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Douglas C. Smith

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Technical Tidbit - April 2010
Identifying Switching Power Supply Noise in Systems
(Frequency Domain Measurements)

test setup with switching power supply

Figure 1.
Test Setup For Measuring Switching Power Supply Induced Noise Current into a Nearby Wire
(Spectrum Analyzer Screen at 10 dB/div Vertically and 10 MHz/div Horizontally)

Abstract: Switched mode power supplies can generate significant harmonics of such an amplitude as to cause EMC emissions problems in the 30 to 100 MHz range and occasionally at higher frequencies. Identifying switching power supply noise is discussed as well as one coupling method that can result in excess radiated emissions.

Discussion: Figure 1 shows the test setup used to generate the data. The setup is composed of a switching power supply, a (black) wire draped across the supply, an Agilent N9320B spectrum analyzer, and an Fischer F-33-1 current probe. The power supply is powering a 48 Watt, 12 Volt incandescent light bulb through the yellow test leads. The black wire passing through the F-33-1 current probe is about three meters long and its center has been positioned over the power supply. The plot on the analyzer screen in Figure 1 extends from zero Hz to 100 MHz at 10 MHz/div.

The noise floor on the screen of the analyzer is about 30 dBuV, a level that corresponds to enough current flowing in a one half wavelength dipole antenna to just hit the CISPR/FCC class B emissions limits in the 30 to 100 MHz frequency range. You can see the level of current near 40 MHz is about 20 dB above that level and could cause a Class B emissions problem. In reality, the power supply exhibits broad band noise, but the current in the wire peaks at the frequency at which the wire is a one half wavelength dipole. At that frequency, the driving point impedance of a dipole is near 70 Ohms, its minimum value, and the induced voltage from the power supply in the black wire is able to produce a current maximum at the center the wire.

The local field from the power supply is probably not physically large enough to radiate efficiently at 40 MHz, but once it induces enough current in the nearby black wire, the wire itself is long enough to be an efficient radiator. That is what has happened in this case.

We know that the source is the power supply for this simple case, but in some cases the source might not be so obvious. One way to discover if a broadband noise, such as shown on the analyzer screen in Figure 1, is due to a switching power supply is to adjust the analyzer as shown in Figure 2.

plot of switching noise at 100 kHz/div

Figure 2. Spectrum Analyzer Screen Centered at 43.5 MHz with a 100 kHz/div Horixontal Scale and a Resolution Bandwidth of 1 kHz

The analyzer has been adjusted in Figure 2 to focus the plot around a center frequency of 43.5 MHz with a frequency span of only one MHz, 100 kHz/div. The resolution bandwidth has also been lowered from 100 kHz in Figure 1 down to one kHz, a value consistent with the horizontal scale of 100 kHz/div. With these analyzer parameters, you can clearly see peaks in Figure 2 separated by about 120 kHz, the fundamental frequency of the switching power supply used for this experiment.

Noise originating in  switching power supplies that operate at a constant frequency will produce a spectrum plot similar to Figure 2, although the spacing of the harmonics may be different. That the noise is actually coming from a specific supply can be determined by holding a small loop probe near the power supply to pick up induced noise. Display the loop output on an oscilloscope or spectrum analyzer to determine the fundamental frequency. If the observed noise has a harmonic spacing the same as the fundamental switching frequency of the power supply, the supply is the likely source of the noise unless there are other supplies in the system with "the same" fundamental frequency. For the case where there are several power supplies with "the same" fundamental frequency, they are likely slightly different and this information can be used to narrow the source of the noise down to one supply out of several.

Summary: By expanding the spectrum plot of a broadband noise, individual harmonics from the source of the noise can be resolved. In many cases, the spacing of harmonics on the expanded plot will identify a switching power supply as the source of the noise.

Additional articles on this website related to this topic are:
  1. Current Probes, More Useful Than You Think (~170K)
    (1998 IEEE EMC Symposium paper)
  2. March 2006, Predicting Cable Emissions from Common Mode Current
Equipment used in this Technical Tidbit:
  1. Agilent N9340B spectrum analyzer
  2. Fischer Custom Communications F-33-1 current probe
A 10+ minute video that shows the live experiment pictured in Figure 1 and discusses implications for system radiated emissions will be posted at in the membership section.

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