5uH LISN for Spectrum Analyzer EMC/EMI work

[georges80]

I've used the LISN for several designs over the past 3 or so months and am happy to report that it truly works well and provides an excellent correlation to the real world.

I've taken a few designs that would wipe out the FM band and with some changes to the design (input power filtering to the led drivers) and some power plane layout changes those drivers are now totally quiet.

On one driver (2 layer board in that case) I could cut pieces of copper from the ground plane under the filter area and the improvements seen on the DSA reflected as improvements in lower FM interference.

So, in summary, the tekbox LISN design DOES work really well and for the cost of the DIY version it's an excellent addition to owning a DSA for quantifying conducted emissions (for *DC* powered circuits) and then observing improvements (or not...) as you implement changes.

cheers,
george.

[HighVoltage]

Hello George
This is very good information you are leaving here, thank you.
Can you share the details of your different designs and how you use the LISN correctly.
May be some pictures of the setup?
Thanks

[georges80]

I've attached a few pictures showing the LISN and DSA in use testing a buck/boost LED driver. The driver is mounted to the back of an MCPCB with 6 XML running 3A a piece, so about 60W to the LEDs.

The picture below shows the noise levels with power turned off. The small peaks are FM stations being received via the DUT power cable. The rectangular aluminium plate is to prevent the LEDs from burning spots in my antistatic mat



The next picture shows the driver powered up and the conducted noise on the power leads to the driver. This noise level is a lot lower than with the original driver that did not have a common mode choke at the input (the black rectangle to middle/right on the PCB).



The next picture has the same setup, but a 0.1uF capacitor is making contact between the IN- of the LED driver board and the aluminium of the MCPCB. Noise levels are much reduced and you can see some of the FM radio station peaks showing through. Noise is now essentially below the FM received strength using the power leads as an antenna

At this level practical tests show the driver (in a housing with the LEDs and longer power leads) will not interfere with FM radio reception even if the power lead is wound around the vehicle antenna - good enough!

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So, this is an example of reducing emissions and being able to observe changes in real time by making changes to filtering and ground paths etc. It's not testing for compliance, but testing for a design that is proven to not interfere in the real world on an actual vehicle.


I am currently working to bring another design through FCC qualification at a local EMC lab. Our first run through had issues and based on what we did in the lab to identify the actual sources we discovered that one of the HDMI cables we used had huge leakage at the connector end (likely not shielded in the connector end where the cables are then soldered) and also that the AC/DC adapter we were using had nasty broadband noise in the 30MHz - 70MHz range.

I then brought my DSA to work along with my LISN and also a beehive probe. The LISN showed the same broadband noise shape with the AC/DC adapter plugged into the DUT end of the LISN. I could then apply a load at the Power supply end of the LISN to see how the noise of the adapter increased. Then we tried various other adapters and found one that was actually FCC approved having essentially no noise emissions.  This proved that the LISN does a great job of identifying conducted noise that will then radiate (as measured by the antenna/emi receiver setup at the EMC lab). I then used the beehive (nearfield probe) and probed various HDMI cables and confirmed all were quiet at the cable and connectors, EXCEPT for the 'bad' HDMI cable.

It has been a learning experience, but with several real world check points between the LISN/DSA and the Beehive, I am becoming confident in what my setup shows and how the unit will behave with nearby radios etc. The test lab results add a lot of credibility and correlation between my setup and their measurements. Certainly these tools allow for pre-compliance testing to at least identify things that will surely cause you to fail.

cheers,
george.

[wemme]

Hello,
I am looking at making a LISN for preliminary scanning MIL461/DO160E.
These are based around 50uH inductors and not the 5uH as used for automotive.

Does any one have any construction notes about building these?

Regards
Bart

[wemme]

Hey Jay_Diddy_B

Thanks for sharing your design,
Any reason you used such a large discrete solution for the attenuator and filter?
filter aside Wouldn't a simple pi or T attenuator be adequate for a 10dB with 50ohm in/out impedance.
Regards
Bart

Hi,
I am going to share my implementation of a 5uH LISN. I used carefully selected SMD inductors instead of the traditional method of winding air-cored inductors. The inductors are Wurth 744314110.

I also incorporated a combination 10dB attenuator, limiter and 9kHz High Pass filter into the design. The attenuator / filter is to protect the spectrum analyzer from damage.

The style of construction was chosen to make the LISN performance very predictable.


Schematic
This is the schematic diagram:



Attenuator / Limiter / Filter Design

LTspice model:



Modeling Results:



Board Design



Construction
The board is designed to replace the lid of a Hammond 1590 diecast box.



Testing
This graph from a HP 3753C Network Analyzer shows the attenuation from Load Connection to the Spectrum Analyzer connection. It is flat within 0.5dB over the range 2MHz to 100MHz.



This Smith chart shows the input impedance from the load connection. After 2.6MHz the input impedance is pretty close to 50 Ohms. This matches the requirements for a CISPR 25 LISN.



