This chapter describes the optional Time Domain feature in the Vector Network Analyzer. General descriptions, key concepts, and examples are presented for time and distance measurements for both coaxial and waveguide media.

The function hard keys in Vector Network Analyzer mode with Option 2 are:

The Option 2 Time Domain feature provides the ability to transform the native frequency domain data (that is measured by the Vector Network Analyzer) into time domain or distance domain information to help in determining the location of impedance discontinuities. Some typical applications are: distance‑to‑fault (DTF) in cables and waveguides, characterizing antennas, isolating and analyzing a desired response in a one‑port or two‑port network, and identifying and analyzing circuit elements.

The relationship between the frequency‑domain response and the time‑domain response of a network is described mathematically by the Fourier transform. The instrument makes measurements in the frequency domain, then transforms that data into its time‑domain response, which can be displayed as a function of time or distance. This computational technique benefits from the wide dynamic range of the instrument (and its measurement data) and from the error correction of the frequency‑domain data.

The transformation technique that is used by the instrument (in most cases) is the chirp‑Z transform of the available frequency domain data for that parameter. Because the transform simply treats the frequency domain values as input data, any S‑parameter can be transformed (including differential S‑parameters). The chirp‑Z transform is (in a macro sense) very similar to the Fast Fourier Transform with the exception that the output range can be variable. This permits you to zoom in on a specific time (distance) range of interest for the data display. A different algorithm is used with the waveguide dispersive media, where the time‑frequency relationship is more complex, but the functionality remains the same.

Two of the fundamental properties of time‑domain conversion are resolution and maximum (alias‑free) range. Resolution is the ability to resolve one discontinuity from another. Resolution is limited by the frequency span of the measurement. Maximum range defines how far you can see discontinuities on the media you are measuring. Beyond the maximum range, the data just repeats itself, and you start seeing the same discontinuities from closer ranges. The maximum range is determined by the frequency step size.

For more details about time domain fundamentals, refer to the following application notes: