An Assessment of Precision Time and Time Interval Science and Technology (2002) / Chapter Skim
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2. State of the Art of PTTI Physics and Devices
2. State of the Art of PTTI Physics and Devices
Pages 12-22
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From page 12... ...
Quartz crystal oscillators are also used as local oscillators for lower-performance atomic standards. High-performance laboratory atomic standards require higher-performance local oscillators such as the rubidium standard or the hydrogen maser.
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From page 13... ...
Tuning Voltage 1 Amplifier T 5 MHz Output Output Frequency FIGURE 2.2 A simple circuit diagram showing the basic elements of a crystal oscillator. PERFORMANCE OF FREQUENCY STANDARDS 13 The performance of any frequency standard, whether atomic or crystal, is described by three characteristics: · Accuracy is a measure of how closely the frequency generated by the standard corresponds to its assigned value (e.g., the atomic transition frequency for an atomic standard)
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From page 14... ...
They are used as clocks in laboratories that contribute to the international atomic time scale, at the GPS ground control station that uploads time to the GPS satellites, and aboard Navy ships. Cesium Fountain These standards are also based on transitions in cesium-133 but differ from the cesium beam standards in that they probe collections of laser-cooled cesium atoms, which are launched upward like a fountain and probed during their free fall.
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From page 15... ...
Rubidium cells flown on GPS block IIR satellites are providing better short-term stability than the cesium beam standards flown on earlier GPS satellites. Rubidium standards are also the principal timekeeping devices on MILSTAR military communications satellites.
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From page 16... ...
The uncompensated crystal oscillator, XO, does not contain means for reducing the crystal' sfversus T variation. The temperature-compensated crystal oscillator, TCXO, uses temperature-dependent reactance variations to produce frequency changes that are equal and opposite to the crystal's temperature-dependent frequency changes that is, the reactance variations compensate for the crystal's f versus T variations.
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From page 17... ...
GPS Common View In GPS common view, observers at different locations receive the same time signal from the same GPS satellite and record the difference between their local clocks and the GPS time. Then by communicating their results and subtracting them, the GPS time drops out, with the result being the time difference between the local clocks.
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From page 18... ...
The equipment for this method is moderately expensive, and satellite time must be scheduled and paid for, so this method is not as easy to use as the others. Time average accuracy ranges from 1 to 5 nanoseconds rms and time stability ranges from about 0.1 to 2 nanoseconds rms, giving a frequency accuracy at 1 day between 0.1 and 2 x 10-~4rms.
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From page 19... ...
In a passive standard used as a clock, if the LO has a step change in frequency due to shock or some other cause, the time output of the clock can have zero offset after the transient dies out if the loop filter contains at least two digital integrators. Vibration Vibration affects quartz LOs mainly through its acceleration effects on the resonator.
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From page 20... ...
Microwave atomic frequency standards are particularly sensitive to magnetic fields because the hyperfine transition frequency that is the basis of the standard is proportional to the energy of the magnetic interaction between the outer electron and the nucleus. All these standards use hyperfine transitions with small quadratic magnetic-field dependence at the operating magnetic field of the unit typically a few thousandths of a gauss.
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From page 21... ...
The atomic resonator in well-designed passive atomic standards determines frequency noise at times longer than the loop time constant. Atomic shot noise in beam standards and shot noise in the optical detector in gas cell standards are the primary source of frequency noise until flicker noise, with spectrum proportional to 1/f, takes over at low frequencies or long times.
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From page 22... ...
LOs with voltage-variable tuning capacitors are particularly susceptible. Input voltage variations will cause variations in the power dissipated in the voltage regulators, and the resulting temperature changes can induce frequency changes.
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