Abstract:
A crystal oscillator circuit has capacitors that govern the resonant circuit and are designed such that they can be connected and disconnected, for frequency adjustment. A respective compensation capacitor is connected opposite and in mirror-image form to these capacitors that govern the resonant circuit. This compensation capacitor influences only the dynamic operating point, but has virtually no effect on the oscillation frequency. In consequence, it is not possible to shift the operating point during adjustment of the oscillation frequency, thus ensuring stability of the oscillating system and operation at the desired operating point.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The following invention relates to a crystal oscillator circuit. 
     Crystal oscillators are electrical systems that can oscillate with a better frequency constancy than LC oscillators. In this case, crystals mechanically oscillate by being stimulated by electrical fields. An oscillating crystal having connected electrodes electrically behaves like a high Q-factor resonant circuit. 
     A so-called Pierce oscillator, which is representative of fundamental frequency oscillators, has two capacitors forming a series resonant circuit that is stimulated by an amplifier. In this case, a complementary metal oxide semiconductor (CMOS) inverter is used, for example, as the amplifier. A Pierce oscillator such as this is specified, for example, in the Tietze, Schenk: Halbleiter-Schaltungstechnik, [Semiconductor circuit technology] 10th Edition, 1993, on pages 466-469. In the Pierce oscillator, one of the two capacitors is designed to be adjustable. The oscillation frequency of the resonant circuit is influenced by adjusting the series capacitor. However, frequency adjustment using capacitors can also be carried out, for example, by connecting or disconnecting capacitor elements in discrete steps. 
     In crystal oscillators such as these, in which frequency adjustment is carried out using capacitors that are connected to ground, it is possible for the operating point to be shifted so far that the phase reserve of the system that can oscillate is no longer adequate, and the system becomes unstable. Instability such as this is disadvantageous in particular in the case of oscillators that have amplitude regulation. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a crystal oscillator circuit having an improved stability response. 
     With the foregoing and other objects in view there is provided, in accordance with the invention a crystal oscillator circuit including: an oscillating crystal having a first and a second connection; a quick-response amplifier having an input connected to the first connection of the oscillating crystal and an output connected to the second connection of the oscillating crystal; a device for operating point adjustment, the device coupling the oscillating crystal to the input of the quick-response amplifier; a reference node; a first capacitive component for connecting the first connection of the oscillating crystal to the reference node; a second capacitive component for connecting the second connection of the oscillating crystal to the reference node; and a compensation capacitor connected to the input of the quick-response amplifier and to the reference node. The first capacitive component and/or the second capacitive component is adjustable. The compensation capacitor is adjustable in a complementary manner to the first capacitive component and the second capacitive component. 
     In accordance with an added feature of the invention, the first capacitive component includes a plurality of capacitive components connected in parallel; the second capacitive component includes a plurality of capacitive components connected in parallel; the plurality of the capacitive components of the first capacitive component and the plurality of the capacitive components of the second capacitive component are connectable and disconnectable in pairs independently of one another; the compensation capacitor includes a plurality of capacitive components connected in parallel; and each one of the plurality of the capacitive components of the first capacitive component is associated with a respective one of the plurality of the capacitive components of the compensation capacitor. 
     In accordance with an additional feature of the invention, each one of the plurality of the capacitive components of the first capacitive component and the respective one of the plurality of the capacitive components of the compensation capacitor associated therewith has a fixed capacitance ratio. 
     In accordance with another feature of the invention, the plurality of the capacitive components of the compensation capacitor can be connected and disconnected independently of one another. 
     In accordance with a further feature of the invention, there is provided, a plurality of switches; and a drive circuit. Each one of the plurality of the capacitive components of the first capacitive component, the plurality of the capacitive components of the second capacitive component, and the plurality of the capacitive components of the compensation capacitor are coupled to a respective one of the plurality of the switches enabling connection and disconnection thereof. Each one of the plurality of the switches has a control connection. The drive circuit supplies control signals in a non-inverted form to the control connections of ones of the plurality of the switches connected to the plurality of the capacitive components of the first capacitive component. The drive circuit supplies control signals in a non-inverted form to the control connections of ones of the plurality of the switches connected to the plurality of the capacitive components of the second capacitive component. The drive circuit supplies control signals in an inverted form to the control connections of ones of the plurality of the switches connected to the plurality of the capacitive components of the compensation capacitor. 
