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

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TimNJ:
A few sanity checks about safety:

The LISN is an unusual piece of test equipment due to it's normal high earth leakage current. 60Hz LISNs may leak close to 1A (depending on the standard). For 400Hz DO-160, the earth leakage current is close to 3-6A (!!!)

However, if my analysis is correct, with an isolation transformer (or isolated AC source), a high current no longer flows in the earth wiring of the building since the conductive path is broken by the galvanic isolation of the transformer. So, in these cases, no more 3-6A flowing in earth wiring which could possibly cause a ground bounce issue that might affect nearby equipment and/or elevate the voltage on nearby earthed surfaces to something unpleasant or dangerous. The earth leakage current through the 10uF caps should be limited by the capacitive reactance/insulation resistance of the isolation barrier (transformer)...which will be on the order of 100s of KOhms at least.

Is that correct?

For an ideal isolation transformer, grabbing either the live or neutral secondary wire is not a risk because there's no way for current to flow through your body. But, with two 10uF caps now tied to ground, a loop can be formed on the secondary side of the transformer. It's is nearly "earth-referenced" with such a huge capacitance between earth and one of the secondary. So, grabbing one of the hot wires allows a current to flow out of the transformer, through your body, through the earth, up through one of the capacitors, and back into the transformer. Danger! Still, this requires actually touching a hot wire, which is not such a likely scenario.

If the LISN is left unearthed with an isolation transformer, touching the housing of the LISN should still be safe since you're still galvanically isolated from mains. From this, it seems that an isolated source provides a good deal of safety.

On the other hand, with a non-isolated source, there seems to be much less room for error. If the LISN is left unearthed, the housing floats to 1/2 the mains voltage with respect to earth. Touching the housing will be nasty, especially since the source impedance will be quite low through the 10uF. And, now, since there is a conducting path back to the mains supply, a large steady state current flows in the building earth wiring/structure.

So, it sounds like isolated supply is really the way to go for two reasons. Does this seem accurate?

Jay_Diddy_B:
Hi

If the noise source is current, not voltage, you get this result:



If you inject and current and measure current you get a flat response from the CT and the expected result from the LISN impedance.

Jay_Diddy_B

TimNJ:
Thanks. Sorry, my circuit image is not showing. I modeled as voltage source with 1R source resistance, same as you did before. But then measuring the current into the LISN. In reality, I don’t know if a “typical noise source” (if one existed) is more closely a current source or voltage source.

chris_11:
Try to give you some practical advice. The first inductor data sheet SRF is the parallel resonance of the inductance with its own parallel capacitance. This is in a LISN a high impedance point (against 50 Ohm) and of no concern. The problem are the real series resonance frequencies (low impedance points) where partial parts of the inductor inductance resonate with partial serial capacitance. Since those points are not given in any data sheet you need to characterise them with your own measurements (VNA or Tracking generator with SA). You are better off with several low inductors in series (same values) because their resonances are higher than larger inductances in the same package. I got good results up to 1Ghz with 6x 800nH SER2009-801 Coilcraft inductors in series. Do not mount them on a PCB, since the connections will add capacitance and move the partial series resonances lower. Solder those 6 inductors in the air in a long strip. You might support them with some low capacitance material on the core if you like too, but due to the flat wire with over 50A capability it is usually not an issue.

br
Christian

TimNJ:
Hi everyone,

Photo of the assembled PCB is below (sitting unmounted in the Bud AN-1322-A enclosure.) I'll report back with NanoVNA measurements soon. If everything's okay, I'll share the KiCAD project files after.

Thanks for all of your help!

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