Re: [SI-LIST] : bypass cap question (long, simple)

Nirmal Jain (nirmal@ansoft.com)
Tue, 03 Feb 1998 14:08:28 -0500

Dear Jeff,


> Just to add a question to this fine discussion, what role
> does the capacitor formed by the laminate between reference
> planes play? A great many designs are organized in strip-
> line pairs, with signals contained in such a cavity.

The capacitor formed by the GND/PWR plane is generally
referred as "Integrated Decoupling Capacitor" and the
frequency response of the integrated decap (very high Q)
is generally much better than the discrete decap.
To increase the capacity of this decap you need to
reduce the thickness of the laminate between the
GND/PWR planes and/or use high dielectric constant materials.

For a 10 cm sq. board with 5 mil (127micron) thick laminate
(diel constant of approx 4) between the GND/PWR plane would
give you approximately 2787 pF.

As to whether you would go for stripline or the microstrip
configuration with GND/PWR plane on one side would be decided
by what are the issues in the designs such as controlled Zo,
cross coupling or switching noise.

Regards

Nirmal

>
> Regards,
> --
>
> Jeff Seeger Applied CAD Knowledge Inc
> Chief Technical Officer Tyngsboro, MA 01879
> jseeger "at" appliedcad "dot" com 978 649 9800

-- 
Nirmal Jain       			Ansoft Corporation
(412)261-3200 X129 (W) 			Four Station Sq., Suite 660
(412)261-471-9427(FAX)			Pittsburgh, PA 15219
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