Abstract:
Provided are a frequency synthesizer and a frequency synthesizing method. The frequency synthesizer includes a ring oscillator, duty buffers, half adders, and a switch. The ring oscillator receives a pair of input signals and generates a pair of oscillating signals. The duty buffers receive the pair of oscillating signals of the ring oscillator and generates output signals with predetermined duty cycles. The half adders receive output signals of the duty buffers and generate an output signal as a result of an Exclusive-OR operation on the output signals of the duty buffers and an output signal as a result of an AND operation on the output signals of the duty buffers. The switch selects one of the oscillating signals of the ring oscillator, the output signals as results of the Exclusive-OR operation, and the output signals as results of the AND operation. By using the frequency synthesizer, it is possible to select one of an oscillating-frequency output signal of a high-frequency ring oscillator, an output signal of a high-frequency that is two times higher than that of the oscillating frequency of the ring oscillator bock, and an output signal of a frequency that is the same as that of an input signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application claims the priority of Korean Patent Application No. 2003-25534, filed on Apr. 22, 2003, in the Korean Intellectual Property Office, the disclosure of which is hereby incorporated by reference in its entirety.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to a semiconductor integrated circuit. In particular, the present invention generally relates to a frequency synthesizer and a frequency synthesizing method.  
           [0004]    2. Description of the Related Art  
           [0005]    With the increasing demand for information communications, the mobile communication system market is expanding rapidly. Thus, a great deal of research is being done in relation to low-cost, low-power consumption, and small-volume systems. As a result, Complementary Metal Oxide Semiconductor (CMOS) process technology, and semiconductor devices using a small-size chip and operating at a high-frequency have been introduced.  
           [0006]    A high-frequency, low-noise, and low-power phase-locked loop (PLL) may be used in various fields such as optical data links and asynchronous transfer mode (ATM) systems. The PLL normally has a high operating frequency, a short response time, small clock jitter, a wide input locking range, and a linear voltage-to-frequency conversion characteristic. These characteristics of the PLL serve as factors in determining the performance of a voltage controlled oscillator (VCO). In general, the VCO is designed using an LC tank circuit or a ring oscillator.  
           [0007]    [0007]FIG. 1 illustrates a conventional LC tank circuit according to the Related Art. Referring to FIG. 1, in an LC tank circuit  100 , two pairs of an inductor L and a capacitor C are series connected between a supply voltage VDD and a ground voltage VSS. Inductors L and capacitors C connect at nodes  102  and  104 , respectively. NMOS transistors MT 1  and MT 2  are connected between nodes  102  and  104 , respectively, and the ground voltage VSS. Gates of transistors MT 2  and MT 1  are cross-connected to nodes  102  and  104 , respectively. In the LC tank circuit  100 , an output voltage Vout typically oscillates according to discharging/charging operations of capacitors C.  
           [0008]    The LC tank circuit  100  normally has a high Q value and may produce a clear signal due to low phase noise and small clock jitter. However, the LC tank circuit  100  typically has a small tuning range and may need a large layout area to form inductors.  
           [0009]    A VCO using a ring oscillator typically may be easily integrated because a ring oscillator typically requires a small layout area. Such a ring-oscillator-type VCO typically has a large operational range. However, the VCO that uses a ring oscillator normally cannot implement high-frequency operation and may undesirably generate large phase noise.  
         SUMMARY OF THE INVENTION  
         [0010]    An example embodiment of present invention provides a frequency synthesizer, which can operate in a wide range of high frequencies can accurately select an operating frequency and/or can exhibit high integration density.  
           [0011]    Another exemplary embodiment of present invention provides a frequency synthesizing method by which a high operating frequency can be generated and selected.  
           [0012]    An exemplary embodiment of present invention provides a frequency synthesizer including a ring oscillator having input thereto a pair of input signals, the ring oscillator for generating a pair of oscillating signals; duty buffers having input thereto the pair of oscillating signals, the duty buffers for generating output signals with predetermined duty cycles; half adders having input thereto the output signals, the half adders for generating an output signal as a result of an Exclusive-OR operation on the output signals of the duty buffers and an output signal as a result of an AND operation on the output signals of the duty buffers; and a switch for selecting one of the oscillating signals of the ring oscillator, the output signal as a result of the Exclusive-OR operation, and the output signal as a result of the AND operation.  
