TDR Horizontal Units and Scaling

Three horizontal unit types are available in both ZTDR™ and XTDR™ software from HYPERLABS: time (ns), distance (ft), and distance (m).

Although the general shape of the waveform doesn't change between unit types, it is useful to know why these units are used and how they are derived.

Units of Time

In time domain waveforms, the most straightforward horizontal unit is time. In all HYPERLABS instruments and software, time is scaled in nanoseconds (ns).

The reason for this is simple: it takes a discreet amount of time for an electrical signal to propagate through a transmission line. This is true whether analyzing a reflected or transmitted signal.

A high-speed signal integrity instrument measures the amplitude of a waveform at very precise intervals (within a few picoseconds, or trillionths of a second). These data are then plotted by the software.

Accordingly, a time domain waveform mapped as a function of time provides a set of discreet points that are independent of the material or insulation of the transmission line.

NOTE: When using TDR mode, the time shown is round-trip, that is, the time it takes the impulse to propagate and return to the sampler.

Units of Distance

Time domain waveforms can also be displayed as a function of distance, provided the dielectric constant (K) of the transmission line is known.

Need a refresher course on dielectric constant or velocity of propagation? See our Application Guide on the topic (coming soon).

The software converts time to distance based on the value of K. This value can be adjusted by the user numerically or by selecting from a list of common materials.

As long as K is correct, distance measurements will be accurate. Inversely, an incorrect K will render distance measurements inaccurate.

NOTE: When using TDR mode, the distance shown is round-trip, that is, the distance it takes the impulse to propagate and return to the sampler. For purposes of distance-to-fault measurements, the distance should be halved.

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