Tech News: Fundamentals of TCXO

  • September 03, 2020

The Temperature Compensated Crystal Oscillator (TCXO), is a form of crystal oscillator used where a precision frequency source is required within a small space, low power supply, which is commonly used in large scale networks.

By applying temperature compensation within the quartz crystal oscillator module, TCXO is designed to enhance on the basic performance of the crystal.

Although crystal oscillators offer a highly stable form of oscillator, they are nevertheless affected by temperature.  The type of crystal cut inside the oscillator can help to minimize the effects of temperature, but they are still affected to some degree.

For a crystal cut known as the AT cut, the drift with temperature can be minimized around normal ambient temperature, but the rate of drift will rise above and below this.

The effects of temperature are, to a large extent, repeatable and definable. Hence  we can design oscillators to compensate for many of the effects using a temperature compensated crystal oscillator, TCXO.

A TCXO adjusts the frequency of the oscillator to compensate for the changes that will occur as a result of temperature changes. To achieve this, the main element within a TCXO is a Voltage Controlled Crystal Oscillator (VCXO). This is connected to a circuit that senses the temperature and applies a small correction voltage to the oscillator.

5 different elements that comprise the overall TCXO:

1. The Compensation network

The compensation network is the key to the operation of the whole system.  The compensation network needs to sense the temperature and produce a voltage that is the inverse of this.

 Early designs would have used analog circuitry and often directly used a network of capacitors, resistors and thermistors to directly control the frequency of oscillation. 

Currently technologies typically adopt an indirect approach where the temperature is sensed in the compensation network, and a voltage is generated to provide a frequency change being the inverse of the temperature curve.

Indeed current technologies often incorporate some form of digital signal processing to be able to generate a far more accurate response, with the possibility of linearizing units separately by programming a ROM with the response of the particular oscillator. The DSP circuitry is often contained within a special ASIC to enable it to be tailored to suit the application without draining too much current.

2. The oscillator pulling circuit

Once the voltage has been generated, the voltage is applied to a circuit that can pull the frequency of the crystal oscillator. Typically this circuit incorporates a varactor diode and some low pass filtering.

3. The crystal oscillator circuit itself

The crystal oscillator circuit is normally a standard circuit, but one that is designed to give the operating conditions for the crystal with ideal drive levels, etc.

4. The voltage regulator

In order to prevent external voltage changes from introducing unwanted frequency shifts, the overall TCXO should incorporate a voltage regulator which itself should not introduce unwanted temperature effects.

5. The buffer amplifier

A buffer amplifier is required to give the increased drive to the output. It should provide isolation to the crystal oscillator from any external load changes that may be seen.

TCXOs normally have an external adjustment to enable the frequency to be reset periodically. This enables the effects of the aging of the crystal to be removed. The period between calibration adjustments will depend upon the accuracy being required, typically around six months or a year. Shorter periods may be used if very high levels of accuracy are required.

Dynamic Engineers offers a diverse range of TCXO’s with a wide variety of packages and mounts (surface, through-hole) designed for 5G, satellite communication, harsh environment, etc.... Some TCXOs are compatible with the dual in line format used for many through hole mounted integrated circuits.

For additional information regarding fundamentals of TCXO or other RF / microwave topics, please contact your local Dynamic Engineers sales representative or