Digital Cable Test Technology


Fluke developed a revolutionary approach to cable testing: digital signal processing or DSP technology. This technology offers unique diagnostic (fault isolation) capabilities, fast test times, compatibility with Accuracy Level II for TIA TSB-67 for both the Channel as well as the Basic Link, the ability to distinguish between noise and crosstalk, accurate TDR measurements, as well as, newly developed Time Domain Crosstalk Analysis (TDX) measurements.

The Contribution Made By Digital Test Technology

NEXT (Near End crosstalk) is the most challenging UTP performance parameter to measure accurately. The Digital Cable Test Technology developed by Fluke leapfrogs the "adapted analog" cable tester performance. Digital delivers Accuracy Level II for both the Basic Link and the Channel.

The Accuracy Level II standard of performance could not be met with any of the analog testers on the market. All of the vendors of analog cable testers have introduced a "second generation" tester. These units are equipped with special cable connectors in the instrument which produces less crosstalk and less signal imbalance than the standard 8-pin modular plugs. This method allowed them to achieve Accuracy Level II for the Basic Link. This adapted analog "solution" does not deliver Accuracy Level II for the Channel as explained later in this section.

New Technology Ensures Fault Isolation

To measure NEXT the DSP-100 sends a pulse that very closely resembles 100 Mbps LAN signals into one pair and captures the signal induced on the other pair as a result of the crosstalk between these two pairs. The captured signal is digitized (amplitude and time - similar to a digital O'scope - and is analyzed using Digital Signal Processing algorithms to provide a very accurate picture of NEXT over the total length of the link (including end-connectors) in both the frequency and the time domain. This data capture does not use any "time gating" as some competitors have tried to describe the DSP technology.

The frequency domain answer obtained by the DSP-100 very accurately tracks the results obtained with analog swept frequency methods of laboratory test tools. The DSP technology always provides a resolution of 100 kHz along the frequency axis. The small step size between frequency points assures that the DSP-100 accurately constructs the NEXT curve in the frequency domain.

If a cable fails the NEXT test, the time domain signal analysis (which we call TDX Analysis) allows the tester to pinpoint the location(s) along the cable where the excessive crosstalk occurs.

Diagram of Fault Isolation
Figure 1

Rather than only tell the technician that a link fails because the "worst case" NEXT value exceeds the limit at say 85.6 MHz (as analog testers do, for example), the TDX Analysis in the DSP-100 also reports the fact that the major contribution to that crosstalk is occurring at 168 feet from the tester (for example). An example of such a diagnostic plot of a Cat 5 link in which a Cat 3 connector had been installed is depicted in Figure 1.

The majority of crosstalk problems occur at cross-connects or terminations. The technician can immediately identify this location and go right ahead to perform the corrective action. This is a far cry from the general "Fail" message which requires that the technicians perform a number of additional measurements to locate the link segment in which the defect is located.

There is a major difference between TDR (Time Domain Reflectometry) and TDX. Many potential customers are confused and/or misled by the "competition". See the section "TDR versus TDX" for more information.
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Fast Test Time with Accuracy

A record test time without sacrificing measurement accuracy is one major way in which digital technology produces tangible benefits for the user.

The method with which current analog testers make NEXT measurements - the swept frequency analog approach - means that the tester sends a series of signals at different frequencies in one pair of the two pairs to be evaluated, and listens on the other pair to measure the crosstalk. Three factors impact the duration of the NEXT test of a link:

  1. the frequency range over which NEXT is to be tested, 
  2. the number of "points" (discrete frequency values) at which NEXT is to be evaluated, 
  3. the number of wire pair combinations for which these tests are to be executed. A typical four pair UTP cable requires that six NEXT tests are to be performed (six different combinations of pairs). 
Diagram of NEXT in Frequency Domain
Figure 2 - Typical NEXT in the Frequency Domain

Many vendors' testers offer a selectable step size with which the tester "sweeps" the frequency range. The more thorough test mode makes the NEXT measurements at more frequencies across the spectrum to produce a more accurate characterization of the link as required by the TIA TSB-67. But, you guessed it, this mode typically takes too much time; consequently, users choose frequency step sizes that produce less accurate - and non-standard compliant - results in the interest of saving time.

