Temperature Compensated Crystal Oscillators

Temperature Compensated Crystal Oscillators


A TCXO is a crystal oscillator where the frequency vs. temperature characteristic of the oscillator is compensated by a compensation network. The two most common techniques for compensation are: 

Indirect Compensation technique ; and
Direct Compensation technique

Indirect Compensation

The typical oscillator using AT Cut crystals has a cubic polynomial frequency Vs. temperate curve. Compensation for the frequency temperature is done by incorporating a network with temperature characteristics opposite to the uncompensated crystal oscillator as shown in figure 1. This results in a oscillator with a temperature Vs frequency curve similar to that shown in figure 2

The indirect compensation technique is based on the use of a varactor diode in series with the crystal. The Compensation network consisting of a two or three thermistors and resistors provide the varactor diode the compensating voltage over the operating temperature range of the TCXO so that the resulting change in the varactor diode capacitance corrects for the frequency drift due to the temperature. The indirect compensation is shown in Figure 3.

Direct Compensation

The crystal is placed in the feedback path of an amplifier and a capacitor is used as a reactive element in series with the resonant current path of the crystal, which shifts the crystal frequency. By using thermistors as switches, the temperature extremes of the crystal can be compensated by switching in or out the reactive element in the oscillator circuit. Typically the series compensation network consists of a cold region network and a hot region network as shown in figure 4.

The resulting compensation will appear as in figure 5.

The direct compensation allows for low power consumption, uses fewer components than the indirect compensation method and can be constructed in small sizes. The indirect compensation method can achieve a better compensation over a wider temperature range and can achieve higher stabilities than the direct compensation method.

Definition of TCXO stability

When a TCXO is specified as ± 1 X 10-6 (1 ppm) over -20° C to +70° C , for example, it denotes that a maximum peak to peak error of 2 ppm over the entire temperature range and not referenced to any particular temperature. If any application calls for a specific reference temperature, then it should be clearly stated that the frequency deviation over the temperature range is referenced to the frequency of the TCXO at, say, 27° C .