           [0013]    An exemplary embodiment of present invention provides a frequency synthesizer including a ring oscillator having a pair of input signals input thereto and, in response to a control signal, the ring oscillator generates a pair of first oscillating signals, a pair of second oscillating signals, and a pair of third oscillating signals being delayed by a predetermined amount of time; duty buffers having the pair of first oscillating signals and the pair of second oscillating signals of the ring oscillator input thereto, the duty buffers for generating first and second output signals having duty cycles being 50%; half adders receiving output signals of the duty buffers, the half adders for generating an output signal as a result of an Exclusive-OR operation on the first and second output signals of the duty buffers and an output signal as a result of an AND operation on the first and second output signals of the duty buffers; and a switch for selecting select one of a third oscillating signal of the ring oscillator, the output signal as a result of the Exclusive-OR operation, and the output signal as a result of the AND operation.  
           [0014]    An exemplary embodiment of present invention provides a frequency synthesizing method including receiving a pair of input signals and, in response to a control signal, generating a pair of first oscillating signals, a pair of second oscillating signals, and a pair of third oscillating signals that are delayed by a predetermined amount of time; receiving the pair of first oscillating signals and the pair of second oscillating signals of the ring oscillator and generating first and second output signals having duty cycles being substantially 50%; receiving output signals of the duty buffers and generating an output signal as a result of an Exclusive-OR operation on the first and second output signals of the duty buffers and an output signal as a result of an AND operation on the first and second output signals of the duty buffers; and selecting one of a third oscillating signal of the ring oscillator, the output signal as a result of the Exclusive-OR operation, and the output signal as a result of the AND operation.  
           [0015]    According to the present invention, by using the frequency synthesizer, it is possible to select one of an oscillating-frequency output signal of a high-frequency ring oscillator, an output signal of a high-frequency that is two times higher than that of the oscillating frequency of the ring oscillator bock, and an output signal of a frequency that is the same as that of an input signal.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The exemplary embodiments of the present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0017]    [0017]FIG. 1 illustrates a conventional LC tank circuit according to the Related Art;  
         [0018]    [0018]FIG. 2 illustrates a frequency synthesizer according to an exemplary embodiment of the present invention;  
         [0019]    [0019]FIG. 3 illustrates a ring oscillator of FIG. 2, according to an example embodiment of the present invention;  
         [0020]    [0020]FIG. 4 illustrates waveforms of signals related to the operation of a ring oscillator of FIG. 3, according to an example embodiment of the present invention;  
         [0021]    [0021]FIG. 5 illustrates an output waveform of a ring oscillator, according to an example embodiment of the present invention;  
         [0022]    [0022]FIG. 6 illustrates a duty buffer of FIG. 2, according to an example embodiment of the present invention;  
         [0023]    [0023]FIG. 7 illustrates a half adder of FIG. 2, according to an example embodiment of the present invention;  
         [0024]    [0024]FIG. 8 illustrates waveforms of signals related to the operation of a half adder of FIG. 7, according to an example embodiment of the present invention;  
         [0025]    [0025]FIGS. 9A-9C and  10  illustrate simulation results obtained using a frequency synthesizer according to an example implementation of an embodiment of the present invention; and,  
         [0026]    [0026]FIGS. 11A-11C illustrate estimation results of a semiconductor chip in which a frequency synthesizer according to an example implementation of an embodiment of the present invention is integrated. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the present invention are shown. In the drawings, like reference numerals are used to refer to like elements throughout.  
         [0028]    [0028]FIG. 2 illustrates a frequency synthesizer  200  according to an exemplary embodiment of the present invention. Referring to FIG. 2, frequency synthesizer  200  may oscillate an output signal of a dual-band frequency, and therefore will be called “a dual-band VCO circuit”. The dual-band VCO circuit  200  may include a four-stage ring oscillator structure  210 , first through fourth duty buffers  220 ,  230 ,  240 , and  250 , a first half adder  260 , a second half adder  270 , a 2:4 decoder  280  and a switch  290 .  
         [0029]    The four-stage ring oscillator structure  210  may receive a first input signal VIN+ and a second input signal VIN− at a pair of input terminals and may output output signals C_OUT and its inversion /C_OUT. The first input signal VIN+ and the second input signal VIN− are generally considered 180° out of phase. The four-stage ring oscillator structure  210  include may include four ring oscillators  212 ,  214 ,  216 , and  218 .  
