Abstract:
An oscillator circuit is provided. The oscillator circuit includes a gated oscillator and a calibration circuit. The gated oscillator is arranged to generate an oscillator signal according to a control signal, and receive a gating signal to align an edge of the oscillator signal with an edge of the gating signal. The calibration circuit coupled to the gated oscillator is arranged to receive a first clock signal and a second clock signal, detect an alignment operation of the gated oscillator according to the first clock signal and a second clock signal and generate the control signal according to the detected alignment operation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/145,765 filed Jan. 20, 2009 and entitled “A Reference-Free, Digital Background Calibration Technique for Gated-Oscillator-Based CDR/PLL”. The entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a calibration technique for a gated oscillator, and more particularly to a background calibration technique for a gated oscillator. 
     2. Description of the Related Art 
     Because gated oscillators can perform instantaneous phase realignment to input signals, they have recently grown in demand. Applications of gated oscillators include Burst Mode Clock and Data Recovery (BMCDR), low noise clock generation . . . etc. A burst mode CDR circuit is a circuit or circuit element that synchronizes or recovers timing information from a burst of formatted data applied to or input to the CDR circuit. However, one drawback of the gated oscillators is that the inherent frequency offset between the gated oscillators and input signals results in BER degradation or unwanted spurs. Conventionally, a Phase Locked Loop (PLL) with a replica gated oscillator can be used to track on-chip Process, Voltage and Temperature (PVT) variations. However, the approach requires additional circuit area for the replica, and a mismatch between the gated oscillator and the replica unavoidably occurs. 
     In another conventional approach, a local reference frequency is included to calibrate the frequency offset between the gated oscillators and input signals. However, the mismatch between the local reference clock and the input data rate still exits, which means a high-precision local clock source that would greatly increase the circuit cost is required. Therefore, an efficient background calibration technique is highly required. 
     BRIEF SUMMARY OF THE INVENTION 
     Oscillator circuits and methods for calibrating a frequency offset of a gated oscillator are provided. An embodiment of an oscillator circuit comprises a gated oscillator and a calibration circuit. The gated oscillator is arranged to generate an oscillator signal according to a control signal, and receive a gating signal to align an edge of the oscillator signal with an edge of the gating signal. The calibration circuit is coupled to the gated oscillator and arranged to receive a first clock signal and a second clock signal, detect an alignment operation of the gated oscillator according to the first clock signal and a second clock signal and generate the control signal according to the detected alignment operation. 
     Another embodiment of an oscillator circuit comprises a gated oscillator and a calibration circuit. The gated oscillator is arranged to generate an oscillator signal according to a control signal. The calibration circuit is coupled to the gated oscillator and arranged to receive a first clock signal and a second clock signal, detect a frequency or period change of the oscillator signal according to the first and the second clock signals, and generate the control signal according to the detected change. At least one of the first and the second clock signals is derived from the oscillator signal, and the second clock signal is a delayed version of the first clock signal. 
     An embodiment of a method for calibrating a gated oscillator is provided, comprising: detecting an alignment operation of the gated oscillator according to a plurality of phases of an oscillator signal generated by the gated oscillator; and generating a control signal to adjust the gated oscillator according to the detected alignment operation. 
     Another embodiment of a method for calibrating a gated oscillator is provided, comprising: generating an oscillator signal according to a control signal; receiving a first clock signal and a second clock signal and detecting a frequency or period change of the oscillator signal according to the first and the second clock signals; and generating the control signal according to the detected change. At least one of the first and the second clock signals is derived from the oscillator signal, and the second clock signal is a delayed version of the first clock signal. 
     Another embodiment of an oscillator circuit comprises a gated oscillator and a calibration circuit. The gated oscillator is arranged to operate according to a control signal. The calibration circuit is coupled to the gated oscillator, arranged to receive a first clock signal and a second clock signal from the gated oscillator, and generate the control signal according to the first and the second clock signals. 
     Another embodiment of an oscillator circuit comprises a gated oscillator and a calibration circuit. The gated oscillator is arranged to operate according to a control signal. The calibration circuit is coupled to the gated oscillator, arranged to receive a first clock signal from the gated oscillator and a second clock signal that is a delay version of the first clock signal, and generate the control signal according to the first and the second clock signals. 
