[USRP-users] spectral leakage ?!

Amina M. Fellan afellan at hotmail.com
Mon Oct 5 11:39:29 EDT 2015


Thanks a lot Marcus and James, that fixed it :) 
I have one more question, when I transmit a BPSK signal with the benchmark_tx.py script I get the spectrum in the attached photo. As can be seen, there is about 5 dB dip of the received signal's power at the center frequency compared to the  center_freq +/- 25 KHz, is that normal ? 
Cheers,


Amina 

 





Date: Fri, 2 Oct 2015 10:24:07 -0400
From: mleech at ripnet.com
To: james.humphries at ettus.com
CC: afellan at hotmail.com; usrp-users at lists.ettus.com
Subject: Re: [USRP-users] spectral leakage ?!



You can use offset tuning to move the DC anomaly out-of-band.  Both in the TX and RX direction:

http://files.ettus.com/manual/page_general.html

All superhet TX/RX have some amount of LO leakage.  With direct-conversion (Zero IF), that leakage happens to be in-band.  So, you can make this more like a non-zero-IF design using offset tuning. There will still be leakage, but it will be out-of-band.

 

 
On 2015-10-02 09:55, James Humphries via USRP-users wrote:


Some LO leakage is typical with the system. When you say sometimes not necessarily at the center frequency, how far away from center is it?
 
-Trip



On Thu, Oct 1, 2015 at 7:53 AM, Amina M. Fellan via USRP-users <usrp-users at lists.ettus.com> wrote:





Hello everybody !
 
I'm using 2 USRPs N210 equipped with SBX daughterboards with GNU Radio to establish BPSK communication over ISM band. For the latter, I started with the benchmark_tx/_rx.py examples. 
 
currently I'm trying to measure the SNR at the receiver. I tried using the SNR estimation blocks in the GRC but they seem to be giving erroneous readings, so I decided to try with a spectrum analyzer to estimate the SNR using (S+N)/N. 
 
I noticed that even when there's no transmission going on, so trying to measure the noise floor, a signal appears on the spectrum and sometimes even not necessarily at the center frequency. This also can be seen using the uhd_fft script from GNU Radio. Attached is a photo of how the spectrum looks if the USRP is on without transmitting/receiving !!
 
Any clues what is causing this problem ?? 
 
Thanks for your time !

Cheers,
 Amina 
  








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