Patent Publication Number: US-7724101-B2

Title: Crystal oscillator circuit with amplitude control

Description:
This application claims priority from German Patent Application No. 10 2007 016 522.8, filed 5 Apr. 2007, the entirety of which is hereby incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The invention relates to a crystal oscillator circuit; and, more specifically, to a crystal oscillator circuit based on the Pierce oscillator architecture. 
   BACKGROUND 
   Crystal oscillators, in particular Pierce oscillators, are widely used in the art to provide stable clock signals. There is a general need to design circuitry that is both suitable for all kinds of applications and highly efficient with respect to the specific requirements of each application. Specifically, electronic devices or electronic circuitry which provide a crystal oscillator should be made to support as many crystals as possible while still providing a constant frequency, high (in absolute value) negative impedance, low phase noise, low power consumption and low power dissipation on the crystal. For integrated solutions, the integrated components and circuitry should be immune to all kinds of noise sources. A large frequency pulling range is also desirable. 
   A detailed discussion and a prior art solution for a Pierce oscillator is given in Vittoz, et al., “High-Performance Crystal Oscillator Circuits: Theory and Application,” IEEE Journal of Solid-State Circuits, Vol. 23, No. 3, June 1988, p. 774. The crystal oscillator circuit presented by Vittoz includes an amplifier stage, an amplitude control stage, an output voltage regulator and an output amplifier. The amplitude control is necessary in order to provide a suitable negative impedance, when looking from the crystal into the respective input pins of the crystal oscillator circuit. The negative impedance should be equal to the equivalent serial resistance (ESR) of the crystal. A negative impedance that is too large has the disadvantages of high power dissipation and fast degradation of the crystal. If the negative impedance is smaller than the ESR, the oscillation may die out or there may be no oscillation at all. 
   The mechanism disclosed by Vittoz to control the ratio between the negative impedance and the ESR is based on control of the bias current through a transistor used as an amplifying transistor for the crystal. The bias current is controlled by an amplitude control circuit (ACC). The ACC monitors the amplitude of the oscillation and generates a control voltage to reduce or increase the amplitude. The control voltage is used to modify the current through the amplifying transistor. Due to the specific control mechanism used by Vittoz, however, the conversion gain is very high. Accordingly, the noise generated by the ACC is amplified and fed into the crystal oscillation loop. This severely degrades the phase noise performance. Furthermore, the crystal oscillator circuit suggested by Vittoz has only limited immunity to substrate noise or cross-talking when implemented on an integrated circuit, in particular in a system on chip (SoC). Finally, the crystal oscillator suggested by Vittoz has only a small pulling range and poor adaptability to multiple different crystals. 
   SUMMARY 
   It is an object of the invention to provide a very flexible crystal oscillator circuit which has a wide frequency pulling range, high immunity to noise, reduced power dissipation, and small crystal degradation. 
   Accordingly, a crystal oscillator circuit is provided which includes a capacitive load stage, an amplifier stage, a pick-up stage, and an amplifier control stage. The capacitive load stage is coupled to a crystal. The amplifier stage includes a bias generator stage and an amplifying transistor coupled to the crystal and to the capacitive load stage for establishing an oscillation. The pick-up stage is coupled to the amplifier stage and to the crystal to generate an oscillator output signal. The amplitude control stage controls the amplitude of the oscillation signal at the crystal. 
   The crystal oscillator circuit according to described embodiments of the invention is based on a Pierce oscillator architecture. The bias generator stage is configured as a degenerated common source amplifier. Using a bias generator stage configured as a degenerated common source amplifier reduces the conversion gain and a corresponding noise injection. By using a degenerated common source amplifier configuration, the amplitude control voltage to bias current conversion is performed by the degenerating element (e.g., resistor or plurality of resistors) rather than by the transistor&#39;s transconductance, thereby reducing the conversion gain. 
