I am not very successful in running the uhd_corr_and_sync_tx/rx.grc example in the gr-digital examples directory (220.127.116.11) using usrps.
In an effort to debug it, i merged the two graphs and eliminated the uhd sink/sources (see attached) and so I am running a simulation without the usrps. I also eliminated the FLL block as I do not have any frequency error and not even any noise.
The behavior I see is as follows:
The transmission seems fine (a nice rrc spectrum followed by silence periodically on and off).
On the reception side the blocks seem to be working fine producing appropriate tags (one every 20Ksamples as expected) which means that the "correlate_and_sync" block works as expected.
When I plot the output of the "polyphase_clock_sync" I expect to see three clear points: +/- A and 0. Indeed this is what I see at first.
Now I do the following: I increase the noise for a while to a large number (~10dB) and then I take it back almost to 0 (-20dB). From that point on, although the "correlate_and_sync" block works as expected (I see the tag offsets back to normal) the picture does not contain three clear points but is blurred indicating that the "polyphase_clock_sync" does not work appropriately and I can never recover from this.
I was hopping that since the "correlate_and_sync" block recovers and adds the appropriate tags to the stream, the downstream block can recover as well. However the behavior is not as robust as I expected.
Can someone shed some light into this.