Identifying Opens, Shorts, and Terminations

Using the TDR functionality of a time domain reflectometer (TDR) or other time domain instrument, it is very simple to identify opens, shorts, and terminations in the device under test. 

When the expected impedance of the transmission line is known, there are three basic rules to keep in mind.

Rule #1: Upward spikes show inductive discontinuities, opens

An inductive discontinuity, in which measured impedance is higher than the rest of the transmission line, is shown as an upward spike in the waveform. Because TDR measures signal reflection, higher impedance causes larger reflections.

A perfect open has infinite impedance, so the entire pulse is reflected back. In a partial or intermittent open, the amplitude of the reflection would be lower than in a perfect open, but still higher than the rest of the transmission line.

In Figure 1 below, an HL1101 Ruggedized TDR from HYPERLABS is connected to 1 meter of 50 ohm coaxial cable in open circuit. Horizontal units are in distance (m) and vertical units are in voltage (mV).

Figure 1: TDR waveform on 1 meter of coax in open (click for full size)

In this image, almost the full 250 mV is reflected back to the sampler because the cable is in a nearly perfect open circuit.

Rule #2: Downward dips show capacitive discontinuities, shorts

Shorts are caused by a signal reaching a region of lower impedance than the rest of the transmission line. The signal is reflected back with opposite polarity, so the waveform dips sharply when the device under test is in short.

In Figure 2 below, a HL1101 Ruggedized TDR is connected to 1 meter of 50 ohm coaxial cable with a high-quality short at the far end. Horizontal units are in distance (m) and vertical units are in voltage (mV).

Figure 2: TDR waveform on 1 meter of coax in short (click for full size)

In this image, the impulse is reflected back with opposite polarity (-250 mV) because the cable has a very high quality short at the end.

Rule #3: Flat waveforms show constant impedance

When a transmission line is terminated with the same impedance as the rest of the transmission line, the waveform will be very flat. If it's of high enough quality, the termination may be invisible (or barely visible) on a TDR waveform.

In Figure 3 below, an HL1101 Ruggedized TDR is connected to 1 meter of 50 ohm coaxial cable with a high-quality termination at the far end. Horizontal units are in distance (m) and vertical units are in voltage (mV).

Figure 3: TDR waveform on 1 meter of coax terminated (click for full size)

In this image, the high-quality termination ensures the flatness of the waveform. The termination itself can barely be seen at about 5.8 m.

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