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Why are most common mode chokes only given with a impedance curve, inductance at a specific frequency and current rating (this is clear).

Wouldn't be S-parameter better? I know it's more work, but what can I calculate with the impedance?

Thanks

winny
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R098
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  • Please give an example and why would an S parameter be useful for common mode impedance? S parameters assume a fixed load impedance and, in many, many cases that would be useless. – Andy aka Feb 11 '22 at 16:50
  • This one for example (the first one I could find without S-Parameter): https://www.mouser.de/datasheet/2/447/P789-2902987.pdf – R098 Feb 11 '22 at 16:53
  • Or like this https://www.mouser.de/datasheet/2/212/1/KEM_LF0030_SS26V-1103810.pdf – R098 Feb 11 '22 at 16:55
  • Aren't Common Mode Chokes common in 50 Ohm or 75 Ohm Networks? – R098 Feb 11 '22 at 16:56
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    Certainly not. Have you tried a search? – winny Feb 11 '22 at 16:58
  • Sure I tried searching , otherwise I wouldn't be here – R098 Feb 11 '22 at 17:00
  • If you needed to use a CM choke then you'd know why they don't give S parameters. Do you have a specific need for one? If so then please reveal. – Andy aka Feb 11 '22 at 17:03
  • What does it give me that I only know 1 specific Inductance at one specific frequency? Or can I calculate something with the Inductance at a frequency+Impedance – R098 Feb 11 '22 at 17:03
  • The graphs tell you the impedance and, from those graphs you can calculate what you need to know for the application you are targeting. – Andy aka Feb 11 '22 at 17:05
  • Please, don't write so mysteries. I'm here to learn – R098 Feb 11 '22 at 17:05
  • Okay, could you give an example or a paper? :) Because this is the answer I'm searching . – R098 Feb 11 '22 at 17:06
  • @Andyaka differential S-Parameters would give you a complete characterization of the common mode as well of diff to common mode / common mode to diff Conversion... please correct me if I am wrong. – schnedan Feb 11 '22 at 17:10
  • @R098 you need to look at what I said on your other question and, take action or explain why you can't take action then, you need to make clear in this question what it is that you are looking to find out. I can invent an application and leave an answer but, if it doesn't correspond with your question then I will rightly receive attention about my answer being inappropriate. I may also get downvoted. – Andy aka Feb 11 '22 at 17:10
  • @schnedan - what will you choose for your reference impedance? – Andy aka Feb 11 '22 at 17:11
  • @Andy aka Okay, I will repeat my question more clearly. I encountered that on some Common Mode Chokes there are S Parameters, and there are some where with non. However almost every Common Mode Chokes has a given Inductance at a specific frequency (for example 1kHz) and a Impedance Plot in the Datasheet. Now I just want to know what I can calculate with this. There are so many, but when I don't know what I can do with the Impedance and the one Inductance than it's useless for me.. than I'm just try and error.. . Even in a 50 Ohm Network, the Impedance seems for me useless if I don't know the – R098 Feb 11 '22 at 17:19
  • Real and Imaginary Part.. – R098 Feb 11 '22 at 17:19
  • I searched online what I can do with it, but there's only explanation Videos and Papers that explain Common Modes in general or with S-Parameters.. – R098 Feb 11 '22 at 17:20
  • @Andyaka OK, problem for a general purpose part, but e.g. for a Ethernet CM Choke, the reference is well defined, isn't it? – schnedan Feb 11 '22 at 17:20
  • @schnedan I'm not going to play a guessing game with you and the OP. The devil is always in the detail and I'm not going to make general sweeping statements about CM chokes. – Andy aka Feb 11 '22 at 17:22
  • I understand the general usage of Common Mode Chokes and also the electrical equivalent circuit. Also the S-Parameter Measurement. – R098 Feb 11 '22 at 17:23
  • @Andy aka than make a example where the Impedance and the one Inductance value would be enough to choose one Common Mode Choke over another. I would appreciate it – R098 Feb 11 '22 at 17:24
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    @schedan that's matches up with my searching, with Ethernet Common Mode Chokes there are mentioned the S-Parameters. But for general purpose ones only sometimes. Some of them could also be used on Power Lines, but there we have my question again? What does the Impedance give me? – R098 Feb 11 '22 at 17:30
  • You don't appear to be listening. I shall state these things again (more simply) 1) Referring to another question of yours you need to take note of what I said 2) you need to amend this question to state exactly what your question is and what you'd like to know and 3) as you've brought up the subject of S parameters you will need to link to a data sheet of a CM choke where you think knowing an S parameter would be useful. Also ,if you `understand the general usage of Common Mode Chokes and also the electrical equivalent circuit` then please do give details. I'm not playing a guessing game. – Andy aka Feb 11 '22 at 17:30
  • I don't want to know an answer on a specific project, I want to know for what I can use the Impedance values in the Datasheet. Forget the Project . Almost all of TDKs Common Mode Chokes have S-Parameters. However there also in use for a specific function, like for Ethernet. But not all have all of them, but everyone of them has the Impedance curve – R098 Feb 11 '22 at 17:32
  • Yeah, there's a petter, power line Chokes have only the Impedance, Signal Line the S-Parameters. Makes sense, but I still don't know how I can work with the Impedance with the Power line Chokes. I just need an example.. – R098 Feb 11 '22 at 17:38
  • https://product.tdk.com/en/search/emc/emc/cmf_cmc/info?part_no=ACT1210L-201-2P-TL00 – R098 Feb 11 '22 at 17:41
  • Keep it simple man, you don't have to explain it very detailed or complex. I juuuust want to know why ever Manufacturer has the Impedance written in their Datasheet.. normally most of them are lazy. They just include the bare minimum of measurement. But why are every Manufacturer seems to care about this. What does it bring me in sense of choosing a Common Mode Choke over another? (Which parameter does the Impedance react with in my Circuit) – R098 Feb 11 '22 at 17:46
  • Why would someone downvote you when I'm just asking for a explanation about the Values (in your case, with an example) and how to calculate something with this. This is the answer what I'm searching. I want to learn about how to choose a common mode chokes. But with thousands of CM Chokes the search is difficult when I'm not understand what this Curve in the Datasheet is for. You can make an example, that's absolutely fine and welcome. – R098 Feb 11 '22 at 17:54
  • @R098 "What does the Impedance give me?" first spara measurements should be carried out with the correct impedance Z0 of a part/system (to my knowledge) to be correct. Once you have the data, you can calculate how the parameters would look like when measured with a different impedance. impedance is always important - even with powerlines. a mismatch in impedance will cause transients to be reflected or worse emitted as EMI and energy is not transported to the point it should. – schnedan Feb 11 '22 at 18:08
  • It's a great question with too many critics but since s parameters are normally used between 50 ohm ports, there is no reason NOT to create s parameters. some day Murata might... for RF suitable parts .. Right eh @Andyaka ? – Tony Stewart EE75 Feb 13 '22 at 18:31
  • R098 Ignore the downvoters - they have a mind of their own. Some explain their reasons but if they don't, and the question has some merit then not worrying about them is your best choice. – Russell McMahon Feb 14 '22 at 12:45
  • Suggestion - people are welcome to critique it: CM chokes are usually a fairly blunt instrument unless you know the characteristic of the source and load reasonably well. If you do then design is possible. If you only have approximations then an impedance at specified frequency is probably good enough as precise design is not possible. – Russell McMahon Feb 14 '22 at 12:46

