Abstract:
Circuits, methods, and apparatus that provide low-noise, high-stability crystal oscillators having controlled-amplitude differential output signals and DC level control. A crystal oscillator circuit has two feedback loops, one for setting the DC level of its signals, the other for adjusting the amplitude of those signals. The DC level feedback loop can set the DC component of the oscillator signals to a voltage midway between two supply voltages. The amplitude control loop sets the amplitude of the output of the crystal oscillator signal to be within a range. The amplitude can be set to provide a maximum swing without clipping the supply voltages in order to provide high-stability and minimal jitter. The amplitude control circuit can also be digital for improved noise performance. The time constants of these two loops can be separated such that instabilities are avoided.

Description:
[0001]     This application claims the benefit of U.S. provisional application No. 60/704,525 filed Aug. 1, 2005, which is incorporated by reference. 
     
    
     BACKGROUND  
       [0002]     The present invention relates generally to crystal oscillators, and more specifically to low-noise, high-stability crystal oscillators.  
         [0003]     Crystal oscillators are extremely useful circuits. They provide clocks and periodic signal sources for telecommunications, wired and wireless networks, and myriad other electronic applications. For example, crystal oscillators are commonly used to time data transfers between integrated circuits. In these applications, crystal oscillator phase noise and jitter degrades performance, causes data transmission errors, and limits data throughput. Thus, it is desirable to provide crystal oscillators having low-noise and high-stability.  
         [0004]     The signal-to-noise ratio for a crystal oscillator can be improved by increasing its signal strength. One way to increase signal strength or amplitude is to generate a differential signal, as opposed to a single-ended signal. A differential signal not only provides a signal that is nominally twice the amplitude of a single-ended signal, but provides a level of common-mode rejection as well, which further reduces noise. Also, a buffer receiving these larger oscillator signals can operate at a lower gain resulting in less noise.  
         [0005]     Unfortunately, excessively large crystal oscillator signals can cause jitter or instability in the oscillator circuit. As these signals become excessive, they may become limited by one or both of a pair of supply voltages for the crystal oscillator. Specifically, electrostatic discharge (ESD) diodes to these supplies can begin to conduct current. This clips the oscillator signals, which adds harmonics and spurious frequency components to the otherwise single-tone signal. These harmonics pull or shift the oscillator operating frequency, resulting in center frequency inaccuracies.  
         [0006]     Also, signals from crystal oscillators typically need to be AC coupled to an integrated circuit that is using the oscillator. If the DC level of the crystal oscillator signals could be well controlled, it would be possible to design an input buffer that could directly connect to the crystal without using the AC coupling capacitors. This would reduce component count, save board space, and reduce costs. This would also help prevent the oscillator signals from being clipped by the ESD diodes.  
         [0007]     Thus, what is needed are circuits, methods, and apparatus that provide crystal oscillators having large, amplitude-controlled differential signal outputs and mechanisms for controlling their DC levels.  
       SUMMARY  
       [0008]     Accordingly, embodiments of the present invention provide circuits, methods, and apparatus that provide low-noise, high-stability crystal oscillators having large differential output signals and DC level controls. One exemplary embodiment of the present invention provides a crystal oscillator having two feedback loops, one for setting the DC levels of its signals, the other for adjusting the amplitude of those signals. Various embodiments of the present invention may incorporate either one or both of these loops, as well as one or more of the features described herein.  
         [0009]     A specific embodiment of the present invention provides a feedback loop arranged to control the DC level of a crystal oscillator&#39;s signals. The DC level can be set to a voltage midway between two supply voltages, to a reference voltage, or to any other appropriate voltage. For example, the voltage may be a ground-referenced voltage that is equal to one-half the minimum supply voltage for the oscillator circuit. This voltage may be a function of either power supply or other condition such as temperature. Alternately, this voltage may be independent of these parameters.  
         [0010]     This embodiment further provides an amplitude-control feedback loop. This loop sets the amplitude of the output of the crystal oscillator signal to be within a range. The amplitude can be set to give a maximum swing without clipping either supply voltage in order to provide high-stability and minimal jitter. The amplitude control circuit can also be digital for improved noise performance. If this control loop is digital, a startup circuit can be included. In a specific embodiment, the startup circuit is an analog control loop that is disabled in favor of a digital control loop once the crystal oscillator circuit starts.  
         [0011]     The time constants or bandwidths of these two loops can be separated such that instabilities are avoided. Specifically, interaction between the loops is minimized by setting the bandwidth of the amplitude control loop to be much lower than the bandwidth of the DC level control loop.  
         [0012]     An exemplary embodiment of the present invention provides an integrated circuit. This integrated circuit includes a means for driving a resonant element to generate the first oscillator signal, means for adjusting a DC level of the first oscillator signal, and means for adjusting an amplitude of the first oscillator signal.  
