(PDF) Time Shared

- JOT]RI{AL PACIGRD HEWTETT o o o JULY1968 more powerful than most usersrequire,and so expensive that very few userscouldjustify owningtheir own system. In the past,mosttime-sharingsystemshavebeenowned A Practical by major users,suchasuniversities or largecorporations, or by commercialtime-sharingservices,where a large numberof users,eachbuyingtime by the hour, supporr Time-Shared the expenseof a largecomputersystem. GomputerSystem SystemDesignObjectives The developmentproject for the Hewlett-packard Model20004 Time-Shared BASIC Systemwasinitiated with the objectiveof developinga systemto fulfill eco_ UsingConversationat BASIC,a new nomicallythe majorityof a user'sneeds,whileleavingthe 16-terminalsystemdoesn,ttry to more complex and expensivefunctionsto be satisfiedby do everythingfor everyone,but stilt larger systems.BASIC was selectedas the systemlan_ safisfiesnearlyall the user,sneeds. guagebecauseof its widespreadapplicationand accept_ anceas a time-sharinglanguage. By ThomasC. poulter.Jr. Certain of the system'sboundswere dictatedby the systemhardwareconsiderations. Togetherwith efficiency Wrrn rnr cRowrH oF coMpurERS IN spEED, powER considerations,the teletype multiplexer desigrrand the ANDpRIcE,'time-sharing' has received increasingaccept_ computer's16-bitword lengthcombinedto fix the maxi_ ance as a means of making the power of a large and mum numberof simultaneous usersat 16. The maximum expensive computer available on-line, to individual users, programlengthwas selectedto equalthe storagecapacity efficiently and economically.Generally, time_sharingsys_ of onedisctrack,or 5440computerwords. tems have been characterized by their large size, multi_ Hewlett-Packard's own experiences with time-sharing lingual capability, complex executive programs, and high computerusecontributedgreatlyto both the decisionto cost. Experience with these systemshas shown that most develop the systemand the designof the systemitself. userspay a high price for featuresthey seldom use. Given Our terminalsare used in circuit analysisand design a choice, most time-sharingusersprefer a simple, conver_ computations, marketprojections,in-process production sational language, such as the Dartmouth College devel_ calculations,and cost analysis,to name a few. About oped BASIC. Yet the implementation of BASIC repre_ 9O% of the in-houseuse of time-sharinghas been in sents a relatively small fraction of the cost of most BASIC. A numberof other computerfacilities are avail_ time-sharing systems. able and are regularly usedfor the larger and more com_ Thus the desire to try to do everything for everyone plex programs. has resulted in a generation of time-sharing systems far SystemHardware The time-sharingsystemis built aroundthe Hp Model Cover:A mobile teteprinterterminatwith a 21168 Computerwith a 16-bitword length(plusparity) telephoneacousticcouplerbringsa computer and 16,384words of magneticcore memory.For time_ into the lab area. Up to l6 usersmay usethe sharing operation the computer is equipped with the computer simultaneouslyin the Hp Modet following: 2000ATi me-Shared Sysfem. Internal r Direct memoryaccess r Memory parity check Inthislssue: A practicalTime-Shared Com- r Extendedarithmeticunit r Time basegenerator pufer Sysfem; page 2. rcC RenamesNoise r Powerfail interrupt r Teletypemultiplexer Contour; page 7. n nuOiAiumVapor Fre_ quency Standard;psge 8. ComparingFre- External quency Standards;page 15, r Magneticdisc memory I Power supply extender : High speedtapereader r Heavy duty teleprinter P R I N T E DI N U . S . A @ HEwLETT-pAcKARD co,, 196a Heart ol the HP Model 2000ATime- Shared System is the HP Model 21168 Com7uter. SPaceis Pro- vided for exPanding the disc mem- oty. With the control telePrinter, tett, the oqetator is able to com- municate with the sYstem and to tog information. The first four internal options provide the high speed words of program storage. The 16 millisecond average data transfer and computation,and the internal check- disc access time, coupled with the executive's optimum ing of power levelsand parity errorsnecessary for best timing techniques assure the efficiency required for han- efficiencyand reliability. The time base generator pro- dling the maximum 16 usersat once. videsa time basefor determiningtime of day, for meas- To provide for rapid loading of the system executive uringsystemusage,andfor timingthe sharingof program or other software systems, the time-sharing system is executiontime. equipped with an optical paper-tape reader' Operating A special teleprinter multiplexer was developedfor at 300 charactersper second,the tape reader can be used the system.Occupyinga singleinput/output channel,it to load the entire system in less than two minutes' Once servicessimultaneouslyall sixteenuserchannels,onewith eachbit of the 16-bitword. For maximumreadingaccu- What Is Time Sharing? racy, the multiplexer sampleseach incoming bit eight Time sharing is broadly defined as the use of a central times.Sincethe teleprinter'sbit rate is 110 per second,a Drocessorfor severalpurposeswithin the same time inter- multiplexersamplingrate of 880 per secondis required' val. This may include many things,such as airline reserva- For bulk high speedmemory' the systemusesa mag- tion, process monitoring and control. But to most engi- netic disc memorywith 348,160words of storage'The neers,time sharing meansquick access to a computerfor s o l v i n g d a y - t o - d a y p r o b l e m s . M u l t i p l e x i n gt e c h n i q u e s disc is usedby the systemfor storageof currentprograms enable two or more users to share the computer from (87,000words),storageof a file copyof the systemexec- terminalswhich may be nearbyor hundredsof miles away' utive program (16,000 words), for storageof system The number of terminalsand the complexity of the prob- tables required for the library and accountingsystem lems that may be handled are largely determinedby the (11,000words),and for storageof saveduserprograms size and capability of the computer, and the level of (235,000words).Discstoragecanbe expanded by adding sophistication of the software system. up to three more disc'units providing ovet 1'25 million 3 H P 2 11 6 8 CO[,4PUTER up to 16 terminals can be operated simultaneously with the Modet 2000A sysfem. Terminars may be tied into the system in one of the three ways shown. The magnetic tape unit is an option, as are the shaded items in the tight cabinet. 