Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Some questions regarding EMC

  1. Mar 13, 2007 #1
    I got some questions regarding Electromagnetic Compatibility:

    1. When we measure transfer impedance for cable shielding , why we have assume that lamda >> L ?

    2. What is the difference between trasnfer impedance & admittance?

    My friend told me that the impedance in purely dependent conductor size , is it correct?

    3. How to convert dbμv to normal v?

  2. jcsd
  3. Mar 14, 2007 #2


    User Avatar

    Staff: Mentor

    For #3, 1dBuV = 20 log (Vpp/1uV)

    So if you have a value for the amplitude of a waveform in Vpp, you calculate the value in dBuV using the equation above.

    As for your transfer impedance questions, I'm going to have to do some reading before I can attempt an answer.....
  4. Mar 14, 2007 #3
    thanks for the reply , I really need to know why lamda >> L .. pls help
  5. Mar 14, 2007 #4


    User Avatar

    Staff: Mentor

    I've been kicking around some PMs (personal messages) with other members, but so far none of us are very familiar with the term transfer impedance, and especially the questions you ask about it.

    The usual reason for a requirement like [tex]\lambda >> L[/tex] would be to avoid the case of standing waves on whatever the cable is that you are measuring. But again, since I know nothing about transfer impedance, I cannot say that's the reason.

    Could you please share with us what you know about transfer impedance so far? What is the context -- audio, Ethernet, some field bus, etc? Can you point us to some online sources of info about the subject? Wikipedia.org was no help for me, and my semi-brief google search wasn't much help either. What class is this from?
  6. Mar 15, 2007 #5
    The main subject of it is about cable shielding in electromagnetic compatibility. I've googling myself but nothing much info about it.

    I got some answers from german , after translate is something like this ( dotn know correct or not ):

    1. reflection produces ripples
    2. to against strong reflection in the end that doenst fit
  7. Mar 15, 2007 #6
    I think you are referring to a "characteristic impedance."

    50 ohms or 75 ohm for coax.

    Admittance is just the reciprocal of impedance, Y = 1/Z

    If you mismatch the source and load impedance with the characteristic impedance, that's when reflections start to occur that degrade your signal.
  8. Mar 15, 2007 #7


    User Avatar
    Science Advisor
    Gold Member

    Transfer impedance Z_t is used to characterize the coupling of external fields to a shielded cable. The voltage induced in the inner conductor by currents flowing in the shield, in a short length dz, is given by
    dV = Z_t * I * dz

    You can find the cuurent induced in a load impedance by integrating over the cable length. If lambda>>L, then the integral becomes simple because I and V are constant throughout L. This is often not a valid assumption, however. Things get really messy if lamdba the same or smaller than L, since wave effects (standing waves, as mentioned earlier), resonances, and electromagnetic scattering then become important.

    Admittance is the inverse of impedance. Most people are familiar with the characteristic impedance of a transmission line, as waht noted. I suppose you could work with transfer admittance in this case if you like.

    I suggest you get a hold of one of the better books on EMC or cable shielding for more info.
  9. Mar 19, 2007 #8
    1. As already quoted if Lambda is large with respect to the cable length the cable acts as an Antenna so you need to look at it differently than just a cable with coupling.

    2. The use of Transfer Impedance is generally used when doing EMC current injection tests. It is effectively treating the coupling from the injection probe as an impedance which makes calculations simpler especially when using lots of calibrated items in the test set-up as you can just add and subtract the impedances in the frequancy domain to determine the coupling applicable to the IUT and to pre-calibrate your test to ensure you get the right test levels into the IUT.

    3. Refer to Berkmans answer
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?