         [0013]     This or other embodiments may further provide means for driving the resonant element by providing a drive signal to the resonant element, wherein the drive signal is responsive to the resonant element. This or other embodiments may further provide means for providing the drive signal with a gain circuit. This or other embodiments may further provide for the gain circuit being a MOS transistor. This or other embodiments may further provide means for adjusting the DC level of the first oscillator signal by comparing the first oscillator signal with a bias voltage, and providing an output responsive to the comparison. This or other embodiments may further provide for the gain element being a MOS transistor responsive to the output of the amplifier. This or other embodiments may further provide means for adjusting the DC level of the first oscillation signal to be between two supply voltages received by the integrated circuit. This or other embodiments may further provide means for measuring an amplitude of the first oscillation signal, and means for providing a measurement of the amplitude of the first oscillation signal. This or other embodiments may further provide means for measuring the amplitude of the first oscillation signal using a peak detector. This or other embodiments may further provide for the amplitude of the first oscillation signal being measured using a diode and a capacitance. This or other embodiments may further provide means for comparing the measurement of the amplitude of the first oscillation signal with a high threshold and a low threshold, and means for providing one or more signals in response to the comparison. This or other embodiments may further provide means for decrementing an output value when the amplitude of the first oscillation signal is greater than the high threshold, means for maintaining the output value when the amplitude of the first oscillation signal is less than the high threshold and greater than the low threshold, and means for incrementing the output value when the amplitude of the first oscillation signal is less than the low threshold. This or other embodiments may further provide means for generating a bias current in response to the output value. This or other embodiments may further provide means for providing the bias current to a gain circuit, the gain circuit providing the drive to the resonant element. This or other embodiments may further provide means for setting the DC level of the second oscillation signal using the DC level of the first oscillation signal. This or other embodiments may further provide means for DC coupling the DC level of the first oscillation signal to generate the DC level of the second oscillation signal.  
         [0014]     Embodiments of the present invention may be implemented in code, for example, code to be used in a digital signal processor or compiled using VHDL. One such exemplary embodiment of the present invention provides code of an oscillator including code for a gain element configured to drive a resonant element, code for a DC control loop configured to adjust a DC level of a signal at an output of the gain element, and code for an amplitude control loop configured to adjust an amplitude of the signal at the output of the gain element.  
         [0015]     This or other embodiments may further provide code for a gain element having an input responsive to a first node of the crystal and a crystal having a second node responsive to the output of the gain element. This or other embodiments may further provide code for the gain element being a transistor. This or other embodiments may further provide code for the transistor being a MOS transistor. This or other embodiments may further provide code for the DC control loop comprising an amplifier configured to compare the signal at the output of the gain element to a bias voltage and provide an output responsive to the comparison. This or other embodiments may further provide code for the gain element being a MOS transistor responsive to the output of the amplifier. This or other embodiments may further provide code for the DC level of the signal at the output of the gain element adjusting to a voltage that is between two supply voltages received by the integrated circuit. This or other embodiments may further provide code for the amplitude control loop comprising an amplitude measurement circuit configured to provide a measurement of an amplitude of the signal at the output of the gain element. This or other embodiments may further provide code for the amplitude measurement circuit comprising a peak detect circuit. This or other embodiments may further provide code for the peak detect circuit comprising a diode and a capacitance. This or other embodiments may further provide code for the amplitude control loop further comprising a comparator configured to compare the measurement of the amplitude of the signal at the output of the gain element with a high threshold and a low threshold, and further configured to provide one or more signals in response to the comparisons. This or other embodiments may further provide code for the amplitude control loop further comprising a counter configured to increment, decrement, or maintain an output value in response to the one or more signals provided the comparator. This or other embodiments may further provide code for the amplitude control loop further comprising a digital-to-analog converter configured to convert the output of the counter to a current. This or other embodiments may further provide code for the current being provided to the gain element. This or other embodiments may further provide code for the DC level of a signal at an output of the gain element being used to set a DC level of a signal at an input of the gain element. This or other embodiments may further provide code for the DC level of the signal at the output of the gain element being DC coupled to the input of gain element using a resistor.  
         [0016]     A better understanding of the nature and advantages of the present invention may be gained with reference to the following detailed description and the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a block diagram of a low-noise, high-stability crystal oscillator according to an embodiment of the present invention;  
         [0018]      FIG. 2  is a block diagram of a low-noise, high-stability Pierce crystal oscillator according to an embodiment of the present invention;  
         [0019]      FIG. 3  is a schematic of a DC biasing loop for a crystal oscillator according to an embodiment of the present invention;  
         [0020]      FIG. 4  is a flowchart showing the operation of the DC biasing loop, such as the DC biasing loop of  FIG. 3 ;  
         [0021]      FIG. 5  is a schematic of a digital amplitude control loop for a crystal oscillator according to an embodiment of the present invention;  
         [0022]      FIG. 6  is a flowchart showing the operation of an amplitude control loop, such as the amplitude control loop of  FIG. 5 ;  
         [0023]      FIG. 7  is a schematic of an analog amplitude control loop used to start a crystal oscillator according to an embodiment of the present invention; and  
         [0024]      FIGS. 8A-8H  illustrate various implementations of exemplary embodiments of the present invention. 
     
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0025]      FIG. 1  is a block diagram of a low-noise, high-stability crystal oscillator according to an embodiment of the present invention. This figure includes a crystal X 1   110 , gain circuit A 1   120 , amplifier A 2   130 , amplitude detection circuit  140 , resistors R 1   150  and R 2   160 , and capacitor C 1   170 . This figure, as with the other included figures, is shown for illustrative purposes and does not limit either the possible embodiments of the present invention or the claims.  
