HP SmartClock Technology. Application Note Improving Oscillator Long-term Stability for Synchronization Applications

Transcription

1 H HP SmarClock Technology Applicaion Noe 1279 Improving Oscillaor Long-erm Sabiliy for Synchronizaion Applicaions Telecommunicaions Power Sysems Merology Manufacuring Calibraion Wireless Communicaions 1

2 Table of Conens Inroducion: The Need for Highly Accurae Timing Synchronizaion... 3 Local and Reference Sources... 3 Clock Technologies... 3 Reference Sources... 4 Oscillaor Performance... 4 Hewle-Packard SmarClock Technology... 5 Timing Sysem Componens... 6 Correcing and Adjusing for Insabiliies... 6 HP SmarClock Implemenaion... 7 Enhanced GPS... 7 Enhanced RAIM... 9 Enhanced Learning Normal Operaion Holdover Operaion HP SmarClock Sysem Benefis Appendix A: Basic Clock Equaion Derivaion of he Clock Equaion Time Error versus Frequency Time Error Resuling from Environmenal Effecs and Noise Appendix B: Applicaions and Producs HP s Timing and Synchronizaion Producs Merology and Manufacuring Synchronizing Wireless Neworks Wide Area Measuremen Synchronizaion Nework Synchronizaion GPS Telecom Primary Reference Source References Glossary HP Relaed Lieraure

3 Inroducion: The Need for Highly Accurae Timing Synchronizaion Modern applicaions in digial elecommunicaions, merology, manufacuring, calibraion, wireless communicaions, and power ransmission require highly-accurae iming. Equipmen funcionaliy and reliabiliy and he qualiy of services delivered can be dramaically affeced by he proper applicaion of precise iming unis. Unil recenly, he cos of highly-accurae iming was prohibiive. Now, excellen nework and iming synchronizaion are available a reasonable cos. Local and Reference Sources Highly-accurae iming sysems mus compensae for he fac ha no single clock can be coninuously available, and ha all clocks lose accuracy over ime. Mos sysems employ a local clock and periodically calibrae his clock agains a reliable reference such as an aomic clock. Universal Coordinaed Time (UTC) is he acceped represenaion of absolue ime and is coordinaed by he Bureau Inernaional des Poids e Mesures (BIPM) in France using informaion from cooperaing imekeeping ceners around he world. Designaed UTC ceners, such as he U.S. Naval Observaory (USNO), he Naional Insiue of Sandards and Technologies (NIST) in he Unied Saes and many ohers hroughou he world, provide local sandard ime sources or synchronizaion references. Wih curren echnologies, a clock can be remoely locaed and sill achieve high accuracy and raceabiliy. The selecion of clock echnology, and he mehod of obaining he reference signal, direcly affec he cos and accuracy of a iming soluion. A variey of alernaives ypically wih accuracy and/or cos radeoffs exiss for deriving and mainaining precise ime. Clock Technologies A local clock keeps ime in erms of a local epoch signal. This may range from an ac moor ied o a naional power grid a eiher 50 or 60 Hz, or an inernally generaed signal similar o ha of he quarz crysal oscillaor in mos wriswaches. The mos accurae clocks use inernal oscillaors ha produce highly reproducible pulses or epoch inervals. Typically he more expensive he oscillaor, he beer he clock s accuracy. Rubidium- or cesium-based clocks or oscillaors, for example, can mainain accurae ime for long periods afer synchronizaion wih a primary ime source. Unforunaely, hey are oo cosly for many applicaions. Quarz oscillaors, while less expensive, require more frequen synchronizaion o a reference or synchronizaion source o mainain he same accuracy. 3

4 Reference Sources In he pas, raveling clocks were aken from places mainaining reference ime scales (like NIST or he USNO in he Unied Saes, or oher naional iming references around he world), and physically ranspored o synchronize remoe clocks. This mehod, while sill used occasionally oday, is expensive and inconvenien. Today, users of ime and frequency can easily receive a reference signal from a number of faciliies. Time raceable o UTC is readily available worldwide via boh erresrial and saellie broadcas echnologies. Some of he more common erresrial (ground-based echnologies) are: VLF and LF ransmissions such as Omega and Loran-C HF ransmissions Television broadcass Telephone and compuer nework disribuion NIST s Auomaic Compuer Time Service (ACTS) Microwave links and opical fibers Some of he more common saellie echnologies are: GOES weaher saellies Navsar Global Posiioning Sysem (GPS) Former Sovie Union GLONASS These echnologies vary grealy in accuracy, expense and accessibiliy. Some ransmissions are suscepible o daily variaions, weaher condiions, or propagaion delays, and may no be available everywhere. Oscillaor Performance Oscillaors vary in heir abiliy o mainain an accurae frequency over ime. Cesium-beam frequency sandards offer he bes long-erm sabiliy he mos sable cesium sandards have been designed o be insensiive o environmenal perurbaions. This resuls in a clock ha is he mos accurae for he longes period of ime. Cesium sandards are expensive (up o approximaely U.S. $70,000) and are ypically applied in he mos criical applicaions or environmens ha require a primary sandard or independen clock. Rubidium sandards have poorer long-erm sabiliy and are more environmenally sensiive, bu acually have beer shor-erm accuracy over periods ranging up o several housand seconds. They are lower in cos (U.S. $1,200 $5,000) han cesium, bu hey require periodic calibraion agains a reference signal or synchronizaion source. 4

