[SI-LIST] Re: subscribe
- From: Lee Ritchey <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "lritchey49" for DMARC)
- To: "dmarc-noreply@xxxxxxxxxxxxx" <dmarc-noreply@xxxxxxxxxxxxx>
- Date: Fri, 26 Feb 2021 23:33:16 +0000 (UTC)
The PCI express protocol required an AC capacitor on all links even if they
stay on the same board. This is, so that at startup the switch fabric chip
knows which outputs have loads on them. Â
Lee RitcheySpeeding Edge
P.O. Box 817Bodega Bay, CA94923408-781-0253lritchey49@xxxxxxx
Worry is like a rocking chairIt keeps you busy, but it doesn't get you
anywhere.
I just took the energy it takes to get madand wrote some blues.Count Basie
It continues to baffle me that so many people who are intolerant and hatefulÂ
call themselves Christians.
-----Original Message-----
From: Sherman Chen <dmarc-noreply@xxxxxxxxxxxxx>
Cc: si-list@xxxxxxxxxxxxx
Sent: Fri, Feb 26, 2021 12:15 pm
Subject: [SI-LIST] subscribe
Regards,
Sherman Shan Chen
SI/PI Group, Kandou BusÂ
5 Queensbridge, Northampton, UK, NN4 7BF
Office: +44 1604 635826Â Email: shan@xxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On
Behalf Of Chuck Corley
Sent: Sunday, August 2, 2020 4:10 PM
To: yourstechnically.yt@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: AC coupling capacitors
Hi Mina,
I see Istvan posted a great link on this subject for you. I suggest
taking his advice and spending some time to study that info before posting
too many additional questions:
You may also find this Samtec gEEk spEEk presentation useful:
https://vimeo.com/426327915
For PCIe case, where we have to place AC cap near Tx do we need to
place the AC Cap on the first/base brd?
On U.2 SSD, we have AC cap in it. In a case like TX->multiple
brds->U.2., do we need AC cap in such a case?
In general only one AC coupling cap is needed between transmitter and
receiver. If the ground reference and signaling levels between devices is
compatible and fixed and equal and never changes you may not need coupling
caps. But you have to make sure all of those things are always going to
be true always. (... My use of "always" twice in the last sentence was
not a typo ... ground levels can float or change between
boards or devices in some systems and then AC coupling is required).Â
But if you're using a standard like PCIe that requires coupling caps in
the standard they may still be required, on the TX side, usually near the
transmitter device.
Chuck Corley
On 2020-08-02 07:48, Mina Tyagi wrote:
Hi Chuck,
Thanks a lot for the explanation.
For PCIe case, where we have to place AC cap near Tx do we need to
place the AC Cap on the first/base brd?
On U.2 SSD, we have AC cap in it. In a case like TX->multiple
brds->U.2., do we need AC cap in such a case?
On Sun, 2 Aug 2020 at 19:59, Chuck Corley <corley@xxxxxxxxx> wrote:
Hi Mina,
There are a few different reasons, but here are the most important
reasons:
Why do we use AC coupling capacitors in high-speed signals?
When signals travel between different boards or units or locations the
destination location could have a different level on it's ground. So
the transmitter and receiver could have different reference levels. A
different ground level would cause the signal to appear to have a
level shift that might make it's level out of range for the receiver.Â
AC coupling allows the receiver to change (bias) the signal reference
levels to be compatible with the receiver's input levels.
Why do we place it near to TX?
It's slightly arbitrary. Often it's defined by a standard and they
pick one end or the other. Sometimes depending on the standard they
are specified on one end or the other end or occasionally on both ends.
(...
But putting them on on both ends is usually not a good idea for
capacitive loading reasons).
Be aware that PCIe (PCI Express) specifies the AC coupling caps on the
TX side, and there are methods it uses for detecting if a downstream
device is connected that depends on that particular AC coupling
configuration.
Hope this helps.
Chuck Corley
On 2020-08-02 06:31, Mina Tyagi wrote:
Hi SI listors,
Why do we use AC coupling capacitors in high-speed signals?
Why do we place it near to TX?
Regards,
Mina
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