         [0026]     The crystal X 1   110  is driven by the gain element A 1   120 . In this and other embodiments of the present invention, the crystal X 1   110  may be a crystal or other resonant element or circuit, for example, it may be an L-C tank circuit. The gain element A 1   120  provides a net inversion and may be as simple as a transistor, though it may alternately be one or more inverters or buffers in series, so long as the combination provides a net signal inversion. The gain element A 1   120  provides the gain necessary to drive crystal X 1   110 .  
         [0027]     In operation, the signal V 2  on line  114 , the output terminal of the gain device A 1   120 , oscillates above and below a DC voltage. This DC voltage is the DC component of the signal V 2  on line  114 ; the oscillation is the AC signal component. Resistor R 1   120  equalizes the DC component of the signal V 1  on line  112  with the DC component of the signal V 2  on line  114 . The two signals, V 1  on line  112  and V 2  on line  114 , are nominally phase shifted by 180 degrees and each are ideally sinusoidal in nature.  
         [0028]     The DC voltage component of V 2  on line  114  is compared to a bias voltage on line  132  by the amplifier A 2   130 . In various embodiments, other voltages can be compared to the bias voltage on line  132 . For example, the DC component of the signal V 1  on line  112  can be compared. In other embodiments, the resistor R 1   120  is a number of resistors in series, and a voltage at a node between two of these resistors can be compared to the bias voltage on line  132 . In a specific embodiment of the present invention, the bias voltage on line  132  is set to a ground-referenced voltage that is equal to one-half a minimum supply voltage for the oscillator. In other embodiments of the present invention, this bias voltage may be equal to a reference voltage. For example, the bias voltage on line  132  may be equal to a bandgap voltage. In other embodiments of the present invention, the bias voltage may be a function of VCC, temperature, or other condition; alternately, the bias voltage on line  132  may be independent of one or more of these parameters.  
         [0029]     The amplifier A 2   130  receives the signal V 2  on line  114 . The amplifier compares the DC component of the signal V 2  on line  114  to the bias voltage received on line  132 . This comparison generates a signal at the output of the amplifier A 2   130 . This voltage is then used to set the DC voltage for the signal V 1  on line  112 .  
         [0030]     The DC control feedback loop operates as follows. As the DC component of the signal V 2  on line  114  increases, the voltage at the output of the amplifier A 2   130  decreases. This lowers the DC component of the signal V 1  on line  112 . Since the signal V 2  on line  114  is DC coupled to V 1  on line  112 , V 2  on line  114  is similarly reduced, thus compensating for the original increase.  
         [0031]     The amplitude detection circuit  140  receives the signal V 1  on line  112 , and provides a bias current or voltage to the gain circuit A 1   120 . The amplitude detection circuit  140  compares the oscillation amplitude of the signal V 1  on line  112  to one or more thresholds. As the amplitude of the signal V 1  on line  112  increases, the amplitude detection circuit  140  decreases the gain of gain circuit A 1   120 , thus reducing the drive to the crystal X 1   110 . This in turn lowers the amplitude of the voltage swing of the signal V 2  on line  114 . Conversely, as the amplitude of the signal V 1  on line  112  decreases, the amplitude detection circuit  140  increases the gain of gain circuit A 1   120 , which increases the amplitude of V 2  on line  114 . In this way, feedback is provided such that the amplitude of the signal V 2  on line  114  is maintained at a certain level (or within a range of levels, depending on the exact implementation.)  
         [0032]     Again, the gain circuit A 1   120  can be as simple as a single transistor in some embodiments of the present invention. When it is a transistor, such as a MOS transistor, this oscillator can be referred to as a Pierce oscillator. In this configuration, the crystal X 1   110  oscillates in the parallel resonance mode. Other types of oscillators may also be improved by embodiments of the present invention. These include Pierce, Colpitts, Hartley, Armstrong, Clapp, and other types of oscillators. An example of a Pierce oscillator is shown in the next figure.  
         [0033]      FIG. 2  is a block diagram of a low-noise, high-stability Pierce crystal oscillator according to an embodiment of the present invention. This figure includes a crystal X 1   210 , transistor M 1   220 , bias current source  230 , amplifier A 1   240 , amplitude detection circuit  250 , resistors R 1   245  and R 2   215 , and capacitors C 1   225 , C 2   255 , and C 3   247 .  
         [0034]     In this configuration, transistor M 1   220  provides the gain necessary to drive crystal X 1   210 . The crystal X 11   210  is AC coupled through capacitor C 1   225  to the base of M 1   220 . This separates the DC level of the crystal oscillator signal V 1  on line  222  from the bias voltage at the gate of transistor M 1   220 . As before, resistor R 2   215  is a large value resistor that biases the DC voltage of the signal V 1  on line  222  such that it equals the DC voltage of the signal V 2  on line  224 . Since the resistor R 2   215  is a large resistor, care should be taken to avoid leakage currents, for example through capacitor C 1   225 , or other capacitors that have been omitted for clarity.  