5 The HP 5071A uses a cesium-conrolled oscillaor o deliver maximum accuracy and sabiliy. High-qualiy quarz oscillaors are inexpensive (priced from U.S. $200 $1,000), bu hey are less sable han eiher cesium or rubidium. Quarz oscillaors are environmenally sensiive, especially o emperaure. Quarz oscillaors and rubidium sandards exhibi a sysemaic change in frequency wih ime. This is called aging. Alhough appearing o be linear over shor periods of ime, in fac aging in eiher oscillaor is more saisfacorily modeled by a logarihmic funcion over long periods of ime. All sandards, cesium, rubidium, or quarz, show random flucuaions wih ime. This is usually expressed by he Allan Deviaion (or roo Allan Variance) of he oscillaor. A ypical Allan Deviaion curve for an HP quarz SmarClock is shown in Figure 6. The curve can be inerpreed as a plo of he uncerainy expeced beween successive measuremens of oscillaor frequency for any given measuremen period. Hewle-Packard SmarClock Technology HP SmarClock echnology improves he performance and accuracy of low-cos oscillaors and enhances he availabiliy and implemenaion of reference sources. HP SmarClock algorihms allow HP o provide highly-accurae iming unis a price poins ha mee he requiremens of many conemporary applicaions. Based in par on work done a Naional Insiue of Sandards and Technology (NIST), HP SmarClock echnology also resuls from Hewle-Packard s 30 years of experience in he design and manufacure of precision oscillaors. 5

6 Timing Sysem Componens Hewle-Packard s precision iming sysems have four componens in common: An oscillaor frequency source An exernal reference signal cesium, GPS, or elecom signals (E1/T1) HP SmarClock algorihms ha characerize sysem behavior and enhance performance A microprocessor ha processes he HP SmarClock algorihms and conrols he oscillaor frequency, should he reference source become unavailable. HP SmarClock algorihms learn an oscillaor s behavior. The resuling iming sysem provides long-erm clock accuracy dependen primarily on he reference used. Correcing and Adjusing for Insabiliies HP SmarClock algorihms, used wih an exernal reference source, compensae for boh aging and emperaure-induced frequency changes. In he case of aging, for example, a quarz oscillaor changes frequency a an approximaely predicable rae, and he resulan deviaion over ime can be deermined. Random noise also mus be considred. HP SmarClock moniors he frequency conrol variable of he inernal oscillaor while i is locked o he exernal reference. This gives a measure of he frequency difference beween he inernal oscillaor, if i is free-running, and he exernal reference over ime. The resuling measuremens include he effecs of random noise in he oscillaor, he measuremen circuiry, and any noise in he exernal reference as well as any aging and environmenal Holdover Mode Exernal Reference Phase Deecor Memory and Aging Predicion Algorihm Sofware PLL Oscillaor Oupu Figure 1. HP SmarClock Technology includes algorihms for seering he oscillaor. The frequency is correced o compensae for he effecs of aging and emperaure. 6

7 Exernal Reference Phase Deecor Sofware PLL Holdover Mode Memory and Aging Predicion Algorihm Oscillaor Frequency Translaor Oupu Figure 2. In his unseered version of HP SmarClock, performance is improved by leing he oscillaor free run and adjusing he oupu frequency of a high-resoluion synhesizer. effecs in he oscillaor. From his informaion, HP SmarClock makes a coninuous predicion of clock error over ime if he exernal reference becomes inoperaive. HP SmarClock acually learns he basic behavior of he oscillaor. HP SmarClock Implemenaion Currenly, HP SmarClock is implemened in wo ways seered and unseered. In seered implemenaions of HP SmarClock (Figure 1), he frequency of a quarz or rubidium oscillaor is compared o ha of he reference signal. An error signal is sen o he microprocessor which, in urn, adjuss he frequency of he oscillaor. The HP SmarClock also learns he behavior of he oscillaor over boh ime and emperaure. This informaion is hen used o seer (or adjus) he oscillaor inelligenly during holdover mode when an exernal reference source is no available. In he unseered version of HP SmarClock (see Figure 2), he frequency ranslaor is implemened in he form of a synhesizer using a quarz oscillaor as is imebase. A coninuous measuremen is made beween he exernal reference and an appropriae signal from he oscillaor. When he sofware knows he measured difference, an appropriae command can be sen o he Frequency Translaor. Enhanced GPS The Global Posiioning Sysem (GPS) provides a highly reliable reference source for HP SmarClock. Wih 24 saellies, each compleing wo earh orbis per day, he GPS worldwide saellie sysem provides posiioning and iming capabiliies for boh miliary and civilian applicaions. Developed by he U.S. Deparmen of Defense, he GPS sysem is subjec o Selecive Availabiliy (SA). SA is he Deparmen of Defense s deliberae degradaion of he GPS broadcas signal in order o deny high accuracy posiion informaion for shor imes o unauhorized users. 7

