PROFICIENCY TESTING ACTIVITIES OF FREQUENCY CALIBRATION LABORATORIES IN TAIWAN, 2009

Transcription

1 PROFICIENCY TESTING ACTIVITIES OF FREQUENCY CALIBRATION LABORATORIES IN TAIWAN, 009 Huag-Tie Li, Po-Cheg Chag, Jia-Lu Wag, ad Chia-Shu Liao Natioal Stadard Time & Frequecy Lab., TL, Chughwa Telecom Co., Ltd., Taiwa 1, Lae 551, Mi-Tsu Road, Sec. 5, Yag-Mei, Taoyua, Taiwa 36 Tel: , Fax: , Abstract I order to meet the requiremets of ISO 1705 ad the demad of TAF (Taiwa Accreditatio Foudatio) for calibratio iter-laboratory comparisos, the Natioal Time ad Frequecy Stadard Laboratory (TL) [1], playig the role of coordiatig lab, has periodically orgaized the proficiecy testig activities to provide opportuities for capability comparisos of domestic frequecy calibratio laboratories. The latest two activities were performed i 006 [] ad this year (009). The Device uder Test (DUT), together with TL s calibratio system, was trasferred to each participatig laboratory accordig to a predetermied schedule. The DUT was measured by the participatig lab s ad TL s calibratio systems simultaeously. The measured results from both systems were the aalyzed ad compared. There were 1 ad 15 participatig labs joied the activities performed i 006 ad 009, respectively. All of them were TAF-accredited laboratories. As usual, the E value was used to evaluate a lab s capability of calibratig equipmet withi its accredited measuremet ucertaity. I these two activities, the absolute E values of all labs were smaller tha 1, which meas their calibratio ability was completely qualified. I this paper, the related details of these two activity ad the result comparisos betwee activities i 006 ad 009 are illustrated. INTRODUCTION The goal of performig the proficiecy testig activity is to provide a chace for the domestic accredited laboratories to compare their calibratio capability with oe aother. Whe performig o-site evaluatio, a assessmet team is orgaized to examie the techical competece of the labs ad their compliace with the requiremets of ISO/IEC 1705 Geeral requiremets for the competece of testig ad calibratio laboratories. The proficiecy testig results are the importat iformatio for the techical assessmet team to evaluate the ability of the laboratory. I proficiecy testig activities [3,4], the ability of each idividual laboratory is supposed to be determied ad able to achieve its accredited level of measuremet ucertaity. I the case that the performace of the Device uder Test (DUT) has good repeatability ad stability, its referece value 313

2 will be assiged by the coordiatig lab ad the compared with measuremet results of the participatig labs, as show i Fig 1. Whe the referece value is located withi a lab s ucertaity rage (e.g., lab 1,, ad 3 i Fig.1), we say that this lab works withi its capability ad its accredited level of measuremet ucertaity is suitable for the DUT beig calibrated. Ref Value 1 3 =Ucertaity Rage N N =Results of LAB N Fig. 1. Measuremet results ad ucertaity rages of participatig labs. However, the characteristic of the DUT i the time/frequecy area (such as a frequecy oscillator) is ostatioary, so the referece value may chage gradually. Therefore, together with the DUT, a calibratio system of the coordiatig lab is usually trasferred to each participat laboratory to get the relative referece value simultaeously with the measuremet. I other words, the DUT is measured by the participat s ad the coordiatig lab s calibratio systems at the same time. EVALUATION OF RESULTS I the proficiecy test activities amog calibratio laboratories, the E value is adopted to idicate how well labs are withi their particular measuremet ucertaity, takig accout of the measuremet ucertaity of the referece value. E stads for Error ormalized ad is defied as: Xi X ref E (1) U ( X ) U ( X ) 95 i 95 ref where X i is the participat laboratory s result X ref is the coordiatig laboratory s result U 95 (X i ) is the participatig laboratory s reported ucertaity (95%) U 95 (X ref ) is the coordiatig laboratory s reported ucertaity (95%) E 1 idicates that the result ad the referece value are i agreemet E 1 idicates that the result is differet from the referece value. Note that the calculatio of E value does ot ited to idicate which lab s result is closest to the referece value, as high-level calibratio labs may have their E values similar to that of some other labs with both larger measuremet errors ad ucertaities. Besides, oe should be aware that the referece value itself has a measuremet ucertaity. The coordiatig lab should have the capability to give a better measuremet ucertaity tha the participatig lab s; otherwise, it will be difficult to evaluate each lab s performace. Cosequetly, the E 1 limit really oly represets the cutoff, below which it is likely that the result is acceptable ad above which it is 314