   In described embodiments, the bias generator stage includes a control transistor and at least one resistive element coupled to the source of the control transistor, together forming the degenerated common source amplifier. The control transistor, having the resistive element as the degenerating element, receives an amplitude control voltage as a control signal from the amplitude control stage to selectively adjust a bias current through the amplifying transistor. Accordingly, the rather fine adjustment of the bias current in response to the amplitude control signal is performed by controlling the gate voltage of the control transistor. Because the gain of the control transistor is low, noise injection via the amplitude control mechanism is reduced. 
   In accordance with one implementation of the invention, the degenerated common source amplifier in the bias generator may be degenerated by means of a plurality of resistors. The crystal oscillator circuit may also include corresponding means (e.g., switches) to selectively switch the resistors into the current path through the degenerated common source amplifier in response to a digital configuration signal, thereby determining the bias current through the amplifying transistor. Switching resistors into the current path, makes it possible to predefine different ranges in response to a digital configuration signal for different crystals or different applications. 
   The capacitive load stage coupled to the crystal may be selectively switched such that the capacitive load effective for the crystal oscillator can be changed in response to a digital configuration signal. A variable capacitive load allows the crystal oscillator circuit to adapt to various different crystals. The load capacity should be selected according to the specifications of the crystal being used. Thus, a wide variety of crystals corresponds to a wide variety of load capacities. 
   By using the circuitry according to the desscribed implementation, the capacity selection can be made merely by switching capacitors. The switches may be implemented by means of transistors, dimensioned so that the quality factor of the capacitors is not significantly degraded for the working frequency. 
   As the resistive elements (that is, for example, the resistors) coupled to the control transistor and the capacitive load are controlled by the same digital configuration signal, a link is established between the bias current (i.e., the transconductance of the amplifying transistor) and the load capacity. Controlling both the capacitive load and the transconductance of the amplifying transistor is particularly useful because the negative impedance depends mainly on the load capacitors and on the transconductance of the amplifying transistor. A large capacitive load needs a large transconductance; but if the transconductance is too high, the negative impedance decreases again. Generally, the transconductance should be selected within a certain range for a determined load capacitor range. Accordingly, it is useful to use the same, or at least related, digital configuration signals for switching the resistors in order to define a specific bias current for the amplifying transistor and for switching the capacitive load. The invention suggests establishing a link between the capacitive load selection signal and the selected current range. This provides optimum adaptability of the oscillator circuit. Within the specific range, the amplitude control stage controls the bias current through the amplifying transistor in order to achieve an optimum transconductance within the preselected range. 
   In another aspect of the invention, the pick-up stage may be adapted to receive and process differential signals. This allows the pick-up stage to be coupled differentially (symmetrically) to opposite sides of the crystal. This approach is particularly useful for integrated solutions, where the crystal oscillator circuit is subject to all kinds of interferences and noise. The noise may cause malfunction of the oscillator, including glitches and double pulses. This aspect of the invention is based on the recognition that the noise primarily shows a common mode behavior, and the input and output signals of the amplifier stage are subject to a phase difference of close to 180 degrees. Thus, a differential pick-up stage significantly rejects all common mode interferences. Furthermore, use of differential stage generally improves phase noise at high frequency and reduces power consumption with respect to the conventional AC coupled inverter stage approach. 
   According to another aspect of the invention, a resistor may be coupled to the output of the amplifier stage on one side of the crystal to achieve improved adaptability of the oscillation frequency (i.e., to increase the pulling range). The resistor together with the load capacitance creates a pole, shifting the phase and allowing the crystal to work in a range where it is easier to adapt (pull) the frequency. 
