Safety Precautions When Working Around Fiber Optics
Contents
1. Figure 13 Metal spring used to ground cable s metal ferrule to the receiver ground plane WWW HPARCHIVE COM 2ND QUARTER 1989 i OT Ch Diy Ta379 2n5 388 nS 0iv Dlyd 50 0nS7 Di Figure 14A This partial pulse caused by overdrive Chi 6 V Div Ch2 5 nV Div MT h 4 DT Ch Diy Te i96 One Main 360 n87Div Divd 36 8n8 Div Figure 14B This partial pulse caused by a missing by pass capacitor You have now checked all parts of the link You checked the transmitter for the correct light output and verified the optical waveform checked the cable and finally the receiver Following these steps of fiber optic troubleshooting should isolate the problem to the circuit or cable I hope that the information from this article plus reading the listed references shows you that fiber optic trouble shooting is straight forward and that you can prepare for applications of fiber optics in present circuits and future uses For a thorough description theory of fiber optics contact your local Hewlett Packard sales office and order our Fiber Optics Handbook HP P N 5952 9654 References 1 Fiber Optics Handbook HP P N 5952 9654 2 User s Manual 6 Hewlett Packard Journal Hewlett Packard HFBR 0100 Fiber Optic Connector Assembly Tooling Kit HP P N 5953 9350 3 Technical Data Sheet The Versatile Fiber Optic Connection HFBR 0501 Series 4 Application Bulletin 78 Low Cost Fiber Optic Links2. course of troubleshooting Fiber optic links as they are referred to are made up of three functional blocks The transmitter is the first block containing an LED light emit ting diode and any needed support circuitry to convert electrical signals to light The second block is the cable and correct connectors for the link which carries the light signal from the transmitter to the receiver which is the third block The receiver con sists of a photodiode phototransistor and support circuitry to convert the light back to an electrical signal 2ND QUARTER 1989 Test Equipment Troubleshooting fiber optic circuits requires specialized test equipment This equipment will allow you to verify the light output of the trans mitter view the waveshape of the optical output accurately vary the light amplitude useful in trouble shooting the receiver and substitute your own optical signal Hewlett Packard manufactures sev eral products that can be used for fiber optic troubleshooting see Figure 2 the first of which and one of the most important is the HP 8152A Optical Power Meter This meter measures light output brightness in dBm so that optical power P or P can be verified The ability to check correct optical modulation makes the HP 81519A Optical Receiver very important Other equipment that may prove helpful include the HP 8158B Optical Attenuator the HP 8154B Optical Source and of course a good dual trace
3. error message HP IB I O failed check power to HP 64120 WWW HPARCHIVE COM HP 64262S 8048 EMULATION SUBSYSTEM 64261A 2 Emulation control board serial prefix 2426A and below Modification to prevent 64261A 8048 microcontroller high frequency port 2 failure HP 64700 SERIES EMULATORS 64700 1 64700 power supply voltage selection procedure 64764A AL 1 64765A AL 1 64764 64764AL and 64765A 64765AL with serial prefix number 2739A Firmware upgrade to correct 64764 65 background monitor problem HP 70001A MAINFRAME 70001A 7A S Serials 2704A01437 and below A pos sible shock hazard may exist in the HP model 70001A mainframe if safety earth ground is defeated and there is a short on the 5 volt supply 70001A 8 Serial prefix 2704A and below Modification to reduce fan noise 70001A 9 Serials 2633A00112 and below Modification to prevent invalid overvoltage fault HP 70594A MODULE DEVELOPMENT KIT 70594A 1A All serials 4 8 module kit HP 70600A uW PRESELECTOR MODULE 70600A 1A Serial prefix 2833A and below Procedure for replacing A12 or A13W1 HP 70601A uW PRESELECTOR MODULE 70601A 1A Serial prefix 2743A and below Procedure for replacing A12 or A13W1 HP 70900A LOCAL OSCILLATOR 70900A 3B Serial prefix 2629A and below HP 70900 60096 memory plus controller board upgrade kit 70900A 17A Serial prefix 2629A and above HP 70900 OPT K91 RAM ROM board firmware upgrade kit 70900 60116 HP 70905A B uW MODULE 7
4. high enough Vpr to forward bias the LED approximately 1 5 volts a high Vp or supply voltage at the anode with the cathode near ground would indicate an open LED At this point any incorrect measurements other than those listed would indicate the support circuitry Check the Optical Power Remembering the safety precautions for the transmitter separate it from the rest of the circuit and check it for correct operation as follows Remove the fiber optic cable from the trans mitter receptacle You should use a meter HP 8152A or equivalent that is calibrated for the wavelength of the LED to check the light output Do this by replacing the original fiber optic cable with a short known good cable approximately one meter in length Make sure the test cable has the correct connectors for the transmitter being tested First verify an output then measure and record Typical Shunt Drive Figure 3A Typical shunt drive Pre bias Figure 3B Pre bias circuit example WWW HPARCHIVE COM Circuit 2ND QUARTER 1989 C the output from the transmitter for later use during the cable test If there is no output the failure has been isolated to this functional block To verify the LED output measure the voltage loop containing the LED refer to Figure 3C then calculate the current through the LED Ip and check this value with the Ip vs P on the graph in Figure 4 Check the Optical Waveform Next check the optica
5. oscilloscope and digital voltmeter the HP 1980B Oscilloscope and HP 3435A Digital Multimeter are good choices Figure 2 HP fiber optic test equipment WWW HPARCHIVE COM Basic Troubleshooting Procedure Note Each failure shown in the fol lowing figures is not an actual failure but is a forced condition for example only One important resource for trouble shooting is the databook from the manufacturer on the specific parts used Databooks are inexpensive free to approximately 15 00 and provide a wealth of data that are useful for troubleshooting Note that for trou bleshooting purposes you only need to look at typical values of the follow ing elements Vp forward voltage measured across the LED ty te for both parts P for specific current Ip usually in graphical form Sensitivity of the receiver Attenuation of the fiber optic cable BENCH BRIEFS 3 Troubleshooting fiber optic circuits requires a logical organized ap proach just like troubleshooting other circuits or instruments But be care ful Even though the approach is similar to many other circuits don t ignore the special safety precautions associated with fiber optics Remem ber that the fact that a fiber optic circuit is used may indicate the pres ence of a high voltage The following steps show the order of the procedure we will use to determine the failure 1 First localize the problem Is it located i
6. possible mechanical hazard HP 3235A E SWITCH TEST UNIT 3235A E 5A All serials HP 3235A firmware revisions 3235A E 6A 3235A serials 2725A00626 and below 3235X serials 2725A00562 and below Additional RAM 512K is now available for the HP 3235A processor HP 3552A TRANSMISSION TEST SET 3552A 18 All serials Preferred replacement for ca pacitors A3C105 and A3C110 HP 3561A DYNAMIC SIGNAL ANALYZER 3561A 3A Serials 2338A01000 and above 12 ROMS replace 24 EPROMS on A40 processor assembly HP 3582A DYNAMIC SIGNAL ANALYZER 3582A 16 Serials 2602A07038 and below Modification that reduces occurrence of main line fuse blowing HP 3708 NOISE AND INTERFERENCE TEST SET 3708A 13 All serials Preferred replacement for A305R2 resistor BENCH BRIEFS 9 HP 3709A CONSTELLATION DISPLAY 3709A 8 All serials Preferred replacement for trans former T1 HP 3709B CONSTELLATION ANALYZER 3709B 1 Serials 2723U00240 and below Preferred replacement for transformer T1 3709B 2 Serials 2723U00240 and below Transformer T1 retrofit procedure HP 3764A DIGITAL TRANSMISSION ANALYZER 3764A 17A Serials 2615U01162 and below Retrofit kit to upgrade the features including CCITT rec ommendation G821 on HP 3764A option 010 3764A 19C Serials between 2615U01162 and 2719U01637 Retrofit kit to upgrade the auxiliary analog input HP 3776A B PCM TERMINAL TEST SET 3776A 33 All serials Preferred replacement of protec tion devices on di
7. 0 HEWLETT PACKARD L H Ril SERVICE INFORMATION FROM HEWLETT PACKARD Fiber Optic Troubleshooting Brett Frymire Hewlett Packard Your voltmeter no longer works You open the cover and begin to trouble shoot You trace the fault to some components with a plastic cable What is this What do you do You have found a fiber optic link that has special parameters and requires special test equipment Follow along as this article covers the parameters equipment and basics of fiber optic troubleshooting with tips and examples Background Fiber optic applications include iso lation video audio or digital data transmission and LANs Local Area Figure 1 Typical HP fiber optic products Pub No 5952 0134 Network In fact the consumer elec tronics industry uses fiber optics to isolate digital TTL from low level analog circuits For example an Onkyo Compact Disc CD player uses fiber optics in this application CD players and the new DAT digital audio tape both use this technology for high noise immunity Look for modern stereo systems from Japan to interconnect the CD player or DAT to any other component with plastic fiber optics A new ANSI standard is being developed for FDDI fiber dis tributed data interface which is a 100 Mbit sec optical LAN As in consumer electronics more and more industry instrumentation uses fiber optics see Figure 1 A good example of an instrument related fi
8. 0905A 2A Serial prefix 2821A and below Procedure for replacing A4 or W13 70905B 1A Serial prefix 2819A and below Procedure for replacing A4 or W13 HP 70906A uW MODULE 70906A 2A Serial prefix 2813A and below Procedure for replacing A4 or W13 70906B 1A Serial prefix 2805A and below Procedure for replacing A4 or W13 HP 85629A TEST AND ADJUSTMENT MODULE 85629A 1B All serials ROM upgrade kit HP P N 85629 60002 HP 85650A QUASI PEAK ADAPTER 85650A 7 Serials 600 1035 1043 1047 Modification to prevent possible HP IB hang ups BENCH BRIEFS 11 Service Note Order Form If you want service notes please check the appropriate boxes below and return this form separately to one of the following addresses Hewlett Packard BENCH BRIEFS 100 Mayfield Avenue Mtn View California 94043 O 1345A 09 O 4948A 07 O 2813B 01 O 4954A 04 O 3235A E 05A O 5316B 01 O 3235A E 06A O 5335A 23 O 3552A 18 O 5335A 24 O 3561A 03A 5340A 22B O 3582A 16 O 5342A 48A O 3708A 13 O 5342A 50A O 3709A 08 O 5342A 52 O 3709B 01 o 5350 51 52A 07 3709B 02 O 5350 51 52B 07 oO 3764A 17A oO 5371A 08C o 3764A 19C O 5501B 5517A B O 3776A 33 5518A 01 O 3776B 39 O 6944A 04 O 6954A 03 O 3776B 40 O 3779C 38 o 8447D F 06A o 3779C 39 O 8554B 08A O 3779C 40 O 8562A 02B O 3779D 45 oO 8562A 03 O 8562A 04 O 4192A 19 O 4195A 01 O 4195A 02 oO 4947A 08 O 4948A 06 oa Oo
9. 2 Service information from Hewlett Packard Company To obtain a qualification form for a free subscription send your request to the above address Reader comments or technical article contributions are welcomed Please send them to the Bench Briefs Editor Editor Jim Bechtold Hewlett Packard 12 BENCH BRIEFS All rights reserved Permission to reprint Bench Briefs granted upon written request to the Editor WWW HPARCHIVE COM Bulk Rate U S Postage PAID Sunnyvale CA Permit No 317 Printed in U S A 2ND QUARTER 1989
10. 25 C VoL 0 5 V Notes 2 38 Chie 2 aV Div da f ac MT Gh 1 DT Ch i Diy Ta 203 2n8 Main 600 n8 70iv Dlyd 46 8nS Div Figure 6 Example of a good optical wave form Note clean pulse fall time line Chi 2 0 nU Div ov were 3 A ee MT Ch 1 DT Ch 1 Diy Ta 263 2ns Main 608 nS Div Dlyd 4 nS Div Figure 7 Example of a poor optical wave form with a long tail effect Note slow fall time line that breaks at approximately the 20 point 2ND QUARTER 1989 z Ai iN PL a Note Be careful not to excessively heat the connector with the light source Inspect the ends of the fiber for dark spots chips or scratches Figure 9 shows an example of a damaged fiber end with embedded metal particles Magnification of at least 50x will be required because of the small fiber diameter about 50 100 microns which cannot be easily viewed with the naked eye Plastic fiber is the exception as it is about 1 mm in diameter Remember Do not use the transmitter as the light source for inspecting the fiber see accom panying article on fiber optic safety If small scratches exist it is possible to remove them with polishing Be careful not to excessively polish the connector as this increases the dis tance separating the cable from the lens or IC which increases the optical power losses at the connection Use the correct polishing equipment for the connector installed on the cable Any attempt t
11. 562B 5 Serials 2750A00128 and below Improved earphone jack operation 8562B 6 Serials 2745A00100 and below Preferred replacement CRT filter front frame assembly 8562B 7 All serials Tech tip for RF input attenuator orientation for more consistent VSWR 8562B 8 Serial prefix 2724A and below Modification to prevent log amplifier oscillations 8562B 9 All serials Modification to prevent RF board and top shield interference 8562B 10 Serials 2750A00201 and above Input con nector replacement kit HP P N 5062 1988 8562B 11 Serials 2745A00200 and below Modification to prevent flashing display at power on 8562B 12 Serials 2745A00200 and below Modification to smooth out power supply start up 8562B 13 All serials New power supply top cover 8562B 14 Serials 2741A00190 and below Preferred replacement of FL1 low pass filter 8562B 15 Serials 2724A00170 and below Modification to eliminate spurs in the 600 MHz reference loop 8562B 16 Serials 2724A and below Modification that improves frequency counter sensitivity by 25 dB 8562B 17 Serials 2724A00160 and below Modification to AQ input attenuator so that it sets to 70 dB at power down 8562B 18 Serials 2750A00209 and below Preferred replacement primary FET heatsink 8562B 19 Serials 2724A00170 and below New side frames and cover assemblies 8562B 20 Serials 2724A00170 and below Modification to prevent YTO loop overshoot 8562B 25 Serials 2809A00228 to 2
12. 809A00327 Mod ification to reduce acoustically coupled 80 kHz power supply sidebands 2ND QUARTER 1989 HP 8568A SPECTRUM ANALYZER 8568A 37A All serials RF attenuators must be replaced with matching calibration ROM HP 8590A SPECTRUM ANALYZER 8590A 1B All serials Instructions on replacing firmware ROMS HP 8702A LIGHTWAVE COMPONENT ANALYZER 8753A 11A Serials 2829A and below Fan assembly upgrade kit HP 8753A B NETWORK ANALYZER 8753A 5 All serials Improving reliability of the A3 source assembly 8753A 7B All serials Modifications to improve A3 source A9 CPU A11 phase lock assembly compatibility 8753A 11A 8753A serial prefix below 2816A or 2950J and any prefix xxxxU 8753B serial prefix below 2808A or 2828J Fan assembly upgrade kit HP 8770A ARBITRARY WAVEFORM SYNTHESIZER 8770A 3A Serial prefixes 2726A and below Synchro nization and triggering capability retrofit 8770A 6C All serials Firmware history and upgrade procedures HP 8780A VECTOR SIGNAL GENERATOR 8780A 04 Serial prefixes 2725A and below Cable assemblies update and kit HP 8901B MODULATION ANALYZER 8901B 3 Serial prefixes 2622A to 2833A Modification to resolve automatic tuning anomaly HP 8902A MEASURING RECEIVER 8902A 7 Serial prefixes 2621A to 2751A Modification to resolve automatic tuning anomaly HP 8903B AUDIO ANALYZER 8903B 3 S Serials 2818A04250 thru 2818A04525 Modification to prevent possible primary wiring har ness sh
