Information > Glossary

The systematic change in frequency over time because of internal changes in the oscillator. For example, a 100 kHz quartz oscillator may age until its frequency becomes 100.01 kHz (see drift). NOTE: Aging is the frequency change with time when factors external to the oscillator such as environment and power supply are kept constant.
In order not to suffer severe aging circuit should be designed with drive level kept at absolute minimum.

Allan Variance or Allan Deviation
The standard method of characterizing the frequency stability of oscillators in the time domain, both short and long term.

Atomic Time (TA) scale
A time scale based on atomic or molecular resonance phenomena.
Elapsed time is measured by counting cycles of a frequency locked to an atomic or molecular transition. Other scales use mechanical reference devices such as quartz crystals or are based on the rotation rate of the earth.

The process of identifying and measuring time or frequency errors, offsets, or deviations of a clock oscillator relative to an established and accepted time or reference frequency standard such as UTC - National Institute of Standards and Technology (NIST)or UTC - Bureau International des Poids et Mesures (BIPM).

Disciplined oscillator
An oscillator with a servo loop that has its phase and frequency locked to an external reference signal.

Drift (frequency)
The linear (first-order) component of a systematic change in frequency of an oscillator over time.
Drift is due to aging plus changes in the environment and other factors external to the oscillator(see aging).

Drive level
The drive level normally referenced in milliwatts is the power dissipated in the crystal's equivalent resistance.
Drive level should be the minimum necessary to begin and maintain crystal oscillation, to assure optimum performance and stability.
Excessive drive can result in breakage of the crystal element, excessive frequency drift, and poor again characteristic.

The rate at which a periodic phenomenon occurs over time.

Frequency analysis techniques
Analysis techniques in the frequency domain, where signals are separated into their frequency components and the power at each frequency is displayed.

Frequency deviation
The difference between frequency values of the same signal at two different times or the difference between the instantaneous signal frequency and the average signal frequency.

Frequency difference
Difference between the frequencies of two different signals.

Frequency offset
The frequency difference between the realized value and a reference frequency value offset is often not referenced to the nominal.
For example, during irradiation testing the offset is referenced to the frequency before irradiation.

Frequency shift
Change in frequency from a standard reference.

Frequency stability
Statistical estimate of the frequency fluctuations of a signal over a given time interval.

◇ Long term stability usually involves measurement averages beyond 100s.
◇ Short term stability usually involves measurement averages from
◇ a few tenths of a second to 100s.

NOTE: Generally, there is a distinction between systematic effects such as frequency drift and stochastic frequency fluctuations.
Special variances have been developed for the characterization of these fluctuations. Systematic instabilities may be caused by radiation, pressure, temperature, and humidity. Random or stochastic instabilities are typically characterized in the time domain or frequency domain.
They are typically dependent on the measurement system bandwidth or on the sample time or integration time.

Frequency tolerance
Frequency tolerance is the amount of frequency deviation(plus or minus) from the desired operating frequency at a specific temperature. Accuracy requirement for crystal tolerance is expressed in percentage.

Frequency standard
A precise frequency generator such as a rubidium, cesium, or hydrogen maser whose output is used as a frequency.

Resistance and Q factor
Resistance is the equivalent impedance of the quartz resonator and it determines Q factor of quartz crystal.
High crystal Q's are obtained by reducing mechanical and acoustic energy losses which is equivalent to R1.

A high Q factor, i.e., low resistance R1 reduces the influence of external parameters, such as variations in supply voltage, load, temperature and oscillator components.

Spurious modes
Spurious modes, which mean unwanted modes, are actually inharmonics modes of vibration of the quartz plate.
Since spurious modes are inherent in every crystal resonator, they are suppressed by special design technique.
Typical spurious specifications are 6dB below the desired mode of oscillations but they are more highly suppressed upon customer's request.