# Impedance mismatch in LISN for EMI/EMC test

I am currently designing LISN for CE-102 test.The schematic of the circuit is provided below.

When I connect the circuit to the network analyzer the impedance is coming as shown below

expected graph is shown below.

I'm unable to understand where is the flaw. After 1 MHz, the impedance should go constant but my circuit it is increasing linearly. Please guide me to stabilize impedance in my circuit After all, LISN is all about providing stabilized impedance for mains network.

• Where did you connect the network analyzer? What frequency range did you measure? Where is 1 MHz on the Smith chart? Commented Aug 13, 2018 at 14:37
• Also, your Smith chart is related to the input impedance, but your theoretical graph is for transfer impedance. Do you understand the difference? Commented Aug 13, 2018 at 14:38
• @Voldemort, It would be good if you could 1) Label the smith chart with frequency 2) Post transfer impedance graph for your LISN Commented Aug 13, 2018 at 17:12
• Conducted noise must be a HPF above 150kHz Commented Aug 14, 2018 at 4:40
• Did you have a question on my answer? Commented Oct 9, 2021 at 1:26

The grid has a zero impedance compared to the DUT.

I modelled your LISN here. Using the proper method of a single sided equivalent to a differential 3 port device with the grid some low impedance like 50 mOhm + 100 uH

I think you are misunderstanding the purpose of the LISN. This is a differential 3 port device. Namely the Plug, The outlet and the coax port to the spectrum analyzer.

When you measure the transfer function from Outlet to coax which I would call ports 2 & 3 the transfer function is a 7 kHz HPF with 50R + 250nF + 50R, but the plug ought to shorted with some small resistance (I chose 50 mOhms and 100 uH of grid cable) If you make it 0 Ohms then you are shorting out 8uF+5R. Then s23 becomes a 75 kHz HPF.

But the whole purpose of the LISN is to attenuation all noise and effects of source impedance and thus raise the impedance of the grid source at RF yet still supply 50/60 Hz power up to xx Amps. The SA measurement should not see any grid RF noise and only from the load-generated RF noise from the outlet transferred to COAX output with a 50 ohm load in parallel with the 1k (which only serves to bleed off any residual HV charge on the caps when the system is disconnected.

The CS102 test is from 10 kHz to 10 MHz for Conducted Emissions such that LISN has a flat 0dB response over this bandwidth. Although the output impedance is not 50 Ohms because of the 1kohm cap discharge bleeder resistor. It is close enough.

• The LISN isolates the power mains from the EUT. The power supplied to the EUT must be as clean as possible.
• The LISN isolates any noise generated by the EUT from being coupled to the power mains.
• To provide a defined impedance to measure the conducted emission test and to provide a measurement port to determine the conducted emission levels

I found this site . which does not make any more sense than your graph. Good luck.