Need help characterizing a DC emi filter - Page 1
Forum section: electronics / projects

author: laydn
author_rank: Contributor
timestamp: May 04, 2021, 12:23:11 pm
content: Hello,I'd like to characterize, as in, plot the insertion loss of a dc emi filter in differential mode. I have access to a scope, a Rigol DSA800 series spectrum analyzer with a tracking generator, a Beehive amplifier, a standalone function generator with sweep functionality (also a Rigol)I have simply connected the tracking output of the Rigol DS800 to the emi filter input, and connected the emi filter output back to the input of the spectrum analyzer. However, the response I see is nowhere near the curves in the datasheet.The filter I want to characterize looks like the following part:https://www.murata.com/en-sg/products/productdetail?partno=BNX012-01Thanks,

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 04, 2021, 01:07:30 pm
content: laydn,First of all welcome to the forum!!Murata measured the filter in a 50environment. When you do this the response is dominated by the capacitor.Here is a quick LTspice model and the results:Compare this against the Murata datasheet:[ Specified attachment is not available ]It shows the filter response is dominated by the 50impedance and the capacitor.If you use this filter with a dc/dc power supply, you will not see this attenuation, because the EMI source is not 50Can you share your results?Regards,Jay_Diddy_B

author: laydn
author_rank: Contributor
timestamp: May 04, 2021, 01:42:18 pm
content: Hello,I am using the tracking generator output of Rigol DSA815, which is specified as 50ohms and and the input of the Rigol DSA815, which is also specified as 50ohms. So I thought I am safe in terms of impedances.I've attached the schematics of the filter and also the insertion loss test circuit from the datasheet.With the exception of the Balun, I think what we are doing is not so far off from this test circuit, but I am probably missing something important...

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 04, 2021, 02:07:16 pm
content: Laydn,What do your results look like?Did you normalize the tracking generator?Are you doing a log frequency sweep?Jay_Diddy_B

author: Terry Bites
author_rank: Super Contributor
timestamp: May 04, 2021, 06:23:35 pm
content: The real measured filter has multiple internal parasitic capacitances and real resitance that are not modeled. The capacitances short the inducutors at higher frequencies lowering the attenuation.Who has controlled 50 ohm power sources and loads in the real world? The device has good comon mode rejection which may matter at least as much as its LPF function. CM filters close magnetic loops and stop junk moving down the line (allegedly). A lone cap just wont cut it.

author: laydn
author_rank: Contributor
timestamp: May 05, 2021, 06:59:19 am
content: Hello,Attached please find our measurement for the filter, with three different spans. Just FYI, this is for a similar filter, BNX002. I've also attached the response characteristics of this filter from the datasheet. To answer your question about the TG, yes, we do the normalization.Thanks for your help.

author: DL2XY
author_rank: Regular Contributor
timestamp: May 05, 2021, 07:32:06 am
content: Quote from: laydn on May 04, 2021, 01:42:18 pmYes, you are shorting (3) and (4) and therefore L1 (which is on the same core as L2) with the Rigol ground path.You must use a balun.

author: laydn
author_rank: Contributor
timestamp: May 06, 2021, 08:40:09 am
content: Thank you @Jay_Diddy_B and @DL2XYI was able to quickly get my hands on the following balun:https://www.mouser.com.tr/ProductDetail/Pulse-Electronics/CX2039LNL?qs=nLJruSqDb95cUa3p4vxxyQ==Although it is characterized starting from 4.5Mhz, I thought I should at least be able to get good results above 4.5Mhz. First question: Do you think this balun is suitable?We connected the balun to the input side as in the schematics in the above post. However, the results are not very different. I've attached the shots below with different spans. At the very least, I think we should see the the highest loss at 30-40Mhz, but we're seeing nothing close to that. Any other ideas?

author: laydn
author_rank: Contributor
timestamp: May 25, 2021, 11:55:08 am
content: Hello,We've retried the experiment with the following balun: ADTT1-6+ (https://www.mouser.com.tr/ProductDetail/Mini-Circuits/ADTT1-6%2b?qs=%2Fha2pyFadujqcwEGQ5cU2a7D%252Bd%252B1HPSbuaPKbhKaKFc%3D)Attached is the trace from the spectrum analyzer, and a picture showing our connections.As can be seen, we are nowhere near the published insertion loss graph. Any ideas?

author: laydn
author_rank: Contributor
timestamp: May 25, 2021, 12:06:09 pm
content: Also, here's a shot with RBW set as low as 3khz.This graphs looks slightly better but the highest loss is at 4Mhz, whereas the datasheet says it is around 30Mhz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 25, 2021, 12:29:33 pm
content: Where's the connection between grounds?Tim

author: laydn
author_rank: Contributor
timestamp: May 25, 2021, 01:25:23 pm
content: Well the TG connector and the input connector of the Rigol is shorted internally. But we'll make a local connection right around the filter, remeasure, and get back to you. Thanks!

author: laydn
author_rank: Contributor
timestamp: May 26, 2021, 06:33:22 am
content: OK so we shorted the grounds. Attached you can find the picture of the new setup. And also the scope trace, which didn't change much.Any ideas?Thanks

author: laydn
author_rank: Contributor
timestamp: May 26, 2021, 11:21:39 am
content: We also got our hands on a proper network analyzer from Rohde Schwarz. We did the same measurement with it. Here is the shot: