Adjustable frequency crystal oscillator with temperature compensation
This is a continuation of application Ser. No. 188,029, filed Sept. 17, 1980 and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an oscillator with a piezoelectric vibrator, in which it is easy to control the oscillation frequency, and in which the oscillation frequency is kept constant over a wide temperature range. 
     For example, in a crystal oscillator, a trimmer capacitor is needed to control the oscillation frequency, so its construction becomes complex. And the oscillation frequency drifts according to the temperature, so it can be used only in a narrow temperature range. Even in an oscillator having the lowest frequency drift rate, its oscillation frequency drifts about ±2 PPM in the temperature range 5˜35° C., and at temperatures outside of that temperature range the oscillation frequency drifts extremely. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide an oscillator having an easily controlled oscillation frequency, without a trimmer capacitor. 
     A second object of the present invention is to provide an oscillator having a very stable oscillation frequency, in spite of switching the switching circuit for connecting and cutting off a capacitance to a load capacitance. 
     A third object of the present invention is to provide an oscillator having an oscillation frequency compensated over a wide temperature range, and for which most of its structure can be integrated in a C-MOS IC chip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an electric circuit for explaining the principle of the present invention; 
     FIG. 2 is a graph showing load capacitance seen from terminal b in FIG. 1 by switching the switching circuit; 
     FIG. 3 is a block diagram showing an electric circuit according to an embodiment of the present invention; 
     FIG. 4 is a frequency-temperature characteristic diagram of a piezoelectric vibrator in FIG. 3; 
     FIG. 5 is a frequency-load capacitance characteristic diagram of a piezoelectric vibrator in FIG. 3; 
     FIG. 6 is a block diagram showing an electric circuit for the temperature compensation according to an embodiment of the present invention; 
     FIG. 7 is a frequency-temperature characteristic diagram before and after the compensation by the electric circuit in FIG. 6; 
     FIG. 8 is a block diagram showing an electric circuit according to another embodiment of the present invention; 
     FIG. 9 is a frequency-temperature characteristic diagram of a ring oscillator in FIG. 8; 
     FIG. 10 is a block diagram showing an electric circuit according to a further embodiment of the present invention; 
     FIG. 11 is a frequency-temperature characteristic diagram of another piezoelectric vibrator before and after the compensation; and, 
     FIG. 12 is a block diagram showing an electric circuit according to an embodiment of the present invention for the compensation in FIG. 11. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention is described below with reference to the drawings. Referring to FIG. 1, a feedback resistance R, a C-MOS inverter V 1  and load capacitances C 1 , C 2  are connected to a quartz vibrator Q. A capacitance C 3  is connected in parallel to a load capacitance C 2 , and a switching circuit S is connected in series to capacitance C 3 . 
     In the aforementioned construction, the connection of the capacitance C 3  to the load capacitance C 2  is controlled by supplying the pulses P to a terminal a of switching circuit S. 
     The capacitance C seen from the terminal b is represented in FIG. 2, and the effective load capacitor seen from the terminal b is varied by changing the duty factor τd/τp of pulses P. Accordingly, the oscillation frequency can be adjusted. 
     Next, it is described below how the effective load capacitor C(t), seen from the terminal b, varies according to the duty factor of the pulses P. 
     The capacitor C(t) is expanded as the Fourier series, as follows. ##EQU1## Provided that, ##EQU2## Consequently, capacitor C(t) is formulated below. ##EQU3## 
     The above formula (1) contains the switching frequency n·fp(n=1, 2, 3 - - - ), and the response of the quartz oscillator to said switching frequency is explained as follows. 
     The time constant τc of the passive elements of the oscillator is formulated as follows. 
     
         τc≈2Q/ω.sub.0 
    
     (Q: Q value of the quartz resonator, ω 0  =2πf 0 , f 0  : oscillation frequency) 
     Accordingly, by predetermining the frequency fp of the switching pulses P to be extremely higher than 1/τc(fp&gt;&gt;1/τc), the frequency fp is negligible in the oscillation frequency spectrum of the quartz oscillator. Therefore, in the formula (1), the terms containing n·fp(fp=1/τp) are negligible. Consequently, the load capacitor is formulated as C(t)=C 2  +C 3  ·τd/τp, namely, the effective load capacitor is decided only by the duty factor of the pulses P. So, the oscillation frequency is controlled by the duty factor of the pulses P. 
     For example, when a quartz oscillator having an oscillation frequency of about 4.2 MHz and Q value of about 3×10 5  to 5×10 5  is used, by predetermining the frequency fp of pulses P to about 10 KHz, the oscillation frequency is not influenced by the above frequency fp, but the oscillation frequency is controlled by the duty factor of the pulses P. 
     Accordingly, the oscillation frequency can be adjusted by the duty factor of the pulses, without losing the stability of the oscillation frequency. 
     In FIG. 3, circuit structure to set the duty factor of pulses P is illustrated. A frequency divider D divides the oscillation frequency, and the output frequencies from the terminals d 1 , d 2  are 1/2 n , 1/2 m  (n&gt;m) of the oscillation frequency, respectively. A differentiation circuit T differentiates the output frequency from the terminal d 1 , and the output pulses from the differentiation circuit T are supplied to the input terminal PE of a K(K=n-m) bits presettable-down counter A. A gate circuit G receives the pulses from the terminal e, and the pulses from the terminal d 2  pass through the gate circuit G. The desired value of K is set by a digital switch DS, and it is memorized in a non-volatile memory circuit M. 
     The operation of the above circuits is described below. By the pulse from the differentiation circuit, the desired value memorized in the memory circuit M is preset in the down counter A. Therefore, the terminal e turns to a logic value &#34;1&#34; (hereinafter simply referred to a logic &#34;1&#34;), so the pulses from the terminal d 2  pass through the gate circuit G, and they are supplied to the down counter A. And, the terminal e turns to a logic &#34;0&#34;, when the down counter A counts the desired value. Then, the content of the memory circuit M is preset again in the down counter A by a next pulse from the terminal d 1 , and the abovementioned operation repeats again. 
     Consequently, from the terminal d 1 , the pulses having a duty factor corresponding to the value set by the digital switch DS are generated and switch the switching circuit S. Consequently the effective load capacitor and the oscillation frequency are adjusted. 
     In FIG. 3, the construction except the digital switch DS, capacitances C 1 , C 2 , C 3  and the quartz vibrator can be integrated, and a trimmer capacitor is not needed, so the stability of the oscillation frequency improves. 
     Next, an embodiment of the temperature compensation circuit is explained below, using an AT cut quartz resonator having a frequency-temperature characteristic which is represented as a cubic curve shown in FIG. 4. In FIG. 4, a frequency difference Δf between the objective frequency f 0  and an actual frequency f is shown on the vertical axis, and the temperature is shown on the horizontal axis. 
     On the other hand, a frequency-load capacitor characteristic is represented as a curve in FIG. 5, so the frequency drift according to the temperature can be compensated by controlling the load capacitor. 
     In FIG. 6, the temperature is converted to voltage by a thermistor Th and a resistance r, and the voltage is supplied to comparison circuits CM 1  - - - CM r . The terminals P 1  - - - P r  of the comparison circuits CM 1  - - - CM r  are connected to respective different predetermined voltages. The outputs from the comparison circuits CM 1  - - - CM r  are converted to binary coded data by an encoder E, and the data is supplied to the down counter A. 
     Consequently, the data corresponding to the temperature is preset in the down counter A, namely, the duty factor of pulses for switching the switching circuit S is set. Therefore, the switching of the switching circuit is controlled corresponding to the temperature, and the effective load capacitor is controlled, accordingly, the oscillation frequency is compensated. The frequency-temperature characteristic in the non-compensated condition is shown in FIG. 7 as a curve l 1 , and it is compensated to a wave-like line l 2  by the above mentioned compensation. Accordingly, the frequency drifting rate Δf/f can be controlled under ±1 PPM in a very wide temperature range. 
     Next, referring to FIG. 8, another embodiment for the temperature compensation circuit is explained below. A ring oscillator RG is composed of C-MOS inverters V 2 , V 3 , V 4 , and its output frequency is supplied to a mixer MR. A difference frequency between the output frequency of the ring oscillator RG and the frequency from a terminal d 3  is supplied to the data converter DT. The frequency-temperature characteristic of the ring oscillator is nearly linear as shown in FIG. 9. At the vertical axis in FIG. 9, the frequency drifting rate Δf/f 25 ° C. (f 25 ° C. ; oscillation frequency at 25° C., Δf=f T ° C. -f 25 ° C., f T ° C. ; oscillation frequency at T° C.) is shown. 
     As the oscillation frequency of the ring oscillator RG drifts linearly according to the temperature, the output frequency from the mixer MR varies according to the temperature. And it is converted to binary coded data by the data converter DT, and the data is supplied to the down counter A. 
     Consequently, the oscillation frequency of the quartz vibrator Q is compensated as shown in FIG. 7. And the construction except the quartz vibrator Q and the capacitances C 1 , C 2 , C 3  can be integrated in a C-MOS IC chip. 
     Further, another embodiment is shown in FIG. 10. The oscillation frequency of a CR (capacitor-resistor) oscillator H which has a thermistor, is counted by a frequency counting circuit CT, and its content is converted to binary coded data by the data converter DT. The oscillation frequency of the CR oscillator H drifts according to the temperature, so the data from the data converter varies according to the temperature. 
     Next, an embodiment is explained below, using a quartz vibrator having a frequency-temperature characteristic which is represented as a quadric curve l 3  in FIG. 11. 
     Referring to FIG. 12, the oscillation frequency of the CR oscillator H and the output frequency from the terminal d 3  are supplied to the mixer MR, and the difference frequency is converted to binary coded data by the data converter DT. The oscillation frequency of the CR oscillator H is predetermined to be coincident with the frequency from the terminal d 3  at 25° C., namely at the center line of the quadric curve l 3  in FIG. 11. Therefore, the difference frequency from the mixer MR is 0 at 25° C. and drifts symmetrically according to the increase and decrease of the temperature. Consequently, the frequency-temperature characteristic shown as the quadric curve l 3  in FIG. 11 can be compensated under ±1 PPM in a very wide temperature range, showing as a wave-like line l 4 . 
     According to the present invention as described above, the parallel connection between a capacitance and a load capacitance is controlled by the switching pulses which have a variable duty factor. Therefore, the oscillation frequency can be adjusted by controlling the duty factor of the switching pulses, a trimmer capacitor is not needed, and the frequency stability improves. 
     Further, since the switching pulses are generated from a frequency divider which divides the oscillation frequency, the oscillation frequency and the switching pulses are not influenced by aging. 
     In addition, since the duty factor of the switching pulses is varied according to the temperature, the oscillation frequency can be compensated and can be kept stable over a wide range of temperature.