[hsdd] High-Speed Digital Design Newsletter - - Tiny Difference
- From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
- To: <hsdd@xxxxxxxxxxxxx>
- Date: Thu, 21 Dec 2006 13:31:43 -0800
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Tiny Difference
HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 9 Issue 08
The snow is falling thick and fast this winter. Our
mile-and-a-half of steep gravel road has become so clogged with
snow that our snowplow operator, Charlie, can no longer push the
snow off to the side. There's no place left to put it! Charlie's
no slouch. He usually drives an F-350 truck with an 8-foot snow
blade. His rig weighs about 9000 pounds (loaded with rocks) and
even with all that he just doesn't have enough traction to get
the job done. So, today we ordered him some heavy machinery. On
Friday he'll be moving snow with a full-sized road grader. This
vehicle weighs 50,000 pounds and has tires that stand almost five
feet tall. I call it the "ultimate off-road vehicle". With that
sort of horsepower at his command Charlie should, as Larry the
Cable Guy says, "Git 'er Done".
Regarding Anjaly's message below, we never satisfactorily
determined the problem with his layout (or if he even had a
problem), but I did come up with some tips that might help yours!
Read on.
_____
Tiny Difference
Anjaly Chandran writes:
I am puzzled by the skew difference on a differential pair and
would like your inputs (see Figure 1). Each purple trace is 24mil
wide and 2.4 mil thick with a 13-mil dielectric thickness. The
board is made from FR408 material.
Each trace is routed as a single ended pair with almost 75mil
clearance between the two traces of each differential pair to
reduce inter-pair coupling. Layer 2 provides a solid ground
underneath the traces. The trace lengths are matched to within 2
mils, but when I measure the skew I find almost 5 ps difference
in delay between the two traces. One thing to point out is that
these traces are tapered as they approach the package pin, but
the tapered length is almost the same, differing only by 3 mils
or so. Also, there is an AC coupling cap (2.2nF) on each trace.
To cause a 5 ps delay on a micro strip requires a 30-35mil trace
length difference (assuming ~140ps/inch).
What could cause the skew between the traces? Any help is highly
appreciated.
_____
Dr. Johnson replies:
Wow! Measuring a tiny time difference like 5 ps can be quite
challenging. Anjaly will need well-matched, skew-calibrated
probes and perfectly symmetric attachments to the board. Before I
discuss the attachments, let's look at his layout.
<http://www.sigcon.com/images/news/9_08fig1.GIF> My rendering
of Figure 1 does not show the SMA launches. They begin just off
the left side of the drawing, connecting through blocking
capacitors to the two purple traces that are the focus of this
newsletter. The SMA layout for the top trace is made just a tad
longer than the one on the bottom. I call that "pre-skewing" the
connections, and it was done for a reason that will become clear
in a moment. From point a, working to the right, segment a-f
comprises four 45-degree turns, two going left and then two going
right. Any skew accumulated in the two left turns is precisely
compensated by the equal and opposite skew accumulated in the two
right turns.
After those turns the signal encounters a straight segment f-g,
also balanced, and then a weird tapered part g-h. The taper
connects the widely spaced, loosely coupled trace pair to the
fine-pitch geometry of the IC soldering pads.
In segment g-h the lower trace appears clearly longer than the
upper trace. Anjaly knew this, so he pre-skewed the SMA
connections to compensate for the skew he knew would accumulate
in the tapered area. Overall, from SMA to IC, the total lengths
are matched to within 0.002 in.
So why does Anjaly measure a 5-ps difference in delay? Here are
some of my guesses.
(1) The ground terminals of the surface-mounted SMA connectors
sit on large layer-1 ground pads that may not be well connected
to the actual ground layer that the traces use, layer 2. Near the
SMA connectors I see only two ground vias in the layout.
Something about the shapes of these layer-1 ground regions and
the inadequacy of their grounding to layer 2 looks suspicious.
Each SMA connector conveys signal current and also returning
signal current (ground current) to your board. Small differences
in the returning signal current paths (i.e., ground pathways) on
the two connectors could easily cause the 5-ps difference he
observes. Recommendation: add more ground vias to the SMA layout,
and make the SMA layouts the same.
(2) Anjaly's test board was likely hand-soldered. The blocking
capacitors may not have been held firmly against the board during
soldering. If one capacitor stands higher above the board than
the other, that would cause skew. A difference of 1/64th of an
inch in height, going up and back down, would contribute the
1/32nd difference in length that bothers him. Recommendation:
test at least three boards to see if the delay differences are
repeatable. [Note -- Anjaly subsequently did test a second board,
from another supplier, and came up with only 2 ps difference].
(3) When measuring tiny time differences, always reverse the
probes to see if the skew stays with the layout or moves with the
probes. Five ps of skew can easily be caused by probe
mis-calibration. Test your probes all by themselves, on the
simplest layout you can manage, to confirm that you can
repeatedly observe skew differences as small as 5 ps. To make the
test, you will be attaching four probes (two in and two out). A
nominal positioning error of, say, 0.01 in. would be more than
enough, when multiplied by four attachment points, to completely
swamp the measurement you are trying to make. Anjaly must get his
probe placement accuracy down to something like 0.003 in. to
obtain a good measurement. That amount represents one-tenth of a
turn on a 36 tpi threaded SMA connector. Therefore, always use a
calibrated torque wrench on SMA connections. At the IC attachment
end of the connection, provide a guide hole on the board
(accurate to 0.003 in.) to help align the probes. You can press a
hardened steel pin into the hole to help hold the microprobes in
precise alignment. Recommendation: make a simple test layout that
is obviously symmetric and practice using the probes on the test
layout.
(4) The layer-1 grounded metal regions may influence the signal
traces. I realize Anjaly kept the layer 1 ground regions at least
75 mil. away from the signal traces at all points. That would be
far enough on an ordinary board, but at his exaggerated trace
height of 13 mils that spacing provides a spacing-to-height ratio
of only about 6:1. That ratio suggests to me perhaps as much as
3% coupling from each trace to its peculiarly-shaped layer-1
ground region. Not much coupling, but it could be having an
effect. And it doesn't contribute anything to the system
performance. Why have these regions? Recommendation: move the
layer 1 ground regions even further away from the signal traces,
or eliminate them altogether except under the SMA connectors
where they are required for SMA soldering.
If you have other ideas for Anjaly, please let me know and I'll
pass them along. Or, if you join my next public seminar Feb. 5-8,
2007 in San Jose, CA, we can talk about them together.
Best Regards,
Dr. Howard Johnson
_____
I'm talking to the folks at National Semi. about filming another
Bob Pease "Analog by Design" show this Spring . If you haven't
checked out Bob's programs, I can highly recommend them. You can
find all his shows, including the ones in which I appear,
archived here: http://www.national.com/nationaltv/.
Join us at our upcoming seminars in San Jose, Feb. 5-8, 2007. A
full schedule of cities and dates appears at: www.sigcon.com
<http://www.sigcon.com/> .
Special offer for our HSDD Newsletter subscribers!
Get a $50 tuition discount
when you sign up for a
seminar in San Jose (Feb 5-8, 2007).
Use the Promo Code: NLS
Register Now at: sigcon.com <outbind://51/seminars/SanJose.htm>
Questions & Comments: All students who attend our High-Speed
Digital <http://www.sigcon.com/seminars.htm> Design seminars
have the opportunity to talk directly with Dr. Johnson about
signal integrity issues.
_____
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