         [0035]     The DC component of the signal V 2  on line  224  is compared to the bias voltage on line  242  by the amplifier  240 . Again, other voltages can be compared to the bias voltage on line  242 . For example, the resistor R 2   215  can be two or more resistors in series, with a voltage at a node between two of these resistors compared to the bias voltage on line  242 . The amplifier  240  provides a voltage output across C 3   247  that is coupled to the gate of transistor M 1   220  by resistor R 1   245 . In a specific embodiment, the amplifier A 1   240  is a transconductance or gm amplifier that provides a current which generates a voltage across capacitor C 3   247 . This output voltage sets the operating point for M 1   220 , which in turn sets the DC component of the signal V 2  on line  224 . Resistor R 1   254  and capacitor C 3   247  provide reverse isolation for the output of the amplifier A 1   240  from the large AC swings on the gate of transistor M 1   220 .  
         [0036]     More specifically, when the DC component of the signal V 2  on line  224  is higher than the level of the bias signal on line  242 , the output voltage of the amplifier A 1   240  is reduced. This reduces the gate-to-source voltage of M 1   220 , which increases the DC voltage of the signal V 2  on line  224 .  
         [0037]     The signal V 1  on line  222  is AC coupled through capacitor C 2   255  to the amplitude detection circuit  250 . The amplitude detection circuit adjusts the bias current provided by the current source IBIAS  230 . As the amplitude of the signal V 1  on line  222  increases, the current provided by the bias current source  230  is decreased, thereby reducing the amplitude of the signals V 2  on line  224  and V 1  on line  222 . Conversely, as the amplitude of the signal V 1  on line  222  decreases, the current provided by the bias current source  230  is increased, thereby increasing the amplitude of the signals V 2  on line  224  and V 1  on line  222 .  
         [0038]     There are various ways in which the DC components of the oscillator voltage signals can be set or controlled. The feedback loops used to accomplish this may be analog, digital, or a combination thereof. One analog circuit that may be used is shown in the next figure. The subsequent figure shows a method of setting these DC components; the method may be implemented in an analog or digital manner.  
         [0039]      FIG. 3  is a schematic of a DC biasing loop for a crystal oscillator according to an embodiment of the present invention. This figure includes a crystal X 1   310 , transistor M 1   320 , current source IBIAS  330 , amplifier A 1   340 , resistors R 1   315 , R 2   317 , and R 3   350 , and capacitors C 1   360 , C 2   365 , C 3   370 , C 4   345 , and C 5   375 . An amplitude detection circuit may be used to adjust the current provided by the current source IBIAS  330 , but has been omitted for clarity.  
         [0040]     The crystal X 1   310  is driven by transistor M 1   320 . The crystal signal on line V 1   322  is AC coupled to the gate of M 1   320  by capacitor C 1   360 . A series combination of resistors R 1   315  and R 2   317  are used to set the DC levels of the signals V 1  on line  322  and V 2  on line  324  such that they are equal to the DC level of the signal V 4  on line  344 . Capacitors C 3   370  and C 5   375  are used to pull or tune the crystal&#39;s frequency. In various embodiments, these capacitors can include arrays of switchable capacitors allowing the crystal&#39;s frequency to be tuned or modulated, for example as part of an FM modulator.  
         [0041]     Again, transistor M 1   320  provides the drive current for the crystal X 1   310 . As the gate voltage of the transistor M 1   320  increases, the drain current of the device increases rapidly. Accordingly, the DC bias voltage of M 1   320  is typically near ground, that it is biased below the threshold of the transistor M 1   320 , such that the transistor M 1   320  is typically off, turning on to provide a pulse of current to the crystal X 1   310  once every oscillation cycle.  
         [0042]     It is desirable for the signal V 1  on line  322  to have a large amplitude. However, if this large signal were AC coupled directly to the gate of transistor M 1   320 , the gate of transistor M 1   320  would require a DC bias below ground, otherwise it would provide excess drive current to the crystal  310 . However, the amplifier A 1   340  is not capable of driving below ground. One alternative is to provide a negative supply voltage for the amplifier A 1   340 , for example with a charge pump. This solution provides excellent noise performance. Alternately, the signal V 1  on line  322  can be reduced in amplitude.  
         [0043]     Accordingly, in this specific example, capacitor C 2   365  is connected from the gate of M 1   322  to ground. In this way, capacitors C 1   316  and C 2   365  form a capacitive divider that reduces the amplitude of the signal seen at the gate of M 1   320 . This allows the gate of transistor M 1   320  to have a DC bias above ground. In a specific embodiment, the DC bias for the gate of M 1   320  is approximately 200 mV, which can be supplied by the amplifier A 1   340  without requiring a negative supply voltage.  
         [0044]     The DC component of the signal V 4  on line  344  is set by a feedback loop including amplifier A 1   340 , resistor R 3   350 , and transistor M 1   320 . Specifically, the voltage signal V 4  on line  344  is compared to the bias signal received on line  342  by the amplifier A 1   340 . The signals V 2  on line  324  and V 1  on line  322  are each large oscillating signals that are 180 degrees out of phase. Accordingly, if the resistors R 1   315  and R 2   317  are equal, the signal V 4  on line  344  has approximately the same DC level as the signals V 1  on line  322  and V 2  on line  324 , but with little or no AC component. Thus, the signal V 4  on line  344  provides a good voltage for comparison to the bias voltage on line  342  by the amplifier A 1   340 .  
