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Binary transitions, power consumption

  1. Jan 28, 2010 #1
    I'm studying the ethernet devices and the transmission system over cable wire and i'm interested especially on the power consumpiot due to the binary transitions from 0 to 1 and viceversa in the transistors. It is possible to estimate how many WATTS (i thinks nW) are necessary to the device to switch from 0 to 1 or viceversa?
    it is due to the charge and discharge of the cable parasitic capacitance?
    thank you in advance
  2. jcsd
  3. Jan 28, 2010 #2


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    Staff: Mentor

    You are not driving the parasitic capacitance so much. You are driving into the characteristic impedance of the cable at high frequencies (Zo):


  4. Jan 28, 2010 #3
    Most coax cables range from 50 ohms (e.g., RG-8) to 125 ohms (e.g., RG-63) characteristic impedance Zc. Zc is the square root of the coax inductance per unit length divided by the capacitance per unit length. The parasitic capacitance cannot absorb energy (other than dielectric "tan theta" losses at GHz frequencies). The best cables for long distance signal transmission are Zc~75 ohms with high signal velocity. The cable should be terminated in impedance Zc to minimize reflections. The power losses are in the termination.

    Bob S.
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