Application Note Troubleshoot Wire Cable Assemblies with Frequency-Domain-Reflectometry S820D Cable and Antenna Analyzer, Mil-PRF-28800F Certified Introduction Imagine being able to troubleshoot defective Wire Cable Assemblies without having to crawl through aircraft bulkheads or remove one access panel after another searching for a cable fault. The Anritsu S820D Site Master Cable and Antenna Analyzer is the answer and Frequency Domain Reflectometry (FDR) is the measurement technique. The Site Master is battery operated so it can be carried to aircraft locations or remote installations where there is no line power available. Better yet, the diagnosis can be carried out at one convenient point where the wire cable end connector is located, such as an equipment bay that is accessible. The S820D Cable and Antenna Analyzer is Mil-PRF-28800F certified for Explosive Atmosphere, Salt Exposure, Drip Proof, Splash Proof, Water Test, Bounce and Loose Cargo. In its normal applications, the Site Master is used extensively to evaluate traditional coaxial cable installations with industry standard coaxial connector interfaces. These include type N, TNC, SMA, K, 7/16 DIN, BNC, UHF, HN, and many others. The FDR diagnostic procedure provides very helpful information on cable or antenna conditions such as damage along the cable length, moisture entry into antennas or connectors, dielectric aging and other common deterioration processes. Frequency Domain Reflectometry It is recognized that when any RF signal traveling down a transmission line encounters a fault or discontinuity, a portion of that signal will be reflected back toward the input connector. For any single frequency data, no determination can be made of the distance down to that fault. However, if a broad band of test frequencies (f1 to f2) are used, the Site Master FDR technique uses powerful data computation algorithms (referred to as an inverse FFT, Fast-Fourier Transform) to compile those individual test signal reflections using their measured phase and amplitude, which displays the integrated reflection and distance information. The FDR advantage over the common pulsed-based TDR technique is that much higher signal power is used and thereby longer cables can be measured. As shown in Figure 1, the integrated amount of power launched is much higher with FDR. Figure 1. Frequency-Domain-Reflectometry (FDR) in the Site Master provides higher test signals in the RF frequencies, thereby making coaxial cable (or waveguide) distance-to-fault measurements more sensitive.
Adapting the FDR Technique to Aerospace Wire Bundle Diagnostics The same Site Master Cable Analyzer capability can now be applied to testing wire cable assemblies and harnesses used in the aircraft industry for airframe electrical control cabling. Aircraft wire testing techniques are basically the same as testing coaxial cables. The major difference is that the (low frequency) wires are tested in pairs and at frequencies well above the frequencies specified by Aircraft Wire Cable manufacturers. A comprehensive industry survey was accomplished in a Master s Thesis which addressed the current status in aircraft wire integrity problems, and reviewed many of the present wire cable troubleshooting and diagnostic practices. [1] Within any common bundle of perhaps 61 wires, any random pair of wires will present some RF characteristic impedance that will vary depending on cable design, with values from approximately 75 to 120 Ohms. You can think of any random selection of two of the many wires as similar to the transmission configuration of the classic 300-Ohm twin-lead home TV antenna cable. Naturally the true twin lead is carefully controlled for 300-Ohms and for consistency along the path. The characteristic impedance of a particular wire pair depends on many factors, which include the diameter of the wire, the spacing between the two wires, the dielectric of the insulation around the wire, the position of all the other wires with respect to each other, and the uniformity of the wire-to-wire positions as they travel down the bundle. Wire cable assemblies evaluation technique using FDR is basically the same as the coaxial cables technique and the process uses the same basic test equipment. Wire cable assemblies (in aircraft wiring for example) contain diverse signal paths that include everything from coaxial cables to a simple wire running the length of the wire cable assembly. Depending on the path design, the wire cable assemblies are used for everything from supplying DC voltages to RF frequencies as high as several MHz. Wire Cable Assemblies with the typical contact pin interface that are associated with the individual coaxial or wire paths have very little or no definable RF performance information. Some pairs are shielded throughout their length and others are merely normal wire insulation against other insulated