Abstract:
A system and method in accordance with the present invention provides a gyroscope incorporating an improved PLL technique. The improved PLL auto-corrects its own reference low-frequency noise, thereby eliminating this source of noise, improving the noise performance of the gyroscope and allowing a compact implementation. The net result is a gyroscope with improved bias stability that can meet noise requirements with a smaller footprint.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Under 35 U.S.C. 119(e), this application claims priority benefit to U.S. provisional application No. 61/551,314 filed on Oct. 25, 2011. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to gyroscopes and more particularly to gyroscopes employing phase- and duty-cycle-locked loops. 
     BACKGROUND OF THE INVENTION 
     Conventional phase-locked loops (PLL&#39;s) utilized in gyroscopes use a single edge of the reference clock for phase comparison. Such PLL&#39;s are vulnerable to low-frequency jitter which arises due to phase modulation of a sinusoidal reference clock source when noise is added to it. This additive noise is converted to duty-cycle error, and subsequently phase noise, when the sinusoidal reference clock is converted to logic levels by a comparator or similar circuit. This becomes a serious limitation when a conventional PLL is used in the demodulator path of a rate gyroscope. Conventional duty-cycle correction methods would directly monitor the duty cycle by averaging the “high” and “low” times of the comparator output, and driving this average to zero. However, such techniques suffer from low-frequency noise added by the duty-cycle measuring circuit. A system and method in accordance with the present invention addresses these issues. 
     SUMMARY OF THE INVENTION 
     A system and method in accordance with the present invention provides a gyroscope based on an improved phase locked loop (PLL) technique. The improved PLL auto-corrects its own reference low-frequency noise, thereby eliminating this source of noise, improving the noise performance of the gyroscope and allowing a compact implementation. The net result is a gyroscope with improved bias stability that can meet noise requirements with a smaller footprint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional MEMS rate gyroscope. 
         FIG. 2  is block diagram of a gyroscope in accordance with the present invention. 
         FIGS. 3A and 3B  are block diagrams of two exemplary conventional phase-frequency detectors (PFD). 
         FIG. 4A  illustrates how low-frequency noise affects a reference source sinusoidal waveform to produce duty-cycle error and phase error. 
         FIG. 4B  illustrates a phase-frequency detector (PFD) in accordance with the present invention. 
         FIG. 5  is embodiment block diagram of a phase and duty-cycle locked loop (PDCLL) in accordance with the present invention. 
         FIG. 6  are exemplary waveforms that illustrate the response of rising and falling-edge PFD&#39;s in the presence of phase and duty-cycle errors. 
         FIG. 7  is an embodiment of a phase-frequency and duty-cycle detector (PFDCD) in accordance with the present invention. 
         FIG. 8  is an embodiment of the present invention incorporating a PDCLL comprising a PFDCD. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to gyroscopes and more particularly to phase- and duty-cycle-locked loops utilized in gyroscopes. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     A system and method in accordance with the present provides the following features:
         Phase comparison operates on both edges of the clock, so that the effect of additive noise on the timing of those edges is averaged out in the PLL. Thereby, conversion of additive low-frequency reference noise to phase noise is avoided.   Duty-cycle is further detected by re-using the phase-frequency detector of the PLL to compare both edges of the clock to the PLL feedback clock and infer a duty-cycle error from timing errors. The duty-cycle error is corrected by applying the detected error to an integrator which adjusts a D.C. offset in the clock comparator. Thereby, spurious PLL products due to performing phase comparisons on both edges of the clock are avoided.   The resulting arrangement comprises a system of two orthogonal feedback loops—one for phase, another for duty-cycle—which both relate to a common phase/frequency/duty-cycle detector circuit.   By detecting and correcting duty-cycle errors by comparing and aligning edge crossings, the present invention provides a significant improvement in noise performance compared to conventional duty-cycle correction techniques, thereby enabling low-noise operation and a compact implementation suitable for a gyroscope.       

