[time-nuts] TBolt vs Twisted pairs

[Magnus Danielson]

On 07/21/2012 01:31 AM, Hal Murray 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?

Oh yes.

An archetypical coax has a conductor within another conductor. Their 
magnetic and electrostatic coupling to the surrounding should be very 
low. Real life is more complex.

An archetypical twisted pair has two conductors with insulator twisted 
around each other. Only for lower frequencies will the twisting cancel out.

A shielded twisted pair combines some of these effects, but the 
shielding cuts of at lower frequencies for magnetics, so the twisting 
needs to handle that.

Maintaining properties in manufacturing and due to bending etc. is 
certainly making a distinction. Use of insulator another.

There is an art in cable making.

There are several books on EMC and noise reduction techniques, which 
gives some of the info. The book by Hendry Ott is a classic.

The PDH E1 and T1 signals attempts to get the most out of the cables, 
and their waveshapes is fairly strictly controlled, as is the receiver 
equalisation. One vendor had a bug in their chip, so they could only run 
shorter distances than the datasheet said. They had a good stock of the 
old revision and hid this fact, only after they had been lectured in the 
lab and shown they didn't meet the spec, they came clean and gave us the 
ordering details for the correct one... they had hid it pretty well, 
since even the public ordering number of the part was the same, but the 
internal ordering number was different.

Cheers,
Magnus