[USRP-users] Not getting white noise in a USRP N210

Harold Daniel Moreno Urbina haroldmk at gmail.com
Fri Sep 5 12:18:33 EDT 2014


Hello Marcus,

Thanks for you detailed explanation (are you a bot?).

I have been reading about LO leakage and direct conversion architecture and
I have found that other systems, configurations and hardware get similar
frequency responses... so it seems the perfect explanation, I have to study
more anyway :D.

I am wondering how will this affect actual measurements.

Thanks for the help.


2014-09-04 16:01 GMT-06:00 Marcus Müller <marcus.mueller at ettus.com>:

> Hi Harold,
> Thanks! Let me comment on this.
>
> On 04.09.2014 23:02, Harold Daniel Moreno Urbina wrote:
> > Sure Marcus,
> >
> > ----------------------------------------------------
> > uhd_fft.grc
> > ----------------------------------------------------
> >
> > center frequency 2.45 GHz: *http://imgur.com/mmsdO0v
> > <http://imgur.com/mmsdO0v>*
> Not too sure what to make of this, but I'll come back to it later
> >
> > center frequency 2.42 GHz: *http://imgur.com/K526vsc
> > <http://imgur.com/K526vsc>*
> This is the classical LO leakage: Incredibly sharp, centered at the RF
> frequency.
> >
> > ----------------------------------------------------
> > Custom USPR2FFT
> > ----------------------------------------------------
> >
> > center frequency 2.45 GHz: *http://imgur.com/3Ukz1F8
> > <http://imgur.com/3Ukz1F8>*
> >
> > center frequency 1.80 GHz: *http://imgur.com/CkZsFyN
> > <http://imgur.com/CkZsFyN>*
> >
> > center frequency 900 MHz: *http://imgur.com/MfVe34m
> > <http://imgur.com/MfVe34m>*
> >
> > center frequency 300 MHz (out of SBX used operational band):
> > *http://imgur.com/zSlv6PD
> > <http://imgur.com/zSlv6PD>*
> These look pretty clean to me, nicely located LO leakage. I don't know
> what exactly happened when you set the frequency to 300MHz, but UHD
> won't let you do that, because it's physically impossible, so it usually
> prints a warning saying what it tuned to instead.
> >
> > ----------------------------------------------------
> > Tried with other frequencies and sample rate
> > I looked to have a center frequency not a factor
> > of sample rate
> > ----------------------------------------------------
> >
> > Center freqency 452 MHz, Sample Rate 9 MSps: http://imgur.com/oLgXmqN
> >
> > Center freqency 2759 MHz, Sample Rate 9 MSps: http://imgur.com/edysiBb
> On the N210, sampling rates must be integer fractions of the 100MHz
> master clock rate.
> Most probably, UHD prints a warning telling you that it used an adjacent
> possible rate -- 10MHz or 9.0909..MHz or so.
>
> But that aside, that's very plausible.
> So tuning with a N210 happens in two steps:
> 1. The driver looks up a frequency that the LO synthesizer can
> physically generate, and configures it accordingly,
> 2. The FPGA eliminates the remaining frequency offset by multiplication
> with exp(2*pi*j*delta_f*n) digitally.
>
> So, for some frequencies, the LO is right in the middle where you see it
> in all of your screenshots, for others, that can not be the case.
> Actually, the synth of the SBX is quite flexible and you most probably
> won't be able to distinguish between the target frequency hitting a
> physically possible LO frequency, and the target frequency being right
> in the middle between two possible LO frequencies at your spectral
> resolution.
>
> So, this explains why the LO can be completely shifted out of your 10MHz
> bandwidth: just manually specify that you want to tune with an |offset|
> > bandwidth/2, and the LO will be out of band.
>
> Comparing the 452 and 2759 MHZ spectra reveals that they look really
> really much alike (which is really comforting to me) but that 2.7GHz
> spectrum is about 4dB lower -- which doesn't surprise much, considering
> that nothing analog really works exactly frequency-independent. So this
> is actually a sign for the USRP being a nice platform, and the SBX being
> a sensitive analog frontend with a wideband input with only little
> distortion.
> > ----------------------------------------------------
> > I used a second USRP N210
