f_main

The LISN must provide the low frequency (f_main) power to the Equipment Under Test (EUT), with no attenuation. Thus, Z₂ should be very low compared to the EUT impedance, and Z₂ & Z₃ (to the ground) should be very high. (f_main << f_noise).

f_noise

Concerning the noise, the LISN must :

• Prevent all high frequency noise coming from the mains from flowing through the LISN.
  Consequently, Z₂ should be very high and Z₁ very low (f_noise >> f_main).
• Impose the noise generated by the EUT to flow through measurement impedance Z₃.