[SI-LIST] Re: Differential microstrip with coplanar ground traces ... unexpected results
- From: Ed Sayre III <esayre3@xxxxxxxx>
- To: welte@xxxxxxxxxx, "silist" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 02 Oct 2002 11:37:32 -0400
Bob,
A quick rule of thumb for CPW (coplanar waveguide) is to have the
grounds at least 5 times wider then the signal conductor. As part of my
doctoral research characterized aluminum CPW transmission lines. I had CPW
lines with 5 line widths wide and 1 line width wide grounds.
I saw a significant increase (~20-25%) increase in the characteristic
impedance for the narrow grounds over the 50 Ohms seen for the wide ground.
When looking at the CPW or microstrip structure in a field solver make
sure that the ground extends well beyond the etch. To really see the
effect of the adjacent ground width of a CPW structure, you will need to
make the distance to the adjacent grounds much smaller then the vertical
distance to a reference plane, above or below.
Best of luck
-Ed Sayre, 3rd
At 07:57 AM 10/2/2002 -0700, you wrote:
>Bob,
>
>You are absolutely correct. Coplanar waveguide can be extremely
>twitchy, especially with narrow ground traces. You generally need to
>have much wider grounds in order to better contain the fields.
> Otherwise, crosstalk from each side will cause some nasty mode
>conversions and crosstalk to occur. In general, CPW or grounded CPW, as
>your case is, is useful for better impedance control and lower loss only
>when you can use a wider ground line. In addition, if your edge rates a
>fairly fast, you may begin to excited a half-wave resonance between your
>vias. For your configuration with vias spaced at 500mils, you can
>expect a resonance point at around 6 to 7 GHz on Fr-4, depending on how
>far away the ground plane is from the microstrip layer. At the
>resonance point, the crosstalk will become extremely high. This would be
>right around the 5th harmonic of your 2.5 Gbps signal.
>
>I use Ansoft 2D to evaluate configurations like this, and treat the
>ground traces as signal traces in the modeling. Then with 3D, you can
>extract the via model and perform a complete simulation of the signal
>and ground traces. I;ve had success with 10 Gbps signals in a package,
>but in that case, we had ample room to form a good ground plane between
>the pairs.
>
>best regards,
>
>scott
>
>--
>Scott McMorrow
>Teraspeed Consulting Group LLC
>2926 SE Yamhill St.
>Portland, OR 97214
>(503) 239-5536
>http://www.teraspeed.com
>
>
>
>Bob Welte wrote:
>
> >Hello to All
> >
> >Would anybody like to share their knowlege or experience with using
> >coplanar ground traces in a microstrip configuration?
> >Are there any "rules" for the width or impedance, and number/spacing of
> >vias for the ground traces of such a configuration?
> >
> >A short explaination of our design:
> >We have 100 ohm diff microstrip; signals are .013", with .015" spacing,
> >ground traces are spaced .010" from signals and are .010" wide, with vias
> >to the ground plane (.012"
> >below) at about every .5". Our digital signals are 2.5Gb/s with the
> >fastest rise time at 100ps.
> >
> >We expected the coplanar grounds to better contain the fields, and allow
> >closer pair to pair spacing without the risk of large crosstalk. We
> >believed that we had taken the coplanar grounds into account when designing
> >the configuration, but now thnk that the software we used assumed that the
> >coplanar grounds were perfect. With the narrow ground traces, and the vias
> >to the ground plane, we now believe that the coplanar grounds are actually
> >quite inductive at our frequencies. We have both microstrip and stripline
> >coupons, and expected the microstrips to be slightly better since they have
> >no vias in the signal path. But in fact the striplines show less
> >discontinuity (TDR), and have less jitter when transporting a pseudorandom
> >bitstream.
> >Thanks
> >Bob Welte
> >IBM Microelectronics
> >
> >
> >
> >
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