         [0045]     The amplifier A 1   340  provides a voltage output across capacitor C 4   345 . The capacitor C 4   345  can be used to limit the bandwidth, time constant, or frequency response of this loop. In a specific embodiment, the amplifier A 1   340  provides a current output that is converted to a voltage by the capacitor C 4   345 . The output voltage of the amplifier A 1   340  sets the DC bias voltage for transistor M 1   320 . The gate-to-source voltage of transistor M 1   320  determines the operating point for the transistor, including its drain voltage, the signal V 2  on line  324 .  
         [0046]      FIG. 4  is a flowchart showing the operation of the DC biasing loop, such as the DC biasing loop of  FIG. 3 . According to this method, a signal from an oscillator is compared to a bias voltage. The comparison is used to set a bias condition for a transistor. The transistor then sets the DC level of the oscillator signal.  
         [0047]     Specifically, in act  410 , a first signal is received from a crystal. The DC level or component of the crystal signal is compared to a bias level in act  420 . Again, this bias level may be set to be between two supply voltages, to a bandgap or other bias voltage, and it may be designed to track or be independent of supplies, temperature, processing, or other condition. For example, it may be set to a ground-referenced voltage that is approximately one-half a minimum supply voltage for the oscillator signal. Alternately, this bias level may be designed to be independent of one or more of these parameters.  
         [0048]     A correction signal based on the comparison is generated in act  430 . This correction signal is then used to set the DC level of the first crystal signal. There are many ways that this may be done, and they may depend on the particular circuit topology that is used. For example, the comparison may be done digitally, where the first crystal signal is filtered, digitized, and compared to a second digital value. In other embodiments of the present invention, the loop is analog.  
         [0049]     In this specific example, the correction signal is used to set a bias voltage for a transistor in act  440 . In act  450 , the transistor is used to set a DC level for the first crystal signal. A resistor is used to set a DC level of a second crystal signal in act  460 . Additionally, other resistors can be used to set other crystal signals.  
         [0050]     Embodiments of the present invention and can include an amplitude detection circuit. The amplitude detection circuit can set the drive level for a transistor or other circuit used to provide gain for a crystal in a crystal oscillator circuit. This loop can be analog, digital, or a combination thereof. Again, to avoid interaction with a DC control loop, the bandwidth of the amplitude detection circuit can set to be lower than the bandwidth of the DC control loop. In other embodiments, other arrangements can be made; for example, the bandwidth of the amplitude detection circuit can set to be higher than the bandwidth of the DC control loop. In one specific embodiment of the present invention, the amplitude detection circuit is predominantly digital, and the bandwidth of the loop is set by a frequency and at which a value of an accumulator or counter is clocked or updated. One specific circuit that can detect an amplitude and use this information to adjust the amplitude&#39;s level is shown in the next figure, while one specific methodology of detecting an amplitude is shown in the subsequent figure.  
         [0051]      FIG. 5  is a schematic of a digital amplitude control loop for a crystal oscillator according to an embodiment of the present invention. The digital amplitude control loop includes an AC coupling capacitor C 1   510 , DC restoration resistor  515 , a negative peak detector made up of a diode D 1   520  and capacitor C 2   530 , window comparator  540 , accumulator  550 , current digital-to-analog converter (DAC)  560 , and a low-pass filter  570 . An oscillator signal is received on line V 1   512  by the AC coupling capacitor C 1   510 . The current DAC  560  generates a bias current that is filtered by the low-pass filter and provided as current IBIAS on line  562 . In a specific embodiment, the bias current on line  562  supplies current to a transistor, such as transistor M 1   320  in  FIG. 3 .  
         [0052]     Again, an oscillator signal V 1  is received on line  512  and AC coupled as signal V 2  on line  512  by AC coupling capacitor C 1   510 . The input signal V 1  on line  512  may correspond to one of at least two signals, for example, V 1  on line  322  or V 2  on line  324  in  FIG. 3 . Detecting the amplitude of V 1  on line  322  provides isolation between the amplitude detector input and IBIAS current output on line  562 . The size of capacitor C 1   510  should be large in comparison to the parasitic capacitances of diode D 1   520  and resistor R 1   515  in order to avoid signal losses that would be caused by the resulting capacitive divider. The resistor R 1   515  sets the DC component of the signal V 2  on line  515  to an appropriate bias voltage, BIAS on line  516  in this example. In an exemplary embodiment of the present invention, the resistor R 1   515  may be connected to a bias line that is midway between two supplies such as VCC and ground. In various embodiments, R 1   515  is connected to the same or similar bias line as the BIAS voltage on line  342  in  FIG. 3 .  
         [0053]     The negative peak of the signal V 2  on line  517  is detected by the diode D 1   520  and capacitor C 2   530  in order to generate a peak detected output signal V 3  on line  532 . In other embodiments, a positive peak detector can be used, for example, by reversing diode D 1   520 . In other embodiments, other peak detectors or envelope detectors can be used. As the voltage of the signal V 2  on line  517  decreases, the voltage of the signal V 3  on line  532  follows. As the signal V 2  on line  517  reaches its minimum value or peak, the signal V 3  on line  532  reaches a corresponding voltage, plus a diode drop caused by the diode D 1   520 . In various embodiments of the present invention, other peak detectors that compensate for, or do not include this diode drop, are used. As the level of the signal V 2  on line  517  increases, the diode D 1   520  reverse biases, and is effectively disconnected from the capacitor C 2   530 , which holds the negative peak voltage.  
