[SI-LIST] Re: AC Coupled Signals

Charles, I would consider putting a capacitor near a connector provided 
it doesn't create excessive impairment and meets some other goal. 

I understand the idea of trying to group "badness" together.  That can 
back-fire.  There is also some surprises that we can run into near 
connectors with anything that is more 3D than etch.  Being fanatics we 
at the Ministry of the Transparent Transmission Channel try to minimize 
peaks and dips.  So we first work to get each element as transparent as 
possible by itself.  Where we find dips and peaks we would rather do 
without we look at our options to:

*Compensate one with another ( Works when Murphy smiles on us and we are 
particularly inspired )
*Minimize the aggravating effect of one on another
 - Relative location, transition design etc
 - Add a compensation mechanism

With that in mind we normally avoid grouping impairments that are 
similar because it tends to aggravate dips and peaks.  The thing that we 
always have to keep our eyes on are electrical distances that translate 
to integer multiples of quarter half-wavelength.


Best Regards,


Steve.
Grasso, Charles wrote:
> Steve - Would you agree that if the impedance mismatch introduced by the
> cap is a lot less than the connector then having the cap at the
> connector makes sense.? =20
>
>
> Best Regards
> Charles Grasso
> Compliance Engineer
> Echostar Communications Corp.
> Tel: 303-706-5467
> Fax: 303-799-6222
> Cell: 303-204-2974
> Pager/Short Message: 3032042974@xxxxxxxxx
> Email: charles.grasso@xxxxxxxxxxxx
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of steve weir
> Sent: Tuesday, September 25, 2007 7:17 AM
> To: signalintegrity@xxxxxxxxxxx
> Cc: Scott McMorrow; jory_mckinley@xxxxxxxxx; leeritchey@xxxxxxxxxxxxx;
> npatel@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: AC Coupled Signals
>
> Stephen, what are you observing that you think is different at one end=20
> versus the other?  What have you done to determine that the difference=20
> you see in your simulations:
>
> * Correlate to real results
> * Is due to position at one end of the line versus the other rather than
>
> some other inadvertent difference introduced in different test cases
>
> I suspect the latter.
>
> Regards,
>
>
> Steve
> Stephen Zinck wrote:
>   
>> Hi Scott,
>> My simulations show that the capacitor is best placed at the receiver
>>     
> end of the transmission-line. Do you disagree? If so, why?
>   
>> Steve
>>
>> Stephen P. Zinck
>> Interconnect Engineering Inc.
>> P.O. Box 577
>> South Berwick, ME 03908
>> Phone - (207) 384-8280
>> Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx
>> Web - www.interconnectengineering.com
>>
>>   ----- Original Message -----=20
>>   From: Scott McMorrow=20
>>   To: signalintegrity@xxxxxxxxxxx=20
>>   Cc: jory_mckinley@xxxxxxxxx ; leeritchey@xxxxxxxxxxxxx ;
>>     
> npatel@xxxxxxxxxx ; si-list@xxxxxxxxxxxxx=20
>   
>>   Sent: Tuesday, September 25, 2007 8:30 AM
>>   Subject: Re: [SI-LIST] Re: AC Coupled Signals
>>
>>
>>   Stephen,
>>
>>   I'm sorry, this is a linear system.  Except for possible resonances
>>     
> that are created by discontinuities and modal conversion (which have
> absolutely zero to do with signal rise time), there is no difference in
> the attenuation of  a capacitor placed at the Tx as opposed at the Rx.
> W.R.T. the receiver, if it is "lost in the rise-time degradation of the
> system", it will be lost wherever it is placed.
>   
>>
>> Scott McMorrow
>> Teraspeed Consulting Group LLC
>> 121 North River Drive
>> Narragansett, RI 02882
>> (401) 284-1827 Business
>> (401) 284-1840 Fax
>>
>> http://www.teraspeed.com
>>
>> Teraspeed(r) is the registered service mark of
