[SI-LIST] Measuring propagation delay on PWB interconnects
- From: Eric Bogatin <eric@xxxxxxxxxxxx>
- To: SI list <si-list@xxxxxxxxxxxxx>, brian.butler@xxxxxxxxxxxxxxx
- Date: Fri, 19 Apr 2002 07:25:17 -0500
Brian-
We have two techniques at GigaTest Labs that work well for
measuring time delay with a TDR. These techniques are described
in papers we've recently given at the IPC expo and are posted as
reprints on our web site: www.gigatest.com. All are welcome to
download them.
1. It's critical to use microprobes that allow you to lift the
probe from the trace under test. Also, this allows small size
vias and pads, which minimize the parasitics. First measurement
is with the probe lifted. The 50% point of the rising edge
defines the beginning of the trace. Lower the probes and you
measure the TDR response. The 50% point of the far end defines
the end of the line. The measured time difference is the round
trip travel time. There are two important parasitics to worry
about. The first is the capacitance of the via pads. That's why
keeping the vias small is important.
The second artifact is from the losses. The losses will degrade
the rise time and give you a measurement of the time delay that
is longer than it really is. As a rough rule of thumb, with FR4
and 50 ohm traces wider than 8 mils, the rise time degradation is
about 10 psec/inch. If the trace is 6 inches long, there could be
as much as 30 psec extra measured delay in extracted TD, just
from the rise time degradation from the losses. This is out of 1
nsec, or about a 3% effect. You can minimize this effect by using
TDA System's IConnect to fit a lossy transmission line and use
the extracted LC values to get the "lossless" time delay. Form
this, the effective dielectric constant can be extracted. From
the effective dielectric constant, the bulk dielectric constant
can be extracted, given the cross section and a 2D field solver.
2. The second method works well if you have to use connectors,
rather than microprobes. On the test trace, add two small
squares- basically pads-, about an inch from the ends of the
lines. If the trace is 6 inches long, the small additional
squares are separated by 4 inches. They should be no larger than
an extra line width on each side. When you look at the TDR, you
see the small squares as small dips, due to their capacitive
discontinuity. You can measure the TD between these dips with hi
precision using cursors. Again, the TD extracted will have the
lossy line artifact associated with it.
It's always useful to simulate the measurement with a SPICE
simulation. This allows you to evaluate the inaccuracies
contributed by connector and lossy effects.
Finally, we find the best accuracy is with VNA- this allows
accurate extraction of the via models in the front and back of
the lines and you can see explicitly where frequency dependent
effects begin.
In addition to having app notes on our web site that review these
techniques, we also teach these methods in our class: GTL260
Creating high bandwidth models for measurements, which will be
offered May 21 and 22 in Sunnyvale. You can find the app notes
and info about the classes on our web site: www.gigatest.com.
--eric
**************************************
Eric Bogatin
CTO, Giga Test Labs
v: 913-393-1305
f: 913-393-1306
e: eric@xxxxxxxxxxxx
26235 W. 110th Terr. Olathe, KS 66061
corporate office:
408-524-2700
134 S. Wolfe Rd Sunnyvale, CA 94086
web: www.gigatest.com
**************************************
From: Brian D. Butler [mailto:brian.butler@xxxxxxxxxxxxxxx]
Sent: Wednesday, April 17, 2002 10:44 AM
To: si_list
Subject: [SI-LIST] Measuring propagation delay on PWB
interconnects
To all:
I wanted to get opinions on the best way to measure propagation
delay
utilizing only a TDR technique (not TDT) on PWB interconnects
(utilizing
only a single interconnect). The methods I have seen to-date are:
Note: Propagation delay is equal to the End point minus the Start
point
along the TDR horizontal time axis. All values obtained are the
"round-trip" delay.
1) Start point is 50% of risetime curve of TDR probe - End point
is 50% of
risetime curve at end of trace.
2) Start point is lowest point of capacitance dip (associated
with launch
via) - End point is 3mvolts over nominal impedance on the
risetime curve at
end of trace.
3) Start point is 40% of risetime curve of TDR probe - End point
is 40% of
risetime curve at end of trace minus 3.2 * SQRT((Ristetime40%-60%
at end of
trace)^2 - (Risetime40%-60% at end of probe)^2) - current IPC
TM-650
method
4) Same as any above but utilize falltime curve (by probing a
short and
shorting end of trace).
Any comments on the above methodologies or alternate methods are
appreciated.
Regards
Brian Butler
Introbotics Corp
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