IPC-TM-650 2.5.5.7

Description / Abstract: This document describes time domain reflectometry (TDR) methods for measuring and calculating the characteristic impedance, Z(0), of a transmission line on a printed circuit board (PCB). In TDR, a signal, usually a step pulse, is injected onto a transmission line and the Z(0) of the transmission line is determined from the amplitude of the pulse reflected at the TDR/transmission line interface. The incident step and the time delayed reflected step are superimposed at the point of measurement to produce a voltage versus time waveform. This waveform is the TDR waveform and contains information on the Z(0) of the transmission line connected to the TDR unit.

Note: The signals used in the TDR system are actually rectangular pulses but, because the duration of the TDR waveform is much less than pulse duration, the TDR pulse appears to be a step.

Applicability

The observed voltage or reflection coefficient change in the TDR waveform is related to the difference between Z(0) of the transmission line and the impedance of the TDR. If the impedance of the TDR unit is known via proper calibration, then the Z(0) of the transmission line attached to the TDR unit may be determined. Thus, the TDR method is useful for measuring Z(0) and changes in Z(0) of a transmission line. These impedance values thus determined can be used to verify transmission line design (engineering development), measure production repeatability, and qualify manufacturers via transfer or artifact standards.

Engineering development requires detailed information on the electrical performance of prototype units to assure the transmission line design yields the expected performance characteristics. Detailed laboratory analysis of the effect of variations in design features expected in actual manufacture can be done to assure the proposed design can be manufactured at a useful quality level.

Measurement System Limitations

Measurements of Z(0) often vary greatly, depending on equipment used and how the tests were performed. Following a specified method helps assure accurate and consistent results. Both single-ended and differential line measurements have limitations in common, including the following:

a. The Z(0) measured units are derived and not directly measured.

b. The value of characteristic impedance obtained from TDR measurements is traceable to a national metrology institute, such as the National Institute of Standards and Technology (NIST), through coaxial air line standards. The characteristic impedance of these transmission line standards is calculated from their measured dimensional and material parameters.

c. A variety of methods for TDR measurements each have different accuracies and repeatabilities.

d. If the nominal impedance of the line(s) being measured is significantly different from the nominal impedance of the measurement system (typically 50 Ω), the accuracy and repeatability of the measured numerical valued will be degraded. The greater the difference between the nominal impedance of the line being measured and 50 Ω, the less reliable the numerical value of the measured impedance will be.

e. Measurement variation (repeatability, reproducibility) may only be a small component of the total uncertainty in the value of the characteristic impedance. For example, if the uncertainty in the characteristic impedance of the reference air line is ± 0.5 Ω (for a 95 % confidence interval), then the uncertainty in the measured characteristic impedance of the test line can be no better than ± 0.5 Ω even if measurement variation is much less.

f. The particular TDR methods described herein are not suited for measuring the characteristic impedance as a function of position along the transmission line (impedance profiling) because signal reflections within the transmission line under test and between the TDR unit and transmission line under test may adversely affect measurement results.

g. The requirements for the length of the transmission line under test given in Section 3 of this test method as well the IPC-2141 must be met.

Further measurement considerations and notes are provided in Section 6.

Sample Limitations

The type of test sample used may also impact Z(0) values (see IPC-2141). The sample-based limitations include:

a. The transmission line under test varies along its length whereas the value of Z(0) obtained assumes a uniform transmission line. Therefore, the measured Z(0) only approximates the characteristic impedance of an ideal line that is representative of the line under test.

b. Lines on a printed circuit board may deviate significantly from design. For example, microstrip lines longer than 15 cm [5.91 in] on boards with plated-through holes often have variations in line width; this variation is due to plating and/or etching variations.

c. If the transmission line is too short, the accuracy of the calculated impedance value may be degraded (see 4.1.2). If the transmission line is too long, skin effect and dielectric loss may cause a bias in the impedance measurement.

d. Depending on where the measurements are made, the value of Z(0) obtained may be affected by dielectric and conductor loss and other effects. The farther away from the interface between the probe and the transmission line under test, the worse these effects will be.

e. Duration of the measurement window (waveform epoch) may need to be adjusted for sample length and location of midpoint vias along the transmission line.