Re: [Discuss-gnuradio] atsc_cpll finally works

[Achilleas Anastasopoulos]

Chuck, Eric,

I think there is a way to perform the cPLL at 8 complex Msps and 
upsample to 16Msps only at the very end when you want to get the Real 
signal out for further processing. I believe this works (i didn't see 
any point where there is a possibility for aliasing) and it can result 
in some savings in complexity without sacrifising performance.
Hopefully I didn't miss anything big...

Take a look at the block diagram here:

http://www.eecs.umich.edu/~anastas/gnuradio/pll.png

Achilleas



Eric Blossom wrote:
> On Fri, May 23, 2008 at 09:00:31AM -0400, Achilleas Anastasopoulos wrote:
>
> > OK, I see.
> >
> > 1) Usually in fractional interpolators the trade-off is: more taps vs finer 
> > sampling. It might be possible to increase the number of taps and still be 
> > able to work with only a 12.8 (or 16) real-Msps.
> > Having said that, I have to admit I haven't looked at exactly what this 
> > particular timing loop is doing.
>
>
> The current resampler requires only 8 taps at each possible fractional
> point (1/128).  It manages this by having a frequency response that is
> unconstrained between Fs/4 and Fs/2.  That's the source of the
> constraint on sample rate and occupied bandwidth.  The filter design
> code is in gnuradio-core/src/gen_interpolator_taps.  See the README
> there for additional info and references.
>
>
> We could use a different fractional interpolator, perhaps the one by
> Julius O. Smith:  http://ccrma-www.stanford.edu/~jos/resample.
> With it, 12.8 real-MS/s or 16 real-MS/s should be possible.
>
> Eric