[time-nuts] TBolt vs Twisted pairs

[Didier Juges]

The Thunderbolt has no problem driving a 100 feet 50 ohm coax cable,  aside from the obvious impedance matching problem (the TB has maybe 5 ohms output impedance), so I am not sure in what context that remark would apply. 

Didier KO4BB



Hal Murray <hmurray at megapathdsl.net> wrote:

>
>(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.
>
>
>
>
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