     In accordance with a further added feature of the invention, the first capacitive component has a capacitance value; the compensation capacitor has a capacitance value; and a quotient of the capacitance value of the first capacitive component and the capacitance value of the compensation capacitor is greater than 5. 
     In accordance with another added feature of the invention, a minimum value capacitor permanently connects the input of the quick-response amplifier to the reference node. 
     In accordance with yet an added feature of the invention, the quick-response amplifier is an inverter. 
     In accordance with yet an additional feature of the invention, the quick-response amplifier is a CMOS inverter. 
     In accordance with yet another feature of the invention, the device for operating point adjustment is a resistor. 
     According to one development of the Pierce oscillator and in the case of the described principle, the oscillation frequency of the oscillator is adjusted by connecting or disconnecting the first and second capacitive component. The introduction of a compensation capacitor makes it possible for the oscillator circuit to maintain a stable response. The compensation capacitor is decoupled from the actual resonant circuit by the device for operating point adjustment, so that the connection or disconnection of the compensation capacitor admittedly has the desired effect on the operating point of the quick-response amplifier, but not on the actual oscillation frequency of the crystal oscillator circuit. 
     The compensation capacitor is adjusted opposite and in mirror-image form to the first and second capacitive component. This means that the compensation capacitor is disconnected when the first and second capacitive component is connected, and vice versa. With a suitable design, this results in the maintenance of the operating point of the quick-response amplifier. The compensation capacitor accordingly affects only the dynamic operating point of the quick-response amplifier. 
     According to one advantageous development of the invention, a number of first and a number of second capacitive components are provided. In this case, the number of first capacitive components are connected in parallel with one another. The number of second capacitive components are likewise connected in parallel with one another. 
     Since the first and second capacitive components can each be connected and disconnected, the parallel circuits preferably each include a series circuit of a switch and of one of the number of first or second capacitive components. 
     The number of first and the number of second capacitive components are preferably provided in pairs. In this case, one first and one second capacitive component are in each case connected or disconnected at the same time. 
     The large number of capacitive components allows the frequency at which the oscillating crystal is oscillating to be adjusted in any desired number of steps. 
     In this case, each pair including a first and a second capacitive component preferably has a respectively associated compensation capacitor. The compensation capacitor in each case has a fixed capacitance ratio with respect to the associated first and second capacitive component, in this case. According to the present principle, in each case one pair of first and second capacitive components are switched at the same time at the associated compensation capacitor, but in the opposite sense and in mirror-image form. This means that, when one pair including a first and a second capacitive component is connected, the compensation capacitor which is associated with this pair and which has a fixed capacitance ratio with respect to the two components is disconnected. With a suitable design, the proposed principle makes it possible first to ensure a simple layout of a crystal oscillator circuit, and second to achieve an exact compensation for the dynamic operating point of the quick-response amplifier. 
     In order to make it possible to switch associated capacitors in the opposite sense and in mirror-image form, a drive circuit is preferably provided which supplies a control signal in a non-inverted form to the control connections of the switches which are respectively associated with the first and second capacitive components. The switch that is connected to the compensation capacitor that is associated with the pair of first and second capacitive components is supplied with this control signal in inverted form at its control connection. This results in the opposite switching, in mirror-image form, on which the present principle is based, of the compensation capacitor with regard to the first and second capacitive components. 
     The capacitor values of the first and second capacitive components are preferably very much greater than the capacitive value of their associated, compensating capacitive component. The capacitor values of the first and second capacitive component, which jointly form a pair, are in this case preferably the same. The quotient of the capacitor value of the first capacitive component and the capacitor value of the associated compensation capacitor is preferably greater than 5. 
     As an improvement, this ensures that the connection or disconnection of compensation capacitors influences only the dynamic operating point of the quick-response amplifier, but not the oscillation frequency of the crystal oscillator itself. 
     According a further advantageous embodiment of the present invention, a minimum value capacitor is provided, which permanently connects the input of the quick-response amplifier to the reference node. 
     Together with the device for operating point adjustment, which is preferably in the form of a resistor, the minimum value capacitor forms an RC element, which has low-pass filter characteristics. A low-pass filter such as this advantageously assists the stability of the crystal oscillator circuit. 
