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
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