[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.



-- 
These are my opinions.  I hate spam.






More information about the time-nuts mailing list