     The described crystal oscillator circuit is preferably constructed as an integrated circuit using complementary metal oxide semiconductor (CMOS) circuit technology. 
     The quick-response amplifier is preferably formed as a CMOS inverter. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a crystal oscillator circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified circuit diagram of a first embodiment of a crystal oscillator circuit; 
     FIG. 2 is an S-parameter diagram illustrating the compensation for operating point shifts in the crystal oscillator circuit based on the real part of the input admittance, plotted against the frequency; and 
     FIG. 3 shows a development of the crystal oscillator circuit shown in FIG. 1, in which an oscillation frequency can be adjusted discretely in a number of steps. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a crystal oscillator circuit including an oscillating crystal  1  with two connections. The first connection of the oscillating crystal  1  is connected, via a resistor  2 , to one input of an inverter  3 . One output of the inverter  3  is connected to the second connection of the oscillating crystal  1 . One connection of a series circuit including a switch  4  and a capacitor  5  is connected to the node between the oscillating crystal  1  and the resistor  2 . Another connection of the series circuit is connected to a reference node  6 , which is connected to reference-ground potential. A further series circuit, which includes a switch  7  and a capacitor  8  having the same capacitor value as the capacitor  5 , is connected between the output of the inverter  3  and the reference node  6 . One connection of a series circuit including a switch  9  and a compensation capacitor  10  is connected to the reference node  6 . The other connection of the series circuit is connected to the input of the inverter  3 , specifically between the resistor  2  and the inverter  3 . A minimum value capacitor  11  is connected from the reference node  6  in parallel with the immediately previously described series circuit including the switch  9  and the compensation capacitor  10 . Together with the resistor  2 , the minimum value capacitor  11  forms an RC element, which acts as a low-pass filter. 
     The switches  4 ,  7 , which effectively connect and disconnect a first capacitive component, namely the capacitor  5  and a second capacitive component, namely the capacitor  8 , are each switched at the same time by the same drive signal since their control connections are connected to one another. The switch  9  associated with the compensation capacitor  10  is switched in inverted form, for which purpose an inverter  12 , which is supplied with the drive signal, is connected to the control connection of the switch  9 . The input of the inverter  12  is connected to the control connections of the switches  4 ,  7 , which are associated with the first and second capacitive components  5 ,  8 , respectively. Accordingly, the switch  9  closes when the switches  4  and  7  open, and vice versa. 
     First and second capacitive components  5 ,  8  are each connected to the reference ground potential connection  6  or the ground connection and are used to adjust the frequency of the resonant circuit formed by the crystal oscillator circuit. This normally results in an undesirable shift in the operating point of the circuit, in particular of the quick-response amplifier  3 , which endangers stability. The quick-response amplifier  3  in the present example is in the form of a CMOS inverter. The relatively smaller compensation capacitor  10  is switched in the opposite sense, but in mirror-image form to the capacitors  5 ,  8  that govern the frequency of the resonant circuit. The compensation capacitor  10  has a fixed capacitor ratio with respect to the capacitors  5 ,  8  that govern the resonant circuit. The capacitors  5 ,  8  have the same capacitor value. Since the compensation capacitor  10  has a smaller capacitor value, and furthermore, is decoupled via the resistor  2  that is used to adjust the operating point of the inverter  3 , it has only a very minor influence on the oscillation frequency, but has a considerable influence on the dynamic operating point. In the present example, the capacitive components  5 ,  8  which govern the oscillation frequency each have a capacitor value of 32 Pico farads, while the capacitor value of the compensation capacitor  10  is between 3 and 4 Pico farads and is accordingly less by a factor of 10 than the capacitor values of the first and second capacitive components  5 ,  8 . 
     The compensation capacitor  10 , which is driven in the opposite sense and in mirror-image form with respect to the capacitive components  5 ,  8 , maintains the stability and drive range of the oscillator. This is advantageous, for example, when additional amplitude or frequency modulation is provided in the crystal oscillator circuit. 
     FIG. 2 is a graph with a logarithmic/linear representation to show the real part of the input admittance of the crystal oscillator, plotted against its frequency. As can be seen, irrespective of whether the capacitors  5 ,  8  with the capacitor value of 32 Pico farads are or are not connected, the same operating point is in each case maintained for the operating frequency of the crystal. This is achieved by connecting and disconnecting the compensation capacitor  10  in the opposite sense and in mirror-image form, as explained with reference to the circuit diagram shown in FIG.  1 . 