4 loaded into core memory the time-sharing software can be stored and reloaded from the disc memory in milli- seconds,or from the optional magnetictape unit in a few seconds.With the power supply extender,the systemhas adequatepower for the full range of disc and tape options' The heavy duty teleprinter servesas the systemcontrol console and is connectedto the computer through a sep- arate l/O interface. It is used for operator communica- tion with the system and for logging systeminformation' Using this system control teleprinter, the operator can also control accessto the system by assignmentof user account numbers and Passwords' User Terminals The teleprinters used as terminals with the system are the Teletype Models 33-ASR or 35-ASR' Communica- tion with the system at rates up to 10 charactersper sec- ond is possible either through the keyboard and printer' or through the paper tape reader and punch' For local service,up to one mile, the terminals can be wired directly to the system.For longer distancesor for greater operat- ing flexibility, the terminals can be connectedto the sys- tem through regular voice-grade telephone lines using coupling equipment such as the Bell System Data Set 103A. Use of the telephonesystemallows a greater num- ber of terminals to be servedby the system;up to sixteen users can be handled simultaneously on a first call-first served basis. System Software The system software can be divided into five major sub-systems: r Executive program r Multiplexer control Program I BASIC languagecomPiler I Accounting system At the upper Interlacecircuitry is on the group of catds,top' r Library system magnetic tape interlace; upper tight is the teletype Ie is the the paper interlace for system control; at the lower lett is at the The time-sharing executive program directs the com- tape reader interlace,and the disc interlaceis shown and puter's support of the following functions: lotwerright. tn the bottom group, the teletype muliplexer disconnect cards are shown at the automatic tetephone I TeletyPe input and output multiplexing memoty upper left, the five cards which make up the direct I Real time clock generatot at the access ui th" ,pp", right, the time base at the r Systemconsolecommands tower left,and the two extended arithmetic unit catds r ljser terminal commands lower right. r Program execution I Accounting records Each user on the system is assignedone of the 16 disc the tracks used for storage of current programs' Of 5440 word maximum program length, approximately 400 c words are reserved for system use. The storage required HP 2000A BASIC Language by a program statement depends upon the statement BASIC is a simple, yet powerful programming language length, but on the average, statements require about designed for on-line conversationalcomputation.A com_ puter program written in BASIC consists ten words of storage.Two words of storageare required of numbered statements.The computer executes these statementsin for each variable or array element. Thus about 5000 sequenceunless an instructionwithin a statementdirects words are available for statement storage (or 500 state_ otherwise.Each statementspecifiesan action to be taken ments) less any storage required for program variables. by the computer,not as the statementis typed, but when the program is executeo. User's programs are brought into core from the disc The program example below, named ,pRIMES,,iltustrates for one of severalreasons: the entry and execution of a BASIC language program. r Addition, deletion, or replacement of The program prints the prime numbers from 2 to 200 by a program testing all odd integersfor factors.While this program is statement neither elegant as a technique nor an efficient use of r Transfer of a program to or from the library computer time, it does demonstratethe ease with which r Reading of a program statement for listing or programscan be written and executedon the system. punching NAilE PRINES r Resequencingaprogram IO PRTNT 2, 29 FoR N.3 TO zOO SlEp a r Running (executing) the program 3S FOR l:3 TO SOR(Nt 4g IF N/I : INT(N/I) THEN ?o 50 NEXT I 5S PRINT N' When in execution, the user's program may be returned 70 NEXT N A9 END to current disc storage for one of several reasons: RUN 235? r Completion of execution rl 13 t7 t9 23 29 41 43 47 53 3l 37 91 tol lo3 to7 59 tog 5t I 13 51 t21 7t r3l 73 79 a3 89 r Filling of the output buffer f57 163 13? I 39 149 t5l t67 l?3 179 lal l9t t93 t91 199 I Input requested from the terminal I Higher priority service required by another user; Hp Modet 2000A Time_ShaledSystem program editing or listing The HP Model 2O0OAis a time-sharedcomputei systemcapableof I End of one-second execute time quantum, s i m u l t a n e o u s l sy e r v i n g u p t o s i x t e e n o n _ l i n eu s e r if other t e r m i n a l s .l t users are waiting for execution. opefates in BASIC language with conversationalprogram enrry, editing, execution,listing, and storage operations. A slystemcon- trol teleprinter provides control over system start-up Programs in core for other than execution are