Calibration of Time Base Oscillators
Contents
1. Figure 2 Effect of Aging on Frequency Stability month then draw a line through the points The slope of the line is the aging rate of the crystal By doing this you have created an historical aging rate of your unit This is an important con cept and each instrument should have its own history record Table 1 summa rizes the oscillator characteristics de scribed utilizing typical specifications of well designed oscillators Time Base Warm Up Under typical operating conditions that is when the instrument s power cord is left connected to the power source there is no warm up because the time base is kept warm or in a standby mode However if the unit has been disconnected from the power source for 24 hours or more the instru ment should technically be warmed up for up to 30 days for it to meet guaran teed specifications This may not be practical Our experience has shown that approximately 85 percent of new units and 95 percent of older units will be within specifications after three days of warm up Of course this is due to the aging process slowing down over time A typical problem you may en counter with some of the older units is that the time base has aged to a point where the mechanical adjustment can no longer compensate the frequency drift because the mechanical adjusting device has reached the physical limit of its travel On other older units drift may be almost non existent The Qu2. Mod A6 power supply to be compatible with A15 assemblies 6003 MR 8560E 10 New A15 RF assy climins high displayed noise level in wide B W 6011 MA 8561E 03B Firmware upgrade kit improves performance 5796 MR 8561E 07 New A15 RF assy climins high displayed noise level in wide B W 6012 MA 8563E 04B Firmware upgrade kit improves performance 5799 MR 8563E 09 New A15 RF assy climins high displaycd noise tevet in wide B W 6013 MR 8564E 01 Firmware upgradc to address flatness instability 6004 MA 8564E 0O1A Firmware upgrade kit improves performance 6004 MR 8564E 02 New 5V reg improves power supply loading when mass mem mod connected 6028 MR 8565E01 Firmware upgrade to address flatness instability 6005 MA 856SED1A Finnware upgrade kit improves performance 6005 MR 856SE 02 New 5V reg improves power supply loading when mass mem mod connected 6029 MR 8590D 04 New PAL device climinates instrument failures 6037 MR 8591C 01 New PAL device eliminates instrament failures 6038 MR 8S91E04 New PAL device climinates instrument failures 6039 MR 8592D 02 New PAL device eliminates instrument failures 6040 MR 8593E 05 New PAL device climinates instrument failures 6041 MR 8594E 05 New PAL device eliminates instrument failures 6042 MR 8595E 05 New PAL device climinates instrument failures 6043 MR 8596E 05 New PAL device eliminates instrument failures 6044 IO E16504 05 Alternative test procedure for TOH and POH ports 6056 I0 E1682A04 Alternative test procedure for TOH and POH
3. 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 at the above address Editor Jim Bechtold Hewlett Packard BENCH BRIEFS J trwienetec All aghts reserved Permission to reprint Bench Brels granted upon wullen request to the E ditor WWW HPA Bulk Rate U S Postage PAID San Jose CA Permit No 1201 Printed in U S A 2ND QUARTER 1994
4. ally see the relationship between suc cessive data points with no dead time between measurements and with no appreciable delay This allows the user DN Saee Case 3 Case 4 STABLE not ACCURATE not not STABLE ACCURATE BENCH BRIEFS 3 to zero in a measurement quickly without the time consuming and frus trating back and forth clockwise a litle counter clockwise a bit clockwise again etc The HP 5372A allows you to measure phase shift continuously over a period of up to 18 hours and provides built in Allan variance calculations Each unit has marker readouts that al low the operator to express frequency differences in the conventional parts in the 10 to the nth notation with a minimum probability of mathematical or procedural error One thing cannot be over stressed the limiting factor for almost all time bases is your ability to keep the temperature constant for the duration of the test This is why the ability to make mea surements and adjustments quickly is so important Once you have created a historical rate of drift aging for your particular in strument you can calibrate it to an off set to take advantage of the known drift For example if you know that the time base drifts from low to high at a certain rate you can calibrate your unit at the low end of its specification so that it will drift from low through center and be at the high end of its specifica tion
5. and permitted time excusion in milli seconds Example 2 A rubidium based time system is to be a 3 2 Eo Total Time Excursion ms T Days Allowable Between Recalibrations Figure 3a Recalibration Chart for Quariz Oscillators 4 BENCH BRIEFS WWW HPARCHIVE COM oS i s oo a 3 E amp 9 he 3 oO x W E E T j L e w N T Days Allowable Between Recalibrations Figure 3b Recalibration Chart for Rubidium Standards maintained within 10 us The drift rate is a positive 1 x 10 month Looking at the appropriate slant line on Figure 3b corresponding to the drift rate yields a recalibration time of 101 days for an excusion of 20 ps Temperature Effects A very small temperature change can drastically affect the frequency of a time base In some cases 1 2 C tem perature change can cause as much drift as 2 weeks of aging Two points are worth remembering with respect to temperature effects First the change of frequency with temperature is usually not a linear function furthermore all crystals even though the same kind may have very different frequency temperature curves Individual oscilla tor frequency temperature curv
6. be analyzed to determine if they have an impact on system operation If they do in fact affect the system operation then appropriate steps can be taken to re duce that impact This article will de scribe the frequency calibration inter val with suggestions on how to mea sure your time base and the effects of how changing environmental condi tions affect time base accuracy Types of Time Base Oscillator There are five basic types of time base oscillators a XO Room temperature crystal oscil lator sometimes referred to as RTXO TCXO Temperature compensated crystal oscillator Display Time Code Output Hewlett Packard 1994 m OvenXO Oven controlled crystal oscillator a Rubidium E Cesium Each type of time base has its own char acteristics The room temperature model would be used in a portable counter Usually the better the time base the longer it takes to verify it the poorer the time base the harder it is to adjust Some time base specifications would be impractical to completely verify so operator judgment is required to identify which parameters have to be checked when to adjust the time base and when to predict final perfor mance based upon rate of change of measured performance Time Base Aging The physical properties of the quartz crystal exhibit a gradual change with time resulting in a gradual cumulative frequency drift called aging See Fig ure 2 The aging rate is dep
7. be repaired and calibrated at the WWW HPARCHIVE COM Hewlett Packard Golden Gate Cus tomer Service Center in California Costs of repair and calibration are not included in this contract price Per incident Usage Per incident service can be purchased at any time you require a loaner by con tacting the HP Golden Gate Customer Service Center administration group at 415 694 2620 Once this contract is set up you can then order a loaner for 350 00 per usage to take the place of your See Loaner Program page 7 2ND QUARTER 1994 New Test Software for the RF Network Analyzer John Vallelunga Hewlett Packard Introduction Hewlett Packard s Microwave Instru ments Division has released a new ver sion of test software for the HP 8711A RF Network Analyzer This software is in tended to replace the previous software HP P N 08711 10009 That version only automated four of the eight required tests did not allow equipment substitution and was difficult to work with The new ver sion has eliminated these shortcomings and has added several improvements some of which are listed below Enhancements a The program includes graphical setup connections customized to your equip ment being used You will no longer need to refer to the manual for proper setup equipment and connections Multiple equipment and mass storage configurations can be saved a Data storage is automatic results can be archived and
8. ports 6057 MR E2411C 01 New capacitor corects clock PLL lock problem at upper freqencies 6008 10 37721A 12 Instructions to retrofit virtual remote facility Opt V01 6017 MR 37724A 01 Power supply modification improves reliability 6018 MR 54502 A 06 Mod eliminates input resistance performance test failure 6020 MR 545034 05 Mod eliminates input resistance performance test failure 6021 MR 54504A 02 Mod eliminates input resistance performance test failure 6022 MR 54510A 01 Mod climinates input resistance performance test failure 6023 MR 54600A 10 Mod replaces defective BNC connector 6031 MR 54600B 01 Mod replaces defective BNC connector 6032 MR 54601 A 10 Mod replaces defective BNC connector 6033 MR 54601B 01 Med replaces defective BNC connector 6034 MR 54602A 02 Mod replaces defective BNC connector 6035 MR 54602B 01 Mod replaces defective BNC connector 6036 IO 54721A 01 Instructions on changing channel or trigger input connector to Type N 6024 MR 89430A 02 New plastic tubing prevents power supply cable short 6026 IO 89440A 06 894 10A 89430A repair strategy 6049 10 89441A 01 89410A 8943 LA repair strategy 6050 Service Note Types I Information Only MA Modification Available MR Modification Recommended SA Safety PS Priority Safety SM Interoffice Service Memo OSM 8 HEWLETT PACKARD COMPANY 100 Mayfield Avenue Mountain View California 94043 BENCH BRIEFS 2nd Quarter 1994 Volume 34 No 2 Service information from Hewlett
