Remote Frequency Calibrations

1 Page 1 NIST Special Publication 250-29 (supersedes 1988 edition) Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Michael A. Lombardi Time and Frequency Division Physics Laboratory National Ins... Open at page → 2 Page 2 Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or... Open at page → 3 Page 3 iii Contents Introduction v Acknowledgements vi Chapter 1. History and Physical Description 1 A. History of the FMAS 1 B. Facilities 7 1. Configuration of Remote FMS Units 9 C. Organizational Control of the FMAS 11 Chapt... Open at page → 4 Page 4 iv Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service B. Software Maintenance 53 C. Customer Maintenance 53 D. Failure Modes 53 1. Hardware Failures 53 2. Software Failures 55 3. GPS Signa... Open at page → 5 Page 5 v Introduction The National Institute of Standards and Technology (NIST) provides its Frequency Measurement and Analysis Service (FMAS) to paying customers on a subscription basis. Subscribers to the service receive a co... Open at page → 6 Page 6 vi Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Acknowledgements Special thanks are due to George Kamas, Andrew Novick, and Lisa Nelson. Kamas conceived the idea of a remote frequenc... Open at page → 7 Page 7 1 Chapter 1 - History and Physical Description The FMAS is a network of individual frequency measurement systems located at each of its customer’s laboratories, and connected by telephone lines to the NIST laboratories i... Open at page → 8 Page 8 2 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service them to the national frequency standard, so their performance was well documented. The results of these LORAN-C comparisons were publis... Open at page → 9 Page 9 3 Chapter 1 - History and Physical Description comparator that this particular customer had previously used as their frequency reference, and commented that “support from NBS is superb.” NBS personnel quickly moved to co... Open at page → 10 Page 10 4 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service could be selected and measured simultaneously, a feature still included in current FMS units. The TIC multiplied its incoming external... Open at page → 11 Page 11 5 Chapter 1 - History and Physical Description and frequency comparisons by 1980 [11], and NBS had launched a GPS-based time service, called the Global Time Service, at about the same time the FMAS began in 1984 [4]. GPS... Open at page → 12 Page 12 6 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service By the time IOC for GPS was declared, work was underway on the third generation FMAS system, this one based on GPS. The first GPS unit... Open at page → 13 Page 13 7 Chapter 1 - History and Physical Description those of the earlier five-channel version, and when the GPS system deactivated its selective availability program in May 2000 [18], the measurement uncertainty was further r... Open at page → 14 Page 14 8 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Maintaining a tightly controlled laboratory environment is not critical for the FMAS operation. The GPS receivers embedded inside the F... Open at page → 15 Page 15 9 Chapter 1 - History and Physical Description this storage area. Parts that are determined to be in complete working order and that are still cosmetically acceptable are returned to inventory and sent to other customers... Open at page → 16 Page 16 10 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Item Description Solid State Flash Memory Disk 16 megabyte or higher capacity with embedded ROM-DOS operating system. Measurement soft... Open at page → 17 Page 17 11 Chapter 1 - History and Physical Description C. Organizational Control of the FMAS The FMAS is managed, controlled, and maintained by the Services Group of the NIST Time and Frequency Division. The Group Leader of the... Open at page → 18 Page 18 12 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service This page intentionally left blank. Open at page → 19 Page 19 13 Chapter 2 - Technical Description This chapter describes the technical operation of the FMAS, including a discussion of the theory of operation and the specific hardware and software used to implement the service. A.... Open at page → 20 Page 20 14 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service 1. Computer System and Storage Devices The FMS computer and measurement hardware are enclosed inside a standard 19 in (48.3 cm) rack-m... Open at page → 21 Page 21 15 Chapter 2 - Technical Description a) Time Interval Counter (TIC) All TICs provide inputs for a start pulse and a stop pulse. The start pulse gates the TIC to start counting, and a stop pulse stops the counting. While... Open at page → 22 Page 22 16 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service When the start pulse arrives, the charging current I REF is sent to integrator A, and the digital counter begins counting periods of t... Open at page → 23 Page 23 17 Chapter 2 - Technical Description where the slope A and the intercept B are determined during the interpolator calibration. During the calibration, a series of readings is taken from the start