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Explanation of TCXOs

1 Temperature-Compensated Crystal Oscillators (TCXO)

When specified voltage is fed, a temperature-compensated crystal oscillator can generate a stable frequency even with changes in ambient temperature. For major applications, it may be used as a standard oscillator for mobile communication equipment's frequency synthesizers. In the mobile communication [PDC (Personal Digital Cellular), GSM (Group system for Mobile communication), and etc.], it is required to assure high stability of ±2.5µ10-6 while frequency tolerance is kept within the range of operating temperatures. Such characteristic requirements cannot be met only by an ordinary AT-cut crystal unit (See Fig.2 on "Crystal Reference" page). In addition, each crystal unit has a little different temperature characteristics since they can change with a cutting angle of the crystal plate. To obtain an oscillator that has high frequency-temperature stability, it is necessary to make compensation, employing a temperature compensation circuit in the oscillator circuit. The crystal unit has a characteristic of changing frequency when a capacitor is inserted in series with the crystal unit, as shown in Fig.7. Utilizing the characteristic, frequency a can be stabilized by inserting temperature compensation circuit, consisting of thermistors, resistors and capacitors, in the oscillation loop as shown in Fig.8. Example frequency-temperature characteristics of temperature-compensated crystal oscillator are shown in Fig.9.

2 Voltage-control Temperature-Compensated Crystal Oscillators (VC-TCXO)

The adjustment of the output frequency is a possible temperature - compensated crystal oscillator in the external control voltage as shown in Fig.10.

3 Phase noise

Taping and Reel Specifications

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Glossary

Center Frequency
A specified reference frequency of the crystal typically denoted in megahertz (MHz) or kilohertz (kHz).

CMOS
Abbreviation for "Complementary Metal Oxide Semiconductor."

Control Voltage
Range of voltage used to pull the frequency.

Duty Cycle
Measurement of output waveform's uniformity; also referred to as symmetry. Measurement of the time in which, the output waveform is in a logic high state, expressed in percentages (%). Must be measured at a specified voltage or at a percentage of the output waveform amplitude.

ECL
Abbreviation for "Emitter Coupled Logic."

Enable/Disable Function
Control function which, enables or disables the output of the oscillator. When the oscillator is in its disabled state, the oscillator is still operating, but no output is coming from the output pin.

Enable/Disable
Time Time between application of the proper voltage to the enable/disable pin

Fall Time
Amount of time it takes the output voltage to go from Logic '1' to logic '0'.

Input Current
Amount of current consumption by an oscillator from the power supply, typically specified in milliamps (mA).

Linearity
Uniformity of the shift in frequency over voltage change, when pulling frequency of a VCXO.

Load Drive Capability (Fan Out)
Maximum load the oscillator can drive. Specified in terms of the number of gates or type of load circuit.

Load Impedance
Impedance presented by a load connected to a generator or any other source.

Logic Levels
Defined as the Output Voltage Logic High or "Logic 1" and the Output Voltage Logic Low or "Logic 0"

Oscillator
A circuit that produces an alternating current of a specific frequency at its output terminals

Output Current
Represented by IOL and IOH; it is the output current at VOL and VOH.

Output Load (Fan out)
Capability of the oscillator to drive other devices. TTL devices are specified in the number of gates that can be driven; i.e., 10 TTL gates. CMOS outputs are specified in pF, i.e., 15pF or 50pF loads.

Output Voltage
Represented by VOL; it is the maximum voltage that output low will be and VOH is the minimum voltage that output high will be.

Overlap Time
Time measured at 50% Vp-p, where the two outputs of a complementary output ECL oscillator overlap.

Pin Through
Only applies to packages with leads. When the device is mounted on a PCB, the leads go through the board.

Pullability
Amount the frequency that can deviate by varying the voltage on the control pin.

Rise Time
Amount of time it takes the output voltage to go from Logic '0' to logic '1'.

Sine wave
Periodic wave which, is represented by a sine curve. The amplitude of such wave is a function of the sine of a linear quantity such as phase or time.

Square Wave
An alternating or pulsating current or voltage whose wave shape is square.

Stability
Maximum amount of frequency deviation of the oscillator; which, includes the tolerance at 25°C; drift over operating temperature range; changes in input voltage; changes in load, shock, vibration; and aging.

Standby Current
Amount of current consumed by the oscillator when it is in a standby mode.

Standby Function
A control function like the Enable/Disable function. In this case, the oscillator actually stops oscillating. This type of oscillator draws less current than an Enable/Disable type oscillator (when both are in their disabled states).

Start-up Time
Period from the instant when a voltage is applied to the oscillator until the oscillator output is stabilized.

Supply Voltage
Voltage required for the oscillator to operate within specification.

Surface Mount
Describes a package with pads that mount to the surface of the PCB.

Symmetry
Measurement of the uniformity of the output waveform

T.C.X.O
Abbreviation for "Temperature Compensated Crystal Oscillator." This type of oscillator contains a temperature sensing network that adjusts the output frequency, thereby allowing the frequency to remain constant over a specified temperature range. Very often referred to as precision oscillator.

Tri-State Output
The ability to turn the output on or off using pin # 1 for control. The output will go to high impedance when disabled, which facilitates the use of Auto Test Equipment (ATE). Tri-state can be substituted for non-tri-state if pin #1 is left open or held high.

TTL
Abbreviation for "Transistor-Transistor Logic."

V.C.X.O
Abbreviation for "Voltage Controlled Crystal Oscillator." This type of oscillator contains a network that employs changes in voltage to change the output frequency.

Warm Up
Time required for an oscillator's frequency to settle within a given tolerance of the frequency after several hours.