       FIG. 1  shows a conventional micro-electromechanical system (MEMS) rate gyroscope system  100 , comprising a MEMS gyroscope  102  that is driven to oscillation with a drive loop comprising a drive capacitance-to-voltage (C2V) amplifier (DC2V)  104  coupled to a MEMS drive-sense terminal  108 C and a drive/AGC block  106  that close a positive feedback loop around the MEMS gyroscope  102 . When oscillating, the MEMS gyroscope  102  provides a second output via a MEMS sense terminal  108 B coupled to a sense C2V amplifier (SC2V)  112  which senses a change in MEMS capacitance in response to a Coriolis force that results when the gyroscope  102  is rotated. Due to mechanical imperfections in the MEMS gyroscope  102 , some portion of the drive signal may inadvertently couple to a sense port  108 B of the MEMS gyroscope  102 . To cancel this unwanted signal, a feedthrough (FT) trim block  110  couples the DC2V  104  output to the SC2V  112  input. After feedthrough cancellation, the output of the SC2V  112  is an amplitude modulated signal whose carrier frequency is the oscillation frequency of the drive loop and whose modulation conveys the rate of rotation. The phase of the desired signal carrying information about the rate of rotation exhibits a 90-degree phase offset compared to the DC2V  104  output. The SC2V  112  output is coupled to a signal input of demodulator  126  and is demodulated to baseband to recover the rate signal  124 . The demodulator  126  has a clock input receiving a phase shifted version of the DC2V  104  output signal so that the demodulator signal and clock inputs have substantially the same carrier phase for proper demodulation. The required phase shift is approximately 90-degrees, owing to the nominal phase offset between SC2V  112  and DC2V  104  outputs. This phase shift can be generated by using a 90-degree phase shifter (such as an integrator  114  or other suitable analog filter), followed by a comparator  116  to convert the sinusoidal output of the phase shifter to a square-wave output for use in demodulation. In some embodiments, this square-wave output may also be applied to a phase-locked loop (PLL)  118  for use as a system clock. 
     A critical issue that arises during this conversion is that low-frequency noise and offsets  120  present at the phase shifter  114  input contribute to duty-cycle error at the comparator  116  output, and this duty-cycle error can be a source of spurious demodulation products and noise. For this reason, a duty-cycle (DC) correction loop  122  monitors the comparator  116  output and feeds back a correction signal to the phase shifter input so that a substantially 50% duty cycle is maintained at the demodulator input. 
     A limitation of the technique of  FIG. 1  is that the accuracy and stability of the phase shift is governed by an analog phase shifter and therefore subject to sensitivity to manufacturing and environmental variation. 
     An improved gyroscope system  200  in accordance with the present invention eliminates an analog phase shifter and its related duty-cycle correction circuitry in favor of a phase-locked loop (PLL)  218 , as illustrated in  FIG. 2 . In this embodiment, the output of the DC2V  104  is coupled to a comparator  116 , which converts the sinusoidal DC2V  104  output signal into a square-wave signal. That signal serves as the reference clock for a PLL  218  comprising phase-frequency detector (PFD)  202 , charge pump (CP)  208 , loop filter (LF)  210 , voltage-controlled oscillator (VCO)  214  and feedback divider  212 . By virtue of the feedback divider  212 , the PLL  218  multiplies the drive frequency by a factor of N. As the PLL  218  output frequency is a factor of N more than the drive frequency, there are N positive-edge transitions of the output clock for each cycle of the PLL reference clock. Considering only positive edge transitions, a phase resolution of 360-degrees divided by N is readily provided by use of a digital delay line. If positive and negative-edge transitions are considered, the resolution is improved to 180-degrees divided by N. The PLL  218  output clock couples to a phase shifter  114  based on this principle. The phase shifter  114  can be programmed by selecting the number of clock cycle delays relating the phase shifter input and output clocks. The output of the phase shifter  114  couples to a frequency divider  216  that divides the phase shifter  114  output clock frequency back down to the drive frequency. With a proper phase selection, the divider  216  output is at the proper frequency and phase for use in demodulation by demodulator  126 . 
     The arrangement of  FIG. 2  has the advantage of providing a very stable and programmable phase shift. The stability of the phase shift owes to the use of clock edges available at the PLL  218  output that precisely span one period of the drive oscillation. 
     A limitation of the embodiment of  FIG. 2  is found in its susceptibility to low-frequency additive noise that may be present at the input of the comparator  116 . 
