Characteristic impedance

Discussion in 'General Electronics Chat' started by bug13, Jul 31, 2014.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    Today, my mind wonder away (as usual), I was thinking for a cable transmitting signal, is it better to use a high characteristic impedance one, or a lower one?

    Just like we want as small resistance as (practically) possible in power cable.

    Thanks guys!!
     
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Use whatever characteristic impedance cable which will effectively match to the impedance of the load with minimum reflected power. Also it is beneficial to use cable with the lowest loss at the frequency of interest. Very different issue than power loss due to current flow in low frequency power distribution systems.
     
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  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    cable tv cables and equipment are usually 75 ohm impedance.
     
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  4. crutschow

    Expert

    Mar 14, 2008
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    A low impedance line doesn't necessarily result in lower losses for RF signals. The choice of impedance depends upon the application. Here's a discussion for coax cable.

    Folded dipole antennas have an impedance of 300Ω and in the early days of television 300Ω twin-lead was often used in fringe are reception for minimum signal loss. Twin-lead is still used today in some RF applications for low loss.
     
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  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    75 ohm coax has lower loss then 52 ohm because the capacitance per foot is lower. 300 ohm twinlead has much less loss than coax. check out the coax suppliers for more information.
     
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  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    So how about for audio? Is there a best characteristic impedance?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Not generally. Characteristic impedance is usually only of concern at high frequencies where it length of the line is a longer than a fraction of the electrical wavelength of the highest frequency signal which only occurs for very long lines at audio frequencies. The only audio impedance standard is 600Ω and that's not much used anymore. Here's a discussion on that.

    For audio equipment there's no reason to match impedances in modern equipment. Power amps, for example, have an output impedance much lower than the speaker impedances to damp speaker resonances.
     
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  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    most shielded cable for audio use isnt even specified for impedance. check the manufacturers websites for specs.
     
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