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Why a DC (direct current) component is undesirable?

  1. Apr 24, 2017 #1
    When choosing Line Codes (e.g. Bipolar or Polar formats) for transmission of bits over a communication channel such as twisted pairs, it is always desirable that no DC component is present for a particular choice.
    According to William Stallings (Digital Communication Book)
    "With a dc component to the signal, there must be direct physical attachment of transmission components.
    With no dc component, ac coupling via transformer is possible; this provides excellent electrical isolation, reducing interference"

    I cannot understand what does he mean in above paragraph. I donot know much about AC/DC circuits and their workings
     
  2. jcsd
  3. Apr 24, 2017 #2

    berkeman

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    Staff: Mentor

    A DC component is undesirable for transformer-coupled communications, because it can cause saturation issues in the transformer cores. You would rather use all of the B-H curve of the transformer for coupling the AC communication information, and not have an offset B-H curve start to limit the AC components.

    You want to use as small a transformer as you can for cost reasons, but as you make it smaller, the AC component of the communication signal starts to get close to the tips of the B-H curve. Adding a DC component to the waveform just pushes you that much farther toward one of the tips of the B-H curve, which is bad. In other words, to accommodate a DC component, you have to make the transformer physically bigger than you would like, which makes it more expensive.

    Does that help?
     
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