[SI-LIST] Re: Advice

Justin,

This is a linear system.  The capacitor (and it's associated transition 
pads, vias and traces) have exactly the same characteristics no matter 
where it is placed.  Your problem has absolutely nothing to do with the 
fact that this is a capacitor.  It is just another discontinuity in the 
transmission channel that must be modeled and simulated to ensure good 
signal fidelity at the receiver.   The capacitor value will impact the 
low frequency cut off.  The size of the capacitor body, location of the 
plates in the body, size of the pads, length of the vias, and trace 
layer, will determine the high frequency performance.  The length of 
traces leading to and from the capacitor will determine the frequency of 
any half-wave resonances formed by the capacitor discontinuities, the IC 
package and the backplane connector, no matter whether it is placed on 
the Tx or Rx side.  Model and simulate each of these.

regards,

Scott


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Smith, Justin D. wrote:
> I'm working on a 6.25Gbps channel SI issue with a constraint that is not 
> ideal. We have a legacy issue which requires that we put decoupling caps on 
> the "transmit" side of a transmission line instead of the receive side (where 
> we'd like them). This must be done to maintain "backwards compatibility" with 
> MODULE 2 that does not have decoupling on the receiver side.
>
> That said, we have this type of setup:
>
> ==================================================================
> #      MODULE 1              BACKPLANE           MODULE 2        #
> # XMT     DCPL        BP.                 BP.               RCVR #
> # |\      CAP         Conn     Trace      Conn              |\   #
> # | \_____||__________[  ]________________[  ]______________| \  #
> # | /     ||          [  ]                [  ]              | /  #
> # |/                                                        |/   #
> =============================Fig 1================================
> (hope that cheesy ascii graphic comes through)
>
> Some questions arise that I would value the SI-LIST opinion on:
>
> 1)Where to put the decoupling cap with the constraint that we have to put it 
> on MODULE 1, which is the XMT side of the overall channel?
>
>       a. Near to the transmitter (possibly greater reflected power into the 
> transmitter?)
>       b. Near the BP connector (as close to the RCV as we can? makes the 
> discontinuity more capacitive - but "lumps" issue with connector 
> discontinuity).
>       c. In the middle (distribute the discontinuities along the path - 
> possibly results in multiple reflections?).
>
> 2) Knowing that the backplane trace is very short (~1.5") does that effect 
> how you'd advise? 
>
> 3) Parameters that I can work with are the length of the trace from FPGA to 
> connector on the module, location of the cap, and cap value/size.
>
> We have Hyperlynx as a SI tool. Thoughts? Anybody ever dealt with an issue 
> like this before?
>
> -justin
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