How is it used
The spectrum analyzer sweeps from 150kHz to 30Mhz and makes a graph of the noise levels. Depending on the device’s category limit lines apply.
Both the L and N line are measured and both must be below the limit line.
To prevent interfering noise comming from the mains wall socket to be measured, a pre-filter is provided.
The design of the LISN circuit is compliant to CISPR 16-1-2 (an international standard used for testing).
If the limit is crossed, the engineer must locate the root cause of the noise and take measures to reduce it. Now having a Common-mode and Differential-mode measurement becomes very important.
If the differential-mode noise is high (as would be expected from an inverter or switched mode power supply), more or better decoupling of the L to N may be the solution. If the common-mode noise remains high, decoupling to the PE (earth) may be useful.
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