Noise source circuits appear to be simple in operation, but there are much more to designing noise sources. Noise source can be used with a spectrum analyzer to characterize filter shapes as an example. For a noise source to be useful, it needs to be flat (+- 0.5 dB) over a wide frequency range. That is easier said than done.
No wonder if you look at any of the HP/Agilent/ Keysight noise sources, they are very expensive. It is not easy to get a source with a flat noise output over a 10 MHz to a couple of GHz. The noise outputs of calibrated noise sources are typically specified in dB ENR (Excess Noise Ratio). They generally also terminate into 50 Ohm. Once you know the calibrated value, you can do NF (Noise Figure) measurements on amplifiers, mixers, etc.
Since the noise level used for measuring noise-figure is often very low, you need to make sure that you eliminate any outside interferences that can corrupt the noise measurements. The noise level we are talking about is low enough that you can’t observe it on a spectrum analyzer since the noise output is often less than the noise floor of the analyzer itself. To observe it on the analyzer, you need to amplify the noise with a low-noise amplifier with gain around 40-60 dB first.
This subject can get mathematically very complicated and there are many articles and books dealing with noise written.
You can read more on the applications of noise sources, with measurement in the link provided.
http://hparchive.com/Application_Notes/HP-AN-57-2.pdfhttps://www.hpmemoryproject.org/an/pdf/an_57.pdf
