Spectrum Analysers for EMC Testing — Key Takeaways

Electromagnetic compatibility testing is much easier to manage when emissions problems are found during development rather than at the test house. A spectrum analyser gives engineers a frequency-domain view of unwanted energy, helping them identify problem frequencies, compare operating modes and check whether a design change has improved the result.

A spectrum analyser is not a complete compliance laboratory, but with the right settings and supporting equipment it can form the centre of a useful EMC pre-compliance bench.

What a spectrum analyser adds to EMC troubleshooting

• Visibility: separate narrowband clock-related emissions from a raised broadband noise floor.
• Repeatability: save traces and compare the same operating mode before and after a design change.
• Prioritisation: identify frequencies closest to a limit or most likely to cause problems during formal testing.
• Diagnosis: combine the analyser with a LISN, near-field probe, current probe or antenna to trace the likely coupling path.

The analyser capabilities that matter

A practical pre-compliance workflow

• Define the frequency range, bandwidth, detector and limit required by the relevant standard or test plan.
• Connect the appropriate transducer: LISN for conducted mains emissions, RF current probe for cable current, near-field probe for local diagnosis, or calibrated antenna for radiated measurements.
• Record an ambient or equipment-off trace where applicable.
• Configure attenuation and preamplification so the noise floor is useful without overloading the front end.
• Run a peak scan in manageable frequency segments and mark the highest or closest-to-limit emissions.
• Remeasure the critical frequencies using the required detector and measurement time.
• Save the settings, correction data, trace and equipment configuration so the result can be repeated.

Common setup mistakes

• Using the analyser’s full-span default settings without checking RBW, frequency spacing or sweep time.
• Treating a probe or antenna as flat when its correction changes with frequency.
• Ignoring cable loss, preamplifier gain or attenuator loss.
• Turning on a preamplifier in the presence of strong signals without checking for overload.
• Comparing an uncorrected bench trace directly with a compliance limit.
• Changing the DUT mode, cable routing or grounding between measurements.

Download the detailed application note

This page summarises the main decisions involved in using a spectrum analyser for EMC work. The detailed Tekbox application note by Michael Mayerhofer includes further discussion of CISPR bandwidths, sweep segmentation, detector behaviour and analyser dynamic range.