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EMC Bench Notes: How to Use Spectrum Analyzers for EMC

Let’s take the basic tools I described last month and use them with a spectrum analyzer to characterize product emissions. A spectrum analyzer measures power (default) or voltage versus frequency. Most digital harmonic emissions occur in the range 10 kHz to 1 GHz and this defines the minimum frequency range for spectrum analyzers. Some harmonics may even extend up to 3 GHz. Two types of harmonic emissions will be observed: narrowband and broadband. Many times, you’ll see a combination of the two.

Narrowband Versus Broadband

Narrowband harmonics are the most common for radiated emissions and most likely to exceed the limits. They appear as a range of narrow spikes and are usually harmonically related (multiples of a clock frequency). For example, Ethernet is usually clocked at 25 MHz, so it’s common to see harmonics every 25 MHz, sometimes exceeding 1 GHz. Figure 1 shows a series of 25 MHz clock harmonics (aqua trace). The yellow trace is the measurement noise floor.

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Figure 1: A combination of narrowband (aqua trace) 25 MHz clock harmonics riding on top of broadband digital switching noise voltages. The broadband noise voltage (violet trace) is characteristic of DC-DC converters, which can switch as fast as 3 MHz and with sub-ns edges. Notice these harmonics extend out beyond 1.5 GHz (GPS frequencies).

Broadband harmonics are normally observed as an increase in the noise floor with periodic broad resonant peaks. The emission is usually highest at the lower frequencies and tapering off gradually. This is normally the result of power conversion circuits, such as on-board DC-DC converters switching in the 100s of kHz to 3 MHz range with transition times of a few ns to sub-ns. Figure 1 shows a typical broadband emission (using Max Hold, violet trace) from a DC-DC converter. Note these harmonics extend out to 1.5 GHz (GPS frequencies).

Setting Up the Analyzer

There will be slightly different settings depending on whether you’ll be measuring radiated or conducted emissions. The most common controls include Frequency > Start and Stop (lower and upper) sweep frequencies, Resolution Bandwidth (RBW), Reference Level and Attenuation. Some of these may be sub-choices under the major setup buttons (Figure 2).

There’s also Center Frequency and Span (Stop minus Start) that would be useful if you want to troubleshoot a specific harmonic.

Figure 2: A typical spectrum analyzer setup screen showing the major controls along the left side.

Optional settings might include activating a built-in low-noise preamplifier (if included), which helps lower the measurement noise floor and reveal smaller harmonics.

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I also like to switch from the default “dBm” (power) vertical units to “dBµV” (voltage), which matches the units used in the compliance limits. You may find this under the Span button. You’ll probably want to readjust the Reference Level when changing units to make the 10 dB steps line up with the vertical divisions. You may need to readjust several of these settings to obtain a centered sweep on the display.

Conducted EmissionsConducted emissions (CE) are the harmonic emissions being conducted out the power cable on products. According to most commercial standards, the analyzer should be set to Start and Stop frequencies of 150 kHz to 30 MHz. The RBW should be set to 9 kHz. However, many stock analyzers may not have the ability to be set to 9 kHz. For troubleshooting purposes, 10 kHz is sufficient.

To measure CE accurately, you’ll need a line impedance stabilization network or LISN. If you don’t already have one available, I recommend the Tekbox (www.tekbox.com) TBCL08, an 8-amp LISN designed for line-operated products, or the TBOH01 10-amp LISN designed for DC-powered products. You’ll need a pair of these DC LISNs in order to measure the positive and return power wires separately.

When I perform benchtop troubleshooting, I’ll tape down sheets of heavy-duty aluminum foil over the top. The LISN (or DC LISNs) should be bonded to this ground plane so common mode currents can return back to the LISN. The product under test is also placed on this ground plane. See Reference 1 for more detail.

Option: In order to measure differential- and common mode harmonic currents separately, Tekbox sells a product called “LISN MATE” (model TBLM01) that separates the two. Knowing which is the dominant harmonic noise current helps determine what kind of filter topology would be beneficial. See Reference 2 for more information on these measurements and the appropriate filter design to use.

