EMC Bench Notes: Troubleshooting with a Nearby Antenna - In Compliance Magazine

Over the last several months, we showed how to use near-field probes to characterize and interpret dominant harmonic energy sources on PC boards and how to use Radio Frequency [RF]Rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">RF current probes to characterize the coupling of these energy sources to power and I/O cables. This time, we’ll discuss how to use a nearby AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna to monitor actual emissions from a product or system.

While many designers attempt to perform Radiated EmissionsThe unintentional release of electromagnetic energy from an electronic device or apparatus." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">radiated emissions troubleshooting at an outdoor site or in a semi-anechoic chamber using a third-party test lab facility, I’ve found a much more efficient method is to perform this using a nearby AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna right on your own work bench (Figure 1). Performing this testing in-house also allows additional tools and resources to be close at hand.

Best of all, a calibrated Electromagnetic Interference [EMI]Disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">EMI AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna is not really required, as all we care about are relative changes! In one case, I was testing an industrial printer and connected a 1m-long piece of wire to the spectrum analyzer and stretched it out nearby. I’ve even had clients use a nearby Wi-Fi AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna for troubleshooting. So long as you can see the harmonic emissions, you can try various mitigations and observe the results in real time!

Some may question the use of an AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna so close to the product under test, as this is within the near field at the lower frequencies. While near field measurements can’t be directly compared to far field measurements, for troubleshooting purposes we’re just looking for relative changes, not absolute. For example, if we know we’re failing at 230 Megahertz [MHz]Unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">MHz by 5 dB, then we’ll strive to lower that on the work bench by 10 dB or more, just to be safe.

If you wish to compare with actual compliance test lab data, you’ll need to space a calibrated AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna at 3m or 10m and account for all the measurement system losses and gains. We’ll describe how to do that next time.

Currently, the AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna I prefer is a log-periodic design built from PC board material. These are available at low cost from Kent Electronics (http://www.wa5vjb.com), and the larger 400 to 1000 Megahertz [MHz]Unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">MHz model I prefer costs just $53 (with attached SMA connector) as of this writing. While not resonant in the range 30 to 400 Megahertz [MHz]Unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">MHz where most of the larger harmonic emissions reside, I’ve found that positioning the AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna about 1m from the Equipment Under Test [EUT]Any electronic assembly under test. Also known as Device Under Test (DUT)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">EUT allows me to see the emissions well enough to troubleshoot.

I published an article a few years ago that describes how to make the PVC fittings to hold the AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna and fasten it to a simple table-top camera tripod (Reference 1).

By now, you should have characterized the emissions profile of the dominant harmonic energy sources within your product using near field probes, as well as characterized the couplings to interior and exterior cables using an Radio Frequency [RF]Rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">RF Current ProbeAn electrical device that can measure amperage without breaking a circuit. Current probes sense the current flowing through a conductor and convert it to a voltage that can be viewed and measured on an oscilloscope." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">current probe.

With this data in mind, it should be easier to identify the specific sources and couplings that result in Radiated EmissionsThe unintentional release of electromagnetic energy from an electronic device or apparatus." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">radiated emissions. The usual “radiating structures” will include I/O and power cables and seams and apertures in shielded products. For the majority of unshielded products, it will be cable or PC board radiation (or both).

Most narrowband emissions will tend to be grouped around 50 to 300 Megahertz [MHz]Unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">MHz and are largely due to radiating structures that start to approach 1/4 to 1/2 wavelength. For example, a 1m long cable (typical USB) will resonate at 45 to 100 Megahertz [MHz]Unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">MHz, depending on whether the shield connects to PC board or line-powered product with a shielded chassis. Refer to my article on cable resonance in Reference 2.

One problem you’ll run into immediately when testing Radiated EmissionsThe unintentional release of electromagnetic energy from an electronic device or apparatus." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">radiated emissions outside a Shielded RoomA room made free from EMI by applying shielding to the floor, walls, and ceiling, and by suppressing interference entering through the power lines. Typical construction shields from 70 dB to 140 dB from 10 kHz to 10 GHz." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">shielded room is the number of ambient signals from sources like Frequency Modulation [FM]Encoding of information in a carrier wave by varying the instantaneous frequency of the wave." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">FM and TV broadcast transmitters, cellular telephone, and two-way radio. This is especially an issue when using external antennas.

I’ll usually run a baseline plot on the analyzer using “Max Hold” mode for a couple of minutes to build up a composite ambient plot. Then, I’ll activate additional traces for the actual measurements. For example, I often have at least two plots or traces on the screen: the ambient baseline and the actual measurement. It greatly helps to become familiar with your area’s Radio Frequency [RF]Rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">RF spectrum usage.

Fortunately, there are three ways around this:

Remember that strong nearby transmitters can affect the amplitude accuracy of the measured signals as well as create mixing products that appear to be harmonics, but are really combinations of the TransmitterAn electronic device which, with the aid of an antenna, produces radio waves." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">transmitter frequency and mixer circuit in the analyzer. You may need to use an external bandpass FilterAn algorithm or device for removing part(s) of a signal." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">filter at the desired harmonic frequency to reduce the effect of the external TransmitterAn electronic device which, with the aid of an antenna, produces radio waves." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">transmitter. An example would be an Frequency Modulation [FM]Encoding of information in a carrier wave by varying the instantaneous frequency of the wave." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">FM broadcast band “stop band” FilterAn algorithm or device for removing part(s) of a signal." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">filter.