8 In iming receivers however, he effecs of SA can be significanly reduced. Observaions of he specral characerisics of SA show ha i has a ime error peak a abou 400 seconds.[1] Thus, any filer ha aemps o reduce SA mus have ime consans ha are significanly longer han 400 seconds. Enhanced GPS is an HP SmarClock digial filering echnique ha explois he observed correlaion characerisics. When properly designed and mached o an inernal frequency reference source, a filer can grealy reduce he effecs of SA. The sandard specificaion for he ime oupu of a 4-saellie GPS posiion soluion, wih SA on, is 170 nanoseconds rms (340 nanoseconds a he 95 percen level). For a muli-channel GPS iming receiver a a fixed posiion, his rms deviaion can be reduced o abou 16 nanoseconds using a high precision quarz oscillaor such as he HP 10811D/E as a flywheel and HP SmarClock echniques. Wih aomic sandards such as rubidium or he HP 5071A Primary Frequency Sandard as a reference, he rms deviaion can be furher reduced. Experimenal resuls wih HP iming receivers using he HP 10811D/E ypically show a deviaion of 16 nanoseconds rms, a enfold reducion in he effec of SA [2]. Figure 3 shows he effec of SA. The daa shows he ime insabiliy of he GPS signal afer averaging wih a 6-channel receiver in posiion-hold mode. Figure 4 shows he resuls of using he SA filer o reduce he effec of SA. In his case, he SA filered daa shows a 13.7 nanosecond rms scaer. Mean value and slope have been removed from Figures 3 and 4. Similar filered daa as measured by he USNO is shown in Figure 7. The key is ha he specral characerisics of SA and he ime-domain sabiliy characerisics of he oscillaor used mus be mached hrough he ypes of filers and he ime consans used in he various conrol Time Difference (ns) RMS Scaer 36.8 ns Time (days) Figure 3. Raw iming daa aken from an HP 58503A Time and Frequency Reference Receiver wih he HP SmarClock filer disabled. The plo shows measuremens of he ime difference beween he 1-pps oupu from he 6-channel GPS engine and ha from an HP 5071A Primary Frequency Sandard versus ime. The measuremens were aken over a period of six days. 8

9 Time Difference (ns) RMS Scaer 13.7 ns Time (days) Figure 4. Time inerval measuremen daa aken by measuring he 1-pps oupu of he HP 58503A Time and Frequency Reference Receiver agains he HP 5071A Primary Frequency Sandard showing he effec of he HP SmarClock filer reducing overall scaer o less han 16 nanoseconds rms. loops. Each oscillaor ype requires a unique filer echnique o opimize he reducion of SA effecs. Enhanced RAIM Receiver Auonomous Inegriy Monioring (RAIM) is a series of algorihms ha coninuously checks each saellie agains all ohers under observaion. RAIM can ake many forms. The GPS engine used in HP iming modules has is own version, T-RAIM, or Time-RAIM. In addiion o T-RAIM, HP iming receivers have an exra layer of RAIM. This algorihm checks iming informaion received from he GPS engine agains inernal iming derived from he uni s own precision oscillaor. Algorihms also monior he overall healh of he iming module, is iming signal, and he signals received from he GPS engine o deermine when he exra layer of RAIM needs o be implemened o preserve he overall iming accuracy. 9

10 HP SmarClock 1 pps 10 MHz GPS Engine Time Inerval Couner Divided by MHz VCXO DAC µprocessor RS 232 Figure 5. Block diagram for he HP 58503A Time and Frequency Reference Receiver. Enhanced Learning During normal operaion, he inernal precision oscillaor, usually a quarz oscillaor, is phase-locked o he GPS signal by comparing he ime difference beween he 1-pps (pulse-per-second) signal from he GPS engine o a similar signal derived from he inernal oscillaor. A block diagram is shown in Figure 5. While locked o he GPS sysem, HP SmarClock employs enhanced learning o measure he aging and emperaure response of he inernal oscillaor. Over a period of ime, changes in he oscillaor frequency caused by aging and emperaure changes are accuraely measured using he reference signal from he GPS engine as filered by he enhanced GPS algorihm. Changes caused by humidiy or pressure can be minimized by using a hermeically sealed oscillaor. Long-erm changes, hose occurring over a period of many hours, are relaed o he aging of he inernal oscillaor. Frequency changes also occur as a funcion of emperaure, which is measured by he hardware. These are measured and sored in inernal memory. Daa relaed o he aging of he oscillaor is sored in RAM and are redeermined each ime he receiver is urned on. Consans relaed o emperaure performance are sored on EPROM, since emperaure performance does no subsanially change during periods when he oscillaor is no powered. Normal Operaion Normal operaion of he HP iming modules begins by racking four or more GPS saellies o deermine accuraely he geographic posiion of he anenna. Iniially, he iming module uses a shor ime consan o 10

11 conrol he oscillaor, hus providing rapid ime seing of he module. Following a series of checks of he overall operaion of he module, he ime consan incremenally increases o is final value. This usually akes from 2 o 18 hours. A his poin, he iming module is fully funcional and should mee all of is specificaions, excluding holdover. While locked o GPS, HP SmarClock echnology in he iming module sars learning he characerisics of he inernal precision oscillaor. The learning algorihm requires wo full days of daa o ensure ha an adequae deerminaion of he aging can be made. Learning coninues as long as he uni is powered and locked o GPS. Daa from he mos recen 48 hours is sored in RAM. Older daa is discarded. The module shares he long-erm sabiliy of GPS when locked. Shorerm, he iming module sabiliy is direcly deermined by he shorerm sabiliy of he oscillaor used. For averaging imes greaer han 24 hours (86,400 seconds), he frequency accuracy is beer han A ypical sabiliy curve is shown in Figure 6. 1 x Time Domain Sabiliy Locked o GPS 1 x Roo Allan Variance 1 x x x Sample Time (seconds) Typical Performance Specificaion Figure 6. Roo Allan Variance of he SA filered ime difference daa measured by an HP58503A GPS Frequency and Time Reference Receiver. 11

