[SI-LIST] Re: AC Coupled Signals
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Stephen Zinck <signalintegrity@xxxxxxxxxxx>
- Date: Tue, 25 Sep 2007 10:36:23 -0400
A slight correction in my previous posting.
"S12 below -20 dB @ 5 GHz, and below -15 dB @ 10 GHz."
should be:
S11 below -20 dB @ 5 GHz, and below -15 dB @ 10 GHz.
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
Scott McMorrow wrote:
> 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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