Now that we can observe the actual emissions from the Equipment Under Test [EUT]Any electronic assembly under test. Also known as Device Under Test (DUT)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">EUT, we need to turn our attention to the coupling paths that connect the internal energy sources to radiating structures. In the near field environment of a typical table top product, we’ll most likely be dealing with capacitive and inductive coupling.

Capacitive coupling is mainly due to large changing voltages with time (high dV/dt) and can be modeled as two plates near each other. A good example is the fast-changing switching voltage of a typical DC-DC converter. The coupling could be occurring between the switch device’s heat sink and a nearby ribbon or flex cable.

Inductive coupling is mainly due to large changing currents with time (high di/dt) and can be modeled as two loops near each other. Cable-to-cable coupling is a good example. Another example would be cable-to-transformer coupling.

If clock harmonics are being coupled to cables and radiating, then you’ll need to look at your PC board layout and stack-up. Are clock traces running too close to I/O traces? Is the clock oscillator or resonator located too close to an I/O connector?

When it comes to internal PC board couplings I find it’s often due to a poor choice in stack-up design. For best Electromagnetic Compatibility [EMC]The branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects (electromagnetic interference, or EMI) that such energy may induce." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">EMC performance, every signal layer should have an adjacent solid return plane. As well, every power plane or routed power layer should also have an adjacent solid return plane. For more information, refer to my series on low-EMI PC board design with part 1 starting in Reference 3.

System-level issues can also lead to Radiated EmissionsThe unintentional release of electromagnetic energy from an electronic device or apparatus." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">radiated emissions. For example, how internal cables are routed can make a big difference. Is there an internal flex cable routed too close to one of your high-energy sources? Are motor drive cables routed in the same bundle as sensor or I/O cables? Known noisy cables should always be separated from quiet signal or I/O cables. A really good troubleshooting technique is to remove I/O cables one by one while monitoring with the AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna.

For shielded products, all sheet metal must be bonded together at frequent intervals. Long seams between enclosure pieces can act as radiating antennas if the length starts approaching 1/4 to 1/2 wavelength. Ott (Reference 4) has a chart of Shielding EffectivenessMeasured using plane waves in the far field (relatively far from the radiating antenna)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">shielding effectiveness versus slot length versus frequency. For example, a slot length of 2 inches (5cm) has a Shielding EffectivenessMeasured using plane waves in the far field (relatively far from the radiating antenna)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">shielding effectiveness of only 10 dB at 1000 Megahertz [MHz]Unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">MHz. A good design goal would be a Shielding EffectivenessMeasured using plane waves in the far field (relatively far from the radiating antenna)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">shielding effectiveness of 20 dB, which would require seam lengths of just 1/2-inch (about 13mm) at 1000 Megahertz [MHz]Unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz)." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">MHz. Keep this in mind for ventilation patterns.

Adhesive copper tape is a good troubleshooting tool when applied over possible seams (Figure 2). A near field ProbeA physical device used to connect electronic test equipment to the device under test." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">probe (either H- or E-field) can help identify longer seams. I typically use a marking pen to identify the beginning and end of a leaky seam. Then, I can measure the length and use that with the dominant frequency to assess whether the seam is approaching resonance.

For products with metal enclosures, apertures, like LCD displays, keyboards, and ventilation holes can also be sources of emissions. A common issue with LCD displays is the lack of BondingThe use of low-resistance material to electrically connect a chassis, metal shield cans, cable shielding braid, and other supposedly equipotential points to eliminate undesirable electrical interaction resulting from high-impedance paths between them." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">bonding between the display housing and product enclosure. Using copper tape or Electromagnetic Interference [EMI]Disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">EMI gasketing are good mitigating techniques. It’s not unusual for me to cover an entire product with heavy-duty aluminum foil during troubleshooting while I carefully cut out around potential apertures one at a time in order to identify the dominant emission source (Figure 3).

The last several installments of Electromagnetic Compatibility [EMC]The branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects (electromagnetic interference, or EMI) that such energy may induce." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">EMC Bench Notes have covered my basic approach to troubleshooting one of the most common Electromagnetic Compatibility [EMC]The branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects (electromagnetic interference, or EMI) that such energy may induce." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">EMC issues, that is, Radiated EmissionsThe unintentional release of electromagnetic energy from an electronic device or apparatus." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">radiated emissions. My process of characterizing energy sources with near field probes and then measuring cable harmonic currents with a Current ProbeAn electrical device that can measure amperage without breaking a circuit. Current probes sense the current flowing through a conductor and convert it to a voltage that can be viewed and measured on an oscilloscope." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">current probe and following up with a close-spaced AntennaA conductor by which electromagnetic waves are sent out or received, consisting commonly of a wire or set of wires often attached to metal rods." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">antenna has proven to be a fast and efficient way to attack emissions over several decades. I’m sure it will help you as well.

Next month, we’ll explore how to set up an in-house or temporary pre-compliance setup for measuring absolute levels of Radiated EmissionsThe unintentional release of electromagnetic energy from an electronic device or apparatus." data-mobile-support="0" data-gt-translate-attributes="[{"attribute":"data-cmtooltip", "format":"html"}]" tabindex="0" role="link">radiated emissions where you can compare to official test limits. While there are obvious issues when testing outside a shielded semi-anechoic chamber, this method will help confirm pass/fail well in advance of final compliance testing.

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