George,
I would recommend
that you start with the below referenced example symbol sync flowgraphs and
parameter settings and tutorials first, get them up and running well, and then tweak the
symbol sync block settings to your desired parameters/settings for your scenario:
- .grc examples:
You can find some symbol sync example flowgraphs with gnuradio
version 3.8.2 installed on usr/share/gnuradio/examples/digital/demod
- GRCon 2019
Slide Presentation: I developed the tutorial presentation at the
following link in order to help get others up and running quickly
with the Polyphase Clock Sync block including the RRC filter taps. See slides #8 and #10 for the
exact parameter settings that may answer your question. It was done
in gnu radio version 3.7.11, but the flowgraph and all its parameter
settings still work in version 3.8.2.
https://www.gnuradio.org/grcon/grcon19/presentations/NASA_Space_Communications_Network_Modem/
- GRCon 2020
Presentation Video: Daniel Estevez’s video tutorial uses the other
symbol sync block in gnu radio with the Gardner TED selection. You
can pause his video at about 1:01:24 with the symbol sync
“properties” box open and see all the exact settings.
You may also want to
check your symbol rate frequency offset setting between your modulator and the
symbol sync block and verify that the frequency offset is not too large for
the loop bandwidth of the symbol sync.
Hope the above is
useful,
Dave
Hello,
My goal is symbol timing
using the Polyphase Clock Sync block. I generated a QPSK waveform which I
input to the Channel Model block with epsilon set to 1.0005 (small
signal timing offset) and pass the signal to the
Polyphase Clock Sync. The Polyphase filter is designed to run with 32
arms and receive the tap coeff's from the RRC Filter Taps. The
parameters for RRC Filter Taps block are are set with defined variables
as follows:
Gain =1
Sample_rate (Hz): sample_rate
Symbol_rate (Hz): sample_rate/sps (where sps is samples/symbol and
defined)
Excess_BW: excess_bw
Num
Taps: 32*(sps*6*2) (designing for overlap of +/-6 symbols
with "sps"
samples/symbol)
1) When I run the GRC and look at the
constellation plot at the output of the Polyphase Clock Sync, all I see
is a dot at the origin of the I/Q plot.
2)
So I tried changing the number of taps to see if things change and only
when I try the number of taps equal to 32*11 do I get something that
makes some sense. I get data over the 4 quadrants
between +/-.04 +/-0.4j, spread out. Of course, it keeps smearing into
one.
Will appreciate any suggestions on how to
use the Polyphase Clock Sync block in conjunction with the RRC
Filter Taps for symbol timing.
Thank you!
Regards,
George