Quartz crystal, composed of silicon and Oxygen (Silicon Dioxide). In
1880, Curie brothers of France discovered the piezo electric propeties
in quartz. Piezoelectricity means when a pressure is applied on the
surface of the crystal, it generates an electrical potential, conversely
when an alternating electric field is applied to the surfaces of crystal,
mechanical vibration or deformation is generated. Base on this properties
of quartz crystal and its extremely stable operation, it becomes an
indispensable electronic components of frequency control elements. GXC
is manufacturer of Lumbered Quartz Bars, Crystal Wafers, Crystal Blanks
and Crystal Resonator Units. Although we cannot expect our customer
to understand fully the knowledge about crystals, in order to correclty
specify the order, a basic understanding of some key glossary and manufacturing
process will help to ensure that user can get the right crsytal performance
for their applications.
PT. Great Microtama Electronis Indonesia (GXC) consistenly manufactures
Wafers, Blanks and Crystal Resonator Units from
high quality As Grown Quartz Bars. With continuous investment in people,
technology and expansion of production plant, manufacturing is able
to keep pace with increasing demand and gain the economies of scale
to ensure a competitive edge of world market.
3. Mode of Vibration and Orientation Angle
Usually quartz crystal blanks vibrate in several simultaneous resonance
modes as shown in table 1 (page 3), in the table you can see the relation
between the mode of vibration, cut angle, frequency range, formula between
wafer dimension and frequency, and capacitance ratio. Fig. 1 and fig
2 show the cut angle of a right handed quartz crystal at which frequency-temperature
coefficient of crystal unit becomes zero near normal room temperature
for the modes of vibration most often used.Following is flow chart of
Fig 1 : Most commonly used cut angle of a right handed quartz
Fig 2 : Frequency-temperature characteristic of some
common crystal cuts
4. Frequency-Temperature Characteristic
Fig 3 shows the Frequency-Temperature Characteristic for a thickness-shear
mode AT-CUT crsytal with the angle of cut as a parameter, since the AT-CUT
Frequency-Temperature Characteristic is approximate to an equation of
the third degree, it exhibits excellent frequency stability over a wide
Note - Broken line
represents loci of temperatures at which Af/AT=0
Fig. 3 : Frequency / temperature curves generalized (AT-cut)
5. Equivalent Circuit Parameters of Crystal Unit
A Crystal Unit consists of a quartz Resonator Blank with metal plating
as electrode. This plating, as shown in Fig 4, is located on both sides
of the crystal blank and is connected to insulated lead on the crystal
package. The Crystal Unit exhibits a piezoelectric response between the
two crystal electrodes as expressed in the simplified equivalent circuit
shown in Fig 5. This simplied equivalent circuit is very useful to explain
the basic concepts governing the crystal's behaviour and performance.
Co represent the Shunt (static) Capacitances and is the sum of the capacitance
between the electrodes and capacitances added by the wire leads and holder.
The R1, L1, C1 elements are referred to as the motional parameters of
the crystal Unit, the mechanical elasticity of the quartz is represented
as a Motional Capacitance C1, the vibrating mass of the crystal is equivalnet
to a Series Motional Inductance L1 and the mechanical losses of the crystal
occur as an Equivalent Series resistance (ESR) R1.
Figure 4 : Inside view of crystal unit & schematic symbol
Figure 5 : Crystal unit simplified equivalnet circuit
6. Series Resonance Frequency (fr)
Crystal unit can be calibrated to two slightly different frequencies (see
figure 6:fr and fa). The Lower one which is called Series Resonance Frequency
(fr) is one of the two frequencies of the crystal unit alone under specified
confoditions, at which the electrical impedance of the crystal unit is
resistive (see figure 6). Figure
6 : Plot oif reactance VS frequency for a crystal unit
7. Anti Resonant (Parallel Resonant)
Frequency (fa) : The higher one which is called anti resonant
or parallel resonant frequency (fa) is one of the two frequencies of a
crystal unit alone, under specified conditions, at which the electrical
impedance of the crystal unit is resistive (see figure 6)
8. Load Resonance Frequency (fL)
Series Resonance or Parallel Load Resonance Crystal Units are physically
the same crystal, but are calibrated to slightly different frequencies.
When a crsytal is placed into an oscillator circuit, they oscillate together
at a tuned frequency. This frequency is dependent upon the crystal design
and the amount of load capacitance (Cl) if any, the oscillator circuit
presents to the crystal. Load capacitance is composed of a combination
of the circuits discrete load capacitance, stray board capacitance, and
capacitance from semiconductor Miller Effects. The presence of load capacitor
shifts the working frequency of the crystal by an amount depending upon
the value of CL and the value of C0 and C1. The figure 7 show the load
resonance frequency (fL) according to series and parallel connections
respectively. For a given value of load capacitance (CL), these Load Resonance
Frequency (fL) is :
9. Load Capacitance (CL)
A standard oscillator with CL, parallel to the crystal as shown in figure
8. The total capacitance external to the crystal is called load capacitance.
The crystal manufacturer needs to know the value of CL in order to adjust
to the specified frequency, the load capacitance is given by : 10.
The pullability of a crystal describes how the operating frequency may
be changed by varying the load capacitance (CL). An approximation of the
pulling limits or load resonance frequency offset can be obtained from
the formula :
No CL is connected
to this the crystal
CL series to the crystal
:fa is uneffected, but fr moves up to a frequency fL
CL parallel to the crystal : resonance
frequency fr is not affected but the antiresonance fa shifts down
to the frequrecny fL.
