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Douglas C. Smith

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Technical Tidbit - June 2014
Radiated Emissions Can be Strongly Affected by Setup, A Problem for Testing Standards
(An Example Using a Class D Stereo Audio Amplifier

Test setup with Class D amp

Figure 1.
Test Setup for Measuring Common Mode Current on the Amplifier Cables

Abstract: Radiated emissions measurements of table top equipment are usually made with the equipment placed on a non-conductive table. But such equipment is often used on conductive surfaces, like a metal desk, and can couple to the conductive surface to enhance emissions, essentially turning the conductive surface into part of an antenna structure. When this happens, standard testing of emissions can understate actual emissions. A Class D (actually labeled Class Z) stereo amplifier is used to illustrate this effect by measuring common mode current on its cables.

Class D (and newer class "Z") digital audio amplifiers are potentially a source of noise strong enough to exceed international radiated emissions limits. Figure 1 shows the test setup for a Lepai LP-2020A+ digital class Z stereo amplifier. Two 8 Ohm power resistors serve as speaker loads and are connected at the ends of the small wires exiting to the right and left. A Fischer F-33-1 current probe is placed over the power and speaker leads from the amplifier. The amplifier is sitting on a blue ESD mat covering the table.

For this test, no input source of audio was used. Behavior of the amplifier driven with an audio signal will be the subject of another Technical Tidbit.

Since the amplifier easily fits into the palm of a hand, it should not be able to drive much common mode current out all the cables in phase as the amplifier case would have to be the other half of the antenna and it is too small to act like an efficient antenna below 100 MHz. Yet in Figure 2, the common mode current on the cables would likely cause emissions that would equal or exceed Class B from 30 MHz up to a little over 100 MHz. The common mode current is plotted from 0 to 300 MHz. The level of common mode current that could cause emissions from the cables to reach the CISPR Class B limit from 30 MHz to just above 200 MHz is at 30 dBuV (left scale on the plot). The plot is at 30 dBuV or more for much of the range from about 30 MHz to about 110 MHz, a likely emissions failure.

The amplifier actually is emitting noise over a wide band but the common mode current flows for those frequencies for which the driving impedance of the cables is low, that is, one or more of the cables is near 1/4 wave resonance. In this case the speaker wires are 2 meters long and that explains the amplitude in the 30-40 MHz range.

CMI plot of Figure 1

Figure 2.
Common Mode Current from Setup in Figure 1

In Figure 3, the amplifier is raised up 9.5 cm on a paper cup over the ESD mat the amplifier had been setting on. The a top view of the setup is shown in Figure 4.

Amp raised above ESD mat

Figure 3. Amplifier Raised 9.5 cm Above ESD Mat

Top view of raised amp

Figure 4. Top View of Amplifier Raised 9.5 cm Above ESD Mat

The resultant common mode current on the cables is shown in Figure 5 and is at or below 30 dBuV across the range, quite a decrease! Apparently the amplifier chassis is coupling to the ESD mat core which is apparently conductive enough (ESD mats often have a conductive core and a high resistance outer surface) to act as the other half of the antenna for the cables, sometimes called a counterpoise. When the amplifier is raised off the mat, this coupling is decreased, and the amplifier cannot drive as much current on the cables because it must rely more on its small enclosure as the counterpoise.

Common mode current from Figure 4

Figure 5. Common Mode Current on Leads of Amplifier Raised 9.5 cm Above ESD Mat

Figures 6 and 7 show the amplifier back down on the ESD mat, but with a ferrite installed and the resulting common mode current on the wires respectively.

Ferrites added to amp on ESD mat

Figure 6. Ferrite Added to Leads From Amplifier

CM current with ferrite on amp leads

Figure 7. Common Mode Current on Leads of Amplifier on ESD Mat With Ferrite on Leads

The common mode current is below 30 dBuV now across the range, and the amplifier with its ferrite core and cables is likely to pass an emissions test, at least from cable emissions due to driving the cables in phase relative to the amplifier chassis.

The important point to be made is that emissions testing for table top gear is normally performed on a non-conductive table. Based on the performance with the amplifier raised on the cup, I suspect the emissions test may be passed. But, what if the amplifier is used on a conductive (metal) desk or placed on the metal enclosure of a cable TV set top box? The proximity of the metal will allow more current to flow on the amplifier leads, causing increased emissions. In this and similar cases, official radiated emissions testing would understate the emissions.

Summary: Class D amplifiers can make good noise sources for experiments. In this case, standard radiated emissions testing for compliance may understate emissions from some devices that couple to conductive table tops.

Technical Tidbit on this site related to this article:
Equipment used in this Technical Tidbit:

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