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Time Domain Reflectometry Theory Application Note 1304-2 For Use with HP 54750A and HP 83480A Mainframes 2 Introduction The most general aproach to evaluating the time domain response of any electromagnetic system is to solve Maxwell's equations in the time domain. Such a procedure would take into account all the effects of the system geometry and electrical properties, including transmission line effects. However, this would be rather involved for even a simple connector and even more complicated for a structure such as a multilayer high speed backplane. For this reason, various test and measurement methods have been used to assist the electrical engineer in analyzing signal integrity. The most common method for evaluating a transmission line and its load has traditionally involved applying a sine wave to a system and measuring waves resulting from discontinuities on the line. From these measurements, the standing wave ratio () is calculated and used as a figure of merit for the transmission system. When the system includes several discontinuities, however, the standing wave ratio (SWR) measurement fails to isolate them. In addition, when the broadband quality of a transmission system is to be determined, SWR measurements must be made at many frequencies. This method soon becomes very time consuming and tedious. Another common instrument for evaluating a transmission line is the network analyzer. In this case, a signal generator produces a sinusoid whose frequency is swept to stimulate the device under test (DUT). The network analyzer measures the reflected and transmitted signals from the DUT. The reflected waveform can be displayed in various formats, including SWR and reflection coefficient. An equivalent TDR format can be displayed only if the network analyzer is equipped with the proper software to perform an Inverse Fast Fourier Transform (IFFT). This method works well if the user is comfortable working with s-parameters in the frequency domain. However, if the user is not familiar with these microwave-oriented tools, the learning curve is quite steep. Furthermore, most digital designers prefer working in the time domain with logic analyzers and high speed oscilloscopes. When compared to other measurement techniques, time domain reflectometry provides a more intuitive and direct look