The invention relates to a dual mode oscillator in which during operation at least a first oscillation having a fundamental frequency and a second oscillation having a second frequency is generated, the second frequency being substantially equal to a, preferably the third, harmonic of the first frequency, both oscil¬ lations generating a first and a second oscillation signal, res¬ pectively, that may be combined to generate a signal having a εub- stantial linear temperature dependency.

Such a dual mode oscillator is known from US Patent 4,859,969 and may, for instance, be applied in a temperature compensated oscillator circuit.

Crystal oscillators having a very high temperature stability are provided with a temperature compensating circuit. Such a cir¬ cuit used to comprise a temperature sensor, a varicap and a control circuit, that could be analogue or digital. The calibration of such circuits in a temperature controlled measurement area took con¬ siderable time because the temperature-frequency characteristic is a function of higher order and, therefore, many points of the char¬ acteristic had to be measured. Because of limitations of a mechan¬ ical nature, the temperature sensor needed for such a system, was always located at a distance from the crystal. Therefore, during the calibration procedure one had to observe a waiting time to guarantee the sensor and the crystal to have equal temperatures.

The oscillator known from US-A-4,859,969 is a so-called dual mode oscillator, oscillating on both the fundamental frequency and the third harmonic and not showing said disadvantage, because the crystal itself is used as temperattire sensor. The frequency differ- ence between the fundamental frequency and the third harmonic (f3- 3*f1 ) shows the feature of having a substantial linear temperature dependency. This frequency difference supplies the needed tempera¬ ture information to the control system.

However, a disadvantage of the known dual mode oscillators is that a number of higher order filters is needed for the crystal to oscillate on both frequencies and to separate both signals after¬ wards. Because of these filters the circuit may only be integrated in a limited way.

The object of the dual mode oscillator according to the in¬ vention is to solve the problems of the prior art mentioned above. Therefore the circuit described in the preamble of claim 1 is char¬ acterized in that the oscillator comprises an electronic control- lable switch connected in such a way that in a first state the oscillator oscillates in the first oscillation and in its other state the oscillator oscillates in the second oscillation, the oscillator having at least one output supplying alternately the first and the second oscillation signal. By applying these measures a fast calibration of the circuit may be achieved, and moreover, the circuit may be integrated in CMOS-technology.

Preferably the dual mode oscillator according to the inven¬ tion is applied in a temperature compensated oscillator circuit. In a preferred embodiment of such a temperature compensated oscillator circuit it comprises a first counter connected to the output of the dual mode oscillator, a processor connected to the output of the first counter and having connected at least one out¬ put to a frequency controlled oscillator, for instance, a voltage controlled oscillator (VCO), a second counter that may be connected to an output of the frequency controlled oscillator through switch¬ ing means either directly or through a frequency divider dividing by three, the output of the second counter being connected to an input of the processor, the processor being arranged in such a way that, in dependency on the signals supplied by the first counter and the second counter, it supplies a control signal to the fre¬ quency controlled oscillator to control its frequency to become substantially temperature independent.

In an other preferred embodiment of such a temperature com- pensated oscillator circuit further switching means are connected between the frequency divider and the frequency controlled oscil¬ lator in such a way that the frequency divider may be connected either to the frequency controlled oscillator or to a reference signal. Moreover, the invention relates to a method to calibrate a temperature compensated oscillator circuit comprising the following steps: a. setting a predetermined temperature;

b. switching the switch and the switching means in such a way that the dual mode oscillator oscillates in the second oscil¬ lation and the second counter is directly connected to the frequency controlled oscillator; c. counting the number of oscillations of the second oscillation by means of the first counter starting from a first initial value; d. counting the number of oscillations (fx) of the frequency controlled oscillator by means of the second counter starting from a second initial value; e. measuring the frequency of the second oscillation (f3) of the dual mode oscillator by means of a reference counter; f. stopping steps c. and d. after a predetermined number of oscillations (=N) of the second oscillation (f3), after which the content of the second counter is equal to fx/N.f3; g. switching the switch in such a way that the dual mode oscil¬ lator oscillates in the first oscillation; h. switching the switching means in such a way that the second counter receives the output of the frequency divider, which frequency divider receives the output of the frequency con¬ trolled oscillator; i. counting downwards the number of oscillations of the first oscillation by means of the first counter during the time period lapsing as the second counter counts down to the sec- ond initial value; j. storing the result of step i. being the value of the first counter into a memory of the processor and storing the value fgew/N.f3, fgew being the desired output frequency of the frequency controlled oscillator and f3 being measured by means of the reference counter; k. repeating steps a. to j. at a predetermined number of differ¬ ent temperatures in order to make up either a look up table or a polynom to determine the relationship between the fre¬ quency of the frequency controlled oscillator and the tem- perature.

