[SI-LIST] Re: ROOKIE: Anti-Pad Size Effect On Signal Integrity:By the formula, as F goes up, Xc goes down (was up by typo)

From: "damarla babu" <sridamarla@xxxxxxxxx>
To: steven.salkow@xxxxxxxx
Date: Tue, 12 Dec 2006 17:56:35 +0530

Hi ,
The crosstalk noise and crosstalk delay are Different ?between these any
relation is there ?

How these two are depend each other ?

regards,
Suresh babu.D

On 12/12/06, Salkow, Steven <steven.salkow@xxxxxxxx> wrote:
>
>
>
> -----Original Message-----
> From: Salkow, Steven
> Sent: Monday, December 11, 2006 1:59 PM
> To: 'PaulClarke@xxxxxxxxxxxxx'; 'si-list@xxxxxxxxxxxxx'
> Subject: RE: [SI-LIST] ROOKIE: Anti-Pad Size Effect On Signal Integrity
>
> Paul:
> I will make this simple are seems reasonable. It does, however, seem to
> me quite extraordinary that a mechanical fellow might be getting
> involved with Gigahertz design of vias.
>
> You're correct the effect does depend on speed. The "anti-pad" is used
> when building plane layers (i.e.: solid layers) using negative planes.
> It is the VOID area between the pad and the copper of the plane. The
> effect is to provide a capacitive reactive effect given by the formula
> Xc= 1/(2*pi*F*C) where f is frequency and C is capacitance. By the
> formula, as F goes up, Xc goes down (was up by typo). The C capacitance
> is given by the formula C = (Area*k*e)/length where length is really the
> distance the two areas are apart (in this case the width of the anti-pad
> (the bigger the gap, the smaller the capacitance). The effects of C is
> cumulative for multiple planes.
>
> If the anti-pad size is very large, are we out of the woods. NO!
> All signals used in modern design as transmission lines have a certain
> desirable impedance. The is the effective "resistance" of the line that
> best matches the driver electronics. When effective "resistance" of the
> line does not match the driver electronics one of two possibilities
> happen:
> The signal has energy reflected back to the source
> Or excessive energy is absorbed by the circuit a too little gets to the
> load.
> Anti-pads are designed to maintain the required effective "resistance"
> (impedance) of a transmission line at a matching value. What's that
> mean?
> If the line impedance and the driver impedance and the load impedance
> are all 50 ohms, then the via should be tuned to the same value.
>
> How do we tune via impedance? We use ground vias nearby and 3D Modeling
> tools that exist to fufill this purpose but that is beyond the scope of
> a short answer.
>
> Steven Salkow
> Lockheed IS&S
> 3130 Zanker Rd, San Jose
> Ca. 94588
> steven.salkow@xxxxxxxx
> salkow@xxxxxxxxxxxx
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Clarke, Paul
> Sent: Monday, December 11, 2006 1:25 PM
> To: 'si-list@xxxxxxxxxxxxx'
> Subject: [SI-LIST] ROOKIE: Anti-Pad Size Effect On Signal Integrity
>
> Hello,
>
> Before you read the question please keep in mind that I am just a lowly
> Mechanical guy that has better odds of selecting the right bolt than I
> do
> designing an LED circuit.
>
> I have a question about how the size of an anti-pad can effect signal
> integrity. The example application could be a backplane @ 5, 10, 20, 40,
> or
> 80 [G] (I am asking for this range because I anticipate the answer may
> depend on the speed).
>
> If you have a BP via for a signal pair of .025" with a pad of .044", how
> much impact can an antipad have on the impendance through a range of
> sizes
> of let's say .054-.060"? Center-Center distance could be 2.1 [mm].
>
> In the case described above, would the antipad size range really have
> any
> effects or is it negligible?
> Is an anti-pad just to keep solder off the pad if you flood the plane?
> Or is
> there an actual SI reason for those things?
> How sensitive is the SI to changes in antipad size?
> Any concerns regarding manufacturing tolerances on antipads?
>
> Thank you for any information and your patience explaining any of the
> above
> questions to a mechanical guy.
>
> Paul Clarke
>
>
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-- 
Thanks and Regards,
SURESH BABU.D


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Follow-Ups:
- [SI-LIST] Re: ROOKIE: Anti-Pad Size Effect On Signal Integrity:By the formula, as F goes up, Xc goes down (was up by typo)
  - From: agathon

References:
- [SI-LIST] Re: ROOKIE: Anti-Pad Size Effect On Signal Integrity:By the formula, as F goes up, Xc goes down (was up by typo)
  - From: Salkow, Steven

[SI-LIST] Re: ROOKIE: Anti-Pad Size Effect On Signal Integrity:By the formula, as F goes up, Xc goes down (was up by typo)

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