For many types of electronic devices, the Federal Communications Commission just requires Part 15 certification. For devices that fall under the Part 15 rules and regulations section, emissions are checked if there are any clocked circuits that conceivably might radiate RF that could cause problems with other devices. Susceptibility (i.e., is the device likely to be interfered with rather than to interfere) is also checked to a nominal 5 volts/meter field strength.
While it may seem reasonable to test at this level of EMC/RFI (electromagnetic compatibility/radio frequency interference), one must examine this more closely, to see what really may be required. For example, I have seen residential housing subdivisions built near 100,000 watt FM and TV transmitters. In this case, many of the items in the homeowner's house would likely malfunction. RF-based wireless remotes for TVs, DVRs, DVD players, and set-top boxes would fail routinely. “Garage door openers” would likely revert to the old-fashioned version — get out of your car and manually raise the door with your arm.
Fortunately, RF energy radiated from a line source (or something close to a line source) diminishes with the square of the distance — so one fourth the energy at twice the distance. However energy can, unfortunately, be concentrated and effectively increase in strength due to reflections and antenna gain.
If one looks into the details of various RF sources a bit more, one learns that a smartphone for any of the 2.5/3/4G networks will produce 40V/m at the end of the phone opposite the antenna. The fairly common two-way radio or walkie-talkie used by safety forces and in commercial settings is often a 5W digital radio. Setting this on the dash of your car while data is flowing can expose the instruments in the vehicle to fields that are as much as 200V/m. Set the handy-talkie near a laptop, tablet, or phone, and problems may occur. An additional complication: Signals from multiple devices can on occasion combine to voltage levels equal to the square of the number of carriers times each carrier's peak power.
How does the RF find its way into the susceptible equipment? Isn't it shielded? At frequencies above 1 GHz, the effectiveness of the mesh shields on coaxial cable and shielded twisted pairs starts to diminish rapidly due to skin effect and the physics of microwaves. Also, energy can sneak in along with other signals and then re-radiate once in an enclosure and cause issues.
What can happen to these devices? For typical analog electronics, this energy can be rectified to DC and cause bias points to shift, rendering circuits inoperable. Op-amps' inputs can develop a mysterious offset voltage and produce an error in their outputs. Linear voltage regulators can be pushed out of regulation. Switchers can switch and regulate at the wrong frequency and voltage.
For the more purely digital devices, there can also be problems. Clock signals can develop timing and logic errors, resulting in improper operation or multi-clocking on a single (now non-monotonic) edge. This can even cause carefully constructed software to malfunction (the logic can become highly illogical). Bits can flip, tasks can hang or die in the code, and variables and calculations can change.
One is left with a serious dilemma. For some of the older classes of logic devices that operate at higher supply voltages, there is higher noise immunity. But the tradeoff is shorter battery life and less functionality. Other systems that use the much more common lower voltages use redundant error correction and detection circuits. Refer to applications information for the new MCUs from Spansion and TI. These offer improved resistance to bit-flipping. They employ methods with appropriate levels of paranoia in the firmware to catch and clean up problems — or restart upon serious errors.
What are your experiences in this area? Have you experienced any erratic operation of electronics due to RFI?