[USRP-users] Problems with setting gain

Sean Nowlan sean.nowlan at gtri.gatech.edu
Tue Apr 9 11:35:20 EDT 2013

I am having trouble with setting gain consistently on the WBX board. 
Sometimes it is set correctly as requested. Many times it is set to the 
minimum gain for WBX (0 dB) and occasionally it is set to the maximum 
(31 dB).

I've observed this problem in several of my own applications, but I also 
observed it using GNU Radio's benchmark_tx. Some examples:

./benchmark_tx.py -m cpm --excess-bw=3 --non-differential -r 2e5 -M 10 
-f 1.36e9 --tx-amplitude=0.2 --tx-gain=10
./benchmark_tx.py -m gmsk --bt=3 --non-differential -r 2e5 -M 10 -f 
1.36e9 --tx-amplitude=0.2 --tx-gain=10
./benchmark_tx.py -m bpsk --non-differential -r 2e5 -M 10 -f 1.36e9 
--tx-amplitude=0.2 --tx-gain=10

I verified that my GNU Radio application is producing samples with 
"sane" values, i.e., complex floats with I and Q in [-1.0, 1.0] and with 
typical values in [-0.2, 0.2]. The problem doesn't go away if I remove 
IQ & DC calibration files (mv ~/.uhd ~/.uhd_backup). Power cycling the 
USRP doesn't help.

My application follows the paradigm of a Python script generated by GRC. 
In the top block __init__() function I instantiate a uhd_usrp_sink 
object and set its freq and gain. Then I instantiate other GNU Radio 
objects and connect them. Finally in the main function I call start() on 
my top block. I called get_gain() after setting it in __init__() and 
verified it was set correctly. However in the main function when I call 
get_gain() again, sometimes the value is changed to 0 or 31. I can force 
it to behave by setting the gain again, but it concerns me that the 
value is getting blown away in some cases and I don't know the root cause.

My next step will be to upgrade to the 3.5.2 release and see if the 
problem goes away, but I wanted to get this message on the list as soon 
as I could.

USRP N200 r4
WBX r3
GNU C++ version 4.6.3
Boost 104800
UHD 3.5.0 (003.005.000-0-g5cb9779d)
GNU Radio 3.6.2

Thanks for your help!


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