[hsdd] High-Speed Digital Design Newsletter - Double Tracking v7_04
- From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
- To: <hsdd@xxxxxxxxxxxxx>
- Date: Fri, 10 Sep 2004 15:55:15 -0700
DOUBLE TRACKING
HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 7 Issue 05
Some of you know that I live way out in the country
in the rural eastern portion of Washington State,
high in the Cascade mountains, in a dry, sunny
region called the Methow Valley. I occasionally get
questions about what it is like to live so far from
the big city, away from the hustle and bustle of
daily city life. In response to those questions, I
have attached at the end of this letter a brief
story about what happened one day here at our ranch
in the town of Twisp, Washington.
Thanks to everyone who has inquired about our upcoming
"High-Speed Digital Engineering Week" in San Jose,
October 4-8. I encourage you and your colleagues
to check this out. It will be a terrific week of
classes, panel discussions and vendor demonstrations.
See: http://www.sigcon.com/seminars/HSDEW.htm
______________________________________________________
DOUBLE TRACKING
Let's begin this discussion looking at the belt-and-
suspenders, super-safe differential stripline
architecture. The application I have in mind is a
backplane, using ZD-style backplane connectors. The
backplane runs horizontally (as in a standard rack-
mounted application), and has about ten connectors.
Figure 1 shows a partial layout for one connector in
plan view (pictures available at
www.sigcon.com/Pubs/news/7_05.htm ). This snippet of
the layout shows 9 wafers, each holding four
differential pairs. At the bottom, I show the
G S+S-G pin configuration, indicating that the
differential pairs (S+S-) are separated by ground
pins (G). The traces draw proceed to the right,
headed toward the next connector.
Figure 1- This 9-wafer ZD-style backplane
connector layout accommodates 36 differential
pairs, four pairs per wafer (figure courtesy of
ERNI GmbH).
Some engineers would call this a four-row connector,
because it holds four pairs in each column. Others
might call it an eight-row connector, because it has
eight signal connections, or perhaps a 13-row
connector, because that is the number of pins in
each column (including grounds). When you order
connectors, make sure you and your vendor are using
the same terminology. Also, note that in this
drawing each column, or wafer, extends horizontally
while rows extend vertically (strange, but that's
the convention). In the clearest terms, this layout
supports four pairs per wafer and shows 9 wafers.
I would like to produce an escape pattern that
routes all four differential pairs away from the
connector using only two signal layers. I call this
a double-track differential escape pattern, because
it provides escape paths for two differential pairs
(four wires total) between each wafer. If I can
accomplish the double-track escape, I can escape the
whole connector (in one direction) using only two
signal layers. The entire backplane will of course
require many more than just two signal layers,
because it must allow some signals to pass through
my connector without touching it, and also account
for the crossing of signals at intermediate
locations.
One possible double-track escape pattern appears in
Figure 2. The figure shows a cross-section view of
the layer stack. The connector's signal-pin vias
appear on either side (only partially visible, in
yellow). The solid planes above and below represent
the minimum width of the web of continuous reference
plane material existing between the columns of
signal pin vias.
Figure 2- (Cross-section view of layer stack) This
double-track (two pair) routing scheme passes two
differential pairs between each column (wafer) of
connector pins.
This pattern uses a 6-6-6 differential pair
configuration. That notation indicates an edge-
coupled differential pair comprising a 6-mil trace,
with a 6-mil space, followed by another 6-mil trace.
In this drawing, possible locations for the
connector-pin signal pads are indicated in light
blue. Some of the signal pads may be missing,
however, we must still provide a 6-mil minimum
clearance for our traces in case those pads are
present.
The available space between the blue pads is 57 mils
wide. I have filled that space with two differential
pairs (red), observing the 6-mil requirement on both
sides, and leaving a 9-mil gap between pairs. The
vertical space required to produce a 100-ohm
differential impedance with this configuration
equals 16 mils above and below the traces, for a
total thickness of 32 mils.
The near-end-crosstalk coefficient (NEXT) for this
configuration is 0.031, or about three percent
crosstalk pair-to-pair. Keep in mind that the total
delay of the coupled portion of these pairs will be
on the order of 127 ps (in FR-4). Any edges faster
than 127 ps will generate fully developed NEXT
waveforms of three percent. You will probably also
encounter additional crosstalk from various other
connector pads along the way, since they approach so
close to the traces. If amount of crosstalk from
this layout seems OK for your design, this is not a
bad way to go. The layout is simple and uses few
layers.
The systems I have worked on recently cannot
tolerate three percent crosstalk. If you are working
on a serial backplane, you probably cannot tolerate
that much, either. Consider that one pair, threading
through ten connectors in a row, picks up three
percent crosstalk underneath each connector it
traverses. Those crosstalk numbers can add up
quickly.
