emissions from the device as shown in Figure 2. In this
example, a Wi-Fi demo board that had already been
through EMI compliance testing was evaluated, so there
were no failures to detect.
If you have set up your testing correctly and nothing
comes close to the limit line, it may mean that you are
ready for compliance testing.
Unintentional Radiator Testing
Figure 3 shows the pre-compliance setup for
conducted emissions testing. In this example, we’re testing
a universal AC/DC power adapter for a laptop computer.
For conducted measurements, instead of antennas you
use an LISN; essentially, a low pass filter which is placed
between an AC or DC power source and whatever you’re
testing to create a known impedance and to provide an
RF noise measurement port. It also isolates the unwanted
RF signals from the power source. Adding a pre-amplifier
is a good way to boost the relative signal levels.
For conducted measurements, background noise
often comes from the power source. While the LISN will
provide some isolation, many times you will need
additional power filtering. In this case, building power
noise was a problem. By adding a power filter, the
incoming noise was reduced to a low enough level for
making conducted measurements.
In this example, it’s worth pointing out that the
interference being conducted on a 60 or 50 Hz
power supply also created issues. While most of
the conducted EMI tests specify a measured
frequency range of 9 kHz to 1 GHz, it can be
useful to measure the signals at lower frequencies
when the need arises.
For best conducted EMI measurement results,
you should ideally use two LISNs: one for defined
impedance to your device; and one to go to the
spectrum analyzer or receiver. However, at least
one LISN is a must.
Given that this was a low cost power supply,
the conducted emission unsurprisingly went above
the limit at approximately 172 Hz as shown in
38 June 2017
FIGURE 3. An LISN is used
for making conducted
FIGURE 2. Broadcast signals are clearly visible in the VHF band; however, no out of limit conditions were
attributable to the device in this example.