Abstract:
A hybrid circuit includes a current-starved voltage-controlled circuit configured to adjust a first type of signal difference, and a phase-interpolated voltage controlled circuit configured to adjust a second type of signal difference. The current-starved circuit and the phase-interpolated circuit cooperate to provide improved operational performance of the hybrid circuit.

Description:
BACKGROUND 
   1. Technical Field 
   The present invention generally relates to a voltage-controlled oscillator and a voltage-controlled delay line, and more particularly, to a voltage-controlled oscillator or a voltage-controlled delay line using hybrid current-starved phase-interpolation. 
   2. Description of the Related Art 
   Voltage-controlled delay (VCD) elements are useful in many applications. VCDs are important blocks in delay-locked loops (DLL) which synchronize an internal clock with an incoming clock. 
   Referring to  FIG. 1 , a typical block diagram of a DLL  10  is shown, which has a phase detector (PD)  14 , a loop filter (LPF)  16 , and a voltage-controlled delay line (VCDL)  12 . The phase detector  14  compares the timing difference between a reference clock edge φ ref  and a clock edge from the VCDL  12 , which is a delayed version of a clock input, φ in . Phase detector  14  generates an error voltage for the VCDL  12  to adjust the phase shift φ out . The phase detector  14  is usually accompanied by a charge pump (CP) in typical IC designs. 
   The loop filter  16  between the phase detector  14  and the VCDL  12  rejects high frequency noise. The feedback operation provides a control voltage V ctr  by a feedback loop  17 , which forces the internal clock edge φ ref  to be aligned to the incoming clock edge φ in . 
   Also, the VCD has been employed to build a ring-oscillator voltage-controlled oscillator (VCO) which is one important building block in phase-locked loop (PLL) design. 
   Referring to  FIG. 2 , a delay line in a phase locked loop  20  includes a ring-oscillator VCO  22  having a plurality of VCDs  12 . A phase-and-frequency detector (PFD)  14  and loop filter  16  are employed to provide a control signal to VCO  18 . 
   For digital clock generation, current-starved ring VCOs  22  shown in  FIG. 3  have been primarily used in the monolithic PLLs since they provide wide tuning range and high integration. (See e.g., I. Young, et. Al, “A PLL clock generator with 5 to 110 MHz of lock range for microprocessors”, IEEE JSSC, November 1992). 
   Referring to  FIG. 3 , the oscillation frequency of the current-starved ring VCO  22  is directly related to the delay time of each delay element  12 , resulting in high sensitivity to process, voltage, and temperature (PVT) variation. The nonlinear voltage-to-frequency transfer characteristic of the current-starved ring VCO  22  results in a highly variable VCO gain characteristic which is undesirable in PLL implementations. 
   The VCO  18  includes a V ref  input to a replica cell  24 . The VCO  18  includes transistors M 1 , M 2 , M 3  and M 4 . M 3  and M 4  are controlled in accordance with the output of the replica cell  24 . Vdd is the supply voltage, and V ctl  is employed to control a variable current source  26  at the foot of the oscillator. VIN and VINB represent V input and V input bar signals, and VO and VOB represent V output and V output bar signals, respectively. M 1  and M 2  are controlled in accordance with VIN and VINB, respectively. 
   Referring to  FIG. 4 , another way of implementing VCD elements is using a phase-interpolation method. An example of a circuit  30  implementation for phase-interpolation is shown in  FIG. 4  and described in J. Savoj and B. Razavi, “A 10-Gb/s CMOS clock and data recovery circuit with a half-rate linear phase detector”, IEEE, JSSC, May 2001. 
   Compared to a VCD using a current-starved approach, a VCD  32  using a phase-interpolation method offers reduced phase noise and more linear voltage-to-frequency characteristics. However, VCD  32  has less tuning range than the current-starved ring oscillator. For this reason, the current-starved method is still widely used in digital clock generation applications. 
