[SI-LIST] Re: Differential microstrip with coplanar ground traces ... unexpectedresults
- From: "Lewandowski, Bob" <blewandowski@xxxxxxxxxxxxxxxxxxxx>
- To: <welte@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sun, 13 Oct 2002 20:52:47 -0500
Bob,
Stripline has the advantage of being a more uniform dielectric media.
However, getting to a buried conductor without adding large impedance
discontinuities due to via hole stubs, and layer transition problems is
quite difficult. I would suggest that it's not as clean a choice as
that. The propagation distortion due to dispersion is pretty small, and
is usually masked by impedance discontinuities within the structure.
Also dielectric loss is higher with stripline particularly with FR-4.
Dielectric loss increases linearly with frequency, while conductor loss
as the square root of frequency. =20
---Bob Lewandowski
Stratos Lightwave
> -----Original Message-----
> From: Bob Welte [mailto:welte@xxxxxxxxxx]
> Sent: Thursday, October 03, 2002 6:09 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Differential microstrip with coplanar ground
> traces ... unexpectedresults
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> Thanks to everyone who responded!
> Based on the responses I think I would now tend to favor a stripline
> structure over microstrip.
> Differential microstrip requires significant spacing between=20
> pairs even
> with a coplanar ground.
> Microstrip structure because of its multi-mode propagation=20
> characteristics
> will cause distortion (resulting in added signal jitter)
> Coplanar grounds, to be effective, are required to have low impedance
> (perhaps 5X width of signals),
> and lots of vias ...up to 20/Wavelength. Using the 3rd=20
> harmonic, this is
> a via every .075" (FR4)!
> Bob Welte
> IBM Microelectronics
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