So, let's try a different layout. Figure 3 gives up
on the double-track idea. Each signal routing layer
places only one signal pair between the wafers. This
is a bulletproof design, with great appeal.
Crosstalk from pair to pair will be (as far as we
digital folks are concerned) non-existent. An analog
guru might be able to measure the crosstalk leaking
through the connector via holes in the reference
plane, but not me. This design works. Notice that we
did not have to quite double the board thickness to
do it. The double-track design used very tightly
coupled pairs, thus requiring either incredibly
tiny, thin traces or a generous spacing between the
planes. This single-track design accommodates a more
widely spaced pair. This 6-8-6 layout is less
heavily coupled, and thus for the same trace width
can make do with more tightly spaced planes. The
plane spacing here is 10 mils above and below. Both
designs produce a differential impedance of about
100 ohms.
Figure 3- This single-track (one pair) routing
scheme passes only one pair between each column
of connector pins. Crosstalk is practically non-
existent.
If you saw my previous newsletter
(www.sigcon.com/Pubs/news/7_04.htm, "Squeeze Your
Layer Stack") you know that I like thin backplanes.
Keeping the 6-8-6 trace configuration fixed, you
can slim down the layer stack in Figure 3 by
deleting every other solid ground layer. That
deletion raises the impedance of the differential
pairs. Jamming the solid planes closer together then
brings the impedance back down to 100 ohms.
This change still leaves at least one good, solid
return path underneath every differential pair, and
it really slims down the board, but there is a
hitch: the crosstalk re-appears. The design in
Figure 4 shows the numbers. With the center ground
plane missing, you get a total thickness of 30 mils
(as opposed to 40 from last time), but the crosstalk
pops right back up to +3.3 percent, assuming the
differential orientation as drawn. Darn. It didn't
help (yet).
Figure 4- This routing scheme uses fewer layers,
and is more vertically compact, than the single-
track approach. Crosstalk (NEXT) is 3.3% pair-to-
pair.
To fix the crosstalk problem you might try
offsetting the pairs from each other. Figure 5 shows
the result. The horizontal offset is 25 mils, the
most we can have while still meeting our 6-mil
clearance requirement on each side. The NEXT
crosstalk, pair-to-pair, falls to only -1.1 percent,
a better value, but perhaps still not good enough. I
want it even lower. What can I do?
Figure 5- Setting the offset to x=25 mils, the
maximum available in this layout, reduces the
crosstalk (NEXT) to a level of -1.1 percent pair-
to-pair.
Reviewing the numbers from Figures diagrams 4 and 5
closely, I see that the crosstalk in Figure 4 is
positive, while that in Figure 5 is negative. This
happens because in Figure 4 the traces line up + to
+ and - to -, so you of course get a positive NEXT
coefficient. In Figure 5 we have offset the traces
so severely that now the + and - traces line up most
closely, generating a negative NEXT coefficient. As
we slide the traces from the Figure-4 position to
the Figure-5 position, nature behaving in a
continuous manner, shouldn't we at some point obtain
a NEXT coefficient of zero?
Surprisingly, that is exactly what happens. There
exists a natural null in the pair-to-pair crosstalk
for pairs on adjacent layers, but it only happens if
you choose precisely the right offset. Figure 6
shows the position of that null. This solution was
obtained empirically, using HyperLynx LineSim V7.
The plot of NEXT crosstalk versus offset in Figure 7
shows that the null is fairly well behaved (i.e., it
is not a narrow, deep notch). A layer-to-layer
offset tolerance of 0.001 in. should allow you to
reliably contain the pair-to-pair crosstalk to a
value below 0.003. In my opinion, that's good
enough. The net result is a board 25% thinner than
Figure 3, using 25% fewer layers.
Figure 6- The crosstalk (NEXT) varies as a
function of the pair-to-pair offset, x. The
particular offset drawn in this geometry produces
zero crosstalk.
This null reminds me a similar effect used in some
German LAN cables. These cables (popular in the
10BASE-T era) contained four wires, held in a rigid
"X" profile (one wire at each tip of the X).
Labeling the wires clockwise as RED, YELLOW, GREEN,
and BLACK, you wired opposing wires into pairs (RED
and GREEN, or YELLOW and BLACK), to make up a 10BASE-
T Ethernet cable. Because the wires were held in a
rigid exact (and highly symmetrical) position they
existed in a kind of crosstalk null much like the
one produced in Figure 6.
Figure 7- Near-end crosstalk (NEXT) between two
differential pairs implemented on adjacent layers
passes through zero as it goes from positive (on
the left side of the figure, representing traces
piled right on top of each other) to negative (on
the right side, representing widely spaced
pairs).