returned and shut_ d o w n ,t h e p u b l i ca n d p r i v a t el i b r a r i e sa, n d t h e a c c o u n t i n g system. to current disc storage at the completion of the service. User terminalsare served through a teletypemultiple""r"uno ,"y be wired direcily to the system if located within one mile, or con- Responseto program statementediting is typically less nectedvia a telephonesystemwith suitablecouplers. system includes all hardware and software necessary The stanoard than 0.3 second. The wait for execute time varies from for normal o p e r a t i o n .S y s t e mo p t i o n s i n c l u d ea d d i t i o n a ld i s c m e m o r y ,o a t a less than 1 second, for a lightly loaded system, to an s e t c a b l e s f o r c o n n e c t i n gt o t e l e p h o n ed a t a s e t s , magnetictape recorder for off_linestorageof the nine_channel average of 4 seconds,for a fully loaded system (i.e., 16 softwaresysrem and. libraries, and a telephone auto_disconnect board for auro_ users). Interactive programs (requiring frequent data m a t i cd i s c o n n e c t i n og f i n v a l i dc a l l e r s . input from the user) will have a shorter wait time, tlpi_ PRICE:HP Model 20004 Time_SharedBASTCSystem, cally I second or less, than programs with extensive R a c k m t . , ( w i t h c a b i n e t )j l s V , 6 0 H z $S9,5OO execution. C O N S I S T SO F : 2 1 1 6 B - M ZM, 5 , M 8 ,M 9 , M 1 . lC o m o u l e r The multiplexer control program operatesin response (K2) 2754A-M3Teteprinterwith 1253jB to the interrupts from the multiplexer and performs the (K3) 2737A punched Tape Readerwith 1 2Sg2A ( K 1 0 ) T i m e B a s e c e n e r a t o r ,1 2 5 3 9 4 following functions: (K34) Z7STA-M1Disc Memory with 22564 and 12561A ( K 3 6 )2 1 6 0p o w e r S u p p r y For each interrupt (880 per second),input a 16_bit 12584A-M1TetetypeMultiprexer word representing the input status of the 16 user 29928-M11 Duat Bay Cabinet (with doors) MODIFICATIONS: channels,and sort the bits according to user channel. ADD: T3 2757A-M1Disc Memorywith 27564 and 12561A Determine for each channel, by analysisof succes_ $20,000 T4 12584-8006Data Set Cabte 50 sive samples,whether the input is a mark or a space, T 5 H O 1 - 3 0 3 0 cM a g n e t i cT a p e U n i t w i t h 1 2 5 5 9 4 18,500 T6 12584A-M2TetephoneAuto-disconnect packing the respective bits into g-bit bytes, repre_ 1,500 MANUFACTURING DtVtStON:PALO ALTO DtVtStON 3 9 5 P a g sM i l l R o a d senting characters. P a l oA l t o ,C a l i f o r n i ag 4 3 0 6 Construct a 16-bit output word, with one bit per user channel, with the following content: 6 - l . For eachuser receivingoutput from the system, transmit from his output bufier the character IEC RenamesNoise Contour string, bit by bit, characterby character. We have just been informed by the chairmanof the lnter- 2, For eachusersendingto the system,echobit by national ElectrotechnicalCommissionthat they have de- 'N' weighting contour for bit, eachcharacterreceived. cided to recommend that the 'D' from now on. This makes sound-levelmeters be called it consistent with the A, B, and C weighting contours Output the word to the teletype multiplexer for a l r e a d yi n u s e . transmission to the users. The D (formerly N) contour was developed mainly 'Loudness Evalua- for monitoring iet aircraft noise. (See Since there is no synchronism between teleprinters, the tion,' Hewlett-Packard Journal, November 1967.) driver must be able to handle each channel as an inde- pendent entity. The BASIC languagecompiler operatesinterpretively. 'condensed' Program statementsare stored in a special that sessionand the total for that accounting period along form and are converted to machine language in sequence with the number of words of disc storage saved by that as required for execution. The condensedprogram form user. is sufficiently similar to the original source program that The library systempermits saving of programs on the a source program listing can be reconstructedfrom the disc in two categories.PUBLIC programs are entered stored program. into the system by the system operator using a special The compiler supports a user'sprogram inputs in three account number-password.Such programs are available phases.The first phase is active while the user is entering to all system users,but can only be modified by the sys- program statements.It checkseach statementfor format tem operator. PRIVATE programs are those saved by and syntax and, if correct, condenses the statement and each user under his own account number and are avail- forwards it for inclusion in the user's program. If the able only to him. statement form is incorrect the statement is discarded and a diagnostic message is sent to the user describing Acknowledgments the nature of the error. The secondphase is active when 'RUNI Some of the The time-sharing system project team consisted of: the user requeststhat his program be Software System - Mike Green (manager), Gerould functions performed by the secondphase include alloca- 'DIM' (dimension) Smith and Lewis Leith. Hardware System Design - tion of spacefor arrays as defined in Stephen Porter. Hardware System Engineering - Al statements,and checking the logical structure of the pro- Marston and Willis Shanks. Industrial Design - Gerry gram for correct statement ordering and loop formation' Priestley. Product Marketing - Tom Poulter and Paul Any errors found during Phase II are listed and control Schmidt. E returned to the user. If the program is correct the opera- tion is transferred to the third phase where the actual execution is performed. ThomasC. Poulter,Jr. The third phase begins with the lowest numbered Tom Poulteris a graduateof statement and executes each statement in turn unless StanfordUniversitY,1957,with a BS degree in