In an other preferred embodiment the invention relates to a method to calibrate a temperature compensated oscillator circuit comprising the following steps:

a. setting a predetermined temperature; b. switching the switch in such a way that the dual mode oscil¬ lator oscillates in the second oscillation (f3) and switching the switching means in such a way that the second counter is directly connected to the reference signal; c. counting the number of oscillations of the second oscillation by means of the first counter starting from a first initial value; d. counting the number of oscillations (fx) of the reference signal by means of the second counter starting from a second initial value; e. stopping steps c. and d. after a predetermined number of oscillations (=N) of the second oscillation, after which the content of the second counter is equal to fref/N.f3, fref being a reference frequency; f. storing the value of the second counter into a memory of the processor; g. switching the switch in such a way that the dual mode oscil¬ lator oscillates in the first oscillation; h. switching the switching means in such a way that the second counter receives the output of the frequency divider, and the frequency divider is connected to the reference signal; i. counting downwards the number of oscillations of the first oscillation by means of the first counter during the time period lapsing as the second counter counts down to the sec¬ ond initial value; j. storing the result of step i. being the value of the first counter into a memory of the processor; k. repeating steps a. to j. at a predetermined number of differ- ent temperatures in order to make up either a look up table or a polynomial to determine the relationship between the frequency of the frequency controlled oscillator and the tem¬ perature.

Furthermore, the invention relates to a method to control a frequency controlled oscillator according to the following steps: a. switching the switch and the switching means, as well as eventually further used switching means, in such a way that the dual mode oscillator oscillates in the second oscillation

(f3) and the second counter is directly connected to the fre¬ quency controlled oscillator; b. counting the number of oscillations of the second oscillation by means of the first counter starting from a first initial value; c. counting the number of oscillations (fx) of the frequency controlled oscillator by means of the second counter starting from a second initial value; d. stopping steps b. and c. after a predetermined number of oscillations (=N) of the second oscillation, and storing the content of the second counter, then being equal to fx/N.f3; e. switching the switch and the switching means in such a way that the dual mode oscillator oscillates in the first oscil¬ lation and the second counter is connected to the output of the frequency divider; f. counting downwards the number of oscillations of the first oscillation by means of the first counter during the time period lapsing as the second counter counts down to the sec¬ ond initial value; g. storing the result of step f. being the value of the first counter into a memory of the processor; h. reading or calculating the calibration value, being the desired value of fx/N.f3 by means of the value stored in step

9-' ^' i. comparing to the value stored in step d.; j . transmitting a control signal to the frequency controlled oscillator to control its frequency at the desired value.

The invention will hereinafter be described with reference to the accompanying drawings which are not meant in a limiting sense, but only show an example of the invention and in which figure 1 shows a crystal oscillator having a temperature compensating circuit according to the invention; figure 2 shows the characteristic of a higher harmonic of a fundamental frequency of the compensating circuit according to figure 1 as function of the temperature; figure 3 shows a flow chart corresponding to the calibration of the circuit according to figure 1 ; figure 4 shows a flow chart corresponding to the operation

mode of the circuit according to figure 1; figure 5 explains the characteristic of the real part of the input impedance of a colpitts-oscillator used in the circuit of figure 1 as a function of the frequency; figure 6 shows an electronic equivalent circuit of the col¬ pitts-oscillator, used in the circuit of figure 1 ; figure 7 shows an example of a colpitts-oscillator used in the circuit of figure 1.

