1、12 The Contents 1.Noise Sources in Digital Equipment 2.Suppressing EMI Noise Emission 3.How to Select and Use EMI Suppression Filters 4.Conclusions3 1.1 Digital signal and harmonic Components 1.2 Radiated Noise from Digital Circuit Boards 1.3 Radiated Noise from Cable 1.4 Causes of Common Mode Noise
2、4 Digital signals are rectangular waves made up of a large number of sine waves.Digital signals waveform contains fundamental waves whose frequency is the signals cycling frequency and harmonic waves whose frequencies are integer-multiples of the fundamental waves frequency.5 When the duty ratio is
3、50%,only the odd order harmonics are included;otherwise,some even order harmonics are also included.6 1.2.1 Noise generated by IC 1.2.2 Radiated noise from patterns 1.2.3 Effect of EMI suppression Filter 7Radiation of noise requires both a noise source where noise is generated and antenna from which
4、 noise can be received.In this experiment,the circuit board contained noise source ICs but no antenna.8 A pattern that will act as the antenna forradiated noise is connected to the noise source IC2.9 an EMI suppression filter to suppress noise that is radiated from a signal pattern acting as an ante
5、nna.10 cable is more efficient in radiating noise.Suppression method11IC4 is added as the interface circuit,a 1 m long signal cable and a grounding cable are connected to its output.12Suppression method:1314 When a signals return current flows to ground,the ground patterns inductance causes a potent
6、ial difference developed between the grounds of IC1 and IC2.It is possible that the noise caused by this potential difference flows through the grounds and the interface circuit to the cable.The common mode noise generated in this manner can be effectively suppressed by reducing the ground impedance
7、,suppressing the signal current,or inserting an EMI suppression filter in the signal line.15 2.1 Approaches to Suppression Emission of EMI Noise 2.2 Principal Emission Suppressing Methods 2.3 EMI Suppression Filters 2.4 Improved Ground Patten 2.5 Changing Component and Pattern Layout 2.6 Shielding16
8、In typical electronics circuits,noise is radiated from both circuit patterns and connecting cables.17 EMI noise emission suppression model18Guidelines to select noise suppressing point Level of direct emission Strong-Weak from noise source Number of noise Many-Few transfer routes(cables)Number of no
9、ise Few-Many sources(ICs)Suppress noise sourceSuppress noise at transfer route1920 It is important to thinking about how to use EMI suppression filters from the design stage and not when the prototype board is completed.More effective noise suppression can be achieved by installing the EMI filters a
10、t the potential noise source and conductive noise paths during the initial design stage and then removing unnecessary filters.21 The basic rule of EMI suppression filter application is to locate it near the noise source or the noise receiving unit so that the effectiveness will not be degraded by co
11、upling of the filters input and output wiring.When applying EMI suppression filters on a cable,the circuit requiring the noise suppression should be considered the noise source or the noise receiving unit and the filter is located at the root of the cable.22 2.3.1 Use Inductor Type EMI Suppression F
12、ilter 2.3.2 Use Capacitor Type EMI Suppression Filter23 Example:Ferrite bead inductor Inductor type EMI suppression filters are inserted in series with the signal path.The inductor type suppression filter generates impedance that is primarily resistive at the high-frequency band where EMI noise beco
13、mes problem and uses this resistance to absorb noise.24 Examples:Three-terminal capacitors,EMI suppression filters for signal lines Capacitor type EMI suppression filter is inserted between the line through which noise is transferring and the ground.The filters ground terminal is attached as follows
14、:25(a)When used at noise source Filters ground terminal is connected to the noise source sides ground,this enables bypassing of normal mode noise to the ground.(b)When used on conductive noise path The common mode noise is the problem,capacitor type EMI suppression filters ground terminal is connect
15、ed to a ground that is very stable.26used at the noise source27used at the noise source28used on conductive noise path29used on conductive noise path30Ground impedance is reduced by making the ground patternbetween the signal ICs input and output wide and short.This minimizes the potential differenc
16、e relative to the ground31 Common mode noise can be minimized by using a wider and shorter ground pattern between the signal input and output ICs to reduce the ground impedance32Improve ground pattern with ground plane33 Component and pattern layout should be done by giving priority to shortening th
17、e length of high-speed signal patterns.Common mode noise that is generated by or emitted from signal patterns is stronger when the signal frequency is higher and the signal path length is longer.34 Cable should be located away from high-speed signal circuit to minimize noise coupling.35 Examples of
18、how signal frequency influences noise levels.The faster the signal frequency,the higher the noise levels extend to higher frequency ranges.Influence of signal frequency 8MHz 16MHz 25MHz36The path length caninfluence radiated noise.Frequency band of noisethat is radiated changes with the pattern leng
19、th L.Normally,minimizing the pattern length results in lowering of overall noise radiation level.37 L=20cmL=10cmL=5cmInfluence of signal path length38 Principle of shieldingThe principle of shielding effect is produced by reflection and absorption of unwanted noise.Reflection is the dominant factor
20、for high-frequency band as is present in electromagnetic field radiation test.The basic shielding materials for this frequency band is aluminum or comparable low impedance material.39 Making the shield cases openings smaller in size and in intervals Heat ventilating openings that consist of interspe
21、rsed smaller holes will provide better shielding effect.40)2log(20lSshielding effectiveness413.1 EMI Suppression Filter performance and Signal Waveform3.2 Circuit Impedance and EMI Suppression Filters Performance3.3 Selecting Capacitor Type or Inductor Type EMI Suppression Filter3.4 Examples of EMI
22、Suppression Filter Use at Noise Source 3.5 Examples of EMI Suppression Filter Use on Conductive Noise Path 42 In addition to suppression noise,EMI filters may cause distortion of signal waveform.When the capacitance value of three-terminal capacitor is increased,impedance of inductor is raised,or cu
23、toff frequency of EMI suppression filter for signal lines is lowered,the noise suppression performance is increase but the distortion of signal waveform also increase.4344 Insertion loss of EMI filter varies depending on thecircuit impedance.45 1.At Noise Source(a)Capacitor type EMI suppression filt
24、er as primary device Line with high circuit input or output impedance Line with high noise level(Ex.Clock line;control bus line)(b)Inductor type EMI suppression filter as primary device Line with low circuit input or output impedance(Ex.Power supply line to which bus controller is connected)Line wit
25、h relatively low noise level(Because filter grounding is unnecessary,and installation is simple)Line requiring current control(Ex.Multiple lines that switch simultaneously and in which large current flow to the ground:Address/data bus;control bus)46 2.Noise on conductive path Generally,both capacito
26、r type and inductor type filters are used in combination for suppression noise in a conductive noise path.When noise level cannot be reduce adequately by using filters in conductive path alone,filters must be used at the noise source as well.47 Bus lines contain many lines that switch on and off sim
27、ultaneously.Especially on data and address bus lines,an instantaneous large current flows into the GND and power supply lines.Therefore,it is necessary to suppress the current flow on the signal lines.48 Current is suppressed mainly by installing the inductor type EMI filter in the oscillator circui
28、t output,when necessary,capacitor type EMI filter is used in combination.49 For EMI noise suppression where signal cables are connected,chip solid EMI filter,a three-terminal capacitor product,and chip Ferrite Bead Inductor are used in combination.50 Both normal mode and common mode noises exists in
29、 power supply line.(in a circuit with relatively stable ground,normal mode noise is the primary noise,in a circuit with unstable ground,common mode noise is also present.)51 Describe various EMI noise source types from simple experiments using realistic circuits.Discuss the approach and the methods of suppressing against these EMI noise.Utilizing EMI suppression filters and examples.52
