Re: [Discuss-gnuradio] Polyphase Clock Synchronization

[Tom Rondeau]

On Fri, May 17, 2013 at 12:37 PM, Nemanja Savic <address@hidden> wrote:
> Hi all guys again,
>
> After few posts and numerous tries to set up M&M sync block, I finally want
> to give up, and before I start with some kind of ad-hoc approach I want to
> ask for help to setup polyphase Clock Synchronizator.
>
> This block is very well documented (by the way, it looks like description of
> the block is shown twice in the property window, at least in my computer),
> but since I am not that much in polyphase filtering, i have following
> doubts:

Good questions.

> - As it is stated in documentation, there are two filterbanks, one for
> signal shaping and one for filtering signal derivative. My question is what
> is the meaning of TAPS property (taps of which filter?)?

Taps are the taps of the prototype filter. Since the other filter is
the derivative filter, you give the block a set of taps (think of it
as the upsampled RRC filter coefficients), and the block will generate
the derivative filter by taking the derivative of the taps you provide
it.

> - What is filter size? (Number of paralel sections?)

Each filter represents a different phase. The algorithm tries to drive
the derivative to zero, which will occur when you have the correct
filter phase. So the number of taps gives you the quantization of the
phase value. Think 2pi/N. You reduce ISI and timing errors by using a
larger number of filters as the expense of complexity and memory.
Generally, 32 should be enough and the get reduced gains by going
higher (unless you have some ridiculous dynamic range).

There is some math you can do to calculate how much error you'd
introduced for N filters and samples with B bits/sample, but I don't
know it off the top of my head (nor want to derive it this morning).

This would be a good topic to put up on the signal processing wiki page:
http://gnuradio.org/redmine/projects/gnuradio/wiki/signalprocessing

> - In which way to setup alpha and beta for best performances (based on my
> symbol rate or whatever)?

Unknown... small. Usually set beta ~= (alpha^2)/4. You'll just have to
play with this.

The new block takes just a loop bandwidth parameter, which is usually
some small increment around the unit circle. I tend to set it at
2pi/100. See this blog for more that:

http://www.trondeau.com/blog/2011/8/13/control-loop-gain-values.html

> As for reminding, I have 19.2ksps baseband signal, sampled with 100ksps.
>
> Thank you in advance
>
> --
> Nemanja Savić

Tom