[SI-LIST] Re: Coaxial Cable crosstalk
- From: "Daniel, Erik S., Ph.D." <Daniel.Erik@xxxxxxxx>
- To: deibele@xxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 5 Feb 2002 23:01:35 -0600
Craig-
Although it's possible you could estimate the crosstalk by solving the
fields in the simple geometries you indicated in one of your e-mails, we've
found that this method grossly underestimates the crosstalk (overestimates
the isolation) in the cases we briefly looked at (again at ~1 GHz in good
laboratory cables). Perhaps in your case where the cable shield thickness
is comparable to or less than a skin depth (as opposed to many many skin
depths as in our case), this would yield more reasonable answers. Scott's
note regarding matching of simulation and measurement in PCB structures
would seem to support this.
I was confused by your last note as you mentioned that you "weren't
interested in any specific cable", yet it seems you have a specific
application in mind coupling of high current transients on one cable or set
of cables to a (presumably a differential) signal on twinax. If you did
have specific cables in mind, again, I'd suggest going the measurement
route. I assume you would want to determine the coupling of the high
current cable to both the even and odd modes on the twinax. We did similar
measurements of crosstalk betweeen two diff pairs using baluns to source /
sense either the even or odd mode on each pair -- a method that seemed to
work well for our frequencies of interest.
- Erik
==================================================================
Erik Daniel, Ph.D. Voice: (507) 538-5461
Mayo Foundation Fax: (507) 284-9171
200 First Street SW E-mail: daniel.erik@xxxxxxxx
Rochester, MN 55905 Web: www.mayo.edu/sppdg/
==================================================================
> -----Original Message-----
> From: C Deibele [mailto:deibele@xxxxxxx]
> Sent: Tuesday, February 05, 2002 10:48 AM
> To: Daniel, Erik S., Ph.D.
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Coaxial Cable crosstalk
>
>
>
>
>
> "Daniel, Erik S., Ph.D." wrote:
> >
> > Craig-
> >
> > I think you're right -- this is a hard problem. In our
> experience with good
> > quality coax cables in the lab, the crosstalk is so low
> (easily better than
> > -100 dB) that I think you'd find it extremely hard to
> predict -- much easier
> > to measure with a good spectral quality signal source and a spectrum
> > analyzer with fairly low noise / narrow resolution
> bandwidth. Because the
> > crosstalk between very good cables is so low, you may find that the
> > effective crosstalk between signals on the two cables is
> totally dictated by
> > what is happening at the cable ends (e.g., connection to a
> PCB where the
> > signal is not totally enclosed by ground).
> >
> > If the cables you are interested in are not as high a
> quality (e.g., thinner
> > ground shield with a looser braid, etc.), it may be even
> easier to measure
> > the crosstalk, but it seems it would still be difficult to
> predict. Can you
> > give some more information on the type of cables you are
> interested in, the
> > approximate level of crosstalk you care about, and the
> frequencies of
> > interest? Our limited experience here in the lab was with
> good quality
> > flexible cables with SMA or 3.5mm connectors, looking at
> frequencies of a
> > few hundred MHz to a few GHz, lookinf for crosstalk at the
> ~-100 dB level.
> >
> > Some cable manufacturers specify crosstalk per foot or some
> such thing
> > (assuming some cable separation I suppose). I think I've
> seen this for RG-6
> > cable before.
> >
>
> Well, I am not interested in any actual cable. I am more
> interested in
> some general theory/theoretical concepts.
>
> We have an application where we have twinax (twinax is like
> coax, except
> it has two conductors surrounded by an outer shield instead of one
> conductor surrounded by an outer shield like coax). The frequency is
> DC-1MHz or so.
>
> These cables are long....separate sources, and they will all be
> collected at a common point where they will share a small space, and
> then separate again.
>
> There is some cables with high currents on them...on the
> order of a few
> thousand amps....which may/may not have switching supplies
> driving them.
>
> So, if I have a small signal cable nearby...how can I reliably predict
> what the crosstalk/might be?
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