Patent Publication Number: US-8120429-B1

Title: Voltage-controlled oscillator methods and apparatus

Description:
This is a division of U.S. patent application Ser. No. 11/437,558, filed May 19, 2006 now U.S. Pat. No. 7,728,674, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to transceiver circuitry. More particularly, this invention relates to voltage-controlled oscillators for use with transceiver circuitry. 
     Periodic clock signals are often used to facilitate the operation of transceiver circuitry (e.g., for data synchronization or modulation). Phase-locked loop (“PLL”) circuitry is often used to generate such a periodic clock signal, which can often be substantially synchronized in frequency and phase to a reference clock signal. One key component of PLL circuitry is voltage-controlled oscillator (“VCO”) circuitry, which can generate a clock signal whose phase and frequency vary according to a voltage of an input control signal. 
     Many challenges exist in the design of effective VCO circuitry. For instance, it is desirable to achieve a relatively high bandwidth, so that the frequency of the output clock signal generated by the VCO circuitry can vary over a relatively wide range. A relatively high bandwidth can be especially important if the VCO circuitry is used in a programmable logic device (“PLD”), because the VCO circuitry must be able to support a relatively large number of communication protocols. As another example, it is desirable to keep the phase noise of the clock signal relatively low. (Phase noise is a metric commonly used to quantify the jitter of a clock signal.) Traditional design goals, such as relatively low power consumption, relatively high performance (e.g., speed), and relatively low circuit area, are also considerations that often need to be taken into account when designing VCO circuitry. 
     Deficiencies exist in existing VCO circuitry that can make them undesirable or unsuitable for certain transceiver applications. For instance, the “LC-tank” VCO architecture relies on an inductance and a capacitance to define a baseline oscillation frequency for the VCO&#39;s output clock signal. Although such an approach can yield relatively low phase noise, the frequency range is also relatively narrow. In addition, the LC-tank VCO architecture often consumes relatively large circuit area and a relatively large amount of power. 
     In view of the foregoing, it would be desirable to provide methods and apparatus that can generate a clock signal with relatively high bandwidth and relatively low phase noise. In addition, it would be desirable to provide such methods and apparatus with relatively low power consumption, relatively high performance, and relatively low circuit area. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, methods and apparatus are provided for generating a clock signal with relatively high bandwidth and relatively low phase noise. Advantageously, such methods and apparatus are provided with relatively low power consumption, relatively high performance, and relatively low circuit area. 
     According to an embodiment of the invention, a circuit can include a pair of transistors serially coupled between a signal of relatively high voltage and a source of relatively low voltage, where a voltage of the signal of relatively high voltage can vary according to a voltage of a variable control signal. A gate of one of the pair of transistors can be coupled to an input clock signal, and an output node between the pair of transistors can be coupled to an output clock signal. The circuit can also include a third transistor, whose drain and source are coupled to the output clock signal, and whose gate can be coupled to a gear input signal. This circuit of the invention can advantageously operate under at least two different gears, each with different bandwidth and phase noise characteristics. (It will be understood that, as used herein, the expression “coupled to” can be used to define either a direct or an indirect connection between two elements.) 
     In another embodiment of the invention, a VCO circuit can include a plurality of VCO cells coupled to each other in a loop. The VCO circuit can include control circuitry coupled to a source of relatively high voltage and operable to generate a control input signal of the VCO cells. The VCO cells can be operable to receive a gear input signal. A delay of at least one of the VCO cells can be based on a voltage of the control input signal and a voltage of the gear input signal. 
     In yet another embodiment of the invention, a method can include setting a control signal of a VCO circuit, where the control signal can be coupled to a source of relatively low voltage through a pair of inverters coupled to each other. The method can also include setting a gear signal of the VCO circuit, and generating a pair of differential output clock signals of the pair of inverters. A frequency of the pair of differential output clock signals can be based on the control signal and the gear signal. 
     The invention therefore advantageously provides methods and apparatus for generating a clock signal with relatively high bandwidth and relatively low phase noise. Advantageously, such methods and apparatus are provided with relatively low power consumption, relatively high performance, and relatively low circuit area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIG. 1  is a circuit diagram showing an illustrative VCO circuit in accordance with an embodiment of the invention; 
         FIG. 2  is a circuit diagram showing an illustrative VCO cell in accordance with an embodiment of the invention; 
         FIG. 3  is a graph showing illustrative relationships between frequency and control voltage in accordance with an embodiment of the invention; 
         FIG. 4  is a graph showing illustrative relationships between phase noise and control voltage in accordance with an embodiment of the invention; and 
         FIG. 5  is a block diagram of a data processing system incorporating the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a circuit diagram showing an illustrative VCO circuit  100  in accordance with an embodiment of the invention. VCO circuit  100  can include voltage regulator circuitry  101 , control circuitry  108 , and VCO cells  110  and  112 . 
