Tool evaluates EMI on power lines

Article By : Kenneth Wyatt

EMI on power lines has become an issue, but this tool can be used with spectrum analyzers or oscilloscopes to evaluate it.

Ever since we’ve switched from incandescent to LED lighting and from linear to switch-mode power supplies, EMI on power lines has started to become an issue, especially for those of us who still enjoy AM broadcast radio and amateur radio in the MW and HF spectrum (0.54 to 30 MHz). Add defective power utility transmission line arcing and corona, and many amateur radio enthusiasts and military operators simply can’t receive national and international stations with the resulting hash on their receivers.

I wrote an article in 2015, How cops are finding “grow ops” with AM radios, about how those who were growing marijuana illegally were purchasing poorly-filtered power supplies (“ballasts”) from Asia for their lighting. These power supplies were producing EMI over a wide range, up to 1 mile away, and the police in Oakland, California, realized they could hear the hash coming through their AM radios, which allowed them to zero in on these grow operations.

Because this broadband high frequency EMI is both conducted and radiated, it’s very difficult to get rid of at this point in time; an unintended consequence of the desire for more efficient power supplies. In the meantime, those using these frequencies are suffering the consequences.

OnFILTER recently released a power line EMI adapter that can be used with spectrum analyzers or oscilloscopes to evaluate conducted power line EMI (Figure 1). They have units that can plug directly into a wall socket or one that includes test leads. I was able to evaluate the one with test leads, as it seemed more versatile. The test leads and accessories are excellent quality, made by Pomona.

photo of the OnFILTER MSN15 power line EMI adapter and test leadsFigure 1 OnFILTER’s new MSN15 power line EMI adapter and test leads can be used to evaluate conducted power line EMI.

When used with a spectrum analyzer, they recommend inserting a 20-dB attenuator at the input port. For use with an oscilloscope, they provide a BNC T-adapter with 50 Ω load. I suspect you wouldn’t need the latter if your scope had a 50 Ω setting, as most do.

Because the unit has galvanic isolation from AC mains, the input leads may be connected between any two connections without worrying about Earth reference. Maximum voltage rating is 250 VAC and the frequency response of the adapter is 30 kHz to 200 MHz, which allows recording of most power line communications (PLC) to be displayed, such as Homeplug AV and AV2, Wave 2, CENELEC EN50065 (all bands), IEEE 1902.2, and ARIB (Japan).

While most use modes might be performed with an oscilloscope, I was hot to try it out with a spectrum analyzer. I also had a Solar 7032-1 isolation transformer and one of OnFILTER’s AFN515FG CleanSweep power line filters to see how well they could reduce the conducted EMI from my own power line (Figure 2).

photo of an isolation transformer and EMI filter on a benchFigure 2 Here’s the setup for comparison testing the Solar 7032-1 isolation transformer and the OnFILTER AFN515FG CleanSweep EMI filter.

The results were quite interesting (Figure 3). For one, I didn’t realize how “dirty” my own power line was. Adding the isolation transformer helped quite a bit, but the CleanSweep filter knocked most of the EMI down to the noise floor of the analyzer, at least out to 12 MHz (it’s upper specification limit).

graph of frequency domain sweep resultsFigure 3 A frequency domain sweep from 100 kHz to 100 MHz shows my power line (violet), the Solar 7032-1 isolation transformer (blue), and OnFILTER AFN515FG EMI filter (green).

The broad peaks from about 15 to 80 MHz are likely ambients and may have been coupled directly into the long test leads. I’ve not investigated this as yet. Placing a ferrite choke around both test leads did knock these down. Figure 4 shows a sample Homeplug-AV2 frequency spectrum on the power line.

graph of the HomePlug AV spectrumFigure 4 This is a sample of the Homeplug-AV2 spectrum on the power line. Source: OnFILTER

Turning now to time-domain captures, let’s look at the older X10 remote controllers using the MSN515 adapter, and expand the time scale out (Figure 5). PLC is easily viewed.

time domain captures of X10 remote controlsFigure 5 This is a time domain capture of one of the older X10 remote controls using the MSN515. Source: OnFILTER

Here’s another example of servo motor EMI in an industrial environment (Figure 6).

graph of a servo motor EMI on a power lineFigure 6 This graph shows servo motor EMI on the power line in an industrial environment. Source: OnFILTER

Overall, I’m impressed with the EMI adapter and CleanSweep filter I was able to try out. I’m still thinking of uses for it other than power line EMI monitoring and evaluation. I suspect it could also be used for measuring different “ground” points in a large industrial system or even to measure EMI on ground return planes or power distribution networks.

Certainly, using one of OnFILTER’s CleanSweep filters would work well to reduce or eliminate transient EMI contaminating power lines from things like soldering stations or industrial machines using servo or variable speed drives.

I do wish there was something that could be done about the general increase in EMI on power lines due to the proliferation of switch-mode power supplies. Perhaps I should start replacing all switch-mode supplies with linears, as a friend has done to improve the background EMI in his remote radio astronomy station!

This article was originally published on EDN.

Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.


  1. Wyatt, How cops are finding “grow-ops” with AM radios, EDN
  2. OnFILTER MSN515 EMI adapter

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