The (RFI) Plot Thickens

24 Feb 2022

SMPS Hash on 40M

40M SMPS hash on IC-705

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.

40M on Battery

40M on IC-705 with battery

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.

DC Common-mode Choke

Ad-hoc DC common-mode choke

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).

DC choke on 40M

40M on IC-705 with DC choke

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.