# Application Notes

■ **Quartz Crystal
Notes**

A
quartz crystal resonates when an

electrical potential is applied to its surfaces, causing mechanical deformation(vibration). This phenomenon is called the piezoelectric effect. When metal electrodes are placed on both sides of a crystal plate, the piezoelectric response between the electrodes can be

expressed as the equivalent electrical parameters shown in Figure 1.

■ **Mode of Oscillation**

The Oscillation Mode
for AT cut quartz crystals is at the fundamental mode or and odd frequency
harmonic of the fundamental frequency. Standard 3rd overtone mode,
followed by 5th, 7th ,9th, etc.

■** Frequency Tolerance
@25℃**

Frequency Tolerance is the minimum and the maximum
frequency deviation allowed from the Target Frequency @25℃. This deviation is usually specified in ±ppm (
parts per million ).

■** Frequency Stability**

Frequency Stability is
the amount of frequency deviation that will occur over the Operating Temperature Range with respect to the frequency @25℃.This deviation is often associated with other
specified operating conditions such as Load Capacitance and Drive Level..
Temperature is a major influence on crystal frequency.

■ **Equivalent Series
Resistance(ESR or R _{1})**

ESR is the resistance (in Ohms) that the crystal exhibits at

resonance. Equation (1)

■ **Shunt Capacitance**

Shunt capacitance (C_{0})
is the capacitance between the crystal terminals. It varies with package,
usually it is smaller in SMD (4pF typical) and is 6pF in leaded crystals.

■ **Load Capacitance **

Crystals can be calibrated by
the manufacturer at either fr, where they appear resistive (or fs which is very
close to fr),or for resonance with a capacitive load, where of course
they must appear inductive. The latter condition is called load –resonant and
is represented in general terms by the symbol fL or ,more specifically, the
symbol f 30 would , for example , represent the frequency at which the crystal
is at resonance with a 30 pF capacitor.

■ **Motional
Capacitance (C _{1})**

Motional Capacitance (C

_{1}) is a parameter largely controlled by the design of the electrode size and shape. When C1 is specified, it should be specified with a maximum and a minimum value in pf of fF. C

_{1}has physical design limitations due to constraints in quartz blank size, mode of operation and nominal frequency. L

_{1}is usually not specified because it is virtually specified by C

_{1}due to the absolute relationship shown in Equation 2.

Equation ( 2 )

■ **Storage Temperature Range**

The Storage Temperature Range is the absolute limits of temperature to which
the device will be exposed in a non-oscillation state.

■ **Pullability and
Change of Load Capacitance****
Frequency **

change as a function of load
capacitance C_{L} in a parallel resonant crystal. Pullability is a
function of shunt capacitance C_{0}, motional capacitance
C_{1}, and size of crystal. When a

crystal is operating at parallel
resonance (Fs<Fr<Fa),it looks inductive in the circuit. As the
reactance changes, the frequency changes correspondingly,
thus change the pullability of the crystal.See
Equation (3) .The same crystal with frequency at 3rd-overtone mode
will have much less pulling because its ’motional
capacitance C_{1}` is approximately 1/9 of C_{1} at fundamental.

Equation (3)

■ **Spurious**

Unwanted
resonances usually above the operating mode, specified in dB max. or number of
times of ESR. Frequency range must be specified. In oscillator applications, it
is necessary to control unwanted modes as lower as possible to prevent circuit
oscillating in the “spurious mode ”.See Figure 3. The design of large
electrodes on crystal to produce large pulling is a common
cause of prompting spurious. A resistance ration of 2:1 or minimum of 3db
separation

is usually adequate.

■ **Aging**

Aging is the change in
operating frequency over a certain

period of time .It is usually expressed as a maximum value in ppmper year. Typical crystal aging:±5ppm per year max..

■ **DLD(Drive Level
Dependency)**

To change a crystal of drive level that will change the frequency or
resistance, the effect is called DLD Usually DLD is a ration between the
largest resistance measured over a user defined power range ,and the
resistance at the nominal power . DLD is a good measure of internal
cleanliness of crystals.

■ **Oscillator Notes**

A crystal oscillator is
a timing device that consists of a crystal and an
oscillator circuit, providing an output waveform at a specific
frequency. When a crystal is placed into an amplifier circuit(as
shown in Figure 4 ),a small amount of energy is feedback to the
crystal, which causes it to

vibrate. These vibrations act to stabilize the frequency of the oscillator circuit.

■ **Supply Current(I _{CC})**

The current flowing into Vcc terminal with respect to ground. Typical supply current is measured without load.

■ **Supply Voltage(V _{DC }max) **

The maximum voltage which can safely be applied to the V

_{DC}terminal with respect to ground .

■ **Symmetry ( Duty
Cycle )**

Symmetry is a measurement of the time that the output waveform is in a logic
high state, expressed as a percentage (%) of the complete cycle. A typical
symmetry tolerance is 40/60%. Tight symmetry is considered to be 45/55%.

■ **Rise / Fall Time**

Rise Time is a measure
of the

transition time from a “Logic 0” to a “Logic 1” level.Fall Time is a measure of the transition time from a “Logic 1” to a “Logic 0” level. Both Rise and Fall Time are typically specified as a maximum transition time in ns. Typical rise and fall time for CMOS 4000 series is 30ns, HCMOS is 6ns, and for HCMOS/TTL compatible) is 3 ns max..( See Figure 5 )

■ **Output load**

Output Load is the
maximum load an

oscillator can drive. It is specified in terms of number

of gates or type of load circuit. An HCMOS load is usually specified as a capacitive load in pF. TTL loads are specified as a number of TTL gates.

■ **Start-up time**

Start-up time is the
delay time between the oscillation starts from noise until
it reaches its full output amplitude when power is applied.
The start-up time varies from microseconds to milliseconds
depending on frequency, ASIC speed and logic. See figure 6.

■ **VCXO ( Voltage
Controlled Crystal Oscillator )**

A VCXO is an oscillator that allows the user to vary the Output Frequency by varying a Control Voltage applied to pin 1.