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Military and Aerospace EMC: Why and How We Use LISNs

First, the Line Impedance Stabilization Network, or LISN (pronounced “listen”), is not intended to be a filter. Although it can function as a bit of a filter, please do not use it in this manner. The original intent was to simulate the line impedance found on aircraft power lines. The work was first performed by Alan Watton, who worked on the Douglas DC-3 type aircraft during WWII.[i]  This was originally a 5 µH LISN design, which the RTCA and most aircraft manufacturers still use. The LISN has been used in military and aerospace testing since the early 1953’s.  The 5 µH LISN was first used in MIL-I-6181B (29 May 1953). It was also used in DO-138 (1967) and may have been used earlier.

This ubiquitous use of LISNs has provided another benefit, which I believe is more important: The use of a standardized test setup and power line impedances allows testing to be performed and replicated at different testing laboratories and hopefully find the same or similar results. When everyone uses the same source line impedance, cable lengths, ground planes, and standoffs, the ability to reproduce the emission measurements provides assurance that the results can be trusted.[ii]

Earlier versions of DO-160 included the schematic for the LISN, whereas DO-160D and later versions included only the impedance curve, with a note stating that a 10 µF capacitor may be needed on the power line side of the LISN to achieve the low frequency impedance required. The modified schematic is shown in Figure 1.

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DO-160 LISN

Figure 1: The DO-160 LISN included the additional 10 µF capacitance required of later versions

The MIL-STD 461D-G LISN (see Figure 2) has a slightly different design and includes a 50 µH inductance. The effect of the larger inductor is to provide a 50 Ω impedance to a lower frequency. Note that while conducted emissions for DO-160 start at 150 kHz, MIL-STD 461D-G starts at 10 kHz.

MIL-STD 461D-G LISN

Figure 2: The MIL-STD 461D-G LISN

Conducted emission measurements made for DO-160 are performed starting at 150 kHz and use a current probe on the power line, although early versions allowed measurements directly from the LISN port. Note that the port provides a voltage measurement, while the current probe provides a current measurement (to state the obvious). However, the two may not directly relate. Increased line impedance may increase the measured voltage but decrease the measured current. For this reason, only one method was chosen for testing.

For MIL-STD 461A-C, current emission measurements were made on all lines using a current probe, but on power lines, this was problematic. At the time, most agencies required the use of 10 µF capacitors on the power line instead of LISNs. However, the capacitor has less than 1 Ω impedance above 16 kHz. With the approximately 1.5 µH of series inductance in the 1 meter input wire, resonances were often a problem, and a filter using capacitors as a first stage was rendered inefficient. Thus, starting with MIL-STD 461D, LISNs were used on power lines. Conducted emissions were voltage measurements made at the port of the LISN and starting at 10 kHz. Due to the low frequency, the 50 µH LISN was used to extend the impedance down in frequency.

For power supplies that provide power to the aircraft bus, it is important to use LISNs on the output of the power supply. This is often neglected. But remember, the intent is to replicate the aircraft bus impedance, and the LISN is the device we have chosen to do this.

Secondly, the coaxial connector port of the LISN must always be loaded into a 50 Ω load, either using a terminator or a measurement device (spectrum analyzer). And ensure the load is rated for the power it needs to handle. For high-level conducted susceptibility testing (DO-160 Category Y, MIL-STD 461 Level 5), the test levels can be up to 600 mA (6 dB over test levels).  If 0.6 Amps of current has to pass through the 50 Ω termination, this results in having to dissipate 18 Watts.  Some current may pass through other components, but a significant amount of current must run through the termination. If only a ½ Watt termination is used, as is often done, the termination will fail, and the test will be invalid. The currents must be allowed to flow from the line to the ground plane and back to the unit, and the path is mostly through the termination. Please ensure the termination is large enough to handle the required current.

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[i] This information was provided by the late Al Parker to Ken Javor. Details can be found in Mr. Javor’s excellent article Line Impedance Stabilization is in its Seventieth Year and Still Going Strong, In Compliance Magazine, June 2023.  See https://incompliancemag.com/line-impedance-stabilization-is-in-its-seventieth-year-and-still-going-strong/

[ii] Arguments can be made if the limits are realistic and should be modified. That is not the point of this article, only that the ability to reproduce the findings makes them trustworthy.

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