>> Teraspeed Consulting Group LLC
>>
>>
>>   Stephen Zinck wrote:=20
>> Hi Jory,
>>
>> I have simulated this at length and concur with your experience that
>>     
> the=20
>   
>> capacitor is best placed at the receiver...
>>
>> In effect, the attenuation associated with the capacitor placement at
>>     
> the=20
>   
>> receiver (parasitics/pads/vias) is lost in the rise-time degradation
>>     
> of the=20
>   
>> system.
>> The classic "don't break it until you have to" rule is applicable...
>>     
> OK this=20
>   
>> is my rule... :-)
>>
>> All the best,
>> Steve
>>
>> Stephen P. Zinck
>> Interconnect Engineering Inc.
>> P.O. Box 577
>> South Berwick, ME 03908
>> Phone - (207) 384-8280
>> Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx
>> Web - www.interconnectengineering.com
>>
>> ----- Original Message -----=20
>> From: "Jory McKinley" <jory_mckinley@xxxxxxxxx>
>> To: <leeritchey@xxxxxxxxxxxxx>; <npatel@xxxxxxxxxx>;
>>     
> <si-list@xxxxxxxxxxxxx>
>   
>> Sent: Monday, September 24, 2007 5:31 PM
>> Subject: [SI-LIST] Re: AC Coupled Signals
>>
>>
>>   I will elaborate a bit on what I have seen. I have measured (time
>>     
> domain)=20
>   
>> in the lab some effects that appears to be location specific in the=20
>> placement of the AC coupling caps at the rcvr.  Now this may be due in
>>     
> part=20
>   
>> to the fact that I am using 50-ohm resistor termination in each lead
>>     
> as=20
>   
>> well and the combination (cap plus rcvr reflection) is giving some=20
>> imbalance depending on distance.  The best rcvr eye that I am seeing
>>     
> is=20
>   
>> when I can move the AC/term as close to the rcvr as I can.  By the way
>>     
>
>   
>> these are 5Gb/s signals.
>> If I have time I will try and isolate what I am seeing and even
>>     
> simulate=20
>   
>> it, has anyone else seen or simulated this?
>> -Jory
>>
>> ----- Original Message ----
>> From: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
>> To: "npatel@xxxxxxxxxx" <npatel@xxxxxxxxxx>; si-list@xxxxxxxxxxxxx
>> Sent: Monday, September 24, 2007 1:06:06 PM
>> Subject: [SI-LIST] Re: AC Coupled Signals
>>
>> Nikil,
>>
>> I have made measurements on test PCBs and the location is not all that
>> important.  In identical pairs, one with AC coupling capacitors and
>>     
> the
>   
>> other without, the loss vs. frequency is virtually identical at leas
>>     
> out=20
>   
>> to
>> 6 GHz.  That would be 12 Mb/S.
>>
>> Lee Ritchey
>>
>>
>>     [Original Message]
>> From: <npatel@xxxxxxxxxx>
>> To: <si-list@xxxxxxxxxxxxx>
>> Date: 9/24/2007 10:21:37 AM
>> Subject: [SI-LIST] AC Coupled Signals
>>
>> Hi all,
>> In case of AC coupled signals does anyone know of an optimum placement
>> for the caps? I mean should they be placed near the source, receiver,
>> middle of  the transmission line?
>> How much difference does it make in the opening of the eye?
>> The signals are differential CML running at 3.0Gbps
>>
>> Thanks,
>> Nikhil
>>
>>
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> --=20
> Steve Weir
> Teraspeed Consulting Group LLC=20
> 121 North River Drive=20
> Narragansett, RI 02882=20
>
> California office
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> (707) 780-1951 Fax
>
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-- 
Steve Weir
Teraspeed Consulting Group LLC 
121 North River Drive 
Narragansett, RI 02882 

California office
(408) 884-3985 Business
(707) 780-1951 Fax

Main office
(401) 284-1827 Business 
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Oregon office
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http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of 
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