Overview

A Temperature Compensated Crystal Oscillator (TCXO) is a crystal oscillator that stabilizes the operation frequency across a temperature range. The oscillation frequency of a regular crystal oscillator shifts as the temperature changes, resulting in a undesirable variation of the oscillation frequency when used as a reference in wireless, GPS, and other applications. A TCXO corrects the frequency shifts by using a temperature compensation network that senses the ambient temperature (typically a thermistor) and adjusts the crystal to the nominal operating frequency (Figure 1).

The temperature response of crystals is approximate to a 3rd order polynomial curve so many compensation techniques have been devised to negate the effects temperature has on the crystal. Direct, Indirect and Integrated Compensation schemes have been developed to produce up to a 5th order polynomial correction that also allow to take into account higher order non-linearities. Temperature compensation schemes allow for a standard VCXO stability of 50ppm to be corrected somewhere between 5ppm and 0.1ppm. (Figure 2)

For the compensation networks to typically work, coefficients have to be entered for the polynomial terms during the TCXO manufacturing process. This can be a tedious and costly process as each crystal’s temperature response has to be measured to the level of desired stability. However, TCXOs are cheaper and lower power than OCXOs and have acceptable stability performance for wireless, IoT and GPS applications.

 

Figure 1

Figure 2

Hexius Products


TM100
Thermal Management IC


    • Low phase noise oscillator
    • < 100 ppb Stability
    • High accuracy temperature sensor
    • Embedded microcontroller and memory
    • Optional external thermistor connections
    • Up to 7th Order correction algorithm
    • Simple software configuration
    • Defined manufacturing flow


Reference Designs

20MHz TCXO Design

  • Low cost
  • <100 ppb stability
  • Low phase noise
  • 5×7 form factor