FreeLists / si-list / [SI-LIST] Re: AC coupling Capacitor (Eye Diagram Issue)

[<jain.nitin@xxxxxxxxx>]

Hello,

We have a interface running at 1.25 Ghz between  a Gigabit Switch &
Optical SFP. The SFP datasheet say that it has got internal AC caps and
100 ohm termination. Thus no cap or termination has been provided on
differntial link between SFP & Switch. We have observed the eye diagram
at input of SFP pins when switch is driving. I assume that since the
Caps is internal to SFP, the observed eye diagram is before the AC caps.
The length of this 100 ohm differential line is maximum of 2inches with
P & N limb very closely matched (skew is less than 50 mils). The
reference plane on one side of differential line is ground & on other
side is 3.3V .=0D
The geometry used is differential stripline with  - width: 4.5mils ,
spacing: 8.5 mils=0D

The eye diagram has a swing of 1200mv (600mV at top & -600mV at bottom).
But during the rise time , the signal goes to 400mV from 0 & then goes
to 300mV before reaching 600mV. The same happens during the fall time as
well (signal goes to -400mV from 0 & then goes to -300mV before reaching
-600mV). When we removed the SFP and put 100 ohm termination resistance
between the pins (where the previous eye was observed), we get a proper
eye diagram. What's the exact reason for this behavior? Will the SFP
actually see a proper eye at its internal 100 ohm termination. I can
send the waveforms if anybody wants to have a look.

Also I have one more basic doubt. Normally the termination which are
used for 100 ohm differential line looks like this- one 50 ohm
resistance connected from 'p' & another 50ohm from 'n'. The common
points at which two resistance are connected in series is connected to a
cap (normally 0.1uF) & other end of cap is connected to gnd. What's the
logic behind this. How is it different from putting a 100 ohm resistance
in between.

With regards,
Nitin=0D
-----Original Message-----
From: Stephen Zinck [mailto:signalintegrity@xxxxxxxxxxx]=0D
Sent: Thursday, May 25, 2006 8:04 PM
To: Nitin Jain (WT01 - Broadband Networks); si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] AC coupling Capacitor

Hi Nitin,

In addition to what the other folks have stated, two AC coupling
capacitors, one at the source and one at the destination, have been
suggested by some vendors for hotswap protection of both driver and
receiver.

Also, in my simulation experience, the capacitor package and pad
parasitic losses that inevitably occur may force placement at the end of
the transmission line so these losses are lost in the rise-time of the
entire system loss. Whether this is required can only be determined by
doing margin analysis with the simulated worst case results.

All the best,
Steve

Stephen P. Zinck
Interconnect Engineering Inc.
P.O. Box 577
South Berwick, ME 03908
Phone - (207) 384-8280
Fax - (207) 676-8676
Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx
Web - www.interconnectengineering.com

----- Original Message -----
From: <jain.nitin@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Thursday, May 25, 2006 7:03 AM
Subject: [SI-LIST] AC coupling Capacitor


>
> Hello,
>
> For a gigabit differential pair AC coupling capacitor is used. But
what
> exactly is the use of AC coupling capacitors? I believe that they are
> used when the bias voltage of driver & receiver are different.=3D0D
>
> Also where exactly AC coupling capacitors should be placed when the
> differential trace is running on a backplane from one daughter card to
> other (at driver end or at receiver end).
>
> What will happen if two AC coupling caps are used. One at driver side
> and other at receiver side.
>
> With regards,
> Nitin
>
>
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The information contained in this electronic message and any attachments to=
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 are not the intended recipient, you should not disseminate, distribute or=
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WARNING: Computer viruses can be transmitted via email. The recipient=
 should check this email and any attachments for the presence of viruses.=
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