12 Table 1. GPS Timing Errors Cause Magniude of Error GPS Sysem (wih SA) 340 ns (a 95%) Propagaion Ionosphere Troposphere Solar Flares User Receiver Horizonal Posiion Errors Verical Posiion Errors Anenna Cable Mulipah Environmenal up o 40 ns up o 20 ns 40 ns o sysem inoperaive, dependen on severiy <20 ns negligible if self-survey is used 3 ns per meer of aliude error up o 3 ns per meer of anenna cable lengh measuremen error up o 50 ns up o 15 ns Timing accuracy is direcly affeced by he errors discussed previously. Assuming ha all of he user-conrolled errors (Table 1) are negligible, HP SmarClock iming modules wih quarz oscillaors achieve iming accuracies beer han 110 nanoseconds a he 95 percen level. As an example of his, Figure 7 shows daa aken from an HP 59551A GPS Synchronizaion Module by he Unied Saes Naval Observaory. This daa was acquired using a direc measuremen beween he USNO maser clock and he 1-pps oupu of he HP 59551A. The peak-o-peak deviaion is 100 nanoseconds. The average offse is 20 nanoseconds. The offse is he resul of a 10-ns GPS engine ime bias used a he ime and a known offse of GPS from he USNO maser clock of anoher 10 nanoseconds. 80 RMS Jier 16.7 ns Time Difference (ns) Modified Julian Dae Figure 7. Direc measuremen of he 1-pps oupu of he HP 59551A GPS Synchronizaion Module agains he USNO maser clock. The smooh line is a daily fi. 12

13 The iming module easily me is iming specificaion of ±110 nanoseconds over his period. Daa shows ha, compared o he USNO maser clock, he iming module was less han ±70 nanoseconds, wih an rms jier of 16.7 nanoseconds. The maximum deviaion of he 1-pps signal was less han 6 nanoseconds over any one-minue period. The sandard deviaion was less han 1.8 nanoseconds over any one-minue period. As deermined from he 1-pps daa, he 24-hour average frequency offse was Holdover Operaion Occasionally, he reference signal is no available. If he reference is a elecom signal (E1 or T1), he signal may be los for various reasons, such as lighning, physical damage, urban canyon and local jammers. In he case where GPS is he reference, he anenna may become unusable because of weaher, broken or damaged cable, or oher causes, or he receiver may emporarily lose rack of he saellies. The saellie sysem may receive a bad daa upload, malfuncion or oherwise be unavailable. Whaever he cause, during loss of he reference, accurae iming signals mus sill be generaed and used o conrol cusomer equipmen. During he loss of he reference, HP SmarClock uses daa learned previously abou he oscillaor o conrol he oscillaor and aemps o mainain all iming oupus a essenially he same level of precision as ha obained while locked o he reference. This form of operaion is called holdover. A conrol loop racks emperaure changes in he module and compues he correcions for he oscillaor o remove emperaure effecs. Anoher loop racks elapsed ime and compensaes for any aging effecs. Oher loops coninue o monior he GPS engine o deermine wheher normal operaion can be resumed. Typical specificaion requires ha he module mainain frequency accuracy o beer han during holdover and accumulae iming errors no greaer han 8.6 microseconds for he firs day of holdover, afer hree days of learning ime. Acual performance is highly dependen on he overall lengh of learning ime available before holdover. The longer he learning period, and he more sable he oscillaor, he more accurae he predicion. Figure 8 illusraes he effecs described above. Here, he value ploed in ligh gray is he elecronic frequency conrol signal ha seers he oscillaor. This uni had previously been operaing for several weeks. A he sar of his es, he memory was cleared of previously learned daa, hen he oscillaor relearning was sared. A he end of day 3, all of he learned daa, including he prediced fuure performance of he uni, was rerieved. 13

14 300 Sar of Day Accumulaed Time Error (ns) Average Frequency (x10 11 ) EFC Command Day Learning Period Time (days) Acual EFC (Elecronic Frequency Prediced EFC Command Conrol) Command Figure 8. Example of holdover showing environmenal effecs. During he nex hree days, acual operaion (he ligh gray curve) was compared o he prediced operaion (he dark curve). This is he mos accurae way of deermining he qualiy of he predicion in all circumsances. Anoher way would have been o disconnec he anenna and observe he resuls. However, i becomes difficul o deermine he cause of any unexpeced ime or frequency error. The daa shown in Figure 8 is a more accurae second-o-second picure of overall performance. Comparing acual o prediced performance allows an easy deerminaion of boh he expeced frequency offse and accumulaed iming errors. The assumpion is ha boh were perfec a he sar of he comparison. The daa shows a diurnal variaion caused by changing emperaure. The uni was operaed in a normal room environmen. During he nigh, he room s climae conrol was urned off, causing a decrease in room emperaure. The large dip in he curve a he sar of he experimen marks he sar of a weekend, when a much larger emperaure change was seen. Compued values show ha a he end of day 4, he firs day in simulaed holdover, he frequency error was and he accumulaed ime error was 1.74 microseconds. A he end of day 6, he hird day in simulaed holdover, he frequency change was and he ime error was 7.5 microseconds. 14

