Precision clock

A chronometer is a very precise clock or watch and, if constructed so that it can be started or stopped (for precise timing) at will, it is known as a stopclock or stopwatch. 

FIGURE 4-5 Precision clock circuit. Each decade counting unit divides the frequency at its input by exactly 10. The accuracy of each output frequei cy is equal to that of the crystal oscillator. 

Solutions using Kalman filters, integrated GPS/LORAN, or a precise clock with only two TOAs, win typically be based on TOAs vs. TDs. 

A sensor that is especially interesting and instructive is made from a quartz crystal microbalance, or QCM. This device is based on the piezoelectric characteristics of quartz. When quartz is mechanically deformed, an electrical potential difference develops across its surface. Furthermore, when a voltage is impressed across the faces of a quartz crystal, the crystal deforms., A crystal connected in an appropriate electrical circuit oscillates at a frequency that is characteristic of the mass and shape of the crystal and that is amazingly constant as long as the mass of the crystal is constant. This property of some crystalline materials is called the piezoelectric effect and forms the basis for the QCM. Moreover, the characteristic constant frequency of the quartz crystal is the basis for modern high-precision clocks, time bases, counters, timers, and frequency meters, which in turn have led to many highly accurate and precise analyTical instrumental svsiems. 

The use of a series of decade scalers with a quartz oscillator provides a precise clock, as illustrated in Figure 4-5. The frequency can be selected in decade steps from the original 10 MHz to as low as 0.1 Hz. Because there is no noise or variation in the counting operation, all the outputs are as accurate and precise as the crystal oscillator used. Integrated circuits are available containing several decades of scaling. Such circuits are programmable with the output decade selected by a binary input. 

The Computer. Modern computer-controlled NMR spectrometers allow the implementation of practically any sophisticated pulse sequences such as INEPT (10) or INADEQUATE (11). Very precise clock timing together with the ability to pulse simultaneously the emitter and the decoupler with their own relative rf phases are then a must for the computer. This task is appertaining to the computer pulse generator which is now a standard item on commercial systems ... 

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