TDR Interconnect analysis – applications, measurement caveats, tips and techniques

February 05, 2018 // By Erik Babbé, Keysight Technologies
The continued sharp increase in high speed digital signals in electronics designs drives the need to characterize high frequency interconnects, failing to do so can result in multiple design re-spins and delays to bring products to market.

The Time domain Reflectometer (TDR) has come a long way since the early days when it was used to locate faults in cables. If your designs involve signals with rise times shorter than one nanosecond, transmission line properties of the interconnects are important. TDR is a versatile and intuitive tool to provide a window into the performance of your interconnects to quickly and routinely answer the three important questions: does my interconnect meet specifications, will it work in my application, and where do I look to improve its performance?

The TDR is not just a simple radar station for transmission lines, sending pulses down the line and looking at the reflections from impedance discontinuities. It is also an instrument that can directly provide first order topology models and S parameter behavioral models. We will look at five most important, from the most common to the more advanced, applications of 1-port TDR.


[1] Measuring characteristic impedance and uniformity of a transmission line

For an ideal, lossless transmission line, there are only two parameters that fully characterize the interconnect: its characteristic impedance and its time delay. This is the easiest and most common application for the TDR. The TDR sends a calibrated step edge of roughly 200 mV into the device under test (DUT). Any changes in the instantaneous impedance the edge encounters along its path will cause some of this signal to reflect back, depending on the change in impedance it sees. The constant incident voltage of 200 mV, plus any reflected voltage, is what is displayed on the screen of the TDR.

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