Explanation of TCXOs
By Macrobizes
June 15, 2004
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
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