page_title: Holdover in synchronization applications
section_title: Implementing holdover
hierarchical_section_title: Holdover in synchronization applications / Implementing holdover
caption_attribution_description: English: A modern GPS Disciplined Oscillator
context_page_description: Two independent clocks, once synchronized, will walk away from one another without limit. To have them display the same time it would be necessary to re-synchronize them at regular intervals. The period between synchronizations is referred to as holdover and performance under holdover relies on the quality of the reference oscillator, the PLL design, and the correction mechanisms employed.
context_section_description: In applications that require synchronization (such as wireless base stations) GPS Clocks are often used and in this context are often known as a GPSDO (GPS Disciplined Oscillator) or GPS TFS (GPS Time and Frequency Source).
  NIST defines a Disciplined Oscillator as:
  An oscillator whose output frequency is continuously steered (often through the use of a phase locked loop) to agree with an external reference. For example, a GPS disciplined oscillator (GPSDO) usually consists of a quartz or rubidium oscillator whose output frequency is continuously steered to agree with signals broadcast by the GPS satellites.
  In a GPSDO a GPS or GNSS signal is used as the external reference that steers an internal oscillator. In a modern GPSDO the GPS processing and steering function are both implemented in a Microprocessor allowing a direct comparison between the GPS reference signal and the oscillator output.
  Amongst the building blocks of a GPS Time and Frequency solution the oscillator is a key component and typically they are built around an Oven Controlled Crystal Oscillator (OCXO) or a 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 During Holdover any remaining clock error caused by aging and temperature instability can be corrected by control mechanisms. A combination of quartz based reference oscillator (such as an OCXO) and modern correction algorithms can get good results in Holdover applications.
  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. The earliest documentation of such an effort comes from the then National 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 caesium 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. In Holdover the learned behaviour of the OCXO is used to anticipate and correct for future behavior. Effective aging and temperature compensation can be provided by such a mechanism 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 filters).
  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.
  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 noise has resulted in tailor-made Holdover Oscillator solutions being introduced in the market.
caption: A Modern GPSDO
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