Abstract:
Briefly, in accordance with embodiments of the invention, switched capacitors may be utilized to emulate resistors in a longer time constant feedback network for amplitude regulation of a crystal oscillator.

Description:
TECHNICAL FIELD  
         [0001]    Embodiments of the present invention pertain to crystal oscillators, and in some embodiments, to processing systems such as wireless communication devices and other systems and devices that use a precision time reference.  
         BACKGROUND  
         [0002]    Crystal oscillators and crystal-oscillator systems are used to provide a precision time reference for many applications, such as processing and computing systems including communication devices. Some conventional crystal-oscillator systems employ large resistors and/or capacitors for low-pass filtering as part of a feedback process to limit the amplitude of the oscillation frequency. One problem with these conventional crystal-oscillator systems is that the large resistors and capacitors usually require a large amount of circuit area on a die. Another problem with these large resistors and capacitors is that they are susceptible to process variations making it difficult to design and fabricate accurate oscillator circuits. Another problem with these conventional crystal-oscillator systems is that their output level may vary over time due to environmental factors.  
           [0003]    Thus, there are needs for improved oscillators and methods. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    The appended claims are directed to some of the various embodiments of the present invention. However, the detailed description presents a more complete understanding of embodiments of the present invention when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures and:  
         [0005]    [0005]FIG. 1 is a block diagram of a system in accordance with embodiments of the present invention;  
         [0006]    [0006]FIG. 2 is a block diagram of an oscillator element in accordance with embodiments of the present invention;  
         [0007]    [0007]FIG. 3 is a circuit diagram of an oscillator element in accordance with embodiments of the present invention; and  
         [0008]    [0008]FIG. 4 is a flow chart of a precision time reference generating procedure in accordance with embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0009]    The following description and the drawings illustrate specific embodiments of the invention sufficiently to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The scope of embodiments of the invention encompasses the full ambit of the claims and all available equivalents of those claims.  
         [0010]    [0010]FIG. 1 is a block diagram of a system in accordance with embodiments of the present invention. System  100  may be part of any computing or processing system including computer systems, server systems, and wireless communication devices and systems. System  100  includes oscillator system  102  to generate precision time reference  104  for use by system elements. System elements which use precision time reference  104  may include, for example, receiver/transmitter elements  106 , data-signal processing elements  108 , and other system elements  110  including phase-locked loops, displays and I/O devices. In some embodiments, elements  106 ,  108  and/or  110  may be on-chip elements.  
         [0011]    Oscillator system  102  may include oscillator element  112  to generate oscillation frequency  114  and a switching-frequency generator  120  to provide switching frequency  116 . Switching frequency  116  may be responsive to oscillation frequency  114 . In embodiments, switching frequency  116  may be a multiple of oscillation frequency  114 . Oscillator system  102  may also include buffer-amplifier element  118  which may instruct switching-frequency generator  120  to generate a multiple of the oscillation frequency when the oscillation frequency is determined to be stable. In some embodiments, the stability of the oscillator frequency may be determined based on a number of clock cycles (e.g., after power up). In other embodiments, the stability of the oscillator frequency may be determined by sampling energy of the oscillation frequency and setting a latch when the energy exceeds a predetermined threshold.  
         [0012]    In embodiments, switching-frequency generator  120  may generate switching frequency  116  as a multiple of oscillation frequency  114 . Buffer-amplifier element  118  may receive oscillation frequency  114  and determine when the oscillation-frequency output is stable. In one embodiment, a system element, such as data-signal processing element  108 , may provide a control signal to switching-frequency generator  120  to tailor oscillation frequency  114  for individual circuit variances. For example, process variation may cause one chip to require a switching frequency of 1.000 MHz for minimum power consumption while another chip may require 1.001 MHz. In embodiments, processing element  108  may initially, continuously, and/or intermittently monitor the oscillation frequency and may adjust the switching frequency to reduce the oscillation amplitude to allow each chip to operate its oscillator at a minimum power level. In some embodiments, processing element  108  may adjust the switching frequency as a battery condition changes, or as the temperature changes to help keep the oscillator at an amplitude level which may minimize power consumption.  