15 HP SmarClock Sysem Benefis HP SmarClock allows selecion of he oscillaor and reference source mos appropriae for he applicaion. During normal operaion, sysem accuracy is deermined by he accuracy of he exernal reference. An HP SmarClock-based sysem, using a rubidium or high-qualiy quarz oscillaor combined wih an exernal GPS reference, achieves nearcesium performance over he long erm. HP SmarClock eliminaes he need for frequen calibraions. Is learning capabiliies and he GPS reference source auomaically ensure ongoing accuracy. This accuracy complemens he field-proven reliabiliy of he HP quarz-based oscillaors ha can claim a Mean Time Beween Failures (MTBF) of more han 500,000 hours. Rubidium and cesium sandards offer moderae reliabiliy wih 50K-150K hours, and 120K-150K hours MTBF, respecively. The exremely high performance and low cos of HP SmarClock unis faciliaes implemenaion of advanced synchronizaion sysems. I can provide he precise frequency reference neded for wireless communicaions sysems. For new digial communicaions sysems like CDMA and Flex Pagers, i also can provide ime synchronizaion. Applicaions in digial elecommunicaions include providing accurae synchronizaion signals for equipmen and neworks a inernaional gaeways as well as andem, long disance, and local offices. I provides he precise iming required for power sysem applicaions ha include raveling wave faul locaion, sequence of evens reconsrucion, adapive relaying, phasor measuremen, and sabiliy conrol, and i can be used as a reference in a sandards lab. As echnology advances and compeiion increases in each of hese areas, a cos-effecive iming soluion is criical o an insallaion s success. HP 10811D/E Quarz HP Quarz wih SmarClock FEI 5650 Rubidium Rubidium wih SmarClock HP 5071A Cesium Aging Per Day in Holdover 2 x x x x N/A Tempco 0 o 50 C 2.5 x x x x <1 x seconds 25 C 3 x 10 2 W 12 3 x 10 2 W 12 3 x 10 8 W 12 3 x x W 42 W Figure 9. Wih HP SmarClock Technology, HP quarz oscillaor performance compares wih more expensive rubidium and cesium alernaives. 15

16 Appendix A: Basic Clock Equaion Wha will be covered is he basic heory of he causes of ime errors by an uncorreced oscillaor. Firs he effecs of aging or drif rae will be examined, hen he environmenal effecs. This will give he basic clock equaion for ime errors. Derivaion of he Clock Equaion In a simplified form, he frequency of an oscillaor a any ime can be expressed as: f = f0 + fr a( ) d (Eq. 1) 0 f f frequency f 2 f 1 f o f r Figure A Oscillaor ime Frequency vs. Time. where: and f = frequency a ime f 0 = iniial frequency a ime =0 f r = reference frequency (f and f 0 are assumed o be abou he same as f r ) a() is he aging rae of he oscillaor expressed as a funcion of ime. Aging rae or drif rae is he fracional rae of change of frequency per uni of ime. Convenionally, we express he frequency of an oscillaor in erms of is fracional frequency. Fracional frequency is defined as: f f f = 2 1 fr fr where: f = f 1 a ime = 1 f = f 2 a ime = 2 Cusomarily we define he fracional frequency by he leer y. By subracing he reference frequency from each side of Equaion 1 and rearranging, we have: y = y0 + a( ) d (Eq. 2) 0 16

17 The erm y 0 is called he synonizaion (or iniial frequency) error. For quarz oscillaors, he aging rae is normally saed in erms of a daily fracional frequency deviaion. As a well-aged quarz oscillaor has a nearly consan aging rae per day, we usually express he daily aging rae as if i were consan; e.g., ± per day. Any oscillaor over a period of ime will exhibi a change in is frequency, y. A clock based on his oscillaor will gain or lose ime because each oscillaor cycle is a lile shor or long. We can deermine he change in ime, or he ime error by inegraing frequency. or: where: and x = y ( ) d +x 0 0 (Eq. 3) x = x + y + d 0 0 a( ) d (Eq. 4) 0 0 x is he ime error in seconds x 0 is he iniial ime error, also known as he synchronizaion error. In he case where he oscillaor aging rae is consan, Equaion 4 becomes: x = x + y a (Eq. 5) 2 x a posiive y fo f r Figure A2. Posiive Frequency Drif. 17

18 Time Error versus Frequency Equaion 5 indicaes ha he accumulaed ime error over any ime depends upon he values of four quaniies; (1) iniial ime error x 0, (2) iniial frequency error y 0, (3) aging rae usually assumed o be consan, and (4) elapsed ime. A plo of Equaion 5 as a funcion of ime is a parabola for which he verical displacemen depends on he value of x 0, Figure A2. The corresponding frequency plo is shown beneah he ime error plo. Noe ha he oscillaor frequency is precisely equal o he reference frequency a he poin corresponding o he verex of he error curve. If he frequency drif or aging were negaive, he parabola would be invered (Figure A3). x a negaive fr y Figure A3. Negaive Frequency Drif. 18

19 Figure A4 shows corresponding plos of frequency and ime errors o clarify heir relaionship. Frequency f = fr fref Time Error seing error Frequency f fr Time Error Frequency fr f Time Error Frequency f fr Time Error Frequency fr Time Error f Figure A4. Frequency offses and resuling ime errors. Example As a specific problem, consider a GPS-based ime sysem ha needs o mainain ±10 µs for 24 hours in he absence of he GPS reference signal (usually called he holdover or flywheel mode). Alhough he GPS saellie sysem iself is very robus, he GPS anenna could be damaged or disconneced. This would cause he loss of he GPS signal. Assume ha his occurred a ime = 0. 19

