CRYSTAL TERMINOLOGY
Aging:
Also referred to as long‐term stability. A measure of the
frequency stability of the crystal
over an extended period of time and is usually expressed in terms of parts per million (ppm) per
day or per year. Aging normally follows an exponential progression so that most aging takes
place within the first few weeks of manufacture.
Allan Variance: Also known as short‐term stability, this is the measure of oscillator stability in
the time domain. Commonly referred to as the Allan variance, it measures the RMS change in
successive frequency measurements for short gate times (milliseconds to seconds) and is
important in timing applications. It typically improves as the gate time increases until it becomes
a measure of the medium to long term drift of the oscillator. This drift is either the result of the
temperature coefficient of the oscillator, and/or the aging.
Angle: It is the angle at which a resonator plate is cut from the quartz stone in relation to the
original crystallographic axes. The angle of cut is critical to the performance of the crystal unit,
particularly for frequency deviation over a temperature range.
AT cut: The commercial designation for a specifically oriented resonator plate, having desirable
and repeatable operating characteristics. The plate is cut from a crystal of quartz such that the
plate contains the X‐axis and makes an angle of about 35 degrees with the optic or Z‐axis. The
"AT cut" is the most popular crystal unit manufactured today.
Bevel: A modification to one or both of the major faces of a resonator plate in which the face is
altered to have a partially spherical configuration.
Blank: A round or rectangular quartz crystal that has been lapped to produce parallel major
surfaces and has minor surfaces machined to the final dimensions required to build the desired
resonator.
BT cut: The commercial designation for a specifically oriented resonator plate, having well
known and repeatable characteristics.
Burst Noise: Burst or “popcorn” noise is believed to be connected to surface defects of the
resonator (deep pits or scratches). Its source is confined to a very small area of the crystal.
C.I.: An abbreviation for "crystal impedance," also referred to as "resistance."
C0: An abbreviation for "Shunt Capacitance."
C1: An abbreviation for "Motional Capacitance."
Calibration Tolerance: The tolerance, in parts per million (ppm), to which the crystal
manufacturer will set the resonator frequency during manufacture. This tolerance is always
specified at a particular reference temperature (e.g. +/‐ 10PPM @+25C).
Can: The upper portion, or cover, of a crystal holder.
Capacitance: The property exhibited by two conductors separated by a dielectric where an
electric charge becomes stored between the conductors. Capacitance is measured in "farads"
and denoted by the letter "C."
Cold Weld: Cold welding is a solid‐state welding process in which joining takes place without
fusion at the interface of the two parts to be welded. Unlike in the fusion‐welding processes, no
liquid or molten phase is present in the joint. This is recommended for crystals facing diverse
and extreme environments.
Contour: A modification to one or both of the major faces of a resonator plate in which the face
is altered to have a completely spherical configuration.
Crystal cut: The orientation of the crystal element with respect to crystallographic axis of the
crystal.
Drive Level: The Drive Level is the amount of power dissipation in the crystal, expressed in
microwatts or milliwatts. Maximum power is the most power the device can dissipate while still
maintaining operation with all electrical parameters guaranteed. Drive level should be
maintained at the minimum levels necessary to initiate proper start‐up and assure steady state
oscillation. Excessive drive level can cause poor aging characteristics and crystal damage.
Etch: A method to improve the surface condition of a crystal and to increase the frequency of a
blank.
FC cut: This cut has an improved temperature and frequency characteristic for ovenized
applications (OCXO). The frequency vs. temperature curve is a sine with the inflection
temperature at ~ +52"C. Please contact our factory regarding your specific requirements.
Frequency Stability: The amount of frequency deviation from the ambient temperature
frequency over the operating temperature range. This deviation may be influenced by a set of
operating conditions such as: Operating Temperature Range, Load Capacitance, and Drive Level.
Frequency stability is specified with a maximum and minimum frequency deviation, expressed
in percent (%) or parts per million (ppm). The cut of the crystal and angle of the cut will
determine the frequency stability. Some secondary factors influencing frequency stability are
the mode of operation, the drive level, the load capacitance, and mechanical design.
Fundamental: The lowest frequency at which a given crystal will oscillate.
G‐Sensitivity or Vibration Sensitivity: Vibration sensitivity refers to the degradation of the close
in noise performance of a crystal under the influence of external mechanical vibrations. Please
inquire with our factory about your requirements.
Holder: The complete housing for a quartz resonator plate.
Impedance: The total opposition presented by a circuit or device to the flow of alternating
current. Impedance is measured in "ohms" and denoted by the letter "Z."
Inflection Point: AT Cut crystals have a temperature vs. frequency characteristic that can be
represented by a third order polynomial. This curve has a point where the slope is zero and the
slope is positive on one side and negative on the other side. This point is defined as the
inflection point.
IT cut: This crystal cut has an improved temperature and frequency characteristic for ovenized
applications (OCXO). The frequency vs. temperature curve is a sine with the inflection
temperature at ~ +78°C. Please contact our factory regarding your specific requirements.
L1: Abbreviation for “Motional Inductance.”
Lapping: Moving a quartz crystal slab over a flat plate on which a liquid abrasive has been
poured, to obtain a flat polished surface or to reduce the thickness a carefully controlled
amount.
Load Capacitance: The value of capacitance used in conjunction with the crystal unit. Load
capacitance is a parameter normally set by the customer, typically expressed in pF (picoFarads).
Long ‐Term Stability (Aging): Long‐term stability is a measure of the frequency stability of the
crystal over an extended period of time and is usually expressed in terms of parts per million
(ppm) per day or per year. Aging normally follows an exponential progression so that most aging
takes place within the first few weeks of manufacture.