     To understand how this additive noise affects the PLL  218  operation, it is useful to consider the operation of two exemplary phase-frequency detectors (PFD), shown in  FIGS. 3A and 3B . In  FIG. 3A , first exemplary PFD  202  comprises two D-flip-flops (D-FFs)  302 A and  302 B and a NAND gate  304 . Referring back to  FIG. 2 , the first exemplary PFD  202  compares rising edges of the reference and feedback clocks in the PLL  218 . The principle of operation will be familiar to one skilled in the art and is summarized here for ease of reference. If a rising edge of the reference input arrives first, it is sensed by the first D-FF  302 A and the UP signal is set. This signal persists until a rising edge of the divider input arrives and is detected by the second D-FF  302 B. At that time, the DN signal is set momentarily. The condition where both UP and DN signals is set triggers a RESET of both D-FF&#39;s  302 A and  302 B. However, because the UP signal was set first, it has persisted for longer, thereby indicating that the reference is early and that the VCO frequency should be increased to allow the divider clock edge to catch up. 
     The opposite situation is found when the divider clock edge arrives first, in which case the reference is considered “late”, and the VCO frequency should be decreased to allow the reference to catch up. When the edges of the reference and divider are aligned, the durations of UP and DN pulses are identical and equal to the reset delay of the D-FF  302 A and  302 B plus the delay of NAND gate  304 . The exemplary falling edge PFD  202 ′ works by the same principle, but operating on the falling edges of the clocks, rather than the rising edges. 
     Conventional PLL&#39;s may typically operate on one edge—rising, or falling—of the reference clock. Operation on both edges is generally avoided because duty-cycle errors in the reference clock will lead to static timing errors that cause instability of the PLL  218  output clock. 
     Now, consider the influence of low-frequency noise on PFD operation with reference to  FIG. 4A . This figure illustrates a reference source sinusoidal waveform. This waveform is exemplary of the type of signal that might be provided by a DC2V. The zero-crossings of this signal may be sensed by a comparator to produce the reference clock shown in the diagram. When low-frequency noise is added to the reference source, the zero crossings are perturbed, resulting in uncertain and jittery edge placements in the reference clock waveform. In the exemplary waveforms of  FIG. 4A , a low-frequency noise with a positive value is added to the reference source. The effect is to advance the positive-going transitions of the reference clock and to delay the negative going transitions, thereby causing duty-cycle distortion. Thus, additive low-frequency noise is converted to duty-cycle noise at the comparator output. A rising edge PFD will generate UP pulses of varying width in response to the rising edges with varying placements. Correspondingly, a falling edge PFD will generate DN pulses of varying width. In either case, the net effect will be to cause the PLL to track the varying reference edge timing, thereby converting duty-cycle noise at the PFD input into phase noise at the PLL output. This phase noise can cause performance degradation in the demodulator when it interacts with and demodulates residual quadrature at the output of the sense amplifier. For this reason, it would be desirable to have PFD whose output does not respond to duty-cycle errors produced by low-frequency noise. 
     One solution in accordance with the present invention is shown in  FIG. 4B . By combining a rising-edge PFD with a falling-edge PFD and summing their outputs, the average PFD output is no longer responsive to duty-cycle errors. This can be seen with reference to  FIG. 4A  which illustrates that rising- and falling-edge detectors produce pulses of equal magnitude but opposite polarity in response to the same low-frequency noise. This behavior derives from the fact that the rising and falling edges of the reference clock are respectively advanced and delayed by equal amounts. 
     In the embodiment of  FIG. 4B , a positive edge PFD and a negative edge PFD are combined. The PFD  400  compares the edge timing of the two input clocks—reference and divider. The resulting pulses at the D-FF  402 A- 402 D outputs are combined using OR gates  410 A- 410 B to produce UP and DN pulses for the phase-locked loop. The phase-UP signal is formed by taking the OR of the two PFD UP signals. That is, the VCO frequency should be increased when the rising or falling edges of the reference clock are early on average with respect to the divider clock. In corresponding fashion, the phase-signal is formed by taking the logical OR of the two PFD outputs. The net effect is that the phase UP and DN signals relate to the sum of the two PFD outputs. 