     Another embodiment of a method for calibrating a gated oscillator is provided, comprising: controlling operation of a gated oscillator according to a control signal; and receiving a first clock signal and a second clock signal from the gated oscillator, and generate the control signal according to the first and the second clock signals. 
     Another embodiment of a method for calibrating a gated oscillator is provided, comprising: controlling operation of a gated oscillator according to a control signal; and receiving a first clock signal from the gated oscillator and a second clock signal that is a delay version of the first clock signal, and generate the control signal according to the first and the second clock signals. 
     By calibrating the gated oscillator, the frequency offset between the natural resonant frequency of the gated oscillator and the received input data can be reduced or eliminated, thereby improving the BER and jitter performance. Compared to the conventional calibration techniques, the replica oscillator and the local reference clock are not necessarily required in the present calibration mechanism. A reduced circuit area and production cost can be obtained with higher precision. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1A  shows a schematic diagram of a gated oscillator; 
         FIG. 1B  shows the timing diagrams of a target clock signal with a target frequency, an oscillator clock signal and the gating signal according to an embodiment of the invention; 
         FIG. 2  shows a schematic diagram of an oscillator circuit according to an embodiment of the invention; 
         FIG. 3  shows a block diagram of an oscillator circuit according to an embodiment of the invention; 
         FIG. 4  shows a block diagram of an oscillator circuit according to another embodiment of the invention; 
         FIG. 5  shows a schematic diagram of a burst mode clock and data recovery circuit with the proposed calibration technique according to an embodiment of the invention; 
         FIG. 6A  and  FIG. 6B  show the transient waveforms of the clock signals CK 1  and CK 2  according to an embodiment of the invention; and 
         FIG. 7  shows a schematic diagram of a phase realignment detector according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1A  shows a schematic diagram of a gated oscillator  101  and  FIG. 1B  shows the timing diagrams of a target clock signal with a target frequency, an oscillator clock signal and a gating signal according to an embodiment of the invention. As shown in  FIG. 1B , an oscillator has a resonant frequency (or free-run frequency) that is not exactly the same with the target frequency. To make the average frequency of the oscillator clock be close to the target frequency, a gating signal whose waveform corresponds to the target clock signal is used. When the pulse of the gating signal is input into the oscillator at time instance t 1 , it forces the oscillator clock signal to be aligned with the gating signal (for example, aligning an edge of the oscillator clock signal with an edge of the gating signal). However, the oscillator clock signal may gradually deviate due to the inherent frequency offset between the free-run frequency of the oscillator and the target frequency, which causes the clock edges of the oscillator clock signal to deviate from the clock edges of the target clock signal. In order to correct the frequency deviation, another pulse of the gating signal is input into the oscillator at time instance t 2  to re-align the oscillator clock signal with the gating signal, keeping the average frequency of the oscillator clock signal equal to the target frequency. Such kind of the oscillator is called a gated oscillator. The gating signal may be generated whenever there is a data transition. However, the alignment operation causes output jitter and deteriorates the bit error rate (BER). To enhance the performance,  FIG. 2  shows a schematic diagram of an oscillator circuit that may reduce the effect resulting from the alignment operation according to an embodiment of the invention. 
     The oscillator circuit  200  comprises a gated oscillator  201  and a calibration circuit  202 . According to the embodiments of the invention, the gated oscillator  201  may be a gated voltage controlled oscillator (GVCO), a gated current controlled oscillator (GICO), a gated digital controlled oscillator (GDCO) . . . etc. The gated oscillator  201  generates an oscillator signal at a resonant frequency, wherein the resonant frequency is adjustable according to a control signal S ctrl . The gated oscillator  201  further receives a gating signal S G  to align an edge of the oscillator signal with an edge of the gating signal. The calibration circuit  202  detects the behavior of the gated oscillator  201  or the oscillator clock signal, and determines if the resonant frequency of the gated oscillator  201  needs to be adjusted. For example, if an alignment operation occurs or a transient change on the period/frequency of the oscillator clock signal occurs, the calibration circuit  202  generates the control signal S ctrl  so as to decrease the frequency offset between the resonant frequency and the target frequency (generally, the target frequency corresponds to a multiple of the input data rate or input data frequency). When the frequency offset becomes smaller, the disturbance on the oscillator clock signal caused by the alignment operation becomes slighter, and therefore the output jitter performance can be improved. 