The performance of this LISN has been compared to a commercial LISN and found to agree very closely.


Jay_Diddy_B

[Jay_Diddy_B]

Hey Jay_Diddy_B

Thanks for sharing your design,
Any reason you used such a large discrete solution for the attenuator and filter?
filter aside Wouldn't a simple pi or T attenuator be adequate for a 10dB with 50ohm in/out impedance.
Regards
Bart

Bart,

I did not invent the 10dB filter / High Pass filter. It is a copy of the HP 11947A. Details of the HP 11947A can be found here:

http://www.keysight.com/en/pd-1000001160%3Aepsg%3Apro-pn-11947A/transient-limiter-with-high-pass-filter?cc=US&lc=eng

The 10 dB is distributed over 2, 3 and 5 dB attenuators to spread the power out in an overload situation.

You can use a standard 10 dB attenuator on the front of the spectrum analyzer, and many people do.

I like the idea that the mixer and attenuator in my spectrum analyzer are protected. It is much easier to rebuild the protection circuit than repair the spectrum analyzer front end.

I have also built just the attenuator and limiter in box, without the LISN.

I first used this circuit in a line voltage LISN. The construction is similar to the low voltage LISN. I had two copies of the entire circuit, one for line and the other for neutral. The key components were:

PHE850ER7100MR03R06L2 for the capacitors on the line side, these are 1uF 300VAC Y2 class

PHE850ED6100MD18R06L2 for the 0.1uF capacitors from the load side to the attenuator. Chosen for safety.

Wurth 7443331000 10uH 9A Inductors, times 5 to get 50 uH. Chosen for high self-resonant frequency and shielding.

Construction was similar to two copies of my low voltage LISN, inductors in the center, attenuators on the outside, about 110mm x 110mm.

I hope that this helps.

Regards,

Jay_Diddy_B

[wemme]

Wurth 7443331000 10uH 9A Inductors, times 5 to get 50 uH. Chosen for high self-resonant frequency and shielding.
Jay_Diddy_B

Hi Thanks for all the information that explains a lot.

Did you have any issue with the cored inductors rather than air?
was your line lisn 110 or 230V?
Regards
Bart

[Jay_Diddy_B]

Hi Bart and the group,

Did you have any issue with the cored inductors rather than air?
was your line lisn 110 or 230V?
Regards
Bart

The inductance of the inductors is not super critical. Ideally the LISN should look like 50 Ohms to the input. The impedance of the 50 uH is 314j Ohms at 1 MHz.

Here are some pictures of my LISN








I did a measurement of the input impedance using an HP 3577A Network analyzer with a s-parameter test set. One of the challenges connecting the power connector to a coax cable. (Where is my precision N to power connector adapter?)



The return loss is 23 dB at 30 MHz



This is a picture of insertion loss. Ideally it would -10db and flat. It is hard to normalize this measurement because the power connectors are not very good at 50 MHz 



Since this is used for pre-compliance testing, you compare the results against the results from the lab.

Regards,

Jay_Diddy_B

[mk_]

Maybe this pdfs are helpful....


michael

[wemme]

Thanks for sharing all the information this is very helpful.
Has anyone built or using a common mode/differential splitter with their lisns?
Regards
Bart

[Jay_Diddy_B]

Bart and the group,
 
It is relatively easy to build a resistive splitter/combiner to distinguish between common mode and differential mode noise. Here are some schematics using WYE (3 x 16.7 Ohm resistors) or Delta (3 x 50 Ohm resistor). Keep you cable lengths between the LISN and the splitter the same to maintain the phase relationship.



If the noise is differential it will be reduced, ideally it will cancel. If it is differential it will increase.

Fairly easy to construct, a box, three connectors and three resistors. I don't find them too useful, because I generally know when to expect differential or common mode EMC.

Jay_Diddy_B

[ivonenand]

Schematic


Jay_Diddy_B

Hi,
I'm just drawing up my own version of this LISN and I have a question regarding the component selection. Are there special L1 (1mH inductor in the filter) requirements? The best ones I've found have the SRF of about 2.5MHz, which is allot lower than the 100MHz line where the LISN is still designed to work. Or am I missing something?

Much appreciate any info,
Ivo

[T3sl4co1l]

As long as the impedance of that inductor is well above 50 ohms at frequencies up to 100MHz, it's okay.

Wirewound inductors tend to have many peaks and valleys above their first SRF, but it's probably not too bad.  To a certain extent, you can put other RFCs in series to cancel those out, but you have to be careful not to make a worse circuit instead (i.e., the parasitic capacitance of the large inductor goes series resonant (low Z) with the next inductor).

Tim

[Jay_Diddy_B]

Hi,
I'm just drawing up my own version of this LISN and I have a question regarding the component selection. Are there special L1 (1mH inductor in the filter) requirements? The best ones I've found have the SRF of about 2.5MHz, which is allot lower than the 100MHz line where the LISN is still designed to work. Or am I missing something?

