an accumulator in (supposedly) mod_2pi
> bounds - so d_freq would indeed be the derivitive of phase (the steeper
> the phase, the greater the frequency) - and a 2nd order control loop has
> a proportional and a derivitive component. Then d_phase is converted
> to sin/cos for output.
>
> It just seeemd strange to me that a very large number, d_freq, which is
> bounds limited to between d_min_freq and d_max_freq, is inside a
> function trying to limit it's output to between +PI and -PI.

Chuck, let me try again.

The function has the right name, mod_2pi, but the wrong implementation ;)
These are commonly called bugs ;)  Hence my comments above about doing the
mod the right way.

> if(100e3 > M_PI)
>   return(100e3-M_TWOPI)
>
> or 99993.7168...   error can get close to zero, but frequency will
> never be less than d_freq_min.

Eric

```