# Glossary - alphabet order - "F"

Feedback resistor

This is a component in a crystal oscillation circuit. See "crystal oscillation circuit".

Reference: Crystal oscillation circuit

Flexure vibration

Flexure vibration is a vibration mode (style) of crystal chips, in which flexure motion of the vibrating piece is used as the oscillation source, such as in tuning fork crystal chips. This type is suited for low-frequency crystal devices.

Reference: Tuning fork crystal unit Flexure vibration

Frequency

The frequency is the density of cycles (waves) per unit of time (one second). The relation between frequency (f) and cycle time (t) is f (Hz) = 1/t (s).

Frequency band

Epson classified the entire frequency spectrum divided into three bands as a matter of practical convenience: the low frequency band, medium frequency band, and high frequency band. Frequency pulling range

This is the range in which the frequency can be changed by input to the Vc pin in a voltage controlled crystal oscillator.

Reference: Voltage controlled crystal oscillator

Frequency stability (Frequency tolerance)

Frequency stability refers to defining a crystal oscillator's specified tolerance ranges (including the tolerances for frequency-temperature characteristics and frequency-voltage characteristics) (Δ f) for specified output frequency (fo) within the specified temperature conditions and operating voltage range. It is usually expressed as a ratio (Δ f/fo).

Frequency versus temperature characteristics

This refers to the variation in frequency due to ambient temperature changes, relative to the reference frequency at +25 ℃. The frequency-temperature characteristics are generally expressed by the following approximate formula.   Example of tuning fork crystal unit's frequency-temperature characteristics Example of AT crystal unit's frequency-temperature characteristics Example of SAW resonator's frequency-temperature characteristics
1. Tuning fork crystal unit. SAW Resonator.
f_tem=B (Ti-θX)2 θX:specified temperature
2. AT crystal unit.
f_tem＝α (θX-25)+β (θX-25)2＋γ (θX-25)3
3. SAW resonator, SAW oscillator:Δf/f = a(θT-θX)2

Seiko Epson offers SAW resonators/oscillators with a normal level of frequency-temperature characteristics as well as those with a half coefficient of frequency-temperature characteristics (New SAW: NS-32R, EG-2121, EG-2102).

Frequency tolerance

The frequency tolerance, when applied to the frequency specification of the crystal units and SAW resonators, indicates the deviation (Δf) of the actual frequency (measured value) from the nominal frequency value (f) within the +25 ℃ ambient temperature and specified conditions. It is usually expressed as a ratio (Δf/f).

Reference: Frequency stability

Frequency voltage characteristics

Using the output frequency at the central value of the operating voltage range as a reference, frequency voltage characteristics describe variation in output frequency. Variation in frequency due to variation in voltage can be caused by changes in crystal strain or changes in the internal constants of the IC that is built in a crystal oscillation circuit or real time clock. The IC generally plays the greatest role in causing this variation.

Fundamental mode

This is the first (fundamental) vibration order among the vibration orders excited by an AT vibration mode. The stable oscillation provided by the fundamental order makes it easier to design a crystal oscillation circuit than if a higher (overtone) oscillation order were used.  Fundamental mode (Reference)Third overtone mode

Reference: Thickness shear vibration , Overtone mode