RFOCXO vs VCXO vs TCXO vs DCXO: Oscillator Differences Explained
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This article delves into the distinctions between four types of crystal oscillators: OCXO, VCXO, TCXO, and DCXO. Crystal oscillators are vital components, serving as frequency references to generate stable frequency sources in various applications. These include frequency synthesizers, local oscillators, and providing stable clocks in modems and other circuits.
However, factors like temperature variations, supply voltage fluctuations, and aging can cause the crystal frequency output to drift. The most significant contributor to frequency drift is temperature change, potentially leading to shifts of approximately 10 PPM (parts per million). Using OCXOs and TCXOs can significantly improve stability, achieving levels around 0.0001 PPM.
OCXO: Oven Controlled Crystal Oscillator
An OCXO, or Oven Controlled Crystal Oscillator, is designed to operate within a temperature-controlled oven. This oven maintains a constant temperature for both the crystal and the oscillator circuit itself.
The stability of an OCXO surpasses that of other crystal oscillator types. Consequently, OCXOs find applications in satellite systems, telecommunications, and other areas demanding exceptionally stable frequency reference sources. OCXOs are available with different output waveforms, including sine, square, and HCMOS. They also come in various supply voltage options, such as 12 V, 5 V, and 3.3 V.
VCXO: Voltage Controlled Crystal Oscillator
A VCXO, or Voltage Controlled Crystal Oscillator, allows control of the crystal output frequency using a DC voltage. In mobile phone designs, VCXOs are often interfaced with a DAC (Digital-to-Analog Converter).
The DAC, in turn, is connected to a microcontroller. The microcontroller provides digital input, which the DAC converts to an analog voltage output. This analog voltage then adjusts the VCXO output. Therefore, any frequency deviations can be compensated for using the voltage control pin.
TCXO: Temperature Controlled Crystal Oscillator
A TCXO, or Temperature Controlled Crystal Oscillator, is designed to operate across a broader range of ambient temperatures.
TCXOs incorporate a thermistor that continuously monitors and compensates for changes in the surrounding temperature, thereby stabilizing the crystal oscillator’s frequency.
Many GSM handsets utilize voltage-controlled temperature-compensated crystal oscillator (VC-TCXO) modules as their system reference oscillator. VC-TCXO modules employ analog techniques to correct frequency deviations. However, VC-TCXOs tend to be more expensive, have a larger footprint, and require external components on the circuit board. These factors can pose challenges for the design of smaller and more affordable mobile handsets.
DCXO: Digitally Controlled Crystal Oscillator
A DCXO, or Digitally Controlled Crystal Oscillator, presents an alternative solution to the drawbacks of VC-TCXOs. Modern GSM mobile phone manufacturers are increasingly adopting DCXOs in their designs to replace VC-TCXOs.
The performance of a DCXO relies heavily on frequency estimation, frequency correction, and frequency tracking algorithms. DCXOs are generally considered more stable than VCXOs and TCXOs.
