The effects of electromagnetic interference (EMI) and radio frequency
interference (RFI) are particularly troublesome when designing printed circuit
board (PCB) assemblies for high-speed military and aerospace systems. The
components on a PCB may be digital and analog. Transmission lines connecting the
two different sections are used to transmit signals back and forth.
Unfortunately, as frequency increases and signals are enhanced, noise related to
those frequencies is also enhanced, thus creating EMI and RFI.
Electronic systems are expected to operate normally within a given environment without internally or externally radiating excessive amounts of electromagnetic energy. In this state, they are called electromagnetically compatible (EMC).
However, when electromagnetic energy is radiated adversely it becomes EMI, disrupts circuit performance and makes overall PCB operation less optimal, falling short of original design goals. The two main sources of electromagnetic radiation from a PCB are radiation from signal loops and circuit tracks.
The radio frequencies that are by-products of an electronic device’s operations range from about 10 kHz to 100 GHz. These can be either radiated emissions through a medium such as an electromagnetic field, or conducted emissions through a medium such as a propagating wave through wire or interconnect cables.
Successfully reducing EMI/RFI calls for careful preparation, planning and close interaction between an OEM and an electronic manufacturing service (EMS) provider. Further, the EMS provider must focus on a set of five key design measures to significantly suppress EMI/RFI effects. These are using differential pairs, multilayer PCB construction, shielding clock signals, properly placing analog and digital components and properly bifurcating power and ground planes (Table 1).
By carefully and correctly implementing these design steps, EMI/RFI can be reduced by 90% or more. Omitting or giving little attention to any one of them can boost EMI/RFI to increase noise and crosstalk levels well beyond a design’s specifications, thus impeding proper signal transmission and reception.
Differential Pairs
Differential pairs refer to the traces that provide paths for transmitting and returning current (Figure 1). These parallel traces are separated by a minimum distance. Tolerance is normally to within 10% in order to reduce EMI. For example, if a transmit line is 10 mils long, then the return path should be 9 to 11 mils long. Hence, the difference between the two should be 10% or less. In military and aerospace applications, differential pair tolerances in extremely high-frequency, high-speed circuitry must be considerably less than this at 5%.
If tolerances exceed these low percentages and increase to 20% or 25%, for instance, the transmit path becomes shorter than the return path by 20% to 30%. Hence, transmit-to-receive ratios become unbalanced, causing increased EMI/RFI. PCB layout software packages, such as Cadence Allegro or Mentor Graphics’ PADS Power PCB, can alert designers to differential pair tolerance variances.
PCB Layer Construction and Geometries
For high-density, high-speed applications, multilayer PCBs containing multiple power and ground planes are used. Compared to single- or double-sided boards, these PCBs greatly reduce the effects of EMI/RFI and allow designers to closely control impedances on certain layers. They also allow high component densities and make it easier to route signal and power tracks.
Layer construction can play a vital role in reducing EMI/RFI. Symmetrical stripline geometry is the best approach in minimizing or eliminating EMI/RFI. A single signal layer is sandwiched between two ground layers so the ground planes absorb virtually all noise and crosstalk. In addition, ground plane redundancy suppresses all noise and crosstalk.
One electrical requirement of an efficient circuit is providing a low impedance path for current to return to its source at both DC and high frequencies. Therefore, it must have low resistance and inductance. Symmetrical stripline geometry delivers an ideal balance for EMC because its solid ground plane provides a stable reference for all the board’s current returns.
Coplanar geometry is RFI-specific, and also helpful for reducing EMI. It creates an antenna effect, or solid closed loop, to provide a clear signal path. As much solid ground is made possible on both sides of a trace so that a signal passing through that trace is heavily guarded. |
