What is the noise figure of a signal generator?
How much does the signal generator raise the noise level


What noise level?

The Spectrum Analyzer (SA) has a noise level of -140 dBm, but that doesn't change. What this -140 dBc means is that you cannot measure below that level, it is the limit of your SA.

Your signal generator (SG) will also have a noise level (look that up in its datasheet). Let's assume it is -90 dBc (90 dB below the signal). Your signal is + 3 dBm so that means that the noise is at -87 dBm.

The 10 dB amplifier amplifies both signal and noise by 10 dB. On top of that the amplifier will add 3 dB of noise (NF = 3 dB).

So the signal at the output of the amplifier will be +3 dBm + 10 dB = +13 dBm (note how I can just add a signal level in dBm and a gain in dB).

The noise at the output of the amplifier will be -87 dBm + 10 dB + 3 dB = -74 dBm (the 3 dB is from the noise figure).

So at the Spectrum Analyzer will just show a + 13 dBm signal with a -74 dBm noise level. That -74 dBm noise level is much higher than the -140 dBm noise floor of the SA so the noise floor of the SA can be ignored.