[Discuss-gnuradio] Burst FSK Receiver Clock Synchronization Problems

[Wilson, Ryan N.]

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

http://oshearesearch.com/2015/05/31/building-a-burst-fsk-modem-in-gnu-radio-with-message-lambda-blocks-and-eventstream/

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