3 ulikely that the result is acceptable. So whe cosiderig ay result with E greater tha 1, all factors should be evaluated to see if there is a systematic bias that is cosistetly positive or cosistetly egative which causes the problem. PROFICIENCY TESTING ACTIVITY TL, playig the role of coordiatig lab, has periodically orgaized the proficiecy testig activities every 3 years sice 003. The latest two activities were performed i 006 ad 009, with HP866A [5] ad HP3350A frequecy sythesizers as the DUT, respectively. Both HP866A ad HP3350A ca offer 5/10 MHz output sigals, so ay laboratory with a calibratio capability for either 5 MHz or 10 MHz ca joi these activities. The agig rate of HP866A is smaller tha /day, which is lower tha that of HP3350A, which is about /day. Sice the properties of a oscillator s sigal are o-statioary, TL s calibratio system, cosistig of a 5071A cesium clock, a SR60 time-iterval couter, ad a recordig computer alog with the DUT, were set to each participatig lab sequetially. The DUT s referece value ca the be obtaied durig the measuremet process i each lab. The above metioed 5071A cesium clock is TL s portable frequecy referece, which is traceable to the atioal frequecy stadard. To make sure that the HP5071A ca satisfy the requiremet of traceability, the performace of this Cs clock was measured i TL before ad after every measuremet trip to the participatig labs. The block diagram of the related system for the proficiecy testig activities is show i Fig.. 5/10 MHz Participat Laboratory s Calibratio System DUT (HP866a / HP3350a) Power Splitter 5/10 MHz Recordig Computer TIC (SR60) Cs Clock (5071a) TL s Calibratio System Fig.. The system arragemet i proficiecy testig activities. The umber of participatig labs that joied the proficiecy testig activities was 1 i 006 ad icreased to 15 i 009, but the same procedure was followed. Before the measuremet trips to the participatig labs i each activity, a opeig meetig was held i advace to discuss all details about the related activities. A cosesus referrig to the schedule for trasferrig the DUT ad TL s calibratio system, the measuremet method, the ucertaity evaluatio, the result expressio, etc. was reached. Sice 006, TL has recommeded that the participatig labs iclude the best measuremet capability of the idividual lab, the DUT s agig rate ad temperature effects, etc. i their ucertaity budgets. 315

