[SI-LIST] Re: AC coupling Capacitor (Eye Diagram Issue)

Hi again Nitin,

I think the phenomena you are experiencing is due to the fact that the 
differential signal is not being "viewed" at the end of the transmission 
line when the SFP is in-circuit. Maybe you can "pop the hood" on the SFP to 
get a differential scope probe on the destination?

Hardware verification and test (HVT) is still useful in some ways, but for 
something like this, the only way to truly know what the signal will look 
like is to simulate the interface across process, voltage and temperature. 
Signal integrity simulation, I think, is the new HVT because most folks just 
can't get scope probes where they need to be these days. In simulation, the 
signal can be viewed not at the ball/pin of the device package (If you are 
able to pop the hood on the SFP) but at the die. There is no better place to 
make these measurements (waveforms will exhibit what you describe even at 
the package pin of some devices). Accurate simulation is the key to success 
though...

The termination network you described is one useful for reducing common mode 
noise...

Best regards,
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>
Cc: <jitender.misra@xxxxxxxxx>; <shreesha.nataraj@xxxxxxxxx>; 
<shankara.chandrashekar@xxxxxxxxx>
Sent: Monday, June 05, 2006 10:39 AM
Subject: [SI-LIST] Re: AC coupling Capacitor (Eye Diagram Issue)


>
> 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
>>
>>
>> The information contained in this electronic message and any
> attachments=0D
>> to=3D
>> this message are intended for the exclusive use of the addressee(s)
> and=3D
>> may contain proprietary, confidential or privileged information. If
> you=3D
>> are not the intended recipient, you should not disseminate, distribute
> or=3D
>> copy this e-mail. Please notify the sender immediately and destroy
> all=3D
>> copies of this message and any attachments.=3D0D
>>
>> WARNING: Computer viruses can be transmitted via email. The recipient=3D
>> should check this email and any attachments for the presence of
> viruses.=3D
>> The company accepts no liability for any damage caused by any virus=3D
>> transmitted by this email.
>> =3D0D
>> www.wipro.com
>> ------------------------------------------------------------------
>> To unsubscribe from si-list:
>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>> or to administer your membership from a web page, go to:
>> http://www.freelists.org/webpage/si-list
>>
>> For help:
>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>> List FAQ wiki page is located at:
>>                http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>>
>> List technical documents are available at:
>>                http://www.si-list.org
>>
>> List archives are viewable at:
>> http://www.freelists.org/archives/si-list
>> or at our remote archives:
>> http://groups.yahoo.com/group/si-list/messages
>> Old (prior to June 6, 2001) list archives are viewable at:
>>  http://www.qsl.net/wb6tpu
>>
>>
>>=0D
>
>
>
>
> The information contained in this electronic message and any attachments 
> to=
> this message are intended for the exclusive use of the addressee(s) and=
> may contain proprietary, confidential or privileged information. If you=
> are not the intended recipient, you should not disseminate, distribute or=
> copy this e-mail. Please notify the sender immediately and destroy all=
> copies of this message and any attachments.=0D
>
> WARNING: Computer viruses can be transmitted via email. The recipient=
> should check this email and any attachments for the presence of viruses.=
> The company accepts no liability for any damage caused by any virus=
> transmitted by this email.
> =0D
> www.wipro.com
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
> List FAQ wiki page is located at:
>                http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
> List technical documents are available at:
>                http://www.si-list.org
>
> List archives are viewable at:
> http://www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>  http://www.qsl.net/wb6tpu
>
>
> 


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: