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EMC Basics #8: A primer on troubleshooting EMI problems

Posted: 18 Jul 2011 ?? ?Print Version ?Bookmark and Share

Keywords:EMI? troubleshooting? electrostatic discharge? radio frequency interference?

Unintended electromagnetic emissions can bring about interference to surrounding communications receiversradio, TV, GPS, WiFi, and more. Many years ago, EMI problems were called RFI, or radio frequency interference. Marconi suffered EMI (RFI) problems over 100 years ago.

Mandatory emissions tests are required for most electronics devices. These include both radiated emissions (RE) and conducted emissions (CE). The RE tests look at electromagnetic fields from the entire system, while the CE tests look at voltages/currents on the input-power mains.

As an aside, although the CE tests are not radiated tests, they still aim to prevent direct radiation from the power lines. The CE tests also aim to prevent interference from directly coupling through the power system to other equipment.

There are two broad categories for emissions tests, with different goals:?
- Commercial limits aim to protect a nearby TV receiver
- /avionic/vehicular limits aim to protect a nearby radio receiver

Since radio receivers are much more sensitive than television receivers, the latter limits are much lower. For example, at 100MHz the military/avionic limits are often 40 dB or more lower than corresponding commercial limits. The exact differences vary depending on the actual environments.

Even if you pass all the emissions test at the EMC lab, you can still have problems in the field. If that happens, don't assume you can hide behind your FCC/CE certifications. In the US, the FCC can invoke the "noninterference clause". You cause a problem, you clean it up.

There are two major components to emissions problems: hidden transmitters and hidden antennas. Even though they are not really hidden, most designers don't see them as transmitters and antennas (RF designers being an exception, of course.) But the electrons don't careif it looks like a transmitter and an antenna, it's time to party.

The primary hidden transmitters in most systems are digital circuits. Highly repetitive signals such as clocks and clock-like signals (busses, repetitive control lines, etc.) generate strong harmonics. But even switching power electronics (power supplies and motor drives) can get into the act. As a helpful hint, we usually assume the first twenty harmonics of any repetitive signal are potential transmitters.

But even lower frequency circuits can be hidden transmitters, thanks to parasitic oscillations. Long a problem with vacuum tubes, we've seen a significant increase these unwanted oscillations in solid state voltage regulators, op amps, and other more. As a helpful hint, these usually occur above 100MHz.

The hidden antennas are highly dependent on physical dimensions and frequency. The higher the frequency and the longer the antenna, the more the radiation. We usually assume anything over 1/20 wavelength is an efficient antenna. That means six inches at 100MHz, and about 19 mm (3/4 inch) at 1GHz.

As such, cables are very likely hidden antennas, followed by traces on circuit boards and even the components themselves in the GHz range. Openings in shielded enclosures can act as "slot antennas", and follow the same guidelines. A two-inch slot at 300MHz leaks like a sieve.

So how do we troubleshoot these emissions problems? Here are five quick suggestions:
- Turn off clocks or change their frequencies to see if the emissions more or disappear. This can isolate the hidden transmitters.
- Use sniffer probes to identify hidden transmitter circuits. These are small hand held magnetic probes you connect to a spectrum analyzer. Since they are quite localized, you can quickly sniff around a circuit board for hot spots for emissions.
- Remove cables to see if the emissions change. This can isolate the hidden antennas.
- Use current probes on the cables. These are high frequency probes you clamp on a cable and connect to a spectrum analyzer. Currents in excess of a few microamps are suspect.
- Shield the entire enclosure with aluminum foil. This is very useful in identifying any hidden antennas in the mechanical enclosure.

Next time, we'll share some troubleshooting techniques for ESD (electrostatic discharge).

- Daryl Gerke
??EMI/EMC consultant
??Kimmel Gerke Associates

To download the PDF version, click here.

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