Engineering Bulletin E-6: Frequency Control with Quartz Crystals

FREQUENCY STANDARDS

Early frequency standards were simply variable tuned circuits, known as wavemeters. These instruments were calibrated against the output of a rotary high-frequency alternator whose frequency, and its harmonics, was determined from the constants of the machine and its speed of rotation at that time, the alternator was the only source of accurate calibrating frequencies. The wavemeter is subject to considerable error and, even with present precision manufacturing and calibrating facilities, the dependable accuracy is in the neighborhood of only 0.1% to 0.25%. While the wavemeter does have a definite place in radio engineering, its inherent inaccuracies are far too great for frequency standardization purposes.

The first true standards of frequency were electrically excited tuning forks. These forks were maintained in vibration by a regenerative vacuum tube oscillator circuit and were temperature controlled to provide the highest degree of frequency stability. Frequency was determined by direct reference to the basic element, time, through the medium of connecting a synchronous motor-driven clock to the output of the oscillator circuit and comparing the time, as indicated by that clock, with true time as determined by astronomical observatories such as the U. S. Naval Observatory. The average frequency of the tuning forks was then calculated from the time-rate of the oscillator clock. The fundamental accuracy of the tuning-fork standard could be held within about 7 parts in one million (0.0007%) which is far greater accuracy than obtainable with wavemeters. For frequency measurement purposes, the output of the tuning-fork oscillator was multiplied by vacuum-tube frequency multipliers to produce a series of standard frequencies.

The development of the quartz oscillating crystal entirely changed the conception of practical frequency stability and accuracy. Quartz crystals, having a large equivalent inductance and a high Q, possess a degree of frequency stability unattainable with other types of oscillator frequency control; the oscillating frequency is almost entirely determined by physical dimensions, it is unaffected by magnetic fields or gravity, and is influenced to only a very small extent (much less than with a tuning fork) by variations in atmospheric pressure. Furthermore, a quartz crystal has the physical, chemical and electrical stabilities which are obvious prerequisites for permanence in a frequency standard.

Frequency standards are divided into two classifications: (1) primary standards of frequency and, (2) secondary standards of frequency. The primary standard, as its name implies, is a fundamental standard against which all other frequency determinations are made. It is an independent standard because it is checked for accuracy and stability by direct measurements against time. Quartz crystal control has so simplified the construction of frequency standards that primary standards are commercially practicable and are regular equipment in many laboratories, schools and government bureaus. The secondary standard has no provisions for checking its frequency directly with time and it, therefore, must be calibrated by reference to some primary standard.