[SI-LIST] Re: De-emphasis in 3GIO
- From: "Ed Priest" <epriest@xxxxxxxxxxxxxxxxxxx>
- To: <esayre@xxxxxxxx>, <Christopher_Brewster@xxxxxxxx>,<g.fuentes@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 21 Feb 2003 16:05:35 -0800
What is needed to compensate for the low pass transfer function of the
interconnect is to provide a high pass zero(s). This zero can be
accomplished with or without gain (without - it's a de-emphasizing of
the low frequencies) with gain you can accomplish a boosting of the high
frequencies and keep the lower frequency gain the same. The goal of both
is to flatten the overall frequency response of the
channel/driver/receiver.=20
Pre-emphasis means at the transmitter and is usually accomplished by
driving the first bit harder that the later bits. I.E. - a 0110 has the
first one driven harder than the second one. This same approach is
called both pre-emphasis and de-emphasis in papers (are you de-emphasing
the second bit or emphasing the first bit?). You can get more
complicated schemes where on a 01110 pattern the first 1 is driven
harder than the 2nd which is driver harder than the 3rd. etc.=20
For both cases you must be careful because the ratio or the high and low
frequencies change - you can get reflections and crosstalk caused by the
harder driven bit hitting the relaxed softer bits. The high low eye
ratio is a function of the overall frequency flatness achieved and is
independent of what approach you take.=20
Thanks
Ed
-----Original Message-----
From: Dr. Edward P. Sayre [mailto:esayre@xxxxxxxx]=20
Sent: Friday, February 21, 2003 2:27 PM
To: Christopher_Brewster@xxxxxxxx; g.fuentes@xxxxxxxxx;
si-list@xxxxxxxxxxxxx
Cc: breda@xxxxxxxx
Subject: [SI-LIST] Re: De-emphasis in 3GIO
Colleagues:
That is not strictly correct. What de-emphasis does is lower the=20
low-frequency spectral content to "flatten-out" the spectrum. It is
what=20
one does in "passive equalization". It is also possible to provide=20
selective high frequency boost to accomplish the same thing, but one has
to=20
be careful not to over-drive the high frequencies and cause the envelope
high-low eye limits to be exceeded. It all looks like control systems
but=20
for non-linear electronics and at a lot higher frequencies!!!
ed
PS: Don't forget DesignCon East June 23, 24 in the Boston area! Great
time=20
to come to New England - bring the family.
=3D=3D=3D=3D=3D=3D=3D
At 02:05 PM 2/21/2003 -0600, Christopher_Brewster@xxxxxxxx wrote:
>Gabriel
>
>As I understand it, and somebody correct me if I am wrong, de-emphasis
is
>just another term for the same concept as pre-emphasis. In most SERDES
>designs, it is called pre-emphasis, however, most communications guys
use
>the term de-emphasis due to the fact that the DC gain is,
"de-emphasized."
>Technically, it means the same thing.
>
>-Chris Brewster
>
>-----Original Message-----
>From: Fuentes, Gabriel [mailto:g.fuentes@xxxxxxxxx]
>Sent: Tuesday, February 18, 2003 10:04 AM
>To: [SI-LIST]
>Subject: [SI-LIST] De-emphasis in 3GIO
>
>
>
>Hi all.
> I am looking for a document describing the de-emphasis technic used
in the
>3GIO physical layer. I need to understand how does it works, how it
helps
>to reduce ISI, etc.
> I understand the Tx pre-emphasis in RS-485 communications, but I have
not
> been able to deduce de de-emphasis in PCI-E
>
>Gabriel
>
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