60-932-446 PERFORMANCE TESTING - Electrical TESTING PROCEDURES As part of the Quality Assurance procedures during the manufacturing operation, MC 2 coaxial cables are sweep tested for structural return loss and insertion loss. TDR (Time Domain Reflectometer) measurements are also made to verify the cable length as well as to locate spot damage of any nature. Although these tests are performed prior to shipment to the customer location, it is still advisable to perform sweep measurement tests upon receipt of the shipment. This process will eliminate any reels which may have been damaged during the shipping or unloading processes. After unloading the reels from shipping, a portion of the cardboard wrap must be opened to expose the outside end of the cable on the reel. It is unnecessary to remove this completely, in fact its design is such that it enables access to the end and can be closed after testing has been completed. The ends of the cable are capped prior to wrapping and the caps must be removed before testing. owever, the cable ends are left as prepared in the plant so that trimming or coring will not be necessary in preparation for the sweep testing. In order to discover any shipping damage as quickly as possible, the customer should perform the TDR test first, then structural return loss and finally, insertion loss. The recommended methods for each of these tests are detailed on the next pages. TDR TESTING Use of the TDR is the quickest and easiest means of locating cable faults and verifying the overall length of the cable. The principle purpose of this operation is to inject an electrical pulse in one end of a cable and observe the display of the pulse by means of a LCD (Liquid Crystal Display). A portion of the initial pulse is reflected back to the injection end if impedance changes in the cable (such as those caused by damage) are encountered. With all TDRs it is possible to locate the damaged area and the cable length. The primary is the velocity of propagation of the cable under measurement. The velocity of propagation is a measurement of speed at which the electrical signal travels through cable. It is defined as a percentage of the speed of light in a vacuum. MC 2, being an air dielectric, has a velocity of propagation of 93%. With this factor properly entered into the TDR, the location of a damaged point is determined by measuring the time required for the injected pulse to reach the damage and reflect back to the input end. This time is converted to distance and displayed as such on the LCD. With certain model TDRs it is possible to not only locate a fault, but to measure the reflected magnitude of the fault as a reflection coefficient,, or as return loss in db. e=e E2 where: E = the reflected voltage E 2 = the incident voltage Return Loss =20 log 0 Since the purpose of the TDR test is to enable a quick location of damage and verification of length, return loss and reflection coefficient will not be covered. Operation of a particular instrument should be per the manufacturer s specifications, however, the following general points should be adhered to (figure 8-). Figure 8-. OUTPUT Time Domain Reflectometer. Calibrate the instrument for the correct velocity of propagation of the cable under test. If the instrument can be dialed in, the MC 2 V. P. is set at 93%. Some instruments have infinitely adjustments. If this is the case, hand measure approximately 50 to 00 of the same cable and adjust the TDR until the display matches the measured length. 2. Set the impedance reference of the instrument for 75 ohms for CATV cables. 3. Adjust the display on the LCD for a sharp, clear baseline and position the leading edge of the injection pulse at a convenient starting point, or graticule, on the screen. 4. Use the pulse width recommended by the TDR manufacturer for a given length of cable. In general the longer the cable to be measured, the wider the pulse width. Narrow pulses are a more critical measurement but may not contain enough energy for long cable lengths. 8-
60-932-446 5. Most TDRs utilize an adapter from RG-6 to the size hardline cable being checked, or use cable leads with alligator clips. Clip one to the center conductor and one to the aluminum sheath, making sure not to short the two at any point. 6. Adjust the footage display so that the far end of the cable is visible on the LCD. 7. Different TDR models utilize different location techniques, however, after the above settings have been made it will be possible to measure the distance to a cable fault or to the end of the cable on the reel. Remember to: A. Make measurements from the leading edge of the injected pulse to the leading edge of any reflected pulse. B. Subtract the length of the test leads from the measurement. (Note: If possible use test lead lengths at least ten to fifteen feet in length to ensure the resolution of any reflection (fault) at the beginning of the cable under test.) Test leads that are too short or pulse widths that are too wide could cover up any faults at the beginning of the reel under test. Always use the TDR manufacturer s settings for the test equipment being used. C. It may be necessary to increase the instrument sensitivity in order to see reflections from the far end of the cable. After the instrument sensitivity is increased, very minor impedance variations will result in a wavy or noisy appearance of the baseline. This is normal and not cause for rejection. Faults are identifiable above the noise and generally display a characteristic response signature. 