20 Assume he quarz reference oscillaor in he ime sysem has an uncorreced, consan aging rae of per day. Since he ime sysem was locked o GPS immediaely before = 0, we will assume ha he following are rue: 1. y 0 = 0 2. x 0 = 0 The accumulaed ime error Equaion 5 becomes: x a = x10 s day day 2 86, 400 ( 1 ) day or x a = 4.32 µs, well wihin he 10 µs specified. Time Error Resuling from Environmenal Effecs and Noise So far he clock equaion has been based upon perfec condiions. However, environmenal condiions are usually a major cause of iming error in precise imekeeping applicaions. In addiion, here is a saisical uncerainy associaed wih he ime domain sabiliy of he oscillaor and wih sysem and measuremen noise. For environmenal condiions, we need o consider: 1. Effecs of emperaure changes on frequency (T) 2. Effecs of pressure changes on frequency (P) 3. Effecs of humidiy (H) 4. Effecs of magneic fields (M) 5. Effecs of graviy (G) 6. Cross-coupled inerdependen effecs. Wih all of hese condiions, we can wrie Equaion 4 as: x = x + y + d 0 0 a( ) d E ( T, P, M, H, G ) d + ε( ) (Eq. 6) 0 where ε() is he ime uncerainy in he reference signal and he measuremen process, and E (T,P,...) is he fracional frequency change caused by he ime-dependen environmenal effecs. When locked o GPS, he ime uncerainy componen of ε() over one day amouns o abou 50 ns. Over he same period, he noise componen of ε() can be assumed o be sochasic and herefore average ou over he period. 20

21 Many of he environmenal effecs can be minimized by he design of he oscillaor and is careful placemen in he operaing environmen. Pressure and humidiy effecs can be virually eliminaed by sealing he oscillaor. Mos of he remaining effecs can be minimized by oscillaor placemen. The one componen no so easily conrolled is emperaure. Example Looking again a our GPS-based ime sysem in holdover, assume i now experiences a emperaure ramp o a new fixed emperaure over a 24-hour period. The emperaure profile is shown below (Figure A5). For simpliciy, we will assume o firs order ha he coefficien of frequency versus emperaure is independen of ime and ambien emperaure (i.e., he emperaure coefficien is linear). We will also assume ha aging is negligible. 50 C 10 C/hr 0 C 0 5 hr 24 hr 1 2 Figure A5. Temperaure Profile. Le 1 = 5 hours (18,000 seconds) 2 = 24 hours (86,400 seconds) R = emperaure ramp of 10 C per hour T 0 = sar emperaure of 0 C T 2 = final emperaure of 50 C T (1) f = firs order frequency-emperaure coefficien of / C. Again, assume ha immediaely prior o he sar of holdover, he sysem was locked o GPS so ha any iniial ime errors are zero. The problem will be broken down ino wo pars, he emperaure ramp and he fixed emperaure offse. 21

22 For he emperaure ramp: x 1 ( 1) = T f R d 0 x T R ( 1) 2 1 = f 2 0 or: x x 12 2 ( x10 / C)( 10 C / hr)( 5hr) ( 3, 600s / hr) = 2 = 045. µ s For he consan emperaure region from 1 hrough 2 : 2 ( 1) x = T f T2d 1 ( 1) x T f T2 2 1 = ( ) 12 x = ( 1x10 / C )( 50 C )( 19hrs )( 3, 600s / hr ) x = 342. µ s Afer 24 hours, he oal ime error due o emperaure changes is 3.87 µs. Now including aging from he firs example, he oal ime error (wors case) is: x = xa + x x = 432. µ s µ s x = 819. µ s Therefore, his iming sysem would have mainained he ± 10 µs requiremen for 24 hours wih loss of lock o GPS and wih he given emperaure profile. 22

23 Appendix B: Applicaions and Producs Digial ranspor neworks are being pushed o he limis wih new echnology such as SONET/SDH. Service providers, anicipaing increased performance, new sandards, and a vasly expanding user base, realize ha nework synchronizaion will be vial for compeiiveness. The radiional iming synchronizaion approach applying piecemeal soluions o problems is being replaced wih well-planned nework-wide synchronizaion sraegies. Wih increased accuracy requiremens, beer synchronizaion soluions are required a more and more locaions in each nework. Neworks are flaening heir synchronizaion hierarchies by using higher performance clocks a regional and even local sies. The arenas of power generaion, ransmission, and disribuion are also becoming increasingly compeiive. Deregulaion, expanding consumer requiremens, and environmenal resricions are placing unprecedened demands on supplying companies and uiliies. Complee and synchronized daa analysis is criical for suppliers o increase power capaciy, qualiy, and services reliably. Synchronizaion producs provide a foundaion for real-ime monioring and conrol applicaions, and deailed nework disurbance analysis. All of he Hewle-Packard iming producs offer a common se of benefis, including: Compliance o indusry sandards Shelf modulariy High reliabiliy and redundancy Excellen shor-erm sabiliy GPS compaibiliy 23

24 HP s Timing and Synchronizaion Producs Hewle-Packard provides a range of iming and synchronizaion producs based on our advanced GPS echnology and HP SmarClock. These producs include: Primary reference sources. A primary reference is ofen referred o as he maser clock. Top level synchronizaion soluions demand he highes accuracy and bes long-erm sabiliy since hey are used o drive muliple lower level synchronizaion unis. Specialized producs and full cusom soluions. HP offers cusomizaion services and specialized producs such as he HP 59551A GPS Synchronizaion Module for synchronizing measuremens for power ransmission sysems. The compac HP 58503A Time and Frequency Reference Receiver delivers excellen accuracy, easy operaion (no periodic adjusmens are required), and exremely low cos of ownership. Synchronizaion unis. These producs deliver synchronizaion signals a specific poins wihin a nework or sie, or disribue a reference source o muliple poins. Since many organizaions require large numbers of hese unis, hey mus be affordable while mainaining accuracy and sabiliy hroughou he synchronizaion infrasrucure. 24

