Ghost in the Machine

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Have you ever had a circuit or design do something you donıt want it to and just canıt explain why it does it? Worse yet, it doesnıt do it all the time, just when the planets are properly aligned? You may just have a circuit haunted by EMIıs (pronounced Emmyıs) ghost. Dealing with EMI is definitely a school of hard knocks course. I thought I might jot down a few cliff notes for those of you that are about to enroll.

EMI stands for electromagnetic interference, and boy does it ever interfere! I remember one of my first bouts with this ghoul. We had recently completed a design of a display that worked great on the bench and even worked most of the time on the product. But about 20% of the time when you turned the motor on, the display would simply freak out. By an all-night process of trial and error, we finally stumbled across a solution to get production up and running again. Since then I have learned a lot about how to pinpoint an EMI problem and resolve it. The things I point out here work well when combined with my trouble shooting course Shotgun Wedding. I doubt anyone has ever dealt with EMI on anything other than a trouble-shooting basis. Letıs face it, we donıt go looking for EMI, it does just fine by itself finding us!

What is EMI?
EMI is basically an unwanted signal entering into your circuit. It is still an electrical signal, it still obeys ohmıs law, and for all its supernatural behavior it is still just a signal. This is good news, it means you can exorcise these demons from your design because they still obey the laws of physics.

Let Me Count the Ways
First of all, how does EMI get into a circuit? There are only two ways: conducted and radiated. In the first case, the unwanted signal has to travel on a trace-wire or other path into the area of disruption. In the second case, the signal propagates without wires. It is important to know how the signal is getting in because that affects the solution you will need to employ.

Conducted EMI
How do you know if it is conducted EMI? The easiest thing to do is disconnect everything part by part till it goes away. Case in point, we were hooking a computer up to a circuit board, both at the audio output of the sound card as well as the serial port. There was this annoying buzz in the speakers that changed tone in sync with the displays on the board. When I unplugged the serial connection, the buzz went away. We had what's known as a ground loop. This is a specific type of conducted EMI. I usually try to detect whether the problem is conducted EMI first, as this is the easiest to check. Donıt overlook the connection to a wall outlet if the AC line powers your device. I once saw a design disrupted every time an overhead projector was plugged in.

Radiated EMI
The best way I have learned to determine radiated effects is to divide them into two camps, the near-field effects and the RF effects. Near field effects can be easily divided further into current and voltage disruptions. Consider this rule of thumb: anything within a wavelength is near field and anything outside that range is RF. Inside the near-field range, magnetic fields induce current fluctuations into a circuit and electric fields produce voltage fluctuations.

Here is a simple test with a piece of equipment that you are likely to have on your bench. Take your oscilloscope probe and with the ground dangling, move it near an AC outlet. Adjust your range and quickly you will see a nice 60Hz sine wave. This configuration is basically a dipole antenna and it responds well to electric fields. Next connect it into a loop by clipping the ground to the probe tip. You will see that the previous signal disappears. But move your new sensor near the power cord of the scope you are using, or some other device that is moving current. Viola, you pick up magnetic fields with this configuration. You can often times use this simple technique to determine the type of EMI you are dealing with. (And you didn’t have to buy expensive sniffers and spectrum analyzers!)

The next page offers some tricks to rid yourself of the EMI demon.

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