64 September 2015
device. We can refer to this as
common-mode breakthrough, where
the interfering signal is picked up by
cables and carried to internal
electronics where it is strong enough
to alter circuit functions.
It is also common for unshielded
electronics in plastic boxes to pick up
and be disrupted by strong signals
without any cables at all! This is
direct detection and it can require
modifications to the circuit to
eliminate the interference.
The general process for assessing
which types of RFI one is dealing
with is outlined in the steps of Figure
3. There are different remedial
techniques for each. Using the wrong
technique or applying it to the wrong
end of the path won’t solve the
problem.
Keeping RF In
At a potential source — such as
digital computer and networking
equipment, microprocessor-controlled
appliances, switching converters, any
circuits that turn large currents on
and off — the first step is to avoid
radiating RF if you don’t have to. This
is not just being altruistic, because if
your system is radiating RF it can also
receive RF by the very same path.
Closing off that path to outbound RF
usually closes it to incoming RF, too!
It’s just good engineering to do that
right in the design.
A good strategy to follow is to be
sure your printed circuit board (PCB)
has filtering applied to any signal or
power connection
entering or leaving
the board. For
example, a 0.01 µF
disc ceramic capacitor to circuit
common on a low frequency signal
or control lines and power
connections will act like a short-circuit at RF. That path will be “cold”
at RF for both incoming and outgoing
RF signals. (Note that any component
connected directly to the AC line
must be “line rated” to avoid fire
hazards.)
Treat your equipment as if all of
the signals going in and out of it
were water, and every connection
had to be water-tight. Start with a
metal enclosure — even for simple
projects. Using a plastic box means
you have two strikes against you RF-wise: You have no shield that will
route noise and RF currents away
from the circuit, and every wire and
PCB trace in the electronics will act
as an antenna.
Use shielded cables and
connectors — including a shielded AC
power connector — with the shield
connected to the enclosure.
Another often overlooked
strategy is to avoid connecting the
circuit common to any conductor
that will leave the enclosure without
being inside a shield. Keep the circuit
common connected to the shielding
enclosure as a separate connection
from any data or signal cable as
shown in Figure 4. (Refer to the
sidebar, “What is the Pin 1
Problem?”)
Keeping RF Out
Let’s say the RFI is being caused
by a receiver — such as an over-the-air TV receiver or scanner — by
fundamental overload from an
otherwise legitimate signal.
For example, you might live
across the street from the local TV
and FM antenna tower — somebody
has to! The strong signal is probably
coming right in the antenna input of
the receiver, along with the signals
you want to hear.
In this case, you can block the
unwanted signal with a DM filter in
the antenna input cable. The goal is
to reduce the level of the signal you
don’t want until it is weak enough for
the receiver to reject.
To do this successfully, you’ll
need to know the frequency of the
signals you want and of the signal
you don’t want. If the unwanted
signal is at a higher frequency, you
can use a low-pass filter to allow the
signals you want into the receiver
while blocking the higher frequency
unwanted signal. A high-pass filter
does the opposite.
In some circumstances, you
might have to use a band-pass filter
that only allows some frequencies
through while blocking all others.
Remember that if the unwanted
signal is on the same frequency as
Cable System Leakage
Cable TV systems carry signals from the very low MHz to upper
UHF, so extra attention has to be paid to the details of making proper
connections. This keeps the cable system’s programs noise-free, and
also prevents the signals getting out and causing interference. The
cable TV channels are not restricted in frequency like over-the-air TV, so
the signals can be directly on top of other over-the-air signals. This is
known as cable leakage and is most often due to loose, dirty, or
improperly installed type F connectors on the cable feed. If you
experience interference to or from your cable TV equipment, making
sure connectors are tight, clean, and properly installed is a great place
to start looking for problems.
FIGURE 3. The
general process
of identifying the
source and type
of interfering
signal. Use the
appropriate DM
or CM remedy at
the “victim” or
“source” as
indicated.