25 Merology and Manufacuring The HP 58503A GPS Time and Frequency Reference Receiver, a lighweigh compac module, fis comforably ino no only conrolled lab environmens, bu also a variey of general-purpose calibraion and manufacuring applicaions ha require precision ime or synchronizaion. The HP 58503A source mainains frequency accuracy of beer han (measured as a 24-hour average), even in he presence of Selecive Availabiliy. This performance, combined wih he uni s low cos, makes i an aracive lab alernaive compared wih more expensive cesium and rubidium soluions. Manufacuring companies are aking advanage of he HP 58503A. Because of is low cos, i can be locaed anywhere precision ime and frequency are needed, eliminaing or simplifying disribuion sysems and expensive cable runs. In operaion, he HP 58503A requires no periodic adjusmen or calibraion. This, along wih is greaer han 100,000-hour MTBF, provides an exremely low cos of ownership. If he GPS signal is los, he HP 58503A auomaically goes ino is inelligen holdover mode. An RS-232 por and a TTL alarm oupu (BNC) allow easy performance monioring. Eiher of hese oupus will enable you o monior he saus of he HP 58503A auomaically. Synchronizing Wireless Neworks Accurae iming is essenial for he evolving digial cellular personal communicaions sysems (PCS) and paging applicaions. Sof handoffs passing a CDMA user from one base saion o anoher require accurae synchronizaion. The HP 58503A general purpose reference source can deliver accurae and sable synchronizaion signals for boh analog and digial base saions. Today, many wireless neworks also require cusomized designs for iming and frequency synchronizaion and disribuion. For nework equipmen manufacurers building volume quaniies of wireless communicaions base saions or oher equipmen, he HP cusomizaion services can shoren implemenaion cycles and lower in-house developmen coss wihou sacrificing qualiy and affordabiliy. Oher feaures HP can bring o he wireless marke include: Shorened ime o marke Lowered coss Improved sysem and service qualiy Reduced projec risk 25

26 Cusom designs use many of he same echnologies incorporaed in he HP 58503A and are ailored o clien specificaions. A cusom design can accommodae he needs of any analog or digial cellular, specialized mobile radio, or personal communicaion sysem manufacurer. Cliens specify he exac form, fi, and funcion of he equipmen needed for synchronizing base saion operaion. Cusom producs are manufacured o HP s qualiy sandards and are suppored worldwide. Wide Area Measuremen Synchronizaion The HP 59551A GPS Synchronizaion Module is he firs produc o provide cos-effecive, wide-area synchronizaion of measuremens. I overcomes he limiaions of radiional measuremen ools and grealy exends applicaions direced a monioring and conrolling power ransmission sysems. The HP 59551A can be placed a any of he criical poins in he power sysem or any oher disribued sysem. The low uni price ensures ha synchronizaion neworks can provide complee field daa for analysis. The resoluion and ime-agging feaures of he HP 59551A make i ideally suied o applicaions such as frequency and magniude measuremen, sae esimaion, sabiliy proecion and adapive relaying. The HP 59551A also provides key daa for newly evolving monioring applicaions such as faul recording, disurbance recording, and verificaion of compuer modeling of ransmission and generaion sysems. Wih he HP 59551A, planners have access o real-ime field daa o predic more accuraely and conrol safely he behavior of power sysems. The HP 59551A GPS Synchronizaion Module is affordable, can be placed a many poins hroughou a wide-area power sysem, and provides essenial real-ime field daa for predicing and conrolling power sysem operaion. 26

27 The HP 55400A Nework Synchronizaion Uni les users configure efficien and coseffecive synchronizaion disribuion soluions for large sies. Nework Synchronizaion The quarz-based HP 55400A Nework Synchronizaion Uni disribues clock signals for elecommunicaions neworks. The rack-mounable uni filers incoming iming references from exernal clocks or GPS reference modules and oupus iming signals hroughou a building or office. The HP 55400A incorporaes quarz and HP SmarClock echnology o provide feaures ha ensure reliable disribuion soluions: Online swichable reference sources Holdover mode during loss of reference signal Ho-pluggable inpu and oupu signal cards Disribuion of muliple oupu signals A maser subrack and expansion subrack can each accommodae en oupu cards. Each card provides 16 oupus for a subrack oal of 160 oupus (or 80 proeced oupus). This funcionaliy suppors he economical disribuion of muliple oupu signals hroughou a large faciliy. 27

28 HP 55300A GPS Telecom Primary Reference Source is a highly accurae, low-cos source of precision frequencies for elecom neworks. GPS Telecom Primary Reference Source The HP 55300A GPS Telecom Primary Reference Source provides highly-reliable, low-cos sources of precision frequencies for elecom digial neworks. The HP 55300A is lighweigh, compac, and fis comforably ino conrolled lab environmens, wireless base saions, and remoe or unaended applicaions. The HP 55300A mainains frequency sabiliy of beer han (measured as a 24-hour average), even in he presence of Selecive Availabiliy. The uni is an excellen soluion for flaening exising digial neworks and delivering performance ha exceeds indusry sandard requiremens for lower-level poins on he synchronizaion hierarchy. The HP 55300A provides a low-cos reference source ha improves nework services and provides scalabiliy o synchronizaion soluions. 28