Figure 2 shows a typical NEXT curve as a function of frequency. The irregular shape of this curve should make it intuitively obvious that unless NEXT is measured at many points along the frequency range, peaks (worst case NEXT) could easily go undetected. Therefore, the TSB-67 defines a maximum frequency step size for NEXT measurements as shown in Table 2.

Table 2 - NEXT Test Sample Spacing.
Frequency Range (MHz) Maximum Step Size (MHz)
1 - 31.25 0.15
31.26 - 100 0.25

With the digital test technique, there is no longer a need to trade thoroughness of measurement against speed. It measures NEXT for six combinations in a four pair UTP cable in approximately 17 seconds, about 2 to 6 times faster than any analog tester. While analyzing the NEXT performance from 100 kHz to 105 MHz in record time, the DSP100 presents data at 1050 points - more than required by the TIA link standard.
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Level II Accuracy for both the Basic Link and the Channel

NEXT is a critical performance parameter for installed UTP links. The NEXT of a link not only depends on the performance of the cable itself, but is very much impacted by the connectors along the link (including the end-connectors), as well as, the workmanship of the connections (the distance over which the pairs of wires have been untwisted). Connectors introduce a point source of crosstalk and are a major contributor to crosstalk in a link.

The ubiquitous 8-pin modular plugs and jacks (usually referred to as RJ-45) make a significant yet unpredictable contribution to the NEXT performance of a link. The measurement uncertainty or effect on accuracy of an 8-pin modular plug at the end of the link (which plugs into tester) has been estimated to be 1.8 dB. The existing analog test technology using a frequency sweep to measure crosstalk at the many points in the spectrum cannot undo or "separate" the effect of the end connection (as required by the TSB67 guidelines) and cannot provide answers with the accuracy specified by Level II requirements. TIA TSB-67 specifies that the Residual NEXT performance of a level II tester be not less than 55 dB at 100 MHz. The modular 8-pin end-connector which is Category 5 compliant has a worst case NEXT limit of about 40 dB at 100 MHz and does not provide the required value for Residual NEXT. This is the reason that competitors using analog swept frequency methods are introducing products with "low NEXT" connectors in the tester. (They are not "NO NEXT" connectors and require special patch cords to connect between these non-standard tester connectors and the link to be tested.) This is their solution to meet Accuracy Level II for the Basic Link. However, they typically do not mention the limitation that this "special connector solution" only can deliver Accuracy Level II performance for the Basic Link.

Diagram of Measurement Accuracy using Analog Tester
Figure 3 - Measurement Accuracy of Channel for an Analog Cable Tester using an Adapter

These same competitors contend that Accuracy Level I was defined to test the Channel. Nothing is further from the truth. Both Basic Link and Channel should be measured with the highest accuracy affordable. These vendors are misleading the market by confusing accuracy limits due to measurement techniques with the performance requirements of the links to be tested.

A tester with a special connector for the link under test must use an adapter to connect and measure a Channel which ends in an 8-pin modular jack - See Figure 3. One end of the adapter accommodates the 8-pin modular jack with its 40 dB NEXT performance limitation at 100 MHz and produces all of the measurement uncertainty and inaccuracy which led to the design decision to change the tester's connector to be able to achieve higher accuracies. So with this adapter, these testers do not and cannot meet Accuracy Level II.

