[SI-LIST] Re: AC Coupled Signals
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: signalintegrity@xxxxxxxxxxx
- Date: Tue, 25 Sep 2007 08:30:37 -0400
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® is the registered service mark of
Teraspeed Consulting Group LLC
Stephen Zinck wrote:
> Hi Jory,
>
> I have simulated this at length and concur with your experience that the
> capacitor is best placed at the receiver...
>
> In effect, the attenuation associated with the capacitor placement at the
> receiver (parasitics/pads/vias) is lost in the rise-time degradation of the
> system.
> The classic "don't break it until you have to" rule is applicable... OK this
> 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 -----
> 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)
>> in the lab some effects that appears to be location specific in the
>> placement of the AC coupling caps at the rcvr. Now this may be due in part
>> to the fact that I am using 50-ohm resistor termination in each lead as
>> well and the combination (cap plus rcvr reflection) is giving some
>> imbalance depending on distance. The best rcvr eye that I am seeing is
>> 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
>> 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
>> 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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