The first EMI standards tried to control both these radio frequency interferences (RFI) coupling paths. Prior to 1953, JAN-I-2255 used a pair of 4 uF bypass capacitors in shunt (8 uF total capacity between power feeder and ground plane) and a 10’ length of power wire suspended not more than ¼” from the ground plane for what they called power supply stabilization (see Figure 1). Because these receivers tuned from 0.15 to 20 MHz, JAN-I-225 conducted and radiated emission measurements covered that same range. The resonant frequency of the 10’ wiring and 8 uF capacity occurred below the test frequency range, so that the impedance looking back into the capacitors through 10’ of wiring was inductive in character.
JAN-I-225 was superseded in 1953 by MIL‑I‑6181B, which included both required impedance (Figure 2) and construction drawings (Figure 3) for the 5 uH LISN. These same drawings, with two minor tweaks, appeared in RTCA/DO‑160 for commercial aircraft avionics, up to 1989.6 After that, they required the extended impedance control as in DEF STAN 59-411, but don’t include the construction details of DEF STAN 59-411. The two tweaks already appeared in MIL‑I‑6181C7 which replaced MIL‑I‑6181B in 1957: a 1 kΩ bleeder resistor from the EMI port center conductor to case and the removal of the 1 Ω resistor in series with the input side 1 uF filter capacitor.
It would surely be gratifying for the originator of the 5 uH LISN to know that his work has gained this much success and acceptance worldwide. Who was this person, and how did the 5 uH LISN come about in the first place? We are indebted to A. T. Parker (1915 – 2000), for the following historical snippet. In 1960, Parker founded Solar Electronics, a designer and supplier of EMI test equipment. Previously he had worked at the Stoddart Aircraft Radio Company, which was the company that produced the first commercial 5 uH LISN. In Parker’s own words:
The DC-3 (military version C-47 “Skytrain”) was all aluminum. Aluminum aircraft return current on structure, except where inductance causes excessive voltage drop. No such problem occurs with dc power. Electrical power was from engine-mounted generators. Engine centerlines were about three meters from the aircraft centerline. Thus, using a nominal value, such as one microhenry per meter for a wire suspended above a ground plane, 5 uH seems a reasonable value if the measurement was taken in the cockpit-mounted breaker boxes, which act as the point of distribution for electrical power in the aircraft.
It is specifically this property of a LISN that allowed it to be used in MIL‑I‑6181B through “D” (the last revision prior to MIL‑STD‑461) in mirror image roles when measuring conducted emissions (Figure 5) and conducted susceptibility (Figure 6).
When all the Service- and platform-specific EMI specifications released prior to 1967 were superseded by the Tri-Service EMI standards MIL‑STD‑46113 and MIL‑STD‑462,14 it was the Navy practice of inserting line impedance stabilization in each power conductor that was adopted for Tri‑Service use. That is, instead of running return current back through the ground plane, it is returned through a wire and LISN instead.
We return once again to Mr. Parker for the rationale behind current measurements in lieu of measuring rf potential across a LISN.18 This is follow-on to the material quoted earlier from Reference 10.
“At any rate, this impedance suddenly began appearing in specifications which demanded its use in each ungrounded power line for determining the conducted EMI (then known as RFI) voltage generated by any kind of a gadget. The resulting test data, it was argued, allowed the government to directly compare measured RFI/EMI voltages from different test samples and different test laboratories.
“No one was concerned about the fact that filtering devised for suppressing the test sample was based on this artificial impedance in order to pass the requirements, but that the same filter might have no relation to reality when used with the test sample in its normal power line connection.
“Not until 1947, that is. At that time, this same Alan Watton, a propulsion engineer having no connection with the RFI/EMI business, decided to rectify the comedy of errors which had misapplied his original brainchild. He was in a position to place a small R and D contract with Stoddart for the development of two probes; a current measuring probe and a voltage measuring probe. Obviously, he felt that one needed to know at least two parameters for a true understanding of conducted
“As it turned out, Stoddart was successful in developing a current probe based on Alan Watton’s suggestions regarding the toroidal transformer approach which is still the primary basis used today. However, the development of the voltage measurement probe suffered for lack of sensitivity. Watton’s hope had been to provide a high impedance voltage probe with better sensitivity than was then available for measurement receivers designed for rod antennas and 50-ohm inputs. Since this effort failed and Watton’s funds (and probably his interest in the subject) faded out of the picture, the program came to a halt.
“This meant that the RFI/EMI engineer could either measure EMI voltage across an artificial impedance which varied with frequency, or he could measure EMI current flowing through a circuit of unknown r.f. impedance. Either way, the whole story is not known. In spite of the unknown impedance, the military specifications began picking up the idea of measuring EMI current instead of voltage…”
The more things change, the more they stay the same!
