Issue 146 September 2002
Killing the EMI Demon

MORE REDUCTION TRICKS

There were other tricks that helped to reduce EMI on the Rabbit 3000. For instance, some of the internal clocks are gated, so they’re only enabled when needed. The use of gated clocks reduces the amplitude of the current surge into the chip.

The external processor bus is designed so that it isn’t required to run the clock around the PC board. The clock is available at a dedicated pin, but in most systems it is turned off because it’s not needed. When it’s needed (e.g., to provide a clock for an FPGA), it can be supplied at full or half the internal clock frequency.

Additionally, the external processor bus cycles are not all the same length. This breaks up the periodicity associated with the external bus.

An internal clock doubler allows the external crystal oscillator to operate at half frequency. This reduces EMI by lowering the frequency of the external clock, which is physically larger with large currents and thus in great danger of generating EMI.

Figure 3—Take a look at the actual EMI measurements for a Rabbit 3000 RCM3010 core module board with clock spectrum on and off. Peak values are shown as dots. Units are in amplitude dB relative to 1 µV per meter at a 3-m distance.

THE RESULTS

Figure 3 shows the EMI measurements on a Rabbit 3000 RCM3010 core module, which is a small microprocessor board operating at 29.5 MHz. The EMI is virtually undetectable; it’s far below the noise floor of the spectrum analyzer as used in the normal FCC-mandated measurements. We were able to use a special measurement technique to ascertain where it was and plot it on the graph.