| 1. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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A fiber based frequency distribution system with enchanced output phase stability
- 2009
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Ingår i: Proceedings EFTF-IFCS2009 joint conference 20-24 April 2009, IEEE catalog number:CFP09FRE-CDR. - 1075-6787. - 9781424435104 ; , s. 1061-1064
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Konferensbidrag (refereegranskat)abstract
- Experimental results on the stability of the output phase of a frequency distribution system from several days of measurement is presented, in addition to a discussion regarding the influence of control loop parameters. The setup handles the issue that the output phase stability of a system depends on perturbations along the transmission length. This is especially critical if the signal is transmitted through optical fiber, at lengths of a few 100 m. An experimental evaluation using a laser based transmitter at a wavelength of 850 nm, and 625 m of multimode fiber where 575 m where placed outdoor, a temperature dependence of 100 ps/°C was detected. Tocompensate for these slow variations in real time, a setup using two-way transmission, in conjunction with an adjustable optical delay, was constructed. This device is adjusted to induce a delay variation of equal magnitude but opposite direction, in comparison to the delay change of the fiber. Calculating the modified Allan deviation of the transmitted signal, it is apparent that without active compensation, the deviation at τ below 1000 s is comparable to the values from the measurement system without transmission. At longer integration times, however, the slow variations in the fiber transmission will deteriorate the modified ADEV substantially. When activating the dynamic adjustment of pre-delay in the system, the deviation at shorter times will increase with a few dB, however, the modified ADEV decreases continuously with τ, eventually below the values for the uncompensated system. In conclusion, activating a dynamically controlled pre-delay in a fiber based frequency transmission system will induce a small penalty on fast variations of the output phase, however giving a remarkable improvement on slower variations. The usefulness of this added functionality must therefore be determined by the application of the signal.
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| 2. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Measurements and Error Sources in Time Transfer Using Asynchronous Fiber Network
- 2010
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Ingår i: IEEE Transactions on Instrumentation and Measurement. - 1557-9662 .- 0018-9456. ; 59:7, s. 1918-1924
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Tidskriftsartikel (refereegranskat)abstract
- We have performed time transfer experiments basedon passive listening in fiber optical networks using Packet over synchronous optical networking (SONET)/synchronous digital hierarchy(SDH). The experiments have been performed with differentcomplexity and over different distances. For assessmentof the results, we have used a GPS link based on carrier-phase observations. On a 560-km link, precision that is relative to the GPS link of
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| 3. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Time Transfer between UTC(SP) and UTC(MIKE) Using Frame Detection in Fiber-Optical Communication networks
- 2011
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Ingår i: 43rd Precise Time and Time Interval (PTTI) Systems and Applications Meeting. - 9781622767953 ; , s. 431-441
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Konferensbidrag (refereegranskat)abstract
- This paper presents recent results from a time transfer method using passive listening and detection of SDH frame headers in fiber-optical networks. The results are based on an experimental fiber-link that is implemented between the national time and frequency laboratories at SP in Borås, Sweden and at MIKES in Espoo, Finland with an intermediate connection at STUPI time and frequency facility in Stockholm, Sweden. The total fiber length exceeds 1129 km and is implemented in SUNET (Swedish University Network) and FUNET (Finnish University and Research Network). The two networks are connected via NORDUnet (Nordic Infrastructure for Research & Education) and the links are DWDM-based (Dense Wavelength Division Multiplexing).Both SP and MIKES maintains local representations of UTC and contributes with clock data to TAI, which gives the opportunity to compare the fiber-based method with those independent methods that are used regularly by the laboratories for the links to UTC. Preliminary results show that a time transfer stability of less than 10 picoseconds is obtained for averaging times of a few hundred seconds. The results also show that the method suffers from daily variations of a few nanoseconds, presumable due to temperature sensitive network equipment and asymmetric fiber paths. Nevertheless, a comparison to GPS carrier phase time transfer over three months shows an rms-agreement of less than 1 nanosecond.
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| 4. |
- Ebenhag, Sven-Christian, 1976, et al.
(författare)
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Time transfer using an asynchronous computer network: Results from three weeks of measurements
- 2007
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Ingår i: European Frequency and Time Forum, 29/5 - 1/6, Geneva, CH.
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Konferensbidrag (refereegranskat)abstract
- We have performed a time transfer experimentbetween two atomic clocks, over a distance of approximately 75km using an 10 Gbit/s asynchronous fiber-optic computernetwork. The time transfer was accomplished through passivelistening on existing data traffic and a pilot sequence in the SDHbit stream. In order to assess the fiber-link clock comparison, wesimultaneously compared the clocks using a GPS carrier phaselink. The standard deviation of the difference between the twotime transfer links over the three-week time period was 243 ps.
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| 5. |
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| 6. |
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| 7. |
- Hedekvist, Per Olof E, 1967, et al.
(författare)
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Accurate time transfer utilizing the synchronization in an SDH-network
- 2006
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Ingår i: 2006 Optical Fiber Communication Conference, and the 2006 National Fiber Optic Engineers Conference; Anaheim, CA; United States; 5 March 2006 through 10 March 2006. - 9781557528032
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Konferensbidrag (refereegranskat)abstract
- A nationwide system for accurate time distribution is being developed, utilizing synchronization in an SDH-network. The first experimental results based on this technique are presented, performed on, but not limited to, STM-64.
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| 8. |
- Jaldehag, R. T. Kenneth, 1962, et al.
(författare)
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A GPS Carrier-Phase Aided Clock Transport for the Calibration of a Regional Distributed Time Scale
- 2009
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Ingår i: Proceedings EFTF-IFCS2009 joint conference 20-24 April 2009, IEEE catalog number:CFP09FRE-CDR. - 1075-6787. - 9781424435104 ; , s. 659-663
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Konferensbidrag (refereegranskat)abstract
- Clock transportation is a historically proven time transfer method for the calibration of time links and time scales. With the establishment of satellite-based time transfer methods, however, clock transportation has become less attractive especially on long baselines. In order to match for instance the GPS common view time transfer method with calibration uncertainties of a few nanoseconds, it is necessary to transport high quality, expensive clocks such as caesium beam frequency standards. The stability of the clock during transportation and the duration of the transport set the limit of the prediction uncertainty. Being able to measure the clock during transportation instead of predicting it would yield some major advantages: (a) the use of less expensive and small clocks such as rubidium or quartz oscillators for transportation, (b) no need for environmental conditioning of the transported clock, and (c) the duration of the transport is not critical as long as the clock can continuously be measured. One solution to the clock measurement problem during transport is the use of GPS carrier-phase observations as described and evaluated in this paper. It is shown that a calibration uncertainty of less than one nanosecond is potentially achievable.
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| 9. |
- Jaldehag, R. T. Kenneth, 1962, et al.
(författare)
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Time and frequency activities at SP in Sweden
- 2009
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Ingår i: 41st Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Santa Ana Pueblo. NM, November 16-19. - 9781617386541 ; , s. 231-251
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The national time and frequency laboratory of Sweden has since 1995 been a part of the measurement technology department at the SP Technical Research Institute of Sweden. The laboratory is responsible for maintaining a realization of Swedish standard time and the dissemination of this time scale in Sweden. The objective of the laboratory is to support and supply Swedish industry and authorities with accurate measures of time and frequency by instrument calibration, knowledge transfer, time dissemination, and research and development. Swedish standard time is connected by law to UTC as maintained by the BIPM. UTC (SP) is the realization of UTC in Sweden and is traceable to UTC via BIPM and time transfer using the GPS and TWSTFT techniques. This paper describes the generation and maintenance of UTC (SP) and the equipment, including clocks and time transfer equipment, needed for this task as well as the concept of a "Distributed Time Scale" using alternate versions of UTC (SP) maintained at sister laboratories in Sweden. The paper presents also activities related to the dissemination of Swedish standard time including GPS time transfer, Network Time Protocol (NTP), telephone time code, and a speaking clock. Finally, research activities including time transfer in optical fiber networks, continuous GNSS carrier-phase processing, and Kalman-filter-based ensemble clock generation are briefly presented.
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| 10. |
- Jaldehag, R. T. Kenneth, 1962, et al.
(författare)
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Time and frequency transfer using asynchronous fiber optical networks: progress report
- 2009
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Ingår i: 41st Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Santa Ana Pueblo. NM, November 16-19.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- SP Technical Research Institute of Sweden has since 2004 been running a project withthe aim of performing time and frequency transfer using commercial asynchronous fiberopticalnetworks. The project is motivated by the need for an alternative and complementarytime transfer method on a national basis with the goal of reaching accuracy and stabilitycomparable to satellite-based methods. Previous results using an OC-192/STM-64 10-Gb/spacket over SONET/SDH network, show that time transfer accuracy of the order of a fewnanoseconds is possible on baselines exceeding 500 km [1]. The method is based on passivelistening on existing data traffic and the detection of certain bit sequences in the SDH frameheaders continuously transmitted by the network routers. By using two-way time transfer, itis possible to estimate and compensate for symmetric delays in the optical fibers. The methodrelies on that time dependent residual delays are small or can be can be compensated forand constant residual delays can be calibrated.This paper briefly revises the method and presents new results in comparison with theGPS carrier-phase technique, with focus on residual effects due to temperature variationswhich have shown to have significant impact on the stability and accuracy. It also discusseshardware miniaturizations as well as new ideas for active time transfer using bit-sequencegenerators/transmitters in dedicated wavelength slots of the optical network. Finally, the useof a subset of the IEEE standard 1588-2008 (Precise Time Protocol, PTP) for data transportis briefly discussed.
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