Cable's specs vs performance

Discussion in 'Accessories, Cables, and Remotes' started by Robert Garrison, Jun 26, 2004.

  1. Robert Garrison

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    Carrying over from a conversation Chu and I started in another thread. We started to debate the actual effect of high capacitance on interconnect cabling.

    Chu, I was just talking high frequency applications since I used 34Mhz in my example. I certainly wouldn't want to try and make a tuned speaker cable. That just wouldn't work (well). Since video signals are fairly close to their stated frequency used I thought that would have been a better candidate for a so called "tuned interconnect".

    I'm assuming when you analyse the pF/FT figures, the pF/ft sums as if it were just one capacitance. Instead of multiple 16pF/ft capacitors in series which actually makes it have less capacitance as you know. Just using 16pF as example number.
     
  2. KurtBJC

    KurtBJC Agent

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    The thing here is: you wouldn't want the cable "tuned," if what you mean by that is that the cable would be designed, by length, capacitance, or otherwise, to resonate at some frequency involved in the signal. The object of creating a cable with a highly consistent impedance, matched with the impedance of the source and load circuits, is to make the cable neutral at all frequencies. The cable shouldn't resonate, and if it does, you wind up with a lot of signal loss in the line; what's worse, since the video signal has some bandwidth, what will happen is that it will resonate better at some frequencies than others, causing an uneven frequency response.

    To calculate the characteristic impedance of the cable, you don't want to use formulas for the impedance of a load; the concept of characteristic impedance is not that the cable itself, in some finite length, presents a load of a given impedance. It is instead that the cable, if extended to an infinite length, would present a load of that impedance; the reason being that we want to be able to terminate the cable into the load at any length without altering the impedance of the load as "seen" at the source end.

    Capacitance will always harm signal quality because it rolls off higher frequencies more than it does lower frequencies. However, because capacitance and impedance are interdependent, the lowest one can get capacitance to go in a 75 ohm video cable is about 16 pf/ft.
     
  3. Chu Gai

    Chu Gai Lead Actor

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    I'll buy what Kurt said. Your queston sounds almost like you're thinking about antennas Robert and looking to tune the antenna to pick up specific frequencies. For a video cable, all you want is that it be shielded, be of the proper impedance, and possess a healthy bandwidth margin with neglible attenuation. To do all that is pretty easy and you can spend a few bucks or a few thousand. It's just one of those things that I don't beat my head with. Make and keep good clean connections and just move on.
     

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