13. ber optic application is the continued on page 3 1 4 OPTICAL TRA HFBR WWW HPARCHIVE COM 2nd Quarter 1989 Safety Precautions When Working Around Fiber Optics Handle fiber optic components with care keeping in mind the following precautions The output from fiber optic links can cause serious damage to the eye and the glass in the cable can pierce the skin Use caution when viewing fiber ends or optical ports under magnification For further precautions and more detail see ANSI 2 136 1 1986 Use caution when viewing the optical port without knowing the optical power level and the wavelength Handle bare fiber with care The core end of the fiber is glass that can pierce the skin and break off This is a hazard only when termi nating a fiber end with a connector or a splice Potential eye problems result from invisible wavelengths collimated light and light intensity of unknown sources As a rule of thumb it is always safer and more accurate to use a meter to measure light output Hewlett Packard 1989 The following list defines a few of the terms used in fiber optics The book Fiber Optics Handbook available from Hewlett Packard HP P N 5952 9654 contains a more detailed list of terms BAUD The symbol rate of the fiber optic link Depending on the encod ing format the symbol rate can be the same or twice the apparent signal rate BER Bit Error Rate The ratio of errors to t
14. ble 4948A 7 Serials 2615U00365 and below Preferred replacement for fan HP 4954A PROTOCOL ANALYZER 4954A 4 Serials 2745A00600 and below Modification to prevent HP IB cable connector to disc controller board from loosening HP 5316B HP IB UNIVERSAL COUNTER 5316B 1 Serial prefix 2816A Modification to correct SRQ HP IB failure 10 BENCH BRIEFS HP 5335A UNIVERSAL FREQUENCY COUNTERS 5335A 23 All serials Tech tip to prevent degraded sensitivity with special option C10 5335A 24 All serials Preferred replacement part for A9CR6 and A9CR7 HP 5340A MICROWAVE FREQUENCY COUNTER 5340A 22B Series prefixes 2250A and below Preamp retrofit kit HP 5342A MICROWAVE FREQUENCY COUNTER 5342A 48A All serials Repair installation of amplitude measurement opt 002 HP pin 05342 80505 5342A 50A All serials Improved IF adjustment procedure 5342A 52 All serials A22 motherboard replacement HP 5350 51 52A MICROWAVE FREQUENCY COUNTER 5350 51 52A 7 All serials LCD replacements kit HP P N 05350 60204 HP 5350 51 52B MICROWAVE FREQUENCY COUNTER 5350 51 52B 7 Serial prefix 2810A and below LCD replacement kit P N 05350 60205 HP 5371A FREQUENCY AND TIME INTERVAL ANALYZER 5371A 8C All serials with firmware revision 2828 installed Firmware revision 2828 anomalies and their workarounds HP 5501B 5517A B 5518A LASER HEADS 5501B 5517A B 5518A 1 See text for serials Modi fication to correct fault in the high voltag
15. de to prevent BASIC command TRACE DATA from returning error message HP 54100A D DIGITIZING OSCILLOSCOPE 54100A D 11 All serials Incrrect sampler bias and sampling efficiency adjustment can cause erratic offset 54100A D 12 54100A serial prefix 2812A and below 54100D serial prefix 2809A and below New ROM s to eliminate intermittent loop failures at power up and incorporate firmware feature upgrade HP 54110D DIGITIZING OSCILLOSCOPE 54110D 11 All serials Incorrect sampler bias and sampling efficiency adjustment can cause erratic offset 54110D 12 Serials 2633A00900 and below Modifi cation to prevent vertical gaps in the displayed trace HP 54111D DIGITIZING OSCILLOSCOPES 54111D 8 Timebase accuracy performance test change 54111D 9 10 MEG OHM 10 1 divider probes may cause damage to the attenuators HP 54112D DIGITIZING OSCILLOSCOPE 54112D 3 ROM revision allows PaintJet support and improves selftest reliability HP 64206A 6301V 6303R EMULATOR POD 64206 4 6301V 6303R emulation pod all serials 64206 66501 timing board modification to prevent error message HP IB I O failed check power to HP 64120 HP 64207A 6301 03X EMULATOR POD 64207 1 6301 03X emulator pod all serials 64207 66501 timing board modification to prevent error message HP IB I O failed check power to HP 64120 HP 64208A 6301 03Y EMULATOR POD 64208 1 6301 03Y emulator pod all serials 64208 66501 timing board modification to prevent
16. e power supply assemblies that drive the HP Laser Heads HP 6944A MULTIPROGRAMMER 6944A 04 All serials Isolating system failures caused by termination plug HP 6954A MULTIPROGRAMMER 6954A 03 All serials Isolating system failures caused by termination plug HP 8447D F AMPLIFIER 0 1 MHz 1300 MHz 8447D F 6A 8447D serials 2648A and below 8447F serials 2634A and below Replacement kit for preamplifier HP P N 5086 7005 HP 8554B SPECTRUM ANALYZER 8554B 8A Serial prefix 2111A and below Precaution on changing A7 YIG oscillator assembly HP 8562A PORTABLE SPECTRUM ANALYZER 8562A 2B All serials Latest revision ROM upgrade kit HP P N 08562 60062 8562A 3 Serial prefix 2745A and below 08562 60069 roller oscillator replacement kit 8562A 4 All serials Modification to the peak detector top shield special screws HP P N 1390 0746 8562A 5 Serials 2724A00885 and below Modification to prevent 80 kHz power supply sidebands 8562A 6 Serials 2805A01787 and below Improved earphone jack operation 8562A 7 Serials 2745A01523 and below Preferred replacement CRT filter front frame assembly 8562A 8 All serials Tech tip for RF input attenuator orientation for more consistent VSWR 8562A 9 Serial prefix 2724A and below Modifications to prevent log amplifier oscillations WWW HPARCHIVE COM 8562A 10 All serials Modification to prevent RF board and top shield interference 8562A 11 Serials 2750A01524 and above Input con n