     Voltage regulator circuitry  101  can be coupled to a source of relatively high voltage VCC (e.g., a power supply with a voltage of approximately 3.3V), and can advantageously be used to regulate the relatively high voltage. For instance, if the source of relatively high voltage is coupled to a plurality of VCO circuits, voltage regulator circuitry  101  can be used to desensitize VCO cells  110  and  112  from noise generated by other VCO circuits. In an embodiment of the invention, voltage regulator circuitry  101  can include resistor circuitry  102 , capacitor circuitry  104 , and transistor circuitry  106 , coupled to each other as shown in  FIG. 1 . In an embodiment of the invention, resistance R 0  of resistor circuitry  102  and capacitance C 0  of capacitance circuitry  104  can be selected to provide at least a certain amount of attenuation (e.g., approximately −30 dB) in power supply noise. Transistor circuitry  106  preferably includes an N-type metal oxide semiconductor (“NMOS”) transistor, which typically has a relatively high output impedance when compared to a P-type metal oxide semiconductor (“PMOS”) transistor. Control circuitry  108  is preferably operable to accept a variable control signal VCTRL of VCO circuit  100 , and to generate a control input signal VCC_CTRL of VCO cells  110  and  112 . Variable control signal VCTRL can be generated by any appropriate circuitry (e.g., a loop filter or phase-frequency detector of a PLL circuit in which VCO circuit  100  resides). By varying the voltage of variable control signal VCTRL, the voltage of control input signal VCC_CTRL can be varied, which in turn can affect the delay through each of VCO cells  110  and  112 . In an embodiment of the invention, control circuitry  108  preferably includes an NMOS transistor whose gate is coupled to variable control signal VCTRL, and whose drain is coupled to control input signal VCC_CTRL. In this embodiment, the NMOS transistor can advantageously further regulate relatively high voltage VCC. For example, if voltage regulator circuitry  101  can attenuate power supply noise by approximately −30 dB, voltage regulator circuitry  101  and control circuitry  108  together might be able to attenuate that noise by approximately −50 dB. 
     Control input signal VCC_CTRL can be coupled to VCO cells  110  and  112 , and can preferably affect the propagation delay through those cells. In the illustrative embodiment shown in  FIG. 1 , VCO cells  110  and  112  are differential inverters that are cross-coupled to each other in a loop, such that their output signals (OP 1 , ON 1 , OP 2 , and ON 2 ) are oscillatory in nature. In another embodiment, single-ended inverters could be used. In addition, although only two VCO cells  110  and  112  are used in the embodiment shown in  FIG. 1  in order to conserve circuit area, it will be understood that any appropriate number of VCO cells could be used. For instance, if three inverting VCO cells were used, the cells would not have to be cross-coupled, as coupling the cells in a loop would already produce the desired oscillation. VCO cells  110  and  112  can be operable to accept a gear input signal GEAR, which can affect the bandwidth and phase noise characteristics of VCO cells  110  and  112 , as described in more detail below in connection with  FIG. 2 . 
     VCO circuit  100  can provide a number of advantages over conventional VCO circuits (e.g., a known “current starved” VCO circuit). For instance, VCO circuit  100  employs relatively few transistors, which can yield relatively low switching noise. In addition, the tuning range of VCO circuit  100  can be relatively wide, and its voltage gain within that tuning range can be relatively uniform. In addition, a current starved VCO circuit often requires a finely-tuned voltage-controlled current source, which is not required in VCO circuit  100 . Finally, VCO circuit  100  can yield improved phase noise performance, especially at relatively high frequencies, when compared to certain known VCO circuits. 
       FIG. 2  is a circuit diagram showing an illustrative VCO cell  200  in accordance with an embodiment of the invention. VCO cell  200  can be used as VCO cell  110 , VCO cell  112 , or both in VCO circuit  100 . VCO cell  200  can be operable to receive control input signal VCC_CTRL, whose voltage can vary according to the voltage of a variable control signal (e.g., signal VCTRL in  FIG. 1 ). Control input signal VCC_CTRL can supply a pair of inverters with a relatively high voltage. In the illustrative example shown in  FIG. 2 , a first inverter can include PMOS transistor  202  and NMOS transistor  206 , which are coupled in series to each other between control input signal VCC_CTRL and a source of relatively low voltage (e.g., a ground voltage, commonly denoted as “GND”). Similarly, in the illustrative example shown in  FIG. 2 , a second inverter can include PMOS transistor  204  and NMOS transistor  212 , which are coupled in series to each other between control input signal VCC_CTRL and a source of relatively low voltage. The first and second inverters can be coupled in parallel to each other. 
