How using near field probes and a spectrum or signal analyzer help with EMI diagnostics.
We know that electronic devices have to be compliant with EMC/EMI standards.
In the product development phases, if a device fails the EMI standards test limits diagnostics are
performed to locate the source of emissions and are typically done using a spectrum/signal analyzer
and near-field probes. This step provides information that is then used to take corrective measures.
In this video, we will use a signal analyzer and near field probes to detect a typical
radiated emission of a laptop computer and locate the source of emission. Today, we are
using the Agilent N9000A CXA signal analyzer with the EMI measurement application.
The CXA is a part of Agilent's X-Series of signal analyzers and is the most economical model.
The sensitivity of the CXA exceeds 160 dB per meter hertz at 1 GHz and it is equipped
with a full range pre-amplifier up to 26.5 GHz. It can make CISPR pre-compliant measurements
with EMI filer, quasi-peak detector and logarithmic X-axis.
There are two types of near field probes. One is E-field probes, which detect electric field.
The other is H-field probes which detect magnetic field from emission sources.
H-field probes are more commonly used to locate emission sources in near field tests because the H-field fades
faster than the E-field as the distance increases. In other words, H-field probes provide better resolution.
Today, we are using an Agilent N9311X-100 or N9311X Option 100 near field probes set.
You can see there are four probes in the N9311X-100 set, they are all H-field probes.
Probe #1 has the largest diameter. It is the most sensitive and therefore has the lowest resolution.
It can be used at distances up to 10 cm from the unit under test.
Probe #2 has a higher resolution and a lower sensitivity than probe #1.
It is suitable for measurements up to 3 cm. Probe #3 is designed for detecting magnetic fields
emitting vertically from the surface area of flat units. It has the lowest sensitivity and the best resolution.
It enables obstructed parts of the printed circuit board to be measured with approximate 2 mm resolution.
Probe #4 acts like a current clamp and is designed for detecting surface and circular
magnetic fields on conducting paths, metalized surfaces, plug-in socket connectors, cables
and component connections. Ok, let's start the test.
For this demonstration, we chose this old laptop computer as our DUT and we need to turn it on.
Now let's set the CXA signal analyzer. When we access the EMI receiver mode, we will see the
display looks different than when the CXA is in normal spectrum analysis mode. In EMI receiver mode, the analyzer
is able to make CISPR pre-compliant measurements which has 6 dB bandwidth filters and selectable
EMI detectors including EMI average, quasi-peak and peak detectors. It has pre-defined test
ranges according to EMI standards. Here we chose range five, 30 MHz to 1 GHz. As mentioned,
radiated emissions are usually low-level signals therefore, we set the signal analyzer to its
best sensitivity, set attenuation to 0 and enable the full pre-amplifier.
When conducting a far field EMI pre-compliance test we can recall the limit lines which are pre-installed
in the instrument according to the EMC standards. For instance, in this case, we choose the
EN55022 standard which applies to laptops. In the near field test, EMI standard test limits
are not useful because of a number of factors that can affect the test readout, like the
probe position and DUT shape. Now let's start the sweeping and adjust the reference level
to better observe the signals.
Once the signal analyzer settings have been selected, we connect the probes.
We begin using probe #1 which has the best sensitivity to obtain the suspicious signal.
The display shows that there are two obvious signals around 800 to 900 MHz,
even though we have not placed the probe close to the laptop; these are cellular signals
which we will ignore in our testing as they are not generated by the DUT. We move the
probe over the back of the notebook at a distance around 2 cm. On the display of the signal
analyzer we see the signals which are radiating from the DUT. The signals may vary when we
move the probe; we notice there is an obvious signal with high amplitude. Let's try to locate
the source of emission of this signal. To do this, we place a marker onto that signal.
Its frequency is about 333 MHz. The signal level varies when we move the probe around,
move the probe over the DUT, then place the probe in the location where the 333 MHz signal
is at its highest level. After our exam of the DUT is done we move the probe to the place
where we believe the 333 MHz emission is highest. Now we change the probe to probe #2,
which has better resolution.
By moving the probe around on the DUT we can narrow the test range.
In this case the highest level of 333 MHz signal is lower than what we observed by using
probe #1. However we get better resolution to locate the source of emission.
Like before, we place the probe where we obtained the 333 MHz at its highest level.
Now we remove the cover to perform deeper analysis.
It is time to use the probe with best resolution.
As mentioned, probe #3 has lowest sensitivity and the best resolution.
This probe helps us see that the emissions are coming from the memory chip slot.
It also allows us to examine the memory chips and components point-by-point while
observing the signal level on the signal analyzer.
The signal varies a lot as the probe moves to different components.
We can switch to the spectrum analysis mode to check the signal and depth, set the center frequency,
span and amplitude, and set the attenuation and pre-amplifier. This allows us to observe
This allows us to observe the signal better. It looks like a broadband digital signal with center
frequency around 333 MHz. We know that it is the clock frequency of DDR 333 SD RAM. We continue testing the
components and finally locate the components which generate the highest emissions. To summarize for this
demonstration, we have used the N9000A CXA with W6141A EMI measurement application and probe set.
This configuration is the most cost-effective X-Series signal analyzer solution for performing pre-compliance,
conducted and radiated emissions, testing for both commercial and MIL-STD requirements.
For more information on conducting emission tests using the CXA EMI measurement application and probe set,
please visit www.Agilent.com/find/cxa-emi.