I'd like to understand better how to measure a noise figure. I've been measuring noise with RMAA which I think is similar to REW. It displays the noise vs frequency and a single dBA number.
For instance,
here I measured the noise with RMAA. I had my sound card plugged in to the input and output (and the green line shows the soundcard at -140 dB). I was bringing the soundcard send level down 42 dB and the preamp was adding 42dB gain.
How is a single noise figure calculated from the noise vs frequency data? For unweighted, it it a RMS average, in this case probably about -105dB? Subtracting the gain from this would make it -147dB, which doesn't seem realistic.
And to find an EIN does the input need to be terminated with a 150 ohm resistor?