[SI-LIST] Re: [SPAM] - Re: AC cap placement on Clocks - Emailfound in subject
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Scott McMorrow" <scott@xxxxxxxxxxxxx>
- Date: Fri, 23 Oct 2009 10:17:02 -0700
Scott,
Thanks for chiming in!
----- Original Message -----
From: Scott McMorrow
To: Lee Ritchey
Cc: Stefan Milnor; wolfgang.maichen@xxxxxxxxxxxx; Sam Pete; icer world;
si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Sent: 10/23/2009 9:37:11 AM
Subject: Re: [SI-LIST] Re: [SPAM] - Re: AC cap placement on Clocks - Emailfound
in subject
Location of the DC block capacitor in PCIe systems has nothing to do with
signal integrity. PCIe capacitors are a special case. The capacitor is used in
the receiver presence detection circuit. By placing the capacitor at the
transmitter side, presence or absence of a receiver can be detected by the
charging time constant when a common mode voltage is applied during
initialization. If no receiver is present, the Tx is disabled.
In systems where the capacitor is not part of a card presence detection
circuit, it may be placed anywhere. I usually recommend optimizing the
capacitor transitions to minimize return loss (reflections) and by placing it
in the area of the system architecture with the lowest routing density. In hub
backplane/midplane architectures, it is advantageous to place the blocking
capacitors on just the line cards, irrespective of whether it is near a Tx or
Rx. This reduces congestion on the hub or switch card where signal density is
often highest.
--
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
Lee Ritchey wrote:
App notes do not always contain instructions that are well proven.
----- Original Message -----
From: Stefan Milnor
To: wolfgang.maichen@xxxxxxxxxxxx;Lee Ritchey
Cc: Sam Pete; icer world; si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Sent: 10/22/2009 1:52:21 PM
Subject: RE: [SPAM] - [SI-LIST] Re: AC cap placement on Clocks - Email found in
subject
Experts -
If the physical placement of the caps does not matter, why do vendors like
Intel advise us to place them close to the transmitters, for PCIe use?
No matter how carefully you place the caps and route the pairs, it seems that
having them (and the vias and layer changes etc) causes an impedance bump of
some sort, and for this reason, it would be better to have them close to the
source package.
Just my amateur opinion - Stefan M. @ Kontron
From: si-list-bounce@xxxxxxxxxxxxx on behalf of wolfgang.maichen@xxxxxxxxxxxx
Sent: Thu 10/22/2009 10:15 AM
To: Lee Ritchey
Cc: Sam Pete; icer world; si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SPAM] - [SI-LIST] Re: AC cap placement on Clocks - Email found in
subject
Hello Sam,
an ideal capacitor will be completely transparent and high-frequency
signal (down to the cutoff frequency - the capacitor C in combination with
the Thevenin-equivalent line impedance of 2*Zo form a high-pass filter
with a time constant of 2*Zo*C). So from that standpoint it does not
matter at all where you place it, as Lee already stated.
Of course a real capacitor always has some parasitic package and mounting
inductance, so if you go to very high frequencies or data rates
(multi-Gbit/sec) you will end up seeing reflections caused by that.
Although at your speeds (below a GHz) that won't be an issue unless you
reeally mess up the design (e.g. use a through-hole capacitor instead of a
good surface mount ceramic one). In that case it will be better in improve
your design rather than try to find a "sweet spot" for the placement which
will make your design very sensitive against any changes (e.g. line
length, other parasitics). Failing that, the best bet is probably to place
it very close (within ~1/4th of the shortest wavelength of interest, given
by the frequency 0.33/rise_time) to either your driver or your receiver.
The second consideration would be whether you put in the AC coupling
purely for signaling reasons (e.g. to avoid debiasing driver or receiver),
or whether it shall also act as protection. If e.g. it shall protect the
driver against short circuits (e.g. if driver and receiver reside on
different boards that get hot-plugged together), it may be better to place
the cpacitor on the driver side. But that willd depend on the specific
design.
Wolfgang
"Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
10/22/2009 06:39 AM
To
"icer world" <icermail@xxxxxxxxx>, "Sam Pete" <cygnul@xxxxxxxxx>,
si-list@xxxxxxxxxxxxx
cc
Subject
[SI-LIST] Re: AC cap placement on Clocks
Form an SI point of view, it does not matter where along the path the
capacitors are placed.
[Original Message]
From: icer world <icermail@xxxxxxxxx>
To: Sam Pete <cygnul@xxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Date: 10/21/2009 8:48:26 PM
Subject: [SI-LIST] Re: AC cap placement on Clocks
It's hard to say where the AC cap should be placed ,so you'd better do a
simulation if you have device models .
I'm in doubt that why you use the AC coupling manner since the driver
and
the receiver are both LVDS level.
________________________________
From: Sam Pete <cygnul@xxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Sent: Thu, October 22, 2009 7:22:17 AM
Subject: [SI-LIST] AC cap placement on Clocks
Hi All,
I have a situation like this:
LVDS Driver, ac cap, LVDS receiver (internal term) 156MHz clock.
|>-----------------||----------------|>
|>-----------------||----------------|>
What is the optimum place to put coupling cap when the clock is
ac-coupled.
Should it be close to driver or should it be close to receiver.
From my understanding, the discontinuity should not be visible to the
electrical length of the signal. Having said that, ac cap should be as
close as possible to Driver.
please share your thoughts.
Thanks
Sam
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
www.qsl.net/wb6tpu
The information contained in this document is CONFIDENTIAL and property of
Kontron. Any unauthorized review, use, disclosure or distribution is prohibited
without express written consent of Kontron. If you are not the intended
recipient, please contact the sender and destroy all copies of the original
message and enclosed attachments.
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
- » [SI-LIST] Re: [SPAM] - Re: AC cap placement on Clocks - Emailfound in subject - Lee Ritchey
