Hi Dave, Thanks again for the help!
I followed your example to set the firdes filter parameters and my GRC flowgraph is now working,...Thank you! I must mention an observation, the I/Q constellation points after timing synchronization are around +/-0.8 on the real and imaginary axes and not +/-1. I know the filter gain scaling you use is correct. The gain must be equal to the number of arms in the polyphase timing synchronizer and I am using 32 arms like you so I set it to 32 (Fred Harris' work makes that clear). At any rate, I am not worried about this because I increase filter gain to a value greater than 32 to get +/-1 on the I/Q constellation axes. I have a question for you: Is there a reason why you scaled the values leaving the Polyphase Clock Sync before they entered the Costas Loop? Thanks for the great help! Regards, George On Tue, Mar 9, 2021 at 4:11 PM Dave Miller <dave.todd.mil...@gmail.com> wrote: > 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. > > https://www.youtube.com/watch?v=RDbs6l4rMhs > > 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 > > _________________________________________________________________________ > > *From*: George Edwards > *Subject*: Questions on Polyphase Clock Sync block > *Date*: Mon, 8 Mar 2021 16:21:22 -0600 > ------------------------------ > 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 >
