How Quiet ?
I am redoing all my Kiwis and antennas before the snow flies. I'm looking at grounding, CM etc. So I wonder if you have your Kiwi with no antenna attached, the input terminated with 50 or 75 ohms, what noise level do you see DC-30 MHz on the spectrum display? If you see discrete carriers, you have a problem. If the noise is flat across that whole range, probably OK. But what level do you read?
Using python3 kiwirecorder.py -s 10.0.1.71 -p 8073 -f **** m iq -L -5000 -H 5000 --s-meter 100
and a bash script that sweeps **** from 10kHz to 30MHz and subtracting 40dB to get to noise level in dBm in 1Hz results in the graphs below for two KiwiSDRs when running on a LiPo 5v output battery pack.
The spectrum display is perhaps not the best way to measure noise since it is an entirely different path with different limitations. I suggest using a known bandwidth, such as 10 kHz wide AM, making sure that it doesn't contain any coherent signals, reading the S-meter and then subtracting 40 dB to account for the bandwidth, as Gwyn describes. I think the spectrum display will give different answers depending upon zoom level and the corresponding bin bandwidth. I'm also not sure that it returns RMS noise as does the communications channel, though it may.