17. ector replacement kit HP P N 5062 1988 8562A 12 Serials 2745A01523 and below Modification to prevent flashing display at power on 8562A 13 Serials 2745A01523 and below Modification to smooth out power supply start up 8562A 14 All serials New power supply top cover 8562A 15 Serials 2741A01344 and below Preferred replacement of FL1 low pass filter 8562A 16 Serials 2733A01023 and below Modification to eliminate spurs in the 600 MHz reference loop 8562A 17 Serial prefixes 2724A and below Modification that improves frequency counter sensitivity by 25 dB 8562A 18 Serials 2724A00885 and below Modification to AQ input attenuator so that it sets to 70 dB at power down 8562A 19 Serials 2750A01694 and below Preferred replacement primary FET heatsink 8562A 20 Serials 2724A00885 and below New side frames and cover assemblies 8562A 21 Serials 2712A00901 and below Modification to prevent YTO loop overshoot 8562A 26 Serials 2809A02037 to 2809A02837 Mod ification to reduce accoustically coupled 80 kHz power supply sidebands HP 8562B SPECTRUM ANALYZER 8562B 1B All serials Latest revision ROM upgrade kit HP P N 08562 60062 8562B 2 Serial prefix 2745A and below 08562 60069 roller oscillator replacement kit 8562B 3 All serials Modification to the peak detector top shield special screws HP P N 1390 0746 8562B 4 Serials 2724A00160 and below Modification to prevent 80 kHz power supply sidebands 8
18. elength The optical spectrum of the light emitted at the transmitter 2ND QUARTER 1989 continued from page 1 HP 3458A Multimeter where fiber optics provide a data link for high voltage isolation between floating measurement hardware and the HP IB section Traditionally pulse transformers have been used in this area Other applications also include RS 232C serial interface connection modules such as the DOC102P and DOC101P which can be used for higher noise rejection over long dis tances Note that a standard hardwire RS 232C connection is limited to ap proximately 50 feet according to EIA Electronic Industries Association specifications The DOC102P and DOC101P are good to approximately 500 meters Hewlett Packard s new Precision Architecture computer uses an optional fiber optic interface for the connection between the disk drive and the CPU The HP 37204A Mul tipoint HP IB Extender also uses an optional fiber optic interface Fiber optics play a big part in isolating the effects of TTL noise from small signal analog electronics Possessing both fiber optic basic knowledge and troubleshooting skills are valuable aids that will help you repair instru ments now and in the future Definition of a Fiber Optic Link Before we discuss fiber optic trouble shooting and equipment you need to understand the basic components of a fiber optic link along with related terms and acronyms that you will encounter in the
19. ely noisy and a metal ferrule is used it is recommended SPRING IS SOLDERED TO RECEIVER GROUND PLANE that a metal spring contact be used to ground the cable s metal connector to the receiver ground plane see Figure 13 If this spring is missing or has a poor connection oscillations can occur that may not be readily visible on an oscilloscope The symp toms may include ringing overshoot or decreased sensitivity Install a spring if it is missing or clean the surface between the connector metal ferrule and spring or resolder the spring to the printed circuit ground plane Another method that reduces the effects of EMI pickup by the fiber optic connector is to use a connector with a non conductive plastic or ce ramic ferrule Note that some receiv ers incorporate a shield into the case or housing of the component so it is important on these receivers that the case is grounded The example of jitter shown in Figure 12 was caused by underdrive which could result from either a contaminated fiber cable or a weak LED Partial pulses see Figure 14 is a classic symptom of a long tail effect although an open by pass capacitor or overdrive may cause a similar effect The problem shown in Figure 14A was caused by overdrive The problem shown in Figure 14B was caused by a missing by pass capacitor COIL SPRING IS INSTALLED OVER THE OPTICAL PORT SO THAT F O CONNECTOR IS GROUNDED WHEN CONNECTED TO THE HFBR 2406 2416
20. for Digital Applica tions up to 150 MBd HP P N 5954 8478 5 Fiber Optic Cable Installation Guide HP P N 5954 0978 June 1989 Volume 40 Number 3 HP P N 5953 8575 b BENGI si awa NDE Need Any Service Notes They re free Here s the latest listing of service notes They recommend modifications to Hewlett Packard instruments to increase reliability improve perform ance or extend their usefulness Use the form at the rear of Bench Briefs to order free of charge service notes for several instruments 2ND QUARTER 1989 If you would like to purchase large quantities of service notes covering a wide range of instruments or if you desire a complete history of all service notes documenting all changes to your instruments Hewlett Packard offers a microfiche library for a one time charge There is also a microfiche update service available that you can order on a quarterly basis to update the library The part numbers for the service note microfiche library and subscrip tion service are 5951 6511 5951 6517 Library Subscription service Contact your local HP Sales Office for ordering information oO WWW HPARCHIVE COM HP 1345A DIGITAL DISPLAY 1345A 9 Serials 2619A and below New A4 high voltage power supply assembly requires new CRT cable HP 2813B QUARTZ PRESSURE PROBE 2813B 1 All serials The possibility of high internal pressure may cause a