         [0013]    In embodiments, buffer-amplifier element  118  may perform signal buffering and amplification to generate precision time reference  104  comprising a square wave suitable for CMOS applications. In some embodiments, element  118  may have hysteresis for noise isolation. In some embodiments, element  118  may instruct switching-frequency generator  120  to vary the characteristics of the switching frequency to change or substantially maintain the level of the oscillation-frequency output, for example, to compensate for environmental changes affecting the oscillator. In embodiments, oscillation frequency  114  and precision timing reference  104  may be almost any predetermined frequency. For example, oscillation frequency  114  and precision timing reference  104  may be around 32 kHz (e.g., 32.768 kHz), around 13 MHz, or around 3.6 MHz (e.g., 3.686 MHz) and multiples thereof. The exact frequency may depend on the requirements of the system or system elements.  
         [0014]    In embodiments, oscillator element  112  may include an oscillator sub-circuit to generate oscillation frequency  114  and an amplitude-limiter sub-circuit to provide feedback to the oscillator sub-circuit to control a level of oscillation frequency  114 . An example of an oscillator element is illustrated in FIG. 2. The amplitude-limiter sub-circuit may include a switched-capacitor network to control the feedback based on switching frequency  116  and the level of oscillation frequency  114 . In embodiments, switching elements of the switched-capacitor network may be switched in opposition when oscillation frequency  114  is stable. In some embodiments, characteristics of switching frequency  116  may be varied to change the level of oscillation frequency  114 . The characteristics of the switching frequency that may be varied include at least the switching rate/or the duty cycle. In embodiments, the oscillation frequency may be controlled by a ceramic, quartz or another piezoelectric type material which have either a crystalline or a non-crystalline structure.  
         [0015]    In embodiments, system  100  may be a wireless communication device and may include receiver/transmitter elements  106 . In these embodiments, elements  106  may receive and/or transmit RF communications using antenna  122 . In these embodiments, system  100  may, for example, be a personal digital assistant (PDA), a laptop or portable computer with wireless communication capability, a web tablet, a wireless telephone, a wireless headset, a pager, an instant messaging device, an MP3 player, a digital camera, an access point, or other device that may receive and/or transmit information wirelessly. In these embodiments, elements  106  may receive and/or transmit RF communications in accordance with specific communication standards, such as the IEEE 802.11 (a), 802.11 (b) and/or 802.11 (g) standards for wireless local area network (LAN) standards, although elements  106  may receive and/or transmit communications in accordance with other techniques including Digital Video Broadcasting Terrestrial (DVB-T) broadcasting standard, and the High performance radio Local Area Network (HiperLAN) standard.  
         [0016]    Antenna  122  may comprise a directional or omni-directional antenna including a dipole antenna, a monopole antenna, a loop antenna, a microstrip antenna or other type of antenna suitable for reception and/or transmission of RF signals. Elements  106  may provide convert RF signals to data signals for processing by data-signal processing elements  108  and for use by other elements  110  as part of the operation of system  100 . Data-signal processing elements  108  may also provide data signals to elements  106  for conversion to RF signals and transmission by antenna  122 .  
         [0017]    Although system  100  is illustrated as having several separate functional elements, one or more of the functional elements may be combined and may be implemented by combinations of software configured elements, such as processing elements including digital signal processors (DSPs), and/or other hardware elements. For example, processing elements may comprise one or more microprocessors, DSPs, application specific integrated circuits (ASICs), and combinations of various hardware and logic circuitry for at least performing the functions described herein.  
         [0018]    [0018]FIG. 2 is a block diagram of an oscillator element in accordance with embodiments of the present invention. Oscillator element  200  may be suitable for use as oscillator element  112  (FIG. 1) although other oscillation elements may also be suitable. Oscillation element  200  may include oscillator sub-circuit  202  to generate oscillation-frequency output  210 , and amplitude-limiter sub-circuit  204  to provide feedback to oscillator sub-circuit  202  to control and/or limit a level of oscillation-frequency output  210 . Oscillator element  200  may also include startup sub-circuit  206  to initially set bias of elements of oscillator sub-circuit  202  and/or amplitude-limiter sub-circuit  204  at power up.  
         [0019]    In accordance with embodiments of the present invention, amplitude-limiter sub-circuit  204  includes switched-capacitor network  208  to help control the feedback to oscillator sub-circuit  202  based, at least in part, on switching-frequency input  212  and the level of oscillation-frequency output  210 . In embodiments, switching-frequency input  212  may be provided by a switching-frequency generator, such as switching-frequency generator  120  (FIG. 1).  
