In high-speed serial communications systems, a designer usually relies on a vector network analyzer (VNA) to measure S-parameters. The VNA measures input/output reflections and transmission by sequentially applying signals at various frequencies and recording the response of the device under test (DUT). The measured values that result are a function of frequency. In contrast, recent advancements in high-speed time-domain reflectometry (TDR) and time-domain transmission (TDT) have created an alternative to frequency-domain testing through the use of time-domain analysis. This approach is explored in a white paper from Picosecond Pulse Labs (Boulder, CO) titled, "S-parameter Measurements with the PSPL Model 4022 High-Speed TDR and TDT System."
A fast rise-time pulse is composed of many different frequency components. As a result, high-speed TDR and TDT measurements may be used to measure S-parameters versus frequency. With this technique, TDR and TDT measurements are initially recorded in the time domain. They are then transformed into the frequency domain for return loss and transmission analysis. Instead of being sequentially tested at single frequencies, the DUT is tested with an entire spectrum of frequencies at once.
The key to obtaining frequency-domain data from time-domain measurements is the Fast Fourier Transform (FFT). Using the FFT, frequency-domain data can be calculated from sampled time-domain measurements. An inverse FFT allows frequency-domain measurements to be transformed into time-domain data.
Commercial TDR/TDT measurement systems typically produce a pulse and measure it on the same channel using a sampler built into the channel. Picosecond Pulse Lab's Model 4022 also includes circuitry to utilize a separate high-bandwidth sampler plug-in. According to the company, system bandwidth is thereby increased because a faster edge is produced. In addition, a high-bandwidth sampler is used to acquire the response.
The note explains how to compare TDR/TDT systems for frequency content. The trick is to measure the incident pulse and calculate the frequency or spectral content of the pulse with an FFT. The second half of the paper is devoted to examples of TDR/TDT-based S-parameter measurements. The measurements made with the Model 4022 demonstrate the viability of using a high-bandwidth TDR/TDT measurement system to acquire S-parameter data.