Holdover in synchronization applications: Difference between revisions

Revision as of 06:28, 22 December 2012

Revision as of 06:38, 22 December 2012

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[[File:GPSDO.png|thumb|left|A Modern [[GPSDO]]]]

Amongst the building blocks of a GPS Time and Frequency solution the oscillator is a key component<ref name="autogenerated2"/> and typically they are built around an Oven Controlled Crystal Oscillator ([[Crystal oven|OCXO]]) or a [[Rubidium standard|Rubidium based clock]]. The dominant factors influencing the quality of the reference oscillator are taken to be aging and temperature stability. However, depending upon the construction of the oscillator, barometric pressure and relative humidity can have at least as strong an influence on the stability of the quartz oscillator. What is often referred to as "random walk" instability is actually a deterministic effect of environmental parameters. These can be measured and modeled to vastly improve the performance of quartz oscillators. An addition of a Microprocessor to the reference oscillator can improve temperature stability and aging performance<ref>{{cite web|url=http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4622980 |title=IEEE Xplore - Improvements in OCXO performance by the use of an on-board microprocessor |doi=10.1109/FREQ.2008.4622980 |publisher=Ieeexplore.ieee.org |date= |accessdate=2012-09-28}}</ref> During Holdover any remaining clock error caused by aging and temperature instability can be corrected by control mechanisms.<ref>{{cite paper|url=http://kunz-pc.sce.carleton.ca/thesis/CrystalOscillators.pdf |title=Frequency Accuracy & Stability Dependencies of Crystal Oscillators |author=Hui Zhou, Charles Nicholls, Thomas Kunz, Howard Schwartz |publisher= |date=November 2008 |accessdate=2012-10-21}}</ref> A combination of quartz based reference oscillator (such as an [[Crystal oven|OCXO]]) and modern correction algorithms can get good results in Holdover applications.<ref name="Lombardi">{{cite paper|url=http://tf.nist.gov/general/pdf/2297.pdf |title=The Use of GPS Disciplined Oscillators as Primary Frequency Standards for Calibration and Metrology Laboratories |author=Michael A. Lombardi |publisher=NCSL International |date=September 2008 |accessdate=2012-10-21}}</ref>

The Holdover capability then is provided either by a free running local oscillator, or a local oscillator that is steered with software that retains knowledge of its past performance.<ref name="Lombardi"/> The earliest documentation of such an effort comes from the then NBSNational Bureau of Standards in 1968 [Allan, Fey, Machlan and Barnes, "An Ultra Precise Time Synchronization System Designed By Computer Simulation", Frequency], where an analog computer consisting of ball-disk integrators implemented a third order control loop to correct for the frequency ageing of an oscillator. The first microprocessor implementation of this concept occurred in 1983 [Bourke, Penrod, "An Analysis of a Microprocessor Controlled Disciplined Frequency Standard", Frequency Control Symposium] where Loran-C broadcasts were used to discipline very high quality quartz oscillators as a Cesium replacement in telecommunications wireline network synchronization. The basic aim of a steering mechanism is to improve the stability of a clock or oscillator while minimizing the number of times it needs calibration<ref>{{cite web|url=http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=199433 |title=IEEE Xplore - Smart clock: a new time |doi=10.1109/19.199433 |publisher=Ieeexplore.ieee.org |date=2011-09-27 |accessdate=2012-09-28}}</ref> In Holdover the learned behaviour of the [[Crystal oven|OCXO]] is used to anticipate and correct for future behavior<ref name="analog1"/> Effective aging and temperature compensation can be provided by such a mechanism<ref>{{cite paper|url=http://www.gmat.unsw.edu.au/snap/publications/tappero_etal2007c.pdf |author1=Fabrizio Tappero |author2=Andrew G. Dempster |author3=Toshiaki Iwata |title=Phase error reduction method for free-run QZSS clock |publisher=[[IEEE]] |date=2007 |accessdate=2012-10-21}}</ref> and the system designer is faced with a range of choices for algorithms and techniques to do this correction including extrapolation, interpolation and predictive filters (including [[Kalman filter]]s).<ref>[http://www.eftf.org/proceedings/PDFs/FPE-0031.pdf ]{{dead link|date=September 2012}}</ref><ref>{{cite web|url=http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=1418510 |title=IEEE Xplore - Adaptive OCXO drift correction algorithm |doi=10.1109/FREQ.2004.1418510 |publisher=Ieeexplore.ieee.org |date= |accessdate=2012-09-28}}</ref>

Once the barriers of aging and environmental effects are removed the only theoretical limitation to holdover performance in such a GPSDO is irregularity or noise in the drift rate, which is quantified using a metric like [[Allan deviation]] or [[Time deviation]].<ref>http://www.leapsecond.com/pages/adev/adev-why.htm</ref>{{Unreliable source?|date=October 2012}}

The complexity in trying to predict the effects on Holdover due to systematic effects like Aging and Temperature Stability and stochastic influences like [[Random walk|Random Walk]] noise has resulted in tailor made ''Holdover Oscillator'' solutions being introduced in the market.<ref>{{cite web|url=http://www.vectron.com/products/modules/MD-023.pdf |title=MD-023: Extended Holdover Crystal Oscillator |publisher=Vectron International |date=June 2011 |accessdate=2012-10-21}}</ref>

The field of precision frequency control remains not unlike that of the violin maker, where as much art as science is in play.

==See also==