4 Note that, i geeral calibratio services, a ucertaity budget usually icluded the DUT s agig to assure the validity of the measuremet result durig the period that the DUT retured to its lab till the ext time the DUT was set for calibratio. However, it is t ecessary to cout the agig for a log period (for example, 1 year) i proficiecy testig activities, because the DUT oly serves as a itermediate for proficiecy testig comparisos ad the measuremet time i each participatig labs is ot loger tha 1 hour. Therefore, we aouced DUT s agig rate for 1 day to each participatig lab for their evaluatio of measuremet ucertaity. While fiishig the measuremet, each participatig lab was required to sed its test report, raw measuremet data, ad ucertaity budget to TL withi 3 days. After the measuremet trips to the participatig labs, collectig the measuremet data ad evaluatig the capability of each laboratory, a meetig was held for fial discussio ad providig the chace for faceto-face commuicatio with the participat labs. RESULTS ANALYSIS AND CONCLUSION The proficiecy testig results of 1 participatig labs i 006 ad 15 participatig labs i this year are show i Tables 1 ad, respectively. X i, X ref, U 95 (X i ) ad U 95 (X ref ) stad for the correspodig measuremet results (frequecy accuracy) ad reported ucertaities from participatig labs ad TL. Usig these four parameters, the E value of each participatig lab could be obtaied. Cosiderig a driftig frequecy stadard may ifluece the measuremet result, it is reasoable for participat labs to iclude the latest calibrated accuracy of their idividual frequecy stadards i the X i measuremets, or separately specify both of them i each participat lab s test report. Results of the latter are adopted i Table 1 ad the metioed calibrated accuracy ΔX i ca be used for the E correctio. The corrected E value could be obtaied usig equatio as below: ( X i X i ) X ref E. () U ( X ) U ( X ) 95 i 95 ref Besides, adjustmet of X ref is egligible because the calibrated accuracy of TL s portable frequecy referece ca reach , about 4 orders lower tha the magitude of X ref itself. For most of the participats i Table 1, the corrected E are smaller tha or equal to the origial oes. This shows that the E correctio is meaigful, which gives more reasoable results for iter-laboratory comparisos. Equatio () ca also be cosidered as the corrected measuremet differece betwee a participatig lab ad TL, divided by their combied ucertaity. I geeral, U 95 (X i ) is the domiat term i the combied ucertaity, so the corrected 316 E is a good idex to show how well a participatig lab is withi its accredited capability. All the corrected E of 1 participatig labs i 006 ad 15 participatig labs this year are smaller tha 1, which idicates that the calibratio capabilities of all the participats are qualified i these two activities. Moreover, oe may foud that most of the corrected E values i Table are smaller tha those i Table 1. This is caused by the performaces of differet DUTs adopted i these two activities. The agig rate ad temperature coefficiet of a HP3350A (DUT of 009) are larger tha those of a HP866A (DUT

5 of 006), so the combied ucertaities are commoly larger ad the correspodet E values become smaller. Geerally speakig, a DUT with smaller agig ad temperature coefficiet would be more suitable for the proficiecy testig applicatio, ad the capability of the participatig labs ca be more properly evaluated. REFERENCES [1] H. T. Li, C. S. Liao, S. Y. Li, W. H. Tseg, C. C. Li, P. C. Chag, D. Wag, F. D. Chu, ad H. Shyu, Highlights of the 008 activities of the Natioal Time ad Frequecy Stadard Laboratory of the Telecommuicatio Laboratories, CHT CO. LTD., Taiwa, i Proceedigs of the Asia-Pacific Workshop o Time ad Frequecy (ATF), 30 October-1 November 008, Jakarta, Idoesia. [] P. C. Chag, H. T. Li, ad C. S. Liao, Proficiecy testig activities of frequecy calibratio laboratories i Taiwa, 006, i Proceedigs of the Asia-Pacific Workshop o Time ad Frequecy (ATF), December 006, New Delhi, Idia. [3] NATA Proficiecy Testig Traiig Course. [4] Guide to the Expressio of Ucertaity i Measuremet ( ISO), [5] HP 866A Sythesized Sigal Geerator Operatig ad Services Maual, Sec. 1 (Hewlett-Packard), pp Table 1. Proficiecy testig results of 1 participatig labs. Report No. X i X ref ΔX i U 95 (X i ) U 95 (X ref ) E E (corrected) TL-PT E E E E E TL-PT E E E E E TL-PT E E-09 3.E E E TL-PT E E E E E TL-PT E E E E E TL-PT E E E E E TL-PT E E E E E TL-PT E E E E E TL-PT E E E E E TL-PT E E E E E TL-PT E E E E E TL-PT E E-09-3.E E E

6 Table. Proficiecy testig results of 15 participatig labs. Report No. X i X ref ΔX i U 95 (X i ) U 95 (X ref ) E (corrected) TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E E E TL-98FMPT E E E-09.00E E TL-98FMPT E E E E E TL-98FMPT E E E E E