Device for detecting the temperature of an oscillator crystal

In a device for detecting the temperature of an oscillator crystal 2, arranged on a carrier, in particular in a mobile radio apparatus, the detected temperature should be as exact as possible a replica of the temperature to which the oscillator crystal 2 is subjected. For this purpose, a temperature sensor 7 is arranged on the carrier 1 in such a way that it is subjected to the same ambient temperature as the oscillator crystal 2 or the oscillator-crystal housing 2′. The temperature sensor 7 and the oscillator crystal 2 are located so as to be electrically parallel.

The invention relates to a device for detecting the temperature of an oscillator crystal that has a crystal vibrator in an oscillator-crystal housing, in particular in a mobile radio apparatus.

Described in JP 2001-077627 is a temperature-compensated, piezoelectric oscillator. In order to achieve a small size, a temperature compensation circuit with a thermistor in thick-film technology is applied to the rear wall of the housing of the oscillator circuit for direct analog temperature compensation. The thermistor is decoupled from the oscillator by the rear wall in respect of the temperature acting upon it. The temperature and the temperature gradient at the thermistor and at the frequency-determining element of the oscillator here deviate from each other to a greater or lesser extent.

Described in U.S. Pat. No. 4,862,110 is a Surface Acoustic Wave (SAW) resonator, which is tuned to its nominal frequency by temperature variation of the resonator. A regulated heating element is provided for this purpose.

It is an object of the invention to propose a device of the kind specified above in which the measured temperature is as exact as possible a replica of the temperature to which the oscillator crystal, or its crystal vibrator as the frequency-determining component, is subjected.

This object is achieved by means of the features as claimed in claim1. Since, with this device, the detection of the temperature and the temperature gradient takes place on the oscillator crystal directly in terms of both time and space, it is precisely the temperature influencing the frequency response of the oscillator crystal that is detected by means of the temperature sensor. By virtue of this direct detection, no delays, inertia or deformations of the impulse response of the temperature propagation between the oscillator crystal and the temperature sensor occur. A precise compensation of the frequency error of the oscillator circuit of the oscillator crystal respectively occurring as a function of the temperature is thereby made possible. A software temperature compensation may take place by means of targeted influencing of the actuators serving for frequency correction.

The electrical parallel connection of the temperature sensor is advantageous because, as a result, the oscillator crystal with temperature sensor is a solely two-terminal component, which can be accommodated on a printed circuit board with minimal wiring requirement. The device permits a minimization of the number of components necessary for the temperature detection. These can be integrated into an integrated circuit. Because of the omission of external components, an inexpensive structure with a small space requirement and low susceptibility to error is guaranteed.

In a mobile radio apparatus, the device described solves the temperature detection problems that arise as a result of the fact that temperature gradients with different signs propagate in the mobile radio apparatus, wherein, on the one hand, self-heating occurs through energy loss and, on the other, heating or cooling occurs through the environment. The device described may also be used with other equipment, e.g. equipment used in entertainment electronics, medical engineering or automotive engineering.

In an embodiment of the invention, the temperature sensor is applied to a constant-current source or a constant-voltage source and an evaluation circuit which evaluates the temperature and/or the temperature gradients for compensation of the temperature-dependent resonant frequency of the oscillator crystal.

InFIG. 1, a printed circuit board1of an apparatus, e.g. a mobile radio apparatus or an apparatus used in entertainment electronics, automotive technology or medical engineering, is provided as the carrier of a frequency-determining circuit, which is equipped with an oscillator crystal. Arranged on the printed circuit board1are an oscillator crystal2and an integrated circuit3. The oscillator crystal2is equipped with an oscillator-crystal housing2′, in which is a crystal vibrator4(seeFIG. 2), which is connected via printed lines5,6of the printed circuit board1to terminals A and B of the integrated circuit3.

Arranged on the printed circuit board1as a temperature sensor is a temperature-dependent resistor, especially the thermistor7, which is electrically connected to the printed lines5,6in parallel with the crystal vibrator4. The thermistor7is arranged isothermally in relation to the oscillator-crystal housing2′ in the area52. In particular, in the vicinity50of the oscillator-crystal housing2′, the thermistor7is arranged on the same side of the printed circuit board1as the oscillator-crystal housing2′ and not separated from it by a wall. It is thereby achieved that the temperature and the temperature gradient at the oscillator crystal2, specifically at its crystal vibrator4, and at the thermistor7are in essence identical. An additional thermal conduction means, e.g. heat transfer compound, may support the immediate vicinity50of the temperature sensor7at the oscillator-crystal housing2′.

