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Time and Frequency from A to Z: St to Sy
An inherent characteristic of an oscillator that determines how well it can produce the same frequency over a given time interval. Stability doesn't indicate whether the frequency is right or wrong, but only whether it stays the same. The stability of an oscillator doesn't necessarily change when the frequency offset changes. You can adjust an oscillator and move its frequency either further away from or closer to its nominal frequency without changing its stability at all. The graphic below illustrates this by displaying two oscillating signals that are of the same frequency between t1 and t2. However, it’s clear that signal 1 is unstable and is fluctuating in frequency between t2 and t3.
The stability of an oscillator is usually specified by a statistic such as the Allan deviation that estimates the frequency fluctuations of the device over a given time interval. Some devices, such as an OCXO, have good short-term stability and poor long-term stability. Other devices, such as a GPS disciplined oscillator (GPSDO), typically have poor short-term stability and good long-term stability. Specification sheets for quartz oscillators seldom quote stability numbers for intervals longer than 100 seconds. Conversely, a specification sheet for an atomic oscillator or a GPSDO might quote stability estimates for intervals ranging from one second to more than one day.
A device or signal used as the comparison reference for a measurement. A standard is used to measure or calibrate other devices. NIST is responsible for developing, maintaining and disseminating national standards for the United States for the basic measurement quantities (such as time interval), and for many derived measurement quantities (such as frequency).
A device (usually a handheld device) used to measure time interval. Most stop watches are manually operated, a button is pushed to start and stop the measurement. The measurement is made using a quartz or mechanical time base. Stop watches are used for simple time interval measurements and calibrations. Their resolution is very coarse compared to a time interval counter, with 10 millisecond resolution being typical.
A clock in a telecommunications system or network that is assigned a number that indicates its quality and position in the timing hierarchy. The highest quality clocks, called stratum 1 clocks, have a frequency offset of 1 x 10-11 or less, which means that they can keep time to within about one microsecond per day. The formal specifications are given in the ANSI standards T1.101-1999 and T1.105.09-1997. Only stratum 1 clocks may operate independently; other clocks are synchronized directly or indirectly to a stratum 1 clock.