8. Typical Response Signatures (figure 8-2) Open - Strong positive pulse similar in shape to the injection pulse. Short - Strong negative pulse similar in shape to the injection pulse. Water - Two part pulse - Similar to a sinusoid with the negative portion firsts. STRUCTURAL RETURN LOSS Structural return loss (SRL) is usually caused when minute cable imperfections (micro-reflections) occur is a repeated or periodic pattern along the cable length. The result is an SRL spike which causes additional and unwanted signal attenuation at the spike frequency. The SRL spike occurs at that frequency where the repeated reflection pattern distance is equal to /2 of the electrical wavelength in the cable. A percentage of the transmitted signal is always reflected back when a mismatch of impedance is encountered. For example, when a cable of characteristic impedance Z is terminated to a device with characteristic impedance Z 2, the reflection coefficient is defined as: e = Z -Z 2 Z +Z 2 This coefficient is a measure of percentage of reflected signal and is converted to db (decibels) of return loss by: db = 20 log 0 Periodic impedance discontinuities may be present in a cable as a result of variations in raw materials or from the manufacturing process. The effect of these periodic impedance discontinuities on the cable is the signal is reflected back and can be in or out of phase at a frequency of one half wavelength of the separation of the discontinuities. When testing coaxial cable for structural return loss, it is necessary to separate the reflections of the connectors from the reflections of the cable. This is accomplished through the use of a return loss. With this device, adjustment of the network varies resistance to match the characteristic impedance of the cable as well as capacitance to compensate for connector mismatch. When using the, the only reflections observable will be those which are structural in origin. To perform this test connect the cable to the return loss whose input and output connect to an RF Analyzer (a combination of an RF sweep generator, coaxial, RF amplifier and detector). The RF swept signal is split between the return loss and leg with standard s. The RF amplifier feeds a OPEN SORT WATER Figure 8-2. 8-2
60-932-446 detector whose output is displayed on a LCD. The result is a dual trace, one being the output of the and the second a reference trace of a specific db level. Matching the reference trace to the peak of a return loss spike determines the return loss of the spike in db. The characteristic impedance of each cable is read from the return loss when the structural return loss test is performed. The specification for MC 2 is 75 ohms ± 2 ohms. Variable Bridge Test Method - Gz. Assure calibration accuracy of all equipment by following the manufacturer s recommendations before beginning the test. 2. Turn the power es on for all pieces of test equipment and allow a warm up period of least 0 minutes. 3. Remove the cable end caps and attach the to one end of the cable and the terminator to the other (figure 8-3). 4. Adjust the sweep range and display to cover the bandwidth of interest, i.e. 5 Mz to Gz. 5. Adjust the system to assure a maximum power transfer into the cable by observance of the LCD. The resistance and capacitance controls are adjusted on both the and terminator until the display is lowered and flattened as much as possible. This establishes the condition of maximum power into the cable. 6. Locate a spike of interest with the frequency markers. 7. Adjust the s until the reference line coincides with the peak of the spike. The return loss is then read directly from the settings of the s, provided that the typical 2 db fixed loss pad to compensate for the loss is in the circuit. If not, subtract 2 db from the settings. 8. The characteristic impedance is read directly from the resistance dial on the. 9. Repeat this process for the other cable end. Network Analyzer Test Method - Gz The CATV Industry is now demanding components and coaxial cable that pass up to Gz and beyond for increased channel capacity. The structural return loss testing standard in the past has been the use of a sweep generator and/or spectrum analyzer. This set up works well at low frequency when coupled to a with a test cable/test connector arrangement. owever, with the increased frequency response to Gz the following precautions are recommended:. Test Leads to the Bridge - One must use high quality, stable coaxial drop cable to present a good (Z) Impedance Match. 