RE: [Discuss-gnuradio] New digital modulation receiver in trunk

[Tom Rondeau]

> -----Original Message-----
> From: Dan Halperin [mailto:address@hidden
> Tom Rondeau wrote:
> > small increase in --costas-alpha. The main issue with these gains is the
> > different samples per symbol, which is corrected for mostly by changes
> in
> > the mu gain in the M&M loop. To keep the logic simple, I set costas-
> alpha
> >
> 
> Using the old versions of the code, I noticed that the SNR performance
> is MUCH MUCH MUCH MUCH better when 64M/bitrate is a power of 2. E.G.
> 125k (decim 512), 250k (decim 256), 500k (decim 128) work great but 200k
> does not. Is the above why?
> 
> Thanks,
> 
> -Dan

Yes, I'm sure these are related. It has a lot to do with how much the phase
rotates between samples, and the Costas Loop corrects the phase for every
sample; if the SNR is high and you have many samples per symbol, many of
those samples are going to have lower amplitude (not the maximum SNR you'd
see when just sampling at the symbol peak); the noise can easily push these
around, which would cause the Costas Loop phase detector to mis-trigger, and
the phase correction for this sample would be wrong.

The new method makes a decision on the sampling time to maximize the SNR,
then the Costas loop part uses this to make the phase and frequency
adjustments.

At least, I think this is what is happening.

Tom