9. what is rea sonable to expect from it Do not try to use your counter to calibrate a cesium beam frequency standard Acknowledgements I would like to thank Chris Franks at the Hewlett Packard Santa Clara Divi sion for his guidance with this article For more information on the subject I recommend the free Hewlett Packard Application Note 52 2 Timekeeping and Frequency Calibration Ask your local HP office for HP P N 5952 7874 Com ments on this article are invited and we will answer each and every one If you have recommendations for future ar ticles about this subject please send them to the editor BENCH BRIEFS 5 Cooperative Support Service for Self Maintainers Hardware Support for Test and Measurement Systems Ken Callender Hewlett Packard Introduction Hewlett Packard Test amp Measurement Cooperative Support service for self maintainers provides all the essential support elements you need to comple ment your internal hardware mainte nance capabilities With this service you will have everything you need to fully address your unique requirements You use your trained maintenance organi zation for labor and rely on HP for training replacement parts diagnostic support tools repair documentation and remote backup support This cooperative support service is available for HP s large test amp measure ment systems Examples are the HP 83000 Series Digital IC Test Systems HP 3060 and 3070 Board T
10. 5952 9777E Support Terms and Conditions HP 8648A B C Loaner Program Eliminates Lengthy Downtimes Henry Jeung Hewlett Packard For U S Customers Only Customers in the U S can eliminate downtime for calibration and reduce downtime for repair from a week to a day Hewlett Packard now offers an affordable way to quickly obtain a loaner signal generator so that you can keep operating when your present sig nal generator needs calibration or re pair If you cannot afford downtime for your HP 8648A B C Signal Generator consider the following two options 6 BENCH BRIEFS Shared Loaner Contract You can purchase a three year loaner contract Any time your HP 8648A B C Signal Generator fails during the three years a loaner instrument will be ex press mailed to you within hours You keep the loaner until your instrument is repaired and returned You can also use the loaner once dur ing the three years while your signal generator is being calibrated Since cali brations are planned in advance you can arrange to have the loaner arrive at your location before disconnecting your generator for calibration Down time will be limited to the time it takes you to connect the loaner in place of the instrument requiring calibration This three year contract is available for 500 00 U S If at any time during the three years you want additional cali brations you can have a loaner deliv ered for 200 00 Your instrument must
11. HEWLETT PACKARD L bilite SERVICE INFORMATION FROM HEWLETT PACKARD 2nd Quarter 1994 Calibration of Time Base Oscillators Jim Bechtold Editor Counting and Clocks Counting intervals has been going on since man s beginning Early time mea surements involved counting the num ber of days in terms of sunrises sun sets or moons Later the day was di vided into smaller increments by using an hourglass candles sundial etc With the discovery of the pendulum clocks were born The accuracy of early clocks was around 1 part in 10 As more accurate clocks were pro duced new uses of time measurement were explored As new uses were dis covered the need for even more accu rate clocks became apparent Atomic Accuracy Current state of the art atomic fre quency standards has attained an ac curacy of 1 part in 10 in the labora tory The specified accuracy in commer cially available atomic clocks has reach 1 part in 10 This unprecedented commerical accuracy is equivalent toa gain or loss of 1 second in a minimum of 400 000 years Definition of a Second Frequency standards and clocks have no fundamental differences they are based upon dual aspects of the same phenomenon The basic unit of time the second is defined as the duration Pub No 5952 3466 of 9 192 631 770 periods of transition within the cesium atom Frequency is determined by counting the number of cycles over the p
12. alibrated out Frequency Calibration Interval In theory a time system based upon a quartz oscillator or a rubidium stan dard of known drift rate can be kept within prescribed limits of error with infrequent adjustments through a sys tematic approach With this systematic approach the os cillator and clock are preset to offsets that will keep the time system operat ing within a selected accuracy for a long time despite the oscillator s drift This drift aging rate must be known measured and must be nearly con stant so that a plot of the frequency over the adjustment interval periods between calibration can be approxi mated by a straight line TN ee 2 gt Case 1 Case 2 ACCURATE ACCURATE and not STABLE STABLE Figure 3 Accuracy What this means in simple terms is that the time base is adjustable and if its drift rate is known it can be set so that the drift remains within specifications for a longer period of time therefore extend ing the interval between calibrations Suggested Method of Calibration There are iwo excellent HP products to help an operator make these measure ments quickly and accurately and ob tain a permanent record or history when connected to a printer They are the HP 5372 Frequency and Time In terval Analyzer and the HP 53310A Modulation Domain Analyzer HP 5372A HP 53310A The HP 53310A often called a fre quency scope allows the user to visu
