What is the plane stackup, and speed of the signals, if I may ask?

I would "no", don't do this.  Add a goundplane, as stripline is supposed
to be sandwiched between two ground planes.

Hard to explain over email.  You see, any signal above another plane
(ground or not) will induce a current in that plane (assuming we're not
talking DC here).  A differential signal may be thought of as two single
ended signals.  Each one will induce a return current in said plane; I
guess you could think of it in terms of parallel plate capacitors.  The
current simply is there.  The only difference for a differential signal
is that two return currents are generated, equal and opposite (unless if
there is an unbalance) -- but they still exist!  There just isn't a net
current, but there is a local current, right next to each trace.

If the signal speed is low, and the split power plane has ac coupling
capacitors bridging the plane *right above your signal*, with an SRF
above the max frequency in your differential signal and enough
capacitive reactance to look like say an ohm below oh 100kHz, then it
might work ok.  But you may be troubled by cross currents on the power
supply itself (unlikely, but it depends upon your noise margins).  And
you may have some discontinuity in the signal from jumping that gap.
And you're counting on the AC bypass capacitors AC coupling your ground
plane to you split power planes in a very big way--they would need to
exist very close to anywhere your signal changes planes or goes into a
connector.

You have to define how good it must be.  And then, if you're allowed to
go back and redesign if it doesn't work first time.

Shawn Upton, KB1CKT
Test Engineer
Allegro MicroSystems, Inc
[log in to unmask]
603.626.2429/fax: 603.641.5336

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Baldwin
Sent: Monday, November 24, 2008 9:36 AM
To: [log in to unmask]
Subject: [TN] Stripline Traces and Plane Splits

Fellow Designers:

I am designing a board with differential pairing in a stripline
configuration.  One the bottom side is a solid ground plane.  On the top
side is a split power plane.

I was told that running traces across the splits is not a significant
problem as there is a solid ground plane on the other side for the
return currents to flow.

Do you agree with this assessment and what has your personal experience
told you.

Thanks in advance for your opinions and response.

David Baldwin CID+

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