Radiated EmissionsRadiated emissions (RE) are the harmonic emissions being radiated from the product under test. According to most commercial standards, the analyzer should be set to Start and Stop frequencies of 30 MHz to 1 GHz. The RBW should be set to 120 kHz. However, many stock analyzers may not have the ability to be set to 120 kHz. For troubleshooting purposes, 100 kHz is sufficient.

For troubleshooting purposes, I use an uncalibrated antenna placed nearby, as described in Bench Notes 2. Space the antenna about 1m away from the product under test. I usually place the antenna and analyzer about 1m apart (not critical) while manipulating cables, adding ferrites, or mitigating leaky seams. So long as you can observe the emissions in real time, you will be able to assess various mitigations quickly. See Reference 3 for more detail.

I often find most narrowband harmonics tend to be dominant between 50 and 250 MHz, so once I see the “big picture” from 30 to 1 GHz, I’ll often reduce the Stop frequency to 300 or 500 MHz so the sweep time is faster. Also, concentrating on mitigating the lower harmonics first will usually do the same for the higher harmonics if related to the same source.

Other Settings

There are three other common settings I use while troubleshooting RE: establish a measurement noise floor, display more than one measurement trace, and use markers.

Measurement Noise Floor – I almost always use Trace 1 to display the measurement noise floor. This helps to visually observe the amplitude of harmonic signals being measured. Disconnect the antenna. You’ll normally see these trace controls under the Display button. Press Display > select Trace 1, and then press Max Hold. This will build up several sweeps. Pressing Store or Freeze will store that trace on the display.

Ambient Measurement – When measuring outside a shielded room or chamber (typically with an RF current probe or antenna), chances are you’ll pick up ambient signals from broadcast radio, two-way radios, DTV, or cellular signals. It’s instructive to display the ambient spectrum usage with the product under test Off. This will display the measurement noise floor, plus any active ambient signals. Selecting another trace with the Product On will take the actual measurement of the product emissions, and you should observe frequencies where the product under test is emitting in between the ambients.

Display Line – Many analyzers have the means to add a horizontal line at an arbitrary amplitude. This is useful when troubleshooting by placing the line at one of the dominant harmonics while you’re trying different mitigations. This makes it easy to visually see whether you’re making progress or not (Figure 3).

Figure 3: An example showing a single harmonic with Display Line adjusted to the peak. The Display Line serves as a reference for quickly determining whether progress is being made during troubleshooting.

Displaying More Than One Trace – Pressing the Display button will allow you to display several traces at once. Normally, you’d make the first measurement, using Max Hold to build up a collection of scans, then pressing Store, Freeze, or View will store and display that trace. The Display button will allow you to display additional traces, storing each on the display simultaneously. This is useful for “before and after” measurements.

Using Markers – Markers are useful for identifying specific harmonics or resonant peaks. There will likely be up to two Marker buttons: one to select particular markers and one to automatically find the highest peak or “Peak Left/Right.” I also use markers on adjacent narrowband harmonics to confirm the fundamental clock as the difference frequency. Some analyzers can display six or more markers, and some can display a Marker Table or otherwise display all the marker amplitudes and frequencies.

Summary

Spectrum analyzers are usually the tool of choice for troubleshooting CE or RE. In recent years, the cost of a basic analyzer has dropped markedly. However, the more expensive lab-quality analyzers will have advanced measurement features that may be worth considering.

Alternatively, some of the more recent digitizing oscilloscopes have built-in advanced spectrum analysis features. Examples include the Rohde & Schwarz RTE, RTO, and MXO series, and the Tektronix Series 4/5/6. They can simultaneously display independent time and frequency domain traces. This ability is quite useful for EMI debugging, where you can relate specific portions of the time domain to a frequency spectrum.

References

  1. Wyatt, Workbench Troubleshooting Emissions, Volume 2, Chapter 3.
  2. Wyatt, Review: Tekbox LISN Mate is valuable for evaluating filter circuits (EDN).
  3. Wyatt, Workbench Troubleshooting Emissions, Volume 2, Chapter 4.

 

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