     FIG. 3 shows a development of the crystal oscillator circuit shown in FIG. 1, in which the oscillation frequency of the crystal oscillator can be switched between two discrete frequencies and a large number of discrete oscillation frequencies can also be selected. This is accomplished by providing a number of pairs of capacitive components  5 ,  8 ;  14 ,  16 ;  21 ,  23 , each of which has an associated compensation capacitor  10 ,  18 ,  25 . 
     The design and method of operation of the crystal oscillator circuit shown in FIG. 3 corresponds largely to the crystal oscillator circuit shown in FIG.  1  and includes all the components that are shown there, with the same advantageous interconnection between them. To this extent, the description of FIG. 3 will not be repeated at this point. 
     In addition to the components that are shown in the crystal oscillator circuit shown in FIG. 1, the crystal oscillator circuit shown in FIG. 3 has further series circuits including in each case a respective switch  13  or  20  and a respective capacitor  14  or  21  (each defining another first capacitive component), which are connected in parallel with the series circuit formed by the switch  4  and the first capacitor  5 . Two further series circuits, each including a respective switch  15  or  22  and a respective capacitor  16  or  23  (each defining another second capacitive component), are also connected in parallel with the series circuit formed by the switch  7  and the second capacitor  8  and are symmetrically connected with respect to the series circuits formed by the capacitor-switch pairs  13 ,  14 , and  20 ,  21 . In addition, further series circuits which each include a respective switch  17  or  24  and a respective compensation capacitor  18  or  25  are also provided in parallel with the compensation capacitor  10 , which forms a series circuit together with the switch  9  that is associated with it, and these further series circuits are connected in parallel with the series circuit formed by the switch  9  and the compensation capacitor  10 . Inverters  19  and  26  are respectively also provided for driving the switches  17  and  24  that are connected to the further compensation capacitors  18  and  25 , in an analogous manner to the driving of the compensation capacitor  10  using the switch  9  and the inverter  12 . 
     In the same way as the capacitors  5 ,  8  that form a pair that is driven jointly and at the same time, the capacitors  14 ,  16  and  21 ,  23  also each form a jointly driven pair of capacitors. 
     The first and second capacitive components  5 ,  8 ;  14 ,  16 ;  21 ,  23  are graduated in binary form, in pairs, with respect to one another. In a corresponding way, the associated compensation capacitors  10 ,  18 ,  25  are also graduated in binary form, with respect to one another, such that the capacitor ratio V of the first or second capacitor  5 ,  8 ;  14 ,  16 ;  21 ,  23  to the associated compensation capacitor  10 ,  18 ,  25  is always the same. In the present case, this capacitor ratio V is approximately 10. 
     The drive circuit  27  controls the connection and disconnection of the first and second capacitive components  5 ,  8 ;  14 ,  16 ;  21 ,  23  and of the compensation capacitors  10 ,  18 ,  25 . If, for example, the pair of capacitors  5 ,  8  is disconnected by the switches  4 ,  7 , by opening the switches, the compensation capacitor  10  which is associated with these capacitors is connected at the same time by closing the switch  9 . In a corresponding manner, the pair of capacitors  14 ,  16  are driven in the opposite sense and in mirror-image form to the compensation capacitor  18  which is associated with them, and finally, the pair of capacitors  21 ,  23  are driven in a corresponding manner in the opposite sense and in mirror-image form to the compensation capacitor  25  which is associated with this pair of capacitors. The capacitors  5 ,  8 ;  14 ,  16 ;  21 ,  23 , which govern the frequency, are provided symmetrically with respect to one another and are graduated in binary form to allow discrete adjustment of the resonant circuit frequency in a very exact manner with little complexity. The respectively associated compensation capacitors  10 ,  18 ,  25 , which are each connected in the opposite sense, result in always maintaining the operating point of the crystal oscillator circuit irrespective of the choice of the capacitors that determine the frequency. The circuit may, of course, also be extended to any desired number of further switchable capacitors, based on the principle described above. 
     As an alternative to the described binary graduation, the capacitors may also, by way of example, all be of the same size. 
     In addition to the described embodiment having in each case three parallel-connected capacitor branches that can be connected and disconnected independently of one another, it is also possible according to the present principle to provide any desired number of further switchable branches.