https://owenduffy.net/calc/RxSensitivityCalc.htm  by VK1OD

I have built a USB automatic switch box for VK1OD "NFM" and G8KBB "Noise meter" with step-up converter to power the noise source from USB and release my regulated power supply for DUT. The USB box was built entirely from my junk box. I cut the MC34063 board from a CATV internet modem, where it generate 33V for the varicaps. The modem was MOTOROLA, so I expect EMI from the pcb to be lower, than one build by me :). The board with PL2303HX is from data cable for old generation mobile phone. FT232BM is better, but for this application PL2303 do a nice job. If you don't have one, just try to search on ebay "PL2303" and mark "Include description". Not a big deal, but they are cheaper than FT232BM IC only. The box works very well, I put 1k load at the place of noise source and the the total current from USB was 330mA, while the 28.00V remain unchanged. The consumption from USB did not exceed 180mA with all of my noise sources. I do not have any problems with ground loops from PC to other equipment, but an option for galvanic decoupling is added for future. This is possible only with external power supply for the nosie source, but if you make a transformer for MC34063 (see the application notes for MC34063) and add an optocoupler for the feedback loop, PC GND will be insulated and the external power supply will be unnecessary. Here is the schematic diagram: https://drive.google.com/file/d/1EgvJQ2YEHr68agjlfruv3_l5z8xNr5S0/view?usp=drive_web

and photos(click for zoom):

the results and hints are here, explanations - later:

 http://s161.photobucket.com/albums/t206/LZ2STO/TEMP/My%20first%20Noise%20Measurements/

to be continue ...