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
An oscillator with a servo loop that has its phase and frequency locked to an external reference signal.
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).
The drive level normally referenced in milliwatts is the power dissipated in the crystal's equivalent
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
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.
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.
Difference between the frequencies of two different signals.
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.
Change in frequency from a standard reference.
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 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.
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, which mean unwanted modes, are actually inharmonics modes of vibration of the
Since spurious modes are inherent in every crystal resonator, they are suppressed by special design
Typical spurious specifications are 6dB below the desired mode of oscillations but they are more
highly suppressed upon customer's request.