The screen capture in Figure: Beatty Standard – Frequency Response versus Impulse Response shows both frequency and time domain measurements of a Beatty standard (a transmission line with a low impedance section in the middle). The left quadrants shows the frequency response of S11 and S21, and the right quadrants show the impulse response of S11 and S21. Time domain responses offer insights about the physical characteristics of the DUT.

For example, in the upper right graph, the negative pulse at approximately 1.3 cm is caused by the reflection from the 50 ohm to low‑impedance step in the transmission line. A positive pulse at approximately 6.3 cm is caused by the reflection from the low‑impedance to 50 ohm step in the transmission line. The bottom right trace shows the impulse response of S21 versus distance. A positive pulse at approximately 7.5 cm indicates the total length of the Beatty standard. The pulse amplitude is slightly less than unity because some of the energy from the transmitted impulse was reflected back to Port 1 (the excitation port of S21 measurements).

 

The screen capture in Figure: Beatty Standard – Impulse and Step Response shows both impulse and step responses of the same Beatty standard. Step responses may seem more intuitive because they are a representation of impedance versus distance. A negative reflection off the leading edge of the low‑impedance section of transmission line causes the purple step response to drop down as the step travels through the low‑impedance portions of the Beatty standard. A positive reflection off the trailing edge of the low‑impedance section of transmission line causes the step response to rise back up. A secondary reflection off the trailing edge eventually brings the step response back to zero. Secondary reflections can be seen with large impedance mismatches such as the Beatty standard.