21. gital assemblies A7 and A12 3776B 39 All serials Preferred replacement of protec tion devices on digital assemblies A107 and A112 3776B 40 All serials Retrofit instructions for adding option H01 quantization distortion noise measure ment to the HP 3776B option H01 deletes the return loss measurement and adds QD noise meas urement capability in its place HP 3779C D PRIMARY MULTIPLEX ANALYZER 3779C 38 Serials 2520U00649 and below Modification to correct unstable CRT display random lines and tearing 3779C 39 Serials 2520U00649 and below Modification to correct error R64 when performing Q D noise measurement on 900 ohm lines 3779C 40 All serials A35 board recommended re placement for fuses F1 and F2 3779D 45 All serials A35 board recommended re placement for fuses F1 and F2 HP 4192 LF IMPEDANCE ANALYZER 4192A 19 Serials 2514J05055 and below How to remove fuse A1F1 HP 4195A NETWORK SPECTRUM ANALYZER 4195A 1 Serials 2737J00219 and below firmware revisions 1 00 Upgrading the ROM based firmware 4195A 2 All serials Frequency response phase shift adjustment in the network configuration HP 4947A TRANSMISSION IMPAIRMENT MEASURING SET 4947A 8 Serials 2750U00336 and below Upgrade instructions for adding the 2 wire return loss measurement HP 4948A IN SERVICE TRANSMISSION IMPAIRMENT MEASURING SET 4948A 6 Serials 2615U00395 and below Recom mended replacement adjustment of power switch ca
22. ies one the average duty cycle is 50 and two it is self clocking Optical Port The portion of the transmitter housing with which the fiber optic cable mates Optical Power P or P The brightness regardless of the wave length of the light measured in either watts or dB sometimes dis played as dBm which is relative to WWW HPARCHIVE COM milliwatts Usually seen when refer ring to the output power of the transmitter Pt or to the power presented to the receiver Optical Power Budget The calcu lated amount of optical power needed to sustain correct operation of the receiver incorporating the losses throughout the link For ex ample the optical power out of the transmitter minus the loss of the cable minus any losses due to splices or bulkhead connectors If the level of light falls below this power level the receiver output is no longer valid See Sensitivity Overdrive The tendency of the receiver to produce erroneous or distorted data when too much light is presented to it P The optical power presented to the receiver Py The optical power at the transmitter Sensitivity The minimum optical power level where the receiver is guaranteed to have valid data This is usually specified at a particular BER Underdrive The tendency of the receiver to produce erroneous data when not enough light is presented to it Ve The forward voltage across the LED portion of the transmitter Wav
23. l waveform with a waveform analyzer Use the HP 81519A Optical Receiver or equivalent and an oscilloscope refer to Figure 5 While attaching the optical receiver verify that the oscil loscope termination matches the ana lyzer requirements usually 50 ohms Example Of Peaking Circuit Figure 3C Peaking circuit example NORMALIZED OUTPUT POWER dB F 10 Ip TRANSMITTER DRIVE CURRENT mA Figure 4 Typical output power vs drive current 2ND QUARTER 1989 WWW HPARCHIVE COM If possible use a dual trace scope and trigger on the input signal Compare the input signal to the output signal The signal from the optical receiver should match the signal seen across the LED Figure 6 is an example of a good waveform It will not be an exact duplicate because of slow re sponses in the LED In particular pay attention to the rise fall times of the transmitter as a long fall time sometimes called long tail effect can cause problems see Figure 7 A long tail response can be recognized by the characteristic changing slope in the fall time At approximately the 20 point on the fall time line the slope changes to a more gradual sloped line Usually the rise time on a long tail LED will also be slow but slow rises are usually fixed by the designer using a peaking circuit see Figure 3C A peaking circuit can be recognized where the output of the drive circuit is connected in series with two resistors one of the re
24. n the fiber optic portion of the instrument 2 Gather information about the prob lem without disturbing the circuit or instrument This is sometimes referred to as milking the front panel This assumes that the prob lem has been traced to the fiber optic portion of the instrument 3 Using the test equipment men tioned earlier isolate the problem to one of the functional blocks of the fiber optic circuit transmitter cable or receiver 4 Determine what is wrong with the defective block Troubleshooting the Transmitter Troubleshooting the transmitter is divided into three parts checking for signals to the transmitter checking the optical waveform and checking the optical power Check Signals to the Transmitter Probe the anode of the LED with an oscilloscope Does a signal exist at this point Depending on the support circuitry a current source might be driving the LED In this case the voltage excursion if it is measurable will be very small on the order of 50 millivolts ac which will ride on a Vr of 1 2 volts de 4 BENCH BRIEFS If the LED is driven by a TTL gate see Figure 3A or other voltage source the ac excursion will range between ground and Vp 1 5 volts or between Vcc and Vp depending on the drive circuit used For example a pre bias circuit see Figure 3B can affect the voltage swings in a different manner Pay attention to the de volt age across the LED as the diode must have a
25. o polish the cable with out the fixture which maintains the cable and connector perpendicular to the polishing paper will increase the loss Make sure that the cable is clean as polishing is a last resort before installing a new connector If 2ND QUARTER 1989 Figure 8 Example of a good clean defect free fiber optic cable end WWW HPARCHIVE COM either of the ends are damaged be yond cleaning or polishing cut off the damaged end and install a new connector The Receiver If the transmitter and the cable test good move to the receiver side of the link First identify the type of re ceiver If the receiver is a PIN pho todiode and depending on the support circuitry you may see a voltage of 200 millivolts or a current of 100 to 400 nanoamps or less If the part is TTL expect typical TTL levels Note that some TTL parts require an ex ternal pull up resistor Check the output Is a signal present Is the signal badly distorted Does the part oscillate or ring Are there extra or doubled pulses Is there a spike on the output Is there jitter on the Figure 9 Example of a damaged fiber optic cable end with embedded metal particles BENCH BRIEFS 7 edges Are mysterious half or partial pulses at half amplitude present All these symptoms hint at where the problem lies As shown in Figure 10 output distor tion can come from several areas Overdrive underdrive receiver pho todiode failure support ci
26. ock hazard HP 8903E DISTORTION ANALYZER 8903E 3 S Serials 2818A00325 thru 2818A00375 Modification to prevent possible primary wiring har ness shock hazard HP 8970A B NOISE FIGURE METER 8970A 13 All serials Improved measurement repeatability 8970B 3 Serials 2745A and below Improved meas urement repeatability HP 10269C GENERAL PURPOSE PROBE INTERFACE 10269C 1 Serial prefix 2723A and below serial prefix 2723G and below Modification to prevent state clock violates overdrive specification error message on 1650A 51A 510A logic analyzers HP 10314B 80386 INTERFACE MODULE 10314B 1 Serial prefix 2731A and below Modification to reduce target system clock loading 2ND QUARTER 1989 HP 16500A LOGIC ANALYSIS SYSTEM MAINFRAME 16500A 4 Serials 2650A00758 and below serials 2650G00541 and below serials 2650J00296 and below New disc drive shields for instruments re quiring CPU board change 16500A 5 Serials 2650A01200 and below serials 2650G00561 and below serials 2650J00329 and below Modification to insure fans start reliably HP 16515A 16516A 1 GHz HIGH SPEED TIMING ANALYZER 16515A 1 16516A 1 All serials Tech tip to prevent ESD damage resulting from probe assembly replacement HP 35660A DYNAMIC SIGNAL ANALYZER 35660A 02 See text for all applicable serial numbers Modification to add HP IB power up SRQ and revise CPU test LED codes 35660A 03 See text for all applicable serial numbers Firmware upgra
27. oo DAGE 8562A 05 8562A 06 8562A 07 8562A 08 8562A 09 8562A 10 8562A 11 8562A 12 8562A 13 8562A 14 8562A 15 8562A 16 8562A 17 8562A 18 8562A 19 8562A 20 8562A 21 8562A 26 8562B 01B 8562B 02 2 E o Ere O 8562B 03 O 8562B 04 oO 8562B 05 8562B 06 8562B 07 8562B 08 8562B 09 8562B 10 8562B 11 8562B 12 8562B 13 8562B 14 8562B 15 8562B 16 8562B 17 8562B 18 8562B 19 8562B 20 8562B 25 8568A 37A Please photocopy this order form if you do not want to cut off the page NAME COMPANY NAME ADDRESS CITY STATE m AP O 8590A 01B O 54111D 09 O 8753A 05 O 54112D 03 8753A 07B O 64206 04 O 8753A 11A O 64207 01 O 8770A 03A o 64208 01 o 8770A 06C O 64261A 02 O 8780A 04 O 64700 01 O 8901B 03 O 64764A AL 01 O 8902A 07 64765A AL 01 O 8903B 03 S oO 70001A 07A S O 70001A 08 O 8903E 03 S 8970A 13 O 70001A 09 O 8970B 03 O 70594A 01A O 10269C 01 o 70600A 01A O 10314B 01 O 70601A 01A O 70900A 03B O 16500A 04 o 70900A 17A O 16500A 05 O 16515A 01 O 70905A 02A 16516A 01 70905B 01A 35660A 02 O 70906A 02A o 35660A 03 70906B 01A O 85629A 01B O 54100A D 11 O 85650A 07 O 54100A D 12 O 54110D 11 oO 54110D 12 O 54111D 08 HEWLETT PACKARD COMPANY 100 Mayfield Avenue Mtn View California 94043 BENCH BRIEFS 2ND QUARTER 1989 Volume 29 Number
