[SI-LIST] Re: AC Coupled Signals
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Stephen Zinck <signalintegrity@xxxxxxxxxxx>
- Date: Tue, 25 Sep 2007 09:44:16 -0400
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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