1 Answers1

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most common-mode chokes only give an impedance curve, inductance at a specific frequency and current rating.... why not s parameters?

TL:DR ;)

Some day you may find s parameters for RF CM chokes. It's a good idea for a limited application with 50 Ohm ports.

But in general, that is not the case as it is the impedance ratios that matter for Z{CM/DM} with some frequency dependant load or the degradation of an INA CMRR (instr. Amp) due to slight wire CM impedance mismatches.

The different purposes of a Balun or CM choke affect the choice of CM choke and its design for noise reduction on Grid, SMPS harmonics, RF interference on Ethernet, VGA noise emissions and converting unbalanced 50,75 lines to more balanced lines. But the primary reason is to raise CM impedance for some load relative to the Impedance of each line. Ideally, a CM choke should not add any differential impedance if perfect coupled lines. (M=1 & DCR=0)

When this happens with CM interference how much is injected? what is the attenuation? Attenuation and reflections are shown with s parameters but may not be relevant for the Choke until the reactive or resistance load is added. You could create s-parameters for a 50 Ohm load, but some chokes are not used for that impedance such as power supplies.

schematic

simulate this circuit – Schematic created using CircuitLab

The more significant parameter not mentioned in datasheets is the degree of mismatch in the CM impedance. It is this overall mismatch that ultimately determines the CMRR or ratio of attenuation of CM/DM into some load with some external CM interference and DM signal.

Low inductance bifilar wound use twisted-pair magnet wire. For DIY one can use a single magnet wire then folded in half and twisted so the inductance in each leg is almost identical within 0.5% then wound around some core with the midpoint split open to form the other port.

High inductance CM chokes will wind the same number of turns on opposite sides but may have a slightly worse matching due to differences in the high permeability MgZn on either side.

reading material

https://www.semanticscholar.org/paper/Insertion-loss-analysis-of-low-voltage-power-line-Kurniawan-Basuki/b8acc43b921d7f78c5d9410e8a96e7af7182a6ba

Magnetics tend to be custom, so there may be a limited selection of stock items, yet there are millions of combinations of material properties and design to meet specs when feasible. In some situations, I have improvised with 1:1 signal transformers with high inductance.

How to do Scattering parameters with CM choke for 50 Ohms.

https://www.researchgate.net/profile/Jorma-Kyyra/publication/224600584_Common-mode_choke_coils_characterization/links/0c96052bddccc487be000000/Common-mode-choke-coils-characterization.pdf

enter image description here

"it is worth nothing that according to (1), when k = 1, i.e. assuming M = L, then Lcm = 0 and Ldm = 4L. Therefore, a coupled DM inductor is very different from two decoupled DM inductors, which would result in Lcm = L/2 and Ldm = 2L."

Tony Stewart EE75
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