How Limiting the Effects of EMI Improves Signal Integrity
- Electromagnetic interference, or EMI as it is commonly known, is a major cause of signal integrity problems for PCBs. All electronic circuits generate EMI and all are affected by it.
- EMI may be caused by unexpected radiated emissions as well as though interference conducted along cables and traces. Common reasons for high EMI in PCB circuits include poor grounding, lack of shielding and poor circuit design. Methods to control EMI include the elimination, attenuation and control of emissions at their source.
EMI is coupled to other circuits through radiation or conduction. In both instances, the degree of coupling and the magnitude of the EMI increases with frequency.
International EMI standards specify the acceptable levels of EMI emissions. Class A devices that are intended for business use have slightly lower, more relaxed limits than Class B devices, intended for domestic use. Class B limits are around 25 percent more rigid than those for Class A. The two main bodies controlling EMI Standards are the FCC and the European CISPR. Although in many ways these standards are similar, the CIPSR requirements are slightly tighter.
Although this article is primarily about EMI susceptibility, for electromagnetic compatibility to exist, products should be immune to generally acceptable levels of EMI and should not generate EMI that exceeds the levels specified in the standards. Make sure to limit excessive EMI radiation from entering or exiting the board at the entrance point.
EMI can originate from external or internal sources and can be conducted over cables, connectors and traces or radiated. EMI sources include:
High frequency signals: High-frequency signals are more prone to excessive radiation, especially if proper shielding is not done. At high frequencies, the inductive effect of a trace or line increases, and as a signal is transmitted, its magnetic field couples the signal to adjacent lines. The effects are exacerbated by higher frequencies associated with fast signal rise time.
Signal harmonics: Square wave signals contain odd harmonics, particularly the third, fifth, seventh and ninth harmonics. These harmonics are sources of EMI.
Transients: External conducted and radiated transients exacerbate EMI. Sources include lightning, circuit breaker switching, power switching devices, power converters and disturbances on power and communication lines.
Internal EMI sources: Fast signal transitions are strong EMI emitters. These include high-frequency clock signals, high-speed data lines and large di/dt variations due to switching of high-speed logic circuits.
Impedance mismatching: High-frequency signals in PCB circuits exhibit transmission line characteristics, and reflection and ringing occur if the receiver and transmitter impedances don’t match the characteristic impedance of the trace or line.
How Is EMI Coupled?
EMI is coupled to other circuits through radiation or conduction. In both instances, the degree of coupling and the magnitude of the EMI increases with frequency. Characteristics of the two coupling methods include:
Radiation: EMI radiation occurs primarily at frequencies above 30 MHz. Internally, EMI can be radiated by the electric field generated by a fast transient signal that utilizes a stub or loop as an antenna. EMI can also be inductively coupled. Traces without a return path will generate common mode radiation and those with a return path differential mode radiation. The radiation far field strength is directly related to frequency in single-ended circuits and to the square of the frequency for differential signal circuits.
Conduction: This occurs when the interfering signal travels along input cables or connecting wires. Coupling frequencies are generally less than 30 MHz. Any line entering or leaving a PCB board is susceptible to coupling radiation. Interconnecting cables between two sources are a good source of EMI transferring one path to the other side of the board. As such, long cables help control EMI.
Designing Circuits to Minimize EMI
Techniques to minimize EMI include the control, attenuation and elimination of emissions at source as well as designing for clean and smooth transitions that are precisely placed in time. Here are several ways this can be achieved:
Good PCB layout and design: Keep trace lengths short and away from the edge of the board (five line widths). Use 45 degree bends, and avoid microstrips but use striplines instead. Avoid layer changes (especially where signal paths are concerned), and don’t route high speed signals over slots. Reduce inductive effects by employing three terminal capacitors, keeping differential traces close and placing other traces at least three trace widths apart.
Adopt shielding: Shield high frequency circuits with metal enclosures and use shielded cables bonded to enclosures.
Apply transmission line design: Design PCB layers and trace widths to provide the right line impedance, and match the source and receiver impedances to the trace impedance using terminating resistors. As a guideline, transmission line theory should be applied to any line length in inches that exceeds the signal rise time in nanoseconds. Although metal traces are resistive at low frequencies, they are largely inductive at high frequencies.
Pay careful attention to grounding: Use a large, unbroken ground reference plane and connect ground islands to the ground plane using numerous vias. Ensure that return paths in the ground plan don’t cross.
Decouple: Employ low-pass filters to attenuate high-frequency signals using ferrite core inductors around cables. Fit decoupling capacitors on fast IC power and ground connections to reduce radio frequency emissions, and consider using ferrite filter sheets to reduce resonance.
Ferrite Beads: Property resistance increases at high frequencies. They prevent high frequency signals to pass from one side of the board to the other.
EMI Propagation: This won’t affect DC or lower frequency signals or components, only higher frequency signals and high impedance.
Avoid antennas: Unconnected stubs and traces without return paths act as antenna as do ground and power loops.
Separate sensitive components: Assign different PCB areas for diverse circuits keeping oscillator circuits away from other components. Keep high speed components away from disturbing signals and from I/O connections.
Choose logic families carefully: Select logic families that have a high noise margin and avoid high speed logic.
Loop Area: To avoid excessive amounts of EMI, make the loop area as minimal as possible.
EMI affects PCB design in two ways, through EMI generation and EMI susceptibility. Although the focus has been on reducing the effect of internal and external EMI within PCBs, these techniques are equally effective in reducing EMI emissions. Limiting the deleterious effects of EMI significantly improves signal integrity on PCB boards.
Eric Bogatin talks about EMI and signal integrity:
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