22 November 2015
that prevents inductive or capacitive
coupling. It can just be a single
plane or a complete metallic box or
cover, and totally block any signal
transfer. As an example, digital clock
oscillators are usually contained in a
fully shielded enclosure or package.
Finding and Fixing EMI
If you experience EMI, you will
usually know it. It manifests itself
by complete equipment failure to
intermittent operation. Data errors
or noisy reception in a receiver are
typical indicators. While you may
experience some EMI effects, it is
typically hard to find the source.
Once you do, you can usually remedy
the situation. Finding EMI is often a
“cut and try” experimental process.
For example, when listening
on my ham receiver recently, I was
experiencing an unusually high noise
level. I thought it might be the AC
power line, but then in an unrelated
act, I turned off a table lamp and the
noise level dropped dramatically.
That lamp used a CFL bulb. I turned
off another CFL lamp and the noise
If you are a ham or shortwave
listener and have this problem, check
for CFLs first. Plain old incandescent
bulbs use more energy but do not
interfere with anything.
Another experimental approach
is to try repositioning equipment to
provide more spacing. Rearrange
Motors are another EMI
generator. Turn off any suspected
appliances or motor-using equipment
and see if the EMI goes away.
At the circuit level, you can easily
find some EMI with an oscilloscope.
An example is detecting power
supply ripple on the DC supply line.
A common offending signal is the
noise from a switching power supply
that has pulse waves from 100 kHz
to several MHz along with their
If you have a wireless EMI
problem, more serious measures are
usually needed. The best way to find
offending RF EMI is to use a spectrum
or signal analyzer. This test instrument
shows a segment of frequency
spectrum where you can identify
your own desired signal, as well as
potential interferers. Unfortunately,
some of these bench instruments
are often beyond the reach of us
common folks. Fortunately, cheaper
portable units for field use are
available similar to the one in Figure
1. The spectrum analyzer becomes
a receiver to ferret out offending
signals. Some manufacturers offer an
antenna or sensing probes you can
use to sniff out the EMI.
If you want to know more
about EMI and RFI, I recommend
the recently published book, Radio
Frequency Interference Pocket Guide
by Kenneth Wyatt and Michael
Gruber. It is published by Scitech
Publishing. It’s a good concise
summary of EMI/RFI and a helpful
solution to some problems, plus it’s a
great book for hams having RFI issues.
n FIGURE 1. This portable spectrum
analyzer is the best way to find EMI
of the RF wireless sort. Courtesy