[USRP-users] Getting a -135 dBm/Hz noise spectral density in USRP N210

Nazmul Islam mnislam at winlab.rutgers.edu
Wed Jun 27 01:51:14 EDT 2012


Hello,

I am trying to measure channel path loss using two USRP N210. I need to
measure the noise floor so that I can find the threshold for pathloss
measurement. I am using the attached flowgraph to calibrate the receiver
signal strength. I tried two ways to calibrate the receiver power and both
show me approximately -135 dBm/Hz noise spectral density.

1. At first I don't transmit anything and measure the power of the floating
point numbers that is coming out of rx USRP block. Then  I transmit a PN
sequence of -5 dBm power at 17 MS/s from an USRP. I connect this USRP
through a 60 dB attenuator to the receiver USRP. This input (-65 dBm) seems
to double the power of the floating point numbers. I infer that the my USRP
noise floor is equivalent to -65 dB at 17 MHz.

2. I also apply an external 1 MHz noise source of -75 dBm power to the
receiver USRP. This leads to an increase of 3 dB power in the receiver FFT
plot. This also suggests I have -75 dBm noise power at 1 MHz.

Both these experiments suggest -135 dBm/Hz noise spectral density in my rx
USRP. However, this is very far from theoretical noise spectral density
(approximately -170 dBm/Hz). My Rx USRP N210 has a WBX daughterboard and my
Tx USRP has an SBX daughterboard. The ettus websites suggest only 6 dB
noise figure (http://code.ettus.com/redmine/ettus/documents/16). If my USRP
daughterboard is not faulty, I must be giving some wrong parameters. I put
the RX gain as 30 dB to get the full range of WBX daughterboards (
http://files.ettus.com/uhd_docs/manual/html/dboards.html). Shall I increase
it more?

Thanks for your help.

Nazmul


-- 
Muhammad Nazmul Islam

Graduate Student
Electrical & Computer Engineering
Wireless Information & Networking Laboratory
Rutgers, USA.
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