**From:** Larry Smith (*[email protected]*)

**Date:** Fri Mar 17 2000 - 13:18:37 PST

**Next message:**Larry Smith: "Re: [SI-LIST] : Fast edges with limited plane capacitance"**Previous message:**Haller, Robert: "RE: [SI-LIST] : GOOD IBIS models!"**Maybe in reply to:**mjs: "[SI-LIST] : Fast edges with limited plane capacitance"**Next in thread:**Larry Smith: "Re: [SI-LIST] : Fast edges with limited plane capacitance"

Ron - see comments in text.

*> Date: Fri, 17 Mar 2000 11:13:44 -0800
*

*> From: Ron Miller <[email protected]>
*

*>
*

*> Larry
*

*>
*

*> I have some issues as follows
*

*>
*

*> >
*

*> > So, if we compare the two power plane pairs below with 4 and 40 mil
*

*> > separations:
*

*> >
*

*> > The velocities are constant.
*

*> >
*

*> > The capacitance of the 4 mil planes is 10X the 40 mil plane.
*

*>
*

*> Capacitance as you mentioned above is proportional to the inverse of the
*

*> square of the distance making 4 mils 100 X the 40 mil plane.
*

Capacitance between two point sources is proportional to the inverse of

the square of the distance. But the capacitance between two parallel

plates of a capacitor is proportional to the thickness of the

dielectric (neglecting the fringe field). This happens as you

integrate over the surface of point charges.

*> > The inductance of the 40 mil plane is 10X the 4 mil plane.
*

*>
*

*> The inductance does not change with dielectric thickness. In fact if you
*

*> look at a smith chart you will note that the low impedance outside circles
*

*> tend to approximate the outer circle as you go out. The outside circle
*

*> follows the electrical length and cannot be made larger then the length
*

*> of the line. This is a limit.
*

I'm not sure about the Smith Chart argument, but the inductance between

planes definitely goes down as the dielectric gets thinner. Remember

the good old equation:

velocity = 1/sqrt(LC) ?

The velocity is constant and the capacitance goes up as the dielectric

between planes gets thinner. Therefor, inductance must go down.

regards,

Larry Smith

Sun Microsystems

**** To unsubscribe from si-list or si-list-digest: send e-mail to [email protected] In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.

si-list archives are accessible at http://www.qsl.net/wb6tpu

****

**Next message:**Larry Smith: "Re: [SI-LIST] : Fast edges with limited plane capacitance"**Previous message:**Haller, Robert: "RE: [SI-LIST] : GOOD IBIS models!"**Maybe in reply to:**mjs: "[SI-LIST] : Fast edges with limited plane capacitance"**Next in thread:**Larry Smith: "Re: [SI-LIST] : Fast edges with limited plane capacitance"

*
This archive was generated by hypermail 2b29
: Thu Apr 20 2000 - 11:35:44 PDT
*