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5uH Aerospace LISN: How dumb would I be to "throw one together"?

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TimNJ:
Thank you all. I realize I need to go back to my LISN fundamentals so I’m spending some time with Henry Ott and this rather insightful pdf from Michigan State: https://www.egr.msu.edu/emrg/sites/default/files/content/module11_conducted.pdf

At least where I work, the LISN is a somewhat “mythicized” black box type thing, so this is forcing me to think about it a little more.

I guess my question now is Jay_Diddy, if it’s seemingly so straightforward to put together the network with off the shelf parts, and with reasonable frequency characteristics, why are there so many LISNs with fancy damping techniques? Maybe that’s what’s required to get true flatness in the 50ohm band.

Your plots were very helpful in my understanding. Initially my thinking was that if the inductor’s SRF was anywhere in the measurement band, it was useless..but now it’s obvious that a resonance will mostly not affect the 50 ohm impedance, and mainly affects the HF roll off of the 50 ohm impedance. Still, there certainly can be many other things to which make an inductor look, err..not so much like an inductor, as Tim showed in his complicated model of the common-mode choke. Clearly, there can be several resonant interactions and an impedance which doesn’t quite rise quite in like with 2Pi*f*L.

Seemingly those Wurth inductors were pretty well behaved, ferrite core and all.

Whats SOL by the way?

2N3055:

--- Quote from: TimNJ on July 25, 2020, 05:21:54 am ---Thank you all. I realize I need to go back to my LISN fundamentals so I’m spending some time with Henry Ott and this rather insightful pdf from Michigan State: https://www.egr.msu.edu/emrg/sites/default/files/content/module11_conducted.pdf

At least where I work, the LISN is a somewhat “mythicized” black box type thing, so this is forcing me to think about it a little more.

I guess my question now is Jay_Diddy, if it’s seemingly so straightforward to put together the network with off the shelf parts, and with reasonable frequency characteristics, why are there so many LISNs with fancy damping techniques? Maybe that’s what’s required to get true flatness in the 50ohm band.

Your plots were very helpful in my understanding. Initially my thinking was that if the inductor’s SRF was anywhere in the measurement band, it was useless..but now it’s obvious that a resonance will mostly not affect the 50 ohm impedance, and mainly affects the HF roll off of the 50 ohm impedance. Still, there certainly can be many other things to which make an inductor look, err..not so much like an inductor, as Tim showed in his complicated model of the common-mode choke. Clearly, there can be several resonant interactions and an impedance which doesn’t quite rise quite in like with 2Pi*f*L.

Seemingly those Wurth inductors were pretty well behaved, ferrite core and all.

Whats SOL by the way?

--- End quote ---

Air core inductors won't saturate.. Cored inductors will also change characteristics as they get loaded. So with cored inductors you have to be mindful of the current going through..

SOL : Short, Open Load calibration of VNA....

Jay_Diddy_B:

--- Quote from: TimNJ on July 25, 2020, 05:21:54 am ---Thank you all. I realize I need to go back to my LISN fundamentals so I’m spending some time with Henry Ott and this rather insightful pdf from Michigan State: https://www.egr.msu.edu/emrg/sites/default/files/content/module11_conducted.pdf

At least where I work, the LISN is a somewhat “mythicized” black box type thing, so this is forcing me to think about it a little more.

I guess my question now is Jay_Diddy, if it’s seemingly so straightforward to put together the network with off the shelf parts, and with reasonable frequency characteristics, why are there so many LISNs with fancy damping techniques? Maybe that’s what’s required to get true flatness in the 50ohm band.

Your plots were very helpful in my understanding. Initially my thinking was that if the inductor’s SRF was anywhere in the measurement band, it was useless..but now it’s obvious that a resonance will mostly not affect the 50 ohm impedance, and mainly affects the HF roll off of the 50 ohm impedance. Still, there certainly can be many other things to which make an inductor look, err..not so much like an inductor, as Tim showed in his complicated model of the common-mode choke. Clearly, there can be several resonant interactions and an impedance which doesn’t quite rise quite in like with 2Pi*f*L.

Seemingly those Wurth inductors were pretty well behaved, ferrite core and all.

Whats SOL by the way?

--- End quote ---

Hi TimNJ,

You are right there are a lot of myths associated with building LISNs. Which has resulted in a lot of 'traditions' and not a lot of engineering. I believe that that there are many reasons for this:

1) MIL 461 one of the earlier standards was introduced in 1967. In 1967 we didn't have the same tools that we have today.

2) Some of the early EMC specifications described the construction of the LISN with air-cored solenoids. This started the tradition of using air-cored inductors.

