Digital circuit PCB EMI control technology --to be continued

With the increased integration of IC devices, equipment gradually miniaturization and device speed is more and more high, electronic products in the EMI problems are more serious. From the system EMC / EMI design perspective, in the equipment PCB design stage handle EMC / EMI problem, is to make the system reach the standards of electromagnetic compatibility, the lowest cost of the most effective means of. This paper introduces the digital circuit PCB design EMI control technology.
 One, the generation and suppression of EMI principle
 EMI is produced because of the electromagnetic interference source through the coupling path of transferring energy to the sensitive system caused by. It includes a through wire or public ground conduction, by radiation or by near field coupling of three kinds of basic forms. EMI harm is done to reduce the transmission signal quality, the circuit or device caused by interference or damage, so that the equipment can not meet the EMC standards required by the technical requirements.
 To inhibit EMI, digital circuit EMI design should according to the following principles:
. according to relevant EMC / EMI specification, decompose the targets to the single board circuit, hierarchical control.
 The three elements of EMI interference from that source, energy coupling pathways and sensitive system these three aspects to the control, the circuit has a flat frequency response, to ensure the normal, stable working circuit.
 The front end design proceed with from equipment, pay attention to EMC / EMI design, reduce design cost.
 In two, PCB EMI control technology in digital circuit
 In the treatment of various forms of EMI, must be a concrete analysis of concrete problems. In the digital circuit in PCB design, can from the following several aspects of EMI control.
 The 1 device selection
 In the EMI design, we must first consider the selection device rate. Any circuit, if the rise time for the 5ns device for rise time for the 2.5ns devices, EMI will raise about 4 times. EMI radiation intensity and frequency is proportional to the square of the highest frequency ( fknee, EMI ) also known as the EMI emission bandwidth, it is the signal rise time rather than frequency function: fknee = 0.35 / Tr ( where Tr is a device to signal rise time )
 The radiation type EMI frequency range from 30MHz to GHz, in this band, short wavelength, the circuit board even very short wiring may also become a transmitting antenna. When EMI is high, the circuit is easy to loss of normal function. Therefore, in the selection of devices, the circuit performance requirements under the premise, should try to use low speed chip, using suitable driver / receiver circuit. In addition, due to a lead of the device pins have parasitic inductance and parasitic capacitance, so in the high speed design, device package form on the signal effect also cannot be ignored, because it is the important factor of EMI radiation. In general, patch the parasitic parameter is less than inserting device, BGA parasitic parameter is less than QFP package package.
 The 2 connector selection and signal terminal definitions
 High speed signal transmission connector is key link, is also easy to produce EMI weak link. In the terminal of the connector design can be arranged more needle, reducing the signal and ground interval, reducing connector produce radiation effective signal loop area, to provide a low impedance return paths. When necessary, to consider some of the key signal ground pin isolation.
 3 layer designAt the cost of licensing under the premise, increase ground layer number, the signal layer adjacent to the ground plane layer can reduce the EMI radiation. For high speed PCB, the power supply layer and ground layer adjacent to the coupling, can reduce the source impedance, thereby reducing EMI.
 The 4 layout
 According to the signal current to flow, reasonable layout, can reduce signal interference. Reasonable layout is the key to control EMI.
 The basic principle of layout is:The analog signal is prone to digital signals, analog circuit and digital circuit should be separated;
 The clock line is the main interference and radiation source, to stay away from sensitive circuit, and the clock line is the shortest;
 The big current, big power circuit to avoid arranged in the center of the board area, at the same time should consider heat and radiation effects;
 The connector to the side of the plate, and away from the high frequency circuit;
 The input / output circuit close to a corresponding connector, the decoupling capacitor is close to the corresponding power supply pin;Fully considering the layout of the power split feasibility, multiple power supply device to cross in the power supply region segmentation boundary placement, to effectively reduce the influence of EMI on plane segmentation;
 The return plane ( path ) does not split.
 The 5 wiring
 The impedance control: high speed signal line will show the characteristics of the transmission line, the need for impedance control, in order to avoid signal reflection, overshoot and ringing, reducing EMI radiation.
 The signal classification, according to different signal ( analog signal, a clock signal, a I / O signal, bus, power etc. ) EMI radiation intensity and sensitivity, so that the interference source and sensitive system as far as possible separation, reducing the coupling.
 The strict control of the clock signal ( especially the high speed clock signal ) line length, number of vias, cross partition, terminal, wiring layer, return path.
 The signal loop, i.e. signal flows to the signal into the formation loop, is PCB design EMI control key, the wiring must be controlled. To understand each of the key signal flow, for key signals close to the return path wiring, to ensure that the loop area of the smallest.
 On the low frequency signal, to make the current flow through the path of least resistance; the high frequency signal, to make the high frequency current flows through the inductor minimal paths, rather than the path of least resistance ( see Figure 1 ). For differential radiation, radiation ( EMI E ) is proportional to the current, the current loop and the frequency of the square area. ( where I is the current, A is the loop area, f is the frequency, R is the distance from the center to the loop, K is a constant. )
 So when the minimum inductance return path just before the signal wire below, can reduce the current loop area, thereby reducing the EMI radiation energy.
 The key signal not across regions.
 The high speed differential signal line as far as possible the tightly coupled method.
 * ensure stripline, microstrip line and its reference plane to meet the requirements.
 The decoupling capacitor lead wire should be short and wide.All signal lines should be kept away from the edge of the board.
 * for point to point connection network, choose the right topology, in order to reduce the letter, reflection, reducing EMI radiation.
 6 power plane segmentation
 The power supply layer segmentation
 In a main power plane having one or more sub power, to ensure that the power supply region of the coherence and enough copper foil width. Segmentation lines need not be too wide, generally 20 ~ 50mil linewidth can be, in order to reduce the gap of radiation.
 The ground layer segmentation
 The ground plane layer should maintain integrity, avoid segmentation. If it must be split, to distinguish between analog and digital ground, noise, and the outlet is connected through a public ground and external connected.
 In order to reduce the power of the edge of the radiation, power / ground plane should follow the 20H design principle, namely planar dimensions than the power plane of large size 20H, such edge radiation strength may be decreased by 70%.