[SI-LIST] Re: matching differential impedance for edge-coupled lines
- From: "S. Weir" <weirsp@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 15 Jun 2001 15:01:29 -0700
Perry, the closer the lines get, the lower the odd mode, and the higher the
even mode. Somewhat conversely, making the dielectric thinner pushes the
even mode down faster than the odd mode. It makes a field solver a nice
program to have.
The problem with only terminating the odd mode is that you will have even
mode reflections in the line, putting you at the mercy of the common mode
performance of the receiver.
Regards,
Steve.
At 05:12 PM 6/15/01 -0400, you wrote:
>Hi! Steve:
>
>I understand that closer spacing between the two trace bring even mode
>impedance
>higher. However, for differential application, can we just terminate the
>line for
>odd mode signal and ignore the even mode ?
>
>Thanks
>
>Perry
>
>"S. Weir" wrote:
>
> > Perry,
> >
> > Tighter coupling between the lines is an advantage within pairs, but it
> > means that the even mode impedance has to be raised above 50 ohms. This
> > means a different layer, or narrower line widths than your single-ended
> > lines. No matter what you do, you are unlikely to get coupling above 15%.
> >
> > Regards,
> >
> > Steve
> > At 03:05 PM 6/15/01 -0400, you wrote:
> > >Hi!
> > >
> > >There has been a few threads about odd/even mode impedance and I would
> > >like to throw in another one:
> > >
> > >Basically there are two types of differential pair for edge-coupled
> > >lines, one is loosely-coupled, with trace spacing much wider than trace
> > >width/height above ground; I find from my 2D field solver that only in
> > >this case you can obtain a 100 ohm differential impedance (or 50 ohm
> > >Zodd as mentioned in TDA's paper); To match the diff pair, you can
> > >either terminate each line with 50 ohm to ground or a 100 ohm across
> > >plus and minus;
> > >
> > >The second type is a tightly-coupled diff pair, with trace spacing
> > >equivalent to trace width and trace height against ground plane. In such
> > >case, I can't get a 100 ohm differential impedance (or 50 ohm Zodd) in
> > >my calculation with a realistic stackup and manufacturing constaints.
> > >Zodd is more likely in the region of 40-45 ohm range. To match the diff
> > >pair in this case, I need to terminate each line against ground with
> > >Zodd or 2*Zodd across plus and minus. (I did this with HSPICE on a very
> > >simple topology: Ideal voltage source (50ps rise time step pulse) ->
> > >transmission line -> load). This is intuitive since we can imagine that
> > >there is a virtual ground between plus and minus.
> > >
> > >My question:
> > >
> > >In reality, I saw many diff pair done in second case because of the
> > >space limit on already very dense board. In such case, I think instead
> > >of using standard 100 ohm termination (e.g., LVDS application),
> > >shouldn't we terminate the diff pair with smaller values to match the
> > >differential signal ?
> > >
> > >Thanks in advance
> > >
> > >Perry Qu
> > >
> > >
> > >
> > >
> > >
> > >
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