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Production Measurement of Frequency Dependent Attenuation and Phase Constant of PCB Traces
Keywords: Performance, Attenuation, Measurement
As clock rates have increased and pulse widths have narrowed, signal loss at the higher frequencies has become a key problem for both designers and printed circuit board fabricators. Current techniques for measuring signal loss during production testing of PCBs cannot provide directly the specific attenuation and phase shift for the trace being measured. A new measurement technique using short pulses has demonstrated in the laboratory that the propagation constant of a trace can be determined accurately and presented as attenuation and phase per unit length as functions of frequency to beyond 20 GHz. This Short Pulse Propagation measurement process has been evaluated as mature enough to transfer from a laboratory environment and become part of a mainstream production measurement system. The technique uses available sampling oscilloscopes, step generators, and hardware differentiators to obtain the high-frequency pulse needed for Time Domain Transmission (TDT) measurements. Since the narrow pulse is impulse-like, it can be easily and accurately transformed from the time domain to the frequency domain to provide the frequency-dependent information that is desired. Two similar traces of different lengths are measured, and the ratio of the frequency-dependent measurement from each trace provides the trace’s attenuation in dB/inch/Hz and the trace’s phase delay in radians/inch/Hz. The use of the ratio of the two measurements also removes the parasitic effects of the test set-up and measurement structure. Since a production application of the Short Pulse Propagation process uses available equipment and proven measurement and mathematical techniques, it can provide an accurate and efficient tool for test, evaluation, and process control. The propagation constant that is obtained from the Short Pulse Propagation measurements also provides the foundational data for additional measurements and modeling to obtain the broadband dielectric constant and loss tangent on product-level, multi-layer PCB wiring.
John DiTucci,
Introbotics Corp.
Albuquerque, NM

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