High Frequency Measurements
Douglas C. Smith
Address: P. O. Box 1457, Los Gatos,
Technical Tidbit - February 2004
Measuring Printed Circuit Board Characteristic Impedance
without a TDR
(when you need a close answer
Abstract: In a crunch, being able to adapt test equipment to get a
close answer fast can be a life saver.
The characteristic impedance of a prototype circuit board was suspected as
the cause of a 64 bit DDR SDRAM bus failing initialization tests and a method to
confirm the board design was needed urgently. This Technical Tidbit describes
the method by which a network analyzer was pressed into service and its use
outlined. This technique has been successfully used for analyzing other assemblies
such as EIDE cables and USB unshielded twisted pairs.
Discussion: The relationship between inductance and capacitance for any
transmission line is defined as (L/C)1/2. What was
needed then was a way to measure the electrical components of the board
estimate the characteristic impedance by using the transmission line formula.
Typically when boards that have impedance sensitive designs are manufactured,
coupons are provided with test data that describes the board electrical details.
Test coupons are small PCBs with exactly the same layer and trace
construction as the main PCB. For example, a coupon will include traces of the
same line width and copper weight on the same layer as the controlled impedances
on the main PCB thereby providing an easy way to confirm the board design. The
coupon is manufactured in the same process as the main board and with the same
aperture code as used for the critical traces. An example of a coupon is shown
is Figure 1.
Printed Circuit Board Coupon
Test Method. The test coupon is a very simple pwb
construction with traces that reflect the board stack-up and trace
construction design. Test coupons will also have, typically, a set of vias
connected to ground and the signal, in close proximity at one end of the
trace, to allow a test probe to be attached. The test coupon shown in
Figure 1 was ideal for testing in the impedance analyzer as not only
are the test points are close to each other but there is a set at each
end of the trace making the modification for the inductance measurement
very simple indeed. The coupon is prepared by attaching wires to one end
of the set of test points as shown in Figure 2 and the wires are used
to attach the coupon to the analyzer adapter as shown in Figure 3. Two measurements
are now taken with the test coupon in two different configurations: with
the second set of vias open (used in the capacitance measurements) and
shorted ( used for the inductance measurement) as shown in Figures 4.
After getting the capacitance and inductance numbers, use the transmission
line formula to estimate the characteristic impedance of the trace.
Coupon with wires attached.
[note the desired impedance!]
Coupon under test
Figure 4. Coupon showing shorted signal.
(click on pictures for a larger image)
From the plots: L = 37.5954 nH and C = 39.1054 pF
Zo (estimated) = 31 Ohms
(measured) = 29 Ohms
Conclusion: When you need a close answer fast, a
network analyzer can be pressed into service in place of a TDR. Typically the
error for measuring the characteristic impedance of a circuit board is
about 5% which can really add up so if you need a more accurate measurement
then a TDR is the best instrument to use.
Editor's note: Be sure to select a test frequency high enough to easily
measure the L and C values, but low enough to avoid standing waves on the
coupon's signal traces. For this article, Charles picked 3 MHz, a suitable frequency for
this case. If you are working with circuit boards 2 cm in length, a higher
frequency would be in order and control of the connection parasitics becomes
more important. Thanks goes to Charles for contributing this article.
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Copyright © 2004 Douglas C. Smith