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

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TimNJ:
September 7, 2020 Update: Please this thread for final design files and discussion: https://www.eevblog.com/forum/projects/lisn-for-rtcado-160-aerospace-emc/

Hi all,

I've been following this lovely thread for a long time: https://www.eevblog.com/forum/projects/5uh-lisn-for-spectrum-analyzer-emcemi-work/

I have two projects that need to comply with RTCA DO-160G (Aerospace) EMC. The end-game is 3rd party testing with "correct" equipment. Like other standards for vehicles, the LISN defined is the 5uH type. I have the option to rent monthly at $600/month for a pair of LISNs. To purchase, it's about $2000-3000. Purchasing is out of the question for the moment. Obviously, the commercial LISNs are already characterized nicely, and since there's already plenty of things that can go wrong with in-house EMC pre-testing, it's nice to keep unknown variables at a minimum. At the same time, I still have a hard time justifying the cost of the commercial LISNs.

For the sake of ballpark pre-compliance testing, how dumb would be to try to DIY the LISN with the simplest possible design? (FYI, I need to pass >12A DC (for project #1) and >3A AC (for project #2). I have a 1.6GHz SA with tracking generator to help characterize. I see most people are using VNAs. I'm not sure the difference in this case.)

-----

A few questions to help me understand whether this is practical or not:

1. The Tekbox 5uH design uses a "segmented" or "staggered" inductor design..with taps that go to RC networks. What gives? Is this to help flatten/control the frequency response? What's the effect of breaking down into segments as opposed to one big "lumped' inductor?

2. Some designs have small value series resistors in line with the 1-10uF capacitors. Same idea as above? Add some damping to the frequency response/prevent oscillation?

3. Generally speaking, are you more likely to have issues with relatively large air-coil inductors or smaller ferrite/powdered core inductors? Seems like air-core is generally preferred, and probably easier to reach at only 5uH, as opposed to 50uH.

4. For designs with series inductors, some have parallel resistors across each. What's the rationale? (Jay_Diddy_B shows 330R across his.)

5. The measurement is taken across a 5K resistor but with a 50R  receiver, so the RC filter is really 0.1uF + (50R || 5K = 49.5R). I suppose the value of that 5K resistor barely even matters then. since it's always paralleled with 50R. (Some LISNs use 1K.) I was planning on copying the HP 11947A transient limiter...Is this generally a "universal design" that can be used with any 50R system? Obviously need to adjust for 10dB attenuation in the receiver.

I know these are pretty naive questions. I think it's obvious I'm still a novice in many of these areas. Maybe that's a sign that now's not the time to dabble in something I'm not too certain about, especially when I have a deadline to meet and customers to please.

Thanks,
Tim

T3sl4co1l:
The answer to most of these, is: dampen resonances of the inductor.

Play around with this, for example; read the supporting information: http://hamwaves.com/antennas/inductance.html
It uses a helical waveguide model, and a bunch of corrections, to estimate the voltage and current through a single layer, wire solenoid.  Apparently this geometry is remarkably difficult to analyze!

If the calculation succeeds (there is some root-finding that can fail, particularly at small pitch angles), you get the complex impedance of the element -- resonant modes accounted for!

The first (parallel) resonant mode is fine, that's what we want -- a high impedance between DC and EUT sides.  What we don't want is the second (series) resonant mode, which acts in parallel with EUT, shunting signal and distorting the frequency response.

Knowing that standing waves are the culprit, we can employ resistors in strategic locations to dampen those modes.  The series resonance has an antinode in the middle (voltage peak), and nodes (current peak) at the ends, which is why we measure a low impedance at the ends, at that frequency.  A resistor from one end to the middle will act in parallel with that mode, dampening it.  The resistance acts in parallel with the voltage peak, and gets 1/4-wave-transformed into a series equivalent resistance at the terminal.  When R = Zo,* parallel R is transformed to series R, and that's simply the resistance you will measure at the port.

*For an ideal transmission line, Zo is simply Zo.  Helical waveguide however is dispersive (velocity varies with frequency), meaning the Zo varies for each resonance; the resonances also aren't harmonically related.  So it's not quite as simple as knowing the wire length of the coil (though that's close for the 1st mode, I think?), and it may be worth playing with the resonances in a calculator such as above, or testing real hardware.

[Note that helical waveguide can have quite high impedances, low kohms -- this makes them useful for bias tees for one, but also useful for delay lines in certain applications.

A vintage application was color TV sets: the impedance must be high (low kohms) to suit to the vacuum tubes used in early sets.  The delay is applied to the luma signal (which is simply detected directly from the radio signal), to "catch it up" with the chroma signal, which ends up delayed due to additional filtering and processing stages.  The required delay was about a microsecond.  A typical delay line was a phenolic tube, wound with fine wire, and lined with a strip of foil -- not a complete wrap-around foil lining, that would make a shorted turn, defeating the helical mode; just a narrow strip to give some ground reference for the travelling wave.]


We can also employ loading materials; if we have a lossy ferrite or powdered iron material, we can use it to both increase the inductance (reducing the number of turns required, potentially raising the resonant frequency) and dampen the resonance (by magnetically coupling material losses to the resonances).

The loss has to be appropriate, of course; a high-mu powdered iron will be too conductive, and actually increase the capacitance more than dampen modes.  A low-mu ferrite might have too high of a Q at these frequencies, and just not do much, or maybe make things worse.  It may take some trial and error to perfect.