28. otal number of bits sent in a data stream which defines the quality of that data e g BER of 1x1E 7 Bulkhead or Splice A special con nector that joins two cables Gener ally a bulkhead connector is used when the cable has to pass through a wall or partition A splice repairs a break in the cable See the photo below ae Degradation The tendency of the light emitting diode LED to produce less light with constant current IF after an extended period of time Encoding The process of translat ing data into a controlled average duty factor range for use in an ac coupled circuit For example NRZ Non Return to Zero data can be sent as Manchester coded 2 BENCH BRIEFS Fiber Optic Terms Eye Pattern A term describing the oscilloscope display when a data pattern is triggered on the clock signal See the photo below Fiber Optics FO The glass or plastic medium used to transmit signals between two points or light via an optical fiber This term can also refer to parts of the link e g the cable transmitter receiver or the complete circuit link FDDI Fiber Distributed Data Inter face A proposed ANSI standard for high speed fiber optic LAN lp The forward current through the LED portion of the transmitter LAN Local Area Network A group of terminals computers linked to gether in a fashion to permit ex change of data with protocols Manchester Code A coding scheme that has two propert
29. rcuitry and LED long tail effects are some of the culprits Underdrive caused the waveform shown in Figure 10 Again some of the same problems that caused a distorted output can cause extra or doubled pulses as shown in Figure 11 Overdrive LED long tail effects and a faulty receiver can cause this problem In the ex ample shown in Figure 11 overdrive was the problem Chi S mV 7Div Ch2 S aV Div i OT Ch 1 Diy Tae37 9 2ns 0 nS 0iv Dlyd SO ns Div Figure 10 This severe pulse width distortion caused by underdrive MT Ch i Main 190 n87 0iv Figure 11 Extra or doubled pulses Receiver may be ringing or oscillating Overdrive caused this problem 8 BENCH BRIEFS Edges with excessive jitter Figure 12 can be a hard problem to solve as the cause can be one or a combination of many defects The first and most obvious cause would be insufficient optical power After that it could be Chi 560 aU Div Ch2 500 aV Div MT Ch i DT Ch 1i Diy Ta 440 ins Main 300 n8 0iv Diyd 1i0 ng Div Figure 12 This pulse edge with excessive jitter caused by underdrive a bad receiver open by pass capacitor overdrive long tail effects or any part of the power supply filter In fact the problem could be noise related where the metal ferrule of the fiber optic connector acts as an EMI source by inducing electrical noise into the receiver If the environ ment in which the fiber optic link is used is extrem
30. sistors being bypassed with a capacitor When the first edge occurs there will be a surge of current causing a fast turn on of the LED Unfortunately this does not help the falling edge Depending on the receiver if the long tail response occurs near the threshold of the amplifier comparator noise which rides on the signal will again pass through the threshold This will cause jitter or multiple switchings on the receiver output Examples of this problem will be shown later The Cable Testing the cable consists of compar ing the optical output from the trans mitter using a short and known good cable to the output of the original cable at the receiver Using the one meter known good cable measure the optical output at the transmitter as was done earlier when you checked the optical power Reconnect the original cable to the transmitter and then move to the receiver Re move the cable end from the receiver and measure the power The power from the cable at this point should BENCH BRIEFS 5 Table 1 Receiver data sheet specifications This is a partial example from the HFBR 0501 Series Technical Data Sheet 0 C to 70 C 4 75 V lt Voc lt 5 25 V Unless Otherwise Specified Ce ee Ay pees and HFBR 2531 Receiver Input Optical HFBR 2522 Power Level for and Logic 0 HFBR 2532 HFBR 2524 and HFBR 2534 be sufficient to ensure correct opera tion of the receiver that is the power should be above the sen
31. sitivity level as listed in the data sheet see Table 1 If the power out of the cable is below the minimum sensitivity level of the receiver and the optical output for the transmitter is correct then either the cable has high atten uation or is broken If the cable is damaged or broken and needs to be replaced refer to the HP Fiber Optic Cable Installation Guide HP P N 5954 0978 for installation tips If the cable has high attenuation first try cleaning the ends of the cable with HP 81519A OPTICAL RECEIVER Fiber Opis ee Cable alcohol If this does not fix the prob lem use the following technique only if the cable is glass and less than one kilometer in length or plastic and less than 200 meters in length Cables exceeding these lengths will have to be tested using another technique Remove the cable and hold one end of the cable up to a light source A 100 watt incandescent light works the best A long cable will have an inter esting effect The light exiting the fiber will be red for glass and green for plastic An example of a good clean defect free fiber end is shown in Figure 8 HP 54110A 0000000 ode Fiber Optic Figure 5 Test equipment set up for optical waveform test 6 BENCH BRIEFS WWW HPARCHIVE COM 0 70 C Vo 0 5 V lot 8mA 25 C Vor 05 V 0 70 C Vo 0 5 V loL 8mA 25 C Vo 05V lot 8mA 0 70 C Vo 0 5 V lot 8mA
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