In order to increase the heat transfer resistance between the heat-emitting circuit3and the oscillator crystal2, and thereby to support the isothermal arrangement in the area52, openings51in the printed circuit board material may be provided in the printed circuit board1.

It is then obvious that, as the temperature sensor, the thermistor7is subjected to the particular temperature and temperature gradient that have a frequency-determining effect on the crystal vibrator4. The integrated circuit3contains the evaluation circuit that is further described below. It is spaced at a distance from the thermistor7, so that its temperature scarcely affects the thermistor7.

FIG. 2shows an oscillator crystal2in Surface-Mounted Device (SMD) design, wherein the thermistor7is arranged not, as inFIG. 1, adjacent to, but inside the oscillator-crystal housing2′. The oscillator-crystal housing2′ of ceramic, metal or plastic, forms an inner chamber8filled with inert gas. Inside this, the crystal vibrator4is located on mountings9. The crystal vibrator4is connected to oscillator-crystal terminals10, which lead outwards. A base panel11of the oscillator-crystal housing2′ is suitable to act as a carrier for the attachment of components and lines, and takes the form of, for example, a printed circuit board.

The thermistor7is integrated into, or applied to, the base panel11serving as the carrier. It is located inside the inner chamber8, as near as possible to the crystal vibrator4, but must not touch the latter, since the oscillation properties could be negatively influenced as a result. The thermistor7is not separated from the crystal vibrator4by a wall, and is provided on the same side of the base panel11as the crystal vibrator4. The thermistor7is connected by connecting leads12to the crystal terminals10in such a way that the crystal vibrator4and the thermistor7are electrically connected parallel to the oscillator-crystal terminals10.

FIG. 3shows the parallel connection of the oscillator crystal2, or the crystal vibrator4, and to the thermistor7. Given the typical values of a crystal-oscillator circuit, the thermistor7does not represent any significant additional load on the oscillator crystal. The thermistor7has, for example, a nominal ohmic resistance of approximately 30 kΩ.

In the case of the alternative shown inFIG. 4, coupling capacitors Ck are connected in series to the oscillator crystal2, specifically the crystal vibrator4. The thermistor7is connected parallel to this series connection. This layout too may be integrated into the oscillator-crystal housing2′ in the case of an arrangement as shown inFIG. 2. The coupling capacitors Ck are then arranged on the base panel11.

In the case of a layout as shown inFIG. 1, this arrangement is applied to the isothermal area52of the printed circuit board1.

In their operational mode, the coupling capacitors Ck separate the direct voltage to be applied to the thermistor7, which is further described below, from the oscillator crystal2, or the oscillator vibrator4.

FIGS. 5,6and7show evaluation circuits and oscillator circuits that may be integrated into the integrated circuit3. With the evaluation circuits, the temperature detected at the thermistor7can be evaluated in such a way that, as the end result, the temperature response of the resonant frequency of the oscillator crystal2is compensated. The thermistor7has a known resistance/temperature characteristic.

A constant current source13(seeFIG. 5) impresses a constant current on the thermistor7. As a result, a direct voltage, which corresponds to the present temperature-dependent resistance value of the thermistor7, arises between the terminals A, B. This direct voltage is detected by an analog/digital converter14, and sent digitally, via a data processing lead61, to a microcontroller15of the apparatus. This microcontroller15determines the real-time temperature, e.g. from a voltage/temperature Table stored within it, which corresponds to the characteristic curve of the thermistor7. The microcontroller15determines the voltage gradient or temperature gradient from sequential measurements.

An amplifier16serves to excite and maintain the high-frequency oscillations of the oscillator equipped with the oscillator crystal2, which, as a Pierce oscillator, is equipped with capacitors17from terminal A and from terminal B to ground. The oscillator crystal2, especially its crystal vibrator4, with the amplifier16and the capacitors17forms an oscillator circuit The capacitors17are preferably integrated into the integrated circuit3. In order to provide the facility of frequency control, the capacitors17may be adjustable in terms of their capacitance value. For the possibility of adjustment, the capacitors17are connected to the microcontroller15via control leads60. The HF oscillations, e.g. 26 MHz, of the oscillator are passed on via a lead34from the terminal A to a phase-locked loop18of the apparatus.