13. at a prescribed point in time Recalibration Charts for Quartz Oscillators and Rubidium Standards Figures 3a and 3b are useful for esti mating the length in days of the recalibration cycle for an oscillator with a known drift rate which will keep the time system based on that oscillator within prescribed error limits A recalibration cycle is the time in days that can be allowed to pass between calibration adjustments A shorter cycle more frequency adjustments is needed to keep a system accurate to 100 ps total time excursion equals 200 js rather than to 1 ms To use the charts select the slant line marked for the aging or drift rate parts per day for quartz oscillators and parts per month for rubidium stan dards of the oscillator Note the in tersection of this line with the horizon tal line corresponding to the permitted error excursion This intersection re ferred down to the horizontal axis gives the recalibration cycle Example 1 A time system is to be maintained to within 10 ms based on a quartz oscilla tor with a positive aging rate at 5 x 10 day Use Figure 3a to estimate the length of the recalibration cycle by lo cating the slant line marked 5 x 10 1 day and note its intersection with the horizontal line corresponding to a to tal time excursion of 20 ms 10 ms The answer read from the chart is 60 days Note that to use Figure 3a aging rate must be expressed in parts per day
14. e and the measurement needs of the user dictate how the time base is to be cali brated and supported On the other hand crystal oscillator clock accuracy is seldom of conse quence in practical time interval mea surements Most electronic counters have a quartz oscillator with an accu racy of 1 part in 10 1 part per million or better As a result the effect of oscil lator stability does not affect a time in terval measurement unless the display has 5 or 6 valid digits of information While it is possible to measure long in tervals with high resolution most prac tical measurements today are the rise time of fast signals propagation time through high speed logic or on narrow pulses Resolving a5 psec interval to 1 nanosecond entails only 4 digits of in formation i e 5000 nanoseconds so an oscillator as poor as 1 part in a mil lion introduces only 1 200th as much error as 1 count for this measurement For shorter intervals the oscillator er ror is proportionally less Time interval averaging increases the number of valid digits but here again usually not to the extent that crystal accuracy is important Short term sta bility may become important when doing time interval averaging on nar row pulses The short term stability specification is statistical in nature so is worse for short averaging times Consider for example a short term ac curacy specification of 1 x 10 fora 1 second averaging time This
15. endent on the inherent quality of the crystals used and goes on all the time Aging is often specified in terms of frequency changes per month since temperature and other effects would mask the small amount of aging for a shorter time pe riod Aging for air crystals is given in frequency changes per month as it is not practical to accurately and correctly measure Over any shorter averaging period For a good RTXO the aging rate is typically of the order of 3 parts per 10 per month For a high quality oven controlled oscillator the aging rate is typically 1 5 parts per 10 per month Aging rate specifies maximum fre quency change with time Any oscilla tor can be much better than specified but will never be worse than the indi cated rate unless it is malfunctioning You may have noticed that HP has two kinds of specifications some oscilla tors are specified as having a daily ag ing rate such as lt 3 x 10 day while others are specified as having a monthly aging rate such as lt 3 x 107 month HP oscillators with a daily ag ing rate specification use ovens that sufficiently buffer the oscillator from the environment To determine crystal aging rate one has to check the oscillator once a day when room temperature is ata constant value plot these points for approximately a 2 BENCH BRIEFS Parts Per 10 Change 5 10 Long Term Stability or Aging Short Term Stability 15 20 Days From Calibration
16. eriod of a second Therefore the definition of a clock can be expressed as a device that counts the number of seconds occurring from an arbitrary starting time From this definition it appears that a clock needs three basic parts First a source of events to be counted This source can be labeled a frequency stan dard frequency source or time inter val standard Second a means of accu mulating these events or oscillations Third a means of displaying the accu mulation of time Figure 1 shows a simple clock block diagram including a method of presetting the arbitrary starting time and obtaining an electri cal time reading from the clock time code generator Errors in Accuracy Accuracy in a timekeeping system is dependent on six major problem areas m Maintenance of accurate frequency m Accurate time transfer Determination of radio propagation path delays a Maximization of the frequency cali Preset Frequency Source Figure 1 Basic Clock Accumulator bration interval m Determination of the effects of noise in frequency generating equipment m Determination of the effects of changing environmental conditions This Article Prior to analyzing the effects and im pact of the above sources of error it is necessary to determine the level of ac curacy required and the tolerances es sential for the individual application Once the essential tolerances have been established the sources of error can