interpolator counter and... Open at page → 24 Page 24 18 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service The stop interpolator performs a nearly identical function as the start interpolator, except that it estimates the interval of T − δt... Open at page → 25 Page 25 19 Chapter 2 - Technical Description on both sides of the counter to a nearly identical shape and amplitude. There are no user adjustable trigger levels, although the TIC software is preconfigured to expect either a sinu... Open at page → 26 Page 26 20 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service from the customer’s primary frequency standard. Typically, the customer has a stable frequency standard, with a frequency stability at... Open at page → 27 Page 27 21 Chapter 2 - Technical Description the software controlling the counter would have to keep track and correct for all the overflows and underflows. This could be a difficult task with some low performance oscillators; f... Open at page → 28 Page 28 22 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service c) Multiplexer Since the inception of the service, all FMS units have included multiplexer circuitry that allows the selection of diff... Open at page → 29 Page 29 23 Chapter 2 - Technical Description The current FMS TIC actually has two multiplexers. One allows the selecting between any of eight frequency inputs as shown in Figure 2.5, but only five inputs are activated in the cur... Open at page → 30 Page 30 24 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service The main disadvantage of the time interval method is that it lacks the resolution to accurately measure frequency or to estimate short... Open at page → 31 Page 31 25 Chapter 2 - Technical Description The GPS receiver searches for, acquires, and tracks satellites on eight parallel channels using the L1 carrier frequency (1575.42 MHz) and the coarse acquisition (C/A) code with a 1.0... Open at page → 32 Page 32 26 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service The receiver outputs a 1 pps timing signal with a 200 ms pulse width (20 % duty cycle) and a 20 to 30 ns rise time from 0 to 5 V. The... Open at page → 33 Page 33 27 Chapter 2 - Technical Description a) GPS Antenna The GPS antenna provided with the FMS was designed specifically for time and frequency applications. The antenna was designed for outdoor installation, and must be moun... Open at page → 34 Page 34 28 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service 4. Printer An inkjet printer with a parallel interface is used to print daily phase graphs from each of the five measurement channels... Open at page → 35 Page 35 29 Chapter 2 - Technical Description a) GPS Software When an FMS unit is first installed at a customer’s site, or whenever it is restarted, a software program named GPS2000 is executed. This software performs the site su... Open at page → 36 Page 36 30 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Subframe 1 contains the GPS week number, six bits that describe the health of the satellite, an estimate of the signal dispersion due... Open at page → 37 Page 37 31 Chapter 2 - Technical Description Receiver Warm Start – As previously mentioned, GPS2000 can perform either a cold start or warm start of the receiver. A warm start is performed whenever the FMS unit is powered on or... Open at page → 38 Page 38 32 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service are still checked to verify receiver integrity, a process that normally takes a few minutes or less. Once this integrity check is comp... Open at page → 39 Page 39 33 Chapter 2 - Technical Description Table 2.3. TIC control registers. Register Read Write R0 Control register 0 Control register 0 R1 Read waveform type (channels 0 to 7) Set waveform type (channels 0 to 7) R2 Count don... Open at page → 40 Page 40 34 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service The TIC is prepared for measurements by first setting the time base frequency and then setting the frequency dividers. The time base f... Open at page → 41 Page 41 35 Chapter 2 - Technical Description 1600 UTC (the customer can change this time). During this stoppage, the collected data are graphed and printed, and the TIC is recalibrated. The collected data are stored every hour,... Open at page → 42 Page 42 36 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Step Action Description 8 Read all measurement channels again. The reading from the current channel is stored in the count array. If n... Open at page → 43 Page 43 37 Chapter 2 - Technical Description While the FMS is taking data, a screen similar to Figure 2.9 is displayed. The top part of the measurement screen is a bar graph that provides the customer with a quick indication of... Open at page → 44 Page 44 38 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service has collected data for more than 24 h, only the most recent 24 h of data are used to estimate the frequency offset. A new data point i... Open at page → 45 Page 45 39 Chapter 2 - Technical Description As previously discussed, the FMS measures time interval, but produces frequency information. This is