   According to still another aspect of the invention, the capacitive load stage may include at least one NMOS transistor to serve as a capacitive load. Conventional solutions suggest the use of MOS varactors as a variable capacity. However, NMOS transistors used as controlled capacitance have a higher maximum-to-minimum capacitance variation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and features of the invention will become apparent from the following detailed description of example embodiments, considered with reference to accompanying drawings, wherein: 
       FIG. 1  (Prior Art) shows a simplified schematic of a Pierce crystal oscillator circuit according to the prior art; 
       FIG. 2  shows a simplified schematic of a Pierce crystal oscillator circuit with an amplitude control according to an aspect of the invention; 
       FIG. 3  shows a simplified schematic of a Pierce crystal oscillator circuit with an amplitude control according to another aspect of the invention; 
       FIG. 4  shows a Pierce crystal oscillator circuit with a differential pick-up stage according to an aspect of the invention; 
       FIG. 5  shows a simplified schematic of a capacitive load stage comprising NMOS transistors according to an aspect of the invention; 
       FIG. 6  shows a simplified schematic of a pick-up stage according to an embodiment of the invention; 
       FIG. 7  shows a simplified schematic of an amplitude control stage according to an embodiment of the invention; and 
       FIG. 8  is an overall block diagram view of a Pierce crystal oscillator circuit according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     FIG. 1  (Prior Art) shows a simplified schematic of a Pierce crystal oscillator circuit according to the prior art. As shown, a crystal XTAL is coupled to an amplifier stage including amplifying transistor M 1 . The output of the amplifier stage AMPOUT is coupled via resistor R 0  to a terminal XIN of the crystal XTAL. The terminals XIN and XOUT of the crystal XTAL are coupled to respective capacitive loads C 0  and C 1 . An amplitude control stage ACC is coupled to the output AMPOUT of the amplifier stage and serves to generate a control current ICNTRL, which is coupled by current mirror M 2 , M 0  to the amplifier stage in order to adapt the transconductance of transistor M 1  in response to the oscillation amplitude. Because the gain of the control loop comprising the current mirror and the amplitude control stage is typically very high, the amplitude control mechanism shown in  FIG. 1  introduces additional and amplified noise into the Pierce crystal oscillator circuit. 
     FIG. 2  shows a simplified schematic of a Pierce crystal oscillator according to an embodiment of the invention. As shown, the amplitude control stage ACC is coupled to a bias generator stage which is now implemented as a degenerated common source amplifier by use of control transistor M 0  and resistor R 1 . Accordingly, the gain of transistor M 0  is reduced and the control voltage VCTRL used to adjust the transconductance of the amplifying transistor M 1  is basically transformed into a bias current IBIAS by transistor R 1 . As a result, the noise introduced by the amplitude control stage ACC is substantially reduced. 
     FIG. 3  shows a simplified schematic of a Pierce crystal oscillator circuit according to another embodiment. As shown, a plurality of resistors R 1 , R 2  and R 3  are coupled in series to the source of NMOS transistor M 0 , which serves as a resistively degenerated control transistor, as explained with reference to  FIG. 2  above. The resistors R 1 , R 2  and R 3  can be selectively bypassed by transistors M 3  and M 4  in response to a digital configuration signal, which is represented by CL 1  and CL 2 . If necessary, more resistors, bypass transistors and more control signals may be provided to give additional configuration ranges. This approach allows the amplifier stage to be tuned roughly by switching resistors R 1  to R 3  into the current path through transistors M 1  and M 0 . A more precise tuning of the amplitude is provided by the amplitude control stage ACC which controls the gate voltage of NMOS control transistor M 0 , that is selectively degenerated by the resistors R 1  to R 3 . 
     FIG. 4  shows a simplified schematic of a further embodiment of the invention. The circuitry of  FIG. 4  is based on the circuitry of  FIG. 2 , except that a pick-up stage PICKUP is implemented as a differential stage coupled by input terminals VIN+ and VIN− respectively with both sides of the crystal XTAL, i.e., with crystal terminals XIN and XOUT. The pick-up stage PICKUP amplifies the differential input signal and transforms the signal into a single-ended output signal VCXOUT which serves as the oscillator output signal OUT. The differential pick-up stage allows common mode noise (substrate noise, crosstalk) to be suppressed. 
