Frequency Stabilization

in radio engineering, the maintenance of a constant frequency in the oscillations in a self-excited oscillator. The frequency of the oscillations in a self-excited oscillator may deviate from an original value as a result of destabilizing factors, such as changes in temperature, humidity, atmospheric pressure, supply voltage, and load resistance. Other factors include the noise of vacuum tubes and semiconductor devices, radioactive irradiation, shocks and vibrations, and the aging of parts.

Frequency deviation, or drift, leads to such undesirable consequences as mutual interference in the reception of signals from radio stations having similar frequencies and drift over time in the tuning of superheterodyne broadcast receivers. The techniques used in frequency stabilization seek to ensure stability in the frequency of oscillations despite the presence of destabilizing factors or, stated differently, to reduce instability in the frequency of oscillations. The instability is characterized by the value of the relative frequency instability Δf/f, where Δf is the deviation in the frequency from the original frequency f (Δf/f is also referred to as the relative frequency stability). A distinction is made between short-term instabilities, defined as frequency deviations during a time interval of less than 1 sec, and long-term instabilities. In practice, concepts pertaining to instability cover periods of minutes, hours, days, months, and years.

An increase in frequency stability in self-excited oscillators, that is, a reduction in Δf/f, can be achieved by increasing the quality factor of the oscillatory circuit driving the frequency and by decreasing the circuit’s temperature coefficient of frequency. Frequency stability can also be realized by a judicious choice of layout, design, and operating mode of the self-excited oscillator, by a thermostatic control of the oscillator, and by a stabilization of the supply voltage.

The most widely used frequency stabilization system utilizes quartz components. A piezoelectric quartz resonator is the electromechanical oscillatory system serving as the oscillatory circuit. Quartz-crystal oscillators, built with transistors, tunnel diodes, or electron tubes, have instabilities Δf/f of 10^–6–10^–10; they have small dimensions and are economical and reliable. The high frequency stability of the quartz-crystal oscillator derives from the low temperature coefficient of frequency of the quartz resonator, the stability of the resonator’s parameters with regard to external effects, and the exceptionally high quality factor (up to 10⁷, with the figure for conventional oscillatory circuits, in most cases, –10²). Frequency-stabilizing devices with quartz components are widely used in radio engineering in medium-power and high-power transmitters, in time and frequency standards, and in the oscillators of multichannel communication systems. Here, decade frequency synthesis is used in wide-band radio equipment.

The highest frequency stability (Δf/f = 10^–11–10^–13) is achieved in quantum frequency standards. This stability can be explained in principle by the high stability of microsystems (atoms, molecules) in comparison with macrosystems (oscillatory circuits, cavity and quartz resonators). In addition, microsystems, in contrast to macrosystems, are not subject to aging and mechanical influences.