[Discuss-gnuradio] DBSRX AGC?

[Robert Miller]

Hi,

I am running an experiment where I would like to monitor the magnitudes of a
bursty, constant-envelope signal using the USRP and a DBSRX receiver.  I am
seeing some quasi-periodic amplitude transitions and wanted to know if this
could be related to an AGC.  Why I believe it is AGC-related is because the
amplitude transitions occur ONLY on leading-edge boundaries (and never
intra-burst).  What I see magnitude-wise in Matlab based on data from
usrp_rx_cfile.py is something like:

_---_---_---_|||_|||_|||_|||_|||_---_---_---_|||_|||_|||_|||_---_---_|||_|||_|||_---
   
...     (magnitude vs. time --- please excuse my ascii art:)

where there are really only two magnitude levels ( ||| and --- , and
interburst 'silence' is denoted as _ ).  Furthermore, I ran another
experiment using the same bursty, constant envelope signal using 2 DBSRX
receivers in a single USRP (with the proper mux of 0x3210).  Both receivers
experienced the same phenomenon, however there was some more 'interesting'
behavior.  When the DBSRX on side A transitioned from high to low, the DBSRX
on side B transitioned from low to high.  This was the case for multiple
runs, and again the transitions only occured at leading edge times.  In
summary my questions are as follows:

1)  Is there an AGC in the DBSRX?
2)  If yes, can I turn it off?

Thanks in advance for any ideas, recommendations, or comments!
Rob

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