Re: [USRP-users] UBX160 TX "noise figure"?

[Lukas Haase via USRP-users]

Hi Marcus,

 

Gesendet: Montag, 07. Dezember 2020 um 15:25 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 12/06/2020 08:33 PM, Lukas Haase 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.

 

This is a UBX-to-UBX loopback, or UBX-to-TwinRX loopback?   The gain ranges on the two are different.

 

 

It is UBX-to-UBX loopback (UBX-160).

 

Thanks,

Lukas

 

 

 

 

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