Coax Cable Impedances

Discussion in 'General Electronics Chat' started by Sparky49, Jan 6, 2012.

  1. Sparky49

    Thread Starter Well-Known Member

    Jul 16, 2011
    Hi all,

    I 'm currently putting together a simple TDR circuit to use with my oscilloscope. However, the instructions say to simple use a BNC connector. Looking around there seem to several impedances of BNC connector - does it matter which one I choose?

    I say this because the circuit has the abilty to measure different impedances via different resistors and a jumper.

    However, it would be pretty useless if I wanted to measure a 50Ω cable, set the circuit to measure 50Ω but the 75ΩBNC connector messed everything up!

    I would post a schematic, but it's from a magazine, and I'm not sure how the copyright works with that.

    It's page 64 of Elektor December 2011, if you've got it.

    Thanks for your time,

  2. MrChips


    Oct 2, 2009
    BNC connectors are different to match the specific diameter cable and are not interchangeable, for example, a BNC connector for RG-58 cable would not fit an RG-62 cable. The connection in the connector itself is so short you would not be able to detect the electrical difference.
    Sparky49 likes this.
  3. Sparky49

    Thread Starter Well-Known Member

    Jul 16, 2011
    Thanks Mr. Chips!

    May I also ask - is there a difference of pricing of different cables? Is one type cheaper than another?

    The reason I'm asking is that I'm not intending to use this (much) for real 'in the field' testing - more as something to 'quench my thirst for knowledge!!!' Hehehe.:rolleyes:

    Perhaps you could suggest a decent cable type?

    Thanks again,

  4. crutschow


    Mar 14, 2008
    If the connector impedance does not match the cable impedance, then you will get a reflection from the mismatch.

    For your experiment just about any coax cable will work. Just pick a cable and connector with the same impedance.
    Sparky49 likes this.