TECHNET Archives

February 1997

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Andy Burkhardt <[log in to unmask]>
Date:
Fri, 7 Feb 1997 18:12:38 GMT
Content-Type:
text/plain
Parts/Attachments:
text/plain (84 lines)
At 13:27 06/02/97 -0500, you wrote:
>Various sources (e.g. IPC) have formulas for propagation
>delay for microstrip and stripline configurations. Does 
>anyone know of a formula for propagation time in a
>DUAL stripline configuration? Or is the stripline
>configuration close enough?
>
>Thanks
>
>Doug Brooks
>[log in to unmask]
>

Doug,

As far as I'm aware DUAL STRIPLINE, OFFSET STRIPLINE & ASYMMETRIC STRIPLINE
are different names for the same thing.  They refer to a controlled impedance
structure where a conductor is sandwiched between two reference planes and the
conductor is closer to one plane than the other.  A special case of this
construction is SYMMETRIC STRIPLINE, where the conductor is equi-distant from
both reference planes (ie in the middle of the sandwich!).

The propagation delay characteristics would be very similar for all of the
above named structures.  The electromagnetic field is bounded by the two
reference planes.  Propagation velocity is inversely proportional to the
square root of the dielectric constant of the material in which the
electromagnetic is travelling.  In a vacuum (Er=1) electromagnetic waves
travel at the speed of light.  In any other medium the propagation velocity
will be slower.  For striplines in FR-4 boards (Er=4 approx) the propagation
velocity will be approximately half the speed of light.

To make things simple, it is common to treat the dielectric in a controlled
impedance structure as if it were a homogeneous material.  This is referred
to as the effective dielectric constant (Er').  So for a surface MICROSTRIP
(a conductor supported above a single reference plane), the electromagnetic
wave travels half in AIR (Er=1 approx) and half through the circuit board
dielectric material.  For a surface microstrip (no solder mask) on an FR-4
board, the effective dielectric constant will be a combination of AIR (Er=1
approx) & FR-4 (Er=4 approx), so the approximate effective dielectric
constant would be about three (Er'=3 approx).

Clearly from the above it can be seen that surface microstrips offer the
designer the fastest propagation velocity (shortest propagation delay) for a
given conductor length.  Unfornately surface traces also radiate more, so EMC
regs are harder to meet - life's full of little compromises!

One more thing which is worth a quick mention:  DUAL STRIPLINE refers to a
controlled impedance structure where the two conductors are sandwiched
between the two reference planes on adjacent layers and the traces on each
of these
internal signal layers are routed orthogonally to reduce inter-layer crosstalk.
(ie in plan view, say L2 is a VCC plane, L3 & L4 are the signal layers and L5
is a GND plane, then L3 traces would be routed East-West and L4 traces would
be routed North-South.)  This structure behaves like two independant offset
single-ended controlled impedance striplines.  This is not to be confused with
a broadside coupled differential controlled impedance stripline structure,
which has the signal conductors routed in parallel on adjacent layers with
the individual traces directly overlayed.

I hope this helps.  You can email me directly, or through TechNet if you
prefer, if you have further questions.

Best regards

Andy Burkhardt <[log in to unmask]>
Tel: + 44 1481 53081
Fax: + 44 1481 52476
http://www.polar.co.uk
----------------------------------------------------------------------
World leaders in PCB faultfinding and controlled impedance measurement
----------------------------------------------------------------------

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************



ATOM RSS1 RSS2