[SI-LIST] Re: Microstrip to stripline transition related question
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: jean-francois hasson <jfhasson@xxxxxxxxxxxxxxxx>
- Date: Wed, 25 Nov 2009 15:04:57 -0800
Jean-Francois,
as you suspected, you do need to stitch together the two reference planes
close to the transition point. Capacitive coupling will help, but by
itself will not be sufficient to guarantee a clean launch. At very high
frequencies only a very small area around the transition is effective (the
signal has to get there and back!) so the actual coupling capacitance is
quite small.
Depending on your frequency range it may be a good idea to simulate the
transition area with a field solver; that will show how many vias and how
close to the trace you need to place (and also show the impact of those
vias - additional metal close to the trace! - on impedance control); below
1 GHz that is probably not necessary; above a few GHz it definitely is.
Wolfgang
jean-francois hasson <jfhasson@xxxxxxxxxxxxxxxx>
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11/25/2009 01:41 PM
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[SI-LIST] Microstrip to stripline transition related question
Hi,
I believe a similar question was already brought up but I want to make
sure I have a good understanding of the situation.
If a single ended signal is first routed in a microstrip configuration
(for instance on layer 2 over a ground plane located on layer 3 and a
(mainly) signal plane on layer 1) and then a local ground plane on layer
1 starts so there is a transition from a microstrip configuration to a
symetrical stripline configuration. I imagine the return current
established on layer 1 local ground plane will need to find a way back
to the driver. Would this situation be an issue provided the local
ground plane and the large ground plane are tightly stitched together at
the transition ? What if there is no stitching, would the capacitance
between both planes if not too separated be good enough (assuming about
8 mil separation and a much bigger local ground plane than the layer 1
to layer 3 separation) ?
Best regards,
JF
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