TCXO, Temperature Compensated Crystal Oscillator

- the TCXO, temperature compensated crystal oscillator is used for providing a much higher levels of temperature stability than are possible with a normal crystal oscillator.

The TCXO, Temperature Compensated Crystal Oscillator (Xtal oscillator) is a form of crystal oscillator used where a precision frequency source is required within a small space and at reasonable cost.

By applying temperature compensation within the crystal oscillator module, it is possible to considerably improve on the basic performance of the oscillator.

In view of their usefulness, a wide range is available from many suppliers in a whole variety of packages from a host of different surface mount varieties through to through hole mounted ones, some of which are compatible with the dual in line format used for many through hole mounted integrated circuits.

Effect of temperature

Although crystal oscillators offer a highly stable form of oscillator, they are nevertheless affected by temperature. The cut of the actual crystal element from the overall grown crystal 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.

Frequency / temperature curve for quartz crystal
Typical frequency / temperature curve for quartz crystal

The effects of temperature are, to a large degree, repeatable and definable. Therefore it is possible to compensate for many of the effects using a temperature compensated crystal oscillator, TCXO.

A typical comparison of the typical or expected performance levels is given in the table below:

Temperature range Basic crystal oscillator TCXO
0C to 70C ±25 ppm ±1.5 ppm
-20°C to 70°C ± 30ppm ± 2.5 ppm
-40°C to 85°C ± 40ppm ± 3 ppm

Note: These performance figures are very generalised and can only be used for a rough guide. Exact figures will depend upon the item used and figures for these should be gained from the manufacturers datasheets.

TCXO solution

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 as shown below.

Block diagram for a typical TCXO
Block diagram for a typical TCXO

There are a number of different elements that comprise the overall temperature controlled oscillator:

  • Compensation network:   The compensation network is the key to the operation of the whole system. An approximate curve for the temperature frequency response of the oscillator is seen above. The actual curve can be expressed approximately in the form of a 3rd order polynomial expression, although a more accurate representation takes into account some non-linearities and works out to be close to a 5th order polynomial. The compensation network needs to sense the temperature and produce a voltage that is the inverse of this.

    Early designs would have used analogue circuitry and often directly used a network of capacitors, resistors and thermistors to directly control the frequency of oscillation. This type of circuit included both blocks on the diagram of the compensation network and the crystal frequency pulling block.

    Compensating voltage for TCXO temperature linearisation
    Compensating voltage for TCXO temperature linearisation

    Currently technologies typically adopt an indirect approach where the temperature is sensed in the compensation network, and a voltage is generated that provides a frequency change that is the inverse of the temperature curve. This can be achieved using analogue components, but current technologies often incorporate some form of digital signal processing to be able to generate a far more accurate response, with the possibility of linearising 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.
  • Oscillator pulling circuit:   Once the voltage has been generated, this is applied to a circuit that can pull the frequency of the crystal oscillator. Typically this incorporates a varactor diode and some low pass filtering.
  • Crystal oscillator :   The oscillator circuit is normally a standard circuit, but one that is designed to give the operating operating conditions for the crystal with ideal drive levels, etc.
  • 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.
  • 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.

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

TCXO performance

Some of the main performance figures are summarised below:

  • TCXO PPM performance:   The TCXO temperature performance is better than that of a normal crystal oscillator. Typically figures of between 10 and 40 times improvement can often be seen. Typical figures are given in the table above for the different temperature ranges. Figures of better than ±1.5 ppm over a 0 to 70°C temperature range are difficult to achieve as they then fall into a high precision category where costs increase significantly.
  • Power dissipation:   The power dissipation of a TCXO will be greater than an ordinary oscillator in view of the additional circuitry required. Additionally the cost is greater. It should also be remembered that it will take a short while after start up for the oscillator to stabilize. This may be of the order of 100 ms, or possibly longer, dependent upon the design.
  • TCXO package:   TCXOs can be supplied in a variety of packages dependent upon the way they have been designed and the requirements of the end user. The most common form of construction is to construct the circuit on a small printed circuit board that can be house in a plat metal package. This is then suitable for mounting onto the main circuit board of the overall equipment. As the crystal itself is sealed, this means that sealing of the overall TCXO package is not critical, or even required for most applications.

    Package sizes such as 5x3.2x1.5 mm or 5x3.5x1 mm are widely used for TCXOs and smaller packages available if required.
  • Output format and level:   With many TCXOs being used for driving digital circuits, most of the small oscillator packages produce what is termed a clipped sine wave. This is suitable for driving a logic circuit, although in many cases it is wise to put it through a logic buffer to ensure it is sufficiently square. Often the output is an open collector circuit. If a sine wave output is required, then this must be chosen at the outset and it will limit the choice available.
  • Power requirements:   The actual power requirements will depend upon the particular device. Many operate from supplies of 3 V, and may draw as little as 2 mA, although this will depend upon the general type, the manufacturer and the particular device chosen.

TCXO types

Although temperature compensated crystal oscillators are normally referred to in this manner, occasionally more detailed descriptions are used. This has resulted in the variety of techniques that can be used to provide the temperature compensation.

  • ADTCXO:   This is an Analogue Digital TCXO. This form of TCXO has been widely used in cell phones. This uses analogue technology to provide temperature correction to the oscillator. It has the advantage that changes take place slowly and no phase jumps are experienced as occurs with some all-digital types.
  • DTCXO:   As may be guessed, this is a digital TCXO. It uses a temperature sensor and then logic and mathematical functions use digital circuitry along with a look up table. The resulting digital correction figure is converted to an analogue signal using a digital to analogue converter, DAC.
  • DCXO:   This is a form of oscillator where any correction is calculated by the host processor within the equipment. In this way, the TCXO is not a separate entity, but the processing is incorporated within that of the overall equipment. This can help save costs in some instances.
  • MCXO:   This form of TCXO uses a microprocessor to provide a considerably increased level of processing to provide more accurate compensation under a variety of circumstances. While performance is a little better, costs are above those of the other forms.

TCXOs are widely used where accurate frequency sources are needed. They are less expensive and smaller than oven controlled oscillators, as such they offer an ideal solution for many portable units requiring a reasonably accurate source.

By Ian Poole

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