• Power limiter for conducted testing
• Near field probes for diagnostics (optional)
Radiated Emissions Testing
An intentional radiator is any device that broadcasts
radio energy (not infrared or ultrasonic energy) to perform
its function. Devices that are intentional radiators are also
subject to unintentional testing requirements. Emissions at
frequencies other than what the device is designed to use
can occur because of internal circuitry.
When evaluating spectrum analyzers for this type of
testing, it is important to select an instrument that can
capture at least the third harmonic (if not more) of the
radiated signals being generated within the device. The
test setup for an intentional radiator is essentially the same
as what’s used for radiated emissions testing. However,
the frequencies of interest are limited to the radiated
frequencies and frequency masks defined by the
specifications, such as Wi-Fi, Bluetooth, etc.
After you have the equipment lined up, the
next problem you’ll face is where to test. From an
RF perspective, it’s noisy out there. Anechoic
chambers like what the FCC labs use are
prohibitively expensive. Fortunately, good results
can be obtained from basements, conference
rooms, or from rural locations. If possible, be sure
to turn off local Wi-Fi, Bluetooth and cellular
transmitters.
Figure 1 shows a block diagram of a typical
setup for radiated emissions testing.
In this example, three very low cost printed
circuit board (PCB) log periodic antennas and a
biconical antenna were mounted on a tripod for
easy placement, and the antenna factors (AF) and
cable loss information was entered into the
spectrum analyzer for field strength correction. A
biconical antenna was used for the 20 to 200 MHz
frequencies.
The longer 20 to 200 MHz wavelengths
require a larger antenna, and the background noise
becomes more of an issue as it includes many
radio broadcast frequencies.
Pre-compliance testing is often done at different
distances from your device, such as one meter and a few
centimeters. Reducing the distance between the device
and the test antenna increases the ratio of the device’s
signal strength to RF background noise. Unfortunately,
near field results do not translate directly into the far field
tests used in EMI (electromagnetic interference)
compliance testing, so be careful about drawing
conclusions. Adding a pre-amplifier is another good way
to boost signal levels relative to external noise.
Prior to turning on your device, take a minute to
evaluate and characterize your test environment. Is there
enough signal room between the limit line and your
ambient noise floor? Are there known signals that can be
reduced? Do you need to move your test setup to a
quieter environment?
Once you are satisfied that your background noise
won’t present a major problem, power-up your device.
The differences between the two measurements are the
Using a Spectrum
Analyzer
By Matt Maxwell
Product Manager
Tektronix Spectrum Analyzers
June 2017 37
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FIGURE 1. Be sure to select a location that will minimize
external signal noises for radiated emissions testing when
using a setup such as this one.