In figure 1 the basic schedule of one example of the oscil- lator circuit to be controlled according to the invention is illus¬ trated. With 1 the (crystal) oscillator to be controlled is indi¬ cated, which oscillator outputs a signal having a frequency fx to be controlled. The output of oscillator 1 is connected to a two state switch S3. The terminal of switch S3 to be switched may either be connected to the output of the oscillator 1 or to a sig¬ nal having a reference frequency fref. The other terminal of switch S3 is fixed to a frequency divider 2, dividing the frequency of the input signal by three. Moreover, the other terminal of the switch S3 is connected to a counter 3 through a switch S2. The switch S2 has two states and connects, in its first state, the counter 3 to the switch S3 and, in its other state, the counter 3 to the output of the frequency divider 2. The counter 3 has four control inputs: a start input 4, a stop input 5, a reset input 6 and an up/down input 7. A signal to the start input 4 starts the counter, a signal to the stop input 5 stops the counter counting, a signal to the reset input 6 resets the counter to the initial value (for instance the value 0) and a signal to the up/down input 7 determines the counter to count upwards or downwards. The output of the counter is connected to a processor 8, comprising among others a memory. It is to be noted that the switch S3 may be omitted in which case there is a direct connection between the frequency controlled oscillator 1 and the frequency divider 2. The reference signal fref is used to calibrate the circuit, however, the calibration may also be executed by means of the output signal of the oscillator 1 it- self, as will be explained hereinafter.

Furthermore, there is provided a switchable oscillator 9 be¬ ing preferably a colpitts-oscillator. The preferred embodiment of a colpitts-oscillator applied in the present case will be discussed

below referring to figure 7. The colpitts-oscillator is connected to a crystal 15 and to a switch S1. At the output of the colpitts- oscillator 9 either a signal φ1 or φ3 appears. φ1 is the fundamen¬ tal harmonic of the signal generated by the colpitts-oscillator and φ3 is a higher harmonic of φ1 , preferably the third harmonic of φ1. The output of the colpitts-oscillator is connected to a counter 10 having four control inputs like counter 3: a start input 1, a stop input 12, a reset input 13 and an up/down input 14. The functioning of these four control inputs is the same as with counter 3 and is not repeated here. The output of the counter 10 is connected to the processor 8. The processor supplies an output signal to the oscil¬ lator 1 to be controlled, being, for instance, a voltage controlled oscillator, and thus controls its frequency.

In the circuit according to figure 1 the known principle of temperature compensating circuits of dual mode oscillators having high temperature stability is used. In such dual mode oscillators the crystal of the oscillator itself is used as temperature sensor. Theoretically, it may be proven that the function f3-3*f1, in which f1 is the fundamental frequency of the oscillation of the crystal and f3 is the frequency of the third harmonic of f1, varies sub¬ stantially linear with the temperature, so the quantity f3-3*f1 may easily be used for temperature compensation. Also in the circuit of figure 1 the quantity f3-3*f1 is used as temperature parameter, whereas f3 provides the frequency information. Both during the calibration phase of the circuit according to figure 1 and during its operation phase the absolute temperature itself is not important, since the information about the absolute temperature is continuously provided by measuring the value of the signal f3-3*f1. Essential to the circuit according to the invention is that the signals f3 and f1 are not available simultaneously, but alternately during short time periods. These time periods are just long enough to measure the frequency of the colpitts-oscillator 9 with enough accuracy. The switch S1 serves to generate the signals f1 and f3 alternately. When the switch S1 is open, as shown in figure 1, the colpitts-oscillator 9 oscillates with frequency f1, whereas it oscillates with frequency f3 when the switch S1 is closed. This will be further explained hereinafter referring to figure 7.

Measuring both frequencies f1 and f3 is done by means of the processor 8 and the counter 10. Furthermore, for the meastirement the output signal fx of the oscillator 1 to be controlled is used. Thus, the oscillator 1 has two functions: one as an oscillator to be controlled and one as a reference oscillator. The oscillator 1 is only required to supply an output signal with a stable frequency during a short time period.

After each measurement cycle, i.e. each time the quantity f3- 3*f1 is measured, the oscillator 1 will be regulated in accordance with the quantity f3-3*f1. f3, being approximately a third order function of the temperature, which is known per se to the person skilled in the art, is used to calibrate the circuit according to figure 1. So, to calibrate the circuit according to figure 1, it is enough to determine four points of the relationship between f3 and the temperature T, since they determine entirely the third order function. As mentioned above, it also holds that the value of f3-3*f1 varies linearly with temperature. Therefore, the function f3=f(f3-3*f1 ) also has to be a third order function. So, to the calibration it may also suffice to determine four points of the function f3=f(f3-3*f1) . When a more accurate approximation than a third order function is required, of course, more points of this function have to be measured to determine in that way a higher order approximation. Figure 2, in which the horizontal axis of the diagram either shows the temperature T or the quantity -(f3-3*f1 ) and the vertical axis shows the value of f3, further illustrates this.

