The (RFI) Plot Thickens
24 Feb 2022
Despite having read for years about the benefit of common mode chokes, and how the lack of them can lead to all sorts of EMI/RFI issues, I’ve never seen a “textbook” demonstration of their effect until today.
I was idly tuning around on my IC-705, and I noticed some of the typical, wide, wandering SMPS noise bands on the spectrum. In a fit of experimentation, first I disconnected the DC changing cable, and the SMPS noise disappeared when the radio was running on battery alone.
I grabbed a Fair-Rite type 75 snap-on ferrite ring, and ran a few turns of the DC charging cable for the IC-705 through it.
Lo and behold, the SMPS hash virtually disappeared, despite this being a quick, ad-hoc choke built using the wrong type of ferrite (type 75 is for LF, MF; this should probably be type 31 or 73).
I did a bit more tinkering to try and find out where the noise is coming from: The DC charging cable for the IC-705 goes to an Anderson Powerpole distribution panel that is connected to my very old, linear DC power supply, and an IC-7300 and an IC-9700. To my surprise, the source of the noise seems to be the IC-9700, regardless of whether the IC-9700 is powered on or off.
I find this surprising: The IC-7300 and IC-9700 are not inexpensive (i.e. “cheap”) radios, and I would think that Icom would have taken care to ensure that they did not create interference for themselves or other nearby radios. However, in their defense, I also realize that none of these devices are properly bonded to each other as they should be.