[SI-LIST] Re: AC Coupled Signals
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Stephen Zinck <signalintegrity@xxxxxxxxxxx>
- Date: Tue, 25 Sep 2007 11:08:14 -0400
Steven,
I would not agree with your following statements.
"I agree in theory with all you state. Assuming a lossless interface to
the capacitor, it shouldn't matter where you place it, given a purely
linear system. But the real world is lossy, even when one makes great 3D
solved structures. Manufacturing and other tolerances tend to take the
trek towards perfection to task."
"Would either of you agree that AC coupling capacitor location may
matter with a lossy interface to the capacitor?"
Insertion loss in a flat impedance linear lossy system will be
independent of capacitor location. Run the math and see. The only time
position matters is in the face of discontinuities. In fact, given a
low loss interconnect with discontinuities and a high loss interconnect
with discontinuities, the low loss system, with it's higher Q, will
often have worse behavior.
An improperly designed 0402 capacitor transition for a 50 ohm line can
easily exhibit a discontinuity of 35 ohms for 50 ps. If attached to
poorly designed via transitions, the discontinuity will be even worse.
When this is coupled closely to a high capacitance receiver input, a
high capacitance transmitter output, a low impedance via stub
discontinuity, or a low impedance connector discontinuity, it can form a
1/2 wave resonant circuit. This is most likely the problem you are
seeing.
If the interconnect has essentially flat impedance, position does not
matter. If the capacitor transition is properly designed, position does
not matter. All of the data we have on this is proprietary at this
time. Our understanding of the physics has been verified by full wave
modeling, simulation and measurement.
regards,
Scott
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 Scott and Steve,
>
> To answer both of your questions, it is the resulting Hspice (with
> S-parameters) differential eye patterns, as viewed at the receiver
> die, that were used to make a comparison of source versus destination
> AC coupling capacitor locations. The system was excited with a string
> of ones, followed by a single zero, followed by a string of ones.
>
> I have not specifically designed a test board that varies the AC
> coupling capacitor location along a trace.
>
> I understand the "shades of gray" here and agree that one can't make a
> "rule of thumb" generalization in our line of work these days.
>
> I agree in theory with all you state. Assuming a lossless interface to
> the capacitor, it shouldn't matter where you place it, given a purely
> linear system. But the real world is lossy, even when one makes great
> 3D solved structures. Manufacturing and other tolerances tend to take
> the trek towards perfection to task.
>
> Do either of you have real world measured results, that you could
> share, that show no marked difference in received signal
> characteristics when the AC coupling capacitor position is varied
> through a 30 inch backplane system (or similar)?
>
> I believe my experience with capacitor location may prove true if the
> capacitor interface is lossy (which is the case). A lot of my
> customers are just looking for quick ways to maximize performance
> using standard component packages and standard layout practices (in
> the end, I don't like to give anything away that is low lying fruit).
> Most of the time I am doing my analysis after the board is in layout,
> where I have limited ability to change the design (unless it is really
> broken). In a perfect world, where I am involved early, the package
> optimization and layout structures can be optimized as you state, but
> only if the margins warrant it (system performance issues are expected
> after initial "what-if" simulations have occurred). The right tool for
> the right job rules the day...
>
> Would either of you agree that AC coupling capacitor location may
> matter with a lossy interface to the capacitor?
>
> 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
> <mailto:szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
> Web - www.interconnectengineering.com
> <http://www.interconnectengineering.com>
>
>
> ----- Original Message -----
> *From:* Scott McMorrow <mailto:scott@xxxxxxxxxxxxx>
> *To:* Stephen Zinck <mailto:signalintegrity@xxxxxxxxxxx>
> *Cc:* jory_mckinley@xxxxxxxxx <mailto:jory_mckinley@xxxxxxxxx> ;
> leeritchey@xxxxxxxxxxxxx <mailto:leeritchey@xxxxxxxxxxxxx> ;
> npatel@xxxxxxxxxx <mailto:npatel@xxxxxxxxxx> ;
> si-list@xxxxxxxxxxxxx <mailto:si-list@xxxxxxxxxxxxx>
> *Sent:* Tuesday, September 25, 2007 9:44 AM
> *Subject:* Re: [SI-LIST] Re: AC Coupled Signals
>
> Stephen
>
> Define "better" and then relate your simulations and conclusions
> to linear system theory and measurements.
>
> I contend that the only difference an AC coupling capacitor can
> possibly have due to position in a linear interconnect is a result
> of impedance mismatch. I contend that the capacitor will form a
> 1/2 wave resonant circuit with other interconnect discontinuities
> (connectors, vias stubs, packages, Tx die, Rx die ... etc) and
> that this interaction is system, chip, connector and package
> design dependent. I contend that it is this 1/2 resonance that
> can cause differences that can be measured, but that there is no
> "rule of thumb", since the position and magnitude of
> discontinuities are different in every system. In some systems
> the receiver constitutes a larger discontinuity than the
> transmitter. In other systems this is reversed. In yet other
> systems, connectors and vias represent larger discontinuites than
> do either the transmitters or receivers. It all "just depends".
> To state a specific rule is just plain incorrect.
>
> I contend that once you remove the magic and myths surrounding AC
> coupling capacitors, analysis of the 3D structure shows that by
> reducing the signal path discontinuity through the capacitor, you
> will necessarily improve performance. An AC coupling capacitor,
> with it's associated via and pad transition design, can be viewed
> as a black box which has insertion loss and return loss, and can
> be modeled quite well using either lumped element approximations
> or (my favorite) S-parameters. As such it will cascade in a
> simulation model just like any other linear element. If we start
> with a system with flat 50 ohm impedance from end to end, it can
> be easily shown that no matter what the position of the capacitor
> along the interconnect is, the insertion loss of the system is
> identical. It is only the return loss, as seen from each end that
> changes.
>
> I've been designing AC coupling capacitor mounting transitions
> properly for quite a few years now and have some 0402 designs that
> keep S12 above -0.2 dB up to 7.5 GHz, S12 below -20 dB @ 5 GHz,
> and below -15 dB @ 10 GHz. For all practical purposes, these
> designs are transparent and may be placed anywhere in an
> interconnect design where there is space, since there is little
> resonance interaction with other devices and structures.
>
>
> Scott
>
> 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 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
>> <mailto:szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
>> Web - www.interconnectengineering.com
>> <http://www.interconnectengineering.com>
>>
>>
>> ----- Original Message -----
>> *From:* Scott McMorrow <mailto:scott@xxxxxxxxxxxxx>
>> *To:* signalintegrity@xxxxxxxxxxx
>> <mailto:signalintegrity@xxxxxxxxxxx>
>> *Cc:* jory_mckinley@xxxxxxxxx
>> <mailto:jory_mckinley@xxxxxxxxx> ; leeritchey@xxxxxxxxxxxxx
>> <mailto:leeritchey@xxxxxxxxxxxxx> ; npatel@xxxxxxxxxx
>> <mailto:npatel@xxxxxxxxxx> ; si-list@xxxxxxxxxxxxx
>> <mailto:si-list@xxxxxxxxxxxxx>
>> *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® 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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>>>>
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>>>>
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