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 < usrp-users@lists.ettus.com> 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): > > > 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" <patchvonbr...@gmail.com> > > *An:* "Lukas Haase" <lukasha...@gmx.at> > *Cc:* USRP-users@lists.ettus.com > *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? > > > 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? > > > > > 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 > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >
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