[SI-LIST] Re: AC Coupled Signals

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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>>>>
>>>>
>>>>
>>>>     
>>>> ____________________________________________________________________________________
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