Abstract:
An apparatus includes a microelectromechanical system (MEMS) device configured as part of an oscillator. The MEMS device includes a mass suspended from a substrate of the MEMS, a first electrode configured to provide a first signal based on a displacement of the mass, and a second electrode configured to receive a second signal based on the first signal. The apparatus includes an amplifier coupled to the first electrode and a first node. The amplifier is configured to generate an output signal, the output signal being based on the first signal and a first gain. The apparatus includes an attenuator configured to attenuate the output signal based on a second gain and provide as the second signal an attenuated version of the output signal.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention is related to integrated circuit oscillators and more particularly to microelectromechanical systems (MEMS) oscillators. 
     2. Description of the Related Art 
     In general a microelectromechanical systems (MEMS) device may be included in an electronic oscillator that converts a direct current from a power supply to an alternating current signal. A typical oscillator includes a core amplifier that senses one (or more) electrode(s) of the MEMS device and drives a restoring voltage on another electrode of the MEMS device. The output of a typical MEMS resonator is a small signal that is amplified by a buffer amplifier to generate a usable signal for other circuits. However, low-power buffering of any of the small electrode signals will introduce substantial noise into the signal. Furthermore, techniques to reduce noise added to a signal by a buffer amplifier generally increase power consumption of the buffer amplifier and do not reduce noise introduced by the core oscillator since the buffer amplifier is outside of the core oscillator feedback loop. Accordingly, improved MEMS oscillator techniques are desired. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     In at least one embodiment of the invention, an apparatus includes a MEMS device configured as part of an oscillator. The MEMS device includes a mass suspended from a substrate of the MEMS, a first electrode configured to provide a first signal based on a displacement of the mass, and a second electrode configured to receive a second signal based on the first signal. The apparatus includes an amplifier coupled to the first electrode and a first node. The amplifier is configured to generate an output signal. The output signal is based on the first signal and a first gain. The apparatus includes an attenuator configured to attenuate the output signal based on a second gain and provide as the second signal, an attenuated version of the output signal. The apparatus may include an automatic amplitude control module configured to generate a feedback signal based on a reference signal level and the second signal. The amplifier may adjust the first gain based on the feedback signal. 
     In at least one embodiment of the invention, a method includes amplifying a first signal on a first electrode of a MEMS device configured as part of an oscillator to generate an output signal. The output signal is based on the first signal and a first gain. The method includes attenuating the output signal based on a second gain to generate a second signal. The method includes providing the second signal to a second electrode of the MEMS device. The method may include adjusting the first gain based on a reference signal level and the second signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
         FIG. 1A  illustrates a circuit diagram of an exemplary MEMS device. 
         FIG. 1B  illustrates a circuit diagram of an exemplary MEMS oscillator circuit. 
         FIG. 2  illustrates a circuit diagram of a MEMS oscillator circuit including an attenuator configured consistent with at least one embodiment of the invention. 
         FIG. 3  illustrates a circuit diagram of a MEMS oscillator circuit including an attenuator and automatic gain control configured consistent with at least one embodiment of the invention. 
         FIG. 4  illustrates a circuit diagram of a MEMS oscillator circuit including a non-linear attenuator configured consistent with at least one embodiment of the invention. 
         FIG. 5  illustrates a graphical representation of a voltage of a signal generated by the non-linear attenuator of the MEMS oscillator circuit of  FIG. 4  consistent with at least one embodiment of the invention. 
         FIG. 6  illustrates a graphical representation of a current of a signal generated by the non-linear attenuator of the MEMS oscillator circuit of  FIG. 4  consistent with at least one embodiment of the invention. 
         FIG. 7  illustrates a circuit diagram of an exemplary non-linear attenuator and associated waveforms consistent with at least one embodiment of the invention. 
         FIG. 8  illustrates a circuit diagram of an exemplary inverting delay element of the non-linear attenuator of  FIG. 7  consistent with at least one embodiment of the invention. 
         FIG. 9  illustrates a circuit diagram of an exemplary inverting delay element of the non-linear attenuator of  FIG. 7  consistent with at least one embodiment of the invention. 
         FIG. 10  illustrates a circuit diagram of an exemplary bias circuit for the inverting pulse generation circuit of  FIG. 9  consistent with at least one embodiment of the invention. 
         FIG. 11  illustrates a circuit diagram of an exemplary bias circuit for the inverting pulse generation circuit of  FIG. 9  consistent with at least one embodiment of the invention. 
