After clocks and resets, we like to focus on voltage regulator circuits when doing an EMC board review. Voltage regulators can cause both radiated emissions and susceptibility problems. Emissions are the result of parasitic oscillations, and susceptibility is the result of RFI (radio frequency interference.)
The first problem, parasitic oscillations, are due to high-frequency feedback from the output to the input. The criteria for an oscillator are (a) 180° phase shift from output to input, and (b) gain greater than one at the feedback frequency. Or, as the old saying goes, "Oscillators won't, but amplifiers will."With today's devices, oscillations typically occur in the 100 to 500-MHz range. We've seen these problems occur with both linear and switching regulators. Both types have feedback, and both have gain. The resulting levels are often high enough to cause radiated emissions failures during EMI testing.
Note that these are free-running oscillations. As such, they will NOT be exact harmonics of any oscillators. Also, the frequencies may vary from system to system. For example, it is not unusual to see an oscillation at 222 MHz in one system, and to see an oscillation at 231 MHz in another. Same circuits, same layout, different parasitics. These are useful clues when troubleshooting radiated emissions.
The second problem, RFI, is due to rectification. Even a small amount of demodulated AC or DC voltage at a critical feedback node can drive the regulator out of range. In digital systems, this can result in unpredictable and unrepeatable behavior, such as system lockups. In extreme cases, the out of specification voltages can even change the state of programmable components.
Like parasitic oscillations, the RF susceptibility problems typically occur at 100 MHz and above. At those frequencies, the dimensions of the circuit boards and traces become efficient antennas. This is particularly problematic for circuit boards that are not in a shielded enclosure.
Fortunately, both problems are easy to prevent. Small high-frequency capacitors placed directly across the component inputs and outputs will "short out" both adverse effects. We typically recommend 1000 pF capacitors at these locations. Keep the leads short!
Note that many regulator circuits have electrolytic capacitors across their inputs or outputs. These capacitors alone are NOT adequate at high frequencies. Most electrolytics are not a good "short" at frequencies above 10 MHz. In those cases, you must add the small capacitors in parallel, located at the device. Just like stereo speakers, think "woofer-tweeter" capacitors.
To recap, add small high frequency capacitors at regulator device inputs and outputs. These provide cheap insurance against unwanted parasitic oscillations, and at the same time protect against RF threats. You will never know when they are working, but if you need them and they are not there, you WILL experience EMI problems.
Previous entries in the seriesEMC Basics #1: Welcome!; and Clocks: critical circuits for EMC
EMC Basics #2: Resets as Critical Circuits
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, and their EDN Designer's Guide to EMC (1994) is still in relevant and in demand. He can be reached via http://www.emiguru.com or his other blog at http://www.jumptoconsulting.com/. 
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