Sonsivri

Hello dear friends!

This circuit produces an electric signal without a definite frequency at J2, the so called "White Noise".
See https://en.wikipedia.org/wiki/White_noise (https://en.wikipedia.org/wiki/White_noise) for further information.
(https://i.imgur.com/Wk0JLQI.png)

How does it works?

We force Q1 to work at breakdown region of operation (transistor basics), putting a lot more of reverse voltage on its emitter compared to its base. The thermal noise generated on Q1 junction is amplified at Q2 and put to use.

You can use any NPN transistor you like, remembering each transistor produces a kind of different noise, and maybe requiring adjustments on its bias.
The gain for the amplifier stage is set at R3. I recommend no less than 100k. The higher value of R3, higher will be the gain.

In this configuration, at 13.8v, you can get about 3.5 to 4v peak-to-peak of noise.

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.pdf (http://hparchive.com/Application_Notes/HP-AN-57-2.pdf)
https://www.hpmemoryproject.org/an/pdf/an_57.pdf (https://www.hpmemoryproject.org/an/pdf/an_57.pdf)

I would rather see this as pink noise audio noise generator. It can have it's uses by all means