Completing our “as time goes by theme,” it is worth noting why MIL‑STD‑462D went with a 50 uH LISN instead of the 5 uH LISN. In fact, the original proposal for MIL‑STD‑462D going in was the 5 uH LISN. The same section of the MIL‑STD‑462D appendix says,
This means that with above ground current return, as shown in Figure 7c, measured single line currents or rf potentials look similar but not identical. The traces are identical for feeder and return when one or the other mode dominates, but where they are of similar amplitude and add on the feeder and subtract on the return, they differ. Separation of cm and dm modes to assist filter design has been a topic of interest since the late 1970s.21,22,23
When we know that current will be returned on a dedicated wire, not on structure, a better technique than controlling emissions on each individual lead is controlling emissions by mode. Separating modes may be done directly off the LISN (References 20 – 22) or using current probes. Regardless, if we control emissions via mode, not line, we can then assign limits based on what the modes actually affect:
- Differential mode noise currents cause ripple, and
- Common mode currents cause radiated emissions
A concrete and illuminating example of the problem of LISN misuse may be found in a report by the author dating to the late 1990s.25 This report showed that the (now obsolete) FCC Class B 48 dBuV conducted emission limit was in fact 20 dB too stringent for differential mode noise but was precisely correct for common mode noise. The problem arose because the original work done to establish the 48 dBuV limit was performed using a single 5 uH LISN, but the FCC test method was based on a pair of (50 uH) LISNs.26 It was not the disparity in the LISN impedance but the mode separation inherent in a pair of LISNs that demonstrated the disparity.
Another modern confusion is using long power leads between the LISN and test sample. Such values range from one meter (for conducted emissions) in MIL‑STD‑462 (1967 – 1993), 2 – 2.5 meters in MIL‑STD‑462D and follow-on versions of MIL‑STD‑461, one meter in RTCA/DO-160, and 1.5 meters in CISPR 25. By way of contrast, the specified length in MIL‑I‑6181B was 24 inches.
Consider the ramifications with respect to measurement uncertainty. First, MIL‑I‑6181B conducted emission limits stopped at 20 MHz. The electrical length of a 24” long wire at 20 MHz is a twenty-fifth wavelength. VSWR will be negligible, and therefore the LISN does in fact control the power source impedance seen by the test sample. MIL‑STD‑462D and follow-on MIL‑STD‑461 versions using a 2.5-meter-long power lead and 10 MHz upper CE102 limit frequency come in at less than a tenth-wavelength, so the LISN controls the power source impedance.
But look at specifications such as RTCA/DO-160 and DEF STAN 59-411, with 400 MHz LISNs and 100 MHz conducted emission control. A one-meter-long power lead is a third wavelength at 100 MHz. And for CISPR 25, using a two-meter-long power wire, the LISN is over a half-wavelength from the test sample. All the work and expense that went into the extended frequency range LISN is wasted when the parasitics controlled within the LISN is simply migrated to the LISN – test sample interconnection.27
- Visit http://www.emccompliance.com to find all specifications, standards, and other sundry documents cited herein that are not copyrighted by others.
- MIL-I-6181B, Interference Limits, Tests and Design Requirements, Aircraft Electrical and Electronic Equipment, 29 May 1953
- Javor, K. “Seventy Years of Electromagnetic Interference Control in Planes, Trains and Automobiles (and Ships and Spaceships, as well),” In Compliance Magazine, May 2023.
- Ministry of Defence Standard 59-411 and the older 59-41 all use a modification of the 5 uH LISN.The modification extends the frequency range of controlled impedance down to 1 kHz and up to 400 MHz. It is less than obvious why the LISN impedance needs to be controlled to 400 MHz when it is placed several meters from the test sample. The mismatch between power wire transmission line characteristic impedance and the 50 Ω LISN is always going to generate reflections, no matter how well the LISN impedance is controlled. If it is desired to have true impedance control, the LISN needs to be within a tenth-wavelength of the test sample power input. A 10 uF feedthrough capacitor would function admirably so used, at a tenth the cost of the 400 Hz LISN.
- JAN-I-225, Interference Measurement, Radio, Methods Of, 150 Kilocycles to 20 Megacycles (For Components and Complete Assemblies), 14 June 1945
- RTCA/DO-160 original through C revision: Environmental Conditions and Test Procedures for Airborne Equipment
- MIL-I-6181C, Interference Control Requirements, Aeronautical Equipment, 06 June 1957
- MIL-I-6181D, Interference Control Requirements, Aircraft Equipment, 25 November 1959
- CISPR 25 all editions, various titles. “Limits and methods of measurement of radio disturbance characteristics for the protection of receivers used on board vehicles” is the 1995 title.
