emission diagnostics use clamp ferrites on harnesses to eliminate effect of
inductors, and ferrites characteristically are used to filter narrow frequency
Ferrites are a ceramic
material having very poor conductivity. Ferrites
act as a combination inductor and frequency-dependent resistor whose resistance
is proportional to frequency. For this reason ferrite beads are great for
eliminating high-frequency noise on (low-current) power supplies and digital
clock signals. Ferrite beads are used to provide high impedance at the
frequencies of the unwanted noise.
circuit designers like to think of signals in terms of their voltage. Signal integrity and EMC engineers must
think of signals in terms of their current.
are two things that every good circuit designer should know about signal
1. Signal currents always return to
their source (i.e. current paths are always loops)
2. Signal currents take the path(s) of
and higher, signal current paths are relatively easy to identify. This is
because the path of least impedance at high frequencies is generally the path
of least inductance, which is generally the path that minimizes the loop area.
Currents return as close as possible to the path of the outgoing current.
(generally kHz frequencies and below), the path of least impedance tends to be
the path(s) of least resistance. Low frequency currents are more difficult to
trace, since they will spread out significant current return paths may be
relatively distance from the outgoing current path.
are some situations where a well-placed gap in the return plane is called for.
However, these are relatively rare and always involve a need to control the
flow of low-frequency currents. The safest rule-of-thumb is to provide one solid plane for returning all
signal currents. In situations where you expect that a particular
low-frequency signal is susceptible or is capable of interfering with the
circuitry on your board, use a trace on a
separate layer to return that current to its source.
general, never split, gap or cut your
board's signal return plane. If you are convinced that a gap is necessary
to prevent a low-frequency coupling problem, seek advice from an expert. Don't
rely on design guidelines or application notes and don't try to implement a
scheme that "worked" in someone else's "similar" design.
times simple board designs that should have had no trouble at all meeting EMC
requirements at no additional cost or effort, wind up being heavily shielded
and filtered because they violated this simple rule.
Why is the
location of connectors so important? At frequencies below a few hundred
megahertz, wavelengths are on the order of a meter or longer. Any possible
antennas on the printed circuit board itself tend to be electrically small and
therefore inefficient. However, cables or
other devices connected to a board can serve as relatively efficient antennas.
Signal currents flowing on traces and returning through solid planes result in
small voltage differences between any two points on the plane. These voltage
differences are generally proportional to the current flowing in the plane.
When all connectors are placed along one edge of a board, the voltage between
them tends to be negligible. However, high-speed circuitry located between
connectors can easily develop potential differences of a few millivolts or
greater between the connectors. These voltages can drive currents onto attached
cables causing a product to exceed radiated emissions requirements.
board operating with a clock speed of 100 MHz should never fail to meet a
radiated emissions requirement at 2 GHz. A well-formed
digital signal will have a significant amount of power in the lower
harmonic frequencies, but not so much power in the upper harmonics. Power in the upper harmonic frequencies is
best controlled by controlling the transition times in digital signals. Longer
transition times are preferred for EMC. Excessively long transition times
can cause signal integrity and thermal problems. An engineering compromise must
be reached between these competing requirements. A transition time that is approximately 20% of a bit period result in
a reasonably good-looking waveform, while minimizing problems due to crosstalk
and radiated emissions. Depending on the application, transitions times may
need to be more or less than 20% of the bit period; however transitions times should
not be left to chance.
are three common methods for controlling rise and fall times in digital logic:
1. Use a logic family that is only as
fast as the application requires.
a resistor or a ferrite in series with a device's output.
3. Put a capacitor in parallel with a
first choice is often the easiest and most effective option. However, the use
of a resistor or ferrite gives the designer more control and is less affected
by changes that occur in logic families over time. Capacitors can actually
increase the amount of high-frequency current drawn by the source device and in
most cases are not appropriate choices.
that it is never a good idea to try to slow down or filter a single-ended
signal by impeding the flow of current in the return path. For example, one
should never intentionally route a low-speed trace over a gap in a return plane
in an attempt to filter out the high-frequency noise.
Ferrite beads tend to be
effective in blocking noise currents in power supplies and typically have
maximum values of impedance of the order of a few hundred ohms. Therefore, in
order for them to be effective, they must be in series with impedances that are no larger than the bead impedance,
since otherwise the bead impedance would be overshadowed by this larger
impedance. The intent is to use the bead to block
noise currents by adding significant impedance to the path. Circuit
impedances tend to be small in power supplies as opposed to other electronic
circuits. Therefore insertion of a bead tends to provide a significant increase
in the circuit impedance in power supply circuits.