[time-nuts] TBolt vs Twisted pairs

[Didier Juges]

I don't think the problem is with the tbolt, see http://ko4bb.com/Test_Equipment/CoaxCableMatching.php

There is a picture of the tbolt driving 50 feet of average quality 75 ohm cable. 

With the right termination, and if the cable is good enough, there should be no problem.

If the cable is junk, there is not much you can do. 

Didier


Chris Albertson <albertson.chris at gmail.com> wrote:

>Yes, of corse twisted pairs can handle fast signals.  Gigabit Ethernet
>over cat-5 cable is a good example.
>
>What I meant when I wrote the below is that a bare t-bolt PPS output
>can't drive 100 feet of cat-5 cable when the far end is connected to
>to typical 74LSxxx t/l chip.  The pulse get smeared.
>
>Yes, you could make it work easy enough.  Place some RS-422
>transceivers on each end and it would work just fine.   But what I
>said is the t-bolt is not able to drive the cable.   I tried it using
>a spare pair in a cat-5 cable that was already installed in the walls
>and up two floor.  It was very unreliable.
>
>
>
>On Fri, Jul 20, 2012 at 4:31 PM, 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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>
>
>-- 
>
>Chris Albertson
>Redondo Beach, California
>
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