   SUMMARY 
   A hybrid current-starved phase interpolation method for a voltage-controlled delay line and voltage-controlled oscillator provides a wide linear tuning range as well as good noise performance. Coarse tuning control is provided by the current-starved method to provide the wide tuning range over process and temperature variations, while the phase interpolation method is used to offer a linear tuning curve as well as low jitter performance. Additional control can be achieved with a programmable reference voltage which will set the output swing. The differential fine tuning control path of the VCD topology based on the phase interpolation method also provides better noise immunity against supply and substrate noise. 
   An adjusting circuit includes a current-starved voltage-controlled circuit configured to adjust a first type of signal difference. A phase-interpolated voltage controlled circuit is configured to adjust a second type of signal difference. The current-starved circuit and the phase-interpolated circuit cooperate to provide improved operational performance of the adjusting circuit. 
   A hybrid circuit includes a current-starved voltage-controlled circuit configured to adjust a first type of signal difference, and a phase-interpolated voltage controlled circuit configured to adjust a second type of signal difference. The current-starved circuit and the phase-interpolated circuit cooperate to provide improved operational performance of the hybrid circuit. 
   A hybrid circuit includes a swing fixing circuit responsive to an input reference voltage for adjusting loads coupled to a supply voltage, and a switching block responsive to an input voltage and the adjusted supply voltage to compare the reference voltage to the input voltage using a phase-interpolated method and adjust a difference between the reference voltage and the input voltage. An adjustment circuit is included for coarse and fine adjustment of the difference using tail current sources in accordance with a current-starved method. 
   A hybrid adjusting circuit includes a current-starved phase interpolation voltage-controlled circuit for adjusting input signals. The voltage-controlled circuit includes a coarse tuning control circuit configured to employ the current-starved method to provide the wide tuning range over process and temperature variations and a fine tuning control circuit configured to employ the phase interpolation method to offer a linear tuning curve and low jitter performance. A swing fixing circuit is responsive to a programmable reference voltage and configured to set an output swing for adjusting loads coupled to a supply voltage. 
   These and other objects, features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
       FIG. 1  is a block diagram showing a known voltage-controlled delay (VCD) for use in a delay-locked loop; 
       FIG. 2  is a block diagram showing a known voltage-controlled delay (VCD) for use in a phase-locked loop; 
       FIG. 3  is a schematic diagram illustrating a conventional current-starved delay cell; 
       FIG. 4  is a schematic diagram illustrating a conventional phase-interpolated delay cell; 
       FIG. 5  is an illustrative schematic diagram illustrating a hybrid current-starved phase-interpolation delay cell in accordance with one embodiment; and 
       FIG. 6  is an illustrative schematic diagram of an example of a hybrid ring oscillator in accordance with one illustrative embodiment. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A hybrid voltage controlled delay (VCD) employs both a current-starved method and a phase interpolation method. The VCD or voltage controlled oscillator (VCO) may include two control inputs, one input for coarse tuning control and the other input for fine tuning control. Coarse tuning control may be performed by the current-starved method to provide wide tuning range over process and temperature variations, while the phase interpolation method is used to achieve a linear tuning curve and low jitter performance. 
   Use of the current-starved method with active field effect transistor (e.g., FET) loads instead of resistor loads enables implementing nested current-starved VCD elements within phase-interpolated delay elements. In this way, the current-starved VCD within the phase-interpolated delay can track phase variations with large tail current variations. 
   Also, the hybrid VCD topology can be further extended to build composite voltage-controlled delay lines (VCDLs) using both current-starved VCD and hybrid VCD or interpolating VCDs and hybrid VCDs. One example is a ring VCO using both current-starved VCD and hybrid VCD. 
   Since the VCD has a dual input, the coarse tuning also reduces the control voltage range of the fine-tuning control. The reduced control voltage range for the fine-tuning path provides linear VCD gain as well as minimum VCD gain variation over a wide tuning range. The reduced voltage range in the loop filter enhances static phase offset performance of phase locked loops (PLLs) or delay locked loops (DLL) by easing charge pump design constraints. 