physics.After graduation instructed otherwise.The executingprogram may request he spent sevenyears in the and accept input from the user and/or output informa- of d e v e l o p m e nat n d e n g i n e e r i n g tion on the user terminal. instrumentationfor ordnanceand The accounting system provides for validating each rocket testing. user as he enters the system by means of an account H e j o i n e dt h e H P D Y m e cD i v i s i o n , number and a password. If the number-password com- n o w t h e P a l oA l t o D i v i s i o ni,n 1 9 6 4 where he was proiect managerfor a bination matches a valid combination then the user is l i n e o f s i g n a lc o n d i t i o n i n g logged onto the system and can work with his programs' instruments.Tom is Presently While the user is connected,the accountingsystemkeeps SystemsProductManagerfor the track of the accumulated terminal time. When the user division. signs oft, the system outputs the accumulated time for A Rubidium-vaporFrequencystandard for SystemsRequiring SuperiorFrequencyStability By DarwinH. Throne T n p l v l I r a e I L I T y o F s T A B L EA N Dp R E C t s E FREeuENCy stability may still have superior short-term stability, and souRcEShas been one factor that has led in recent years vice versa."Short term" as usedherercfers to the interval to progressin such fields as deep-spacecommunications, neededto makc a measurement,typically from a fraction satelliteranging,doppler radar and others.Basically,the of a second to several seconds. frequency problem to be dealt with in many of these ap- A type of frequency sourcethat does have a high order plicationsis to have high short-termstabitityoften along of short-ternt and long-term frequency stability is the with long-term stability. The combination of these two rubidium-vapor or rubidium gas-cell standard. Fig. I requirements in one frequency source is not so readily shows a recently-designedstandard of this type which achieved. It may not be generally realized, for exantple, embodies advancesthat ntake it particularly suited for that a frequency sourcewith lessthan superior long-tcrm work of the sort mentioned above. The new atomic standard has the short-term stability for which rubidium-type standards are recognized. In addition it has a '''.!s:#f{#rr!.dJlrf {}!t4"!,i1r11t.."q\1\!..: high and specifiedlong-term stability together with simple adjustability oi tr, '''iti- , output frequency.This group of char- lic 3. acteristics makes the standard val- uable for use where even the best 1i a,1 iF; quartz standards are inadequate or Sd where better short-term stability than #' '*--s- that of other standardsis required. The simplifiedmcthod of adjusting ,l the standard'soutput frequency con- sistsof a set of quasi-calibratedcon- trols which, if desired,can be usedto Fig. 'l. New rubidiumiype frequency standard provides high_ stability output signal tor use in precision frequency work. change the standard from an atomic- tir.nescale to the UTC scale (300 X l0 '" parts below the atomic scale). The new standard further has a small size and modest weight that enable it to be easily transpslted - by air_ plane, if desired. A relatively fast warm-up time of less than two hours complements the transportability. Still higher portability is achieved in a secondversion of the instrument having battery standby. Stability Much of today's work involving precision frequencies is concernedwith a high degree of short-term frequency stability. The signal being transmitted by a satellite or spaceprobe, for example, must often be stable to parts in l0'1 for the system to extract the desired information Fig.2. Fundamentaloperatingaruangementof new r ubi d i um-vapor f requency standaft . from the signal. This sort of stability is achievedonly by a few of the highest-quality types of frequency sources, of which the rubidium standard offers some special ad- vantages.The most important of these is that it achieves The high stability of the standard results from the use very good short-term and long-term stability at a price of an atomic resonator operating with a flywheel in the level considerably below that of contending types. The form of a very-high-quality quartz-crystal controlled os- new standard achieves,for example, a rated short-term cillator (Fig. 2). The closed-loop bandwidth of the con- stabilityof lessthan 1 part in 10rr rms frequencychange trol systemis limited - about 2 Hz - so that the stabil_ for 1-secondaveraging and a long-term stability of less ity of frequency output for intervals (averaging times) than 2 parts in 10" frequency change per month. This of a fraction of a second is essentiallythat of the crystal level of performance makes the rubidium standard a so- oscillator. The short-term stability of this oscillator is lution for many frequency-stability problems. very high - equal to that of HP's most stable crystal Fig' 3. Typical stability curve (solid Iine) ot HP Modet 5065ARubidium Standardtor wide range of averaging times. Long averagingtimes (lower right) were measured againsta hydrogen maset; data obtainedtrom record shownparly in Fig. B. by the dashed line. The systematic noise level is of con- sequence because the instability being measured is ex- tracted by the measuring system as noise and measured in those terms.