2. Test Connector Interface - N type test connectors are recommended to interface the cable under test. 3. Bridge Selectivity - Use a high quality ( Gz). wide band V sweep generator 2 db pad coaxial RF amplifier terminator Figure 8-3. SRL test method The topics that follow will incorporate the use of a network analyzer to test coaxial cable and drop wire for SRL, attenuation and TDR functions, without disconnecting the cable under test. Structural Return Loss Structural return loss is the ratio of the incident signal to the reflected signal in coaxial cable. It is possible to mathematically analyze the forward or (Incident Signal) with the reverse or (Reflected Signal) to attain the electrical characteristics of coaxial cable. SRL can also be defined as the reflection coefficient of a cable, referenced to the cable s impedance. Cable Impedance Cable impedance is the ratio of the voltage to current of a signal traveling in one direction down the cable. The ratio of the inner and outer conductor diameters and the dielectric constant of the material (air) between the inner and outer conductors determines cable impedance. Where = dielectric constant D = Sheath Inside Diameter d = center conductor outside diameter RF ANALYZER V detector oscilloscope 8-3
60-932-446 Network Analyzer REFERENCE A B RF OUT IN PORT PORT Rf splitter A PORT s-parameter test set Rf B PORT fixed fixed Figure 8-4. Network analyzer with s-parameter test set Calibration Calibration of a typical network analyzer will require a set of calibration standards. With the network analyzer in the learn mode, these standards are used to teach the responses of a calibrated short, open, load and thru. Once completed, a two port analysis of coaxial cable or drop wire may be performed. Time Domain Gating External to the network analyzer, the test connectors interfacing to the coaxial cable under test, is the highest potential contributor to the overall SRL measurement error. Time Domain Gating is the option that can be used to effectively eliminate the response of the test connectors to obtain a more accurate SRL measurement. This technique can reduce or eliminate SRL s caused by the connector interface to the cable under test. Above is a typical network analyzer block diagram with an S-Parameter Test Set (figure 8-4). Equipment required:. 5-000 Mz network analyzer 2. S-Parameter test set 3. 50 or 75 ohm Calibration standards to match cable Impedance (Z) 4. Miscellaneous cables and test connectors. Conclusion The network analyzer, because of its fast response time and high accuracy, has the proven ability to be today s industry standard. With software and a personal computer, this system can be fully automated. Testing procedures are available through the Interface Practices Sub Committee at the national headquarters of the Society of Cable Telecommunications Engineers. ATTENUATION Insertion Loss Test By definition, insertion loss is the reduction of signal strength measured when a given piece of equipment is inserted into a line. This reduction of signal strength or attenuation is measured in the cable, in units of db/00 ft. The following procedure describes a method for measuring insertion loss of a coaxial cable in the frequency range of 5 Mz - Gz. In order to measure insertion loss the following pieces of equipment will be needed: Wideband sweep generator RF Switch Amplifier Detector Reference Attenuator Oscilloscope Interconnecting cables Cable connectors Shorting adapter Perform the test by using the following procedure:. Assure the accuracy of calibration of all equipment by following the manufacturer s recommendations. 2. Connect the test equipment as shown (figure 8-5). 3. Turn the power on all the test equipment and allow to warm up at least 0 minutes. 4. Adjust the sweep range and display to cover the bandwidth of 5 Mz to Gz and adjust the controls for a clear sharp trace. 5. On an alternate channel, using the appropriate shorting adapter, short the two cables coming from the RF coaxial. 6. Adjust the reference to 0 db loss and insure that the reference trace and test trace are the same level. 7. Remove the shorting adapter and return the RF to the channel being used for the test. 8. Remove the cable end caps from the cable being tested and attach the test leads to the cable ends using the test connectors. 9. Locate a frequency of interest with the frequency markers. 8-4
60-932-446 0. Adjust the reference until the reference trace coincides with the test trace at the point of interest.. If greater accuracy is desired, the RF output level may be increased with the vertical amplifier of the oscilloscope. 2. The setting of the reference is the insertion loss at the frequency of interest for that length of cable. 3. Convert the insertion loss to attenuation with the following formula: Attenuation in db/00 ft. = 00X Insertion Loss (db) Cable Length (Ft.) NOTES The listed attenuation for MC2 coaxial cable is measured at 68 F. Since attenuation is related to temperature, the test should be performed at 68 F to achieve comparable attenuations. If testing is performed at a temperature other than 68 F, the listed attenuation must be adjusted by ± 0.% per degree Fahrenheit for a proper comparison. V CRT oscilloscope RF ANALYZER wide band V sweep generator coaxial RF amplifier detector Figure 8-5. Insertion loss test 8-5