     
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A , exemplary microelectromechanical system structure  100  includes MEMS device  102  formed using substrate  103 , which may include a CMOS integrated circuit. MEMS device  102  may be any device that falls within the scope of MEMS technologies. For example, MEMS device  102  may be any mechanical and electronic structure having a critical dimension of less than approximately 250 microns and fabricated above a substrate using lithography, deposition, and etching processes. MEMS device  102  may be a device such as, but not limited to, a resonator (e.g., an oscillator), a temperature sensor, a pressure sensor or an inertial sensor (e.g., an accelerometer or a gyroscope). MEMS device  102  may have a portion suspended from the substrate (i.e., a suspended mass), which includes an integrated circuit (not shown). In one embodiment, the suspended portion of MEMS device  102  is suspended feature  101  having a resonant frequency. For example, the suspended portion of MEMS device  102  is a feature such as, but not limited to, a beam, a plate, a cantilever arm or a tuning fork. In a specific embodiment, MEMS device  102  includes a feature  101  that is anchored to the substrate by an anchor portion  111 , is flanked by a driver electrode  105  and a sensor electrode  107 , and resonates in direction  109 . 
     Referring to  FIG. 1B , an exemplary MEMS oscillator  100  includes MEMS device  102  coupled to amplifier  110 . Sense electrode x 1  generates a signal based on energy transfer from a vibrating mass of MEMS device  102 , thereby converting mechanical energy into an electrical signal. A large feedback resistor (R F ) biases amplifier  110  in a linear region of operation thereby causing amplifier  110  to operate as a high-gain inverting amplifier. MEMS oscillator  100  sustains vibrations of MEMS device  102  by feeding back the output of amplifier  110  to a drive electrode of MEMS device  102 . Amplifier  110  receives a small-signal voltage on a gate of device  106  and generates a voltage on drive electrode x 2  that causes the mass of MEMS device  102  to continue to vibrate. The feedback loop of MEMS oscillator  100  results in a small signal that is received by buffer  108 . MEMS device  102  in combination with capacitances C 1  and C 2  form a pi-network band-pass filter that provides 180 degrees of phase shift from the drive electrode (i.e., electrode x 2 ) to the sense electrode (i.e., electrode x 1 ) at approximately the resonant frequency of MEMS device  102 . 
     Buffer  108  detects the small-signal output of amplifier  110  and converts it to a usable signal (e.g., a digital signal) having appropriate signal levels. For example, buffer  108  may convert the output of amplifier  110  into a CMOS signal or another signal format suitable for other applications. In addition, buffer  108  transfers a voltage from MEMS oscillator  100 , which has a high output impedance level, to a second circuit with a low input impedance level. Buffer  108  prevents the second circuit from loading the MEMS oscillator unacceptably and interfering with its desired behavior. In an ideal buffer  108 , the input resistance is high and the output resistance is low. In addition, buffer  108  is linear and has a low latency over the expected range of signal amplitudes and frequencies. 
     In general, low-amplitude signals are more susceptible to noise than other signals because the noise is more likely to affect zero-crossings of low-amplitude signals than higher-amplitude signals. Input-referred noise of buffer  108  may affect zero-crossings of an output clock signal that is based on a small signal input to buffer  108 , resulting in jitter in the output clock signal. Accordingly, output signals of amplifier  110  having greater amplitudes generally result in lower jitter output clock signals than output signals of amplifier  110  having lesser amplitudes. Thus, if the gain of amplifier  110  is increased to double the signal swing of the small signal output (i.e., the input to buffer  108 ), the output jitter is reduced by half. However, a typical MEMS device  102  has power handling limitations that are handled by restricting the gain of amplifier  110 . For example, an increase in small-signal voltages on drive electrode x 2  may increase the electromechanical forces that cause mechanical movement of the mass of MEMS device  102 . Substantial increases in the mechanical movement of the mass can cause the mass to move non-linearly. If the voltages are large enough, the mass could even hit the electrode(s). Although an increase in the gap between the mass and the electrodes can avoid that non-linear behavior of MEMS device  102 , the increase would cause the mass to resonate at a higher amplitude and would also increase a required body voltage of MEMS device  102 . As a result, the reduction in jitter by increasing the amplitude of the signals on the drive electrode is a tradeoff with increased power consumption of MEMS oscillator  100 . 