   In addition to the coarse tuning, another control can be achieved with a programmable reference voltage which will set the VCD output swing. Since the VCO output swing in the PLL design is one important parameter in determining noise performance as well as the tuning range, the controlled output swing can be used to optimize the noise performance over, for example, the process variation. 
   The coarse tuning path can be controlled digitally by having a current digital-to-analog converter. The fine-tuning path needs to be controlled by an analog voltage. However, depending on PLL topology, both coarse-tuning path and fine-tuning path can be controlled either in digital or in analog. Noise sensitivity should be minimized for the control path. 
   In integrated circuits, it is advantageous to employ differential signals to provide increased immunity to on-chip noise and signal coupling. The VCD topology of the present embodiments may be based on the phase interpolation method and can accommodate differential control for the fine-tuning path. Therefore, compared to conventional current-starved ring VCDs, the present embodiments for VCD provide better immunity to supply noise, wider linear tuning range, less VCD gain variation, and better noise performance. 
   Embodiments described herein may be implemented in a circuit. The circuit as described herein may be part of the design for an integrated circuit chip. The chip design is created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
   The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
   Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIG. 5 , an exemplary embodiment of an adjusting circuit  100  illustratively depicted as a hybrid VCD cell or circuit according to aspects of the present invention is shown. Transistors M 1 -M 8  perform phase interpolation with active loads M 7  and M 8  for fine adjustment. Transistors M 9 -M 13  generate delayed signal paths for phase interpolation, the function of which is similar to that of the VCD  32  shown in  FIG. 4 . 
   A swing fixing circuit  102  is implemented by a replica cell  108  and p-channel CMOS load transistors M 7 , M 8 , M 12  and M 13 . Sources of transistors M 7 , M 8 , M 12 , and M 13  are coupled to supply voltage V DD  and the gates of transistors M 7 , M 8 , M 12  and M 13  are coupled to replica cell  108 . The reference voltage V ref  provided by replica cell  108  is programmable to fix the voltage swing V SW  across load transistors M 7 , M 8 , M 12 , and M 13 , wherein V ref =V DD −V SW . The replica cell  108  senses the voltage swing and adjusts the effective load resistances of M 12 , M 13 , M 7  and M 8  (e.g., the resistances of the transistors: RM 12 , RM 13 , RM 7 , RM 8 ) to set the voltage swing V SW  to Vdd—V ref . The effective load resistance for a transistor is defined as: R M =1/(μ p C ox (V GS —V T ) (W/L); wherein μ p  is hole mobility, C ox  is the capacitance of the gate oxide, V GS  is the gate source voltage, V T  is the threshold voltage of the transistor, W is the channel width and L is the channel length. The voltage swing limits the swing of the VCD output signal (OUT or OUTB). 
   Control can be achieved with a programmable reference voltage (V ref1  through V refn ) which will set the VCD output swing (V SW ). Since the VCD output swing is an important parameter to determine noise performance as well as the tuning range, the controlled output swing can be used to optimize the noise performance over process variation. 
   The voltage swing V SW  limits the swing of the VCD output signal OUT. Hence, control can be achieved with the programmable reference voltage (V ref1  through V refn ) which will set the VCD output swing V SW . 
   A switching block  110  is implemented by n-channel CMOS switching transistors M 1 -M 4  and M 9 -M 10 . Switching block  110  compares and adjusts the input voltages with the reference voltage(s) in accordance with the load adjustments in transistors M 12 , M 13 , M 7  and M 8  and the coarse and fine adjustments input from a bias control circuit  112 . 