         [0054]     The window comparator  540  compares the signal V 3  on line  532  to two thresholds, a high threshold and a low threshold. When the voltage of the signal V 3  is lower than the low threshold, signal VL on line  546  is active. When the voltage of the signal V 3  on line  532  is between the high threshold and the low threshold, the signal VM on line  544  is active. When the voltage of the signal V 3  on line  532  is higher than the high threshold, the signal VH on line  542  is active. In various embodiments of the present invention, the signal VM on line  544  is not required. In various embodiments, the window comparator can be two comparators, one that compares the signal V 3  on line  532  with a high threshold, and one that compares the signal V 3  on line  532  with a low threshold.  
         [0055]     The accumulator  550  can be an up/down counter that provides a digital word to the current DAC  560 . When the signal VL on line  546  is active, the accumulator  550  counts down by one bit. When the signal VH on line  542  is active, the accumulator  550  counts up by one bit. When the signal VM on line  544  is active, the accumulator  550  does not change value. In other embodiments, the accumulator may count in a different manner, so long as the peak detector, accumulator  550 , and DAC  560  operate together to properly control the amplitude of the oscillator signals.  
         [0056]     The accumulator can be clocked by a signal that controls the rate at which the accumulator output can change state. The frequency of this clock signal controls the bandwidth of the amplitude detection circuit. In one specific embodiment of the present invention, in order to avoid interactions with a DC control loop, the bandwidth of this amplitude detection circuit is set to be lower than the bandwidth of the DC control loop. The accumulator can alternately be an analog-to-digital converter, such as a flash converter. Also, more complicated functions can be implemented. For example, transfer functions that include poles and zeros can be implemented to more specifically tailor the frequency response of the amplitude detection circuit. The locations of these poles and zeros can also be programmable or otherwise adjustable.  
         [0057]     The current DAC  560  receives a digital word from the accumulator  550 . The digital word can be binarily weighted or thermally decoded, or have some other weighting or combination thereof. The current DAC  560  is typically a number of switches each configured to turn a current source on or off. The resulting current can be filtered and provided to a gain element or transistor, such as transistor M 1   320  in  FIG. 3 . The filtering is performed in this specific example by the low-pass filter  570 . This filter removes the high frequency components of the current DAC output, protecting the oscillator gain element from these transients. The current sources may be configured to be independent of supply, temperature, or processing. In one embodiment of the present invention, as the digital word increases in value, the DAC provides more current to the gain device. In other embodiments, the DAC may provide less current as the digital word increases.  
         [0058]     In other embodiments, the voltage of signal V 3  on line  532  is compared to a single threshold. In this case, a single output indicating whether the voltage of signal V 3  on line  532  is higher or lower than the threshold is provided. In this configuration, during operation, the comparison signal tends to alternate between one state and another, causing the accumulator to toggle between two levels, and resulting in the current DAC  560  switching between two bias current levels. This tends to add digital switching noise to the oscillator circuit. Using two thresholds provides a window in which the device may operate without changing the output of the accumulator  550  or the resulting bias current level provided by the current DAC  560 .  
         [0059]      FIG. 6  is a flowchart showing the operation of an amplitude control loop, such as the amplitude control loop of  FIG. 5 . In this embodiment of the present invention, an oscillation signal from a crystal is peak detected and compared to a high and a low threshold. The comparison results are used to control an accumulator, which in turn provides an output that is converted to a bias current, the bias current used to drive the gain device or circuit in the oscillator. The peak detection described here detects positive peaks, though negative peak detection can alternately be used.  
         [0060]     Specifically, in act  610 , an oscillation signal is received from a crystal. This signal is AC coupled, such that its DC component is removed in act  620 . In act  630 , the DC component of the oscillation signal is peak detected.  
         [0061]     The peak detected level is then compared to a high and a low threshold in act  640 . In act  650 , it is determined whether the peak level is above a high threshold. If it is, the accumulator is decremented in act  660 . If the peak level is not above a high threshold, it is determined whether the peak level is below the low threshold in act  670 . If it is, the accumulator is incremented in act  680 . If the peak detected value is lower than the high threshold, but higher than the low threshold, the value in the accumulator is maintained in act  690 . Again, in various embodiments of the present invention, the accumulator may increment or decrement in different ways according to the exact implementation used.  
         [0062]     The value of the accumulator is converted into a current in act  695 . Again, this current can be used to drive a transistor or other circuit that is providing gain to the crystal that is generating the oscillation signal.  
         [0063]     The amplitude detection circuits of the previous figures may have a stable state where the crystal does not oscillate or provide an output signal of sufficient amplitude to properly clock the accumulator  550  in  FIG. 5 . Although the presence of noise typically starts these oscillators, in order to provide a robust and fast start-up, an analog amplitude detection circuit can be used. Once the oscillator is running, the analog amplitude detection circuit can be disabled in favor of a digital amplitude detection circuit, such as the circuit shown in  FIG. 5 . One analog amplitude control circuit that may be used at start-up is shown in the following figure.  
