Delay Distortion: Velocity of Propagation Varies with Freq.

[gnome]

I read (in W. Stallings "Data & Computer Communications") that

delay distortion occurs because the velocity of propagation of a signal through a guided medium varies with frequency. For a bandlimited signal, the velocity tends to be highest near the center frequency and fall off toward the two edges of the band.

Does that second sentence make sense? That seems to imply that if, for example, several signals of, say, 6 MHz bandwidth each are multiplexed in a cable, so maybe one signal is in a 1-7 MHz range, and another is at 10-16 MHz, and another at 19-25 MHz, that the portions of the signals at 4 MHz and 13 MHz and 22 MHz would be propagating faster than the frequencies in between these levels. (I have no idea whether or not these specific numbers are realistic; I'm just using them as an arbitrary example.) Why would the signal velocities vary up-down-up-down-up-down... as frequency increases?

 

[berkeman]

I don't know what medium Stallings is referring to. For many data communications cables, you can look up the propagation velocity versus frequency. The general effect is called "dispersion", and is worse in cheaper grade cables.

 

[gnome]

Is there anyplace online where I can find this info for some "representative" coax cable, twisted pair and optical fiber media.

 

[berkeman]

Belden makes a lot of cables, so I checked out their website. It looks like they make twisted pair and fiber, at least. Here is a link into their technical info area:

http://bwccat.belden.com/ecat/jsp/I...efined&P4=undefined&P5=undefined&P6=undefined

 

[gnome]

Thanks berkeman. The Belden website gives "nominal velocity of propagation" for many of their electrical cables, but I didn't find any more detailed info -- nothing that shows how the velocity might vary by frequency. I'll just have to keep looking. Please post if you come across any info of that nature.