COPYRIGHT (c) 1995, 2003 - ALL RIGHTS RESERVED
Ron Brewer
THREE SIMPLE STEPS TO EMC COMPLIANCE
The military and European EMC specifications control both the emission and susceptibility of a system. Because the radiated emission and susceptibility characteristics are interrelated, a coordinated design program is required to meet the combined design requirements. There are three simple steps to meeting these requirements. All three steps are inter-linked.
STEP 1: MINIMIZE CIRCUIT AMPLITUDE/BANDWIDTH
Slowing down is not a popular option, but many circuits are operating at high speeds, and many low speed circuits are using high speed logic devices, when it is not required. Since radiated emission is a function of frequency squared, using slower speeds (with reduced bandwidth) will reduce emissions as well as help to desensitize the circuit to external susceptibility fields.
Choose a logic family with limited rise time. Design the circuit to operate at its slowest speeds consistent with the functionality of the system (minimize bandwidth). Keep in mind that the higher speed devices have lower noise margins than low speed devices. This results in greater susceptibility. If greater processing capability is required use low speed parallel processing rather than high speed serial.
Reduce the circuit bandwidth through the use of lowpass suppression components, i.e. filters, ferrite beads, and bypass capacitors. These devices will significantly decrease high frequency harmonic amplitudes. Unfortunately there will also be a corresponding undesired decrease in both the processing capability and system reliability.
Design the circuit utilizing lower drive current components. Both the conducted and radiated emission are directly effected by current demand. Unfortunately this improves emission while simultaneously increasing susceptibility.
STEP 2: MINIMIZE PCB COUPLING LOOP AREAS
High speed systems require high frequencies with wider bandwidths. Reduction of the highest frequency circuit loop areas is most important.
For two sided PCB's, layout the clock, LSB, and sensitive analog loop areas first. Provide segregation and isolation between the analog and digital circuits. Use high quality decoupling capacitors at each active device, and select their value based on switching time, demand current, and self resonant frequency (SRF). SRF should be equal to or greater than the 5th harmonic.
Utilize multilayer PCB's whenever possible. Multilayer PCB's reduce loop areas by as much as 40 to 60dB, permit controlled impedance design, and allow isolated ground planes to be fabricated into the board.
Place high frequency circuits with signals that leave the PCB close to their respective connectors, otherwise separate as much as possible.
STEP 3: SHIELD PROBLEM CIRCUITS.
Shielding is the only non-invasive suppression technique and can be used for both radiated emission and/or susceptibility signals. Since shielding is not inserted into the circuit it does not effect the high frequency operating speed of a current design and will not effect redesigns.
For existing designs, shielding can frequently be used as a stand alone solution to the EMC problem. In this case the shield design must provide 35 to 50dB additional attenuation to compensate for higher levels of emission and/or susceptibility at the PCB. The greatest problem is holes/seams in the enclosure.
In new designs, shielding should be used in conjunction with or after circuit layout and bandwidth reduction measures have been implemented. With good PCB design the shield attenuation can be 35 to 50dB less, and can take the form of small metal boxes located directly on the PCB.
Shielding frequently eliminates the need for EMI filters. In any case, if filters are required for conducted emissions, the shield provides a low inductance ground sink, and isolates filter inputs and outputs, preventing RF energy bypass. Shields also provide an isolated ground reference to reduce internal circuit coupling.
The primary design requirements for the shield are: constructed from a good conductor, completely encloses the problem circuit, and has no RF holes. Hole performance deterioates with increasing frequency. High frequency attenuation is determined by aperture size and spacing.
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