1 EMI - The Basics

In today’s portable and consumer applications, space has become a premium, and engineers are often required to eliminate enclosures and shielding, and suppress EMI and noise through other means such as better segregation at the circuit level. Smaller space and higher functionality require high-density PCBs, and the use of wafer-scale packaging with tiny PCB design rules makes EMI more of a concern.

EMI encompasses two aspects. Emissions refer to the scope to which equipment generates radiated noise. Susceptibility is the scope to which equipment is affected by emissions generated from other electromagnetic waves. The degree to which a designer controls unintended emissions may make the task of susceptibility easier. Emissions are generally classified as radiated and conducted emissions. Radiated emissions leave a circuit board, trace, or wire, and propagate through the air in the form of electromagnetic waves to interfere with a nearby receiver. It is important to note that a "receiver" refers to any circuit whose operation can be affected adversely by the reception of electromagnetic energy - such as a PCB trace or even the lead of an IC. Conducted emissions refer to energy which escapes, or is conducted, out of a circuit through wires or cables. Conducted emissions may cause problems directly or manifest themselves as radiated emissions.

In order to understand emissions, it is important to understand antennas. In Figure 1-1, the well-known physical relationship between wavelength and frequency is shown:

Figure 1-1 Wavelength as a Function of Frequency

The shortest length required to be an efficient antenna is λ/4. In the case of air, permittivity is 1, but in the case of FR4 or glass-epoxy PCBs, permittivity is reduced to approximately 4.8. The effect causes a signal traveling a trace to slow once it reaches the dielectric gradient created by the FR4 material, causing essentially, a "wavelength- shortening" effect. For example, a 200 MHz signal has a quarter wavelength in air of 16.7 cm.

In an inner-layer PCB trace, it is: 16.7/4.8^(1/2) = 7.6 cm.

A PCB trace can act as an unintentional antenna even at lengths shorter than λ/4, increasing both emissions and susceptibility. Surface traces also exhibit this wavelength shortening effect, as one side of the dielectric serves to change the overall permittivity of the transmission.

Unintended antennas, such as PCB traces, are the key culprit behind radiated noise in digital systems. As you will see, the Class D audio amplifier is, in essence, a digital system from the perspective of radiated emissions. One key principle underlying electromagnetism is that of reciprocity. A flow of current can create an electric field, and a change in electric flux can induce a current to flow. Likewise, an antenna that is good for receiving is also good for transmitting. If any of the dimensions of an unintentional antenna approach quarter wavelength at the frequency at which that antenna is excited by noise current, radiated emissions can be expected.

Figure 1-2 Common Antennas

These are two common antenna designs: the simple dipole and whip shown in Figure 1-2. An interesting observation is that the whip is essentially one half of a dipole with the horizontal ground essentially being induced to act as the other half of a dipole.

Antennas are intended to transmit and receive signals through the radiation of electric energy. But, as illustrated in Figure 1-3, unintended antennas in circuit boards can include:

• Long traces
• Vias
• Component leads and pins
• Unpopulated PCB board connectors and headers

Figure 1-3 Unintended Antennas in PCBs

An unterminated surface trace or an unterminated buried trace can become an unintended whip antenna. Segments of traces at different RF potential due to poor layout can become unintentional dipole antennas. Also, the conductive layers of the PCB can act as the other leg of a dipole antenna with the plane itself being coupled into the electric field.