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Topology and Characterization of a DC Line Impedance Stabilization Network

Foundations

In this month’s column we discuss the topology and characteristics of a dc Line Impedance Stabilization Network (LISN), often referred to as an Artificial Network (AN), shown in Figure 1. This LISN is specified by the military standard MIL-STD-461 and used for conducted emission in the frequency range of 10 kHz to 10 MHz.

Figure 1: MIL-STD-461 LISN

 

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The LISN schematic is shown in Figure 2. Figure 3 maps the schematic to the LISN physical ports.

Figure 2: MIL-STD-461 LISN schematic

 

Figure 3: Physical ports vs. LISN schematics

 

The objective of the LISN is to provide a constant line impedance of over the required frequency range. Additionally, the LISN should minimize the measured noise generated by equipment other than the Equipment Under Test (EUT), effectively acting as a low-pass filter.

The impedance of the LISN can be measured with the ports A and B short-circuited or open-circuited. Figure 4 shows the LT-Spice model used for the short-circuited configuration and Figure 5 presents the corresponding impedance plot.

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Figure 4: Circuit used for the short-circuited simulation

 

Figure 5: LISN impedance with ports A-B short-circuited

 

The low-pass filter effectiveness of the LISN can be determined by investigating its insertion loss. Typically, when describing electric filters the insertion loss is defined with the help of the circuits shown in Figure 6 [1].

Figure 6: Circuits used to define the insertion loss of a filter

 

The insertion loss is defined as

   (1)

Using this definition can determine the insertion loss of the LISN from the circuits shown in Figure 7 .

Figure 7: Circuits used to define the insertion loss of the LISN as a filter

 

The resulting plot of the insertion loss is shown in Figure 8.

Figure 8: Insertion loss of the LISN

 

The insertion loss defined in Eq. (1) is a positive quantity, as shown in Figure 8. In order to compare this simulation result to the measurement result in the next section let’s plot the inverse of this plot, ie.,

   (2)

This is shown in Figure 9.

Figure 9: (Inverted) Insertion loss of the LISN

 

In circuit theory the more appropriate terminology for the plot in Figure 9 would be the gain plot rather than the loss plot. Nevertheless, when describing the frequency response of a LISN we refer to it as its insertion loss.

Verification

The measurement setup used for the LISN impedance measurement is shown in Figure 10.

Figure 10: Measurement setup for the LISN impedance measurement

 

Figure 11 shows the front and back connections at the LISN.

Figure 11: Front and back connections at the LISN for impedance measurement
Figure 11: Front and back connections at the LISN for impedance measurement

 

The impedance measurement result is shown in Figure 12. This measurement closely correlates to the simulated result shown in Figure 5. The measurement setup for the insertion loss measurement is shown in Figure 13.

Figure 12: LISN impedance measurement
Figure 12: LISN impedance measurement

 

Figure 13: Measurement setup for the LISN insertion loss measurement

 

Figure 14 shows the front and back connections at the LISN.

Figure 14: Front and back connections at the LISN for insertion loss measurement

 

Note that both the impedance and insertion loss measurements require special adapters, shown in Figure 15.

Figure 15: Special adapters needed for the measurements

 

The insertion loss measurement is shown in Figure 16.

Figure 16: LISN insertion loss measurement

 

This result shown in Figure 16 is reasonably close to the simulated one, shown in Figure 9, thus validating the insertion loss model presented in Figure 7.

References

  1. Bogdan Adamczyk, Foundations of Electromagnetic Compatibility with Practical Applications, Wiley, 2017.

 

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