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"?
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):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!LukasPS: 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:For the attenuator term, just assign it a NF (in dB) of (31.5 - TXGAIN).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?
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.
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