[SI-LIST] AW: Re: More discussion on AC coupling capacitor

  • From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
  • To: "scott@xxxxxxxxxxxxx" <scott@xxxxxxxxxxxxx>, "emcesd@xxxxxxx" <emcesd@xxxxxxx>
  • Date: Tue, 25 Sep 2012 07:41:29 +0000

Putting the caps at the receiver side has one other little advantage, and thats 
component density. Due to the low signal amplitude at the receiver, the 
coupling between capacitors is lower, thus you can place them closer together 
and still have the same x-talk level as if they were at the transmitter side.

BR
Gert


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-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im 
Auftrag von Scott McMorrow
Gesendet: Montag, 24. September 2012 17:04
An: emcesd@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx; Rose, Michael
Betreff: [SI-LIST] Re: More discussion on AC coupling capacitor

"Does placing the AC capacitor near the receiver really help SI?"
Let's dispel this "fact" once again.  In a linear interconnect that is 
well-matched, it does not matter where a DC blocking capacitor is placed.
 What matters are potential reflective resonances that occur between mismatched 
structures on the interconnect.  Dies, packages, connectors, vias, and 
capacitors tend to be the major impedance discontinuities in an interconnect.  
It does not matter if dispersion happens before or after a discontinuity, or on 
both sides of it.  The only thing that matters is keeping the discontinuities 
as small as possible. This can be done with DC blocking capacitors by 
performing optimization of the transition in and out of the capacitor with 
electromagnetic modeling tools.  It is quite possible to keep 0402 capacitors 
"flat" out to 10 GHz and 0201 capacitors "flat" out to 20 GHz with good designs.


"Which is the better place for placing the decoupling capacitor, near the 
transmitter or near the receiver?"

If the capacitor transition design is not well-matched, then "best"
placement depends upon how it interacts with other structures with impedance 
discontinuities.  This might be packages, drivers, receivers, or connectors.  
It just depends which is the worst, how close the capacitor is to the 
structure, and the amount of interconnect loss in between them.
 Loss is good, since it will de-Q resonances and improve return loss in the 
region of the discontinuities by 2 times the interconnect loss between them.  
For this reason, I like loss line and switch cards, and prefer to use low loss 
practices in backplanes.  Thus the best place to place an DC blocking capacitor 
is as far away as possible from other discontinuities.
 Oftentimes the worst discontinuity is the receiver, due to excessive 
capacitance.  Placing a capacitor near the receiver can sometimes be 
counterproductive.


Finally, what is the correct value ... it depends on the low frequency limit of 
your serial encoding pattern.  The capacitor must be large enough to reduce low 
frequency wander and jitter problems.

Regards,

Scott

--

Scott McMorrow
Teraspeed Consulting Group LLC
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On Mon, Sep 24, 2012 at 3:05 AM, Tesla <emcesd@xxxxxxx> wrote:

> Hi, Experts
>
> In the last thread,Michael said "Placing them at the RX side generally
> helps to minimize impact of the impedance discontinuity given the
> degradation of edge rate from dispersion"
> Does place the AC capacitor near the receiver really help SI?
> Which is the better place for placing the decoupling capacitor, near
> the transmitter or near the receiver?
> How to choose the value of the decoupling capacitor? it is based on
> the data rate, code style or other things? can we calculate it manually?
>
> Thanks.
>
> Tesla.
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