     FIG. 5  shows a simplified schematic of an example capacitive load stage according to an aspect of the invention. Either one, or both, of capacitors C 0  and C 1  may be implemented according to the circuitry shown in  FIG. 5 . A plurality of NMOS transistors M 11 , M 12 , M 13 , M 14 , M 15  and M 16  can be switched by respective PMOS transistors M 5 , M 6 , M 7 , M 8 , M 9  and M 10  to the TOP rail. The switching is done by a digital configuration signal represented by CCL 1 , CCL 2 , CCL 3 , CCL 4 , CCL 5  and CCL 6 . This allows the capacitive load to be selectively switched in response to a digital control signal. The illustrated configuration may also be adopted to allow the resistors R 1 , R 2 , R 3 , . . . shown in  FIG. 3  to be switched using the same digital configuration signal used for the capacitive load transistors shown in  FIG. 5 . By establishing a link between the switching of the resistors and the switching of the capacitive loads, it is possible to find an optimum operating range for the crystal oscillator by defining an optimum negative impedance. The indicated TOP rail may be coupled to the oscillator terminals XIN and XOUT and the indicated BOTTOM rail may be coupled to either receive the control voltage signal VCTRL or to a ground potential. 
     FIG. 6  shows a simplified schematic of a differential pick-up stage according to another aspect of the invention. Transistors M 17  and M 18  receive the differential input signals VIN− and VIN+ (cf.,  FIG. 4 ) and serve as a differential pair. Transistors M 21  and M 22  represent the load to the differential pair M 17  and M 18 , while transistors M 23 , M 24 , M 25  and M 26  constitute the output stage. Transistors M 19  and M 20  provide a feedback connection, such that the whole circuitry operates as a bistable circuit, producing an alternating clock signal at the single-ended output OUTS. The output OUTS is coupled to digital circuitry represented by an inverter INV 1 . Additional logical cells may be provided as appropriate. 
     FIG. 7  shows a simplified schematic of an example amplitude control stage ACC according to the invention. In the ACC, the signal sampled from XOUT is detached from its original DC value by a capacitor C 2  and attached to a DC value defined by resistors R 9  and R 10 . It is then fed into a peak detector which will store its highest voltage (the peak voltage) value VPK. This peak voltage is then compared to the voltage defined by the resistive divider made up by resistors R 6  and R 7 , resulting in the control voltage signal VCTRL. The difference between the voltages defined by the resistive divider R 9 , R 10  and the resistive divider R 6 , R 7  defines the amplitude to which the oscillation will be regulated. Resistor R 8  serves as a pull-down resistor for capacitor C 3 . 
     FIG. 8  is a block diagram view of a Pierce crystal oscillator circuit implementation according to the invention. A crystal can be coupled to terminals XIN and XOUT as suggested in connection with  FIG. 2 . The amplifying transistor M 1  and the resistor R 0  provide the necessary amplification to establish an oscillation. The capacitors C 0  and C 1  ( FIG. 2 ) are implemented as capacitor banks C 0 BANK and C 1 BANK, respectively, in a manner as shown in  FIG. 5 . The block ISET is the bias generator stage. It includes the switchable resistors (including the switches and additional logic gates to provide appropriate switching signals for the switches) that define the several selectable biasing points, as well as the transistor M 0 , thereby configuring the degenerated common-source amplifier. 
   The pick-up stage PICKUP is coupled to the XIN crystal terminal and to the amplifier output AMPOUT as shown in  FIG. 4 . However, in contrast to  FIG. 4 , a resistor R 4  is coupled between the XOUT crystal terminal and the input VIN− of the pick-up stage PICKUP. The resistor R 4  serves to enlarge the pulling range by shifting a pole of the transfer function, as explained above. The digital configuration signal CL is coupled to the capacitive load bank C 0 BANK and C 1 BANK and also to the bias current setting stage ISET. CL is used to provide digital control signals CL 1 , CL 2  shown in  FIG. 3  and CCL 1  to CCL 6  in  FIG. 5 . The number of switchable resistive and capacitance elements in the configurations of  FIGS. 3 and 5  may advantageously be chosen so that their corresponding control signals have the same number of lines (or bits). Accordingly, the control mechanisms for setting the transconductance of transistor M 1  and the setting of the capacitive loads are linked to each other in order to achieve optimum performance. If some of the resistors in block ISET are switched, some capacitors in C 0 BANK and C 1 BANK are likewise switched. The block VCSWITCH is used to switch the potential on nodes BOTTOM of C 0 BANK and C 1 BANK between ground and VC when working in VCXO mode.