Jameco Electronics Rabbit 3000 - Manual (Page 220)

User’s Manual 211
15. EMI C
ONTROL
EMI or electromagnetic interference from unintentional radiation is of concern to the
microprocessor system designer.
One concern is passing the tests sometimes required by the U.S. Federal Communications
Commission (FCC) or by the European EMC Directive. For example, in the U.S. the FCC
requires that computing devices intended for use in the home or in office environments
(but not industrial or medical environments) not have unintentional electromagnetic radia-
tion above certain limits of field strength that depend on frequency and whether the device
is intended for home or office use. This is verified by measuring radiation from the device
at a test site. The device under test (DUT) is operated in a typical fashion with a typical
mechanical and electrical configuration while the electromagnetic radiation is measured
by a calibrated antenna located either 3 or 10 m from the device. The output of the antenna
is connected to a spectrum analyzer. For the purposes of the test, the spectral power is
measured by using a filter with a bandwidth of 120 kHz. The peak power is measured by
using a “quasi peak” detector in the spectrum analyzer. The quasi peak detector has a
charge time constant of 1 ms and a discharge time constant of 550 ms. In this manner the
peak radiated signal strength is measured. The tests required by the FCC and the EC are
practically identical.
The Rabbit 3000 has important features that aid in the control if EMI.
• The power supply for the processor core is on separate pins from the power supply for
the I/O buffers associated with the processor and various peripheral devices.
• A spectrum spreader in the clock circuit can be enabled to spread the spectrum of the
clock by varying the clock frequency in a regular pattern.
• The built in clock doubler allows the external oscillator circuitry to operate at 1/2 the
ultimate clock frequency.
• In most cases it is not necessary to route the system clock outside the package, although
a pin is provided for this purpose in the unusual circumstances where it might be neces-
sary. The high speed clock on PC board traces is a major cause of EMI.
If all the EMI suppression features of the Rabbit 3000 are properly utilized and low EMI
design techniques are used on the printed circuit board, system EMI will likely be reduced
to a very low level, probably much lower than is necessary to pass government tests.