Document the DC LISN Configuration Before Comparing Conducted Noise

A conducted-noise trace belongs to a defined supply-interface measurement. Changing the impedance network, cable arrangement or measurement port can change the result even when the PCB is untouched. Record the setup before using a DC LISN trace to compare filters or board revisions.

Identify the network and its intended use

Tektronix’s DC LISN information describes a network placed between the supply and device under test, with a separate measurement port. The specified voltage, current, frequency range and intended configuration belong to that model. Another LISN is not automatically interchangeable.

Record manufacturer, model, calibration or characterization record, port labels and the measurement method being followed. Have the responsible laboratory determine the appropriate network for the product and evaluation. Do not select one only because its connector fits the available analyzer cable.

Draw every connection

Include the DC source, supply conductors, return path, ground plane where required, network ports, receiver and external loads. Mark unused-port treatment according to the equipment and method instructions. Photograph cable lengths and placement so that a later comparison can reproduce the arrangement.

Keep the actual product operating mode with the drawing. A converter driving a resistive laboratory load may not produce the same pattern as the complete assembly switching between active and sleep states. Identify which case the trace represents.

Check the measurement path

Review receiver input limits and the network’s connection procedure before powering or reconfiguring the setup. Use the specified protective and coupling arrangements. A measurement port is part of an instrument system with its own limits, not a general-purpose oscilloscope connection.

Apply the relevant cable and network correction data when reporting absolute levels. Record whether displayed values are raw or corrected. A comparison between corrected and uncorrected traces can create an apparent filter improvement that is entirely due to processing.

Establish a background reference

Characterize the setup background using a procedure that preserves meaningful source and instrument conditions. Investigate peaks that remain when the product is not generating the suspected activity. Keep ambient or source-related observations separate from the board’s contribution rather than subtracting traces without a justified method.

For a component experiment, retain identical network, port and cable conditions and change one design variable at a time. Confirm promising changes across the required operating states. The final record should say what was measured, where and under which configuration. That supports a useful conducted-noise comparison without stretching a development setup into an unsupported compliance claim.