Is there an electronic product or circuit that’s not susceptible to electromagnetic interference (EMI)? For that matter, are any devices EMI-free? Simply put, no. EEs wish it wasn’t the case, of course, but it’s a fact of life in electrical engineering— and it’s one of those things they typically don’t teach you in school.
Most engineers find out about EMI on the job, where expunging it often takes more time than the original design itself. And don’t forget that federal and international regulations mandate EMI control according to a wide range of rules. This requires the testing and certification of any equipment before it can be sold.
EMI DEFINED
EMI is the general term used for electrical signals that interfere with the normal operation of electronic equipment. All electric and electronic systems and equipment generate some kind of signal that could potentially interfere with the normal operation of another nearby piece of equipment.
Also, EMI can degrade the performance of equipment, introduce errors or operational faults, or cause complete failure. It may be self-generated, where one circuit interferes with another inside the equipment, or externally generated by some other device or equipment within the same environment.
The earliest form of EMI, radio-frequency interference (RFI), consisted mainly of harmonics and spurious signals from radio transmitters that interfered with other radios. For example, ham and citizen’s band radios generated harmonics that produced television interference (TVI).
We’re expected to see an exponential rise in EMI due to the growing number of wireless devices and standards, including cell phones, Wi-Fi, Bluetooth, ZigBee, WiMAX, and GPS. While RF sources still cause interference, digital equipment and computers have become the preeminent cause of EMI. The very high clock speeds with fast rise and fall times cause digital signals to generate huge bandwidths of large harmonics.
Switch-mode power supplies also generate EMI. Increased use of microwave frequencies for radio as well as digital applications has produced even more EMI. What was once a relatively minimal problem has turned into a challenge for most design engineers.
Electromagnetic compatibility (EMC) occurs when a piece of equipment can operate normally in an environment with other electrical and electronic equipment. With EMC, the equipment won’t generate EMI, nor is it susceptible to EMI from another source in the operating environment. The goal is to eliminate EMI or greatly suppress it to avoid interference and achieve EMC.
Still, questions and concerns abound. Can your TV operate without interference from a nearby PC? Will your security system be compromised by radiation from some nearby RF source? Can your garage door be opened by the harmonics of a passing two-way radio transmitter? Will your DSL interfere with your telephone operation? Does your cell phone really interfere with the navigation or landing systems on an airliner?
Can radios operate reliably in an environment where every piece of equipment has some electronics? How do you keep all the radios inside a cell phone from interfering with one another? Recently, while filming some videos at a conference, I experienced one of the many kinds of EMI when two other nearby wireless microphones interfered with my microphone. And just imagine the EMC issues experienced on the battlefield. Will radar systems interfere with one another or missile systems or satellites or vice versa?
TYPES AND SOURCES OF EMI
The two main types of EMI are conducted and radiated. Conducted EMI is noise or interference that’s passed over wires and cables from a source or emitter to the receiver or “victim” device. The most common conducted path is via power lines. The ac lines are a major source of EMI because of spikes and noise generated by a wide range of devices connected to the line. Motors and switching sources are the main examples.
Then there are the signals that leak out of a power supply on to the ac line to be passed along to another device connected to the ac line. Power connections from power supplies to circuits are other sources. A huge amount of conducted EMI is passed through poor grounds.
Radiated EMI is the wireless transmission of signals from source to victim. Capacitive and inductive coupling from one circuit to another is one type of radiated EMI. This type of coupling is often called near-field interference. One example is crosstalk from one wire to another in a cable. All closed-loop circuits carrying current generate a magnetic field that can potentially induce a voltage into an adjacent circuit.
Far-field interference is the reception of electromagnetic or radio waves. A radio wave is a combination of a magnetic field (H) plane at a right angle to an electric field (E) plane. Both travel perpendicular to those planes. These fields behave as James Clerk Maxwell explained them back in the 19th century.
As the waves travel together, the magnetic field recreates the electric field and vice versa. The far field begins approximately at a distance beyond ?/2p meters. For example, one wavelength is 30 cm at 1 GHz. Therefore, the far field begins at about 30/6.28 = 4.78 cm from the source and beyond. Inside that range is the near field, which consists mainly of a predominant magnetic or electric field.
Any kind of noise is also a form of EMI. Noise is either external or internal. Manmade noise, coming from sources like fluorescent lights and auto ignitions, is the worst type of external noise. Spikes on the ac line caused by switching loads such as motors, contactors, or relays off and on are instances.
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