         [0030]    [0030]FIG. 3 illustrates the first ring oscillator  212  of FIG. 2 as a representative of the four ring oscillators  212 ,  214 ,  216 , and  218 , according to an embodiment of the present invention. Referring to FIG. 3, the first ring oscillator  212  may include PMOS transistors M 1  and M 2 , PMOS transistors MP 1  and MP 2 , PMOS transistors MP 3  and MP 4 , NMOS transistors MN 1  and MN 2 , NMOS transistors MN 3  and MN 4 , NMOS transistor M 3 , and NMOS transistor M 4 . The PMOS transistors M 1  and M 2  may be connected to a supply voltage VDD and their gates may be connected to a first bias signal PBIAS. Gates of the PMOS transistor MP 1  and the NMOS transistor MN 1  may be connected to the first input signal VIN+. Gates of the PMOS transistor MP 4  and the NMOS transistor MN 4  may be connected to the second input signal VIN−. The gate of the NMOS transistor M 4  is normally connected to a control signal VCON. The gate of the NMOS transistor M 3  may be connected to a second bias signal NBIAS.  
         [0031]    NMOS transistor M 4  may be connected between commonly connected source terminals of the NMOS transistors MN 1  and MN 4  and the ground voltage VSS. Similarly, NMOS transistor M 3  may be connected between commonly connected source terminals of the NMOS transistors M 2  and M 3  and the ground voltage VSS.  
         [0032]    The drain terminal of transistor MP 2  may be connected at a node  302  to the drain terminals of transistors MN 1  and MN 2 , its own gate terminal and the drain terminal of transistor MP 1 . The source terminals of transistors MP 2  and MP 1  may be connected to the drain terminals of transistor M 1 . The drain terminal of transistor MP 3  is connected at node  204  to its own gate terminal and the commonly connected drain terminals of transistors MN 3  and MN 4  and to the source terminal of transistor MP 3 . The gates of transistors MN 2  and MN 3  are normally cross-connected at nodes  304  and  302 , respectively. Connection points of the source terminals of transistors MP 3  and MP 4  are connected to the drain terminal of transistor M 2 . A first output signal VOUT+ is applied to node  304 . A second output signal VOUT− is applied to node  302 .  
         [0033]    The first bias signal PBIAS, the second bias signal NBIAS, and the control signal VCON can together control the first ring oscillator  212 . Waveforms of signals PBIAS, NBIAS and VCON are depicted in FIG. 4, according to an embodiment of the present invention. As shown in FIG. 4, the first ring oscillator  212  operates such that the second output signal VOUT− is generated as a high logic value (logic high) and the first output signal VOUT+ is generated as a low logic value (logic low) when the first input signal VIN+ is logic low and the second input signal VIN− is logic high. When the first input signal VIN+ is logic high and the second input signal VIN− is logic low, the second output signal VOUT− is generated as logic low and the first output signal VOUT+ is generated as logic high.  
         [0034]    The voltage level of the control signal VCON may control the current flowing through the NMOS transistor M 4  such that when the voltage level of the control signal VCON increases, the current flowing through the NMOS transistor M 4  increases. As shown in the waveforms of FIG. 5 (according to an embodiment of the present invention), as the voltage level of the control signal VCON increases, transition slopes of the second output signal VOUT− and the first output signal VOUT+ become generally steeper (or, in other words, the oscillation speed increase).  
         [0035]    Referring back to FIG. 2, the four-stage ring oscillator structure  210  normally operates based on the operation of the first ring oscillator  212  described above. In response to the first input signal VIN+ taking a value of logic low, the second output signal VOUT− of the first ring oscillator  212  is generated as logic high, and accordingly the second output signal VOUT− of the second ring oscillator  214  is generated as logic low, the second output signal VOUT− of the third ring oscillator  216  is generated as logic high and the second output signal VOUT− of the fourth ring oscillator  218  is generated as logic low. Similarly, when second input signal VIN− takes a value of logic low, ultimately the first output signal VOUT+ of the fourth ring oscillator  218  is generated as logic high. The first output signal VOUT+ of the fourth ring oscillator  218  may be fed back to and added to the first input signal VIN+ of the first ring oscillator  212 . The second output signal VOUT− of the fourth ring oscillator  218  may be fed back to and added to the second input signal VIN− of the first ring oscillator  212 .  
