Crystal oscillators are generally classified into four types according to the difference in the general circuit.

Simple Packaged Crystal Oscillator (SPXO)

A simple packaged crystal oscillator is a crystal oscillator that is not temperature compensated or temperature controlled.
AT-cut crystal is generally used for the crystal unit, and the frequency temperature characteristic is a cubic curve.
The frequency temperature characteristics of the simple packaged crystal oscillator is almost the same as that of the crystal unit, resulting in a cubic curve.
Major applications are as reference oscillators for communications equipment, measuring instruments, controllers, etc. and as crystal clock oscillators for computers, office automation equipment, information terminal equipment, etc.

Temperature-Compensated Crystal Oscillator (TCXO)

Temperature compensated crystal oscillator is a crystal oscillator with a built-in temperature compensation circuit designed to obtain good frequency temperature characteristics over a wide temperature range.
There are two types of temperature compensation circuits: one uses a thermistor as a temperature-sensitive element in a temperature compensation circuit and combines it with a resistor and capacitor, and the other uses an element in an LSI as a temperature-sensitive element.
In the case of an LSI with a temperature compensation circuit, the whole oscillator circuit is usually composed of single-chip LSIs.
Figure 1 shows an example of frequency temperature characteristics of a temperature-compensated crystal oscillator.
TCXO is characterized by its excellent frequency temperature characteristics , low power consumption, compact size and light weight, and short start-up time.
It is used as a reference oscillator for various communication devices (mobile phones, GPS, land mobile radio, microwave communications, satellite communication systems) and measuring instruments such as frequency counters and synthesizers.
In addition, VC-TCXO (Voltage Controlled TCXO) with frequency control function (VC/AFC) by external voltage is also treated as TCXO.

Figure 1 Example of the frequency-temperature characteristic of a temperature-compensated crystal oscillator

Voltage-Controlled Crystal Oscillator (VCXO)

A voltage controlled crystal oscillator is a crystal oscillator in which a variable capacitance diode is inserted in series with a crystal unit, and the capacitance of the diode is varied by externally applied voltage to obtain a frequency variable amount according to the load capacitance characteristics of the crystal oscillator.
Our products achieve a wide range of variable amounts and good frequency linearity by considering the combination of the applied voltage of the variable capacitance diode, the capacitance characteristics of the crystal resonator, and the oscillation frequency.
The temperature characteristic is basically the same as that of the simple packaged crystal oscillator (SPXO), and is the sum of temperature characteristic of the crystal unit and the temperature characteristic of the oscillator circuit, resulting in a cubic curve.
It is mainly used for synchronization (PLL) and demodulation (jitter filter) of transmission equipment and switching equipment.

Oven-Controlled Crystal Oscillator (OCXO)

Oven Controlled Crystal Oscillator is a crystal oscilator in which the ambient temperature of the crystal unit is maintained at a constant level by a thermostatic oven. Compared to other crystal oscillators, the temperature characteristics are several orders of magnitude better.
The temperature characteristics of the crystal oscillator are mostly determined by which part of the crystal oscillator is housed in the thermostatic oven and the temperature characteristics of the thermostatic oven, as the temperature inside the oven is set to the zero temperature coefficient point (inflection point of cubic curve) of the crystal unit.
The lower limit of the operating temperature range is determined by the maximum power consumption, while the upper limit is determined by the temperature set in the thermostatic oven and the thermal contribution of the circuit power consumption.
Oven Controlled Crystal Oscillator are also commonly referred to as high-stability crystal oscillators.
The crystal units used in these oscillators are developed and manufactured using advanced technology under extremely favorable environmental conditions.
By using these excellent crystal units, crystal oscillators with excellent frequency aging characteristics, short-term stability, frequency repeatability, phase noise, and temperature characteristics can be manufactured.
Oven Controlled Crystal Oscillator are ideal as reference oscillators for various communications equipment such as mobile base stations, frequency counters, spectrum analyzers, and other electronic measuring instruments.