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

Jason Roehm jasonr at 3db-labs.com
Wed Jun 27 09:22:10 EDT 2012

Do you have the correct antenna port selected? I believe that RX2 is the 
default for the SBX. If you are injecting signal into the port that 
isn't selected, then you'll see ~30-40 dB of isolation in the front-end 
antenna switch. That seems to be in the neighborhood of the difference 
that you're seeing.


On 06/27/2012 01:51 AM, Nazmul Islam wrote:
> 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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> USRP-users at lists.ettus.com
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