         [0036]    By performing such operations repeatedly, the output signals C_OUT and /C_OUT of the four-stage ring oscillator structure  210  are oscillated. Also, as explained hereinabove with reference to FIG. 5, the oscillating speed of the output signals C_OUT and /C_OUT may increase as the voltage level of the control signal VCON increases, and thus, the output signals C_OUT and /C_OUT may have high frequencies.  
         [0037]    In FIG. 2, the first through fourth duty buffers  220 ,  230 ,  240 , and  250  may be used to set duty cycles of input signals of the first and second half adders  260  and  270  to 50%. As a representative of the first through fourth duty buffers  220 ,  230 ,  240 , and  250 , the first duty buffer  220  is shown in FIG. 6, according to an embodiment of the present invention.  
         [0038]    Referring to FIG. 6, the first duty buffer  220  may include a PMOS transistor  601 , an NMOS transistor  603 , a PMOS transistor  605 , an NMOS transistor  613 , a PMOS transistor  611 , an NMOS transistor  615 , a PMOS transistor  607 , an NMOS transistor  609 , and inverters  620  and  630 . The PMOS transistor  601  may be connected to the supply voltage VDD and its gate may be connected to a first input signal IN+. The PMOS transistor  607  may be connected to the supply voltage VDD and may be connected to a second input signal IN−. Drains and gates of NMOS transistors  603  and  609  may be respectively connected to PMOS transistors  601  and  607  at nodes  642  and  644 , respectively. A gate of the PMOS transistor  605  may be connected to node  642 . A gate of the PMOS transistor  611  may be connected to node  644 . The NMOS transistors  613  and  615  are generally connected to the PMOS transistors  605  and  611  in the shape of a current mirror, respectively, such that gates thereof are connected to a node  646 . The inverters  620  and  630  may be serially connected with each other, an input of the inverter  620  being connected to the drains of the PMOS transistor  611  and the NMOS transistor  615  at a node  648 .  
         [0039]    The operation of the first duty buffer  220  is performed as follows. The PMOS transistors  601  and  607 , the PMOS transistors  605  and  611 , and the NMOS transistors  613  and  615  are designed in such a way that they are substantially symmetrical to each other. Drain currents of the PMOS transistors  601  and  607  may be given by  
               I                   d        (   601   )         =       1   2          K        (   601   )            (     W   L     )          (   601   )            (       Vsg        (   601   )       -          Vt        (   601   )              )     2               (   1   )                 I                   d        (   607   )         =       1   2          K        (   607   )            (     W   L     )          (   607   )            (       Vsg        (   607   )       -          Vt        (   607   )              )     2               (   2   )                               
 
         [0040]    Since the PMOS transistors  601  and  607  are substantially symmetrical to each other, Id ( 601 ) is treated as being substantially equal to Id ( 607 ), Id ( 601 ) is treated as being substantially equal to Id ( 603 ), and Id ( 607 ) is treated as being substantially equal to Id ( 609 ). Therefore, the following Equations 3 and 4 can be obtained.  
               I                   d        (   601   )         =       1   2          K        (   603   )            (     W   L     )          (   603   )            (       Vsg        (   603   )       -          Vt        (   603   )              )     2               (   3   )                 I                   d        (   601   )         =       1   2          K        (   607   )            (     W   L     )          (   607   )            (       Vsg        (   607   )       -          Vt        (   607   )              )     2               (   4   )                               
 
         [0041]    The ranges of voltages between respective sources and drains of the NMOS transistors  603  and  609  may be given as:  
               Δ                   Vds        (   603   )         =       Δ                   Vgs        (   603   )         =           2                 Δ                 I                   d        (   601   )             K        (   603   )            (     W   L     )          (   603   )           +          V                   t        (   603   )                          (   5   )                 Δ                   Vds        (   609   )         =       Δ                   Vgs        (   609   )         =           2                 Δ                 I                   d        (   601   )             K        (   609   )            (     W   L     )          (   609   )           +          V                   t        (   609   )                          (   6   )                               
 
         [0042]    If clock signals whose duty cycles are accurately 50% are input as the first and second input signals IN+ and IN−, ΔVds( 603 ) and ΔVds( 609 ) may be identical. Accordingly, the following Equation 7 can be obtained as:  
                     2                 Δ                 I                   d        (   601   )             K        (   603   )            (     W   L     )          (   603   )           +          V                   t        (   603   )                =           2                 Δ                 I                   d        (   601   )             K        (   609   )            (     W   L     )          (   609   )           +          V                   t        (   609   )                        (   7   )                               
 
         [0043]    Assuming that |Vt( 603 )|=|Vt( 609 )|, Equation 7 can be expressed as  
                 (     W   L     )          (   603   )       =         K        (   609   )         K        (   603   )              (     W   L     )          (   609   )               (   8   )                               
 
         [0044]    That is, an output signal OUT whose duty cycle is substantially accurate 50% may be obtained by controlling a W/L ratio of the NMOS transistors  603  and  609 .  