* In examining the stability curve shown in Fig' 3 it is evident that for short averaging times the new standard's stability does in fact follow that of the crystal flywheel for averaging times up to approximately the time constant of the control loop (100 milliseconds). The short-term instability is, indeed, so near to that of the measuring system that the performance of the standard may be better than that shown. As the averaging time becomes longer than that of the loop time constant, the measured frequency stability im- Fig.4. Basic citcuit attangement proves as 1,/t/v z being the averaging time' This trend ol rubidium lrequencY standad. continues to about 100 secondswhere the stability curve tends to become more constant. For averaging times longer than2 to 3 hours, the stability continues at about the same level under room temperature conditions' Under harsher environments, temperature effects and other en- vironmental factors begin to influence stability' Basis of OPeration Fig. 4 shows the general arrangement of the electronics of the new standard. The rubidium gas cell is excited by the frequency-multiplied output of the 5-MHz crystal oscillator, which in turn is voltage-controlled by a dc signal derived from the output of the gas cell' The gas cell or resonator has an absorption characteristic similar to that shown in Fig. 5. The dip in the cell's output current at its resonant frequency is utilized by phase-modulating Fig. 5. Outputcutrcnt characteristic trom rubidium- the frequency (6,835.685 MHz) that drives the cell' An cet! opticaldetectoras a functionof drivelrequency' error current at the modulating frequency derived from the cell is amplified, phase-detected' and applied as a control voltage to the crystal oscillator from which the standard. As longer averaging times are considered, the RF signal was obtained. control system constrains the crystal oscillator to the superior long-term stability of the atomic resonator. The Optical Pumping and the Rubidium Resonator result is the combination of superior long- and short-term The rubidium resonator is shown schematically in frequency stability. The bandwidth of the control loop is Fig. 6. In operation, the RF oscillator produces in the fixed at a value that yields the full short-term stability of spectral lamp a plasma in which the rubidium atoms are the crystal oscillator without degradation by atomic * T h ec u r v es h o w n i n F i g '3 i s i n g e n e r aalg r e e m ewnitt ht h e d e t a i l eadn a l y s im s ade resonator noise. Other bandwidths would provide little U yi . i . C u t t . ,a n dC . L ] S e a r l eorf f r e q u e n cf lyu c t u ? t i o n i nsf r e q u e n cs yt a n d a r d u se t i gm e s s d i c a t et hs a ti o r a v e r a g i n l a r S i e n c o m p a r i swo i n t h the improvement in either short- or long-term stability. i 6 * i J r , i t i , a n a l y si n .ioriU ll,op nrnO*riOn the fracti'nalfrequency deviation variesas 1/V-n assuming t h a tt h e a t o m i cr e s o n a t n oor t s ei s w h i t en o i s eF. o ra v e r a g i nt igm e sl o n g etrh a nL 0 0 r . r o n o t ,i t r p p r r o t h a tf l i c k e rn o i s eb e g i ntso d e t e r m i nt hee . f r e q u e ns ct ay(bi bi l ii dt yT he Stability vs Averaging Time . r r r u i . r r n i i ' r t o , n ni n F i g .3 w e r em a d ia s i n d i c a t ebdy C u t l ear n dS e a r l e )a n d , n e i r i n i r y r r oo n a n N : - 2 b a s i sa s o u f l i n e d b y A i l a n zT . h e m e t h o c d a l c u l a t et h se i nsi r e q u e n coyf t w oo s c i l l a t o r s ' The measured stability typical of the new standards is r m sv a l u e o s f t h e s u c c e s s idv ief f e r e n c e 'Some - . L . S . C u t l ea 1 r n dC . L ' S e a r l e , A s p e c tosf t h e T h e o ray n dM e a s u r e m o e fn t shown in the heavy curve of Fig. 3. This stability was i r r q r e n c yF l u c t u a t i oinnsF r e q u e nSc tya n d a r d s , ' P r o c e eodfi tnhgesI E E EV' o l '5 4 ' N0.2, pp. 136-154, FebruarY, 1966. measured by comparing two of the new standards against 2 . D a v i dW . A l l a n , ' s t a t i s t i cosf A t o m i ct r e q u e n cSyt a n d a r d s , ' P l o c e e doifntghse one another in a system that had a noise level indicated IEEE,Vol.54,No.2, pp' 221-230, February, 1966' 10 Fig. 6. Schematic reptesentationof rubidium_vapotcell anangement. energized to an excited state. As the atoms then relax, very high, the cell cannot truly be called a primary stand_ they emit light at two closely-spacedwavelengths. The ard of frequency. The cell and the lamp are temperature_ filter cell is located in the light beam from the lamp and controlled to minimize frequency changes. absorbs one of these spectral radiations. The remaining radiation falls on the absorption cell where it is absorbed Phase Plot by the Rbs' atoms of the lower (F = l) ground state A curve of considerable interest for precision fre_ (Fig. 7) which are then pumped into the optical state. quency work is shown in Fig. 8. This curve is a magnified These atoms shortly decay because of collision with continuousplot of the phasedifferencebetweenan output other gas moleculesand fall into either of the two ground frequency from a representativesampleof the new stand_ states.Since atoms have been taken only from the lower ard and a very stable reference signal of the same fre_ ground state, and only part of the decaying atoms return quency obtained from a hydrogen maser. Such masers to this state, this state becomes depopulated. The cell are consideredto be the most stablefrequency generators then becomestransparentto the radiation from thelamp/ in existence.Phase change appears as the fine structure filter combination. This radiation is sensedby the solar in the recorded line. The general slope of the plot is not cell that follows the absorption cell. By now applying a of interest since it merely indicates that the two signals magnetic field to the cell at a frequency corresponding were not precisely identical in frequency. to the transition from the upper to the lower ground The resolution of the record is high. It can be deter_ state (6,834.685 MHz), atoms in the upper ground state mined from the reference line drawn on it with a slope are causedto decay to the