[SI-LIST] : Re: delay lines with PCB traces

Ryszard Vogel (Ryszard.Vogel@cubt.ericsson.se)
Fri, 26 Sep 1997 07:58:41 +0200

> From owner-si-list@silab.Eng.Sun.COM Thu Sep 25 16:18 MET 1997
> Date: Thu, 25 Sep 1997 10:12:10 -0400 (EDT)
> From: Xingchao Yuan <yxc@Cadence.COM>
> To: bgrossma@td2cad.intel.com, mchin@cisco.com
> Subject: Re: [SI-LIST] : Re: delay lines with PCB traces
> Cc: si-list@silab.Eng.Sun.COM
>
> Michael and Brett:
>
> One way to derive a model for the delay lines is to use an 2.5D
> full-wave Electromagnetic solvers. Commercial available tools
> include Sonnet from Sonnet Software, MOMENTUM from HP-EEsof,
> Strata from Ansoft, and IE3D from Zeland.
>
> These tools solve full-wave electromagnetic fields in stratified
> media and extract scattering parameters in frequency domain.
> They all include the skin effect by using so called impedance
> boundary conditions. By examine the phase of the transmission
> coefficient (S12), one can obtain the dispersion characteristics of
> the line. To use the model in time domain simulations, one needs
> to figure out a way to convert to the s-parameters to an
> equivalent circuit. I believe HP-EEsof provides such an utility though
> it may require some tweaking.

Comment:

> The HP-EEsof (MDS - HP Impulse included with the Picosecond Suite) allo ws to use S-parameters in time domain simulations. The generation of an approximate SPICE model representing S-parameter data is also possible.

Ryszard Vogel,
Ericsson Telecom, Sweden

> Because those tools are specialized for stratified media, they are
> very fast in general. For a typical delay line, it may only require
> a couple of minutes per frequency on a SUN Sparc workstation.
>
>
> -Xingchao Yuan, Ph.D.
> Cadence Design Systems, Inc.
>
>
>
> > From owner-si-list@silab.Eng.Sun.COM Wed Sep 24 23:37 EDT 1997
> > Date: Wed, 24 Sep 1997 17:49:21 -0700
> > From: Michael Chin <mchin@cisco.com>
> > To: bgrossma@td2cad.intel.com
> > Subject: [SI-LIST] : Re: delay lines with PCB traces
> > Cc: si-list@silab.Eng.Sun.COM
> > Sender: owner-si-list@silab.Eng.Sun.COM
> > Content-Type: text
> > Content-Length: 2232
> >
> > Brett,
> >
> > > From bgrossma@td2cad.intel.com Wed Sep 24 17:01:57 1997
> > >
> > > One suggestion in Howard Johnsons 'Black Magic Handbook': there is a
> > > suggestion to implement delay lines with a chamfered corner or a rounded
> > > corner. The argument is based on increased capacitance on a right-angle
> > > due to the change in effective trace width. I am sure the praises have
> > > been sung enough for this book, but I make sure all my designers get it.
> > > A very good working understanding without wallowing in the e-mag world.
> >
> > Thanks for the reference to Howard's book. This was exactly
> > what I did. In fact, by going to a wider gap, the rounded radius
> > become more smoother. I followed this rule for all PCB trace corners,
> > although a lot of my colleagues did not think that these would make
> > a big difference.
> >
> > > Some additional questions:
> > >
> > > 1. Does anyone on the LIST have PCB design rules for this geometry
> > > (serpentine) that they have validated on the bench? (e.g. not just a
> > > rule of thumb)
> >
> > Well, I valididated on the bench the two delay line routings.
> > The first case was a 4 mil trace with 6 mil gap in the serpentine,
> > and about 10% decrease in the expected trace delay was measured.
> >
> > When the gap was increased to 24 mil in the serpentine, the trace
> > delay was in line with our expectation.
> >
> > We did not have time to develop a PCB design rules but a rule of
> > thumb I used is that as the trace gap is at least 3x the trace
> > width, the coupling between two adjacent tracks dimenished to
> > some insignificant level.
> >
> > > 2. Or, have a decent model to comprehend the effects of this geometry
> > > (cross-coupling due to parallelism, any additional degradation of rising
> > > edge, etc?)
> >
> > We once had suspected a coupling problem between two traces in
> > a similar situation. And, we started to work on a model to
> > simulation this behavior. But, after the trace gap was widen
> > to 3x the width and the design worked, our energy was diverted
> > to other areas. Thus, we never completed the modelling effort.
> >
> > What we learned was that this kind of coupling does occur even on
> > slow signal (50Mhz range) if the edge rate was fast enough and
> > the trace separation was not at least 3x the width.
> >
> > Michael Chin
> > Cisco Systems, Inc
> >
>