# High-Speed Digital System Design ~ A Handbook of Intercon..

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Figure 11.19: Measurement response for determining Z
probe
.
Now that the probe impedance is determined, we can calculate the actual DUT impedance
by following a similar procedure. Measure the incident step size with the probe open and
then measure the reflected step response while probing the DUT. The DUT characteristic
impedance is extracted using the equation
(11.7)
In many cases the discontinuity of the probe being used is too small to determine the
impedance. Completing the same measurement as discussed previously using the cable as
the reference instead of the probe can provide good results, on the order of less than 1-
difference. This is especially true in cases where the probe impedance effects are quite
small.
Offset Method.
Another technique of measuring impedance is the offset method. The offset method simply
uses a fixed value that accounts for the difference between the standard and the actual
readings. A standard characterized impedance reference (such as the airline) is measured
and the difference between the measurement and the known impedance is the offset. When
measuring the DUT, the offset is applied to the measurement result to account for errors.
This technique is especially useful for inexperienced users. This assumes a linear
relationship within the given specification window which is not necessarily correct. Although
inferior to the back-calculation method, it does produce relatively accurate results with less
time [
McCall, 1999
].
Impedance Coupon.
Depending on the topology of the bus design, it may or may not be possible to measure the
actual signal traces to determine the bus characteristic impedance. This will depend on the
length of the traces and bus impedance. The larger the difference in Z
o
between the DUT
and TDR, the longer the settling time to obtain a flat response. Furthermore, it may not be
possible to probe the signal traces on the final board design without using a probe with a
large inductive ground loop. To circumvent this problem, an impedance coupon can be used.
In PCB manufacturing, impedance coupons are used to dial in the process to obtain the
target impedance. To characterize the characteristic impedance, it is important that the
coupon geometry replicates the bus design. It is also critical that the coupon is designed
properly for obtaining accurate measurements.
11.4.2. Measurement Region in TDR Impedance Profile

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Summary :

## Now that the probe impedance is determined, we can calculate the actual DUT impedance by following a similar procedure. The DUT characteristic impedance is extracted using the equation (11.7) In many cases the discontinuity of the probe being used is too small to determine the impedance. A standard characterized impedance reference (such as the airline) is measured and the difference between the measurement and the known impedance is the offset.

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