Design Review Request for a DIY AC Mains Line Impedance Stabilization Network - Page 1
Forum section: electronics / projects

author: dmitko97
author_rank: Newbie
timestamp: September 14, 2023, 06:05:52 pm
content: Hello Everyone,I need help with designing an LISN (Line Impedance Stabilization Network) for pre-compliance testing.I need the LISN to be designed with respect to FCC/CISPR 16 requirements (Mains Network). I am currently at the stage of starting the Layout, but I would like to maybe get some pointers beforehand.Here are some of the simulations I have done in order to give me some confidence with the design. All components that were selected in the simulation have parasitic elements.I am attaching my Kicad Schematic that I would like to get reviewed.

author: TimNJ
author_rank: Super Contributor
timestamp: September 17, 2023, 10:32:36 pm
content: I’m trying to understand the parallel 0.1uF and 4.7uF on the EUT side. Frequency response will be off with 4.7uF?As for the 1K resistor, in your simulation, it is across the BNC output, but in your KiCAD schematic it is not. 10x10K generally seems unnecessary. What do you hope to gain there?Flatness of the response at higher frequencies will depend on inductor construction. In practice you may need to add some parallel damping or RC networks here and there.

author: dmitko97
author_rank: Newbie
timestamp: September 18, 2023, 08:31:11 pm
content: Thank you for your reply!!!The goal of the pi-filter at the input is to provide a lower ESR and just extend the bandwidth of the cap.. I guess it is not needed... I should remove the 4.7uF?In theory the 1k is basically the 10x10K, I thought it can again offer a better response.regarding the inductor I plan on using SMD components that have a SFR of about 40MHz as my sweep ends at 30MHz.Where should I be adding these RC networks?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2023, 09:09:27 pm
content: Have you read through the LISN topics on this forum yet? You should find essentially complete designs around, and ample discussion around them.Your simulation and schematic differ in fundamental ways; I don't understand where you would think a series resistor is a parallel one, or multiple capacitors (of larger total value) equals a smaller one?Parallel resistors for frequency response is basically only something audiophools do on superstition; it's not relevant here, and in fact makes things worse as shown.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: September 21, 2023, 04:16:16 pm
content: Quote from: dmitko97 on September 18, 2023, 08:31:11 pmI think you should re-simulate what you drew in your KiCAD schematic see how the response differs from what you originally simulated. In general, it's probably best to stick to the CISPR16 standard unless you have good reason to not. The 1K resistor across the BNC is actually not that critical to start with. As far as I know, it is mainly to keep the voltage at the BNC tip at a reasonable level when there is nothing connected. (Think of it as a pull-down resistor.) Otherwise, most of the RC response is coming from the 50ohm input impedance of the spectrum analyzer. (1K || 50ohm = 47.6ohm).On the power input side, the LC values are not totally critical though I still don't recommended messing with them... but on the EUT side, the EUT needs to "see" the proper impedance as defined by the CISPR16 curve (or whatever standard). If you make C10 = 4.7uF, the impedance at low frequency (e.g. 150KHz - 1MHz) will be reduced. The noise level registered at the BNC is directly related to the impedance seen by the EUT. So, it will introduce error.