EDNE June 2012 - (Page 31)

bakEr’s bEst By Bonnie Baker in the frequency domain. However, FCCand European CISPR-certified companies must perform all radiated EMI measurements before the release of their products to the market. This requirement ensures that the test results are accurate according to FCC or CISPR regulations. Test methods include environmental conditions, along with calibrated EMI test equipment and antennas. The FCC and CISPR require that the radiated signals that your equipment transmits comply with specified values. FCC- and CISPRrelated documents include EN 55011, EN 55013, EN 55014, EN 55015, EN 55022, and EN 50081-1.2. Figure 2 shows Class A limits for electronic equipment for use in commercial, industrial, or business environments. Class B limits apply to electronic equipment for use in the residential environment. Equipment in residential environments may also be subjected to the Class A limits. Class B limits are more restrictive because of the likelihood of the equipment’s close proximity to TV and radio receivers. We are getting close to the devices in your circuit. Next month’s column will discuss EMI-conducted radiation.EDN Read the first parts of this series; www.edn-europe.com/article. asp?articleid=5477 2 www.edn-europe.com/article. asp?articleid=5508 1 H EMI problems? Part three: strength of EMI-radiated signals ow far away from radiating sources do you need to be so that the radiated signal does not interfere with your system? As you answer this question, consider both the amount of radiated energy from a source and your system’s electromagnetic-interference-protection circuitry. Radiating EMI signals propagate from a source to a receiving element (references 1 and 2). The power, or voltage level, of these signals as they hit your sensitive circuits depends on the transmitter’s power and antenna gain and the distance between the source and the receiver (Figure 1). Electric-field strength quantifies the magnitude of the interfering voltage at the source. This narrowband or broadband EMI signal unit of measure is in volts per meter. You can modify the units for the electric-field strength to your liking by converting them to decibels referenced to microvolts, where dBμV=20log(V)+120 μV. A narrowband EMI signal typically is a repetitive signal or pulse train. The equation in Figure 1 allows you to quickly get a worst-case prediction of the radiated voltage, ER, at a specific distance from the EMI source. A broadband EMI signal typically is a single pulse, such as a lightning strike, an electrostatic-discharge event, ER(V/m)=5.5√(PS)/D. or a spark gap. These pulse-type events contain multiple frequencies. Broadband signals are difficult to measure because they are nonrepetitive and fast. Radiated-power-density units can also describe narrowband events. The unit of measure for the EMI narrowband, radiated-power density is watts per square meter. Communications engineers use the power-density representation of an EMI signal for their narrowband EMI issues. You can convert the radiated-power-density units to decibels referenced to milliwatts, where dBm=10 log(W). You can use an oscilloscope to observe EMI signals in the time domain and a spectrum analyzer to evaluate EMI signals 50 45 PR(W/m2)=P /(4πD2). S AREA P S D PR ER P S D 40 FIELD STRENGTH (dBμV/m) 35 30 (a) (b) 25 10 100 FREQUENCY (MHz) FCC CLASS A FCC CLASS B CISPR CLASS A CISPR CLASS B 1000 Figure 1 The power, or voltage level, of these signals as they hit your sensitive circuits depends on the transmitter’s power and antenna gain, the distance between the source and the receiver, and whether the circuit is using a narrowband EMI signal at electric-field strength (a) or a broadband EMI signal at radiatedpower density (b). PS is the source power in watts, D is the distance in meters, ER is the receiver’s electric-field level, and PR is the receiver’s power in watts. Figure 2 Class A limits are for electronic equipment for use in commercial, industrial, or business environments. Class B limits apply to electronic equipment for use in the residential environment. www.edn-europe.com JUNE 2012 | EDN EuropE 31 http://www.edn-europe.com/article.asp?articleid=5477 http://www.edn-europe.com/article.asp?articleid=5508 http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE June 2012

Cover
Agilent Technologies
Contents
International Rectifier
RS Components
Masthead
International Rectifier
Comment
Pulse
Analog Devices
Digi-Key
Farnell
NXP
Test & Measurement
Silicon Labs
Digi-Key
Test-driven development for embedded C: why debug?
Digi-Key
Baker’s best
Cover story
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Rohde & Schwarz
Pico-projector design uses color LEDs
Digital isolation in smart energy metering applications
Mechatronics in design
Teardown
Design Idea
Product Roundup
Tales from the cube

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