We’ve been discussing signals from the perspective of their bandwidth relative to the bandwidth of the receptor and have not considered them from the EMC point of view. RF signal energy can get from the emitter to the receptor by conduction or by radiation. We’ll discuss that later. Regardless of the energy transfer process, using the FCC classification scheme we can divide RF signals into three categories: (1) intentional, (2) unintentional, and (3) incidental.
Intentional emitters deliberately generate RF energy with the specific intent of transferring it to a receptor located some distance from the emitter. Looking at radiated emissions, which are easy to visualize and are also the most EMC problematic, Bluetooth, 802.11 Wi-Fi, cellular telephones, and garage door openers are common examples of intentional RF signals. The devices that produce these signals must be FCC certified and they are required to operate within specific frequency ranges and RF power levels. In general, the signals from these devices are narrowband.
Unintentional emitters deliberately generate RF energy, but only for internal use. That doesn’t mean that it always stays bottled up inside the box, but it’s supposed to. The fact that it doesn’t mind well is why we often hear that EMC means Easy Money for Consultants. Anyway computer clocks and receiver local oscillators are good examples of unintentional RF signals. The clocks are impulsive broadband signals and the local oscillators are narrowband signals.
Incidental emitters do not deliberately generate RF energy. Unfortunately every time there is a rapid voltage, current, or impedance change, electrons are accelerated and RF energy is created. Examples of these emitters are fluorescent lights, electric razors, ignition systems, thermostats, etc. Most of the time incidental emitters generate broadband emissions.
Overall it is a lot easier to design for and suppress narrow band signals. However, for intentional and unintentional emitters, suppression of the signals may prevent the system from operating properly. With incidental emitters, those unwanted signals aren’t required for the operation of the device so we can work diligently to eliminate them. Not needing the signals makes a big difference in the level of attenuation that can be applied. Always ask, do I need that signal for the operation of my device? If yes, then find out if it needs to be that strong, switch that fast, or operate at such a high frequency?
The system must meet its appropriate EMC requirements, or it can’t be sold. In fact, there are special restrictions placed on unapproved new units that must be met or they can’t even be shown as soon to be available. Even an approved device has to be able to accept interference from intentional, unintentional, and incidental emitters and cannot cause interference to licensed broadcast services.
This may sound funny, but unless the device is being used by the DOD or being shipped out of the country, it can interfere with other things as long as it’s not interfering with broadcasts or is a health/safety hazard. Even then, I would not recommend ignoring an interference problem, because the user won’t like being maligned by their neighbors. Check out the FCC rules in Title 47, CFR Section 15.5 General Conditions of Operation for more information about interference from incidental emissions.
– Ron Brewer