wires. Some might be shielded twisted pairs for noise avoidance. The S820D, when calibrated to 100 Ohms impedance will display return loss, VSWR, distance-to-fault and transmission performance of the wire pairs referenced to 100 Ohms. The wire pair characteristics described in the above paragraph is typical of any quality manufactured wire bundle assembly. Since the Impedance and Transmission losses can be well defined and verified very accurately with a high performance instrument, defects along the length of the bundle can be easily located with the S820D Site Master. And the biggest advantage of this procedure is that the test can be carried out right on one end of the bundle at the entry connector, thus avoiding physical inspection along tens or hundreds of feet of inaccessible wire bundles buried in the airframe. Here are some potential wire defects and discontinuities that are easily found with the S820D: Shorts Opens Kinks or sharp bends in the wire Insulation damage Damaged shield Compressed shield Defective splices Defective interconnect plugs and jacks 2
Fabricating an Interface Adapter from Coaxial to Wire Pairs The design considerations for creating an adapter from unbalanced 50 Ohm coax which is the test port connector for the Site Master is much like the common TV cable adapter called a balun. The inexpensive and widely used balun provides a transition from 75 Ohms unbalanced coaxial cable to 300 Ohms balanced twin-lead. In the case of wire cable assemblies, the balanced impedance is designed for 100 Ohms. For the more precise Site Master measurement adapter, see Figure 2. This is more sophisticated than it looks, with the ability to not only convert the 50 Ohm test signal to a balanced wire configuration, but the balanced probes can be adjusted to match the spacing of the chosen two connector pin locations. The two probe wires come out pointed to the lower left. Figure 2. Coaxial to wire cable adapter which electrically matches the 50 Ohm unbalanced Site Master test port to a balanced wire characteristic impedance of approximately 100 Ohms. In addition the spacing of the two probes can be adjusted to match the pin spacing at the launch connector. The interface from the S820D and the wire pairs under test are a very important part of a quality and repeatable measurement. Figure 3A shows an early proof of technique setup which takes the two balanced signals and routes them down to the cylindrical connector where contact is made with two chosen wires as a pair. Figure 3B shows the 100 Ohm precision Balanced Termination Device. As shown in figure 3, the probe uses the same pins that are actually used in the mating connector at the usual cable assembly. A standard unbalanced open/short/load traceable calibration is performed using high quality coaxial connector devices prior to the connection of the balanced wire probe. This method of calibration and measurement insures that results are correct and very repeatable. Probes are provided to accommodate various spacing and pin diameters, including male or female connections. The Site Master instruction manual includes detailed setup and step-by-step instructions for calibration and test. Figure 3A. This picture shows an early proof-of-performance setup with the balun and the two balanced wires connecting to the cable-input connector pins in the middle of the picture. Figure 3B. This picture shows a proof-of-performance 100 Ohm balanced termination installed at the opposite end of the cable. 3
Measurement Considerations To satisfactorily evaluate Wire Cable Assemblies using RF measurement techniques, first consider how the particular wire path through the cable assembly is configured. The path might consist of individual wires routed though the Wire Cable Assembly, alternately, pairs of wires routed parallel to each other; a pair of wires twisted together; multiple pairs of twisted wires routed together; shielded coax, and many other versions of shielded and unshielded wires. Figure 4 shows a variety of wires; Twinax, shielded wire pairs and several wires inside a single shield. There are a variety of connectors that are used to terminate the wire cable assemblies, but the most popular is the cylindrical connector used for decades in Aerospace and Military applications, highly reliable and with known performance. Information on the overall length of the wire cable assembly is also necessary. After the construction of the individual paths is determined, a test plan can be formulated for each test path. Cylindrical connectors are supplied in a very large variety of configurations. They can be supplied with just a few pins to as many as 61 pins. The cylindrical connectors can contain up to nine different contact pin sizes for both male and female connections. The larger size pins are typically used for applying AC or DC power to installed equipment. The smaller contacts, male