   Bias currents of transistors M 5  and M 6  are controlled by a fine tuning control voltage, Vfine, by way of a voltage-to-current (V-to-I) converter  104 . The bias current of M 11  is controlled by a coarse tuning control voltage, Vcrs. A reference current, I, the value of which may be established by a current digital-to-analog converter (DAC) (not shown) is generated by a variable current source  106 . Current source  106  may be controlled using a control signal (Vtune). Since the voltage swing V SW  is determined by the tail current I and the effective load resistance, R M , the current source  106  helps to control the delay between the input signal (IN) and the output signal (OUT). For example, increasing the tail current, I, makes the effective load resistance R M  reduced by the swing fixing circuit  102  to maintain the fixed voltage swing V SW , which in turn reduces the delay time. Hence, transistors M 11 , M 5  and M 6  function as tail current sources and provide the ability to coarsely and/or finely adjust the differences between signals. 
   The hybrid VCD  100  employs the advantages of both the current-starved method and the phase interpolation method. The VCD or VCO includes two control inputs Vcrs and Vfine, one for coarse tuning control and the other for fine tuning control, respectively. Coarse tuning control is done by the current-starved method to provide a wide tuning range over process and temperature variations, while the phase interpolation method is used to offer a linear tuning curve and low jitter performance. 
   Use of the current-starved method with active PFET loads (e.g., M 7 , M 8 , M 12  and M 13 ) instead of resistor loads enables implementing nested current-starved VCD within the phase-interpolated delay element. In this way, the current-starved VCD within phase-interpolated delay can track phase variation with large tail current variation using variable current source  106 . 
   Also, the hybrid VCD topology can be further extended to build composite voltage-controlled delay lines (VCDLs) using both current-starved VCD and hybrid VCD. Having similar kinds of active PFET loads, the hybrid VCD can track the common-mode voltage of the current-starved VCD, which in turn is advantageous to implement the composite VCDL. One example of a ring VCO using both current-starved VCD and hybrid VCD is shown in  FIG. 6 . 
   Since the VCD  100  has a dual input, Vcrs and Vfine, the coarse tuning reduces the control voltage range of the fine-tuning control. The reduced control voltage range for the fine-tuning path provides linear VCO gain as well as minimum VCO gain variation over a wide tuning range. Since VCO gain variation mostly determines PLL loop dynamics variation, having linear VCO gain helps to provide uniform PLL bandwidth. Maintaining uniform PLL bandwidth is one important consideration to minimize random jitter (RJ) variation. The reduced voltage range in a loop filter (not shown) also enhances static phase offset performance of phase locked loops (PLLs) or delay locked loops (DLL) by easing charge pump design constraints. 
   In addition to the coarse tuning, another control can be achieved with a programmable reference voltage (V ref ) which will set the VCD output swing. The controlled output swing V SW  can be used to optimize the noise performance over the process variation. Converter  104  may include a digital to analog converter such that the coarse tuning path can be controlled digitally by having a current digital-to-analog converter. The fine-tuning path should be controlled by an analog voltage. However, depending on PLL topology, both coarse-tuning path and fine-tuning path can be controlled either in digital or in analog. 
   Referring to  FIG. 6 , an example of a hybrid ring-oscillator  200  using current-starved and phase-interpolation methods is illustratively shown; however, it is to be understood that the present invention may be employed in a plurality of other circuits, e.g., in a VCO, DLL, PLL, etc. In this case, the ring VCO  200  includes a three stage VCD. A first VCD  210  and a third VCD  214  are based on current-starved delay cells for coarse tuning, and a second VCD  212  is based on a hybrid phase-interpolated VCD. This topology makes the VCO  200  have up to four independent control inputs, which could provide flexible VCO input control in a PLL design. 
   The four independent inputs include a Vcrs for coarse adjustment to VCDs  210  and  214 , a Vcrs for coarse adjustment of VCD  212  and two Vfine inputs for VCD  212  for fine adjustment. 
   Having described preferred embodiments of hybrid current-starved phase-interpolation circuit for voltage-controlled devices (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.