[SI-LIST] Re: TO define the "Preemphasis"
- From: JC Darden <jcdarden@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sun, 07 Jul 2002 22:16:44 -0700
I'll be surprised if anyone can justify the xilinx definition (explanation)
of pre-emphasis in it's entirety.
Certainly this type of signal "pre-emphasis" (pre-compensation) can improve
hi freq losses if properly done. What it does to phase and ISI may be
another story. The results will show up in the eye pattern of a digital stream.
What is happening, it appears to me, is that hi-freqs are being emphasized
in an attempt to overcome a Lossy Low Pass Filter (LPF) characteristic of a
transmission line. Note: LPF's have "dispersion"
So, call it pre-emphasis -- but it is more like what is generally call
pre-compensation or pre-distortion. This is not, of course, the analog
signal pre-emphasis concept where the notion of de-emphasis is employed on
the receiving end - to improve overall Signal to Noise ratio.
To say this compensates (overcomes?) LPF dispersion may be correct if it
sufficiently pre-compensates or pre-distorts in such a way that the result
after the Filter (transmission line) is more like the original signal -- in
an important way.
Good Luck dong this as a general case. But it is possible in specific
cases. The eye diagram will tell the story.
Additionally, I have not seen a case where dispersion was a problem in PCB
traces. If you or anyone find such a case, let me know please as I have
been searching for this.
Let's see what other have to say.
Sincerely,
JC Pat Darden
JC Darden Consulting
At 09:39 AM 7/8/02 +1000, you wrote:
>There is something about that Xilinx text (it can be found at=20
>http://www.xilinx.com/publications/products/v2pro/handbook/ug012_glossary.p=
>df) which left me thinking.
>
>The thing that puzzles me is that they specifically mention dispersion. No=
>w, I understand how pre-ephasis will help in the presence of all the othe=
>r effects mentioned, but how can pre-emphasis "magically" counter dispers=
>ion?
>
>Could somebody please share his/her insight into this?
>
>Thanks,
>
>Sandor
>
>---
>Sandor Daranyi
>Senior Design Engineer
>Aristocrat Technologies Australia Pty Limited
>
>
> > -----Original Message-----
> > From: DoraBabu - CTD, Chennai. [mailto:dbabu@xxxxxxxxxxxxxxx]
> > Sent: Saturday, 6 July 2002 2:27
> > To: lincj@xxxxxxxxxxxxxxxxxxx; hpc@xxxxxxxxxx
> > Cc: SI_LIST (E-mail)
> > Subject: [SI-LIST] Re: TO define the "Preemphasis"
> >=20
> >=20
> >=20
> >=20
> > Got it from Xilinx Signal Integrity glossary :=20
> >=20
> > Pre emphasis is magnitude boosting of high frequency spectral=20
> > components
> > before launching the signal (wave) onto the Transmission=20
> > Line. Transmission
> > Lines embedded in most standard PCB materials (FR4, Rogers=20
> > 43xx, Nelco and
> > Rogers) suffer varying degrees of dispersion and loss in the=20
> > 1 gigahertz
> > spectrum. This is mostly due to conductance losses (leakage=20
> > from the copper
> > trace to any other conducting structure) and Skin Effect.=20
> > Dispersion is a
> > phenomenon whereby spectral components travel at different=20
> > velocities. The
> > waveform looks smeared when it arrives at the receiver.=20
> > Both of these "characteristics" play into a diminished and=20
> > poorly received
> > signal. By boosting the high freq. spectral components, the=20
> > magnitude of
> > these components can be diminished as the wave travels through the
> > Transmission Line, but since it starts out larger than the=20
> > lower frequency
> > components, the composite signal arrives at the receiver=20
> > looking the way it
> > was intended.=20
> >=20
> > Pre Emphasis is done by simply increasing the maximum amplitude of the
> > signal for one bit period. If the signal is 1 bit in
> > duration, the amplitude
> > is allowed to rise to a value which is some percentage
> > greater in magnitude.
> > At this point, if the signal is to stay at the same logic
> > state, the driver
> > sends a decreased magnitude signal, or nominal logic level.
> > Every time a
> > transition occurrs, the greater magnitude level is used. For
> > all times after
> > this that the same level is to be transmitted, the nominal
> > magnitude is
> > used.
> >
> > --Regards
> > Dorababu
> >
> > -----Original Message-----
> > From: Miller [mailto:lincj@xxxxxxxxxxxxxxxxxxx]
> > Sent: Friday, July 05, 2002 5:38 PM
> > To: hpc@xxxxxxxxxx
> > Cc: SI_LIST (E-mail)
> > Subject: [SI-LIST] Re: TO define the "Preemphasis"
> >
> >
> >
> > You can also check xilinx's Multi-Gigahertz Signaling FAQ
> > http://www.xilinx.com/xlnx/xil_prodcat_product.jsp?title=3Dsi_gigfaq
> >
> > Best Regards
> >
> > Miller Lin
> >
> >
>
>
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