The essence of EMC noise
[introduction]Spectrum is decompose electromagnetic wave for sine wave heft, press the spectrum that wavelengh order arranges, decompose the thing that has complex composition namely (spectrum analyzer) for pure composition, the volume that measures these part by its feature is arranged according to foreword (part not plan) , cross axle serves as frequency, x-axis serves as power or voltage.
The meaning of spectrum
Spectrum is decompose electromagnetic wave for sine wave heft, press the spectrum that wavelengh order arranges, decompose the thing that has complex composition namely (spectrum analyzer) for pure composition, the volume that measures these part by its feature is arranged according to foreword (part not plan) , cross axle serves as frequency, x-axis serves as power or voltage.
From the graph 1-1 can see, digital weaveform is formed by a variety of frequency overlay, oscillograph and spectrum appearance obtain the different information of weaveform of a number from two perspectives respectively, oscillograph obtains Bit information, spectrum appearance obtains spectrum information or energy news.
In the pulse weaveform that represents switch signal in graph 1-2, include Tw (pulse width) with Ts (rise / decay time) .
Graph 1-3 is the theoretic pulse form spectrum that is based on Fourier commutation, this is one changes spectrum continuously, amplitude as frequency elevatory and damply, attenuation slope changes as Tw and Ts. The spectrum after blue line states the Ts of pulse slows changes, slope turns into – when 40dB/dec
Frequency is reduced (deflection of towards the left) , final result is ever since amplitude decreases, become namely the amplitude attenuation of the spectrum when Ts defer, spectrum drops.
Affect the factor of spectrum
Nethermost graph is amplitude is
logarithm graph, 
It is to be based on with 1
Voltage is worth for the DB of fiducial electric pressure ratio (1 ＝ 0dB
) .
Graph 1-4: Initiative and undee
Graph 1-5 is the spectrum when increasing frequency 2MHz, from frequency- – amplitude (
) concern plan can see clearly, when base when wave frequency heighten, whole spectrum is met right (a side with high frequency) deflection.
Graph 1-8: Only Tr (rise time) decelerate
Brief summary:
High frequency change—> spectrum whole increases
Rise / drop speed slow down—> low frequency Duan Cui is decreased – 40dB/dec
Duty change—> produce even number second tall second harmonic, but do not have an influence to the peak value of spectrum, base wave drop
Rise only speed slow down—> ascendant composition is decreased in low frequency Duan Cui
Frequency is lower—> rise / drop slower, spectrum is lower
EMC concept is defined
EMC (Electromagnetic Compatibility) , electromagnetism compatibility, namely incorrect other equipment generates electromagnetism interference, when and accept arrival be being disturbed from the electromagnetism of other equipment, the system moves insusceptible, still hold original property. EMI (Electromagnetic Interference) , electromagnetism interference, because IC job produces noise EMI, bring the electromagnetic wave that disturb or disturbs a gender to circumjacent IC and system, so need designs the circuit that does not produce EMI.
EMS (Electromagnetic Susceptibility) , electromagnetism disturbs susceptibility or electromagnetism sensitivity, the ability that although also won’t be caused by EMI influence,disturbs and be able to bear or endure suffer a gender, need design can bear the dependability circuit of EMI. EMI cent is two kinds inside test domain, conduct noise (Conducted Emission) with radiation noise (Radiated Emission) .
Conducting noise is point to by route system or the noise that PCB board wiring conducts, radiation noise is to point to discharge (radiation) the noise in the environment. To these noise, have respectively in EMS be able to bear or endure requirement sufferring a gender, call conduct resistance (Conducted Immunity) with radiation resistance (Radiated Immunity) their concern is as follows:
Graph 1-9: EMC branch
Radiation mechanism
Graph 1-12: Concept of displacement electric current
To the standard that radiation is worth, each domains, the level of each national basises is different, world general standard is CISPR, and Japan: VCCI Class, United States: FCC, Europe: EN, still have even if serve as CISPR fiducial come from definition set to provide a value, the different fractionize of products of each standard basis electron is each seed standard.
Article origin: EDN electron technology is designed
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