Use a low-cost noise source as a replacement for a tracking generator
https://www.edn.com/use-a-low-cost-noise-source-as-a-replacement-for-a-tracking-generator/?utm_content=buffera18c1&utm_medium=social&utm_source=edn_facebook&utm_campaign=buffer

A noise source is simply a circuit that produces a very broad band of “shot noise” energy over a range of frequencies. This one is advertised to cover 200 kHz to 2 GHz and is available from eBay for $29 [2]. The circuitry is mounted in a sturdy aluminum box with 12-V power, an LED power indicator, and SMA RF output.
Now, what would one use a noise source for? Well, as an inexpensive replacement for a “tracking generator.” A tracking generator is simply an RF signal generator that produces a CW signal and tracks with the sweep of a spectrum analyzer. Many spectrum analyzers include an optional tracking generator and I always recommend clients adding this on to their purchase. I wrote an article on how to use tracking generators a few years ago [3]. They can help plot out the frequency response of an RF filter, for example.

https://www.edn.com/using-a-tracking-generator/

When it comes to radio frequency oscillators, crystal controlled is the way to go when you want frequency precision. But not every slab of quartz in a tiny silver case is created equal, so crystals need to be characterized before using them. That’s generally a job for an oscilloscope, but if you’re clever, an SDR dongle can make a dandy crystal checker too.

RTL-SDR USB receiver – cheap tool for matching crystals
https://www.youtube.com/watch?v=F6pNEsxSvHQ

working on my home made HF 6-band SSB transceiver and this little thing helped me to made 8MHz IF filter. It also helped me in company with wide band noise generator to tune front-end bandpass filters.