         [0020]    [0020]FIG. 3 is a circuit diagram of an oscillator element in accordance with embodiments of the present invention. Oscillator element  300  may be suitable for use as oscillator element  200  (FIG. 2), although other circuits may also be suitable. Oscillator element  300  comprises oscillator sub-circuit  302 , amplitude-limiter sub-circuit  304  and startup sub-circuit  306  which may correspond respectively with oscillator sub-circuit  202  (FIG. 2), amplitude-limiter sub-circuit  204  (FIG. 2) and startup sub-circuit  206  (FIG. 2). Amplitude-limiter sub-circuit  304  includes switched-capacitor network  308  which may correspond with switched-capacitor network  208  (FIG. 2).  
         [0021]    Oscillation sub-circuit  302  includes crystal  310  and transistor element  312  which may amplify and invert a voltage which may develop across crystal  310 . Element  312  may provide oscillation-frequency output  314 . Transistor element  316  may be a current source for element  312 , and may be a current mirroring element which mirrors current of transistor element  318  of amplitude-limiter sub-circuit  304 . Crystal  310  may comprise almost any ceramic, quartz or other piezoelectric-type material, and may have a crystalline or non-crystalline structure. In embodiments, crystal  310  may have a high “Q” mechanical resonance.  
         [0022]    Amplitude-limiter sub-circuit  304  includes transistor elements  318 ,  320 ,  322  and  324 , which, along with switched-capacitor network  308 , may perform an amplitude-regulating function to limit and/or control the amplitude of the oscillation frequency at output  314 . Startup sub-circuit  306  may include transistor elements  326 ,  328  and  330  and may initially provide a bias for elements  316 ,  318  and  320  at power up. Switched-capacitor network  308  includes switching elements  332  and  334  along with capacitive elements  336 ,  338  and  340 . In operation, switching element  332  may transfer charge from capacitive element  336  to capacitive element  338 , and switching element  334  may transfer charge from capacitive element  338  to capacitive element  340 . In embodiments, switching elements  332  and  334  may be alternatively switched (e.g., driven in opposition) by a switching frequency received at their inputs. The switching frequency may be a multiple of the oscillation frequency, and may be provided by switching-frequency generator  120  (FIG. 1). Initially, switching elements  332  and  334  may be closed to create a DC path between the inputs of elements  322  and  324  to help establish initial startup conditions.  
         [0023]    The amount of charge provided to element  324  changes the bias of element  318  and the current through element  318  may be mirrored in element  316 . The current in element  316  may be proportional to the current through element  318  depending on the relationship of the sizes of the devices. In embodiments, element  316  may be a factor larger (e.g., 5×) than device  318  and may carry proportionally more current.  
         [0024]    Before oscillation amplitude has grown large, elements  318 ,  320 ,  322  and  324  provide DC bias to supply element  312  with current. As the amplitude of the oscillation frequency at output  314  increases, element  312  drives an AC signal onto the input of element  322 . If the AC signal has a certain amplitude, element  322  may pull current proportional to the amplitude squared off of capacitive element  336 . This may cause a voltage on the input of element  324  to drop, reducing the current available to element  312 , reducing the AC amplitude until a static operating point is achieved.  
         [0025]    Switching-capacitor network  308  may help reduce the semiconductor die area of element  300  while providing low-power amplitude limiting of the oscillation frequency. Although switching elements  332  and  334  are illustrated as specific-type transistor switching elements, this is not a requirement. In embodiments, switching elements  332  and  334  may be almost any switch or switching element, including, for example, a MEMS relay. In some embodiments, element  332  and element  334  may comprise transmission gates and may use both an NMOS and a PMOS device for better charge transfer. Capacitive elements  338  and  340  may be almost any size as their ratio and the switching frequency primarily may control the filter characteristics of network  308 .  
