Forum: GNU Radio Polyphase clock synchronization issue

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Irfan Ullah (Guest)
on 2014-05-13 17:14
(Received via mailing list)
Attachment: My_Flowgraph_pictutre.png (200 KB)
Attachment: constellation_with_osps_1.png (200 KB)
Attachment: constellation_with_osps_2.png (200 KB)
Hi all,

I have a problem here with polyphase clock synchronization block when i
the output samples per symbol to one the constellation have much more
and noise and when output sps value is 2 the constellation have no noise
and no ISI.I am using the firdes.root_raised_cosine filter design in
polyphase clock synchronization block with the follwoing parameters

Polyphase clock synchronization block parameters
filter size =32
firdes.root_raised_cosine(nfilts,1.0,1.0/(nfilts*sps), rolloff,
int(11*sps*nfilts))  with  sps=2  nfilts=32  rolloff=0.7

My flow graph picture is attached with this mail.
please tell me if there is any problem in my parameters (I m using QPSK
using the chunks to symbol block with const.points() function for mod
constellation decoder block with const.base() function in the
object parameter for demod)
If there are any errors in my flowgraph parameters please guaid me.

Irfan Ullah
Tom Rondeau (Guest)
on 2014-05-14 16:34
(Received via mailing list)
On Tue, May 13, 2014 at 11:11 AM, Irfan Ullah

> firdes.root_raised_cosine(nfilts,1.0,1.0/(nfilts*sps), rolloff,
> Irfan Ullah

You have the output SPS set to 2. Set that to 1. Basically, at 1 sps
you'll get the sampled symbol that you then pass to the Costas loop for
phase and fine frequency recovery. If you output at 2 sps like you have
now, then there's nothing that takes that down to the single bit or
sample that you want. The 2 sps output is specifically designed to go
an equalizer as the next stage, which will take 2 sps in and produce the
symbol out after equalization.

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