The TDR (Time Domain Reflectometer) has been used in cable networks as an effective tool to find faults for decades. So why consider changing anything? If it isn’t broke don’t fix it, right?  As TDR instrument development has been neglected, there is now an opportunity for improvements as can be seen in these points:

  • Any outage situation obviously requires speed, and a TDR test may be required, so quick testing and a smooth process make a difference here.
  • TDR test results are often used in deciding about whether to replace or splice, and in certifying a newly installed span. Utilizing up-to-date data communication and cloud storage makes this process much smoother, more efficient and quicker.
  • Accuracy and sensitivity in TDR measurements is important, as in many cases the cable is underground, requiring excavation. Implementing step technology that is typically only found in lab-grade instruments brings the hand-held TDR a major improvement in sensitivity and accuracy, meaning that smaller holes can be dug.

TDRs with older technology are coming up short. With a fresh look at the technology available, VIAVI saw that a field instrument could be developed to offer significant improvements in measurement capability and a faster decision-making process with cloud data management.

As a time-saving, essential tool for testing coaxial cable, every maintenance tech should have a TDR.  Customers rely more and more on always-connected internet access, so the urgency of repairing faults as quickly as possible is increasing. When a tech has a TDR at hand, the process can happen quickly without having to call and interrupt another tech who has a TDR and have them bring the instrument for the test and then return to what they were working on before they were interrupted. This takes two techs out of commission when only one is needed for the job.  With the speed and efficiency of the DSP TDR, the time savings bringing systems back online, the elimination of unnecessary waiting time, and the elimination of wasted tech time, it’s time to equip every tech with a DSP TDR.

1. Lab-Grade Technology in a Field Instrument – Find faults anywhere in the cable

Measurement sensitivity and accuracy is improved by using step TDR technology, which is typically only available in laboratory TDRs that are not ideal for field applications.  The VIAVI DSP TDR implements step-type technology in a rugged, hand-held field instrument perfect for use in the cable outside plant.  Take a look at this blog entry for information about the advantages of step over pulse TDR technology.  One big advantage of step technology is its ability to locate water migration in the coax cable.  See events that others miss

2.  Find Water in Coax Cable

Water in a coax cable span can result from a variety of things, including poor craftsmanship, loose connections, insufficient weatherproofing, cracked or damaged cable, etc.  Moisture in coax cable causes excessive loss and, while the loss has a detectable impact, without proper test equipment, the location of the water within the cable can be difficult to find until the cable has significant corrosion damage. In some cases, techs compensate for the extra loss by increasing gain on successive amplifiers. Also, don’t know whether to dig, or replace span.

Figure 1  Trace showing full length of cable, ending at 120 meters.

Figure 2  Zoomed in, showing tap to tap.

Figure 3  Zoomed in more, focusing on water cavity.

Figure 4  Event list showing all events.

Fortunately, the VIAVI DSP TDR with step technology has the functionality needed to detect water in cable, so when excessive cable loss is detected, the water damaged section can be repaired, avoiding the expense of replacing the entire cable span.

3. No Blind Spot – See close-in faults

A “blind-spot” is inevitable on a pulse-type TDR, and its length depends on the pulse width.  The pulse-type TDR doesn’t receive and transmit simultaneously and must wait for the pulse transmission to end before “listening” for the reflection. The longer the pulse width, the bigger the blind spot.

The VIAVI DSP TDR uses step-type technology and has no blind spot. So why is this a big deal? Often a tech doesn’t have any idea where in the cable the problem is, and that’s why they’re using a TDR. A common practice is to take an extra-long jumper (the same length as the expected blind spot) and use that between the instrument and the cable under test. The tech must keep track of this jumper to ensure that it will be available when the test is needed, and the jumper could get damaged in the truck, without the tech’s knowledge and begin to impact measurement results. One common example of cable damage close to the cable end is when a pedestal is “slammed” down into place and kinks the cable. A TDR without a blind spot can find these kinds of cable problems.

4. On-Screen Event List – Save time locating faults

Using a TDR can take some skill that’s acquired with experience. The trick is to identify events in the TDR trace, and determine which ones are causing the problem. One thing that helps in this situation is the “event list” in which all reflective events are listed, with an indication of the return loss of each reflection. Some TDRs only make this list available via PC software, which makes getting the info so cumbersome that techs end up just doing their best with trace analysis. The VIAVI DSP TDR provides a simple on-screen events list that enables the tech to see exactly how many events are present on the cable, and easily determine whether they are splices, splitters, or damage to the cable.

Figure 2 DSP TDR on-screen events list shows tech location of events and severity of the reflection.

5. Simplified Documentation – Save time

Documentation is used to qualify new cable spools before use, or to document the quality of a replaced cable span that is ready for activation (contractor proof of work). Technicians document bad span(s) to show need for replacement, as opposed to taking time to repair multiple faults or trying to repair unfixable faults. Technicians also document before and after TDR test results to show work done.

When there is damage to a cable, it’s often desirable to communicate the severity of the damage to a decision maker (management) in order to approve the action required to make the repair.  Traditionally the trace would be saved on the TDR, transferred to a PC via proprietary file type with a data cable, opened with installed software application and then sent to a manager. This all takes time and isn’t really something a tech wants to do out in the field. The VIAVI DSP TDR has a built-in WiFi adapter, and with an internet connection, communicating this data to the manager is much simpler. VIAVI StrataSync cloud enabled data management is included with each DSP TDR, so the tech can just sync the data, and it becomes instantly available to the manager, permitting much quicker decisions.  The trace data can also be held indefinitely (Plus option) for future reference.

Figure 3 Trace data as stored in StrataSync

Why a new TDR?

As discussed here, the VIAVI DSP TDR offers many advantages over conventional TDRs commonly used in the field today. With sensitive and accurate step technology built-into a rugged, hand-held field instrument, techs can find faults anywhere in the cable span, from right up close, to the far end of the cable. This sensitivity allows for finding water migration in cables well before corrosion starts, and before conventional TDRs can see them. The up-to-date user interface makes operation simpler, speeding up fault resolution, and built-in WiFi technology and cloud-based data management enable quick decision-making to resolve issues faster.

Now is the time to upgrade your existing TDRs, but also to equip every maintenance technician with a TDR, to enable them to quickly and efficiently fix issues, saving time and lowering operating costs.

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