17. es must be made to determine a particular unit s actual performance Second the effects of temperature change can be reduced by providing a more constant ambient temperature controlled room temperature when better performance 2ND QUARTER 1994 is required Line Voltage Change Crystal oscillator frequency is also in fluenced by line voltage changes often because the instrument s power dissi pation increases which causes the tem perature inside of the instrument to rise Good circuit design proper buffering and good mechanical design can reduce these effects Operators needing better performance can use a line regulator to better control line voltage fluctuations Cun rey oummary Each time base ages differently You should create a history file for each of your instruments and plot its drift Determine how the unit is going to be used If your measurements require extreme accuracy the time base will have to be calibrated more often Either way when you calibrate the time base adjust it to the extreme end of its speci fication and let it drift through center to the other end of its specification WWW HPARCHIVE COM maintaining calibration for the longest period of time Keep your instruments plugged into the power source to maintain constant internal temperatures and use a line regulator to buffer line voltage changes Above all think about how you are using the instrument and
18. est Systems and the HP 9470 9472 Power Mixed Signal Test Systems to name a few More will be added to this list in the future Features HP T amp M Cooperative Support service for self maintainers provides the fol lowing features for HP Test and Mea surement systems designated by HP as eligible for this service a On site start up visit a Semi annual on site reviews m Electronic access to service notes through HP SupportLine License to use hardware diagnostics and updates Remote hardware troubleshooting assistance Parts replenishment 03W only ex cluded for 03X m One customer initiated on site HP visit to repair hardware failure on eli gible HP Test and Measurement Sys tems s Direct access to technical assistance Measurement Systems Knowledge Center Benefits a Increase the availability of your test and measurement systems Improve the productivity of your system engineers and managers a Maximize your organization s abil ity to maintain HP T amp M systems Significantly enhance the productiv ity of your internal service organi zation Accurately predict your annual maintenance costs The optional parts replenishment is available under the 03W option elec tron only For More Information Ask your HP office for the following documents Description HP Cooperative Support User s Guide 5962 9778E HP Cooperative Support Data Sheet 5962 8520E HP Cooperative
19. estion Do you have to calibrate an oven time base even though the manual for the instrument being calibrated does not have a performance test for it The an swer is YES The oven time base needs to have a drift test performed The de cision to remove offset depends upon the needs of the customer and the type of time base being calibrated Calibra tion should always be checked after repair after being shipped shock can cause an offset of 1 part in 10 and you don t know in which direction or pe riodically to ensure that measurements Table 1 Typical specifications of the five types of oscillators Time Base Typical Type Aging xO 3x 107 per month 1x107 Typical Shift for 5 C 5x 10 TCXO 1x 10 per month 5x10 per day 1x10 per month None Primary Std OvenXO 5x 107 Rubidium 5 x 10 Cesium 3 x 10 Allan Variance T 1 sec Allowable Offset Typical Warmup 60 Hz 30 minutes 1x 10 13 Hz 3 hours 1x10 150 mHz 3 days 5 x 10 400 Hz 4 hours 7 x 10 6p Hz 45 minutes 1x10 21 5 nsec hour made with these devices are within specifications ACCORDING TO THE USE OF THE INSTRUMENT This is an important concept Many users expect an oven time base to have a written specification to tell them exactly when the cumulative offset caused by aging has crossed some specification bound These oscillators are just not specified in this manner The use of the time bas