Figure 3 shows a flow chart to calibrate the circuit of fig¬ ure 1, the switch S3 being switched to connect the frequency con¬ trolled oscillator 1 to the frequency divider 2. In step 20 a pre- determined desired temperature is set, in this case one of the four temperatures to determine the curve of figure 2. Next, in step 21 f3 and fx are selected by closing the switch S1 and switching the switch S2 to the right in figure 2 and switching the switch S3 to the left in figure 2, respectively. Then both counters 3 and 10 are reset on the initial value. In steps 22 and 23 the counters 3 and 10 are counting, the counter 3 counting with the frequency fx and the counter 10 counting with the frequency f3. Counting continues until counter 10 is full, after which counter 3 is directly

stopped. The content of counter 10 is a measure of the value of f3 being determined in step 25 in the processor 8. Then, in step 26 f1 and fx/3 are selected by opening the switch S1, switching the switch S2 in figure 2 to the left and switching switch S3 in figure 2 also to the left, respectively. In steps 27 and 28 counter 3 counts downwards with the frequency fx/3 and counter 10 counts downwards with the frequency f1. This continues until the counter 3 reaches the initial value. Then, in step 29, the content of counter 10 is read directly. Since the frequency fx in frequency divider 3 is divided by 3, counting downwards has lasted three times as long as counting upwards in the first part at the steps 22 and 23. Therefore, the content of counter 10 now corresponds to f3-3*f1 (and the content is a measure for the temperature) . Since at step 25 the value f3 has become available now the first value of the curve of figure 2 may be filled in. The cycle of figure 3 is repeated as long as desired, e.g. four times for a third order approximation.

Thus, during the calibration according to the scheme of fig¬ ure 3 the oscillator 1 to be controlled is used as reference oscil- lator. This is somewhat complicated. In practice the reference sig¬ nal Vref having a predetermined reference frequency fref may also be used instead. Then, in the scheme of figure 3 in step 21 instead of "select fx" one has to read: "select fref" and in step 26 in¬ stead of "select fx/3": "select fref/3". The commands "select fref" and "select fref/3" respectively correspond to switching the switch

S2 in figure 2 to the right and to switch the switch S3 in figure 2 to the right, and to switch the switch S2 in figure 2 to the left and to switch the switch S3 in figure 2 to the right, respectively.

It is to be observed that f3 and f1 are therefore measured after one another to determine the function f3-3*f1. Switching the switches SI , S2 and S3 occurs about once per second, so usual tem¬ perature fluctuations may be easily followed. Only during very fast temperature changes the compensation circuit is not reliable.

In the usual operation mode the measured values from figure 2 may be used. It is also possible to convert the measured (four) values into coefficients of the (third order) polynomial or to make up a "look up" table. The determined values will then have to be converted into a control signal for the oscillator 1.

Figure 4 shows a flow chart for the operation mode of the circuit according to figure 1. In step 40 the signals f3 and fx are selected by closing the switch S1, switching the switch S2 to the right in figure 2 and switching the switch S3 to the left in figure 2. Besides, in this step both counters 3 and 10 are reset to their initial value. In steps 41 and 42 both counters 3 and 10 are count¬ ing, counter 3 counting with the frequency fx and counter 10 count¬ ing with the frequency f3, until counter 10 is full. Then, in step 43 counter 3 is read directly supplying the value fx. Then, counter 10 comprises a value corresponding to f3. In step 44 f1 and fx/3 are selected by opening the switch S1 , switching S2 to the left in figure 2 and switching the switch S3 to the left in figure 2. In steps 45 and 46 both counters 3 and 10 are counting downwards: counter 3 counting with the frequency fx/3 and counter 10 counting with the frequency f1. Counting down stops when counter 3 reaches the initial value. Then, like during the calibration cycle, counter 10 comprises a value corresponding to f3-3*f1. The content of coun¬ ter 10 is read in step 47 and stored. The calibration data corre¬ sponding to this value of f3-3*f1 are determined in step 48, for instance, by means of the look up table or the calculated third order function. In step 49 the required compensation of the oscil¬ lator to be controlled is determined, which is possible because in step 43 the value fx is determined and in step 47 the value f3-3*f1 is determined, so the relationship between them is also known. At last, in step 50 a regulating signal is applied to the oscillator 1.