and female sizes 22, 20, 16 and 12, are often used for transporting low frequency signals for control and data transfer between electronic equipment. This application note addresses smaller contacts used for signal paths but similar techniques can be used for even AC and DC power delivery. The cylindrical wire cable connector is the most popular, but any connector but any connector that utilizes the standard contact sizes 22, 20, 16 and 12 can be tested using the coaxial to wire adapter. Figure 4. This picture shows the typical internal construction of a cylindrical connector based cable assembly. Wires and cables shown in the photo are blue Twinax cable, several shielded 2 wire pairs, shielded bundle of 8 wires and individual wires routed through the cable. Site Master Adapters, Calibration and Test Accessories The S820D Cable and Antenna Analyzer, completely equipped with the option 22xF, adapters and calibration devices fully equips the user to test VSWR, return loss, distance to fault as well as end to end two-port transmission insertion measurements. Users can configure the S820D to cover any part of the 2 MHz to 20 GHz frequency range for any of the above measurements. The typical frequency range utilized for testing Wire Cable Assemblies should always start at 2 MHz and typically ends at 250 MHz. Depending on the configuration of the path, desired distance resolution and distance, a variety of high end frequency selections can be applied. The key to performing quality and repeatable measurements on the wiring enclosed inside a cable assembly is to perform a measurement that is least affected by the wire s surroundings. The many wires, grounds and coaxial cables surrounding the wires under test can strongly influence the measurement result if not approached correctly. Evaluating wires in pairs using balanced mode conditions minimizes the influence of the surrounding construction. Wire pairs present characteristic impedances that vary depending on cable ddesign (typically in the 75 Ohms to 120 Ohms range), thus the furnished coax wire adapter is designed for the nominal center, 100 Ohms. The calibrated S820D with the Coax to Wire adapter allows the user to perform 100 Ohm balanced measurements of the chosen wire path. The S820D will display return loss, VSWR, distance-to-fault and transmission performance of the wire pairs referenced to 100 Ohms. Many attributes of the wire pairs being tested will affect the displayed test results on the S820D, allowing the user to assess the effect of these attributes on the wire assembly performance. Among these attributes are the diameter of the wires, the spacing between the wires, the dielectric of the insulation around the wires, the position of the wires with respect to each other, opens or shorts along the wires, and the uniformity of the wires traveling through the wire cable assembly. 4
In addition to the coax-to-balanced signal adapter, Anritsu also makes available a 100 Ohm termination that is used to identify and verify performance at the opposite end of the wire cable assembly. The Instruction Manual for Site Master with Option 22XF provides detailed step-by-step instructions for the calibration process, and measurement procedure, both for distance-to-fault and for 2-port insertion loss processes. Typical Performance Data The following measurements were made with the Model S820D Site Master. Tests were performed on a multi-wire Aircraft Cable that measured 20 ft long, as shown in the test setup of Figure 5. Figure 5. This Site Master test setup used a 20 foot wire cable assembly with the coax-to-wire adapter shown at the cable connector (left side of picture). It used an early version of the 50 Ohm unbalanced to unbalanced wire probes. On the right side of the picture, a 100 Ohm termination is installed on the far at the opposite end of the wires under test. Distance-to-Fault Measurement Displays Figure 6 shows a distance-to-fault measurement, made on wires C& D at Connector #1 shown in Figure 4. The measurement was made with the opposite end port 2 open. Note the useful information included with the display. The display distance range starts at 0 on left side and ends at 28 feet on the right side. The return loss at the top of the display is 0 db and -40 db at the bottom. Propagation velocity is 0.7. The insertion loss is 0.110 db/ft. Markers are displayed at approximately 0.76 ft (marker 1) and 20 ft. (marker 2). The return loss at marker 1 is approximately -13 db, a good performance value, and marker 2 is approximately 0 db caused by the open cable at the opposite end. A cursory look at the time reflections from marker 1 to 20 feet shows that the wire pair being analyzed has NO significant reflections or apparent defects. Figure 6. This display shows the results of an FDR measurement made on the equipment setup shown in Figure 4. Marker 2 at 21 feet shows the effect of an open circuit on the far port, a reflected signal 0 db down from the launched signal. 5
