Muriel,
Coupling/Decoupling networks (of which LISN's are a subgroup) can affect DUT
performance. I myself have not experienced this with a conducted emissions
LISN, but I have experienced it with an EFT generator.
We made a product that had an internal thermal printer. We EFT test in
house. Part of my EFT testing included verifying the operation of this
printer during the EFT threat. During testing, it seemed as though the
printer was failing. I would start the test, try to access the printer and
be denied. However, the second time I accessed the printer (and any time
thereafter) I would be allowed access and the unit would print fine. I
thought it was an EFT failure.
However, if I left my product connected to the EFT generator with the EFT
turned off, its printer would show the same problem. Why?
The EFT generator has a coupling/decoupling network with some huge in-line
inductors and some capacitors to ground. I did some probing around with an
oscilloscope and found that the first time I accessed the printer, my
product would draw a large, instantaneous current which charged up the
capacitors in the printer's power supply. The second time I accessed the
printer, the caps were already charged and this instantaeous current wasn't
needed.
This initial current draw was being limited by the in-line inductors in the
EFT generator. As such, the power supply voltages in my DUT were sagging
while they were waiting for the required current. During this voltage sag,
my printer logic was getting locked up.
So, any coupling/decoupling network with in-line inductors (which include
LISN's) can and will limit instantaneous current changes to the products
connected to them. If your product depends upon these currents to maintain
regulation of its power supplies, then the product may experience
difficulties.
Happy Holidays!
Chris
> -----Original Message-----
> From: Muriel Bittencourt de Liz [SMTP:mur...@grucad.ufsc.br]
> Sent: Friday, December 22, 2000 11:05 AM
> To: Lista de EMC da IEEE
> Subject: Doubt with conducted emissions measurement
>
>
> Hello Group!
>
> First of all, I wish a merry christmas and a happy new year for the list
> members.
>
> Second, I'd like to solve a doubt. It concerns the methodology of
> conducted emissions tests.
>
> Let's suppose a power electronic equipment (static converter) that has a
> boost converter in the entrance that's used for power factor correction
> (PFC). When I make a conducted emissions test, I plug the converter in a
> LISN, then I plug the LISN in the outlet, this way (the classical way):
>
> Equipment => LISN => Outlet (Mains)
>
> Well, the LISN consists of a RLC network that has the purposes of:
> - prevent that external interference from the mains contaminate the
> measurement
> - create a stabilized impedance (50 ohms) in the frequency range of
> interest (150kHz-30MHz) to make results repeatable, from site to site.
>
> Ok, what I've said until here is well known and is present in every book
> about this subject of EMC.
>
> My doubt is this: The LISN can't interfere in the functional operation
> of the converter? Things like resonance, extra ripple can't occur? And,
> imagining the worst scenario, can the LISN make my equipment not work
> according to what's expected?
>
> Another thing I was thinking about is the fact that the military tests
> os conducted emissions are done with a probe, and not using a LISN.
>
> I'm very curious about this subject because I was asked about this
> question and I became very surprised, because I have never thought about
> this. We are, generally, so interested in results that we forget to ask
> the "basic questions" sometimes.
>
> Well, I think that's all. Thanks in advance for those who can help me.
>
> Best Regards
>
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> Eng. Muriel Bittencourt de Liz
> EMC Testing and Troubleshooting
> Group of Conception and Analysis of Electromagnetic Devices
> Federal University at Santa Catarina
> Florianópolis, SC, Brazil
>
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