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] * ***************************************************************************