         [0045]    [0045]FIG. 7 illustrates the first half adder  260  of FIG. 2, according to an embodiment of the present invention. Half adder  260  includes a logical XOR gate  702  and a logical AND gate  704 . The first half adder  260  may receive output signals X and Y of the first and second duty buffers  220  and  240  of FIG. 2 as input signals S 1  and S 2  and may generate output signals EX_OUT and AND_OUT.  
         [0046]    The operation of the first half adder  260  will be described with reference to the waveforms of FIG. 8, according to an embodiment of the present invention. In FIG. 8, the input signals S 1  and S 2  are received and operated upon by XOR gate  702 , which outputs the first output signal EX_OUT. Similarly, input signals S 1  and S 2  are also received and operated by gate  704  which outputs the second output signal AND_OUT.  
         [0047]    The second half adder  270  may be similar to the first half adder  260 . But it is to be noted that the second half adder  270  is arranged to output the inversions of signals EX_OUT and AND_OUT, namely /EX_OUT and /AND_OUT.  
         [0048]    Referring back to FIG. 2, one of the first output signal EX_OUT (from first half adder  260 ), the second output signal AND_OUT, (also from first half adder  260 ) and the output signal C_OUT (from the four-stage ring oscillator structure  210 ) may be selected by the switch  290  according to an output signal of the 2:4 decoder  280 , and a selected signal may be output as a high-frequency output signal OUT of the dual-band VCO circuit  200 . The 2:4 decoder  280  itself may receive a frequency selection signal SEL&lt;1:0&gt; which can cause the 2:4 decoder  280  to select one of the signals EX_OUT, AND_OUT, and C_OUT. Similarly, another 2:4 decoder and switch could be provided to controllably select among signals /C_OUT, /EX_OUT and /AND_OUT.  
         [0049]    [0049]FIGS. 9A-9C and  10  illustrate simulation results obtained using the dual-band VCO circuit  200 , according to an example implementation of an embodiment of the present invention. FIGS. 9A-9C illustrate waveforms of the output signal C_OUT of the four-stage ring oscillator structure  210  and the first output signal EX_OUT and the second output signal AND_OUT of the first half adder  260 , where the dual-band VCO circuit  200  of FIG. 2 is simulated in the condition that the supply voltage VDD is 3.3V and the voltage level of the control signal VCON is 3.0V. The output signal C_OUT has a frequency of 1.07 GHz, the first output signal EX_OUT has a frequency of 2.1 GHz, and the second output signal AND_OUT has a frequency of 1.05 GHz. The output signal C_OUT and the second output signal AND_OUT have similar frequencies whereas the waveform of the second output signal AND_OUT is clearer than that of the output signal C_OUT.  
         [0050]    [0050]FIG. 10 illustrates an output frequency of the first output signal EX_OUT with respect to the voltage level of the control signal VCON of an example implementation of an embodiment according to the present invention. Referring to FIG. 10, the output frequency of the first output signal EX_OUT generally increases substantially linearly when the voltage level of the control signal VCON ranges from 0.8V to 2.7V. However, the output frequency of the first output signal EX_OUT typically does not change significantly after the voltage level of the control signal VCON reaches 2.8V.  
         [0051]    [0051]FIG. 11 shows frequency characteristics of the output signal C_OUT of the four-stage ring oscillator structure  210  and the first output signal EX_OUT and the second output signal AND_OUT of the first half adder  260 , which are estimated in an example implementation of a semiconductor chip in which the dual-band VCO circuit  200  of FIG. 2 is integrated. Referring to FIG. 11, similarly with the simulation results of FIG. 9, the output signal C_OUT has a frequency of 1.072 GHz, the first output signal EX_OUT has a frequency of 2.057 GHz, and the second output signal AND_OUT has a frequency of 1.051 GHz.  
         [0052]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.