         [0064]      FIG. 7  is a schematic of an analog amplitude control circuit used to start a crystal oscillator according to an embodiment of the present invention. This figure includes gm amplifier  710  and a p-channel current mirror including transistors M 1   720  and M 2   730 , and decoupling capacitor C 3   725 .  
         [0065]     An input signal V 1  is received on line  702  by the gm amplifier  710 . The gm amplifier  710  provides a current output that is mirrored by the p-channel current mirror transistors M 1   720  and M 2   730 . Transistor M 2   730  can be connected in parallel with the current DAC in the digital amplitude detector circuit. In a specific embodiment, the signal V 1  on line  702  is the negative peak detected signal V 3  on line  532  in  FIG. 5 , though in other embodiments, it can be a different signal. As the amplitude of the crystal oscillator signals increase, the voltage V 1  on line  702  decreases, thus decreasing the current provided by the gm amplifier  710  to the p-channel current mirror.  
         [0066]     Again, once the oscillator is running, this circuit can be disabled in favor of an amplitude detection circuit, such as the amplitude detection circuit shown in  FIG. 5 , or other detection circuits consistent with embodiments of the present invention. This circuit can be disabled in favor of a digital amplitude detection circuit when the crystal oscillator signals are of sufficient amplitude to properly clock the accumulator circuit. Hysteresis can also be used to avoid a condition where this circuit toggles between its on and off states.  
         [0067]     The transistors in the above examples are shown as MOS transistors. In other embodiments of the present invention, the devices may be bipolar, HBTs, MESFETS, HFETs, or other types of devices. The capacitors shown may be metal-to-metal capacitors, thin-oxide capacitors, or any other appropriate capacitors, such as the gate of a MOS device. The resistors may be polysilicon resistors, base resistors, implant resistors, or other appropriate type of resistor. The crystals may be crystals operating in parallel or series resonance modes. Alternately, they may be other resonance devices.  
         [0068]     Referring now to  FIGS. 8A-10G , various exemplary implementations of the present invention are shown. Referring to  FIG. 8A , the present invention may be embodied in a hard disk drive  800 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 8A  at  802 . In some implementations, signal processing and/or control circuit  802  and/or other circuits (not shown) in HDD  800  may process data, perform coding and/or encryption, perform calculations, and/or format data that is output to and/or received from a magnetic storage medium  806 .  
         [0069]     HDD  800  may communicate with a host device (not shown) such as a computer, mobile computing devices such as personal digital assistants, cellular phones, media or MP3 players and the like, and/or other devices via one or more wired or wireless communication links  808 . HDD  800  may be connected to memory  809 , such as random access memory (RAM), a low latency nonvolatile memory such as flash memory, read only memory (ROM) and/or other suitable electronic data storage.  
         [0070]     Referring now to  FIG. 8B , the present invention may be embodied in a digital versatile disc (DVD) drive  810 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 8B  at  812 , and/or mass data storage  818  of DVD drive  810 . Signal processing and/or control circuit  812  and/or other circuits (not shown) in DVD  810  may process data, perform coding and/or encryption, perform calculations, and/or format data that is read from and/or data written to an optical storage medium  816 . In some implementations, signal processing and/or control circuit  812  and/or other circuits (not shown) in DVD  810  can also perform other functions such as encoding and/or decoding and/or any other signal processing functions associated with a DVD drive.  
         [0071]     DVD drive  810  may communicate with an output device (not shown) such as a computer, television or other device via one or more wired or wireless communication links  817 . DVD  810  may communicate with mass data storage  818  that stores data in a nonvolatile manner. Mass data storage  818  may include a hard disk drive (HDD) such as that shown in  FIG. 8A . The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. DVD  810  may be connected to memory  819 , such as RAM, ROM, low latency nonvolatile memory such as flash memory, and/or other suitable electronic data storage.  
         [0072]     Referring now to  FIG. 8C , the present invention may be embodied in a high definition television (HDTV)  820 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 8C  at  822 , a WLAN interface and/or mass data storage of the HDTV  820 . HDTV  820  receives HDTV input signals in either a wired or wireless format and generates HDTV output signals for a display  826 . In some implementations, signal processing circuit and/or control circuit  822  and/or other circuits (not shown) of HDTV  820  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other type of HDTV processing that may be required.  
         [0073]     HDTV  820  may communicate with mass data storage  827  that stores data in a nonvolatile manner such as optical and/or magnetic storage devices. At least one HDD may have the configuration shown in  FIG. 8A  and/or at least one DVD may have the configuration shown in  FIG. 8B . The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. HDTV  820  may be connected to memory  828  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. HDTV  820  also may support connections with a WLAN via a WLAN network interface  829 .  
         [0074]     Referring now to  FIG. 8D , the present invention implements a control system of a vehicle  830 , a WLAN interface and/or mass data storage of the vehicle control system. In some implementations, the present invention implements a powertrain control system  832  that receives inputs from one or more sensors such as temperature sensors, pressure sensors, rotational sensors, airflow sensors and/or any other suitable sensors and/or that generates one or more output control signals such as engine operating parameters, transmission operating parameters, and/or other control signals.  
