LISN

A LISN (Line Impedance Stabilization Network) is used in conducted radio-frequency emission and susceptibility tests.

Most electronic devices have to be manufactured so that RF frequency emissions are limited according to the relevant standards.

Testing for compliance to the standards is very expensive, so some sort of pre-compliance testing is normally undertaken to ensure success the first time compliance testing is undertaken.

The homemade LISN was constructed to give the builder some idea of the conducted emissions on the 5VDC USB power supply. It was constructed with available materials so the only real expense was time. The inductor is wound with 1mm diameter wire wound on a cardboard core that was dipped into shellac to hold it together. All components were measured with an accurate LC bridge and are within +-2% or better. The aluminum box affects the Q of the coil, good shielding is necessary with a 50kW AM transmitter only 3 km away.

As no spectrum analyser was available, a 100Mhz digital oscilloscope was used. The FFT (Fast Fourier Transform) function works surprising well, if a short antenna is connected; one can easily spot the nearby radio transmission frequencies.

The colloidal silver maker uses a 6.8 microhenry inductor as specified; the 4.7 microfarad capacitor has been replaced by two capacitors in parallel: a 100nF and a 10uF (low ESR monolithic ceramic).

For the sake of testing, the 10uF was removed and the conducted emissions rose above -60dBV. When the 10uF capacitor was refitted the conducted emissions were better than -80dBV.

Using a scope probe directly on the +Vin to the DC-DC converter, one could easily observe the fundamental operating frequency of ~280kHz the harmonics were quite strong.

30 May 2017