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How much do you know about the classification of commonly used crystal oscillators?

2022-08-17

01 What is SPXO?

SPXO (Simple Packaged Crystal Oscillator), commonly known as OSC or XO, is a simple crystal oscillator and is widely used. The frequency control element is a quartz crystal, and no other control and compensation means are adopted. Its temperature characteristics are completely determined by the temperature characteristics of the quartz chip.

Typical AT-CUT CRYSTAL temperature characteristics curve

By adjusting the cut angle of the crystal, a smaller frequency-temperature deviation can be obtained.

Frequency-temperature characteristic curve of SPXO

The frequency-temperature characteristic curve of SPXO is basically consistent with the curve of CRYSTAL it uses.

Application fields of SPXO

Mainly used in communication, measurement, instruments, meters, remote control, telemetry, and other technical fields.

Application industries cover: post and telecommunications, aviation, aerospace, communications, military, metrology, medical care, industrial control, transportation, etc.

02 What is VCXO?

VCXO (Voltage Controlled Crystal Oscillator) is a crystal oscillator whose output frequency is biased or modulated by an external voltage. When the voltage of the control pin is changed, the output frequency will change according to a certain slope. This slope is usually a positive slope, that is, as the control voltage increases, the frequency increases. The temperature characteristics of VCXO are the same as those of PXO and are determined by the temperature characteristics of the quartz chip inside.

VCXO’s traction capabilities

Definition method 1: Absolute traction range (APR). He defined the sum of the deviation between the output frequency of the crystal oscillator and the nominal frequency and the effects of frequency stability, voltage changes, load changes, aging, etc.

Definition method 2: only refers to the difference between the frequency deviation affected by the control voltage and the frequency deviation under the reference voltage, and does not take into account factors such as temperature, Vdd, load changes, aging, etc.

Frequency-temperature characteristic curve of VCXO

The frequency-temperature characteristic curve of VCXO is basically consistent with the curve of CRYSTAL used in it. Under different control voltages, its temperature-frequency curve shifts.

03 What is TCXO

TCXO (Temperature Compensated Crystal Oscillator) is an oscillator that uses analog or digital compensation methods to compensate the frequency-temperature characteristics of the crystal by using changes in the crystal load with temperature to reduce its frequency-temperature offset.

VCTCXO (Voltage Controlled Temperature Compensated Crystal Oscillator) is an oscillator that adds a voltage control function on the basis of TCXO. It can change the output frequency by changing the control voltage and has a lower frequency-temperature deviation.

How TCXO works

Through the change of load reactance at different temperatures, the frequency-temperature deviation of the chip is compensated, so that the frequency-temperature deviation of the oscillator is very small.

Frequency-temperature characteristic curve of TCXO

Through the temperature compensation network, the output frequency-temperature deviation of the oscillator can be controlled within ±0.5PPM.

04 What is OCXO?

A crystal oscillator that places a quartz wafer in a constant temperature bath to keep the temperature of the wafer basically constant. It usually uses two types of quartz wafers, AT-cut and SC-cut, to make the oscillator.

OCXO basic principles

For AT-CUT wafers, there are two points with a temperature coefficient of 0, one in the low temperature area, usually below zero, and the other in the high temperature area. Finding this 0 temperature coefficient point in the high temperature zone and placing the wafer in a constant temperature bath at this temperature will achieve very high stability.

For SC-CUT wafers, there is a 0 temperature coefficient point in the high-temperature area, and the curve at this point is flatter than that of AT-CUT, and it can achieve higher stability than AT-CUT.

Typical application fields of OCXO