     The first inverter can be operable to receive input signal INP and generate output signal OUTN, which can have a logical value that is the inverse of the logical value carried by input signal INP. Similarly, the second inverter can be operable to receive input signal INN and generate output signal OUTP, which can have a logical value that is the inverse of the logical value carried by input signal INN. Input signals INP and INN preferably belong to a differential pair of input signals, whose logical value is conveyed by the relative voltages of signals INP and INN. For example, a logical “1” might be indicated by input signal INP carrying a voltage that is substantially higher than the voltage of input signal INN. On the other hand, a logical “0” might be indicated by input signal INP carrying a voltage that is substantially lower than the voltage of input signal INN. Similarly, output signals OUTP and OUTN preferably belong to a differential pair of output signals, whose logical value is conveyed by the relative voltages of signals OUTP and OUTN. 
     When input control signal VCC_CTRL carries a relatively low voltage (preferably still higher than the relatively low voltage coupled to transistors  206  and  212 , e.g., GND), both inverters can switch relatively slowly, resulting in a longer delay through VCO cell  200 . In this scenario, the VCO circuit in which VCO cell  200  resides can generate an output signal of relatively low frequency. On the other hand, when input control signal VCC_CTRL carries a relatively high voltage, both inverters can switch relatively quickly, resulting in a shorter delay through VCO cell  200 . In this scenario, the VCO circuit in which VCO cell  200  resides can generate an output signal of relatively high frequency. 
     VCO cell  200  can preferably include NMOS feedforward transistors  208  and  210 , which can be cross-coupled to each other as shown in  FIG. 2 . This NMOS cross-coupling can advantageously conserve circuit area and power consumption relative to full complementary metal oxide semiconductor (“CMOS”) cross-coupling, which would also include a pair of cross-coupled PMOS transistors. In addition, employing NMOS cross-coupling instead of full CMOS cross-coupling can advantageously keep the output loading of VCO cell  200  relatively low. Transistors  202 ,  204 ,  206 ,  208 ,  210 , and  212  can be sized to optimize the performance and functionality of VCO cell  200  according to any appropriate criteria. For example, the sizes of these transistors can be chosen in order to ensure that the delay through VCO cell  200  can vary within a desired range in response to varying VCC_CTRL, such that the frequency of output signals OUTN and OUTP can vary within a desired bandwidth. 
     In accordance with an embodiment of the invention, VCO cell  200  can include gear transistors  214  and  216 , which can be coupled to output signals OUTN and OUTP, respectively. In addition, gear transistors  214  and  216  can be coupled to gear input signal GEAR, as shown. In the example shown in  FIG. 2 , gear transistor  214  can be an NMOS transistor whose drain and source are coupled to output signal OUTN, and whose gate is coupled gear input signal GEAR. Similarly, gear transistor  216  can be an NMOS transistor whose drain and source are coupled to output signal OUTP, and whose gate is coupled gear input signal GEAR. 
     When gear input signal GEAR is set to a logical “0,” both transistors  214  and  216  can enter depletion mode (e.g., be “deactivated”), thereby yielding relatively small capacitive loads on output signals OUTN and OUTP and a relatively short delay for VCO cell  200 . Thus, the VCO circuit in which VCO cell  200  resides can generate an output clock signal of relatively high frequency. Viewed another way, varying input control signal VCC_CTRL can vary the frequency of the output clock signal within a relatively wide frequency range. However, using relatively small capacitive loads at gear transistors  214  and  216  can reduce the amount of charging and discharging at output signals OUTN and OUTP, which can yield relatively large amounts of phase noise (e.g., due to flicker noise, wire noise, or thermal noise). 
     On the other hand, when gear input signal GEAR is set to a logical “1,” both transistors  214  and  216  can enter inversion mode (e.g., be “activated”), thereby yielding relatively large capacitive loads on output signals OUTN and OUTP and a relatively long delay for VCO cell  200 . Thus, the VCO circuit in which VCO cell  200  resides can generate an output clock signal of relatively low frequency. Viewed another way, varying input control signal VCC_CTRL can vary the frequency of the output clock signal within a relatively narrow frequency range. However, using relatively large capacitive loads at gear transistors  214  and  216  can increase the amount of charging and discharging at output signals OUTN and OUTP, which can make those signals relatively robust to phase noise (e.g., due to flicker noise, wire noise, or thermal noise). 