I haven't done this specifically for solenoids, but I have plotted a few resonances on a 100% coverage, single layer toroid winding.  Here, the boundary condition forces the first mode to be a full wave, and it is series resonant.  Again, the resonant impedance is quite high, which makes it difficult to dampen at the terminals -- if we're using this toroid for a current transformer, we can't afford much burden resistance.  Maybe the resistor needs to be a few ohms for the application, but the correct value for damping would be 300 ohms, or a few kohm even -- highly impractical to dampen. :(  Knowing that it is a resonant mode, however, it can be shunted by simply distributing the applied current around the core.  Instead of using one loop in one place, use two loops, wired in parallel but positioned in quadrature (at 90 degrees to each other on the toroid).  Or hexature, etc.  (I was using a high-mu toroid, which is rather lossy at the 10s of MHz these modes were showing up, so the 4th mode was already very weak; I didn't try looking any higher.)


Anyway, what we're after, is knocking out series resonances (impedance dips at the EUT/RF port) and replacing them with resistances, hopefully resistances that are high enough not to worry about (i.e., R >> Zo).


If we have discrete inductors that manifest as lumped single RLC networks (i.e., effectively some simple equivalent parallel capacitance, without having ugly modes at higher frequencies), we can employ the same methods, applied to a lumped-constant circuit, without having to worry about waves and modes necessarily.  Here's a bias tee by Picotronics, flat to, as their name suggests, some picoseconds (many GHz):
https://www.seventransistorlabs.com/Images/Picotronics/MVC-349X.JPG
https://www.seventransistorlabs.com/Images/Picotronics/Schematic.JPG
It seems they opted for the parallel damping resistance instead.  I think that can save a little on impedance -- you don't have the lossy cap-to-ground loading it down.  Which one is best probably depends on the inductors used, and desired flatness.

Don't forget, too -- inductors have capacitance between their terminals, and to ground or free space; a 2-terminal model of an inductor or capacitor can be erroneous at these frequencies.  This may also be a factor.


As for some other questions -- air core is preferred over ferrite because ferrite saturates.  Rod cores aren't too bad here, as they can be made with quite high saturation currents, and the advantage is small anyway (mu_eff maybe only 2-5 say), enough to be helpful without getting in the way.

Saturation current should be quite high indeed for a mains LISN or CDN -- loads with poor power factor can draw quite high current peaks, and those current peaks are also likely where most of your EMI is transmitted (the FWB acts as a PIN diode), so you don't want your network pooping out on the peaks!

Tim

Jay_Diddy_B:
TimNJ and the group,

I can probably help design you a LISN that will work in this application.

Your current specification is higher than the maximum for the LISN that I shared before.

I have a few questions for you:

1) Does the EMC specification that you are testing require one or two LISNs?

2) What is the maximum frequency that is required in the EMC specification for conducted emission?

3) Which EMC specification are using attempting to meet?

Regards,
Jay_Diddy_B

Jay_Diddy_B:
Hi,

From here: https://www.com-power.com/uploads/pdf/LI-325C-1.pdf

I find this:



So it looks like you need two LISNs.

and this is impedance curve limits:



The limit lines are in red. It looks like 5uH // 50 \$\Omega\$ +/- 20% to 152MHz.

This is all that is needed to design the LISN.

Regards,
Jay_Diddy_B

TimNJ:

--- Quote from: T3sl4co1l on July 24, 2020, 07:59:41 pm ---The answer to most of these, is: dampen resonances of the inductor.

Play around with this, for example; read the supporting information: http://hamwaves.com/antennas/inductance.html
It uses a helical waveguide model, and a bunch of corrections, to estimate the voltage and current through a single layer, wire solenoid.  Apparently this geometry is remarkably difficult to analyze!

....

The first (parallel) resonant mode is fine, that's what we want -- a high impedance between DC and EUT sides.  What we don't want is the second (series) resonant mode, which acts in parallel with EUT, shunting signal and distorting the frequency response.

Knowing that standing waves are the culprit, we can employ resistors in strategic locations to dampen those modes.  The series resonance has an antinode in the middle (voltage peak), and nodes (current peak) at the ends, which is why we measure a low impedance at the ends, at that frequency.  A resistor from one end to the middle will act in parallel with that mode, dampening it.  The resistance acts in parallel with the voltage peak, and gets 1/4-wave-transformed into a series equivalent resistance at the terminal.  When R = Zo,* parallel R is transformed to series R, and that's simply the resistance you will measure at the port.

*For an ideal transmission line, Zo is simply Zo.  Helical waveguide however is dispersive (velocity varies with frequency), meaning the Zo varies for each resonance; the resonances also aren't harmonically

...
...


Tim

--- End quote ---

Wow, thanks Tim. As always, you find time to give the most informative answers possible and I appreciate that. I guess the takeaway is that as resistance (or lossy material) added to an LISN (or any similar style network) is to add some damping to keep incidental tank circuits from messing with the "flatness" of the frequency response. From playing with the calculator you attached, and the GCI-Wcalc mentioned in the other thread, I've noticed that making a single 5uH coil with a reasonable diameter (let's say 10mm) might push the SRF to <100MHz. (The conducted EMI measurement goes from 150KHz-152MHz.) But, if you break up the 5uH coil into 4 or 5 coils, then each coil's SRF will be >200MHz. Presumably, if the coils are far enough apart (physically) any proximity effect related issues (that would contribute to parallel capacitance) would be practically gone.

Perhaps I should do some SPICE simulation based on the results from Serge Stroobandt's solver. Even if the SFRs of each coil are above the frequency band of interest, I suppose there could be a way the 4 or 5 coils interact to cause distortion at a lower frequency...maybe? (Talking out the wazoo here.)

By the way, is the "first (parallel) resonant mode" you refer to the "intended" LC interaction as shown in a typical LISN schematic. And the "second (series) resonant mode", that's the interaction between the Cp and the Ls of the coil?

Thank you!

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