The Dark Force Of Evil In Electronics: Electromagnetic Interference

I/O connections can sometimes be made with special shielded connectors. Be on the lookout for the many available versions of these special connectors. Also, power wiring is harder to protect, so keep it short and bypassed. Use feedthrough capacitor/ low-pass filters where possible to keep out the bad signals.

TESTING AND TROUBLESHOOTING
Some EMI can be tracked with a standard oscilloscope that has sufficient bandwidth. But scopes don’t show the whole picture. Here, a good spectrum analyzer is needed, one with a bandwidth beyond what your harmonics and spurious signals may occupy. Signal analyzers like Agilent’s X-Series (EXA/MXA) are designed for EMC testing (Fig. 3).

Along with an analyzer, a set of antennas and probes for radiation detection is essential. A biconical dipole with its superior bandwidth is a common antenna for up to about 300 MHz. A log periodic antenna is also available for higher frequencies up to about 7 GHz. Horn antennas are used for signals up to 40 GHz. Most far-field radiation tests are conducted at a distance of 3 to 10 m.

The antenna is often mounted on a non-conducting tripod, and the equipment under test (EUT) is placed on a rotating table. The measurement is field strength in V/m or dBµV/m. An antenna factor derived from the antenna in use is applied to the analyzer reading to get a direct measurement in dBµV/m. Special probes detect near-field magnetic and electric fields at close range. These help you track leakage at package seams and openings, as well as cable radiation.

You also will need a line impedance stabilization network (LISN) for conducted EMI. This standardized low-pass filter is inserted between the ac power line and the EUT to keep any power-line EMI from entering the EUT or to minimize any EUT-generated EMI from getting to the ac line. The LISN also has a circuit that will pick off any EMI generated by the EUT and send it to the analyzer input.

There are two basic rules for EMI testing. First, test often. Initial testing can come during initial breadboarding, then PCB layout testing, prototype evaluation, and in final production. Second, be sure you know the standards to which you are testing—that is, know your regulatory objectives (see “EMI/ EMC Regulations”). The Federal Communications Commission (FCC) rules typically state specifically what the test setup and parameters are for compliance.