the one you do want, no filtering is
possible. You’ll have to figure out
how to get rid of the interfering
signal at the source.
In the previous column, I talked
about ferrite cores and beads, and
how great they were for suppressing
RFI. Ferrite chokes — placed at the
right location in a system of
electronics — can do wonders at
blocking CM signals getting into your
system on the shields and common-mode paths of wiring and cables.
Figure 5 shows a typical
computer in which any piece of
equipment can be generating and
receiving noise — a distressingly
common situation — with
recommended locations for the
chokes.
Ferrite chokes consist of a clamp-on core or a toroid with cables or
wires wound on them. Don’t be
satisfied with just passing a wire
through a core once. Wind as many
turns on the core as you can to
increase the impedance created by
the choke.
Remember from the previous
column that ferrite impedance
changes with frequency, so be sure
to use the right type or mix of ferrite.
If you are mostly concerned with RF
below 30 MHz, use a type 31 mix
ferrite.
For higher frequency signals, use
type 43. (The various
recommendations are covered in the
Fair-Rite catalog, available online at
www.fair-rite.com.)
More
About
RFI
I’ve just barely scratched the
surface of a topic to which engineers
devote entire careers! You may just
want to get rid of some interference
or keep a system from interfering
with some other system. Or, maybe
you are designing for reliable
operation — good for you!
Hams have been dealing with
RFI since the first transmitter was
built, so you can find a great deal of
website for hams at
www.arrl.org/radio-frequency-interference-rfi. Jim Brown’s tutorials
mentioned in the sidebar are good,
and there is an excellent chapter on
RF interference in the ARRL
Handbook’s recent editions.
Design for EMI quality — to keep
the RF both in and out — at the
outset of building your system. You
won’t regret it! NV
FIGURE 5. The use of ferrite chokes in a computer
system where any piece of equipment can either be an
RFI source or RFI victim. Ferrite chokes should be placed
as close to the equipment as possible. Graphic courtesy of ARRL.
September 2015 65
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FIGURE 4. For RF immunity and to avoid radiated RF,
electronics should be constructed inside a conductive
enclosure such as aluminum. Connectors should be
shielded, with the shield connected to the outside of
the enclosure.
