From: Bob Lewandowski ([email protected])
Date: Fri Nov 10 2000 - 19:09:53 PST
Jerry,
Belden makes a type of "semi-rigid" coax that has a tin dipped round wire braid
shield, a teflon dielectric layer, and a solid center conductor. They call it
"Conformable Coax" and it comes in a variety of outer diameters down to .047
in. It's easily stripped with an Exacto knife, and because of the tin dip the
shield acts like a solid tube. You can solder the shield directly to a ground
pad area on a circuit board, and the center conductor to a trace. By keeping
things very short, you can get a very clean (low reflection) launch into the
cable. It's significantly more flexible than the solid jacketed semi-rigid
coax, and has pretty good impedance characteristics.
Check out their web site for details: http://www.belden.com/products/np140.htm
It's great stuff!
---Bob Lewandowski
Vixel Corp.
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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