- Parker, A. T. “A Brief History of EMI Specifications,” presented at the 1992 IEEE EMC Symposium. The Army Air Corp to which Parker refers was the forerunner of the US Air Force. The Army Air Corp became the United States Air Force in 1947.
- Some exceptions that prove the rule are many spacecraft line impedance simulation networks that appear to be designed to include the dedicated wiring to the test sample itself. See the line impedance simulation section of older print Solar catalogs (they no longer supply spacecraft LISNs, so the on-line catalog is of no value here). Pay special attention to the series resistance value. Values above a few tens of milliohms mean they are simulating the entire power distribution network, not the main bus. As Mr. Parker said in his catalogs, in his gentlemanly way, “Spacecraft designers do not always agree on the characteristics of the d.c. power source aboard the vehicle. The inductance in series with the load, the resistance across the inductor, and the series resistance in each leg of the unit are variables specified by different spacecraft engineers.”
- MIL-I-16910A, Interference Measurement, Radio, Methods and Limits: 14 Kilocycles to 1000 Megacycles, 30 August 1954
- MIL‑STD‑461, Electromagnetic Interference Characteristics, Requirements, Electrical for Equipment, 31 July 1967
- MIL‑STD‑462, Electromagnetic Interference Characteristics, Measurement of, 31 July 1967
- MIL-STE-826, Electromagnetic Interference Test Requirements and Test Methods, 20 January 1964
- MIL‑STD‑461D, Requirements for the Control of Electromagnetic Interference Emissions and Susceptibility, 31 January 1993
- MIL‑STD‑462D, Measurement of Electromagnetic Interference Characteristics, 31 January 1993
- Solar Electronics Application Note AN622001, “Using the Type 6220-1A Transformer for the Measurement of Low Frequency EMI Currents.” The application note used to be included in Solar Electronics catalogs. The excerpted portion is still found on their website under “Audio Isolation Transformers” under “History.” https://www.solar-emc.com/6220-1B.html
- Author’s comment about the 1947 date cited in this paragraph. 1947 seems too early. That is before MIL-I-6181, which used JAN-I-225, which didn’t include the 5 uH LISN. The date 1957 fits better, because MIL-I-6181C released in that year for the first time includes an alternate conducted emission test method and limit based on the use of a current probe, for cases when line current exceeds the 50 ampere LISN maximum current rating. But there is no way to know for certain if this was a typo, or bad memory or some other explanation.
- For much more on the topic of conducted emission mode separation, see the expanded version of this article on the author’s website, and other articles by this author and those listed as references on this topic.
- A. A. Toppeto, “Test Method to Differentiate Common Mode and Differential Mode Noise,” Proc. 3rd Symposium on Electromagnetic Compatibility, Rotterdam pp. 497-502, May 1979.
- M. J. Nave, “A Novel Differential Mode Rejection Network,” IEEE International Symposium on Electromagnetic Compatibility, Denver, May 1989.
- LISN UP Application Note, Fischer Custom Communications, 2005.
- Two spacecraft specifications follow this approach, where it is known that no current of any sort returns on structure. These spacecraft don’t operate radios in the bands where conducted emissions are controlled; the imposition of a common mode limit is based purely on controlling crosstalk. The resulting common mode limit is sufficient to the task and represents a large relaxation relative to typical radiated emission limits that protect against radio frequency interference.
- GSFC-STD-7000B, General Environmental Verification Standard (GEVS) for Goddard Space Flight Center Flight Programs and Projects, 29 April 2021
- GP 11461, Gateway Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, 06 November 2019
- Javor, Ken. “Investigation Into the Susceptibility of Radio Receivers to Power-Line Conducted Noise” EMC Compliance, 1998. Technical committee presentation and demonstration at 1998 IEEE EMC Symposium, Denver
- CBEMA/ESC5/77-29, Limits and Methods of Measurement of Electromagnetic Emanations from Electronic Data Processing and Office Equipment, 20 May 1977
- MIL‑STD‑462 (1967) had a unique approach to this conundrum. It specified one-meter-long wires for conducted tests, and two-meter-long wires for radiated. The only problem with that approach was not enough people followed directions; opting for one or the other length for both conducted and radiated measurements. The end result was that when the “D” revisions came along, they “dumbed down” the standard to one length optimized for radiated and reduced the conducted frequency range accordingly. Hence the 10 MHz upper limit for conducted emission control, unique amongst conducted emission limits. The goal for standardized test results outweighed the desire to control conducted emissions out to the traditional 30 MHz.