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Re: [Xylo-SDR] Signal Integrity and Board stackup issues


Sorry that I did not reply to this earlier, but I did NOT forget. Again,
thank you for your contribution. To my way of thinking, this project depends
on the Atlas - 4 layer board we are designing (by expense and loose
agreement). It is the road that other project boards will travel. Also many
other FPGA boards can use the Atlas to work. I think that Bill - KD5TFD
recognized it when he named it! Your obvious design expertise is needed here
in our 'amateur' effort, and would benefit us all. 

In my gross design suggestion I did suggest a 'socket' for termination which
now extends to linked Atlas boards (just move the resistor pack.). However
we first need to design the board before we can design which lines to
terminate. As you point out, the I2C speed won't have to be terminated, but
we would like some high speed tracks to go over this board. 

I think we have agreed that we are not looking at ultra high speeds, or
powers be switched on the Atlas. However it will be a minimum of 4 inches.
If you would volunteer to be project leader on this design team, I'll
promise to get this board produced! I will try to post a Visio of the gross
design we are talking about on Dale's SVN. We can go from there.


-----Original Message-----
From: xylo-sdr-bounces@lists.ae5k.us [mailto:xylo-sdr-bounces@lists.ae5k.us]
On Behalf Of Ray Anderson
Sent: Thursday, January 26, 2006 1:34 PM
To: xylo-sdr@lists.ae5k.us
Cc: Ray Anderson
Subject: Re: [Xylo-SDR] Signal Integrity and Board stackup issues


Whenever an interconnect must be considered a 'transmission line' (i.e.
when the time delay of the line is greater than about 20% of the signal
risetime) then it is generally necessary to terminate the line to
control reflections.

For the case of boards with FR4 dielectric (where the Er is approx 4 and
signals travel at about 6 inches/nsec) the above can be simplified to
produce a rule-of-thumb that says that termination is required when the
line length (in inches) is > than the risetime (in nsec).

So for a typical FPGA driver (say LVDS) that has a risetime of about
350psec  (.35nsec) then for lines greater than about a third of an inch
you should utilize termination.

For slow risetime signals the line can be correspondingly longer before
termination becomes an issue. A 10MHz clock signal with a 10nsec
risetime wouldn't require termination for line lengths less than about
10 inches.

The USB signals have a risetime speced as > 500psec I believe. So lines
over about a half an inch will need termination. (the standard defines
recommended schemes I believe)

Fast I2C specs the ristime at about 300nsec so termination isn't an
issue for lines less than 25 feet. (though there other issues to be

The simplest termination scheme is a resistor of the appropriate value (
Zo) at the far end of the transmission line. For differential lines
(LVDS) the termination scheme is a bit more involved since you need to
terminate both the even and odd modes (this takes 3 resistors).
Resistive terminations dissipate power. If this is an issue the addition
of an appropriately selected capacitor provides the benefits of AC
termination with no DC power being consumed. Some busses can benefit
from series termination, but there are other considerations for this

So what's the downside from not terminating a line? Basically you can
multiple reflections (standing waves) bouncing back and forth on the
line adding noise, trashing your signal, causing unwanted data
transitions and all others sorts of undesirable effects.

I'll take a look at the posted documentation and block diagrams and see
if I can come up with a simple set of design rules.

-Ray  WB6TPU

-------------Begin Original Message----------------
Message: 4
Date: Wed, 25 Jan 2006 21:34:51 -0500
From: "Eric Ellison" <ecellison@comcast.net>
Subject: Re: [Xylo-SDR] Signal Integrity and Board stackup issues
To: "'Xylo-SDR Discussion'" <xylo-sdr@lists.ae5k.us>
Message-ID: <006301c62221$19df0830$7501a8c0@Development>
Content-Type: text/plain;	charset="us-ascii"


Can you tell me anything about the art of termination, do we need it?


---------------End Original Message-----------------

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