018-019_EETE-VF

EETE JANUARY 2013

TEST & MEASUREMENT Near field analysis for EMI/EMC By Deepak Kumar Behera, Sumit Varshney , Sunaina Srivastava and Swapnil Tiwari EMI/EMC CharaCTErIzaTIon is essential for many electron- ics products. The high speed signals may induce not only signal integrity and power integrity issues, but also cause radiation and electromagnetic interference problem. For low frequency signals (up to 100Mhz) radiated and conducted emission can be ignored but as wavelength becomes comparable to the trace length circuit approximation is no longer valid. The trace geometry and its separation become important in case of PCB. The signal’s rise time, fall time, period and pulse width play the role in determining the EMC of PCB’s. a lack of knowledge of fundamental EMI sources also makes anticipating potential EMI problems at the design stage troublesome. The designer should take care of EMI sources so that the product complies with FCC limits. Conducted & radiated emissions Fig. 1: Near field at 200 MHz The two most common EMI types are conducted and radiated. Far field & near field Conducted emissions are the radio frequency noise present in The electromagnetic field surrounding an antenna can be the physical wiring and traces of any electronic system. This re- divided in 3 regions i.e. reactive near Field, radiating near sults in unwanted common mode and differential mode currents Field and Far Field. Far field patterns give information about within a system. The frequency range where conducted emis- the radiation. at the most they can inform about the geometry sions are regulated is typically lower than the frequency range by using scattering methods but they are not helpful in locating where radiated emissions are regulated. The longer wavelength the source of EMI. The antenna used for measuring the far field where conducted emissions are a problem needs a much larger is too big to detect the source. In contrast, near field probes antenna to radiate and receive electromagnetic interference are used to locate the EMI sources. They can locate the exact than the shorter wavelengths studied for radiated emissions. source of emission on the spot. however there is very little Conducted emissions are regulated by the FCC over the fre- correlation between the near field and far field measurements. quency range 450 khz to 30 Mhz and the CISPr 22 conducted Though, the higher the radiation in the near field, the higher it emission limits extend from 150 khz to 30 Mhz. a Line Imped- will be in the far field. The test performed in the near field gives ance Stabilization network (LISn) is used to perform conducted the data which can be used to predict the behaviour of the far emission measurement. field. This can be used to find out the frequency band where the radiation will be maximal. In this article we use the same radiated emission refers to the unintentional release of property in our simulation to correlate the near field and far field electromagnetic energy from an electronic device. In general data. radiated emissions are usually associated with non-intentional radiators, but intentional radiator can also have unwanted The FCC measurement distance is 3m for Class B products emissions at frequencies outside their intended transmission and 10m for Class a products. For CISPr 22 (En55022) the frequency band. These intentional and unintentional radiators measurement distance is 10m for Class B products and 10m can be monopole, dipole or loop antennas. The frequency range for Class a products. The lower frequency of 30Mhz is one of measurement for domestic radiated emission is from 30 Mhz wavelength at 10m, whereas the frequency of 1Ghz is one to 1 Ghz. In this measurement, product under test is placed at wavelength at 30cm. When we are measuring the far field we some distance from a well defined antenna and the amount of are looking for uniform plane waves whose phase is equal at an radiation is measured across the required frequency band. In instance of time. The distance from where the far field of any this process a receiving antenna is kept at the far field of the antenna start is 2D2/λ, where D is the maximum linear dimen- radiating body. Far field is the field where radiation pattern does not change shape with distance. This region is dominated by ra- diated fields where E and H fields are orthogonal to each other and the direction of propagation. Deepak Kumar Behera - (deepak@freescale.com) and Swapnil Tiwari (b36245@freescale.com) are Design Engineers at Freescale – www.freescale.com Sumit Varshney (b17255@freescale.com) is staff Design Engineer. Sunaina Srivastava (b18098@freescale.com) is Lead Design Fig. 2: Far field radiation at 200 MHz Engineer, also at Freescale 18 Electronic Engineering Times Europe January 2013 www.electronics-eetimes.com


EETE JANUARY 2013
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