I think you’d mentioned you have a spectrum analyzer. What does it have to say about the SNR of your signal?
Sent from my iPhone > On Dec 8, 2020, at 2:41 PM, Lukas Haase via USRP-users > <[email protected]> wrote: > > > Hi Marino, > > Thank you for confirming the -139dBm/Hz. Indeed, with my sampling rate of 5 > MSps, this is -139+dB10(5e6)=-72dBm, which was exactly my prediction from my > initial message. > > For the RX line noise floor, I am sure it's the usual -169dBm/Hz (5dB NF) ... > at least if I disconnect the TX->RX connection, that's what I get. > > Thanks again for your input! If you could think of anything else I could try > to narrow down things please let me know. > > Thanks, > Lukas > > > > > Gesendet: Dienstag, 08. Dezember 2020 um 03:29 Uhr > Von: "cyberphox" <[email protected]> > An: "Lukas Haase" <[email protected]> > Betreff: Re: [USRP-users] UBX160 TX "noise figure"? > I work out the TX noise floor is around -139dBm/Hz at roughly similar step > attenuator setting, this results in similar Snr you predict > > Your setup seems fine unfortunately at the moment I can’t think of what else > at the moment. > > Are you sure about the RX line up noise floor? > > Will keep head scratching ... > > Have a good day! > > Marino > > > >> On Mon, 7 Dec 2020 at 18:00, Lukas Haase <[email protected]> wrote: >> Hi Marino, >> >> I don't think it's a BW/RBW related issue ... My measurements (noise, signal >> etc.) are pretty consistent otherwise. >> I think I am doing things right in my setup. >> >> >> That's why I am asking if maybe something is wrong with my original question >> about the "TX SNR" of the USRP. Maybe our conclusions on the noise figure in >> the TX are wrong? >> >> Thanks, >> Lukas >> >> >> >> Gesendet: Montag, 07. Dezember 2020 um 04:47 Uhr >> >> Von: "cyberphox" <[email protected]> >> An: "Lukas Haase" <[email protected]> >> Betreff: Re: [USRP-users] UBX160 TX "noise figure"? >> The power remaining from the phase noise skirt contributes but even without >> it as you show is still quite a lot different. It’s not a measurement >> bandwidth/rbw related issue? >> >> >> >>> On Mon, 7 Dec 2020 at 08:56, Lukas Haase <[email protected]> wrote: >>> Hi Marino, >>> >>> Ok, so when my TX is set to 20dB and the full range is 31.5, then the >>> attenuator should be set to 11.5dB. >>> >>> How I get the information from my plots: >>> The SNR=38.7dB value is taken by taking the DFT and setting all bins but >>> the signal bin to zero and calculating 20*log(std(ideal)/std(ideal-orig)). >>> Signal period is integer multiple of sampling period and leakage does not >>> exist (at least to the degree it could be relevant). Of course, this SNR >>> includes everything, incl. phase noise skirt >>> The orange signal is generated by adding white Gaussian noise to the ideal >>> signal such that it has XdB SNR. I sweep over multiple SNR values until I >>> get visual correspondance. From my plot you can see that the white noise >>> floor is this signal (yellow) matches the received signal (red). The SNR >>> for the yellow signal is 50dB. Therefore I can conclude that the SNR of the >>> signal is 50dB if I only take thermal noise into account >>> Yes, I have measured the noise floor with signal off. It matches exactly >>> with my expectations (-102dBm at RX gain=20) >>> >>> Thanks, >>> Lukas >>> >>> Gesendet: Montag, 07. Dezember 2020 um 03:46 Uhr >>> >>> Von: "cyberphox" <[email protected]> >>> An: "Lukas Haase" <[email protected]> >>> Betreff: Re: [USRP-users] UBX160 TX "noise figure"? >>> Hi Lukas, >>> >>> Thanks for your reply! >>> >>> I was meaning the attenuator of the TX before the amplifiers. >>> >>> Also from your plots were do you take your noise floor from? Are you >>> reading from the top of the phase noise skirt? >>> >>> Have you measured the noise floor with signal off? >>> >>> Also it is worth doing this measurement with different power levels just in >>> case you have some non-linearity issues >>> >>> Kind regards >>> Marino >>> >>>> On Mon, 7 Dec 2020 at 08:33, Lukas Haase <[email protected]> wrote: >>>> Hi Marino, >>>> >>>> Which digital attenuator do you mean? >>>> >>>> The 46.43dB attenuation that I use in the TX-RX loopback is just composed >>>> of static, passive attenuators >>>> The