Algebric solution of a LISN impedance - Page 1
Forum section: electronics / rf-microwave

author: BelgianEngineer
author_rank: Contributor
timestamp: January 30, 2021, 11:33:55 am
content: Hey all,I am looking into building a DIY LISN. But first, I would like to calculate the expected theoretical impedance curve algebraically so I can later compare my built performance with requirements from the standard (attached pic).The ideal circuit is pretty simple, so it should be easy, right? Well, I've spent already an afternoon on it and I can't get a resulting curve which is similar to that displayed in the standard. So first question:For the impedance curves given in the standard, is the power supply usually assumed to be a short? Is the displayed curve really matching the displayed circuit?If you feel like doing some easy maths, mind having a doublecheck at my calculations (attached pic)? The Laplacian s is converted to frequency domain by s = 2*PI*freq.Seen the simplicity of the circuit I have the feeling that I am missing something basic...Sorry if this is not the appropriate section of the forum. I first posted in metrology and was recommended to post here so I try my chance

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2021, 01:41:35 pm
content: I would say yes, at least at DC. But that's DC, and at AC it's going to be greater than zero. Which also means it's going to be inductive (impedance rising with frequency). What inductance it has, then, is a fine question, particularly in regards to C1+R1...MIL-461 would seem to leave this question unanswered (at least from the above snippet); some others (mains related, e.g. CISPR22 I think?) suggest 250uH source inductance, which is usually realized as an inductor inside the network, but I would take it to mean at least part of the mains supply's internal inductance. Which if you sum up the wiring between receptacle and power station*, is probably a good enough guess, or at least in the right ballpark, like 100uH say. A little extra (e.g. the remaining 150uH, or more) inside the network would seem prudent.Anyway, after the bulk inductor, the bulk capacitor -- with substantial ESR, like 10uF + 5 ohms -- serves to stabilize line impedance as a first go, also keeping those reactances well damped. This sets the leftmost "toe" region of the curve, and the network's main EUT passband being a decade or so higher (i.e., 10x the impedance (50 ohms) and 10x the frequency, give or take), make the passband relatively insensitive to source impedance.*Given a known transmission line impedance, we know the inductance per length. So, just estimate the length of romex and conduit and so on up to the pole, and then between poles up to the transformer, etc. The pole/pad transformer is the first big unknown, in terms of its stray/leakage inductance. On the other hand, its impedance ratio is high enough (e.g. 240:4800V <--> 1:400 impedance ratio) that we don't have to pay much attention to the distribution lines' inductance. Good, it might be meters or kilometers to the nearest substation, who knows! Or to put it another way, the amount of power available via distribution lines is massive (~MVA), while we're talking tiny fractions of that capacity (~kVA), so the change in voltage, there, ought to be pretty small, for piddly little loads like this.Tim

author: BelgianEngineer
author_rank: Contributor
timestamp: January 30, 2021, 02:27:06 pm
content: Hey T3sl4co1l,After taking a good break and starting again it seems that the mistake was coming from bad complex algebra on my side. So problem solved on this part! Thanks a lot for your input anyway, just like a wikipedia page, you opened a path to more interesting stuff to look into than what I was initially here for

author: coppercone2
author_rank: Super Contributor
timestamp: February 10, 2021, 04:13:53 am
content: I am not sure that in real life trying to estimate the power pole behavior is useful because the power company can always change things, move the transformer to another pole, etc. The last thing you want to do is police that part of the system or be responsible for it.I built my lisn with 250uH inductors in the front but every standard makes it a little different. My chassis is still stuck though, after 2 years I decided to continue the project that got ruined by metal warping.. by using flame straitening on the metal box I made and surprisingly its some what of a box now. One of the most dreary projects I have attempted when you look at what the LISN is for.

author: richard.cs
author_rank: Super Contributor
timestamp: February 10, 2021, 11:00:59 am
content: Quote from: coppercone2 on February 10, 2021, 04:13:53 amWell quite. The point of a LISN is that over the frequency range of interest the impedance on the DUT is repeatable and largely independent of the impedance on the power line input. Calculating what it looks like at lower frequencies may be academically interesting, but is not all that real-world useful, other than the realisation that it adds non-zero impedance in the transition region. This can occasionally cause power supplies that work just fine in the real world to fail to start or to exhibit weird behaviour when connected via a LISN for EMC test.