Fast Forward today, we have

affordable VNAs
Simulation tools
Modern components
Internet

Concept


The concept of a LISN is simple. It provides a standard impedance to the DUT. In this case 5uH in parallel with 50 \$\Omega\$.
The requirement is that the LISN looks like 5uH//50 \$\Omega\$

Myth destruction

1) You can use inductors above their self-resonant frequency.

At high frequencies the LISN impedance is 50 \$\Omega\$ so the requirement is that the impedance of the inductor is 'large' (say 200 \$\Omega\$) compared
with 50 \$\Omega\$

2) There always benefit in using a string of different inductors with different SRFs.

This is NOT true.

If you connect inductor in series with different SRFs then they can interact.

3) Inductance changing with DC current

This is not an issue providing the impedance remains 'large' compared to 50 \$\Omega\$

Air cored Inductors


Air cored solenoids typically need many turns to achieve the desired inductance. This long winding may transmission line effects within the target frequency range.
The air cored inductors need to be in a large case, typically 3 diameters of the coil, to avoid coupling the case.

Cored Inductors

Cored inductors are really air cored. They have an air gap or a distributed air gap. The magnetic material is equivalent to a conductor in an electrical circuit.
The winding in a cored inductor is much shorter. This helps eliminate transmission line effects in the target frequency range.
The flux is confined to the core, reducing the effects of coupling.

Inductor Measurements

I am going to measure the Wurth 4.7uH Inductor, 7443640470:



I have solder 2x 100 \$\Omega\$ resistors in parallel and a BNC connector:



This allows the impedance to be measured with my VNA.




This is a log frequency sweep from 10kHz to 200MHz.
The vertical axis is linear 5 \$\Omega\$ /div




This is the LTspice simulation of the same circuit. The LTspice simulation uses lumped parasitic components.

There is excellent agreement between the model and the measured results.

This implies that we can accurately predict the performance of a LISN built this way.

I don't have the proposed inductor 7443630220 in my inventory, or I would have measured it.

SOL is  Short, Open , Load. This one a techniques used to calibrate the VNA.


Regards,
Jay_Diddy_B

TimNJ:
Thank you very much.  I think I may give it a try with either 2x2.2uH or 3x1.5uH of the Wurth high current inductors. Seems their models are pretty good, which may be important Do these models account for effects at high current? Thank you for running some tests on your VNA!

Any reason I can't verify the same parameters with a spectrum analyzer with tracking generator?

A few more questions related more to the actual test standard/requirements:

1. The standard calls for 2x10uF connected between line and ground plane, and between neutral and ground plane. The setup is as below. For our normal CISPR setup, our LISN is actually earthed to the building's earth wiring. But in this case, if I was to earth the chassis/ground plane, wouldn't that be a ton of earth leakage current, possibly messing up other devices, not to mention potentially dangerous?

I'm trying to figure out if these capacitors are really only to be attached to a "local ground plane", insulated from actual earth. Does that make any sense?

2. On that same note, any reason why I can't put the two LISN halves in the same box? Traditionally, two boxes are used, but I think that's mostly because there are certain cases where you only need one. (Although, from reading the other thread, some people have vocalized opinions about why only using 1 LISN is a bad thing.)

Thanks again.

Jay_Diddy_B:

--- Quote from: TimNJ on July 25, 2020, 06:45:33 pm ---

Any reason I can't verify the same parameters with a spectrum analyzer with tracking generator?

A few more questions related more to the actual test standard/requirements:

1. The standard calls for 2x10uF connected between line and ground plane, and between neutral and ground plane. The setup is as below. For our normal CISPR setup, our LISN is actually earthed to the building's earth wiring. But in this case, if I was to earth the chassis/ground plane, wouldn't that be a ton of earth leakage current, possibly messing up other devices, not to mention potentially dangerous?

I'm trying to figure out if these capacitors are really only to be attached to a "local ground plane", insulated from actual earth. Does that make any sense?

2. On that same note, any reason why I can't put the two LISN halves in the same box? Traditionally, two boxes are used, but I think that's mostly because there are certain cases where you only need one. (Although, from reading the other thread, some people have vocalized opinions about why only using 1 LISN is a bad thing.)

Thanks again.



--- End quote ---

Hi TimNJ,

1) I am not aware of an easy way to do the impedance measurement that I did with the VNA with a SA and TG. You can probably get an idea if you have a directional coupler or a return loss bridge.

2) Most of the LISNs that I have built have a capacitor in the position of the 10uF capacitor. It may not be quite as big as 10uF but it is certainly an effect RF short.

3) There is no reason why you can't put two LISNs in one box. Here is a picture of a low current dual 50uH 50 \$\Omega\$ LISN (CISPR 32) that I have built:



It works very well.

Regards,
Jay_Diddy_B

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