Digital circuit PCB EMI control technology - the follow-up

In three, EMI the other means of control
 1 power supply system design
 The design of low impedance power system, ensure below fknee frequency range of the power distribution system impedance is lower than the target impedance.
 The use of filters, control the conduction interference.
 The power supply decoupling. In the EMI design, to provide a reasonable decoupling capacitor chip, can make reliable work, and reduce the power of high frequency noise, reduce EMI. Because the wire inductance and parasitic parameters, power supply and power supply wire slow response speed, which will make the required to drive in high speed circuit instantaneous current. Rational design of bypass or decoupling capacitor and a power layer of the distributed capacitance in power, before the response, with a capacitor energy storage function quickly for device provides current. Correct capacitor decoupling can provide a low impedance power path, which is the key to reduce common mode EMI.
 The 2 ground
 Grounding design is to reduce the whole board EMI key.
 Confirmed by single point earthing, earthing or earthing methods of hybrid.The digital, analog, the noise to be separated, and determining a suitable public ground.The dual panel design without a ground layer, is the rational design of ground grid is very important, should ensure that the wire width > power line width > signal line width. Can also use a large area of the floor, but it should be noted that in the same layer on a large area of coherence.The design for multilayer boards, should ensure that there is a ground plane layer, reducing the total impedance.
 The 3 Series Damping Resistors
 In the circuit timing required to allow for the premise, suppression of interference source of the basic technology is in the key signal output terminal string into a small resistor, usually with 22 to 33 ohm resistor. These output series resistor can slow down the rise / fall time and the overshoot and undershoot signal becomes more smooth, thereby reducing the output waveform of high frequency harmonic amplitude, achieve the purpose of effectively inhibit EMI.
 The 4 shield
 The key device can use EMI shielding materials or shielding network.* to key signal shielding, can be designed into a strip line or in the key signal to ground on both sides of the isolation.
 5 spread spectrum
 Spread spectrum ( DSSS) method is a new effective method to reduce EMI. Spread spectrum is the signal modulation, signal energy is extended to a relatively wide frequency range. In fact, the method of clock signals in a controlled modulation, this method can not obviously increase the clock signal jitter. Practical application proves that the spread spectrum technology is effective, can be reduced by 7 to 20dB radiation.
 6.EMI analysis and testing
 The simulation analysis
 To complete the PCB wiring, you can use the EM I simulation software and expert system simulation analysis, simulation of EMC / EMI environment, to assess whether the product satisfies the standard requirements of relevant electromagnetic compatibility.
 The scan test
 Using the electromagnetic radiation scanner, the assembly and power machine disk scanning, PCB electromagnetic field distributions in the diagram ( Figure 3, shown in red, green, blue and white area indicates that the energy of electromagnetic radiation from low to high), according to the test results of improved PCB design.
 Four, conclusion
 With the new high speed chip development and application, the signal frequency is also more and more high, and bearing the weight of their PCB plate may be increasingly small. PCB design will face more grim challenge EMI, only to constantly explore, innovation, to make PCB board EMC / EMI design success.