______________________________________________________
Questions & Comments: all students who attend the
upcoming High-Speed Digital Engineering Week
seminars have the opportunity to talk directly with
a variety of experts about all aspects of signal
integrity and high-speed system performance. We'll
have independent experts there available to answer
your questions about system design, high-frequency
problems, EMC, measurements, noise, system design
and simulation workflow.
The original High-Speed Digital Engineering Week
event held at the University of Oxford last June
produced comments like these:
A very good exchange of ideas. I've learned a
lot...,
[The session] -Covered a good range of issues.
Good to hear different viewpoints..,
[The session was] -Informative and useful. It was
good to hear other peoples' "real" issues and
their solutions..,
Very useful to see a variety of opinions on how
to solve various problems.
Ninety seven percent of the attendees said they
would recommend it to their colleagues.
You can attend the High-Speed Digital Engineering
Week panel discussion session at our upcoming
seminar in San Jose, Oct. 4-8, 2004.
http://www.sigcon.com/seminars/HSDEW.htm
______________________________________________________
Personal log, dog-date: "First Day of School"
Val chose the first day of the new school season to
teach herself an important lesson. Val is a healthy,
vigorous, four-year old Australian Shepard. A
swirling mass of brown, black and white fur, this
breed of dog relishes the outdoor life; constantly
zipping between the barn, house, and the water hole.
If something moves within the visible horizon, she
and her twin sister Blackie are right on top of the
action.
When I'm packing my truck for a day's outing in the
pastures, Val always appears, like a genie, smiling
and panting, in the bed of the pickup. How she
vaults over the side of the vehicle I will never
know, as it happens too quickly to observe with
human eyes.
On this particular morning, Val and her sissy woke
early and threaded their way downstairs past piles
of new-school materials to the front door for their
morning constitutional. The stacks of notebooks and
paper and backpacks had been all neatly organized
and prepared by our daughters, Katy and Allie, the
night before so as not to be late for the first day
of school. Allie, 12 years old, going into Junior
High for the first time, insisted especially that
there be no last-minute delays leaving the house on
this crucial morning.
Out on patrol, the dogs make their way, sniffing and
barking, checking tracks left by the bear, deer,
coyote, and what-have-you that tramped through the
yard during the night-time hours. Inside, the girls
wolf down their cereal and brush their teeth. Allie
straightens her new, bright-white, first-day-of-
school blouse.
Suddenly, a round of furious barking shatters the
early-morning calm. This is not an unusual
occurrence at our house. The dogs could have turned
up anything - a mouse, a skunk, a man on
horseback - regardless the source they dutifully obey
an ancient instinct that lets everyone know they saw
it first.
Within moments, barking turns to yelps and snarls as
an epic battle rages outside.
The children run out to see and come back screaming
and in tears, then I hear my wife, Liz, add her shouts
to the ruckus. I, oblivious to all, and being quite
accustomed to the increasingly frequent emotional
outbursts of my pre-teens, merely focus my attention
more forcefully on the surgical extraction of the
last remaining portion of a small splinter jammed in
my thumb.
The level of general panic swirling about the house
reaches a level, which even I cannot ignore, so I
trundle downstairs to the front porch, curious to
see what is causing all the excitement. There,
spread before me, lies a tableau of pain and
suffering. I hear the screams of girls and the
snarls of dogs. A high level of general hysteria
prevails. Liz sits with pliers in hand. Blood is
splattered in every direction on new school
clothing. In the center of the hubbub, defiant, even
arrogant, apparently impervious to pain, still
barking, stands Val, the wonder dog, looking
particularly fearsome as she now sports a hideous
array of spikes emanating from her snout.
My first reaction is one of disbelief, and early-
morning confusion. I don't remember Val having
spikes growing out of her head, although they do
look like they would deter the advance of any
rational-minded creature wanting to avoid getting
stuck. Slowly it dawns on me that the spikes point
the wrong way, blunt ends sticking forward. Val has,
apparently, encountered her first porcupine.
It takes a lot of patience to pull porcupine quills
out of a dog's snout. First, you have to catch the
dog. Then you must calm her down and then slowly,
gently as you can, you grasp each quill one at a
time with pliers and YANK. Dogs like this even less
than kids like doing homework.
After all the excitement died down, the kids packed
their stuff in Liz' car (more than a little late)
and left for school. As they all sped away down the
hill, with admonitions from the children to never
let the dogs go outside ever again, I found myself
hoping, with the eternal optimism of parenthood,
that somehow, during their school day, the children
would learn a lesson even half as valuable as the
lesson Val learned on her first day of school.
H. Johnson
Twisp, WA
Sept. 08, 2004
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