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Home of  RadCon Immunity Software

Updated 5-30-04.

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EMC: Electro-Magnetic Compatibility

ElectroMagnetic Compatibility: The ability of two or more electrical/electronic devices to coexist with one another.

While it is possible for natural events (like lightning and static) to interfere with products, it is also possible, with so many consumer products being electronic in nature, that one electronic/electrical device may interfere with another.  Computers, for instance, have many electronic clock frequencies which can radiate out and interfere with televisions or radios.

What are those lumps in your computer cables?

Lumpy cables. Everyone has seen them but most people don't have a clue what those lumps are for. The lumps are ferrite beads (or just "ferrites") and are there because the manufacturer of the computer (or whatever the device) had a problem passing EMC compliance standards.  The cables coming from an electronic product can act as radiating antennas for radio frequencies that exist inside any microprocessor based product.  These frequencies can interfere with other electronic devices, particularly radio and television receivers.  The ferrite acts as a small transformer to the radio frequency current flowing on the cable, in effect "choking" the current flowing on the cable. This significantly reduces the cables ability to act as an antenna. In some cases, the ferrite may be there to keep interference out of a device as well.

In the picture above, the leftmost "lump" is on a USB cable for my camera. The next one is a cable for my PDA.  The next is a telephone cord which I added a clamp-on ferrite that was an attempt at keeping interference out of an answering machine, while the last cable is a video output cable from my camera.  I guess they had a slight EMC compliance problem with their camera design!

A well designed product usually does not need ferrites on the cable for it to comply with emissions limits.  Quite often an emissions problem may not be discovered until the product is already in the compliance testing phase. Adding a ferrite to a cable to get the product to pass is often much cheaper than changing the product design and starting compliance testing over again.  That is why you see ferrites on cables.  (back to top)

Surge Protectors

"Surge" refers to a high voltage spike that can occur on a powerline when lightning strikes nearby. A surge protector is a device that goes between the powerline and any device that you want to protect.  A lot of consumer products intended to be sold in Europe that carry the "CE" marking have some level of internal surge protection.  There are even whole-house surge protectors than can reduce the possibility of a surge coming into the house and destroying home electronics.  Keep in mind that nothing, short of unplugging a device when it is not in use, can completely protect a device from lightning damage.  A direct lightning strike is going to destroy something! However, most surge protectors will prevent damage from lightning that strikes the ground nearby.  (back to top)

ESD or Electro-Static Discharge.

You can build up a significant static charge on your body when you walk across the carpet on a day when the humidity is extremely low. Touch something and you may get an arc. That arc can cause electronic equipment to lock-up, malfunction, or even be damaged.  Immunity standards exist that define how strong of an ESD event a product must be able to withstand and continue to work or not be damaged. The automobile standards are very strict in this regard since, as everyone knows, you can get one heck of a shock on a cold dry day when you get out of your car.  You would not want to touch the steering wheel and have the airbag deploy!

ESD immunity is determined by simulating a static discharge that would come from a person with a static charge touching a product. This is done by using a defined resistor/capacitor network to simulate a charge built up on the human body. Typically the charge voltage will be anywhere from 2 to 15 kv.  (back to top)

Radiated and Conducted Immunity

Ever notice a TV that has a bad picture when a vacuum cleaner was running? Or have you been talking on a phone when all of a sudden you hear "breaker, breaker good buddy"? These are examples of RF (radio-frequency) susceptibility.  In the case of the vacuum cleaner, the RF noise could be either radiated or conducted into the TV set. "Radiated" means the interfering signal is coupled to the TV by signals radiated through the air, while "conducted" refers to signals coupled to the TV by the wiring. In a lot of cases, the interfering signal will radiate through the air and be coupled onto the cables of a device, which then "conduct" the signal into the device.

Radiated immunity tests will determine if a product is immune (good) or susceptible (bad) to a radio frequency signal being radiated through the air. Conducted immunity will determine the same thing for signals conducted through the cables.

Why the separate tests?  Basically it has to do with the wavelength of the signals that could interfere with a product, and the most likely way the signal would couple to the product. The higher the frequency, the shorter the wavelength.

When the wavelength is very large (low frequency) relative to the size of the product, it is unlikely that the signal will couple directly to the product. Instead, it is more likely that any cables coming from the product will act as antennas, which will then conduct the signal into the product.

When the wavelength is very small (high frequency) relative to the size of the product, it then becomes possible the signal will excite the product itself, or even leak in through openings or seams in the product if these openings are large relative to the wavelength. It is still possible that the signal will couple onto cables, however.

Another reason for the separate tests has to do with the most economical or practical ways to perform these tests in a lab. It is more difficult to perform radiated immunity tests at low frequency. It is also less repeatable. When the distance from the antenna is short relative to the wavelength, then the product is in the "near-field" where the coupling properties are difficult to predict.  Also, for an antenna to be efficient in generating a field, it must be at least as long as a quarter wavelength.  Generally speaking, radiated tests are not performed below 26 MHz, and conducted tests are not performed above 230 MHz. Obviously, there are exceptions to this.

While radiated tests use antennas to radiate a signal towards a product, conducted tests use some way to couple the signal directly onto cables which, for the lower frequencies, require much less power and are more easily measured than a radiated field. Some conducted tests use direct injection, or capacitive coupling, to inject a signal. Other conducted tests may use inductive coupling through a transformer or current clamp. The more repeatable tests will have some way to insure directivity, which means the interfering signal will go into the product and not also back out along the cable into other equipment connected to the product.  A good example of this is a CDN (coupler-decoupler network).  (back to top)

Radiated and Conducted Emissions

"Emissions" refers to any type of electromagnetic disturbance that can emanate from a product. As with RF immunity, RF emissions are usually divided into conducted and radiated tests, for the same reason as RF immunity. Most emissions standards sets the breakpoint at 30 MHz.  In most cases, conducted emissions testing covers the frequency range from 150 kHz to 30 MHz, while radiated emissions cover 30 MHz to 1 GHz or higher, depending on the product and the test standard.

It used to be rare that a product was tested above 1 GHz, but microprocessors running at ever higher clock frequencies, along with more wireless communications at frequencies higher than 1 GHz, makes the possibility for interference more likely.

Radiated Emissions levels are determined by pointing an antenna at a product and measuring the signals. Conducted Emissions levels are determined by connecting the product to a LISN (line impedance stabilization network) and measuring the RF voltage levels, or by using a current clamp which measures the RF current flowing on the cable. Usually a spectrum analyzer is used since it can easily sweep the frequency range of interest. Correction values are applied to the measured signal level to correct for the antenna factors and cable loss. The corrected levels are then compared to limits established by the governing body responsible for the type of product being tested. In the United States, this is most often the Federal Communications Commission.  If the product passes the emissions limit, then there is a reasonable assurance that the product will not cause interference.   (back to top)

Other EMC phenomena include EFT or Electrical Fast Transients, VDS or Voltage Dips and Sags, etc.  I will be adding more information about those as I get a chance.

My job is to test products for emissions as well as immunity, or susceptibility, to electromagnetic interference.  My particular area of expertise is RF, or Radio Frequency, immunity testing.  I have written various computer programs to automate the RF immunity test performed in the lab.  (back to top)