[time-nuts] TBolt vs Twisted pairs
Hal Murray
hmurray at megapathdsl.net
Fri Jul 20 23:31:01 UTC 2012
(From a month ago.)
albertson.chris at gmail.com said:
> Take my word for it, the T-Bolt is not able to drive a 100 foot long twisted
> pair cable
I don't think that's quite the right way to phrase it.
What type of twisted pair were you using and/or what sort of setup did you
try? How well did it work and/or what were you expecting?
Yes, you may get much better results if you use differential
drivers/receivers. But that's if you have common mode problems.
-----------
I remember, many years ago, when I got an interesting lesson in this area.
The difference between junk twisted pair and good stuff was impressive.
We were installing a T microwave link. On T1, a 1 is a pulse, a 0 is an
absence of a pulse during a bit slot. Pulses alternate polarity to keep a DC
balance. T1 is 1.544 megabits/second or 647 ns per bit. I don't remember
the details, but the ballpark is a 200 ns pulse has to get through. So the
rise time has to be in the ballpark of 20-50 ns.
We had to go a few hundred feet. My first try with a spool of whatever I
found in the lab was a joke. The spool of good stuff that we ordered worked
fine. I'm pretty sure the good-stuff was Belden Datalene but, again, it was
a long time ago and I don't remember any details. (I wonder if the cable is
still there.)
Does anybody have a good URL on lossy transmission lines? Is there any
obvious reason why twisted pairs should be different from coax?
-----------
Does anybody know what the PPS driver in a TBolt is? I assume it's a typical
CMOS logic family. Is it one section or several in parallel? What
chip/family? ...
It clamps reflections. In any case, it's not linear.
----------
I collected some cable and tried a few experiments.
Theses are all nominally 100 feet long. I didn't measure any of the lengths.
Coax:
http://www.megapathdsl.net/~hmurray/time-nuts/coax/Coax-20ns.png
http://www.megapathdsl.net/~hmurray/time-nuts/coax/Coax-100ns.png
http://www.megapathdsl.net/~hmurray/time-nuts/coax/Coax-1us.png
Twisted Pair:
http://www.megapathdsl.net/~hmurray/time-nuts/coax/TP-20ns.png
http://www.megapathdsl.net/~hmurray/time-nuts/coax/TP-100ns.png
http://www.megapathdsl.net/~hmurray/time-nuts/coax/TP-1us.png
The Belden 8723 is 52 ohms. (I probably used 50 ohms.) The other twisted
pairs are 100 ohms.
I'll have to try harder to find some really-junky twisted pair.
The scope is the standard Rigol 100 MHz. I had to work a bit on the setup to
get clean pictures. An early attempt with several feet of clipleads and such
added a lot of garbage.
The difference between 3 ft of brand-X RG-58 and 1 ft of good RG-58 from the
TBolt to the scope is easy to see. The brand-X isn't 50 ohms.
--
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