> > ----------------------------------------------------
> >
> > Center freqency 2759 MHz, Sample Rate 9 MSps: http://imgur.com/pwFFCPB
> >
> > Center freqency 2760 MHz, Sample Rate 9 MSps: http://imgur.com/NrBbugz
> >
> > Center freqency 2760 MHz, Sample Rate 10 MSps: http://imgur.com/2NTmi2f
> >
> > Center freqency 2760 MHz, Sample Rate 10 MSps: http://imgur.com/CZ4oHOF
> >
> >
> > I can see that when I use 10 MSps the signals is much more flat than the
> > signals captured with 9 or 11 MSps.
> That could be the CIC filter's rolloff that you can only see with odd
> decimations (10MS/p is 1/10 of the 100MHz master clock rate, 9.091MS/s
> is 1/11).
> Maybe that's again where Ian could throw in his experience
>
> Greetings,
> Marcus
>
>
> PS: regarding the first picture: yes, there are more peaks than the LO,
> but they seem to be so random and the current PSD seems so uncorrelated
> that I daresay that I cannot say anything.
> > 2014-09-04 14:33 GMT-06:00 Marcus Müller <usrp-users at lists.ettus.com>:
> >
> >>  About the pictures: Would you mind uploading them somewhere else? It
> >> really takes ages for me to see a single one, and it blocks parallely
> >> waiting for multiples, so I must admit I only saw the first two before I
> >> gave up. Maybe imgur.com?
> >>
> >> Greetings,
> >> Marcus
> >>
> >> On 04.09.2014 22:12, Ian Buckley via USRP-users wrote:
> >>
> >> Good point, I answered that very quickly without looking at the
> downloads. Do you understand Marcus's point about direct conversion
> architectures? It was perhaps more appropriate.
> >>
> >>
> >>
> >> On Sep 4, 2014, at 1:04 PM, Harold Daniel Moreno Urbina <
> haroldmk at gmail.com> <haroldmk at gmail.com> wrote:
> >>
> >>
> >>  But neither 2.45 GHz nor 2.42 GHz are factor of 100 MHz. The other
> frequencies used are (1.8 GHz, 900 and 300 MHz).
> >>
> >>
> >> 2014-09-04 13:53 GMT-06:00 Ian Buckley <ianb at ionconcepts.com> <
> ianb at ionconcepts.com>:
> >> Very likely it's a harmonic of the 100MHz clock used extensively in the
> N210.
> >> Try tuning to a frequency thats not a factor of 100MHz and see if it is
> still there.
> >>
> >> On Sep 4, 2014, at 12:24 PM, Harold Daniel Moreno Urbina via USRP-users
> <usrp-users at lists.ettus.com> <usrp-users at lists.ettus.com> wrote:
> >>
> >>
> >>  Hello,
> >>
> >> When using USRP N210 without an antenna I would expect to receive white
> noise, right?. But consistently I am getting signal at center frequency
> well over 20 dB compared to other frequencies and 30 or dB over the signals
> near the edge.
> >>
> >> Anyone knows what is happening here? Is something wrong in the USRP or
> in GNU Radio? or me? Thanks.
> >>
> >>
> >> I am using USRP N210, GNU Radio 3.7.2.1 (version of last friday) over
> Ubuntu 14.04.
> >>
> >> I uploaded some screenshots of the FFT of the received signal, always
> 10Msps and getting about 5 minutes of signals. I used the example included
> in GNU Radio uhd_fft.grc and a custom flowgraph icluding only a USRP source
> and a FFT sink.
> >>
> >> ----------------------------------------------------
> >> uhd_fft.grc
> >> ----------------------------------------------------
> >>
> >> center frequency 2.45 GHz:
> https://mega.co.nz/#!WFE3UTRZ!AO04usGbl9TBa5-mCUPz7FYW2IabY5nbo5zZqSWwaM4
> >>
> >> center frequency 2.42 GHz:
> https://mega.co.nz/#!Hc0UzBJA!fSfgScdXPJ_phKdIWUKcNseonlKQt3m64yvYMR0bDhs
> >>
> >> ----------------------------------------------------
> >> Custom USPR2FFT
> >> ----------------------------------------------------
> >>
> >> center frequency 2.45 GHz:
> https://mega.co.nz/#!Hd9RyApS!FzsY5yxr5e6dJaAAO7Ww2O4BFefyCk6MDPN6ffcOs_g
> >>
> >> center frequency 1.80 GHz:
> https://mega.co.nz/#!LQcTkDjb!NQ7rIe1PDgRFCp547_U3mhxxxzsvQilPV-esTo7nHa4
> >>
> >> center frequency 900 MHz:
> https://mega.co.nz/#!eZ1TFbSY!nkt-n1mlC54aFfDwvqoJIJd7-a7gMB2GBLLhyclxxcs
> >>
> >> center frequency 300 MHz (out of SBX used operational band):
> https://mega.co.nz/#!mJ8EESwR!lI_0r4RvvTncxwwMnpEVAvrwsVhcHzLX7H6C7TA4Cj0
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> >>
> >>
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