     According to one embodiment of the invention, the calibration circuit  202  detects the alignment operation or the transient change on the period/frequency of the oscillator clock signal according to the phase(s) of the oscillator signal. Reference is now made to  FIG. 3 , which shows a block diagram of an oscillator circuit according to an embodiment of the invention. The calibration circuit  302  comprises a monitor  303  (which may be implemented by a time-to-digital converter or a phase detector) and a feedback controller  304 . The monitor  303  receives a first clock signal CK 1  and a second clock signal CK 2 . According to an embodiment of the invention, the first clock signal CK 1  is derived from the gated oscillator  301 , and the second clock signal CK 2  is a delayed version of the first clock signal CK 1 . For example, the first clock signal CK 1  may be an output signal from an output node of the gated oscillator  301 , or a signal generated by modifying or processing an output signal from an output node of the gated oscillator  301 . The second clock signal CK 2  may be an output signal of the delay unit  305  that receives the first clock signal CK 1 . The delay unit  305  may be implemented by a delay cell, a delay line, or any other circuit capable of producing a delay. The monitor  303  detects the phases of the two clock signals CK 1  and CK 2  or phase difference between the two clock signals CK 1  and CK 2 , and generates an indication signal S ind  according to the detected result. The detected result indicates a positive or negative frequency offset between the gated oscillator and the input data (i.e., whether the resonant frequency of the gated oscillator is higher or lower than the target frequency), and the indication signal S ind  accordingly adjusts the resonant frequency. Alternatively, the monitor  303  may samples the first clock signal CK 1  according to the second clock signal CK 2  or samples the second clock signal CK 2  according to the first clock signal CK 1  to detect a positive or negative frequency offset between the gated oscillator and the input data, and generate the indication signal S ind  accordingly. Detailed explanations will be described in the later paragraph. The feedback controller  304  receives the indication signal S ind  and generates the control signal S ctrl  according to the indication signal. The gated oscillator  301  further tunes its resonant frequency, for example, through tuning the varactors on each oscillating nodes, according to the feedback control signal S ctrl  so as to calibrate the frequency offset. It should be noted that although the gated oscillator  301  shown in  FIG. 3  is a NAND-type gated voltage controlled oscillator, the invention should not be limited thereto. As one of ordinary skill in the art will readily appreciate, the gated oscillator  301  may be any type of gated oscillator performing substantially the same function or achieving substantially the same result as described in the embodiments of the invention. When the gated oscillator  301  is voltage-controlled, the control signal is in a voltage form; when the gated oscillator  301  is current-controlled, the control signal is in a current form; when the gated oscillator is digitally-controlled, the control signal is in a digital form. Moreover, although in this embodiment the feedback controller  304  converts the indication signal S ind  of the monitor  303  into control signal S ctrl , in other embodiments, the feedback controller  304  may be omitted if the indication signal S ind  can be directly utilized as the control signal S ctrl , or the feedback controller  304  may be integrated in the gated oscillator  301 . 