         [0075]     The present invention may also be embodied in other control systems  840  of vehicle  830 . Control system  840  may likewise receive signals from input sensors  842  and/or output control signals to one or more output devices  844 . In some implementations, control system  840  may be part of an anti-lock braking system (ABS), a navigation system, a telematics system, a vehicle telematics system, a lane departure system, an adaptive cruise control system, a vehicle entertainment system such as a stereo, DVD, compact disc and the like. Still other implementations are contemplated.  
         [0076]     Powertrain control system  832  may communicate with mass data storage  846  that stores data in a nonvolatile manner. Mass data storage  846  may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown in  FIG. 8A  and/or at least one DVD may have the configuration shown in  FIG. 8B . The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Powertrain control system  832  may be connected to memory  847  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. Powertrain control system  832  also may support connections with a WLAN via a WLAN network interface  848 . The control system  840  may also include mass data storage, memory and/or a WLAN interface (all not shown).  
         [0077]     Referring now to  FIG. 8E , the present invention may be embodied in a cellular phone  850  that may include a cellular antenna  851 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 8E  at  852 , a WLAN interface and/or mass data storage of the cellular phone  850 . In some implementations, cellular phone  850  includes a microphone  856 , an audio output  858  such as a speaker and/or audio output jack, a display  860  and/or an input device  862  such as a keypad, pointing device, voice actuation and/or other input device. Signal processing and/or control circuits  852  and/or other circuits (not shown) in cellular phone  850  may process data, perform coding and/or encryption, perform calculations, format data and/or perform other cellular phone functions.  
         [0078]     Cellular phone  850  may communicate with mass data storage  864  that stores data in a nonvolatile manner such as optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown in  FIG. 8A  and/or at least one DVD may have the configuration shown in  FIG. 8B . The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Cellular phone  850  may be connected to memory  866  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. Cellular phone  850  also may support connections with a WLAN via a WLAN network interface  868 .  
         [0079]     Referring now to  FIG. 8F , the present invention may be embodied in a set top box  880 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 8F  at  884 , a WLAN interface and/or mass data storage of the set top box  880 . Set top box  880  receives signals from a source such as a broadband source and outputs standard and/or high definition audio/video signals suitable for a display  888  such as a television and/or monitor and/or other video and/or audio output devices. Signal processing and/or control circuits  884  and/or other circuits (not shown) of the set top box  880  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other set top box function.  
         [0080]     Set top box  880  may communicate with mass data storage  890  that stores data in a nonvolatile manner. Mass data storage  890  may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown in  FIG. 8A  and/or at least one DVD may have the configuration shown in  FIG. 8B . The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Set top box  880  may be connected to memory  894  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. Set top box  880  also may support connections with a WLAN via a WLAN network interface  896 .  
         [0081]     Referring now to  FIG. 8G , the present invention may be embodied in a media player  872 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 8G  at  871 , a WLAN interface and/or mass data storage of the media player  872 . In some implementations, media player  872  includes a display  876  and/or a user input  877  such as a keypad, touchpad and the like. In some implementations, media player  872  may employ a graphical user interface (GUI) that typically employs menus, drop down menus, icons and/or a point-and-click interface via display  876  and/or user input  877 . Media player  872  further includes an audio output  875  such as a speaker and/or audio output jack. Signal processing and/or control circuits  871  and/or other circuits (not shown) of media player  872  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other media player function.  
         [0082]     Media player  872  may communicate with mass data storage  870  that stores data such as compressed audio and/or video content in a nonvolatile manner. In some implementations, the compressed audio files include files that are compliant with MP3 format or other suitable compressed audio and/or video formats. The mass data storage may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown in  FIG. 8A  and/or at least one DVD may have the configuration shown in  FIG. 8B . The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. Media player  872  may be connected to memory  873  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. Media player  872  also may support connections with a WLAN via a WLAN network interface  874 .  
         [0083]     Referring to  FIG. 8H , the present invention may be embodied in a Voice over Internet Protocol (VoIP) phone  883  that may include an antenna  839 . The present invention may implement either or both signal processing and/or control circuits, which are generally identified in  FIG. 8H  at  882 , a wireless interface and/or mass data storage of the VoIP phone  883 . In some implementations, VoIP phone  883  includes, in part, a microphone  887 , an audio output  889  such as a speaker and/or audio output jack, a display monitor  891 , an input device  892  such as a keypad, pointing device, voice actuation and/or other input devices, and a Wireless Fidelity (Wi-Fi) communication module  886 . Signal processing and/or control circuits  882  and/or other circuits (not shown) in VoIP phone  883  may process data, perform coding and/or encryption, perform calculations, format data and/or perform other VoIP phone functions.  
         [0084]     VoIP phone  883  may communicate with mass data storage  502  that stores data in a nonvolatile manner such as optical and/or magnetic storage devices, for example hard disk drives HDD and/or DVDs. At least one HDD may have the configuration shown in  FIG. 8A  and/or at least one DVD may have the configuration shown in  FIG. 8B . The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. VoIP phone  883  may be connected to memory  885 , which may be a RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. VoIP phone  883  is configured to establish communications link with a VoIP network (not shown) via Wi-Fi communication module  886 . Still other implementations in addition to those described above are contemplated.  
         [0085]     The above description of exemplary embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.