     As discussed above, the nonlinear effects associated with a MEMS device set an upper limit of the resonator dynamic range. Noise levels determine a lower limit of the resonator dynamic range. In addition, the resonant frequency of MEMS device  102  may vary as a function of vibrational amplitude. An exemplary application requires frequency accuracy of plus or minus 10 parts-per-million (ppm) at room temperature (plus or minus 40 ppm at 85 degrees Celsius or less). If the amplifier ages and, as a result, the amplitude of the output signal changes, then the driving force changes and non-linear behavior changes the frequency of vibration of MEMS device  102 . Mechanical and electrical nonlinearities associated with the MEMS device may result in pulling of the resonant frequency via changes in vibrational amplitude. Moreover, operating in a nonlinear region can degrade the phase noise of the system at frequencies close to the carrier frequency. Accordingly, a target vibration amplitude of MEMS device  102  is a vibration amplitude that is outside the range of amplitudes of a resonant-frequency sensitive region of operation (i.e., a vibration amplitude that is lower than a resonant-frequency sensitive range of amplitudes). 
     To achieve a target low vibration amplitude requires that drive electrode x 2  receive a low amplitude signal from amplifier  110 , which conflicts with the requirement of buffer  108  receiving a higher amplitude signal to reduce noise in the output signal. A technique divorces the amplitude requirements of a feedback signal provided to a drive electrode of a MEMS device in a MEMS oscillator from conflicting amplitude requirements of a signal provided to an output buffer of the MEMS oscillator. Referring to  FIG. 2 , the oscillator technique includes a high gain amplifier  210  that provides a large swing version of a signal on sense electrode of MEMS device  102  to buffer  108  and attenuator  212  that provides an attenuated version of the signal on the sense electrode of the MEMS device as a feedback signal to the drive electrode of the MEMS device. Accordingly, the jitter of clk OUT  is reduced, as compared to a MEMS oscillator system of  FIG. 1  that includes a lower gain amplifier  110 , without substantially increasing power consumption of buffer  108 . Inclusion of attenuator  212  in a MEMS oscillator is contrary to typical efforts to improve or maintain dynamic range of the output signal. In at least one embodiment, attenuator  212  includes a capacitive voltage divider. For example, attenuator  212  includes an additional capacitor C ATTEN  coupled to amplifier  210  and drive electrode x 2 . However, other voltage divider circuits or other attenuators (e.g., resistor-capacitor filter) and configurations may be used. 
     Environmental conditions (e.g., process, voltage, temperature, and aging variations) may impact the amplitude of the output of amplifier  210  and the amplitude of the signal provided to drive electrode x 2 . Referring to  FIG. 3 , MEMS oscillator  300  includes variable gain amplifier  310  in an automatic amplitude control loop that maintains the amplitude of the signal provided to drive electrode x 2  in a well-controlled range. For example, rectifier  504  senses the peak voltage of the signal generated by variable gain amplifier  310  and provides an indicator thereof to comparator  312 . Note that in other embodiments of MEMS oscillator  300 , instead of sensing the peak voltage of the signal generated by variable gain amplifier  310 , rectifier  504  receives the signal generated by attenuator  502  and provides comparator  312  with an indicator of the peak voltage of the signal provided to drive electrode x 2 . Comparator  312  compares the sensed peak voltage to a reference signal (e.g., target peak voltage, V PKREF ) and generates a control signal (e.g., a difference) based thereon. Variable gain amplifier  310  adjusts the gain applied to the signal on sense electrode x 1  based on the control signal (e.g., increases or decreases the gain of variable gain amplifier  310  in response to the control signal V CTL ) and provides the amplified signal to analog attenuator  502 , which attenuates the output of variable gain amplifier  310  to generate a drive signal. In at least one embodiment, attenuator  502  applies a gain having a value of approximately one half to approximately one one-thousandth. In at least one embodiment, attenuator  502  applies a predetermined gain value that was selected based on a power budget of the apparatus. A combined gain of the MEMS device and amplifier  310  is approximately a reciprocal of the gain of the attenuator (i.e., the loop gain equals one to maintain oscillation at the resonant frequency of MEMS device  102 ). Feedback resistor R f  sinks current as needed. 
