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

Test setup with Class D amp and iPhone signal source

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

Abstract: Radiated emissions from electronic equipment can be a strong function of how the equipment is operated. In the case of class D audio amplifiers, the audio signal driving the amplifier can have a large effect on emissions. Test results and implications are discussed as they relate to the philosophy of radiated emissions testing.

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, speaker leads, and audio input leads from the amplifier. The amplifier is sitting on a blue ESD mat covering the table. The June 2014 Technical Tidbit showed that this amplifier couples to the blue conductive mat as a reference to drive common mode current out any leads connected to the amplifier. In this article, I will show that the common mode current is a strong function of the audio signal source content and has implications for how equipment is tested for radiated emissions.

Figure 1 shows the test setup including an iPhone running the app "Tone Gen Pro." The app can output a number of waveforms at audio frequencies and amplitudes. Figure 2 shows a close-up of the test setup.

close-up of test setup

Figure 2.
Close-up of Test Setup for Measuring Common Mode Current on the Amplifier Cables

Figure 3 shows the common mode current for the amplifier turned off, but with the iPhone on to make sure the iPhone was not contributing to the displayed common mode current being investigated. The small peaks shown correspond to signals in the FM broadcast band and are not due to the iPhone.

CMI plot of Figure 1

Figure 3.
Common Mode Current -Amplifier Off
(15 MHz/div from 0 to 150 MHz, Max Hold mode)

The amplifier was then turned on, but idling with no audio input. The resulting common mode current is shown in Figure 4. Given the sensitivity of the F-33-1 current probe, the common mode current that would begin to cause a possible Class B (residential) emissions problem is a horizontal line at 30 dBuV (left scale). At about 55-60 Mhz and at about 90-100 MHz, the common mode current reaches about 33 dBuV, potentially signaling enough common mode current to cause an emissions problem at those frequencies.

The amplifier is likely producing broadband noise throughout the frequency spectrum displayed (0 to 150 MHz), but cable resonances cause common mode current to peak at those frequencies shown in the display.

Top view of raised amp

Figure 4. Common Mode Current - Amplifier On, Idle State, No Audio Signal
(15 MHz/div from 0 to 150 MHz, Max Hold mode)

Figure 5 shows the iPhone Tone Gen Pro app set to produce a 400 Hz sinewave. The amplitude was set so that the amplifier was working hard but not overdriven to saturation.

iPhone App supplying 400 Hz tone

Figure 5.
400 Hz Continuous Tone Supplied from iPhone App

Figure 6 shows the resulting common mode current. Now the common mode current peaks up at about 44 dBuV, enough to potentially cause a Class A (industrial) emissions failure at about 50 MHz. Since this amplifier is a Class B device, it is very likely to show a Class B failure in a radiated emissions test performed at an official EMC test lab.

Common mode current with 400 Hz tone

Figure 6. Common Mode Current - Amplifier On, 400 Hz Tone Supplied from iPhone App
(15 MHz/div from 0 to 150 MHz, Max Hold mode)

Figure 7 shows the iPhone set to output a popular rock song from the 80s (The Breakup Song by the Greg Kihn band) which has a high average power level as well as high peaks. Figure 8 shows the resuting common mode current. All the spectrum analyzer plots, including Figure 8, were made in the "Max Hold" mode.

Ferrites added to amp on ESD mat

Figure 7. Music Supplied from iPhone App

CM current with ferrite on amp leads

Figure 8. Common Mode Current - Amplifier On, Music Supplied from iPhone App
(15 MHz/div from 0 to 150 MHz, Max Hold mode)

The common mode current peaks at about 47 dBuV, a likely Class A and definite Class B radiated emissions failure. Quasi-peak weighting may help here somewhat, but the common mode current would not likely fall below the 400 Hz sine wave level in Figure 6.

This brings up a philosophical question for radiated emissions testing (and a lot of other testing as well). If I were at a test lab trying to get my design tested for radiated emissions, and this amplifier was my product, what audio signal should I use? A quiet classical piece, hard rock, a sine wave, a square wave, or something else. And should the level be at max volume without saturating the amplifier?  From the data above, the choice of audio source could make the difference between passing and failing.

The same would likely hold true for a digital device. What bit patterns should be used? I have seed the radiated emissions status (pass or fail) depend on the bit patterns used in data that was being moved around.

In both cases, should worst case be used even if it is very unlikely (400 Hz tone for instance in an amplifier) or should typical sources (music at a reasonable volume setting) be used? Human voice has a very high peak to average ratio so the amplifier would be more likely to pass using it, especially if Quasi-Peak averaging were used (as allowed in radiated emissions testing). What should be done?

Summary: Class D amplifiers can make good noise sources for experiments. In this case, the choice of audio source and its amplitude had a significant effect on common mode current on leads from the amplifier and therefore on radiated emissions. The philosophical question is: what conditions should I use in emissions testing of any device? Worst case conditions or likely conditions?

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

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