[SI-LIST] Re: AC cap placement on Clocks - Emailfound in subject - Email found in subject
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Stefan Milnor <stefan.milnor@xxxxxxxxxxxxxx>
- Date: Fri, 23 Oct 2009 17:43:35 -0400
Stefan
The capacitor is placed on the Tx side of a separable interconnect.
When the interconnect is separated (board removed) there is no common
mode termination, and therefore no charging path. When the interconnect
is mated there is 50 ohm termination on each line with a charging time
constant of about 5 microseconds.
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
Stefan Milnor wrote:
> I see in the PCIe base spec section 4.3.5.7 the receiver detect scheme
> described by Scott McMurrow, but I did not see that the spec says anything
> about where the cap has to be placed to make this scheme work.
>
> And in section 4.5.3.1 I see the statement "Capacitors must be placed on the
> transmit side of an interface that can be plugged and unplugged. In a
> topology where everything is located on a single substrate, the capacitors
> may be located anywhere along the channel"
>
> Also, charging a cap is a very slow event, so location would not seem to be
> important.
>
> ________________________________
>
> From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> Sent: Fri 10/23/2009 10:17 AM
> To: Scott McMorrow
> Cc: Stefan Milnor; wolfgang.maichen@xxxxxxxxxxxx; Sam Pete; icer world;
> si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
> Subject: [SPAM] - Re: [SI-LIST] Re: [SPAM] - Re: AC cap placement on Clocks -
> Emailfound in subject - Email found in subject
>
>
> Scott,
>
> Thanks for chiming in!
>
>
>
> ----- Original Message -----
> From: Scott McMorrow <mailto:scott@xxxxxxxxxxxxx>
> To: Lee Ritchey <mailto:leeritchey@xxxxxxxxxxxxx>
> Cc: Stefan Milnor <mailto:stefan.milnor@xxxxxxxxxxxxxx> ;
> wolfgang.maichen@xxxxxxxxxxxx; Sam Pete <mailto:cygnul@xxxxxxxxx> ; icer
> world <mailto:icermail@xxxxxxxxx> ; 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 <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>
> <mailto:leeritchey@xxxxxxxxxxxxx>
> Sent by: si-list-bounce@xxxxxxxxxxxxx
> 10/22/2009 06:39 AM
>
> To
> "icer world" <icermail@xxxxxxxxx> <mailto:icermail@xxxxxxxxx> ,
> "Sam Pete" <cygnul@xxxxxxxxx> <mailto: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>
> <mailto:icermail@xxxxxxxxx>
> To: Sam Pete <cygnul@xxxxxxxxx>
> <mailto:cygnul@xxxxxxxxx> ; <si-list@xxxxxxxxxxxxx>
> <mailto: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>
> <mailto: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
> <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
> <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
> <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
> <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
> <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
>
>
>
>
--
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
------------------------------------------------------------------
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:
