Re: [SI-LIST] : Parallel tracks on Flexible PCB without Ground Plane

Adam (adam.w@softsim.com)
Fri, 02 Jul 1999 18:37:03 +0100

Cai,

> Dear experts,
>
> Sorry to disturb you again. :) The following figure explains the case more clearly:
>
> ________ ________ ________ ________
> |________| |________| |________| |________|
> GND Signal GND Signal
>
> So they are not driven differentially. General tools calculate differential impedance, right? Can just setting the distance to the plane very high get correct result?

I see your problem and I don't think there's any way to express the
impedance of an individual signal track. You have a coupled system, ie
some of the signal on one signal track will appear on the other, and the
only complete description is with a 2x2 impedance matrix. However, your
ground tracks between the signal tracks will mean that the coupling will
be fairly small and you can calculate a good working approximation for
individual impedances. If I may shamelessly plug my company's product
'TrackSim', I used it to calculate the impedances of your 4 track
system. I entered rectangular tracks with a track height and spacing
between tracks equal to the 40% of the track width. I also assumed the
tracks to be embedded in a thick PCB layer with rel. perm. = 4. The
impedances came out as roughly 37 ohms for the left hand signal track
and 58 ohms for the right hand track. The coupling is about 10%.

If you have any further questions I'd be happy to try and help.

Best wishes,

Dr Adam Woolfe,
Softsim.

Email address : adam.w@softsim.com
Web for demo downloads : http://www.softsim.com

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