     VCO circuit  100  and VCO cell  200  are preferably laid out to achieve symmetrical geometry and substantially equal loading on all VCO cell outputs (e.g., OP 1 , ON 1 , OP 2 , and ON 2  in  FIG. 1 ). VCO cells  110  and  112  can be placed on top of a deep N-well to advantageously improve noise isolation. In addition, the substrate surrounding the deep N-well can be coupled to a dedicated power pin carrying a relatively low voltage (e.g., GND) to further enhance noise isolation. Decoupling capacitors can be placed around VCO circuit  100  to reduce noise even further. Additional layout considerations, such as device location, orientation, use of a guard ring, N-well/substrate taps, and dummy devices can also be varied as appropriate in order to further reduce the sensitivity of VCO circuit  100  and VCO cell  200  to noise. 
       FIG. 3  is a graph showing illustrative relationships between frequency and control voltage in accordance with an embodiment of the invention. In particular,  FIG. 3  illustrates how the frequency of an output clock signal of a VCO circuit according to the invention can vary with the voltage of a variable control signal VCTRL. As discussed in connection with VCO cell  200  of  FIG. 2 , setting gear input signal GEAR to a logical “0” can reduce output capacitive load and yield a relatively wide bandwidth as variable control signal VCTRL is varied. Similarly, setting gear input signal GEAR to a logical “1” can increase capacitive load and yield a relatively narrow bandwidth as variable control signal VCTRL is varied. 
       FIG. 4  is a graph showing illustrative relationships between phase noise and control voltage in accordance with an embodiment of the invention. In particular,  FIG. 4  illustrates how the phase noise of an output clock signal of a VCO circuit according to the invention can vary with the voltage of a variable control signal VCTRL. As discussed in connection with VCO cell  200  of  FIG. 2 , setting gear input signal GEAR to a logical “0” can reduce output capacitive load and yield relatively high phase noise as variable control signal VCTRL is varied. Similarly, setting gear input signal GEAR to a logical “1” can increase capacitive load and yield a relatively low phase noise as variable control signal VCTRL is varied. 
     Inspecting the relationships illustrated in  FIGS. 3 and 4 , one can see that it is often desirable to set gear input signal GEAR to a logical “1,” as that setting can desirably yield a lower degree of phase noise. However, this setting might not allow the VCO output clock signal to oscillate beyond a certain frequency. Thus, setting signal GEAR to a logical “1” may be desirable when the VCO circuit only needs to operate at relatively low frequencies. If the VCO circuit needs to operate at higher frequencies (e.g., to support operation with certain communication protocols), gear input signal GEAR can be set to a logical “0” to increase the bandwidth of the output clock signal generated by the VCO circuit. Although this setting may result in a relatively large phase noise at certain frequencies, this tradeoff may be acceptable in order to achieve the desired higher output clock frequencies. It will be understood that, even with gear input signal GEAR set to a logical “0,” the phase noise of the VCO circuit of the invention can still be lower than that of many known VCO circuits, and the phase noise is only seen as high relative to the other gear setting. 
     Thus, a VCO circuit and VCO cells are provided in accordance with the invention to generate an output clock signal with relatively high bandwidth and relatively low phase noise. Advantageously, the circuitry of the invention can achieve relatively low power consumption, relatively high performance, and relatively low circuit area when compared to conventional approaches. It will be noted that the embodiments described herein are merely illustrative, and other embodiments could be used. For example, the concepts of the invention can be applied to a VCO circuit with greater than two gears. As another example, level shifter circuitry can be employed to properly adjust the common-mode voltage (i.e., the average voltage of two differential output clock signals) of VCO circuit  100  as appropriate. In addition, current steering circuitry can be coupled to output clock signals of the VCO circuit  100 . This current steering circuitry can preferably improve phase noise characteristics of the output clock signals, though it may also increase power consumption. It will be understood that other embodiments can be used without deviating from the spirit or scope of the invention. 
       FIG. 5  illustrates an IC  506 , which incorporates VCO methods and apparatus in accordance with this invention, in a data processing system  540 . IC  506  may be a PLD, an application-specific IC (“ASIC”), or a device possessing characteristics of both a PLD and an ASIC. Data processing system  540  may include one or more of the following components: processor  502 ; memory  504 ; I/O circuitry  508 ; and peripheral devices  510 . These components are coupled together by a system bus  512  and are populated on a circuit board  520  which is contained in an end-user system  530 . 
     System  540  can be used in a wide variety of applications, such as computer networking, data networking, instrumentation, video processing, or digital signal processing. IC  506  can be used to perform a variety of different logic functions. For example, IC  506  can be configured as a processor or controller that works in cooperation with processor  502 . IC  506  may also be used as an arbiter for arbitrating access to a shared resource in system  540 . In yet another example, IC  506  can be configured as an interface between processor  502  and one of the other components in system  540 . 
     Thus it is seen that methods and apparatus are provided for generating a clock signal with relatively high bandwidth and relatively low phase noise. One skilled in the art will appreciate that the invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.