Cable network maintenance techs and contractors at times need to find and fix cable breaks or faults, which are likely to occur in network locations that impact many subscribers, with a degradation in service or even a complete outage. This means that quick, simple, accurate and high-resolution testing is required. Sometimes the cable is underground, so having an accurate test with good resolution is critical to keep the hole required for splicing as small as possible. Cable breaks and faults can be caused by a wide variety of events, such as storm damage, excavation activities, rodent damage, etc. A time domain reflectometer (TDR) test is typically the best way to find cable breaks and faults.

A TDR works by sending electrical pulses down the cable and sampling reflected energy. Any impedance change in the cable causes some energy to reflect back to the tester and will be displayed. The degree of impedance change determines the reflection amplitude. There are two types of measurement technology used in TDRs – “pulse,” and “step.”

Pulse Technology

Pulse technology is most commonly used in TDRs, and in this method the transmitter sends a single pulse, then shuts off while the receiver is activated to listen for the reflection.

This creates a “Dead Zone.”  A longer pulse increases the measurement range, but the longer the transmitted pulse, the longer the (time) distance before the receiver can start looking for reflections.  In selecting the pulse width, the operator must consider that a short pulse reduces dead zone but limits range, and a long pulse increase range but makes the dead zone larger.  Another limitation is that pulse TDR’s have less signal energy than Step TDR’s which reduces signal-to-noise ratio and provides a less detailed cable test result.

Step Technology

Step technology is what is used in the VIAVI DSP TDR, and in this method the transmitter sends signals continuously while the receiver listens simultaneously for reflected signals.  Unlike pulse technology, this means there is no “dead zone,” allowing the receiver to measure the entire cable.  The constant step signal detects information including impedance along the entire length of the cable.  The higher energy of the step signal TDR improves the signal-to-noise ratio and with digital averaging, effectively eliminates interfering noise which degrades the received signal.

Learn more about the DSP TDR here.

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