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.
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