Figure 7 : Load resonant frequency (fL) when CL is connected
to crystal unit
Figure 8 : Load
capacitance (CL) parallel connected to the crystal unitFractional load
resonance frequency offset DL can be easly calculated
Frequency pulling range
A f L1, L2 between two load capacitance CL1 and CL2 can be obtained using
the formula Fractional
pulling range is as following :
11. Pull-Sensitive (S)
Pull-Sensitivy is defined as the incremental fractional frequency change
for an incremental change in the load capacitance, the formula is :
12. Quality Factor (Q)
The "Q" of a crystal unit is the Quality Factor of the motional
arm at resonance. The maximum stability that can be attained by the crystal
unit is directly related to Q, the higher the Q the smaller the bandwith
and the steeper the reactance slope (fr-fa), see figure 6. External circuit
reatctance value changes have less on a high Q crystal (less pullability)
than lower Q devices :The formula of Quality Factor is : 13. Nominal
The frequency assigned by the specification of the crystal unit.
14. Working Frequency (fw)
The operational frequency of the crystal unit together with its associated
15. Frequency Tolerance or Calibration Accuracy
The amount of frequency deviation from a specified nominal frequency at
ambient temperature (referenced at 25ºC). This parameter is specified
with a maximum and minimum frequency deviation, expressed in parts per
16. Frequency Stability
The amount of frequency deviation from the ambient temperature frequency.
This deviation os associated with a set of operating conditions including
: operating temperature range, load capacitance and drive level. This
parameter is specified with a maximum and minimum frequency deviation
expressed in ppm.
The frequency stability is determined by the following factors : type
of quartz cut, angle of quartz cut, mode of vibration, and mechanical
dimensions of quartz blank.
17. Operating Temperature Range
The range of temperature as measured on the enclosure over with the crystal
unit must function though not necessary within the specified tolerances.
18. Operable Temperature range
The range of temperature as meassured on the enclosure over which the
crystal unitmust function though not necessary within the specified tolerances.
19. Storage Temperature
The minimum and maximum temperature that the crystal unit can be stored
or exposed to when in a non-oscillation state.
After exposing or storing the crystal unit at the maximum or minimum temperatures
for a lenght of time, all of the the operating specifications are guaranteed
over the specified operating temperature range.
20. Reference Temperature
The temperature at which certain crystal measurement are made, for controlled
temperature units, the reference temperature is the mid point of the controlled
temperature range. For non-controlled temperature units, the refrence
temperature is normally 25ºC ¹ 2ºC
21. Equivalent Series Resistance (ESR)
The resistive element of crystal unit, the ESR measurement is made only
at the series resonant frequency fr, not at some predetermined parallel
resonant frequency (fL), the ESR can be represented by a formula :Generally,
the lower the resistance value of crystal, the more active it is and less
drive is required to activate it. How ever, lower values need additonal
process to achieve it causing increase of unit price.
22. Load Resonance Resistance (RL)
the resistance of the crystal unit in series with a stated external capacitance
at the load resoannce frequency fL.
23. Drive Level
rated drive level is the amount of power dissipation in the crystal, usually
expressed in milliwatts or microwatts, it is fuction of the driving or
excitation current flowing through the crystal,therefore in special cases,
the level of drive may be specified in terms of crystal current or voltage.
Drive level can be calculated in the following formula :Where
I stands for current to pass in the quartz crystal Re for effective resistance
of quartz crystal and Re = R1 (1+C0/CL)2. Operating the crystal unit at
drive levels which are too high or too low can result in improper performance,
let say if the drive lvel (P) exceed the specified level, oscillation
frequency will shift , crystal activity dips, excessive aging, or in extreme
cases, physical failure of the resonator. This occurs because of excessive
level of power causes stress for the crystal and causing temperature rise.
If excessive drive level of power is applied to the quartz crystal unit,
this may deteriorate or damage its characteristics. On the other hand,
if the drive level is too low, the crystal unit may fail to oscillate
or have degraded phase nois performance (this is exhibited to a greater
degree in SC-Cut crystals).
Maximum power is the most power the crystal unit can dissipate while still
maintaining operation with all electrical parameter guaranteed. Drive
level should be maintened at the minimum level necessary to assure proper
start up and steady state vibration, thus avoiding poor aging characteristic
and crystal damage.
In addition, the maximum specified equivalent resistance of the crystal
is affected by and is measured at a predetermined drive level. Therefore
it is important to understand the effect of drive level on crystal performance
and to operate the resonator at a suitable drive level.
The changes in operating frequency over a certain period of time, normally
expressed as a maximum value of frequency change in parts per million
(ppm) per year or per day.
25. Spurious or Unwanted Modes
Vibration at frequencies which are not fundamental or overtone modes are
called Spurious or Unwanted modes. Spurious Modes is specified in terms
of minimum resisitance of the spurious mode, or in terms of minimum resistance
ration between the spurious mode and the main mode, or also in terms of
the trasmission response in dB. When specifying spurious mode suppression
it should be noted, not only can it add dramatically to the crystal cost,
but it sometimes can not be done without affecting other parameter such
as C1 and R1. Therefore if spurs are of your concern, please talk to us
before finalying your specification.