Issue 146 September 2002
Killing the EMI Demon

Sources of EMI

EMI on a typical microprocessor board is related to the clock. If the clock is a square wave, it contains frequencies at the clock frequency and harmonics. A perfect square wave clock would have harmonic frequencies at f, 3 × f, 5 × f, 7 × f, and so on. For a perfect square wave, or any string of pulses with a fast rise time, the strength of the harmonics declines inversely with frequency. So, the eleventh harmonic would be one-eleventh as strong as the fundamental frequency. This corresponds to a decline in harmonic amplitude of 20 dB per decade.

Real time clocks are not perfect square waves, and pulses do not have infinitely fast rise times. As a result, the higher harmonics of any real waveform start dropping faster than 1/n at higher frequencies, generally dropping as 1/(n2), or 40 dB per decade, after the frequency is high enough. You can see this in Figure 1. The antenna efficiency of PC board structures or cables increases 20 dB per decade as frequency increases and wavelength gets shorter and closer to the size of structures found on typical PC boards. As a result, the beginning part of the radiated spectrum tends to be uniform, the 20 dB per decade decline in harmonic strength being balanced by the 20 dB per decade increase in antenna efficiency, until a high enough frequency is reached where the curve takes a bend and harmonics start declining at 40 dB per decade zone (see Figure 1). Above this frequency, the radiated spectrum starts declining by 20 dB per octave. But, the amplitudes of the real harmonics of a real device are often quite irregular because of resonances that weaken some and reinforce others.

Figure 1—Here you can see the sources of EMI in a typical microprocessor and the resulting spectrum.

What is not usually understood is that the biggest source of EMI is not the clock directly, but a train of pulses generated on both edges of the clock when current surges into the microprocessor for a nanosecond or two when the clock transitions up or down. This pulse train has a frequency that’s double the clock frequency. It seeps out of the processor chip into the power supplies and generally infects the board with high-frequency EMI. It also gets into the output lines emanating from the processor package; therefore, it’s further spread around the board and to cables and devices connected to the board.

The current surges on both clock edges are related to the clock tree. The clock tree is a system consisting of a branching network of buffers that distribute the internal clock around the silicon die. Because these buffers drive considerable capacitance and have both polarities of the clock present, there is a surge of current on both edges of the clock. This occurs as current flows into the chip to charge up the capacitance in the part of the clock tree that is transitioning from 0 V to the power supply voltage. On-chip devices, such as flip-flops, also contain internal gates and buffers where both polarities of the clock are present and contribute to the current surge.

An additional current surge is related to the crossover current when both the N and P transistors in a CMOS buffer are momentarily conducting during a logic transition. The silicon chip tries to suck in the required current to service these fast transients through its power supply pins. However, these connections have inductance created by the bond wires and lead frame, so the voltage drops briefly on the die, creating an on-chip power supply voltage drop with an amplitude on the order of a few tenths of a volt and the duration of a nanosecond or so.

If this same on-chip power supply drives the output buffers that carry signal lines out of the chip, these lines will also be infected with the fast pulses present in the power and ground supplies. This is because the power supply noise is directly transmitted through the buffer power inputs to the output lines. The on-chip current surges create fast noise that passes out through the power supply pins to the power and ground planes on the PC board, further spreading the infection.

The amplitude of the harmonics of the periodic noise pulses, at least at lower frequencies, declines inversely with frequency (1/f). Unfortunately, the effectiveness of a short antenna, such as a PC board trace, increases directly with frequency (~f). The result is that the radiated EMI tends to be flat across the spectrum. Fortunately, the amplitude of the harmonics starts declining more rapidly than 1/f; it’s more like 1/(f2) at some higher frequency determined by the finite rise time of the pulses in the pulse train. The balance of these countervailing effects is such that the most trouble is often found in the area of 100 to 300 MHz for lower-speed 8- and 16-bit microprocessor boards.

Decoupling capacitors and the intrinsic capacitance of the power and ground planes can be used to short circuit or filter noise on the power supply. However, this technique loses effectiveness above 100 MHz, because the decoupling capacitors have inductance of about 1 nH, giving an effective resistance of about 0.5 W at 200 MHz. The large currents involved will develop millivolt-level voltages across such capacitors.