# What is the noise figure of a signal generator?

I'm wondering what's the noise figure or the overall noise contribution of typical signal generators? For example, if there is a test system with a signal generator that outputs 3 dBm signal (50 ohms), an amplifier with 3 dB noise figure and 10 dB gain and a spectrum analyzer with lets say -140 dBm noise floor with 100 Hz RBW. How much does the signal generator raise the noise level and how much is the noise floor when the generator and amp are connected to the analyzer?

• How big is a 3 dB signal? How small is a -140 dB noise floor? Mar 4, 2020 at 10:38
• I mean, with those power levels, filters might be needed, but if I want to measure very low level signal, what's the noise contribution of signal generators... currently I can't access to the measuring devices so I can't do any measurements to check to situation..
– Hugo
Mar 4, 2020 at 10:42
• Your first step is to understand noise. Noise is basically a signal that is generated by any circuit which handles signals. Look up what "Noise figure" means, noise figure specifies how much noise is added to a signal that goes into a device and then comes out of that device, for example an amplifier. Does a generator take an input signal? So how can we talk about Noise figure then. a signal generator that outputs 3 dB signal Also study what dB means. A "signal of 3 dB" means nothing. 3dBm in 50 ohms does have meaning. Mar 4, 2020 at 10:44
• Sorry, forgot the m from dBm:s, so 3dBm in 50 ohms and -140 dBm noise floor...
– Hugo
Mar 4, 2020 at 11:00
• @Hugo: then you should edit the question accordingly.
– Curd
Mar 4, 2020 at 11:28

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.

• Thanks for the clarification, this helped a lot!
– Hugo
Mar 4, 2020 at 14:12

If the whole output of the signal generator is considered to be the wanted signal, the generator adds no noise. Well specified signal generators should have some data how much the signal contains noise and how it's distributed across different frequencies.

You should calculate the total noise power in watts, not in dBm:s. You should add together in watts what's coming from the generator and amplified, what the amp outputs (=theoretical thermal noise amplified by the gain and noise figure) and the equivalent input noise of the analyzer. All must be limited to the bandwidth in use.

Without knowing how much your generator creates noise as summed to the payload the calculations must be based on guesses.

Noise figure isn't useful quantity for signal generators as others have already stated in their comments.

I think that your question is academic. And if what you are really wanting to know is how much noise is contained in your Signal Generator's output I would use a spectrum analyzer with the ability to digitally noise blank the noise floor of the SpecAn then directly input your Signal Generator output into the SpecAn's input and see if it is clean or not. Perhaps this answers what you are wanting to know.

• Yeah, I Basically wanted to know the noise level of SG. I found out that one high end SG had 140 dBm/Hz noise. So with 10 Hz RBW it should be around 130 dBm in the screen of SA.
– Hugo
Mar 6, 2020 at 8:28