I may have posted this before, but the issue of modeling a shielded twisted pair has come up again.
It needs three txlines. This model checks well against a TDR of the real cable.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
It is probably not true, in a typical cable, that the differential and commom-mode velocities are equal. That makes things a lot messier.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
It's a good thing, though, because if they were the crosstalk would be a lot worse.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
In a multipair cable, like CAT5 UTP, the pairs generally have different twists to scatter the velocities and minimize crosstalk.
But envision putting a fast photodiode at one end of a shielded pair, and doing a single-ended bootstrapped drive at the other end. I basically refused to do that.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Did you need those 1 milli-ohm resistors? Why? Did you mean Meg?
Those are just there for ease of snooping currents.
The 1M things keep LT Spice from complaining about a floating node. But the latest version doesn't seem to mind.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Wise. Putting photodiodes on cables is so tempting to some folks, but is one of the many roads to perdition. :(
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
I think that's where Perdita got lost.
NT
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When I did it, I put common base transistors on the end of the cable, and hooked the photo-diodes up to the collectors.
The write-up talked about pseudo-terminating the cable. The currents and vo ltages differed by the right amount between the on- and off-states to look as if the cable had been terminated with it's characteristic impedance, but the impedance didn't look resistive during the transition.
We were trying to get 10MHz data transfers over 18 metres of shielded ribbo n cable, and got it. The previous arrangement had been designed for 5MHz an d 6 metres of cable, and hadn't been doing well at the higher rates (for a variety of reasons).
The boss got my set-up to work at 12MHz, which scared me silly - it had bee n toleranced for 10MHz, and I hadn't done the work to be sure that the wors t case devices would still work at 12MHz.
One did fail in the field, but it turned out that one of the HP optocoupler s involved wasn't meeting it's data-sheet spec. Replacing that single devic e solved the problem, but the (slightly psychopathic) manager involved got another engineer to repeat my tolerancing, and he came to the same conclusi ons that I had.
-- Bill Sloman, Sydney
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