     When the PFD is operating in its steady-state condition in the absence of reference duty-cycle error, the rising and falling edges of reference and divider clocks will be in very close alignment such that the average outputs of each PFD is zero. In this ideal operating condition, the two PFD&#39;s produce pulses of minimum pulsewidth on alternating half-cycles of the reference clock in “ping-pong” fashion, and thus the UP and DN outputs of OR gates  410 A- 410 B produce minimum pulsewidth pulses on every half-cycle of the reference clock. The minimum pulsewidth is mostly determined by the reset delay through flip-flops  402 A- 402 D and the propagation delay through gates  404 A- 404 B and  406 A- 406 B. 
     An alternate steady-state operating point is found where the average of the sum of the PFD outputs is zero, but the individual PFD outputs are non-zero. This lock point corresponds to the condition where the reference and divider clocks are 180-degrees out-of-phase and is associated with simultaneous half-period pulses of opposite polarity being produced by the PFDs. To avoid undesirable operation at this lock point, the PFD of  FIG. 4B  includes an “inhibit” circuit comprising an additional NOR gate  408 A and AND gate  406 B that allows the rising-edge PFD to inhibit operation of the falling-edge PFD. This circuit excludes any possibility of the PFD&#39;s producing simultaneous pulses and thereby enforces the “ping-pong” operation corresponding to the desired operating point. 
     By employing the improved PFD of  FIG. 4B  in the gyroscope system of  FIG. 2 , the generation of low-frequency phase noise and phase error by the PLL  218  in response to low-frequency noise and offset  120  is suppressed. However, although low-frequency noise and phase error are suppressed, the improved PFD nonetheless produces a pulsetrain comprising UP and DN pulses of alternating polarity in the presence of reference duty-cycle errors. This pulsetrain disturbs the operation of the PLL, producing objectionable high-frequency spurious products at the PLL output. For this reason, it would be desirable to suppress low-frequency noise directly at the comparator input. 
     An improved embodiment of the present invention that addresses this issue is shown in  FIG. 5 . In this diagram, the PLL  118  of  FIG. 2  is augmented with a duty-cycle correction loop that operates with a detector  504 . As one loop operates with respect to errors in reference clock phase and the other loop operates with respect to errors in reference clock duty-cycle, this system of both loops could be referred to as a phase-and-duty-cycle-locked-loop (PDCLL)  502 . This system extends the PFD of  FIG. 4B  to provide a detector capable of simultaneously determining both phase and duty-cycle errors. The resulting detector is referred to as a phase-frequency-and-duty-cycle-detector (PFDCD)  504 . In such a detector, both phase and duty-cycle errors are determined by comparing the relative timing of the rising- and falling-edges of the reference and feedback divider clocks. 
     Referring to  FIG. 5 , the duty-cycle loop comprises the PFDCD  504  coupled to a charge pump  508 A and loop filter  510 A, whose output is fed back to the input of the comparator  516 . The PFDCD  504  measures the difference in duty-cycle between the divider and reference clocks. The error between them is converted to a charge by the CP  508 A and this charge is integrated on the loop filter (LF)  510 A. The LF  510 A output is subtracted from the comparator  516  input, and by virtue of feedback action the low-frequency noise  520  at the comparator  516  input is cancelled, resulting in equalization of the reference and divider clock duty-cycles. The PFDCD  504  also measures the phase error between reference and divider clocks, and this phase error is driven to zero by the PLL loop. 
     A key component of the PDCLL is the PFDCD  504  circuit. This circuit compares the reference and divider clocks and infers the duty-cycle and phase errors using comparisons of the rising and falling edges of both clocks. The principle of operation may be understood with reference to  FIG. 6 . Exemplary waveforms on the left of the figure illustrate the response of rising and falling-edge PFDs with respect to a phase error among two clocks with matching duty cycles. In such a situation, both detectors will produce pulses of the same polarity in this example; they are “UP” pulses, as the reference phase is ahead of the divider phase. In contrast, the right of the figure illustrates the response of rising and falling-edge PFDs with respect to a duty-cycle error among two clocks of matching phase. In this case, the detectors produce pulses of opposite polarity—in this example, “DN” pulses for the rising edge detector and “UP” pulses for the falling edge detector. 
