Re: [SI-LIST] : More on differential impedance calculations

Fred Townsend (fred@luxtron.com)
Thu, 05 Feb 1998 17:03:29 -0800

Mark Nass wrote:
>
> From the advice I heard so far I used a microstrip topology with
> 2 - 5 mill wide traces, 5 mills apart and 10 mills over a plane.
> Using XFX I get 3 impedances:
>
> 85.77 is the trace to plane impedance
> 109.75 is the even mode impedance
> 60.13 is the odd mode impedance
>
> This appears to be what I want. Any comments?

1/Z1 + 1/Z2 + 1/Z3 = 1/ZT ... Actually they are complex impedances so
you need to do a vector sum but never mind adding up all the
impedances. Can you do 2-5 mil traces with 5 mil air gap? That sounds
like less than 5 and 5 to me. My PCB vendor is only certified to 6 and
6. Your normal shop can't do this one.

> Also it seems to me that I want to have some amount of coupling between
> the two traces so that any noise that gets injected into these traces will
> have a canceling effect.

Don't worry about this kind of noise. That's why you use differential
transmission. Common mode signals are automatically removed.

Fred Townsend
DC to Light Consulting Services
Silicon Valley