Figure 7 shows the same Distance-to-Fault measurement made on wires C& D at Connector 1, but this time the measurement condition was made with the opposite end port 2 terminated with a 100 Ohm load. Note that all of the settings are the same as Figure 5. The only difference is the 100 termination is installed at the opposite end. Markers again are displayed at approximately 0.76 ft (marker 1) and 20 ft. (marker 2). The return loss at marker 1 is approximately -13 db the same as before, a good performance value, and marker 2 is approximately -17 db a good result of a quality 100 Ohm termination installed at the opposite end. This is especially so, since the characteristic impedance of a test wire pair is almost never exactly 100 Ohms, but that does not impede a valuable measurement. Figure 7. This display results from the identical test setup as Figure 5, except that the far end port is this time terminated in a 100 Ohm load, which shows a -17 db reflection at 20 feet. End to End Insertion Loss Scalar Measurement An end-to-end 2 port insertion loss (or continuity check) can be performed on any pair of wires installed in a multi-wire cable. The distance from end to end can be separated by hundreds of feet during measurements and it is basically limited to a total loss of about 40 db. The S820D mode is set to the Cable Loss Two Port and the S820D source test output option 22 is connected to the input connector end of the pair of wires. The scalar detector then connects to the opposite end of the cable to complete the measurement. Figure 8 shows the measurement result, again for wires C and D from end to end. This loss measurement is made over the full 20 ft length. This test could obviously NOT be performed on an installed wire cable assembly where the opposite end would be too far from the Site Master port 2 detector. Again note the useful display information. Average wire pair loss is 2.52 db, over the frequency range of 2 MHz to 250 MHz. As indicated at marker 1, the insertion loss is less than 1 db and marker 2 about 5 db. This information is typical of a properly performing pair of wires with a very low loss at the low frequency and a gradual slope to the higher frequency. Depending on the wire design and installation, the loss at the higher frequency can vary considerably. The numerical value of insertion loss, measured with these RF test frequencies could be particularly useful for wire cable functions of carrying signals with video components or HF frequencies (2-10 MHz). 6
Figure 8. This display shows an RF insertion loss plot vs distance from Connector #1, for the same wire cable assembly of Figure 5. OK, All You Wire Cable Assemblies! We Have Your FDR Signature For those technical managers who are tasked with creating new testing strategies for wire cable maintenance, across the globe and across decades of the lifetime of operating aircraft, there is an intriguing maintenance process to consider. What if cable pairs would be measured and documented during or after manufacturing to capture a reference performance for future troubleshooting comparisons? Every wire pair even in good working condition will present a unique signature based on the total environment of the wire pair and for that INDIVIDUAL pair. Because the S820D is a very high performance instrument that can be calibrated with precision calibration devices and used with a precision interface, the measurements are extremely repeatable. Measurements might be performed and compared over many years to find faults, verify performance and predict potential failures. By saving the installation signature in a maintenance database, future troubleshooting procedures on an identified wire cable could be enhanced, system by system. Conclusion For an end user, the wire cable assembly analyzer capability adds significantly to the already high value of the Model S820D Site Master. The wire path evaluations with precision interface adapters insure measurement accuracy and repeatability that allow the end user to perform unparalleled high quality measurements. Wire cable assembly manufacturers can measure and document individual wire paths for future customer troubleshooting and verification references. The end user of the Cable Assembly can troubleshoot and verify the wire paths with a high quality, portable, battery powered, user friendly instrument that will evaluate almost everything used for transporting low frequency signals, RF and Microwave up to 20 GHz. References [1] Kiptinness, Susan Jeruto, An Analysis of the Conventional Wire Maintenance Methods and Transition Wire Integrity Problems Used in the Aviation Industry, MS Thesis presented to the Dept. of Technology, East Tennessee State University, August, 2004 7
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