I have very similar versions of this noise source. They are a very basic design utilising a noise junction and MMIC amplification. The performance of the unit is dictated mainly by the MMIC amplifiers so take a look at their frequency response plots. They will not be flat ! “Flat” is a poor term to use anyway. There should be a +/- dBm figure across the stated operating range.
That said, my units have proved useful over the years and you can always normalise the noise generator output on an SA if using them in a frequency response test.
Professional nose generators are actually quite simple in terms of the circuits used, but there is good reason fir the, being very expensive. They have their output manually calibrated fir the required flat response. The cheap eBay units just rely on the MMIC specs to get a reasonably flat output over a range of frequencies. It is often possible to get a decently flat noise signal if you only use a small frequency range rather than trying to sweep the full 2GHz.
These are cheap and useful devices, but you need to carefully test and plot the output noise level over frequency if wishing to use them for anything approaching serious use.
If you want to learn about noise sources, the Noisecom web site was always an interesting read.
https://noisecom.com/I own several of their 1000 series noise sources and they look relatively simple in PCB design, but they are very carefully designed and manufactured devices.
https://noisecom.com/Portals/0/Datasheets/NC1000_Datasheet_Rev719a.pdfFraser
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