My mod was based on this article here http://www.rtl-sdr.com/improving-hf-reception-disconnecting-switching-power-supply-rtl-sdr/
I had no success using diodes myself so instead I used an LD1117A 1.2v regulator, which I think is a better solution too. This is the RTL-SDR (R802T) after fitting the LD1117D during tests, note I just turned the inductor round and reused it to feed the 1.2v DC;
These 2 plots show the difference in the number of carriers (spuri + signals) between all 3 dongles (R802T unmodded, R802T with SMPS mod & the new R802T2).
This shows less noise (carriers generated by the SMPS) on the modded RTL-SDR dongle, the AGC has then brought the average noise floor up so it looks a little more shaded and gives the impression of more noise, but that’s what you would want here.
I don’t think this mod makes a huge difference above 100MHz as the SMPS harmonics don’t go that high. The new R802T2 however has better sensitivity at VHF and UHF so is a better choice for those frequencies.