Technical Tidbit - February 2004

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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.

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.

Figure 2. Coupon with wires attached.
[note the desired impedance!]

Figure 3. Coupon under test

Figure 4. Coupon showing shorted signal.

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.