     Please refer to  FIGS. 6A and 6B , which show an example of how the monitor  303  detects the alignment operation according to the first and second clock signals CK 1  and CK 2 . As shown in the figures, the first clock signal CK 1  is sampled according to the second clock signal CK 2 . Since there is a predetermined delay (time difference) between the clock signals CK 1  and CK 2  (in this embodiment, the predetermined delay is 180°), an alignment operation that causes an edge to shift its position will be first observed on the first clock signal CK 1  and then be observed on the second clock signal CK 2  after the predetermined timing delay. Therefore, there will be a time period in which the first clock signal CK 1  has already been realigned but the second clock signal CK 2  is not realigned yet. When the monitor  303  detects that a currently sampled value of CK 1  is different from an expected value such as a previous sampled value, the monitor  303  generates the indication signal according to transition of the sampled values to adjust the resonant frequency. For example, in  FIG. 6A , an alignment operation occurs at the time instance T 1 , and the edge of the first clock signal CK 1  is lagged. A value ‘0’ is sampled by the monitor  303 , meaning that the resonant frequency of the gated oscillator is faster than the target frequency, the monitor  303  generates the indication signal so as to decrease the resonant frequency. On the other hand, as shown in  FIG. 6B , if an alignment operation occurring at the time instance T 2  leads the edge of the first clock signal CK 1 , a value ‘1’ will be sampled. The monitor  303  generates the indication signal so as to increase the resonant frequency. A slight sampling offset can be assigned to avoid metastability. 
     According to another embodiment of the invention, an alignment operation of the gated oscillator  301  may also be detected according to the variation of the phase difference between the first clock signal CK 1  with respect to the second clock signal CK 2 . Since there is a predetermined phase difference between the clock signals CK 1  and CK 2 , the alignment operation of the gated oscillator  301  may be detected when a current phase difference between the first and the second clock signals has deviated from the predetermined phase difference. As an example, assuming that the predetermined phase difference between CK 1  and CK 2  is 180°. When the monitor  503  detects an additional phase lead of the first clock signal with respect to the second clock signal, as an example, the current phase difference becomes 120°, the monitor  503  generates the indication signal S Ind  so as to increase the resonant frequency. On the other hand, when the monitor  503  detects an additional phase lag of the first clock signal with respect to the second clock signal, as an example, the current phase difference becomes 240°, the monitor generates the indication signal S Ind  so as to decrease the resonant frequency. 
       FIG. 4  shows a block diagram of an oscillator circuit according to another embodiment of the invention. According to the embodiment of the invention, the calibration circuit  402  comprises a monitor  403  and a feedback controller  404 . The monitor  403  receives a first clock signal CK 1  and a second clock signal CK 2 . Compared to the embodiment shown in  FIG. 3 , the first clock signal CK 1  and the second clock signal CK 2  are derived from two different nodes of the gated oscillator  401  in this embodiment, and the second clock signal CK 2  is a delayed version of the first clock signal CK 1 . The delay between the two clock signals CK 1  and CK 2  is generated by inherent delay in the gated oscillator  401 . For example, the first clock signal CK 1  and the second clock signal CK 2  may be output signals from output nodes of the gated oscillator  301 , or signals generated by modifying or processing output signals from output nodes of the gated oscillator  301 , while a predetermined time difference exists between the first and second clock signals CK 1  and CK 2 . The monitor  403  detects the phases of or phase difference between the first and the second clock signals CK 1  and CK 2 , and generates an indication signal S Ind  according to the detected result. The feedback controller  404  receives the indication signal S Ind  and generates the control signal S ctrl  according to the indication signal. The gated oscillator  401  further tunes its resonant frequency, for example, through tuning the varactors on each oscillating nodes, according to the feedback control signal S ctrl  so as to calibrate the frequency offset. It should be noted that although the gated oscillator  401  shown in  FIG. 4  is a gated voltage controlled oscillator, the invention should not be limited thereto. As one of ordinary skill in the art will readily appreciate, the gated oscillator  401  may be any type of gated oscillator performing substantially the same function or achieving substantially the same result as described in the embodiments of the invention. Similarly, the feedback controller  404  may be omitted if the indication signal S Ind  can be directly utilized as the control signal S ctrl , or may be integrated into the gated oscillator  401 . 