lower ground state. The atoms that fall into the lower ground state then absorb some of the incident optical radiation, causing the cell to become more opaque at this frequency and producing the dip in the curve of Fig. 5. A small dc magnetic field parallel to the RF field is used to separate theZeeman statesof the hyperfine levels and causes the transition to occur be_ tween the AMp: 0 states. To immerse it in the RF magnetic field, the cell is located within a suitable microwave cavity. The cell is partially filled with a buffer gas in addition to the rubi_ dium gas to lengthen the lifetime of the Rb atoms in the cell and thus increase interaction time. The presenceof the buffer gas somewhat ofisets the natural frequency of Fig. 7. Simplitied diagram ol the energy levels of the cell, so, although the performance of the svstem is Rb"' which are of significance in operation of celt. 11 by comparingphaseot output Fig.8. Recordol typicatstabilityof new standardmade it n'Varcgenmaser (comparisonmade at 100 MHz)' High signatwith that ot ourpii stabilityofnewstandardisshownbycompatingwithstopeot2.2xl0'2reterenceline. Time Scale Changes corresponding to a frequency offset of 2'2 x 10 "' To 'instantaneous' slope of the fine evaluate the plot, the A special objective in the new standard was to provide structure can be compared with the reference line' The for simple means of changing time scales' Two time long averaging time data for the rubidium-standard sta- scales are presently in wide usage' One of these' the record by 'atomic time' scale, was defined by the 13th General biliiy curve in Fig. 3 were derived from this averaging the phase change for various intervals' Conference on Weights and Measures, stating it is to be In the measurement arrangement (Fig' 9) used to make determined by the transition between two hyperfine levels the phase difference plot, the 5-MHz output of the stand- of the ground state of cesium.* of ard was multiplied some 20 times, as was the output The second time scale is the UTC or Universal Time a hydrogen maser of the same frequency' This provides Scale. This is related to the rotation of the earth and is two frequencies near 100 MHz. The phase difference set yearly in international agreement'For the years 1966 between these signals was measured by applying the sig- through 1968 this scale has been 300 parts in 1010 nals to an HP Model 84054 Vector Voltmeter whose longer than the atomic scale. output then fed an analog recorder' The arrangement shown in Fig' 10 has been used in the standard to facilitate changesof time scale' Fig' 10 shows a detail of the synthesizer portion of the diagram of Fig. 4. The output frequency of the synthesizer cir- cuitry is controlled by four thumbwheel switches' These Fig' switches control the four preset decade dividers in 10 and thereby determine the appropriate subharmonic of 5 MHz for comparison with the 5'316 ' ' MHz oscillator frequency in the phase detector' The control voltage out of the phase detector thus locks the VCXO of to a harmonic M of fr UH, which has the stability the main crystal osciliator' The circuitry is such that adjustments of frequency in steps of approximately I \ 10-g can be made by the thumbswitches' Finer adjust- ments are made by changing the magnetic field inside the optical package. This is implemented by a potentiometer Joutnal' Hewlett-Packard Fig. 9. Equipment atrangement usecl *'Atomic SecondAdoptedby lnternational Conference" 1968. Vol.19.No.6, February, in making record ot Fig' 8. 12 which controls the field bias so as to provide a linear frequency change with dial reading. Fig. 11 shows a typical plot of frequency change as a function of dial reading. The maximum deviation from a straight line is only 2.5 X 10-" over a range of Z y 1g-o.The overall range is adjusted at the factory to provide a change of 2 X I0 o full scale. Warm-Up Characteristics A particular advantageof a rubidium standard is that its warmup characteristicis much faster than that of pre_ cision crystal standards.Fig. 12, for example, shows the measuredwarnup of two of the new rubidium standards after they were baked in a 70oC oven for 24 hours. As a starting-point for a warmup test, this constitutesa ,worst_ case' condition. After t hour of warmup the standards were within 1 X 10 10of the original frequency and after 15 hours they were within I ;1 1g-". After 30 hours one standard was within 9 X l0 r: of its pre_storagefre_ quency; the second was within 7 X 10 1, of its pre_ storagefrequency. Time Pulse Output Circuitry has been designedso, if desired,the standard will deliver a time pulse at the rate of I pps for situations where time information is to be provided by the standard. Fig. 11. A specialprovision in the new standardis that the output frequency is adjustable by means ot a cati_ A master pulse is derived from the I MHz signal avail- brated dial. Above curve shows typicat linearity ot diat's able in the standard and is divided to i pps by a set of eftect on output ftequency. six integrated-circuit decade dividers. This pulse is used technique synchronizes the output pulse to within 10 to preset a second set of six decade dividers. -f 1 microseconds with respect to the external reference The output pulse to the front panel is controlled by pulse. the second set of six decadesand thumbwheel switches. The optional divider circuitry also operatesa 24-hour By this means the output pulse can be delayed up to 1 clock movement on the instrument front panel. second for phasing purposes. In addition, the output pulse can be automatically synchronizedwith an external Battery Operation pulse by applying the external pulse to a rear panel jack One version of the instrument is operated by a battery and pressingthe 'sync' button on the digital divider. This in a standby arrangement. The battery is automatically Fig. 10. Diagram ol circuit artangementof synthesizerportion ol new standatd's citcuit. 