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From: | Wilson, Ryan N. |
Subject: | [Discuss-gnuradio] Burst FSK Receiver Clock Synchronization Problems |
Date: | Mon, 7 Dec 2015 18:31:57 +0000 |
Hi all,
I am currently attempting to get a bit stream from a captured RF signal that I know is from a burst FSK transmitter( 433.92 MHz carrier, 10kHz deviation, Manchester). Using GNU Radio 3.7.8.
I have isolated the signal and would say I have successfully demodulated the signal. I am not sure of the bit rate but approximated it from the preamble and then ran the signal through a matched low-pass filter (I approximated the bit rate of 20k) giving me this output below:
https://www.dropbox.com/s/jubmjxt0y4lrg95/200ksamp_quad_demod_lpf_20khz_cutoff_output.png?dl=0
It is with the clock recovery/binary slicer that things are not going my way. I have been digging a lot and see people have questions about the M&M clock recovery. I have found some paper and blogs that I have followed which seem to be working with signals very similar to mine:
http://www.inguardians.com/pubs/GRC_signal_analysis_InGuardians _v1.pdf
https://nccgroup.github.io/RFTM/fsk_receiver.html
I have also read many threads from the gnuradio mailing list and it seems like the M&M clock recovery works the best with around 2-4sps (I have not adjusted any of the other values from this block at this time). The amplitude of my signal is between +/- 1 going into the clock recovery .So I have
decimated to give me a sampling rate of 100k and 5sps and ran my demodulated signal into the M&M clock recovery block: https://www.dropbox.com/s/o1eyhmgpzsaaghf/mm_clock_recovery_5sps_output.png?dl=0
I am not sure why it looks so bad. So then I found others recommending the polyphase sync block instead of the M&M clock recovery block. I have tried loop bandwidth between 2pi/100 to 2pi/200, 5 sps, and low-pass filter taps with 20khz cut-off with similar results:
https://www.dropbox.com/s/oqbnqar7cvfi9tf/pfb_5sps_output.png?dl=0
I do not know much about the polyphase sync block and I could have not configured it correctly. And now that is why I am worried that I may have something else misconfigured. Here is my flowgraph:
https://www.dropbox.com/s/yc530rjs7vqvmn6/fsk_demod_flowgraph.png?dl=0
I just ran into the work that was done for better support burst modems (which is what my transmitter is): https://gnuradio.org/redmine/projects/gnuradio/wiki/Hackfest1310
So I will be attempting to get my flow graph running with this FSK burst modem and report back with results. But any other input or recommendations would be greatly appreciated.
Thanks,
Ryan Wilson |
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