possible since the relationship between frequency and time interval is known. Dime... Open at page → 46 Page 46 40 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service A frequency estimation of frequency offset can be made by recording a single TIC reading, waiting for a specified length of time, one... Open at page → 47 Page 47 41 Chapter 2 - Technical Description (y values) contain the accumulated phase differences taken from the five measurement channels during the 1 h interval. The units for these data are microseconds, but since five digits... Open at page → 48 Page 48 42 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service The least squares line is drawn on all graphs produced on channel one of the FMS. If r = 1 or r = −1, the least squares line is nearly... Open at page → 49 Page 49 43 Chapter 2 - Technical Description The change in phase, DF, is plotted as the y-coordinate. However, since phase is expressed in units of time (microseconds), this is actually the change in time, or the ∆t quantity des... Open at page → 50 Page 50 44 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service phase steps from a phase graph with a single keystroke. This feature works utilizing a simple “3-sigma, 2-pass” filter. First, the mea... Open at page → 51 Page 51 45 Chapter 2 - Technical Description device. It can be used to graph the “warm-up” curve of an oscillator that has just been turned on. It is also useful for a quick calibration or adjustment of oscillators with a freque... Open at page → 52 Page 52 46 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service These graphs use a “log-log” format, with the y-axis scale indicating the dimensionless frequency stability, and the x-axis scale indi... Open at page → 53 Page 53 47 Chapter 2 - Technical Description produce nearly the same frequency day after day. In Figure 2.14, UTC(NIST) is being compared to GPS, and the daily frequency offset estimate seldom exceeds 1 × 10 -13 . A similar grap... Open at page → 54 Page 54 48 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service already been done by FMSFILER, a graph of the daily frequency values is generated by TREPORT. The format of the monthly calibration re... Open at page → 55 Page 55 49 Chapter 3 - Operational Procedures This chapter discusses the hardware and software maintenance required by the FMAS. It also discusses failure modes, record keeping, security issues, and how calibration reports are c... Open at page → 56 Page 56 50 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Figure 3.1. Verifying an oscillator to oscillator comparison with a known frequency offset. Figure 3.2. Verifying a GPS to oscillator... Open at page → 57 Page 57 51 Chapter 3 - Operational Procedures In addition to having the measurement integrity verified, each FMS unit must pass a series of other tests prior to shipment. A checklist of these tests is prepared for each unit and... Open at page → 58 Page 58 52 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service 2. Verification of the FMS after shipment After the FMS is installed by the customer and is successfully taking measurements, it is ca... Open at page → 59 Page 59 53 Chapter 3 - Operational Procedures B. Software Maintenance The software maintenance is simplified by the fact that all remote FMS units contain identical software, although the software might be configured slightly di... Open at page → 60 Page 60 54 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Table 3.3. Hardware failures experienced by FMS units at customer sites. Part name Symptom/Cause Critical to measurements? Action take... Open at page → 61 Page 61 55 Chapter 3 - Operational Procedures 2. Software Failures When software failures occur, the software is fixed, and new software is uploaded and installed over the modem link (3.B). If these failures adversely impacted t... Open at page → 62 Page 62 56 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service If a single GPS satellite is broadcasting bad data, there is a good chance that it will not be noticeable to the FMAS customer. The FM... Open at page → 63 Page 63 57 Chapter 3 - Operational Procedures backed up at NIST, but a copy remains on the customer’s system. Therefore, if those data are lost at NIST, it is usually possible to download them again. The raw data consist of two... Open at page → 64 Page 64 58 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Each FMS comes with remote communications software preinstalled. When a call is received from NIST, a series of commands allows NIST p... Open at page → 65 Page 65 59 Chapter 3 - Operational Procedures If a status code other than 0 is assigned, it is accompanied by a brief comment that explains the situation. If a status code of 0 is assigned it means that the calibration is consid... Open at page → 66 Page 66 60 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service The bottom of page 3 contains logos for the United States Department of Commerce and NIST, and signatures and telephone numbers from t... Open at page → 67 Page 67 61 Chapter 3 - Operational Procedures Figure 3.3.a. FMAS calibration report (page 