EMC Conducted Emissions Testing and Setup

• Disconnect the LISN output from the analyzer input
• Connect the power cord from the DUT to the LISN. Do not coil the power cable. Make sure it lies flat and does not run between the DUT and the horizontal ground plane.
• Power on the DUT
• Re-connect the LISN RF Output to the Spectrum Analyzer RF input.NOTE: These steps add another layer of protection to the sensitive analyzer front-end. Some LISNs include features that can minimize these transients and in such cases, these steps can be considered optional.
• Observe the scan and note any peaks that are within 3dB or exceed the limit line. These are potential problem areas.NOTE: It is a good idea to capture a copy of the display either using a camera or a screen capture/print along with details notes of the experimental setup, instrument settings, and any DUT information.
• Example: A preliminary scan showing peaks that exceed the Class B limits for conducted emissions, per the FCC subpart 15 for unintentional radiators.

The preliminary scan described above will provide a good indication of potential fault frequencies. But, the settings are not identical to those indicated in most specifications. We suggested that the following analyzer settings: RBW = 10kHz, Detector = Positive Peak, Span = 30MHz. This allows you to perform quick analysis of problematic areas and quickly gain a basic idea of the conducted emission performance of the DUT.

Here are optional techniques that can help provide more insight:

1. Most spectrum analyzers do not have pre-selection filters. If you are using a spectrum analyzer without pre-selection filters, the peaks you observe may not be real. Analyzers without pre-selection filters can create false peaks due to out-of-band signals mixing with the observed signals.

You can test the validity of a peak by adding an external attenuator (3 or 10dB should do). Real peaks will fall by the amount of the attenuator. If the peak falls by more than the attenuator, it is likely to be a false peak. Make a note of the false peaks for comparison with your compliance test results. You can also use pre-selection filters or an EMI receiver, but these tend to be cost prohibitive for most quick testing.

Figure 5 below shows a typical peak confirmation test. The yellow trace was collected without an attenuator. The Pink trace was collected with a 10 dB attenuator added to the RF input of the analyzer. In this case, the peaks drop the same amount as the added attenuation. This helps affirm that the peaks are likely real and not products of out-of-band signals.

2. Some analyzers have toolkits that more closely match the performance of true compliance receivers.

These toolkits are usually unlocked instrument options that include:

• EMI filter (6 dB shoulder vs. standard 3 dB standard IF frequency shape)
• RBW bandwidths of 200 Hz, 9 kHz, and 120 kHz
• Quasi-Peak (QP) detector

If you expect that the DUT has bursts of RF, intermittent digital communication, or transient outputs, it may be worthwhile to re-run a scan using the EMI filter, RBW bandwidth, and QP detector, if available.

Zoom in to the failing peak by setting the center frequency of the analyzer to the peak frequency of interest. Set the span to 10x the RBW specified in standard (if the specified RBW is 9 kHz, then set the span to 90 or 100 kHz), and then change the filter to EMI, the detector to QP, and RBW to 9 kHz. And observe the scan.

Scans performed using the QP detector will take much longer to perform due to the nature of this detector. QP scans will never exceed scans with a Positive Peak detector, but using the QP can limit the time spent over-engineering a solution.

3. Many spectrum analyzers have Max Hold trace types that will continuously hold the highest amplitudes of each frequency scan. You can enable a single trace as Clear Write to show active RF performance and enable a second trace as Max Hold. This allows you to compare changes in the DUT to the “worst case” data collected and “frozen” using Max Hold.

4. You can use markers and peak tables to clearly indicate peak frequencies and amplitudes, if available:

RESA analyzer with Peak table and markers activated.