This file is raw output from pdftotext and may not be ideal for distribution. If you are a maintainer for Hackipedia, please sit down when you have time and clean this text version up. Source PDF: /mnt/main/jmc-storage/docs/SCTE/ANSI SCTE 050 Test Procedure for Measuring Regularity of Impedence of Coaxial Cable (2007).pdf Like all conversions the text below should be fully readable as UTF-8 unicode text. --------------------------------------------------------------- ENGINEERING COMMITTEE Interface Practices Subcommittee AMERICAN NATIONAL STANDARD ANSI/SCTE 50 2007 Test Procedure for Measuring Regularity of Impedance of Coaxial Cable NOTICE The Society of Cable Telecommunications Engineers (SCTE) Standards are intended to serve the public interest by providing specifications, test methods and procedures that promote uniformity of product, interchangeability and ultimately the long term reliability of broadband communications facilities. These documents shall not in any way preclude any member or non-member of SCTE from manufacturing or selling products not conforming to such documents, nor shall the existence of such standards preclude their voluntary use by those other than SCTE members, whether used domestically or internationally. SCTE assumes no obligations or liability whatsoever to any party who may adopt the Standards. Such adopting party assumes all risks associated with adoption of these Standards, and accepts full responsibility for any damage and/or claims arising from the adoption of such Standards. Attention is called to the possibility that implementation of this standard may require the use of subject matter covered by patent rights. By publication of this standard, no position is taken with respect to the existence or validity of any patent rights in connection therewith. SCTE shall not be responsible for identifying patents for which a license may be required or for conducting inquiries into the legal validity or scope of those patents that are brought to its attention. Patent holders who believe that they hold patents which are essential to the implementation of this standard have been requested to provide information about those patents and any related licensing terms and conditions. Any such declarations made before or after publication of this document are available on the SCTE web site at http://www.scte.org. All Rights Reserved © Society of Cable Telecommunications Engineers, Inc. 2007 140 Philips Road Exton, PA 19341 i Table of Contents 1.0 SCOPE .......................................................................................................................................... 1 2.0 EQUIPMENT ............................................................................................................................... 1 3.0 GENERAL: STEP FUNCTION RETURN LOSS ....................................................................... 1 4.0 STEP FUNCTION RETURN LOSS PROCEDURE ................................................................... 2 5.0 GENERAL: PULSE FUNCTION RETURN LOSS .................................................................... 3 6.0 PULSE FUNCTION RETURN LOSS PROCEDURE ................................................................ 4 7.0 APPENDIX A ............................................................................................................................... 5 ii 1.0 SCOPE This document outlines the procedure for determining the regularity of impedance for coaxial cables using telemetry methods. The regularity of impedance is return loss in the time domain. With basic expertise in the use of time domain reflectometers (TDR), the tester can determine return loss of discontinuities (impedance changes) at specific points along a coaxial cable. There are two methods detailed in this document. The selection of one method over another is governed by the following criteria: (A.) The type of TDR that is available to the tester. (B.) The length of cable being tested. (C.) The degree of resolution that is required. 2.0 EQUIPMENT The equipment required to perform this type of measurement is listed as follows: • BNC to F-Female Adapter • F-Female Terminating Load • (2) Pin to F-Male Adapter (if testing hardline coaxial cable) • TDR incorporating a Step or Pulse Function stimulus • Variable standard line • Variable terminator 3.0 GENERAL: STEP FUNCTION RETURN LOSS This method incorporates the use of a TDR with a step function stimulus. The method can be used for coaxial cables up to a maximum of 2000 feet. This type of TDR provides a high degree of resolution. The resolution of this type of TDR can distinguish discontinuities, in a short (<100 feet) length of high quality coaxial cable, up to ≈1 inch apart. This method involves applying a step function (voltage) to a Cable Under Test (CUT). The ratio of the reflected step voltage to the applied step voltage is the reflection coefficient (ρ) as illustrated below: ρ=Vref/Vapplied Where: Vref=Reflected step voltage by an irregularity at a distance from the cable input. Vapplied=Applied step voltage to the CUT. The step function return loss (as) is calculated as follows: as=-20log(ρ) 1 The rise time (tr) determines the resolution of the measurement. The rise time is defined