13 DarwinH. Throne DurinQhis studiesat the University of Minnesota,Darwin Thorne did researchin cosmic radiation' sturying light nuclei in PrimarY radiationfor his Master'sthesisand working on other cosmic-ray programsduring summer Periods. ln 1965,after receivingBSEE and MS degressat MinnesotaU., he joined the HP FrequencYand Time DivisiondeveloPmentlaboratorY where he has since Performed developmentwork on the new Rubidium-VaPorFrequencY of two rubidium stand- Fig. 12. Warmupcharacteristic Standard.Darwin is a memberof ardsmeasuredunder wotst-ca$econditions.Warmupis Tau Beta Pi and Eta KaPPaNu. fast enoughthat standardsmay be used in only about one hour attet turn-on. switched into operation in case of ac-power failure and Credit is also due to B. E. Swartz who did much of the provides at least 15 minutes of standby power. A battery electronics design. warning light indicates failure of ac power. The battery Richard A. Baugh did much of the development of the can be charged by means of a switch on the panel. optical package and was the original group leader on the project. Acknowledgments Bruce Fowler acquired most of the long-term aging Much credit is due to Ilhan Gozaydin who was the data on the gas cells and is also assistingwith the produc- mechanical engineer and product designer on the project' tion of the HP Model 5Q65A. Rex Brush assistedin the Ilhan also analyzed much of the short-term stability data' analysis of the short-term stability data. 6 HARMONICDISTORTION:(5 MHz, 1 MHz, 100 kHz) >40 APPROXIMATEINPUT: 33 WAITSdC. SPECIFICATIONS dB down trom mted oulput. WITH OPTION01: Add 8 watis. HP Model 50654 NON-HARMONICDISTORTION:(5 MHz' 1 MHz' 100 kHz) WITH OPTION02: No addilionalPower. >80 dB down from rated output. OIMENSIONS:57a in. high; 167a in. wide; 16% in. doep' Rubidium-Vapor Frequency Slandard SIGNAL-TO-NOISE RATIO: For 1 and 5 MHz, >87 dB at WEIGHT:Net,37 lb; Option 01 add 2 lb; Option02 add rated output {in a 30 kHz noise bandwidth, S MHz output 3 . 5l b . tilter bandwidthis approx.1OOHz); for'100 kHz' >60 dB FREOUEI{CYSTABILITY: in 30 kHz noise bandwidth. PRICE: $7,500.00. LONG TERI,:- <2 x 10-rr per month (maximumlimit oi dritt rato). OPtion 01 Time Standard averaging ll standaid Ouartz Oscillalor OnlY CLOCKPULSE: Tlm. I Devielion (Rubidium Vapol Resonatot Turnec! Olf) RATE:1 pulse Per second. 1ms <6 x 10 rorms A M P L I T U D E+: 1 0 V P e a k t l o % . AGING RAIE: <5 x 10-roper 24 hours. 1oms <l x 1o-lo WIDTH:20 ps min. <1.5 X 10 ri FREOUEilcY ADJUSTTE}ITSI RISETIME: <50 ns. FINE ADJUSTMENT:5 x 103 range, with dial reading 1s <5 x10r? FALL TIME: <1 ps. 1 0s <1.5 x 10 r' D a r t si n ' l 0 r o . JITTER: <20 ns. 100s <5 x10 1l COARSEADJUSTMENT:1 pail in 106,screwdriveradjust- All specs are with 50 g load 1000s <5 x 10 L3 mont at front Panol. SYilCHRONIZATIOI{: 10 rs (tl ps) delaved lrom relerence STABILITY: caltBnaTloN ACCURACY:Set at lactory to (lt x '10-t' input pulse (rear BNC). Referencepulse must be >+5 V' AS A FUNCTIONOF AMBIENT TEMPERATURE:<2.5 X ol specified time scale. 10_etotal lrom 0" to +50'C. with a rise time <50 ns. llilE SCALE: Set at factory lo UTC untess specilied ditfeF AS A FUNCTIONOF LOAD: <t2 x 10-rr for open circuat cLocK tlovEMENT:24 hrs., Patek Philippe ently. to short, and 50 O R, L, C load change. PRIcE: option 01, add $1,500.00. AS A FUNCTIONOF SUPPLY VOLTAGE: <15 X 1O-II fOT TUNABILITY: COABSEFREQUENCY SYNTHESIZER ADJUSTMENT: 22 to 30 V dc, or for 115/230V ac, l10o/o. Option 02 Standby Power SUPPIY RANGE: loOOx 10-rorelerencedto aiomic time scale' CAPACIfY: 15-minuteminimum at 25'C after lull charge' RESOLUTTON: <2 x 10-e,thumbwhe€ladiustable. General SPeciticalions CHARGE CONTROL: Frcnt panel FasLFloaLReset charge FINE FREOUENCYMAGNETICFIELD ADJUSTMENT: RANGE: 2 x 10-t. ENVIRONI'ETTAL TEMPERATURE, OPERATING:O.-50.C.StAbIIityiS INDICATOR:A tront panel light tlashos when ac pow€r is RESOLUTION:2 x l0-r2. 'olf' time <a5 X 10-rr over this range. interrupted and batlery is being used. WARM-UP:Operationalin one hour atter 24 hours TEMPERATURE, NON-OPEFATING: -40' to +75'C. PRIcE: $300.00. OUTPUTS: TESTSPASSEOBY UNITS: FREOUENCIES:5 lVHz, 1 MHz, 100 kHz and isolated 100 HUMIDITY:0 to 9570 relativ€humidity. Option 03 kHz clock drive lor external clocks. MAGNETIC FIELD: <1 x 10-r' change for 1 gauss (Combines Oqtions 01 and 02) VOLTAGELEVELS:1 V rns into 50 ohms at 5 MHz, 1 MHz' change in uniform magneiic lield' 100 kHzi 0.5 v rms inlo 1000 ohms at 100 kHz, clock VIBRATION:MIL-Std-l67. PRIGE: Option 03, add $1,800.00. ELECTROMAGNETIC COMPATIBILITY(EMC): ilA]{UFACTURINGDIVISION: HP FREQUENCYAND 'l CONNECTORS:BNC Front and Rest tor 5 MHz, MHz' MtL-1-6181D. TIME DIVISION '1OOkHz; BNC Rear for 100 kHz clock drive. 1501PageMill Road POWER:115 V/220 V rms a10%, 50 to 400 Hz or 22 to Palo Alto, Calilornia94304 'All unitsagedto neet this specification beforeshipnelt. 