Re: [SI-LIST] : Question about Thin Flexible Coaxial cable

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Fri Nov 10 2000 - 17:11:05 PST


Jerry:

I don't know about the cable, but I can give you an idea of skewing clocks
within nanosec.

See, TX line is a resistor-like-behaving thing. So, tap into it right befor
the termination - and you have practically no additional delay from the tap
place to your RX input.
Use another identical line (or the same) and take your signal right after
termination. You create a delay of a front equivalent of that by RC =
Rchar_imped*C_load. Which at 50 Ohm times 10pF is 0.5ns

You just skewed your clocks by 0.5ns.

Mike.

Jerry Martinson wrote:

> Hi,
>
> I need to skew some clocks on a board to correct a minor clock skew
> issue with a prototype that I have. Actually this situation comes up a
> lot in the lab where you want to skew a clock or a net a nanosecond or
> so to verify that some intermittent error is or isn't from a setup/hold
> violation on a
> certain bus, etc... To me the obvious choice (assuming no roboclock) to
> skew things around a bit in the lab is to use a small length coaxial
> cable cut to the appropriate length. The problem is that most coax
> (even RG174) is pretty hard to work with because it is so thick and has
> this gnarly braided shield that is a real pain to solder with. Despite
> the best efforts
> of tinning the shield, half the time these little braided strands fall
> off and do interesting things between the pins of fine pitch PQFP's.
> All I need is a small length of cable so I don't really care about
> having really good cable but it is nice to have the beneficial
> properties of using coax rather
> than a unshielded wire.
>
> I used to work for a company that had a huge spool of this wonderful
> micro coax wire that was perfect for the job. It had blue (Teflon?)
> insulation and had a foil shield with a wire running with the shield
> that could be easily soldered to the closest ground. It has a very
> elastic di-electric.
> The cable was very flexible and maybe 2mm in diameter. It was nominally
> 50 ohms and maintained pretty good SI for TTL at lengths of up to 2m
> before the signal distorted significantly. It was great for debugging
> all sorts of nasty problems because it was so easy to work with that it
> would only take 1
> minute to change the length of the cable on a prototype. I wish I would
> have written down the manufacturer of that cable before I left that
> company because now I'm trying to find a coax company that carries this
> stuff. I've searched the catalogs (Belden etc...) but I couldn't find
> anything that matched this description.
>
> Does anybody know where I could find this micro coaxial cable that I'm
> describing is or does anybody have any other suggestions for doing some
> lab skewing?
>
> Thanks,
> -Jerry
>
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