receive gain is set to 20dB. Hence I assume I do not make use of the >>>> 6dB "digital gain" in the ADC and from the remaining 31.6dB gain I use 20. >>>> Hence I the "digital attenuator" should be 11.5dB if that answers your >>>> question? >>>> At a receive gain of 20dB, the noise figure is ~5dB, according to specs >>>> and measurements. -174+5+dB10(5e6)=-102dBm which is exactly what I measure >>>> without a signal applied. >>>> Thanks, >>>> Lukas >>>> >>>> Gesendet: Montag, 07. Dezember 2020 um 03:23 Uhr >>>> Von: "cyberphox" <[email protected]> >>>> An: "Lukas Haase" <[email protected]> >>>> Cc: "Marcus D. Leech" <[email protected]>, [email protected] >>>> >>>> Betreff: Re: [USRP-users] UBX160 TX "noise figure"? >>>> Hi Lukas, >>>> >>>> What setting do you have the digital attenuator set to? >>>> >>>> >>>> Kind regards >>>> >>>> Marino >>>> >>>> >>>>> On Mon, 7 Dec 2020 at 02:05, Lukas Haase via USRP-users >>>>> <[email protected]> wrote: >>>>> Hi Marcus, >>>>> >>>>> Thanks again! >>>>> >>>>> I did now the following experiment: I connected TX to RX back-to-back >>>>> with 46.43dB attenuation in between. I set TX gain and RX gain to 20dB >>>>> and transmit a single CW at -3dBFS. >>>>> This means my output power is Pout=11.44dBm (cross checked with spectrum >>>>> analyzer) and on RX I sould have Pin=-34.99dBm. Indeed, calculating the >>>>> RMS of the received signal and converting to dBm, I get Pin=-35.0224dBm. >>>>> Spot on! >>>>> >>>>> The red line is what I receive on the PSD (blue is the TX that I send): >>>>> >>>>> <mime-attachment.png> >>>>> >>>>> As you can see from the annotation, the measured "SNR" of the received >>>>> signal is only 38.7dB. I think this is mainly caused by the phase noise >>>>> skirt (and potentially the I/Q image). >>>>> In order to keep only consider thermal noise, I add random noise to the >>>>> original CW (using randn(...)+1i*randn(...) in MATLAB) until it matches >>>>> roughly the white noise floor of the received signal. It's SNRoutput=50dB >>>>> (yellow line). >>>>> >>>>> Now, according to our discussion below, at Gtx=20, we should have >>>>> SNRoutput=72dB (assuming thermal noise only). >>>>> >>>>> Where could the 22dB difference in SNR come from? >>>>> >>>>> Thanks! >>>>> Lukas >>>>> >>>>> >>>>> PS: I am aware of phase noise, DC offsets, I/Q imbalance etc. But as you >>>>> can see from my plot, I am only considerung thermal noise. The thermal >>>>> noise of the receiver should be orders of magnitude lower (at least >>>>> -102dBm) so the receiver noise should not limit the results either. >>>>> >>>>> >>>>> Gesendet: Montag, 30. November 2020 um 17:08 Uhr >>>>> Von: "Marcus D. Leech" <[email protected]> >>>>> >>>>> An: "Lukas Haase" <[email protected]> >>>>> Cc: [email protected] >>>>> Betreff: Re: [USRP-users] UBX160 TX "noise figure"? >>>>> On 11/30/2020 01:54 PM, Lukas Haase wrote: >>>>> Hi Marcus, >>>>> >>>>> That makes sense, thanks. >>>>> >>>>> Would you be willing to confirm if what I am doing here is correct? >>>>> >>>>> <mime-attachment.png> >>>>> >>>>> To first order, the DAC has an SNR of 98dB (16 bit). Then I use Fries' >>>>> equation to get the NF of the following stages (for the filter and the >>>>> attenuator, the noise figure is equal to its attenuation). The NF is >>>>> dominated by the 2nd and third term. >>>>> Then I subtract the NF from the SNR which gives me an output SNR >>>>> somewhere between 92dB and 67dB. Does that sound right? >>>>> >>>>> <mime-attachment.png> >>>>> >>>>> >>>>> For the attenuator term, just assign it a NF (in dB) of (31.5 - TXGAIN). >>>>> >>>>> The noise figure of an attenuator is just the attenuation >>>>> value--similarly for the filter. Just pretend it's a fixed attenuator >>>>> with 0 gain. >>>>> >>>>> So the 'noise figure' after the DAC is just 2+(31.5 - TXGAIN) then >>>>> factor in the gains and noise figures of the amplifiers. >>>>> >>>>> >>>>> >>>>> _______________________________________________ >>>>> USRP-users mailing list >>>>> [email protected] >>>>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > _______________________________________________ > USRP-users mailing list > [email protected] > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
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