The way in which is switched between the signals f1 and f3 is essential to the compensation circuit. The colpitts-oscillator 9 realized in CMOS technology, as shown in figure 7, and forming a parallel resonance circuit with the crystal 15, constitutes the basic circuit. The application of parallel resonance has the advan¬ tage that there may be applied a high impedance circuit reducing the dissipated power.

The colpitts-oscillator of figure 7 comprises a crystal 15 connected in parallel to a resistor R3 and a resistor R1. Resistor R3 is connected to ground through a switch S1. The signal supplied by the crystal 15 is applied to the gate electrode of a PMOS tran¬ sistor T1, the drain electrode of which is connected to ground and

the source electrode of which is also connected to ground through a capacitor C2. Besides, there is provided a capacitor C1 between the gate and the source electrode of transistor T1 and the source elec¬ trode receives a predetermined current from a current source I. The colpitts-oscillator of figure 7 illustrates just one example of an oscillator being appropriate to be used as a switchable oscillator 9. As to the oscillator used it is only important that it may switch from a stable oscillation in the fundamental harmonic to a stable oscillation in a higher harmonic, preferably the third har- monic, in a comparatively short time - for instance 1 second - by switching the switch S1.

Although a colpitts-oscillator as such is known to a person skilled in the art, the following is to be noted with respect to the colpitts-oscillator used. By means of comparatively simple net- work analysis it may be proven that a colpitts-oscillator may be conceived as a negative resistor in series with a capacitor in the frequency range around the oscillator frequency. This negative resistor compensates the resonance resistance of the crystal resulting in the oscillation. The curve of the real part of the input impedance [Re(f)l of the oscillator as a function of the frequency strongly depends on the DC input resistance [Rdc] of the circuit for the DC input resistance (also) determines the position of a zero point of Re(f).

In figure 5 the curve of the real part of the input impedance of two values of Rdc is shown. If Rdc decreases Re(f) will become less negative for low frequencies. In other words, in figure 7 a lower DC input resistance Rdc of the colpitts-oscillator corres¬ ponds to the curve 60 than to the curve 61. So, there is such a Rdc=Rl , that there is a minimum in the curve for a frequency f1. There is also a Rdc=R3 for which there is a minimum in the curve at a frequency f3.

With respect to modelling a fundamental harmonic and a third harmonic, a crystal has an electrical equivalent circuit comprising the parallel circuit of two LCR circuits (figure 6). By compensat- ing the resistance of one of both circuits the crystal will oscil¬ late & the corresponding frequency. Crystals meant to oscillate at the fundamental harmonic have a much lower resistance at this fun¬ damental harmonic than at the third harmonic. Therefore, such a

crystal when connected in an oscillator circuit will preferably oscillate at the fundamental frequency. A crystal realized to oscillate at a harmonic frequency will show a lower resonance resistance at this harmonic frequency. If f1 (figure 5) corresponds to the fundamental harmonic of a crystal and |Re1 (f1 ) |>Rs1 the oscillator may oscillate at frequency f1 (with respect to Rs1 see figure 6). If f3 equals a harmonic (e.g. the third harmonic) of the same crystal and |Re3(f3) | >Rs3 the oscillator may oscillate at f3 (with respect to Rs3 see figure 6). If R3 and the crystal are connected to the oscillator the circuit will oscillate at frequency f3, because |Re3(f3) | >Rs3. |Re3(f1 ) | will be small or even positive, therefore the oscillator will not operate at frequency f1. If R1 and the crystal are connected the oscillator will oscillate at frequency f1, since |Re1 (f1 ) | >>Rs1. Although also |Re1 (f3) | >Rs3 holds the oscillator will still oscil¬ late at frequency f1, since |Re1 (f1 )/Rs1 | >>|Re1 (f3)/Rs3| . By now switching between both resistors the oscillator will be switched between both frequencies. The most convenient solution is to apply a fixed resistor R1 and a switchable resistor R3*, so that R1//R3*=R3. Based on this the circuit of figure 7 is designed. Both the switch S1 and the resistors R1, R3* may easily be integrated in CMOS technology.

It will be understood by the person skilled in the art, that the colpitts-oscillator shown in figure 7 is just an example of a possible switchable oscillator in the circuit of figure 1. In accordance with the invention, for instance, also other modified colpitts-oscillators may be designed. The switchable oscillator 9 is only required to be able to switch from one stable oscillation to an other stable oscillation within a relatively short time period (for instance 1 second) by switching the switch S1.