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RF COMPONENTS PROTECTION CHAPTER 7: SURGE PROTECTIVE DEVICES 7.6.1.3 FILTER-BASED COAXIAL PROTECTORS Filter-based lightning protection devices are characterized by their broadband performance characteristics and can cover multiple transmit/receive frequency bands. These cavity-based designs can be characterized as bandpass filters. They consist of tuned inductive capacitance (LC) networks operating on a bandpass/band-reject principle, with an operating band as broad as 2 GHz. Utilizing the inductive current discharge circuit to shield/ground, followed by a DC blocking capacitive component on the center pin, ensures not only broadband RF performance, but also the desired surge suppression characteristics. Designs based on this principle allow a closer match to 50 ohms over a broader range than the quarter-wave technique. These devices cannot be utilized in applications where DC currents and voltages are supplied to the RF electronics via the coaxial cable's center conductor. Note that a modified form of this filter-based technology can be deployed in applications requiring DC power for the tower top electronics. There are designs combining DC blocking for the RF while passing DC power for the tower top electronics. The design is similar to the Bias-T concept. While the RF surge performance relies on the filter concept, the DC path is isolated from the RF path, conditioned, and reinjected at the appropriate port. While selecting this type surge protector, attention should be paid to the resistance of the DC protection circuit. The typical DC resistance range could be from a few milliohms to a few ohms. Some tower top amplifier configurations incorporate a control and alarm cable in addition to the RF transmission line and sample port (test) cable. This control and alarm cable is typically a multiconductor cable connecting the tower top amplifier unit collocated with the receive antenna to the control chassis collocated with the receiver multicoupler. This cable is typically installed and routed with the receive antenna transmission line. Any shield within this cable shall be bonded to the MGB. All conductors of this cable shall be protected by installation of a suitable SPD within a maximum of 610 mm (2 ft.) of the point of entry into the shelter, room or equipment area. The ground (earth) terminal of the SPD shall be bonded to the MGB with a 16 mm 2 csa (#6 AWG) or coarser, green jacketed, stranded copper conductor. The specifications for this SPD will be dependent on the design parameters of the tower top amplifier system, including the operating voltage and number of conductors within the control or alarm cable. The proper SPD shall be selected for the specific application. The device selected should have a turn-on voltage that is nominally 20% higher than the maximum DC operating voltage; however, the turn-on voltage shall be low enough to provide suitable protection. The tower top amplifier system and SPD manufacturer should be consulted for guidance in selecting the proper SPD. CCTV and Cable TV system cables entering a communications facility shall have SPDs installed. These cables, like RF transmission lines, should by design enter the shelter, room or equipment area at the same location as the RF transmission lines. When there are no RF transmission lines at the facility where these systems are installed (dispatch and command centers) the cables should enter as close as is practicable to the point where the power and telephone conductors enter the facility. The shield of these cables shall be bonded to the MGB. The SPD shall be installed within 610 mm (2 ft.) of the point of entry of the cable(s) into the shelter, room or equipment area. The ground terminal of the SPD shall be bonded to the MGB with a 16 mm 2 csa (#6 AWG) or coarser, green jacketed, stranded copper conductor as described in Chapter 5. The proper SPD shall be selected based on the application. Some CCTV systems will provide DC power to the remote camera via the coaxial cable. The cable TV service provider or the CCTV manufacturer, as well as the SPD manufacturer, should be consulted for guidance in selecting the proper SPD for the specific application. 7-40 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES RF COMPONENTS PROTECTION 1 50.00 mV/D 0.00000 V -2.0000 ms 23.0000 ms 40.0000 ms 5.00 ms/DIV REALTIME VMAX ( 1 ) 154.686mV VMIN ( 1 ) 162.500mV FIGURE 7-21 FILTER-BASED RF PROTECTION DEVICE, OSCILLOSCOPE SET AT 0.05V/DIV V AND 5µS/DIV H, 8X20 µS/3KA 68P81089E50-B 9/1/05 7-41
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© 2005 MOTOROLA, INC. ALL RIGHTS R
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TABLE OF CONTENTS Chapter 1. Introd
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STANDARDS AND GUIDELINES FOR COMMUN
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CHAPTER 1 INTRODUCTION 1 This manua
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CHAPTER 2 SITE DESIGN AND DEVELOPME
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CHAPTER 3 COMMUNICATION SITE BUILDI
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CHAPTER 4 EXTERNAL GROUNDING (EARTH
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CHAPTER 5 INTERNAL GROUNDING (EARTH
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CHAPTER 6 POWER SOURCES 6 6.1 LOCKO
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Page 299 and 300: STANDARDS AND GUIDELINES FOR COMMUN
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APPENDIX A ELECTROMAGNETIC ENERGY I
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APPENDIX B SOIL RESISTIVITY MEASURE
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APPENDIX C PROTECTING AGAINST ELECT
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APPENDIX D GROUNDING (EARTHING) ELE
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APPENDIX E GENERAL CONVERSIONS AND
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APPENDIX F R56 COMPLIANCE CHECKLIST
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I NDEX Index Index AC power cabling
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