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EMC Basics #9: Solving RFI EMC problems

Posted: 15 Aug 2011 ?? ?Print Version ?Bookmark and Share

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

Radio frequency interference (RFI) is a rapidly increasing EMC problem, thanks to the proliferation of wireless devices. These range from low power Wi-Fi and cell phone transmitters to high power radio/television broadcast transmitter to extremely high powered radar systems.

As an aside, commercial testing has resulted in more robust products. Prior to 1996, when CE testing became mandatory, we often saw RFI problems in the field. These were due to both broadcast transmitters (radio and television) as well as mobile transmitters (cell phones, hand held VHF/UHF transceivers, vehicular transmitters, and similar. Today, however, most RFI problems get caught prior to market release. Some, however, still make it into the field.

RFI as an EMI source
Transmitter power is not the sole issue, but rather a combination of power and proximity. The lowly cell phone a few inches away may cause more problems than the broadcast transmitter a mile away. The key parameter is the magnitude of the electromagnetic field at the victim.

In the EMI world, we focus on the electric field magnitude. The field levels can be easily measured with suitable equipment. But here is a simple approximation that will get you in the right ball park for an initial assessment:

E = [5.5 (PA)] / d,

where
= Electric field in Volts/meter
P = Transmitter power in Watts
A = Antenna gain relative to isotropic
d = Distance from transmitter antenna to victim in meters

This formula assumes a point source and a "far field", both valid for most RFI situations.

For example, a 1W radio at 1 meter with a relative gain of 1 (good assumptions for a hand-held radio or cell phone) produces a field level of 5.5 V/m. In fact, it is this hand-held transmitter model that results in the 3 V/m and 10 V/m limits for commercial equipment. Higher field levels reflect higher transmitter levels, such as the 200 V/m limit common for many military/automotive environments.

RFI coupling paths
Since radio transmitters work by electromagnetic radiation, the primary path is radiated. As "hidden antennas" are involved, physical dimensions are critical. For frequencies 300 MHz, everybody gets in the act: circuit board traces, enclosure openings (slot antennas), and even components themselves.

A good rule of thumb is to assume any conductor greater than 1/20 wavelength long is a potential antenna. We've seen smaller antennas, but this criterion is widely used in the EMI community. This means six inches (15 cm) at 100 MHz, two inches (5 cm) at 300 MHz, and ? inch (2 cm) at 1 GHz.

Direct conduction is also possible, but less likely. Nevertheless, when troubleshooting an RFI problem, don't overlook this possibility. We've seen it happen.

RFI victims
The primary RFI failure mode is rectification, but failure levels vary with circuit types. The more sensitive the circuit, the lower the thresholds will be. Here are some typical levels:
- Analog circuits: 0.1 to 1 V/m
- Power circuits: 1-10 V/m
- Digital circuits: 10-100 V/m

These are simple guides. As the saying goes, "Your mileage may vary..."

Different circuits exhibit different symptoms, which may be helpful in troubleshooting:
- When analog circuits are upset, you may get errors in sensor information, but everything else works fine.
- When power circuits are upset, the system often locks up or exhibits other strange behavior.
- When digital circuits are upset, repeatable upsets are typical: resets, unwanted interrupts, or memory upsets. The digital upsets are similar to those seen with ESD and other transients that "flip" critical bits. .

Troubleshooting RFI problems
If you fail an RFI test at the lab, you already have details of frequency, amplitude, and failure mode. In you fail in the field, the picture is less clear. You may need to make some measurements, or to use approximations as described above.

Here are five quick RFI troubleshooting suggestions:
1. Remove cables to see if RF susceptibility changes. As an alternate, apply clamp on ferrites for frequencies above 100 MHz.
2. Wrap the unit under test in aluminum foil. This will show if shielding is adequate (or will help if Unit Under Test is unshielded.)
3. Harden critical circuitsferrites and 1000 pF capacitors are very helpful above 100 MHz. Apply to inputs, power, and even outputs.
4. At the systems/box level, troubleshooting is best done in a shielded enclosure with suitable equipment. In a pinch, a hand-held VHF/UHF radio can be useful. If testing in the field, keep transmissions short (1-2 seconds) on unused frequencies.
5. At the PCB level, a signal generator connected to a sniffer probe can also be helpful in "injecting" a signal at the component/trace level. Not new, this technique was used 50 years or more ago by those who repaired radios and televisions. It still works today.

Next time, we'll look at troubleshooting power disturbances.

To download the PDF version, click here.

About the author
Daryl Gerke, an EMI/EMC consultant since 1987, along with business partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel have been chasing EMI problems for over 80 years (combined, of course.) He is a published author and columnist as well.





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