Forum: GNU Radio Polyphase Clock Synchronization block add some noise to constellation

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8d8a4dc47ea64675b37d810212889b66?d=identicon&s=25 Irfan Ullah (Guest)
on 2014-05-08 12:08
(Received via mailing list)
Hi all,
   I am using Polyphase Clock Synchronization block of GRC for symbol
timing synchronization.currently I am doing simulations of QPSK using
channel modal block. Ployphase clock synchronizer block work properly
it will synchronize the timing offsets in the signal remove the ISI from
constellation but the problem is that it will add some noise to the
constellation so could some one tell me how to solve this problem.
The picture of my flow graph is attached with this email you can see the
parameters of polyphase clock synchronization I have used the
firdes.root_raised_cosine_filter() for filter design in this block.
C539637020fd56193dd6daec746c4a84?d=identicon&s=25 Tom Rondeau (Guest)
on 2014-05-08 15:54
(Received via mailing list)
On Thu, May 8, 2014 at 6:07 AM, Irfan Ullah <>

A few things. First, yes, there will be some noise added to the received
signal. The PFB clock sync block very specifically produces noise based
the settings. The default value of using 32 filters in the filterbank is
designed such that the aliasing noise it creates is below the
noise of a 16 bit number. The length of the RRC filter per filterbank
will determine how much ISI you will accept. You can trade these issues
between computational performance and signal fidelity.

From your flowgraph, my guess is that you have something wrong with your
filter taps and settings. The value of 44 for the number of filters is
strange. It feels like you think that that's the number of taps per
instead of the number of filters in the filterbank. The RRC prototype
filter (rrctaps, I think, in your flowgraph) should be the normal RRC
filter upsampled by the number of filters you use. So rrctaps will be a
really long filter, but the filterbank partitions it among the N
so each arm becomes a reasonable filter again.

Also, you are using 2 for the output samples per symbol. The Costas loop
works better off 1 sps as will your constellation.

Check out the documentation for this block to try to understand better
it's doing and how it works:

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