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

Nazmul Islam mnislam at winlab.rutgers.edu
Wed Jun 27 11:17:44 EDT 2012


You are exactly right! We were using TX/RX antenna port in the WBX receiver
daughterboard. I get almost -170 dBm/Hz noise spectral density when I
select RX2. This is almost perfect!

Thanks! Your email increased my pathloss calculation range by 30 dB :)

Nazmul

On Wed, Jun 27, 2012 at 9:22 AM, Jason Roehm <jasonr at 3db-labs.com> wrote:

>  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.
>
> Jason
>
>
> 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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>


-- 
Muhammad Nazmul Islam

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