1). Open at page → 68 Page 68 62 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Figure 3.3.b. FMAS calibration report (page 2). Open at page → 69 Page 69 63 Chapter 3 - Operational Procedures Figure 3.3.c. FMAS calibration report (page 3). Open at page → 70 Page 70 64 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service The FMAS calibration report contains the necessary elements listed in Section 5.10 (“Reporting the Results”) of ISO/IEC Standard 17025... Open at page → 71 Page 71 65 Chapter 3 - Operational Procedures 5.10.8 Format of Certificate accommodates each type of calibration carried out 1, 2, 3 5.10.8.Note 1 Layout designed for ease of assimilation by reader 1, 2, 3 5.10.8.Note 2 Headings... Open at page → 72 Page 72 66 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service The page intentionally left blank. Open at page → 73 Page 73 67 Chapter 4 - Customers This section describes the number and location of FMAS customers, the type of customers, the potential capacity of the service, and the ways that NIST interacts with and supports the customers. A... Open at page → 74 Page 74 68 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service oscillators as their primary frequency standard, denoted by the letter “C” in Figure 4.1. The letter “R” denotes that a rubidium oscil... Open at page → 75 Page 75 69 Chapter 4 - Customers laboratory that routinely calibrates a large number of frequency counters. When a frequency counter is sent to the lab for calibration, its time base can be measured on the FMS, and the collected... Open at page → 76 Page 76 70 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service during the 20 year span (1984 to 2004) that the service has been operational. NIST also interacts with customers through its web page... Open at page → 77 Page 77 71 Chapter 5 - Measurement Uncertainties This chapter discusses the measurement uncertainty of the FMS unit, the uncertainties of measurements made with and without GPS, and how the service establishes traceability to th... Open at page → 78 Page 78 72 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service When looking at the GPS data in Figure 5.1, or at GPS data collected by the FMS over its 1 d calibration interval, it becomes obvious... Open at page → 79 Page 79 73 Chapter 5 - Measurement Uncertainties The two statistics should produce essentially the same result if the noise processes are white, an assumption that is basically true for the dominant FMS noise source at 1 d, whic... Open at page → 80 Page 80 74 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service is used to estimate the Type A uncertainty because the FMS treats each 1 d period as an independent calibration, and a frequency offse... Open at page → 81 Page 81 75 Chapter 5 - Measurement Uncertainties delays to be reduced since the counter and divider circuitry is housed in a single integrated circuit [16], an advantage over using discrete components. The TIC itself introduces... Open at page → 82 Page 82 76 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Figure 5.4. Measuring a 1.25 × 10 -14 frequency offset with TIC PLL as stop pulse. Figure 5.5. Measuring a 1.25 × 10 -14 frequency off... Open at page → 83 Page 83 77 Chapter 5 - Measurement Uncertainties An uncertainty statement is shown in tabular form in Table 5.1. This statement is identical to the uncertainty statement that appears on page 3 of the calibration report pictured... Open at page → 84 Page 84 78 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service satellites since 1980 [11], and prior to that time with signals from LORAN-C, television, WWV, and other sources. The common-view meth... Open at page → 85 Page 85 79 Chapter 5 - Measurement Uncertainties a laboratory whether a GPS receiver is working properly or meeting its specifications [44]. The FMAS solves these problems since the uncertainties of its GPS measurements have bee... Open at page → 86 Page 86 80 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service Products or companies named in this document are cited only in the interest of providing a complete technical description of the FMS,... Open at page → 87 Page 87 81 References [1] G. Kamas, Unpublished report sent to Rome Air Development Center (RADC), 19 p, October 1983. [2] Current and past editions of the Time and Frequency Bulletin can be found at http://tf.nist.gov/pubs/ bul... Open at page → 88 Page 88 82 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service [13] V. S. Zhang, D. D. Davis, and M.A. Lombardi, “High Resolution Time Interval Counter,” Proc. 1994 Precise Time and Time Interval M... Open at page → 89 Page 89 83 References [27] Hewlett-Packard Company, “Fundamentals of the Electronic Counters,” Hewlett-Packard Application Note 200, 5965-7660E, 44 pp., March 1997. [28] D. A. Howe, D. W. Allan, and J. A. Barnes, “Properties of... Open at page → 90 Page 90 84 Remote Frequency Calibrations: The NIST Frequency Measurement and Analysis Service [42] ANSI/ISO/IEC Guide 17025, General Requirements for the Competence of Testing and Calibration Laboratories, International Organiza... Open at page →