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 <[email protected]> wrote: > >(From a month ago.) > >[email protected] 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. > > > > >_______________________________________________ >time-nuts mailing list -- [email protected] >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. -- Sent from my Motorola Droid Razr phone with K-9 Mail. Please excuse my brevity. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
