Digital coax cable?

Discussion in 'General Electronics Chat' started by electronewb, Jul 31, 2013.

  1. electronewb

    Thread Starter Member

    Apr 24, 2012
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  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Low loss/Attenuation..
    MUCH different between the 2 cables
    See "Nominal Attenuation" chart
     
  3. electronewb

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    Apr 24, 2012
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    And that is due to?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    There is no specific difference between "digital" and "analog" coaxial cable. The coax doesn't know or care what type of signal it is. The main characteristic of interest is the maximum frequency content of the signal and the attenuation factor of the coax for those frequencies.

    It's the same type of hype that is promoted for "digital" TV antennas. "Digital" antennas are no different then the standard "analog" antennas. It's the frequency of the RF signal being received that determines the antenna design, not the type of modulation carried by the signal.
     
  5. electronewb

    Thread Starter Member

    Apr 24, 2012
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    So what can cause some frequencies to have more attenuation than others?

    I can see that the attenuation is higher with the 1505A but is it due to the size of the center conductor?
     
  6. mcgyvr

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    Its mostly due to the density/size of the dielectric between the center conductor and shield. Less density = less losses
     
  7. electronewb

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    Apr 24, 2012
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    Interesting so basically they should make all coax cables with less density? Why ar ell those frequencies tested for attenuation? Is it for multiplexed signals?
     
  8. nsaspook

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    Aug 27, 2009
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    Some cables are designed for power transmission, some are designed for lower loss and other factors. The characteristic impedance of the cable is set by the inner conductor size/spacing and the dielectric constant of the inner insulator so 'less density' is not aways better when size is a important factor. For air the minimum attenuation is at about 75-77Ω for a coax, for other common dielectrics the minimum attenuation is at about 52-64Ω so 50 and 75Ω cables are usually used with RF. With high-speed digital signals it's common to see higher impedance cables (rg-62/63 cable) used to reduce load capacitance and improve rise times with current limited drivers.

    http://www.rfcafe.com/references/electrical/coax-chart.htm
    http://www.prioritywire.com/specs/c-4.pdf
     
    Last edited: Aug 1, 2013
  9. electronewb

    Thread Starter Member

    Apr 24, 2012
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    Speaking of... what is the Ω represents in a cable? Resistance that the center conductor represents like a resistor or it's more complex than that?
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    The impedance is the same regardless of the length of the cable.
    No, it is not the resistance of the center wire.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    It is the characteristic impedance of a transmission line. It is mainly determined by the distributed capacitance and inductance of the line, and for an ideal lossless line it is Z = \sqrt{L/C}.

    If you apply a step voltage source to an infinitely long transmission line, the current drawn from the source will equal V/Z.
     
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