GB2210746A - Temperature controlled crystal oscillator - Google Patents

Abstract

The crystal 2 of the oscillator is in a temperature-controlled oven 5 which is run at a temperature below the maximum temperature of the range of ambient temperature to which the oscillator is to be subjected. For example if the intended maximum is 105 DEG C, the oven is run at 80 DEG C. Hence as the temperature approaches and exceeds the oven temperature the oven temperature control circuit loses control. The system has a sensor associated with the crystal which controls an electronic compensation circuit 34 which maintains the oscillator frequency constant when the ambient temperature exceeds the oven temperature. By running the oven at a lower temperature ageing of the oscillator components is reduced.

Description

Temperature-Controlled Crystal Oscillator The present invention relates to temperature-compensated crystal oscillators.

Wnen better than that provided by an uncompensated crystal is needed, it is possible to use a temperature compensation technique. This uses a variable reactance device, such as a Varicap diode, in the feedback path of the oscillator such as to compensate for changes in the natural resonant frequent of the crystal due to changes in the ambient temperature. One example of such a circuit is described in our Patent No. 2121629B (G.H.S. Rokos et al 4-1), in which the base-emitter voltage of a transistor in an integrated circuit is used to provide substantially linear temperature-sensitive outputs which are used to generate a series of polynomical functions of Chebyshev-like form which are summed and used to control the Varicap diode.

For the ultimate in precision a temperature controlled environment for the cyrstal is provided by placing the crystal in an oven whose temperature is controlled to control the oscillation frequency.

An oven-controlled arrangement gives high accuracy but has disadvantages: (a) As the oven can only heat the crystal it has to operate at a temperature at least as high as the highest expected ambient temperature. In fact it usually has to run at a higher temperature than just mentioned. This reduces the reliability of the crystal and its circuit due to continuous high temperature operation.

(b) The oscillator needs a significant time to reach working temperature during which the crystal frequency is significantly in error.

(c) The oven consumes a great deal of power.

An object of the invention is to produce a crystal controlled oscillator in which the effects of the above disadvantages are reduced or even eliminated.

According to the present invention, there is provided a temperature-compensated crystal oscillator circuit, which includes 2 crystal oscillator whose crystal is located in an oven, a first control circuit which controls the temperature of the oven so as to be at a defined temperature for a range of ambient temperatures, which defined temperature is within but below the maximum expected ambient temperature, a further control circuit associated with the crystal and arranged to control-electronically controlel#r#ic#lly the frequency at which the crystal oscillates and a temperature sensor associated with the crystal and responsive to the temperature of the crystal to effect stimulation of the further control cIrcuIt, the arrangement being such that when the ambient temperature exceeds said defined temperature and the first control circuit loses control of the oven temperature said further control circuit maintains the oscillation frequency of the crystal substantially constant despite changes in the ambient temperature.

Such an arrangement overcomes the main disadvantages of the conventional oven system referred to above. Thus for many applications the crystal only has to operate occasionally at temperature extremes.

Hence the need to operate the oven above the maximum ambient temperature is a nuisance. As an example, if the maximum ambient temperature needed is 105 C, a conventional oven system has to run at a constant oven temperature of 120 C. This causes semiconductor devices to operate in conditions which cause a significant reduction in life.

The basis of our invention is that an oven controlled crystal oscillator has additional circuitry enabling normal variations in crystal frequency to be oven controlled. The oven 5 control circuit is conventional except that the oven 5 operating temperature is less than the maximum ambient temperature. Thus when the ambient temperature is in this high range, the oven loses control and its temperature rises from its normal constant value at the same rate as the ambient temperature rises.For example, assume that the ambient temperature range extends to +100 C and the oven temperature is set at 80 0C for the region where the ambient temperature is less than 70 C, in which case the oven temperature would be constant at 800C until the ambient temperature approaches 80 C. However, with the ambient temperature in the region of 700C and 1000C, oven temperature tracks ambient.

In the present system, with the oven set to run, in the present example at 800C, the temperature sensor associated with the crystal controls an additional control circuit which thus "knows" the actual crystal temperature and electrically controls the crystal frequency such as to produce an approximately constant frequency. The control voltage is a polynomial function of temperature and can be generated, for instance, by an integrated circuit arranged as in our above quoted Patent.

An embodiment of the invention will now be described with reference to the accompanying drawing, in which: Fig. 1 is a simplified block diagram of the temperature control system of the above-quoted Patent.

Fig. 2 is a simplified block diagram of â crystal controlled oscillator arrangement embodying the present invention.

The arrangement shown in Fig. 1 includes an amplifier 1 with a positive feedback path including a quartz crystal 2 in series with a variable reactance element 3 such as a Varicap diode. The reactance of the element 3 is controlled by a temperature sensitive compensating circuit 4. As explained in our above-quoted Patent, the circuit 4 varies the reactance of the element 3 in such a way as to alter the oscillator frequency such that the oscillation frequency is substantially unchanged with variation in ambient temperature.

To improve the accuracy of control of the frequency of a crystal oscillator, the amplifier 1, crystal 2, variable reactance element 3, and circuit 4 are all located inside a temperature controlled oven indicated at 5. #e#ve;:##ba:s-##e#ter indicated at 6, which maintains the oven temperature at a temperature lower than the maximum ambient temperature. Thus if the latter temperature is, say, 105 C, the oven is maintained by the heater at 80 C. The control of the oven is exercised by an ambient temperature sense and control circuit 7, which controls the heater 6 via an amplifier 8.

An additional sensor is provided which, although not shown is associated with the crystal so that it responds to changes in the temperature thereof.

In operation, the dominant control of the crystal's environment is by the circuit 7, until the ambient temperature exceeds about 700C. When this occurs, the circuit 7 progressively loses control. However, the sensor associated with the crystal responds to the condition of the crystal, and controls the oscillator frequency via the circuit 4. As in the above-quoted Patent this is effected electronically by making a suitable adjustment of the reactance of the element 3.

This control function also operates to a limited extent while the oscillator is, in effect, setting itself into stable operation when the system is switched on. Thus during the warm-up phase the frequency comes into a workable tolerance in a much shorter time than for an uncompensated oven oscillator. This gives a useful "trade-off" for the power consumed by the oven.

There are several ways in which the above-mentioned principle can be applied: (a) the temperature sensing device associated with the crystal may be used both for the oven temperature control and for the compensation circuit 4.

(b) in a different operational mode a quartz crystal, e.g. of the SC cut type, has a second operating frequency which exhibits a high temperature coefficient is used as the temperature sensing element.

(c) the temperature sensing element is packaged in a similar fashion to the quartz crystal. Hence any temperature gradients set up during warm up are similar for the sensing device and the crystal. Eris optimises the effectiveness of the compensation circuit.

Where a temperature sensor additional to the crystal is used, it senses variations in ambient temperature before they have been attenuated by the thermal insulation of the oven. Hence a fast indication of ambient temperature change is available to the control and compensation circuitry to improve transient performance.

Claims (4)

CLAIMS:

1. h temperature-compensated crystal oscillator circuit, which includes a crystal oscillator whose crystal is located in an oven, a first control circuit which controls the temperature of the oven so as to be at a defined temperature for a range of ambient temperatures, which defined temperature is within but below the maximum expected ambient temperature, a further control circuit associated with the crystal and arranged to control electronically the frequency at which the crystal oscillates and a temperature sensor associated with the crystal and responsive to the temperature of the crystal to effect stImulation of the further control circuit, the arrangement being such that when the ambient temperature exceeds said defined temperature and the first control circuit loses control of the oven temperature said further control circuit maintains the oscillation frequency of the crystal substantially consW mbr*nes?-te- chasyes in the ambient temperature.

2. A circuit as claimed in claim 1, and In which the output of said sensor controls the first control circuit in addition to the further control circuit.

3. A circuit as cline in claim 1 or 2, in which the crystal is a quartz crystal and the sensor is another quartz crystal with 2 second frequency which exhibits a high temperature coefficient, and in which said crystal performs the functions of the temperature sensing element.

4. A temperature compensated crystal oscillator circuit, substantially as described with reference to Fig. 2 of the accompanying drawing.