Microphonic Noise: Vibration‐induced noise in the otherwise frequency‐independent noise
floor range (up to 30kHz). It consists of discrete spurious peaks that are usually the result of the
crystal resonator and support resonances. Microphonic noise can be significantly reduced by
proper choice of resonator cut and geometry, bonding techniques and support configuration.
Modes of Vibration: Quartz crystals naturally vibrate in several simultaneous resonance modes
referred to as the fundamental or overtone modes. Usually one of these modes is designed to
be dominant at the desired operating frequency. The fundamental frequency of vibration is a
function of the resonator physical dimensions and angle of cut while the overtone modes occur
at odd numbered harmonics of the fundamental mode and include the 3rd, 5th, 7th, 9th and 11th
harmonics.
Motional Capacitance: Abbreviated as "Cm" or "C1", motional capacitance illustrates the
electronic equivalence of the mechanical elasticity of the unit.
Motional Inductance: Abbreviated as "Lm" or "L1", motional inductance illustrates the
electronic equivalence of the mechanical mass of the unit.
Nominal frequency: The specified "name plate frequency" of a crystal or oscillator.
Operating Temperature Range: A range of temperatures over which the crystal will meet the
specified frequency stability.
Ovenized crystals: Any crystal designed to operate at a temperature above the anticipated
ambient temperature (typically +50°C to +110°C) in order to eliminate changes in frequency due
to the change in temperature.
Overtone: The odd numbered multiples of the fundamental frequency, including the 3rd, 5th, 7th,
9th and 11th harmonics.
Parallel Resonant: One of the two modes a crystal will vibrate in. represented by ‘fa.’
Phase Noise: Phase noise is a term used to describe instability in the phase or frequency of a
crystal unit in periods of time of a few seconds or less. It is measured as the ratio of power in the
noise to that in the carrier at a specified offset frequency (Fourier frequency) in a specified
bandwidth. The measurement bandwidth is usually normalized to 1Hz. By knowing the inherent
phase noise of a crystal resonator, oscillator designers can predict the lowest oscillator phase
noise attainable using that resonator.
Plate: A quartz blank or resonator.
Polish: The process used in the manufacture of some types of quartz crystals resulting in a very
fine surface finish.
Pullability: The pullability of a crystal describes how the operating frequency may be changed by
varying the load capacitance. The pullability specification helps you decide how much trimming
will be required to compensate for circuit component variations. Since there are several
methods in which to express crystal pullability, please consult our factory for product
specifications.
Q: Quality Factor is the ratio of energy stored in a system divided by the energy dissipated in the
system. The quality factor characterizes the acoustic loss in quartz crystal resonators.
Quartz Crystal: Synthetic quartz is composed of Silicon and Oxygen (Silicon Dioxide SiO2) and is
cultured in autoclaves under high pressure and temperature. Quartz exhibits piezoelectric
properties that generate an electrical potential when pressure is applied on the surfaces of the
crystal.
Resistance Weld: Procedure involving pressure sealing with electricity and back filling with
nitrogen to force out oxygen and moisture. This process provides superior aging characteristics.
Resistance: The opposition to current flow in a circuit represented by the letter "R" and is
measured in "ohms."
SC‐Cut: A doubly rotated crystal cut plate (theta = 34 degrees and seven minutes and phi = 21
degrees and fifty six minutes) that is used in precision oscillators. It is placed in an oven and
operated at the crystal’s lower turning point, around 100° C.
Series Resonant: One of the two modes a crystal will vibrate in. represented by ‘fr.’
Short‐term Stability: The measure of oscillator stability in the time‐domain. Commonly referred
to as the Allan variance, it measures the RMS change in successive frequency measurements for
short gate times (milliseconds to seconds) and is important in timing applications. It typically
improves as the gate time increases until it becomes a measure of the medium to long term drift
of the oscillator. This drift is either the result of the temperature coefficient of the oscillator,
and/or the aging.
Shunt Capacitance (C0): Shunt capacitance is the element in the resonator equivalent circuit
representing the electrostatic (parallel‐plate) capacitance of the electrodes plus the holder
capacitance. Shunt capacitance is also referred to as static capacitance.
SMD: Abbreviation for “surface mounted device.”
Spur: A substitution for the term "Spurious Frequency Response," a frequency occurring at some
point higher than the desired mode but lower than the next overtone.
Spurious or Unwanted Modes: Variations at frequencies which are not fundamental or
overtone modes are referred to as spurious or unwanted modes. These unwanted responses are
influenced by many factors, including the dimension of the quartz wafer, the surface finish, the
size and thickness of the electrode and the mounting technique.
Tape and Reel: Refers to the packaging method used to accommodate automated pick‐andplace
equipment.
Temperature Stability: The measure of the frequency change due to temperature changes. It is
measured by placing the oscillator in a temperature chamber and allowing it to stabilize. After
the frequency is measured, the temperature is changed and the sequence is repeated until the
desired temperature range has been covered. The most common method of specification is
from the room temperature value, as the oscillator is normally calibrated at room temperature.
X‐Axis: Known as the “electrical axis,” the X‐axis is parallel to a line bisecting the angles between
adjacent prism faces.
Y‐Axis: Known as the “mechanical axis,” the Y‐Axis runs at right angles through the prism face as
well as at right angles to the X‐Axis.
Z‐Axis: Known as the “optical axis” and is an axis of threefold symmetry. All physical properties
repeat each 120 ̊ as the crystal is rotated around the Z‐axis.