     In at least one embodiment of the MEMS oscillator technique, rather than use a linear analog attenuator, a non-linear attenuator is used. Referring to  FIGS. 4-6 , an exemplary non-linear attenuator includes pulse generator  506 . Pulse generator  506  reduces power consumption of MEMS oscillator  400  as compared to MEMS oscillator  300  by exploiting properties of a periodic signal. Since any periodic signal can be decomposed into a sum of simple oscillating signals, (e.g., sine and cosine), rather than providing a sinusoidal signal or square wave signal with a 50% duty cycle (or an approximation thereof) to the drive electrode of MEMS device  102 , pulse generator  506  generates a non-symmetrical, non-sinusoidal signal with a relatively narrow pulse-width, Δt (e.g., Δt&lt;&lt;T o , where T o  is a period of the signal). For example, the output of pulse generator  506  has a pulse width Δt and a resonant frequency ω o , where ω o ×Δt&lt;&lt;1. The resulting signal has a relatively small amount of energy that is concentrated at the resonant frequency of MEMS device  102 . Since higher harmonics are ignored by MEMS device  102 , this approach results in the attenuator generating a full-swing signal, but consumes substantially less power than symmetrical, sinusoidal signals generated by other embodiments MEMS oscillator  300 . 
     Note that since the pulse generator  506  generates a full amplitude (V PK ) pulse, pulse generator  506  also incorporates an automatic amplitude control function and no separate control loop is needed (e.g., comparator  312  and rectifier  504  of  FIG. 3  are excluded). Referring back to  FIGS. 4 ,  5 , and  6 , once amplifier  210  feeds a full-swing square wave to pulse generator  506 , any additional input current on sense electrode x 1  does not affect the pulse generator output width. That is, pulse generator  506  behaves like an amplitude clamp since the maximum amplitude is completely determined by the pulse generator and not based on the input signal amplitude. For example, V PK (ω o )≈V PK (2×Δt/T o ). Since the attenuator output is a full-swing signal, the signal on the sense electrode of MEMS device  102  is a full-swing signal and, in some embodiments of the MEMS oscillator technique, buffer amplifier  108  may not be needed. Instead, a unity gain buffer may be used, a buffer may be eliminated entirely, and/or a divider circuit (e.g., FDIV2  504 ) may be used to generate a clock signal with a 50% duty cycle. 
       FIGS. 7-11  illustrate exemplary pulse generator implementations, although other non-linear attenuators may be used. Pulse generator  602  logically-nors an inverted, delayed version of the input signal with the square wave input signal to generate a pulsed signal having the same frequency as the input signal but having a pulse width Δt, which is much less than the pulse width of the input signal. Exemplary inverting delay elements  700  are illustrated in  FIG. 8  and in  FIGS. 9-11 . Inverting delay element  700  may include an RC filter, as illustrated in  FIG. 8 , or a cascode inverter, as illustrated in  FIG. 9 .  FIGS. 10 and 11  illustrate exemplary bias circuitry for the cascode inverter of  FIG. 9 . Other pulse generators, e.g., delay-locked-loop based pulse generators that lock to the resonant frequency of MEMS device  102  and generate one charge and one discharge pulse per cycle, may be used. 
     Accordingly a technique for protecting a MEMS oscillator from signal overdrive that generates a low jitter output signal within a power budget is described. The amplifier gain is based on output requirements, which may be determined by other circuitry coupled to a MEMS oscillator. The attenuator gain may be based on a power handling limit of the MEMS device. Referring back to  FIGS. 2 ,  3 , and  4 , although the electronic attenuator for oscillator overdrive protection techniques have been described in embodiments that include a Pierce oscillator configuration, other embodiments of MEMS oscillators that use other oscillator configurations and/or additional elements may be included. For example, an isolation resistor may be included between the output of the amplifier and MEMS device  102 . Although amplifier  210  is illustrated as an n-type device biased by current source  204 , amplifier  210  may be implemented using other amplifier designs. 
     While circuits and physical structures have been generally presumed in describing embodiments of the invention, it is well recognized that in modern semiconductor design and fabrication, physical structures and circuits may be embodied in computer-readable descriptive form suitable for use in subsequent design, simulation, test or fabrication stages. Structures and functionality presented as discrete components in the exemplary configurations may be implemented as a combined structure or component. Various embodiments of the invention are contemplated to include circuits, systems of circuits, related methods, and tangible computer-readable medium having encodings thereon (e.g., VHSIC Hardware Description Language (VHDL), Verilog, GDSII data, Electronic Design Interchange Format (EDIF), and/or Gerber file) of such circuits, systems, and methods, all as described herein, and as defined in the appended claims. In addition, the computer-readable media may store instructions as well as data that can be used to implement the invention. The instructions/data may be related to hardware, software, firmware or combinations thereof. 
     The description of the invention set forth herein is illustrative, and is not intended to limit the scope of the invention as set forth in the following claims. For example, while the invention has been described in an embodiment in which electrostatic (capacitive) actuation is used, one of skill in the art will appreciate that the teachings herein can be utilized using other actuation techniques (e.g., piezoelectric actuation). Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.