30 V dc. 14 ComparingFrequencyStandards Four types of precision frequency standardsare in wide No single type is best for all applications. The following use today. They are: tables and charts summarize the advantages,Iimitations, r Quartz crystal oscillators and characteristics of presently available standards and r Rubidium-gas-cell-controlledoscillators indicate some of their more frequent uses. r Cesium-beam-tube-controlledoscillators r Hydrogen masers. Advantages,Limitations, and uses of precisionFrequencySources Quailz Crystal Rubidium-cas-Cetl" Cesium.Beam.Tube- Oscillator Controlled Oscillatol Contaolled Oscillator Hydrogen Maser Description Active oscillatof. Output derived from quartz Outpul derjved from quartz Active atomic oscillator. Frequency determined by osci I lator. oscillator. Frequency determined by quartz crystal. Frequency determined by Frequency determined by atomic resonance of hydrogen passive atomic resonator. passive atomic resonator_ Advantages LOW-COSt,COmpact Compact, light-weight, lower Compact, portable. Most stable oscillator known, lighlweight, good cost than cesium or Excellent long-term stability. long or short term. short-term stability hydrogen, good short-term Primary standard - has high Prjmary standard - has high (< 100 s). stability with lower drift intrinsic reproducibitity. intrinsic reproducibility. rate than quartz oscillator. Relativelyfree from environmental and systematic variations. Limilations Systematic drift (aging). Secondary standard - needs Short-term stabitity in range Size, weight, cost Secondary standard - periodic calibration against 0.1 to 100 seconds, when needs periodic calibration pflmary standard. time constant ot loop is against primary standard. made short lo increase environmental immunity. Higher cost than quartz or rubidium. Use6 Frequency and time standards. Communications systems. Present U. S. frequency and Radio and radar astronomy Spectral analysis of oscillators Narrow-band and security time standards. (especially very-long- and multipliers. systems. 'Flying clocks.' baseline interlerometry). Microwave spectroscopy. Aircraft collision-avoidance Timekeeping with microsecond Super-accurate timekeeping Doppler measurements. systems. accuracy. Deep-space tracking. Communications and Doppler radar. House standards. Tests of Einstein's theory ol navigation systems. Radar and radio astronomy, Radio and radar astronomy, relativity. Systems which multiply the including long-baseline including long-baseline Exlremely stable frequency output frequency many times. interferometry. interferometry. source, Coherent signal sources. Navigation systems. Precision timekeeping. Doppler space-probe tracking. House trequency standards. Propagationstudies. Calibration laboratories. Typical stability curves lor HP 5061A cesium-Beam Frequency standard (two loop time constants..zo: 7 ,""- ond or 60 seconds), Hp 5065A Rubidium-vapor Frequency standard (dritt removed), and Hp 105A Quanz oscillator (drilt removed), and latest stability measurementstor Hp Hydrogen Maser (under devetopment). 15 Characteristicsof PrecisionFrequencySources Ouailz Crystal R u bi d i u m - Ga s - C el l - Cesium-Beam-Tube' Controlled Oscillatol Hydrogen Maser Oscillalor C o n t r o l l e dO s c i l l a t o l HP 50654 HP 5061A HP (see relerence 7) Model HP 1O5A 9 1 9 2 . 6 3 17 7 0 M H z 1420.405751 786 4 MHz 5 i.4Hz 6 8 3 4 . 6 8 26 0 8 M H z Resonator FrequencY None detected within res- None detected within res- systemalic Drifl <5 X 10 t0per 24 hl <2 X 1o-il permonth o l u t i o n o l c u r r e n tm e a s u r e - olutionol current measure- ments. menIs. Estimated at <3 x 10 12 E s t i m a t e da i ( 1 x 1 0 - t z for life. for life. Short-term StabilitY (rms frac- tional lrequencY fluctuations lor typical unils in conslanl Loop time constantto = envi ronments)* 60s 1s A v e r a g i n gT i m e : 5x10r0 5x10ro 1ms 5 x 10 r0 5x10r0 .l x 10 t0 1x10r0 1x 10-10 10ms 1X10r0 ' 1 . 5X 1 0 - r l 4x1o-rr 1X10-rl 5 x 1 0 1 2E x t r a p o l a t e d 100ms 1X10Ll 4x10-rt 5x10-12 5 x 10-rr Measured 1s 5X10 12 5 X 10-12 7X10-t7 5x10-12 7 x 1 0 - r sM e a s u r e d l min 5x1012 6 x 10-rr 1X10-12 1x1012 7 x 1 0 - r sE s t i m a t e d thr 5 x 10 12 5X1013 2X10-13 2x10rl 7 x 1 0 - r sE s t i m a t e d 1 day 5x1013 1 . 4f t t 13.3 ft3 0.38fF 0.83 ftl Volume 60 lb 600 lb 16lb 37 lb Weighl Power ' 1 5 0W 48W 43W C o n s u m p t i o n( r r 5v a c l $ 14.800 N o t p r e s e n t l yc o m m e r c i a l l Y s 15 0 0 $7500 Cost available. * S y s t e m adt irci f tr e m o v ef rdo mq u a r tazn dr u b i d i ufm igures' Bibliography 'A Historical Review of Atomic Frequency 'A Survey of Atomic Frequency 5. R. E. Beehler, 1. A. O. McCoubrey, Standardsl Proceedings of the IEEE, Vol. 55, No' 6, June Standardsl Proceedings of the IEEE, Vol' 54, No' 2, Febru- 1967. ary 1966.* 'Some Aspects of the 'The Relative Merits of Atomic Fre- 2. L. S. Cutler and C. L. Searle, 6. A. O. McCoubrey, Theory and Measurement of Frequency Fluctuations in quency StandardslProceedingsof the IEEE, Vol' 55, No' 6' Frequency Standardsl Proceedings of the IEEE, Vol' 54' June 1967. 'Progress in the Develop- No. 2, FebruarY 1966.* 7. R. Vessot, M. Levine, et al., 'A Survey of Some Performance Charac- 22nd Annual 3. L. N. Bodily, ment of Hydrogen Masersl presented at the Symposium on Frequency Control' Atlantic City, N' J'' terstics of a Large Number of Cesium-Beam Frequency 1968' Standardsl Hewlett-Packard Journal, Vol' 18, No' 2, Oc- Apnl 22-24,1968. See also Frequency, July tober 1966. 'A Rubidium-Vapor Frequency Standard 8. D. H. Throne, 'An Intercomparison of 4. R. Vessot, H. Peters, et al.' for Systems Requiring Superior Frequency Stabilityl this Hydrogen and Cesium Frequency Standardsl IEEE Trans- lssue. actions on Instrumentation and Measurement, Vol' IM-15' * M a n yo t h e r . p a p € rosn l r P q u e n csyt a b r l i t yc a n b e f o u n di n t h i s i s s u e0 f t h e P r o c e e d r n g s0 t t n e l t r E . No. 4, December 1966. @uw le6svotume JOURNAL HEWLETT-PACIGRD 11 ls'Number 1501 PAOE MILL FOAO' PALO ALTO CAL]FORNIA 94304 T H E H E W L E T T - P A C K A F OC O M P A N Y P U B L ! S B E O A T TECHNICAL INFORMATION FROM THE LABOSATOBIES OF SNYOER AilDJIECIOI:F'A FAICXSON E d i l O I I A ] S I A i l : FJ . B U R X H A R D B ' P D O L A N L D ' S H I B G A L ] S ' F ' H HP Archive This vintage Hewlett-Packard document was preserved and distributed by www.hparchive.com Please visit us on the web! On-line curator: John Miles, KE5FX