20. later retrieved at any time a Simple immediate one disk operation Although installation to a hard drive is preferred itis not required This is NOT an STE 9000 program a Elimination of the special option of the power sensor The HP 8481D H70 is no longer required a normal 8481D will suffice m Includes a quick HP IB scan to verify equipment setups DUT can be on a different bus from test equipment a Also includes several handy service utilities a Can be run ona PC using an HP BA SIC Language Processor Card How to Obtain the Software This program can be ordered as HP part number 08711 10011 through your local HP office The price is approxi mately 50 00 U S This new version can now be used in place of all the documented performance tests in the manual which will save a con siderable amount of test time Other Important HP 8711A News HP recently released firmware revision A 02 10 for the HP 8711A This firmware now allows the use of a standard HP 8481D power sensor instead of the previously re quired HP 8481D Option H70 This sensor is used in ALC adjustment 104 on those instruments with a built in attenuator Op tion 1E1 Together with the above test soft ware the need for this special option sen sor has been eliminated However care must now be used when performing ad justment 104 Following the procedure in the manual could result in anon function ing ALC For further informatio
21. n order service note 8711A 05 from Hewlett Packard Bench Briefs 100 Mayfield Ave Mt View CA 94043 The new firmware is available as HP P N 08711 60063 through your local HP office Service Manual Omissions Several commonly used part numbers for 75 ohm HP 8711As were inadvertently omitted from the service manual Please add the following 75 ohm parts Description HP P N Front panel assembly 08711 60071 Front dress labei 08711 80004 Test port assy 08711 60039 conn and brackets J2 to RF out cable 08711 20047 Also omitted was the entire chassis frame and interconnect board The HP P N is 08711 60009 for all instrument versions Oo Loaner Program continued from page 6 HP 8648A B C Signal Generator while itis being repaired or calibrated at the HP Golden Gate Customer Service Center For more information about either of these programs contact the HP Direct Marketing Services group at 1 ee 835 4747 option 1 1994 Bench Briefs Instrument Service Note Index HP FIRST SN SN Abstract Type No Document ID No MR 346C 02 Mod climinatcs 425MHz spurious oscillations 6027 10 3336A B C 17 Instructions for replacing new Al4 PC assembly 6048 MA 3589A 0 A HP 3589A firmware revision history 5254 10 3779C 45 Improved GvL measurement when high noise levels are present 6014 10 3779C 45A Improved GvL measurement when high noise levels arc present 6014 10 3779D 49 Improved GvL measurement whe
22. n high noise levels are present 6015 10 3779D 49 A Improved GvL measurement when high noise levels arc present 6015 IO 3785A 26 Notification of part replacement 6016 10 3787B 14 Preferred replacement for A3 CRT display module 6055 MR 4145B 02A SMU board mod prevents spike when measumg amps with a range change 6047 10 4276A 07 Instructions on replacing A4Q10 power transistor 6009 10 4277A 11 Instructions on replacing A4Q10 power transistor 6010 IO 4396A O1A Firmware update and Al CPU repair information 5642 10 4935A 7A New volume control assembly for older instruments 5397 10 5335A 17B Instructions on replacing front end Schmitt amplifiers 5597 MR 5342A 35C Front panel replacement kit 6019 MR 53474 09 New front membrane panel and new casting improve performance 6045 MR 5348A 09 New front membrane panc and new casting improve performance 6046 MR 5508A 02A EPROM fimmware revision 6006 2ND QUARTER 1994 WWW HPARCHIVE COM BENCH BRIEFS 7 SN SN Abstract HP FIRST Type No Document ID No MA 6010A 09 AC input filter assembly replacement 6051 MA 6011A 10 AC input filter assembly replacement 6052 MA 6030A 17 AC input filter assembly replacement 6053 MA 603lA 19 AC input filter assembly replacement 6054 MA 8112A 02A Replacing 24v supply fuses with jumpers improves reliability 6007 MA 8 16A 03B Replacing 24v supply fuscs with jumpers improves reliability 5875 MA 8S60E 04B Firmware upgrade kit improves performance 5797 JO 8560E 09
23. would be 1 x 10 for a 1 psec averaging time 5 nanoseconds for the example above and would be greater for an oscillator with poorer short term stability Aging rate is generally of consequence only in an application where the counter is used to make phase or time measurements to compare high preci sion frequency standards In this appli cation a counter is used to measure the time variations between once per sec ond time ticks from the two different 2ND QUARTER 1994 frequency standards Each time inter val measurement may be as long as 1 second 1 x 10 usec on frequencies that are stable 5 parts in 10 or better so a stable crystal oscillator is needed Accuracy and Stability Accuracy may be defined as the close ness of ameasurement to the true value as fixed by a universally accepted stan dard The measure of accuracy how ever is in terms of its complementary notion that is deviation from true value or limit of error so that high ac curacy has a low deviation and low accuracy a high deviation The plots shown in Figure 3 show successive measurements for four cases The read ings in case 2 are more spread out This could be due to noise or the operator s inability to consistently read an analog dial The readings in case 3 are stable but offset from actual value The im portant thing is that this offset is a sys tematic error that can be removed by calibration The random errors of case 4 cannot be c
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