In order that the operation of the amplifier16is not impaired by the direct-voltage path of the thermistor7, coupling capacitors19are provided at the output and/or at the input of the amplifier16.

In addition to the measurement direct voltage, the high-frequency signal voltage of the oscillator function is also present at the terminals A, B, and therefore also at the analog/digital converter14. The high-frequency signal voltage is eliminated by signal processing measures, e.g. a low-pass filtration, in the analog/digital converter14or in the microcontroller15, so that only the direct-voltage measurement signal is used for the further processing in the microcontroller.

In a further embodiment, the determination of the temperature or temperature change from the direct-voltage measurement may be assigned to the analog/digital converter14itself if it can have direct recourse to the characteristic curve of the thermistor7. The microcontroller15can then signal, via the lead34, that large temperature gradients are currently occurring.

In the case of the evaluation circuit as shown inFIG. 6, a constant-voltage source20is provided instead of the constant-current source. In addition, a resistor21is integrated into the integrated circuit3. Together with the thermistor7, the resistor21forms a voltage divider, so that, here again, a direct voltage that is dependent on the temperature arises between the terminals A and B.

In the case of the embodiments as shown inFIGS. 5 and 6, the direct-voltage measurement serving for temperature detection and the oscillator operation take place simultaneously. Conversely, in the embodiment as shown inFIG. 7, the direct-voltage measurement and the oscillator operation take place sequentially in cycles. An enable-signal lead22, controlled by the microcontroller, is provided for this purpose. Either the constant-current source13and the analog/digital converter14are switched on via this, or the amplifier16is switched on via an inverter23for the enable signal. In this way, the temperature can be detected before the operation of the oscillator circuit, and, if applicable, a temperature compensation of the oscillator frequency or a temperature calibration can be undertaken.

With this layout, the coupling capacitors19(seeFIGS. 5 and 6) and the coupling capacitors Ck (seeFIG. 4) are superfluous. For the sake of clarity, the control leads60and61are omitted inFIG. 7, although they are used as shown inFIGS. 5 and 6.

In another embodiment, the analog/digital converter14may be arranged with spatial separation from the microcontroller15. If the resistance/temperature characteristic curve of the thermistor7is known to it, the analog/digital converter14can detect that temperature gradients presently occurring exceed a previously-defined or programmable limit value. The analog/digital converter14can then signal this to the microcontroller15via a control lead.

FIGS. 5 to 7show the case where no coupling capacitors Ck are provided in the isothermal area52. The evaluation circuits ofFIGS. 5 to 7may, however, also be used if coupling capacitors Ck are arranged in the isothermal area52(seeFIG. 4). The evaluation circuits inFIGS. 5 to 7may also be used if, as shown in figure the thermistor7is arranged inside the oscillator-crystal housing2′ with or without coupling capacitors Ck.

FIG. 8shows the principle of the phase-control loop18(seeFIGS. 5,6and7). It derives the output frequency occurring at an output30of a voltage-controlled oscillator31from the reference frequency occurring on the lead34, i.e. at terminal A, with phase-locking and frequency-locking. To develop the phase-locked loop, a divider35, a phase comparator33and a loop filter32axe provided adjacent to the voltage-controlled oscillator31.

The divider35can set fractional-rational division ratios in virtually any degree of fineness-stepping. It is a known fractional N divider. In the alternative as shown inFIG. 8, there is a special feature in that the reference frequency originating from the crystal oscillator circuit (seeFIG. 5) on the lead34is not, as described above, tracked to its nominal value by variation of the capacitances of the capacitors17. Instead, the reference frequency maintains its temperature-dependent frequency departure and it is achieved, through an appropriate, finely-stepped reprogramming of the divider35, that the frequency at the output30exhibits the nominal frequency value. The reprogramming of the divider35takes place via a data lead (not shown) from the microcontroller15. The temperature information present in the microcontroller15hereby brings about a temperature compensation of the output frequency at the output30.

The frequency tracking, i.e. temperature compensation, can therefore take place either via a readjustment of the capacitors17or, alternatively, via an appropriate reprogramming of the divider35.

The temperature information obtained may also be used for other purposes. For example, it may additionally be used in a mobile radio apparatus for calibrating other temperature-dependent parameters or for safety shutdown when a battery is charged.