         [0026]    When oscillator element  300  is first powered up, both element  332  and element  334  may be closed, creating a DC path between the inputs of element  322  and element  324 . The current source is biased which starts element  312 . When the oscillation becomes stable, element  332  and element  334  may be driven in opposition at a multiple of the oscillator frequency. Charge may then be transferred onto capacitive element  338  when element  332  is closed and element  334  open setting the voltage on capacitive element  338  to the voltage on capacitive element  336 . When element  332  opens and when element  334  closes, capacitive element  338  modifies the voltage on capacitive element  340  and the voltage on the input of element  324 . In this way, capacitive element  338  behaves as a resistor, and the value of resistance may be controlled by the frequency of switching of element  332  and element  334 . A reduction in semiconductor die area may be achieved over conventional low-pass filters with may use large resistances and capacitances. In embodiments of the present invention, capacitive element  338  is ratioed to capacitive element  340  and capacitive element  336 , and since process technology may control capacitance much better than resistance, an increase in precision may also be achieved.  
         [0027]    In one embodiment, element  332  and element  334  are switched with the open and close times at a different duty cycle and/or switching rate than the primary frequency. In this way, a processor may control the switching and an adaptive algorithm may tailor the duty cycle and/or switching rate to accommodate environmental changes. Although circuit  300  is illustrated to include resistors and capacitors, one or more of such resistors and/or capacitors may be implemented with active devices, such as transistors, rather than with passive devices.  
         [0028]    In one embodiment, the oscillation frequency may be provided at output  315  which is an input node for element  312 . Output  315  may be instead of or in addition to output  314 . In some cases, output  315  may provide a cleaner sinusoid signal for subsequent amplification than the sinusoid signal at output  314 . In this embodiment, output  315  may provide oscillation frequency  114  (FIG. 1) and may correspond with oscillation-frequency output  210  (FIG. 2).  
         [0029]    In some embodiments, a switching element may be placed between the output of element  312  and the input of element  322  to disconnect amplitude limiter sub-circuit  304  from oscillator sub-circuit  302  until the oscillation has stabilized and the switching frequency  116  (FIG. 1) has been generated. When the oscillation is stable and switching frequency is available to amplitude limiter sub-circuit  304 , the switching element can be closed allowing amplitude limiter sub-circuit  304  to sample the level of oscillation.  
         [0030]    [0030]FIG. 4 is a flow chart of a precision time reference generating procedure in accordance with embodiments of the present invention. Precision time reference generating procedure  400  may be performed by an oscillator system, such as oscillator system  102  (FIG. 1), although other systems may also be suitable for performing procedure  400 . Procedure  400  may be used for generating almost any precision time reference signal for use in almost any computing or processing system or device.  
         [0031]    In operation  402 , a startup bias may be provided at power up to allow an oscillator to begin generating an oscillation frequency. Operation  402  may be performed by startup circuitry, such as startup sub-circuit  206  (FIG. 2). In operation  404 , an oscillation frequency is generated. Operation  404  may be performed by oscillator circuitry, such as oscillation sub-circuit  202  (FIG. 2).  
         [0032]    In operation  406 , a switching frequency may be generated and provided to switching elements of a switched-capacitor network used as part of an amplitude limiting sub-circuit of an oscillation system. The switching frequency may be a multiple of the oscillation frequency and may be generated when the oscillation frequency is stable. Operation  406  may be performed by switching-frequency generator  120  (FIG. 1) and buffer-amplifier element  118  (FIG. 1) may determine when the oscillation frequency is stable.  
         [0033]    In operation  408 , amplitude-limiting feedback may be generated to control or limit the amplitude of the oscillation frequency. The amplitude-limiting feedback may be generated by an amplitude limiter, such as amplitude-limiter sub-circuit  204  (FIG. 2) based on the switching frequency generated in operation  406  as well as the amplitude level of the oscillation frequency generated in operation  404 . Operations  404 ,  406  and  408  may be performed substantially concurrently to generate a precision time reference.  
         [0034]    In some embodiments, operation  410  may be performed. In operation  410 , the characteristics, such as the duty cycle and/or switching rate, of the switching frequency may be varied to change the level of the oscillation-frequency output. Operation  410  may be performed, for example, by buffer-amplifier element  118  (FIG. 1). Although the individual operations of procedure  400  are illustrated and described as separate operations, one or more of the individual operations may be performed concurrently and nothing requires that the operations be performed in the order illustrated.  
         [0035]    It is emphasized that the Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims.  
         [0036]    In the foregoing detailed description, various features are occasionally grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the subject matter require more features that are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate preferred embodiment.