At some point, you've probably wondered why some airlines require you to turn off all electronic equipment for 10 minutes or so prior to takeoff and landing.
In fact, some airlines don't allow such equipment to be turned on during any part of the flight. In reality, neither your CD player nor your child's video game is likely to disrupt the communication or navigation signals because the cockpit is shielded. The real concern involves the large amount of windows. If 300 cell phones were turned on at the same time, the airplane would become a huge source of interference to other planes. That problem worsens when planes are aligned in parallel awaiting takeoff.
Similarly, most circuits and systems are subject to interference from internal "unwanted" transmitters and receivers intrinsic to the design. That interference can radiate to other systems. Needless to say, electromagnetic interference (EMI) has received lots of attention over the past few years. Regulatory bodies (the FCC in the U.S. and the EC, or Conformite Europeenne, in Europe) govern higher-frequency products so they comply.
One of the most popular directives, the EMC (electromagnetic compatibility) directive (code 89/336/eec), prevents radiation that can interfere with neighboring applications. But while it's crucial to pass all mandatory certification tests, it's also important to consider optimizing the design for performance.
This article describes methods and tips on how to suppress EMI in your design and still maintain high performance levels. It underscores the importance of component selection. Also discussed is the use of multilayer boards to reduce loop areas that cause radiation to be propagated and transmitted.
HIGHLIGHTS
The Most Suitable Devices TTL and ECL (bipolar) devices are preferable in terms of RFI and radiation. Ringing does occur, though, particularly if there's a heavy capacitive load at the output of an op amp. Discussed are two methods that can eliminate such noise.
Preventing Rectification Interference should be kept to a minimum to avoid rectification. The most direct approach is to use filters at the op amp's output. Amplifier oscillation can be a problem, however, so a couple of quick fixes are presented.
Board-Level Protection EMI is propagated in the presence of electric and magnetic fields. The most effective way to reduce these fields is to enclose the board in a metal box. Also, using a multilayer board reduces the loop areas that can cause radiation to propagate and transmit.
Choosing The Op Amp If the design requires a precision amplifier, and the source impedance is relatively low, it's preferable to choose an amplifier with low voltage noise, high common-mode rejection ratio (CMRR) versus frequency, high power-supply rejection ratio (PSRR) versus frequency, wide dynamic range, and high phase margin.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
Electronic Design UPDATE provides readers with late-breaking news, opinions from industry experts, and timely technology stories. It's a unique opportunity to get your product message in front of engineers, engineering managers, and corporate managers while they're reading about critical information online.
Reprints
