The Solar Electronics LISNs use a series inductor between the test sample and the power source to provide the impedance-versus-frequency characteristic. A coaxial connector with D.C. isolation is provided for connection to the associated frequency selective EMI meter. The power source end of the inductor is bypassed to ground.
When measuring conducted radio interference voltages from active power lines to ground, it is essential to know the line impedance so that repeatable tests can be made by more than one laboratory. Artificial line impedances are specified in MIL-STD-462, V.D.E., C.I.S.P.R., C22.4, NACSEM 5100, ANSI C63.2 and other EMI specifications.
The characteristic impedance of the five microhenry and 50 microhenry LISNs brackets the mean value of power line impedance which has been measured by independent researchers. These two inductance values in parallel with the 50 ohms of the EMI meter fall between the minimum and maximum line impedance values which have been measured. The mean value would be represented by a twenty microhenry inductor in parallel with 100 ohms.