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Does low current and poor termination affect a transmission line?

Discussion in 'General Electronics Discussion' started by pgiles, Oct 14, 2014.

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  1. pgiles


    Jul 16, 2014
    I have a transducer that outputs a 2nS wide pulse, I send this down a 50R transmission line, and terminate it at a oscilloscope in 50R.

    When I connect the 50R transmission line to a BNC with a Short stub of coax at 50R I get some ringing

    When I connect the 50R tranmsission line to a SMBA PCB connector and connect via a short coax to the osciloscope teminated in 50R the ringing gets worse

    When I connect a longer coax lead to the SMA then to the osciloscope the ringing reduces and shifts to the right

    Mt question is does low current transmitted at high frequency down a transmision line require that any point in that transmission line has to be properly terminated in a 50R connector before passing on to another length of coax,, otherwise the signal will be degraded and suffer from ringing.
  2. Laplace


    Apr 4, 2010
    Do you see ringing or reflections? When connecting two lengths of coaxial cable for high frequency transmission one should use connectors with geometry designed to maintain the basic transmission impedance of the cable. Any discontinuities will result in signal reflections proportional to the magnitude of the discontinuity. Time Domain Reflectometers (TDR) use this effect to search a cable run for imperfections in the signal path. From the description I'm not certain where you are terminating the cable. If you terminate a cable before connecting to another cable, the signal pulse will see the discontinuity of the parallel combination of the termination and the cable extension; 50 Ω becomes 50 Ω in parallel with 50 Ω so the signal on the 50 Ω path sees a discontinuity to 25 Ω and a portion of the signal reflects at that point back to the origin, a portion dissipates in the termination resistance, and the remaining signal travels down the new cable path.
    Arouse1973 likes this.
  3. Gryd3


    Jun 25, 2014
    I looked that up the other day to see if I could build a cable length tester...

    Still unsure how (but that's not for this thread)
  4. pgiles


    Jul 16, 2014
    I understand about the discontinuities that will result when I connect two lengths of coaxial cable together without connectors of the designed geometry, especially when the currents are very low, charge times of capacitance have a big effect and resistance mismatch

    To be clear I am only joining the coax together without use of an approved connector, in two of my cases, I am not sure about the impedance match for the SMA PCB connector

    Thanks for your Help by the way
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