Electronics > Projects, Designs, and Technical Stuff

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

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Pitrsek:
Paralleling same value capacitors works well. Paralleling same capacitors actually it works much better than the 1uf 100n 10n triad of resonance...
Make yourself a impedance test pcbs so you can see for yourself (Or buy them https://www.sv1afn.com/en/products/rf-experimenter-s-pcb-panel-of-8-pcs-diy-kit.html).
Each piece of trace adds an inductance. Depending on the inductance of the capacitor and added inductance this might have big or negligible effect. If you have power plane/ground plane pair with very tight spacing, and caps connected directly to the planes, added inductance is marginal. In case of 2L board, spacing is not tight and added inductance might not be negligible at all. Ie. If you add 2cm of trace you will multiply inductance of a small mlcc capacitor. It's a same here, just capacitor inductance is rather big to start with, so the effect is not as huge.

I did a quick test - small pcb, top was signal, bottom ground capacitor(330n/630v mkt, rather big package) connected between the two. SMAs on both ends. 2 samples - one with solid planes on both sides, second with plane slots so it forced flow through pins of capacitor. Difference in attenuation at HF was cca 1.5db. This is not much but: big package - with smaller package the effect would be bigger, and if you have multiple capacitors/multistage filter and on each cap you can gain 1.5db of attenuation....
In your case it is a detail, but depending on components size and layout, it might have bigger effect.

Imagine you are electron traveling from the fuse to the first small cap. In you original design, you go through the trace(inductance) and through the capacitor. If you want to go the output, you do not travel through the same inductance. You have basically added extra ESL to you capacitor.

Actually there will be a HUGE resonance between the 470pF and 10uF. At some point it will be marginally better than just 10uF, and at some much, much worse than just 10uF. For more of the topic I can recommend something from Steve Sandler about flat output impedance. Or Istvan Novak - distributed matched bypassing.

But this all is more of curiosity/academic discussion, unless you have extremely noisy power supply your lisn will be fine will be fine either way.                     

T3sl4co1l:
I don't think quibbling about nH is all that useful here.  The body of the capacitor will be some pF to surroundings, thus having a LCL equivalent with a cutoff at some point, and whatever impedances.  Perhaps more intuitive is to consider it as three TL segments: the leads and body.

About all you can do about the body is remove ground under it.

Tim

TimNJ:
Thanks to everyone for their input. I've ordered a few PCBs based on some recommendations here. I will try to characterize the design once I receive the NanoVNA (and once I watch enough W2AEW tutorials to figure out how to use the thing).

Pitrsek, regarding your explanation:


--- Quote ---Imagine you are electron traveling from the fuse to the first small cap. In you original design, you go through the trace(inductance) and through the capacitor. If you want to go the output, you do not travel through the same inductance. You have basically added extra ESL to you capacitor.
--- End quote ---

Which capacitor? I guess I just can't seem to wrap my head around how a cutting the polygon pour (in my head, increasing the  average distance traveled) makes the inductance lower.

---

Side question: The main difference between a 5uH and 50uH LISN is the low frequency impedance, below roughly 5MHz...If the noise source is modeled as an ideal current source, then the voltage at the RF port of the LISN in theory should always be worse with a 50uH LISN than with a 5uH LISN. I thought about it for a while and figured there was a hypothetical situation where the wiring from the DUT, its input EMI filter, etc. may resonant with the LISN's impedance in such a way that the EMI level is worse with 5uH than it is with 50uH.

However, after trying a number of LTSPICE simulations, the only time I could get resonant peaking (with higher RF port voltage on 5uH vs 50uH) was with a relatively large common-mode capacitance (line to earth), in the range of ~5nF and above. I feel this would generally be a rare scenario, especially for mains power supplies or anything with earth leakage requirements.

Perhaps a situation still exists, but my question is: Is it generally true that a system passing EMC standard on 50uH should most likely pass on 5uH?

Thanks.

Jay_Diddy_B:
Hi,

Let me address the differences between the 5uH and 50uH LISNs.

Historically the 5uH LISN were used in small vehicles like cars and planes where the wiring is short. 50uH LISNs were used for line voltage and ship board applications where the wiring is longer.

At high frequencies the impedance of both LISNs is the same, it is 50 \$\Omega\$. So for high frequencies the results will be identical.

LTspice Model

I am going to turn to LTspice to illustrate the differences between the LISNs. I am stepping the value of the inductor between 5uH and 50uH:



Results



The bottom graph shows how the impedance changes with frequency.

The middle graph shows the voltage output if the emission has a 50 \$\Omega\$ source impedance.

The top graph shows what happens if the emission source is low impedance. I have chosen 1 \$\Omega\$. It could be lower than 1 \$\Omega\$. In the case the choice of LISN has very little effect on the voltage at the output of the LISN.

Regards,
Jay_Diddy_B

TimNJ:
Thanks, as always, for your insightful responses. I understand 5uH may closer approximate a more compact distribution system , and 50uH may closer approximate the wiring in a building. That part makes sense from a historic/rationale perspective. But if the response to EMI is so similar..why were two impedances ever created?

What if we actually measure the EMI current going into the LISN, instead of the voltage across the 50R resistor? (This is as measured in DO-160 and for signal wires in CISPR25). In this case, the 5uH inductor presents a lower impedance and thus the current below ~5MHz will be higher for a given noise voltage signal. No? By contrast, if measuring at the RF out port, since more EMI current is now shunted through 5uH, then less signal reaches the RF out port.

So, by that logic, seems you will show lower EMI with a 5uH LISN if using RF measurement port, but you will show higher EMI if using current transformer method. Once in the 50R region, then both methods should show equivalent readings. Am I making sense?



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