     In the presence of phase error alone, the average of the sum of the two detector outputs gives a good indication of the phase error. Note that the average of the difference of the two detector outputs is zero, as both detectors produce pulses of the same duration. On the other hand, in the presence of duty-cycle error alone, the average of the difference of the two detector outputs gives a good indication of the duty-cycle error, whereas the average of the sum of the two outputs is zero. Thus, we see that a PFDCD can be constructed using a combination of positive and negative edge PFDs. 
     An embodiment of a PFDCD according to the present invention is found in  FIG. 7 . In this embodiment a positive edge PFD and a negative edge PFD are combined in a similar fashion to the PFD of  FIG. 4B . The embodiment of  FIG. 4B  is extended to provide UP and DN signals for the duty-cycle loop by the addition of OR gates  710 C and  710 D. The phase-UP signal is formed by taking the OR of the two PFD UP signals, as before. That is, the VCO frequency should be increased when the rising or falling edges of the reference clock are early on average with respect to the divider clock. The duty-cycle UP signal is formed by taking the OR of the rising edge PFD DN signal with the falling-edge PFD UP signal. That is, the duty cycle of the reference should be increased when the rising edge of the reference is late or the falling edge is early, on average. In corresponding fashion, the phase-DN and duty-cycle-DN signals are formed by taking the logical OR of the corresponding PFD outputs. The net effect is that the phase UP and DN signals relate to the sum of the two PFD outputs, whereas the duty-cycle UP and DN signals relate to the difference. 
     When the PDCLL is operating in its steady-state condition, the rising and falling edges of reference and divider clocks will be in very close alignment, even in the presence of low-frequency noise and offset, such that the average outputs of each PFD is zero. In this ideal operating condition, the two PFD&#39;s produce pulses of minimum pulsewidth on alternating half-cycles of the reference clock in “ping-pong” fashion, and thus the four outputs of OR gates  710 A- 710 D produce minimum pulsewidth pulses on every half-cycle of the reference clock. The minimum pulsewidth is only limited by the reset delay through gates  702 A- 702 B,  704 A and  706 A for the rising-edge PFD and by the reset delay through gates  702 C- 702 D,  704 B and  706 B for the falling-edge PFD. Thus, the high-frequency spurious products formerly produced by low-frequency noise and offsets are suppressed by mutual action of the phase-locked and duty-cycle locked loops. 
     As in  FIG. 4B , to avoid an undesirable lock point in which the reference and divider clocks are 180-degrees out-of-phase, the PFDCD of  FIG. 7  also includes an “inhibit” circuit comprising a NOR gate  708 A and AND gate  706 B that allows the rising-edge PFD to inhibit operation of the falling-edge PFD, thereby enforcing the “ping-pong” operation associated with the desirable in-phase lock point. 
     An embodiment of the present invention incorporating a PDCLL comprising a PFDCD appears in  FIG. 8 . In this embodiment, a rate gyroscope  802  comprises a PDCLL in place of the PLL of  FIG. 2 . This preferred embodiment retains all of the aforementioned benefits of the embodiment of  FIG. 2  with the added benefit that low-frequency noise  520  at the comparator  516  input is rejected by feedback action of the duty-cycle correction loop. The low-frequency noise arriving at the input to comparator  516  is now limited by the CP circuitry  508 A and  508 B in the PLL and duty-cycle loops. In steady-state operation, the CPs  508 A and  508 B will produce brief and simultaneous UP and DN pulses whose duration is set by the propagation delay of the D-FF and combinatorial logic gates in the RESET path of the PFD&#39;s. As these delays can easily be limited to a very small fraction of the reference clock period, the CPs  508 A and  508 B will only produce noise for brief intervals, and so the effect of low-frequency noise  520  generated by the CP  508 A and  508 B circuitry is greatly reduced by the “time-gated” operation of the PFDCD. This operation compares very favorably to conventional duty-cycle correction techniques, such as that illustrated in  FIG. 1 , in which the duty-cycle error is measured by measurement of the difference in “high” and “low” intervals of the comparator output. Such conventional techniques forego the noise reduction benefit of time-gating as the “high” and “low” intervals are continuously monitored, making low-noise operation much more difficult to achieve. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.