29 References 1. D.W. Allan and W.P. Dewey, Time-Domain Specrum of GPS SA, Proceedings of he ION GPS-93, The Insiue of Navigaion, Sepember 1993, pp J.A. Kusers, e al, A Globally Efficien Means of Disribuing UTC Time and Frequency Through GPS, Proceedings of he 26h Annual Precise Time and Time Inerval (PTTI) Applicaions and Planning Meeing, December 1994, pp J.A. Kusers, The Global Posiioning Sysem and HP SmarClock, HP Journal, Vol. 47, No. 6, December Glossary Aging Rae The frequency aging of an oscillaor refers o he change in he frequency of oscillaion caused by changes in he componens of he oscillaor, eiher in he resonan uni or in he accompanying elecronics. Aging differs from drif in ha i does no include frequency changes due o changes in he environmen. BIPM Bureau Inernaional des Poids e Mesures. The mainenance of Inernaional Aomic ime (TAI) and of Coordinaed Universal Time (UTC) has been he responsibiliy of BIPM since January 1, Locaed in Sevres, France, BIPM provides he coordinaion and compuaion of iming and sabiliy daa from abou 230 aomic clocks kep by 65 laboraories worldwide. Cesium clock A primary ime reference which uses a fundamenal propery of he elemen cesium as a reference o produce a highly accurae and sable ime reference. Epoch Usually denoes he inerval elapsed from an arbirary origin; his origin is cusomarily saed in erms of a naural phenomenon common o all observers. Epoch also may denoe a paricular insan of ime. GPS The Unied Sar Deparmen of Defense's Global Posiioning Sysem. I feaures 24 saellies in semipolar orbis using six orbial planes. The saellies broadcas heir precise ime and posiion o users whose receivers use his informaion o deermine boh accurae ime and locaion. 29

30 Glossary (coninued) HF High Frequency, usually assumed o range from 3 o 30 MHz. LF Low Frequency, usually assumed o range from 30 o 300 khz. Loran-C A series of 29 ransmiing saions comprising 12 Loran-C chains operaing in he khz frequency band. Provides local and long range navigaion capabiliies and a iming signal. Major user is he Mariime communiy, wih exensive use also in aviaion. NIST Naional Insiue of Sandards and Technology. NIST provides he merology sandards for he Unied Saes. NIST is he primary source of frequency accuracy and conribues o aomic ime sabiliy as mainained by BIPM. NRL Naval Research Laboraories. Omega Developed and implemened by he Unied Saes Deparmen of he Navy wih assisance of he Coas Guard and 6 parner naions. I provides worldwide, all-weaher radionavigaion capabiliy, operaing in he 9-14 khz band. Rubidium clock A ime reference which uses a fundamenal propery of he elemen rubidium as a reference o sabilize a quarz oscillaor. Rubidium clocks are much less sable han cesium clocks as hey exhibi a slow frequency drif as well as greaer sensiiviy o he environmen. Because of hese insabiliies, rubidium clocks mus be periodically calibraed. Synchronize To se he ime or phase of wo or more clocks o each oher or he same reference value. Synonize To se he frequency of wo or more sources o precisely he same value. USNO Unied Saes Naval Observaory. Boh he official ime for he U.S. and he ime reference for he GPS navigaion sysem are mainained by he USNO. he USNO conribues o he UTC ime scale. USNO MC This is he designaion of he maser clock a he USNO. The maser clock is an ensemble consising of 42 Primary Frequency Sandards. UTC Coordinaed Universal Time (Temps Universel Coordonne'). The world ime scale mainained by he BIPM. UTC is mainained by averaging ime daa from major imekeeping ceners locaed around he world. The BIPM is also responsible for oher basic sandards for inernaional commerce. VLF Very Low Frequency, usually defined o be less han 30 khz. 30

31 HP Relaed Lieraure For more informaion: HP Innovaion for 21s Cenury Nework Synchronizaion brochure Applicaion Noe 1272: GPS and Precision Timing Applicaions Applicaion Noe 1289: The Science of Timekeeping 31

32 H For more informaion abou Hewle- Packard es and measuremen producs, applicaions, services and for a curren sales office lising, visi our web sie, hp:// You can also conac one of he following ceners and ask for a es and measuremen sales represenaive. Unied Saes: Hewle-Packard Company Tes and Measuremen Call Cener P.O. Box 4026 Englewood, CO Canada: Hewle-Packard Canada Ld Specrum Way Mississauga, Onario L4W 5G1 (905) Europe: Hewle-Packard European Markeing Cenre P.O. Box AZ Amselveen The Neherlands (31 20) Japan: Hewle-Packard Japan Ld. Measuremen Assisance Cener 9-1, Takakura-Cho, Hachioji-Shi, Tokyo , Japan Tel: (81) Fax: (81) Lain America: Hewle-Packard Lain American Region Headquarers 5200 Blue Lagoon Drive 9h Floor Miami, Florida U.S.A. Tel: (305) (305) Fax: (305) Ausralia/New Zealand: Hewle-Packard Ausralia Ld Joseph Sree Blackburn, Vicoria 3130 Ausralia Tel: (Ausralia) (New Zealand) Fax: (61 3) Asia Pacific: Hewle-Packard Asia Pacific Ld /F Shell Tower, Times Square, 1 Maheson Sree, Causeway Bay, Hong Kong Tel: (852) Fax: (852) Daa Subjec o Change Prined in U.S.A. March 1998 Hewle-Packard Company Copyrigh E