as the period of time to go from 10% to 90% of the step amplitude. The faster the rise time the greater the detail that can be deciphered between discontinuities in the cable. The resolution (δ) can be determined by the following: δ=(150×10-6)(tr)(Vr) (Meters) Where: tr=Rise time (picoseconds) Vr=Velocity ratio of CUT dielectric (i.e. 0.66 for solid PE) The tr for TDRs using a step function stimulus is typically a fixed quantity that is unique to the TDR. A typical TDR will have a rise time ≤200 picoseconds. The Characteristic Impedance (Z0) on the ends of the cable can be determined. This is accomplished by using a standard line and a terminator that is matched to Z0 of the CUT. The impedance of the cable ends is determined by placing a standard line between the CUT and termination at the far end of the CUT. The use of the standard line and termination makes the impedance at the ends of the CUT more easily observed. 4.0 STEP FUNCTION RETURN LOSS PROCEDURE 4.1 A TDR (i.e. Tektronix 1502C) which utilizes a step function stimulus should be arranged with the CUT as depicted in appendix A. The CUT (w/ F-male connectors attached) should be connected to the BNC to F (female) adapter. Note: When testing hardline coaxial cable, a pin to F-Male adapter can be used to make connection to the TDR. 4.2 Turn “On” the power to the TDR. 4.3 Adjust noise filtering and vertical scale per the TDR manufacturer’s recommendation. 4.4 Set the TDR to the velocity of propagation associated with the CUT. 4.5 Adjust the distance/division setting to view the reflected pulse (discontinuity). 4.6 Adjust the TDR to provide results in terms of decibels. 4.7 Adjust the vertical scale of the reflected pulse to be 2 divisions high. 4.8 Read and record the return loss in decibels from the TDR display. 2 5.0 GENERAL: PULSE FUNCTION RETURN LOSS This method incorporates the use of a TDR with a pulse function stimulus. This method is used with long lengths of cable. A typical TDR that utilizes a pulse function stimulus can test lengths of coaxial cable up to 50,000 feet but with less accuracy as compared to the step function method. A δ of ≈1 foot between discontinuities, in a short (<100 feet) high quality coaxial cable, is achievable with this TDR. This method involves applying a pulse function to a CUT to obtain the pulse function return Loss (ap). This pulse is typically a ½ sine pulse that is unique to the TDR that is used. The tester can however modify the pulse width (tp). The tp is defined as the duration in time between the ½ height points of the applied pulse. The pulse function return loss (ap) is defined as follows: ap=-20log(urx/us) (dB) Where: urx: is the voltage of the pulse reflected by a discontinuity at a distance x from the input. us is the voltage of the sending pulse at the input. The corrected pulse return loss (apc) is the return loss measured at the input end of the cable minus the pulse attenuation from traveling to and from the measured discontinuity. The corrected pulse return loss is defined as follows: apc=ap-(2αx/100) (dB) α: the attenuation constant in dB/100 meters at the frequency (fp) which the main part of the pulse energy is concentrated. x: is the measured distance (in meters) to the discontinuity. fp: frequency at which attenuation of the cable is derived from. The frequency is determined as follows: fp=250/tp (nS) (MHz) The resolution (δ) is the minimum distance between discontinuities and can determined by: δ=0.15tpVr (Meters) Where tp: is the pulse width in nanoseconds. Vr: is the velocity ratio of the CUT (i.e. 0.66 for solid PE dielectric). 3 6.0 PULSE FUNCTION RETURN LOSS PROCEDURE 6.1 A TDR (i.e. Tektronix 1503C) which utilizes a pulse function stimulus should be assembled with the CUT in the arrangement shown in appendix A. 6.2 The CUT (w/ F-Male connectors attached) should be connected to the BNC to F- Female adapter. Note: When testing hardline cable, a Pin to F-Male adapter can be used to make connection to the TDR. 6.3 Turn “On” the power to the TDR. 6.4 Adjust the TDR to Z0 of CUT (i.e. 50Ω or 75Ω). 6.5 Adjust noise filtering per the TDR manufacturer’s recommendation. 6.6 Adjust the velocity of propagation (Vp) to match the CUT. 6.7 Adjust the pulse width (tp) in accordance with the length of the CUT. 6.8 Adjust the distance/division setting to see the reflected pulse. 6.9 Adjust the standard line and termination for minimum reflection of the pulse. 6.10 Note the height of the incident pulse in terms decibels (dB). 6.11 Increase the vertical scale (gain in dB) until the reflected pulse height equals the height noted in Step 5.10. 6.12 The additional gain necessary to match the height of the incident pulse is the pulse return loss in dB to be recorded. 6.13 To compensate for the pulse loss over a long length of cable, the corrected pulse return loss (apc) can be calculated by first determining the frequency (fp) in which the main part of the pulse energy is concentrated. This frequency is calculated as follows: fp=250/tp (nS) (MHz) The attenuation constant (α) in dB/100 meters is determined at this frequency. The x variable is the distance to the discontinuity. The corrected pulse attenuation is calculated as follows: apc=ap-(2αx)÷100 (dB) 4 7.0 APPENDIX A 5