Much appreciate any info,
Ivo


Hi,

Before I built the LISN, I built just the attenuator / High Pass filter portion of the circuit. I too was concerned about the 1mH inductor. Here are some pictures:











The frequency response was measured with an HP3577A Network Analyzer. The measured results show no significant deviations from the results predicted by modelling.

I have tried several inductors, it does not seem to be critical.

Regards,

Jay_Diddy_B

[NikWing]

Jay_Diddy_B, the maximum DC input voltage is limited by the 2 caps close to Vin, right?
The tekbox LISN can handle 200V IIRC. I might need at least 50V (more wouldn't be that bad though, just in case), so all I have to do is to use caps that can handle this voltage?
C2 seems to be an elcap and C1/C3/C4 are MLC?

Next question is, 4148 are fast enough? I've seem other LISN using ESD protection diodes or TVS diodes.
Since you're already using it, I guess it's no problem, right?
And, all 4 should be mounted?

[Jay_Diddy_B]

Jay_Diddy_B, the maximum DC input voltage is limited by the 2 caps close to Vin, right?
The tekbox LISN can handle 200V IIRC. I might need at least 50V (more wouldn't be that bad though, just in case), so all I have to do is to use caps that can handle this voltage?
C2 seems to be an elcap and C1/C3/C4 are MLC?

Next question is, 4148 are fast enough? I've seem other LISN using ESD protection diodes or TVS diodes.
Since you're already using it, I guess it's no problem, right?
And, all 4 should be mounted?

Hi,

just to make sure we are talking about the same thing, here is the schematic that I am going to talk about:



and this construction:




This type of LISN is normally used for low voltages 48V and less. It is not a line voltage LISN.

I used MLCC for all the capacitors. The blue component is the inductor L1. I used a 250V capacitor for C3 and 100V capacitors for C4 and C5. C1 and C2 can be low voltage components.

The diodes are small signal diodes, you need low capacitance.

The limiter is styled after the HP11947A. The documents can be found here:

http://www.keysight.com/en/pd-1000001160%3Aepsg%3Apro-pn-11947A/transient-limiter-with-high-pass-filter?pm=PL&nid=-32400.536881615&cc=CA&lc=eng

HP used 1901-0050 diodes, which are 1n4750, they have 2.5pF of capacitance.

I did not invent the circuit, I changed (slightly) the attenuators to use E24 series resistors.

It works great.

Regards,

Jay_Diddy_B

[NikWing]

yeah, we're talking about this one, well nearly XD
I found this thread while doing some SA researches, then I googled "5µH LISN" and found this github page:
https://github.com/LibreSolar/5uH-DC-LISN
and it links back to this thread, so I somehow thought it's an improved version of the stuff you posted earlier (because the files are 7 months old and the 1st similar looking circuit was from 2014)

so finally I'm mixing up people, sorry ... a bit overworked at the moment XD

Okay, so the point is low capacitance for the diodes.

But beside that, if I would use higher rated capacitors on the input and decoupling path (and maybe also replace the elcap with MLCC), there wouldn't be any drawbacks if I understand it correctly?
since the DC part of the supply voltage is decoupled and we're just looking at the alternating voltage part.
Or do I overlook something?

[Bud]

HP used 1901-0050 diodes, which are 1n4750, they have 2.5pF of capacitance.

Are you sure, 1n4750 seems to be a 27v zenner

[Jay_Diddy_B]

HP used 1901-0050 diodes, which are 1n4750, they have 2.5pF of capacitance.

Are you sure, 1n4750 seems to be a 27v zenner

Sorry, small mistake.

HP 1901-0050 = 1n4150

link to datasheet: http://www.vishay.com/docs/85522/1n4150.pdf

Regards,

Jay_Diddy_B

[PDonchev]

Hi,
Great thread! I already made two 5uH LISNs according Jay_Diddy_B design and they helped me a lot in my first EMC troubleshooting steps.
I have one question to Jay_Diddy_B:
What are these elements on the picture (in the red circles)?

[Jay_Diddy_B]

Hi,

In each red circle there are two 475K resistors and a piece of track. They are to discharge the 1uF capacitors.




Regards,

Jay_Diddy_B

[dcarr]

Jay_Diddy_B,

Is there a place where I can download a copy of your LISN gerber files?  Your design looks great and I'd like to build one for myself.

It seems silly to take the time to re-CAD the whole thing if I can use your proven design files.

Thanks,
David

[PDonchev]

Hi dcarr,
NikWing posted a link to github with project of 5uH LISN almost identical to Jay_Diddy_B's LISN.
Actually, I used that project to build my LISNs.

[PDonchev]

Hi,
Thank you for your reply.
I have one more question, did you mount the 330 ohm resistors?

Hi,

In each red circle there are two 475K resistors and a piece of track. They are to discharge the 1uF capacitors.




Regards,

Jay_Diddy_B

[NikWing]

just a quick update:
I got the PCBs now and most of the parts
after I've built the 2 LISN boxes, I'll report back