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 Post subject: For dummies : Impedance
PostPosted: Mon Apr 26, 2010 7:33 pm 
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HamelnStock Survivor and Midi Guru
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What is impedance ? Is cable impedance the same as cable resistance ?

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PostPosted: Mon Apr 26, 2010 7:41 pm 
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Robbie The Botkiller
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It's not really the same. The cables characteristics should match the things that are connected to it. If a cable's impedance is let's say 75 Ohms, you will measure 75 Ohms over the two connections of the wire when you make it infinitely long. That way, it'll electrically behave the same way as a device that has a 75 Ohm input or output inpedance. When you connect something with different impedance, some of the energy will reflect. The bigger the difference, the bigger the reflection.

A long long time ago, when I was an ignorant young boy, I replaced an ethernet cable section of just a few feet by an optically identical section of arcnet cable. The network didn't work and it took me a whole afternoon to find out why. Arcnet and ethernet cable have different impedances so I had a lot of signal reflections on the cable which were not understood by the ethernet cards. At least I'm not young anymore :)

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PostPosted: Tue Apr 27, 2010 7:42 am 
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Mr. Electonica Dude
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In a nutshell resistance is the opposition of current through a resistor . In circuits that have coils , capacitors and resistors in them that are subjected to AC phased current oposition due to frequency. Filters have high impedance (oposition to current flow) to signals that fall outside of the frequency they are tuned for, i.e. bandpass filters , tone controls on a guitar , etc.

Like Rob said. The output of a device , cable , antennae should match the input to a connected device of some of the power will be reflected back to the source due to impedance mismatching.

(impedance = Z in ohms)

A good example in our business would be not to connect a 4 ohm speaker to a 16 ohm amp output. The speaker input Z should closely match the amplifier's output Z for optimal power to exchange places. Excessive power reflection back to the amp causes overheating of the output stage and cause a very short life to it's component parts.

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