[USRP-users] spectral leakage ?!

Marcus D. Leech mleech at ripnet.com
Mon Oct 5 12:58:24 EDT 2015


On 10/05/2015 11:39 AM, Amina M. Fellan wrote:
> 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
>
It is expected with a BPSK signal for there to be somewhat more energy 
in the sidelobes than closer to the notional carrier
   frequency.


> *
> *
>
>
> ------------------------------------------------------------------------
> 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 <mailto: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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