       FIG. 5  shows a schematic diagram of a burst mode clock and data recovery (BMCDR) circuit with the proposed calibration technique according to an embodiment of the invention. The BMCDR  500  comprises a gated oscillator  501  with an edge detector  506  to instantaneously align the edges of the gated oscillator  501  with the edges of the input data. The edge detector  506  generates a pulse of T/2 whenever there is a data transition, where T corresponds to one bit period. The DFF  508  receives the recovered clock from the gated oscillator  501  and recovers data from the input data according to the recovered clock. According to the embodiment of the invention, the calibration circuit  502  may comprise a monitor  503 , a counter  504  and a converter  505 . The monitor  503  receives two clock signals CK 1  and CK 2  from two different nodes of the gated oscillator  501 , wherein the second clock signal CK 2  is a delayed version of the first clock signal CK 1 . As an example, the predetermined delay between two clock signals CK 1  and CK 2  may be 180°. It should be noted that the second clock signal CK 2  may also be obtained from a delay unit as shown in  FIG. 3  and the invention should not be limited thereto. It should also be noted that although the gated oscillator  501  shown in  FIG. 5  is a gated voltage controlled oscillator, the invention should not be limited thereto. 
     According to an embodiment of the invention, the monitor  503  samples the first clock signal CK 1  according to the second clock signal CK 2  and/or samples the second clock signal CK 2  according to the first clock signal CK 1  so as to detect the alignment operation. As an example, the phase alignment/realignment occurs at the node outputting the first clock signal CK 1  every data transition with one gate delay. The phase alignment/realignment then occurs at the node outputting the second clock signal CK 2  with a delay of T/2. The monitor  503  generates an indication signal S Ind  according to the detected alignment operation. The counter  504  and the converter  505  together provide substantially the same functionality with the above-mentioned feedback controller. For example, the indication signal S Ind  may contain a digital number varying corresponding to the detected alignment operation. The digital number may be ‘1’ or ‘0’ when a positive or negative alignment operation is detected. The counter  504  maintains a count value, and the converter  505  converts the count value into control signal S Ctrl  so as to adjust the resonant frequency of the gated oscillator  501 . In one embodiment, the converter  505  is implemented by a digital-to-analog converter (DAC) for converting the digital count value into an analog control voltage or control current. In this way, the frequency offset between the gated oscillator and the input data rate is reduced, thereby reducing the BER or output jitter of the BMCDR  500 . The BMCDR  500  could be designed without using local reference clock or replica oscillator, and could be background calibrated. Moreover, when the input data shown in  FIG. 5  is replaced by a periodical switching signal (that is, the edge detector  506  generates the gating signal S G  according to the periodical switching signal), the circuit  500  can be utilized as a clock generator. 
     According to the embodiments of the invention, the monitor may be a phase detector, such as a phase realignment detector, a bang-bang phase detector, or any type of monitor performing substantially the same function or achieving substantially the same result as the monitor described in the embodiments of the invention. It should be noted that when the monitor is capable of detecting the amount of the phase difference/frequency offset rather than determining the polarity (such as positive or negative) of the phase difference/frequency offset, the counter may be omitted.  FIG. 7  shows a schematic diagram of an exemplary phase realignment detector according to an embodiment of the invention. The phase realignment detector  700  may comprise two sets of D flip-flop (DFF)  701  and  702 . The DFF  701  is clocked by the clock signal CK 2 _A and samples the clock signal CK 1 _A, wherein the clock signals CK 1 _A and CK 2 _A are the inverted versions of the clock signals CK 1  and CK 2 , respectively. The DFF  701  is introduced to detect whether the rate (resonant frequency) of the oscillator signal is slower than the input data rate. On the other hand, the DFF  702  is clocked by the clock signal CK 1 _B and samples the clock signal CK 2 _B, wherein the clock signals CK 1 _B and CK 2 _B are the inverted versions of the clock signals CK 1 _A and CK 2 _A, respectively. The DFF  702  is introduced to detect whether the rate (resonant frequency) of the oscillator signal is faster than the input data rate. It is noted that according to an embodiment of the invention, a slight sampling offset may be assigned for sampling DFFs in the phase realignment detector  700  to avoid metastability problems and the invention should not be limited thereto. 
     According to the embodiments of the invention, a background calibration technique for the gated oscillator is proposed. This approach reduces or eliminates the frequency offset between the gated oscillator and the input data/reference clock to reduce the bit error rate or output jitter. It demonstrates an error-free operation for a (2 31 −1) Pseudo Random Binary Sequence (PRBS) and tolerates more than 253 consecutive identical digits (CIDs). 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.