Diagram of DSP-100 Connector Compensation
Figure 4 - DSP-100 Connector Compensation

The time domain information in the captured crosstalk signal allows the digital processing algorithms in the DSP-100 to analyze and compensate for the NEXT contributed by the end-connectors of the link under test (See Figure 4). The Residual NEXT specification for the DSP-100 with this connector compensation scheme exceeds the TIA Accuracy Level II requirements. Consequently, the DSP-100 can measure all links ending in 8-pin modular connectors with the highest degree of accuracy by simply plugging the 8-pin modular jack into the tester.

The TSB-67 document states that "The accuracy level shall be specified for both the basic link configuration and the channel configuration." (TSB-67 A.1 Accuracy Requirements, page 15). All competitors are violating this requirement for obvious reasons. The calculated tester accuracy levels for NEXT for each of the link configuration per the TSB-67 formulas is as follows:
Level Channel Basic Link
Accuracy Level I 3.4 dB 3.8 dB
Accuracy Level II 1.5 dB 1.6 dB

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Ability to distinguish between noise and crosstalk

Some competitors have attacked the performance of the DSP-100 based on the following statement: "Digital testers use broadband receivers and are susceptible to noise on the cable."

Another example of how the competition has attacked the superior performance of the DSP100 and - in their ignorance - tried to turn it into a DSP-100 disadvantage! The truth is that the DSP-100 is actually more immune to noise on a cable than any of the competing products on the market today. And the DSP-100 is the only tester that identifies the presence of noise as part of the Autotest.

When measuring NEXT, the DSP-100 indeed uses a broadband receiver. To prevent noise from affecting the measurement's accuracy, the DSP-100 executes multiple measurements, compares the results of consecutive tests to detect the presence of noise, and averages the measured values - a common technique to remove the effects of noise. If noise exceeds a threshold value, the DSP-100 displays a message on the screen telling the user that noise has been detected on the cabling link under test. Analog swept frequency testers, on the other hand, have no way of telling the difference between a signal induced by noise or by crosstalk when measuring NEXT. Even a few millivolts of noise can cause these competitive cable testers to erroneously fail the NEXT test of a link.

Several customers who encountered NEXT failures with other testers, have learned using a DSP-100 that the real problem was noise on their cabling system. Retesting these failing links with the DSP100 showed the real NEXT performance of the cabling; it was well under the test limits. But the DSP-100 detected noise on the cabling plant. This is a problem that requires a different corrective action than poor NEXT.
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TDR versus TDX

The DSP-100 supports a true TDR (Time Domain Reflectometry) measurement and displays the TDR plot graphically. In addition, the DSP-100 provides a unique troubleshooting capability which Fluke has called "TDX" or Time Domain Crosstalk Analysis. (The name was chosen in analogy to TDR.)

If the characteristic impedance along a pair of the cabling link is not constant, some of the transmitted signal is reflected (on the same pair) at the point of change in the characteristic impedance. The TDR test allows the DSP-100 to identify such points and locate them with accuracy. Reflections due to impedance problems along the cable are captured and reported as anomalies. Most competing testers offer this capability with more or less the same accuracy. Also, the DSP-100 TDR function is equal to the "down-line impedance" measurement as this test is named in some competitive products.

There are a number of more subtle problems that do not show up in the TDR test because the impedance change is minute, yet the cable will not transmit the LAN signals very well (If at all!). Such problems are due to point sources of crosstalk and may be caused by untwisting cable pairs more than the recommended 0.5 inch at a termination or connection point. All of these problems are points where crosstalk is excessively high.

The TDX function performs the NEXT measurement (transmit the test signal on one pair and capture the response on another pair). Rather than report the results of this NEXT test in dB and MHz - which do not assist in "fixing the problem," TDX provides the answer in feet or meters and plots the profile where, along the cabling link, Crosstalk is being generated. This picture tells the technician where this more subtle, but critical cabling error, is located.

Conclusion: We are talking about two totally different tests, each very valuable!

  • TDX is a unique and different presentation of the NEXT tests executed by the DSP-100 
  • TDR is a test for consistency in the Characteristic Impedance over the length of the cable 
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