Author Topic: IEC 61000-4-6 Immunity to conducted disturbances - CDN questions  (Read 2173 times)

0 Members and 1 Guest are viewing this topic.

Offline AndersJ

  • Frequent Contributor
  • **
  • Posts: 253
  • Country: se
IEC 61000-4-6 Immunity to conducted disturbances - CDN questions
« on: February 10, 2016, 09:34:25 pm »
I am having problems with a digital input.
It connects with a 2 conductor cable.
One is signal ground.
The other one is the active high input.

The test is done by injecting 10 Vrms common mode RF with a CDN.
The RF is amplitude modulated, 80% at 1 KHz.
Since the signal input is common mode there should be no
differential voltage between the two conductors.
BUT there IS a differential voltage.

My circuit is unbalanced, thus requiring a CDN-AF2 for unscreened non balanced lines.
I suspect that the testhouse is using the wrong CDN,
a CDN-T2 intended for a BALANCED pair.

My question therefore is:
What output signal will a CDN for balanced lines create in the unbalanced scenario described above?

Many thanks for your comments.
Anders J
« Last Edit: February 10, 2016, 11:06:59 pm by AndersJ »
"It should work"
R.N.Naidoo
 

Online joeqsmith

  • Super Contributor
  • ***
  • Posts: 6588
  • Country: us
Re: IEC 61000-4-6 Immunity to conducted disturbances - CDN questions
« Reply #1 on: February 11, 2016, 01:22:35 am »
You know, reading your post I don't think I have ever sent something to a lab to be tested without being present.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline Helix70

  • Supporter
  • ****
  • Posts: 250
  • Country: au
  • VK4JNA
Re: IEC 61000-4-6 Immunity to conducted disturbances - CDN questions
« Reply #2 on: February 11, 2016, 10:28:04 am »
As the CDN-T2 has two anti-phase transformers in the HF path, this will cause line A to be disturbed in the opposite polarity to line B, referenced to ground. That is for a given pulse, A will be driven positive while B will be driven negative (a balanced signal) with respect to the ground.

By contrast, the CDN-AF2 omits the anti-phase transformers, so the disturbances will be driven in the same polarity with respect to ground, that is, in common mode.

If they are connecting your ground wire to ground, and the other to A (or B), then not a lot of difference will occur between the T2 and the AF2 i wouldn't think.

If they are connecting to A & B without ground, you will see differential voltages due to external capacitive coupling to ground and the series inductance of the cables. Is your circuit externally earthed?
 

Offline AndersJ

  • Frequent Contributor
  • **
  • Posts: 253
  • Country: se
Re: IEC 61000-4-6 Immunity to conducted disturbances - CDN questions
« Reply #3 on: February 12, 2016, 04:29:00 am »
Thanks Helix70 for commenting on my issue.
I'm not sure how to interpret your last 3 sentences.
A rephrasing would be much appreciated.

Your comments confirm my initial suspicions.
As an experiment i "built" my own CDN-T2,
and it DOES create a unbalanced ground referenced signal.

I am pretty sure this explains why the test house managed to
fail my device by creating a differential signal input where it
should have been common mode, which my input would have ignored.





"It should work"
R.N.Naidoo
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 15285
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: IEC 61000-4-6 Immunity to conducted disturbances - CDN questions
« Reply #4 on: February 12, 2016, 10:41:08 am »
It sounds like your input/termination impedances are unequal, therefore a common mode current (present on both conductors simultaneously) will result in a nonzero differential voltage, as seen by your circuit.

Heavy filtering on external inputs goes a long way towards preventing these sorts of problems.

What does your input circuit look like?

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline Helix70

  • Supporter
  • ****
  • Posts: 250
  • Country: au
  • VK4JNA
Re: IEC 61000-4-6 Immunity to conducted disturbances - CDN questions
« Reply #5 on: February 12, 2016, 10:56:37 am »
If the lab connected your signal wire to "A" and your ground wire to "Ground" on the CDN-T2, it will not be very different to the CDN-AF.

If the lab connected your signal wire to "A" and your ground wire to "B", then you will see differential voltage.

Do you have the test report?
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf