[0001]     This application claims priority from Korean Patent Application No. 2005-12083, filed on Feb. 14, 2005, the entire content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a quadrature voltage controlled oscillator, and more particularly to a quadrature voltage controlled oscillator having low phase noise and excellent output swing characteristics.  
         [0004]     2. Description of the Related Art  
         [0005]     A quadrature oscillator outputs four signals having different phases. The quadrature oscillator is generally used in a transceiver circuit for communication devices such as cellular phones, wireless telephones, wireless networking devices, and blue-tooth communication devices.  
         [0006]     The above-described devices use a direct conversion method or a low-intermediate frequency method rather than a super heterodyne method to convert a radio frequency (RF) signal to a baseband signal. This is because the direct conversion manner or the low-intermediate frequency manner is advantageous over the super-heterodyne manner in terms of cost and integration. Accordingly, there is a need for a quadrature voltage controlled oscillator capable of generating in-phase output signals and quadrature-phase output signals.  
         [0007]      FIG. 1  is a schematic circuit diagram of a conventional quadrature voltage controlled oscillator (VCO). Referring to  FIG. 1 , the quadrature VCO is composed of two VCOs having the same structure and constituting a feedback structure. This conventional quadrature VCO is disclosed in U.S. Pat. No. 6,462,626 entitled “Quadrature Output Oscillator,” issued on Oct. 8, 2002. U.S. Pat. No. 6,462,626 is hereby incorporated in its entirety by reference.  
         [0008]     One of the two VCOs as described above outputs a positive in-phase signal and a negative in-phase signal and the other VCO outputs a negative quadrature-phase signal and a positive quadrature-phase signal. Each of the two VCOs comprises a constant current source (i.e., NMOS transistor biased by a Vbias voltage) for supplying constant current.  
         [0009]     However, continuous operation of the NMOS transistor in a saturation region causes a flicker noise to increase, which in turn causes a phase noise to increase. The flicker noise is caused by a phenomenon that electrons passing a channel of the transistor are captured by the gate of the NMOS transistor through which constant current continues to flow.  
         [0010]     Further, in the left VCO of  FIG. 1 , the minimum point of a voltage swing of the output voltages IP and IN and the minimum point of a voltage at the drain of the NMOS transistor biased by the Vbias voltage are not aligned to each other, and thus the voltage swing of the output voltages IP and IN is limited. Similarly, in the right VCO, the minimum point of a voltage swing of the output voltages QN and QP and the minimum point of a voltage at the drain of the NMOS transistor biased by the Vbias voltage are not aligned to each other, and thus the voltage swing of the output voltages QN and QP is limited.  
       SUMMARY OF THE INVENTION  
       [0011]     An aspect of the present invention is to provide a quadrature voltage controlled oscillator having a low phase noise characteristic.  
         [0012]     Another aspect of the present invention is to provide a quadrature voltage controlled oscillator having an excellent output swing characteristic.  
         [0013]     According to an aspect of the present invention, there is provided a quadrature voltage controlled oscillator, according to an exemplary embodiment of the present invention, which comprises a first voltage controlled oscillator for outputting a positive in-phase output signal and a negative in-phase output signal; a second voltage controlled oscillator for outputting a positive quadrature-phase output signal and a negative quadrature-phase output signal, the second voltage controlled oscillator having a symmetrical structure with the first voltage controlled oscillator and constituting a feedback loop together with the first voltage controlled oscillator; a first constant current source for supplying constant current to the first voltage controlled oscillator in response to the output signals; and a second constant current source for supplying constant current to the second voltage controlled oscillator in response to the output signals.  
         [0014]     The first current source may supply the constant current in response to the positive in-phase output signal and the negative in-phase output signal.  
         [0015]     The first current source may include two NMOS transistors, and the positive in-phase output signal and the negative in-phase output signal may be input to gates of the NMOS transistors, respectively.  
         [0016]     The second current source may supply the constant current in response to the positive quadrature-phase output signal and the negative quadrature-phase output signal.  
         [0017]     The second current source may include two NMOS transistors, and the positive quadrature-phase output signal and the negative quadrature-phase output signal may be input to gates of the NMOS transistors, respectively.  
         [0018]     The first current source may supply the constant current in response to the positive quadrature-phase output signal and the negative quadrature-phase output signal.  
         [0019]     The first current source may include two NMOS transistors, and the positive quadrature-phase output signal and the negative quadrature-phase output signal may be input to gates of the NMOS transistors, respectively.  
         [0020]     The second current source may supply the constant current in response to the positive in-phase output signal and the negative in-phase output signal.  
         [0021]     The second current source may include two NMOS transistors, and the positive in-phase output signal and the negative in-phase output signal may be input to gates of the NMOS transistors, respectively. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The above and/or other aspects of the present invention will be more apparent by describing certain exemplary embodiments of the present invention with reference to the accompanying drawings, in which:  
         [0023]      FIG. 1  is a schematic circuit diagram of a conventional quadrature voltage controlled oscillator;  
         [0024]      FIG. 2  is a schematic circuit diagram of a quadrature voltage controlled oscillator according to an exemplary embodiment of the present invention;  
         [0025]      FIG. 3  is a schematic circuit diagram of a quadrature voltage controlled oscillator according to another exemplary embodiment of the present invention;  
         [0026]      FIG. 4  is a schematic circuit diagram of a quadrature voltage controlled oscillator according to still another exemplary embodiment of the present invention;  
         [0027]      FIG. 5  is a schematic circuit diagram of a quadrature voltage controlled oscillator according to still another exemplary embodiment of the present invention;  
         [0028]      FIG. 6  is a comparative diagram of characteristic curves of phase noises in the conventional quadrature voltage controlled oscillator and in the quadrature voltage controlled oscillators of  FIGS. 4 and 5 ; and  
         [0029]      FIG. 7A  is a graph illustrating a waveform produced by the quadrature voltage controlled oscillator of  FIG. 4 ;  
         [0030]      FIG. 7B  is a graph illustrating a waveform produced by the quadrature voltage controlled oscillator of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0031]     Certain exemplary embodiments of the present invention will be described in greater detail with reference to the accompanying drawings.  
         [0032]     In the following description, same drawing reference numerals are used for the same elements even in different drawings. The matters defined in the following description, such as a detailed construction and elements, are provided to assist in a comprehensive understanding of the invention, and only refer to exemplary embodiments of the present invention. Thus, it is apparent that the present invention can be carried out without those defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
         [0033]      FIG. 2  is a schematic circuit diagram of a quadrature voltage controlled oscillator (VCO) according to an exemplary embodiment of the present invention.  
         [0034]     In  FIG. 2 , the quadrature VCO is composed of two symmetrical VCOs  200   a  and  200   b.    
         [0035]     The VCO  200   a  includes an inductance-capacitance (LC) circuit  203   a,  a negative resistance part  205   a,  a coupling circuit part  207   a  that couples between the two VCOs  200   a  and  200   b,  and a constant current source  209   a.    
         [0036]     Similarly, the other VCO  200   b  includes an LC circuit  203   b,  a negative resistance part  205   b,  a coupling circuit part  207   b  that couples between the two VCOs  200   a  and  200   b,  and a constant current source  209   b.    
         [0037]     In this exemplary embodiment, the VCO  200   a  generates two output signals, e.g., a positive in-phase output signal IP and a negative in-phase output signal IN, and the other VCO  200   b  outputs two output signals, i.e., a positive quadrature-phase output signal QP and a negative quadrature-phase output signal QN. The VCO  200   a  and the VCO  200   b  have a symmetrical structure and are interconnected to constitute a feedback loop.  
         [0038]     The LC circuit  203   a  of the VCO  200   a  is composed of inductors L 1  and L 2  and capacitors C 1  and C 2  for determining frequencies of the in-phase output signals IP and IN. Variation of inductance or capacitance of the LC circuit  203   a  may lead to a change in the frequency (i.e., resonance frequency) of the output signals. In this exemplary embodiment, the capacitors C 1  and C 2  are varactors having voltage-dependent capacitance.  
         [0039]     Alternatively, the resonance frequency may be changed by adjusting the inductance of the LC circuit  203   a.    
         [0040]     When the LC circuit  203   a  is ideal, a steady state voltage generated by the LC circuit  203   a  is perpetually oscillated. However, an actual LC circuit  203   a  has positive resistance (not shown) existing in the inductors L 1  and L 2  and the capacitors C 1  and C 2 . Accordingly, the LC circuit  203   a  has an output voltage exhibiting a response characteristic of an RLC circuit. That is, the output voltage is damping-oscillated.  
         [0041]     The negative resistance part  205   a  provides negative resistance that cancels out the positive resistance of the above-described LC circuit  203   a.  In the exemplary embodiment, the coupling circuit  207   a,  which will be described later, may also contribute to the provision of the negative resistance. The LC circuit  203   a,  the negative resistance part  205   a  and the coupling circuit  207   a  constitute an ideal LC circuit.  
         [0042]     In this exemplary embodiment, the negative resistance part  205   a  is composed of two N-channel Metal Oxide Semiconductor (NMOS) transistors M 1  and M 2  that are connected to each other. As shown, the negative in-phase output signal is input to the gate of the M 1  transistor and the positive in-phase output signal is input to the gate of the M 2  transistor.  
         [0043]     The coupling circuit  207   a  is composed of two NMOS transistors M 5  and M 6 , coupling between the two VCOs  200   a  and  200   b.  That is, the positive quadrature-phase output signal QP from the other VCO  200   b  is input to the gate of the M 5  transistor and the negative quadrature-phase output signal QN from the other VCO  200   b  is input to the gate of the M 6  transistor.  
         [0044]     In this exemplary embodiment, the constant current source  209   a  is composed of NMOS transistors M 9  and M 10  for supplying constant current to the VCO  200   a.  In the constant current source  209   a,  the positive in-phase output signal IP is input to the gate of the M 9  transistor and the negative in-phase output signal IN is input to the gate of the M 10  transistor. Accordingly, if the M 9  transistor is turned off, the M 10  transistor is turned on and if the M 10  transistor is turned off, the M 9  transistor is turned on, allowing constant current to be supplied. Periodically turning on and off the M 9  transistor and the M 10  transistor reduces the flicker noise, which in turn reduces the phase noise.  
         [0045]     The function of the other VCO  200   b  will be now described. The LC circuit  203   b,  the negative resistance part  205   b,  the coupling circuit  207   b  and the constant current source  209   b  have similar structures as those of the above-described VCO  200   a,  and accordingly only different portions from the VCO  200   a  will be described.  
         [0046]     In the VCO  200   b,  the positive in-phase signal IP is input to the gate of the M 7  transistor in the coupling circuit  207   b  and the negative in-phase signal IN is input to the gate of the M 8  transistor. The negative quadrature-phase signal QN is input to the gate of the M 11  transistor in the constant current source  209   b,  and the positive quadrature-phase signal QP is input to the gate of the M 12  transistor. The VCO  200   a  and the VCO  200   b  constitute a feedback structure.  
         [0047]     The constant current source,  209   b  may supply constant current to the VCO  200   b  in response to the quadrature-phase output signals, similarly to the constant current source  209   a.    
         [0048]      FIG. 3  is a schematic circuit diagram of a quadrature voltage controlled oscillator according to another exemplary embodiment of the present invention.  
         [0049]     The quadrature VCO of  FIG. 3  has the same structure as that of  FIG. 2  except for constant current sources  309   a  and  309   b.    
         [0050]     The constant current source  309   a  supplies constant current to the VCO  300   a  in response to the quadrature-phase output signals QP and QN, unlike the constant current source  209   a  of  FIG. 2 . The constant current source  309   b  supplies constant current to the VCO  300   b  in response to the in-phase output signals IP and IN, unlike the constant current source  209   b  of  FIG. 2 .  
         [0051]     With the configuration as shown in  FIG. 3 , the minimum point of a swing of the in-phase output voltage signals IP and IN and the minimum point of a voltage swing at a node e are aligned to each other, and the minimum point of a swing the quadrature-phase output voltage signals QP and QN and the minimum point of a voltage swing at a node f are aligned to each other. As a result, the output signals have a larger swing width than that in the exemplary embodiment of  FIG. 2 . Further, since the phase noise of the VCO is inversely proportional to the width of the output signal, it is further reduced.  
         [0052]      FIG. 4  is a schematic circuit diagram of a quadrature voltage controlled oscillator according to still another exemplary embodiment of the present invention.  
         [0053]     The quadrature VCO of  FIG. 4  has the same structure as that of the quadrature VCO of  FIG. 2  except for negative resistance parts  405   a  and  405   b.    
         [0054]     The negative resistance part  405   a  is composed of two PMOS transistors M 13  and M 14  (which forms an “N-channel” part) and two NMOS transistors M 1  and M 2  (which forms a “P-channel” part), unlike the negative resistance part  205   a  of  FIG. 2 . The negative resistance part  405   b  is composed of a P-channel part having two PMOS transistors M 15  and M 16  and an N-channel part having two NMOS transistors M 3  and M 4 .  
         [0055]     With a push-pull configuration having the N-channel and P-channel parts, it is possible to obtain a larger output voltage swing width using a lower power supply voltage, like the configuration shown in  FIG. 4 . That is, with respect to the same power supply voltage, the output voltages IP, IN, QP and QN of  FIG. 4  have a swing width greater than that of the output voltages IP, IN, QP and QN of  FIG. 2 .  
         [0056]      FIG. 5  is a schematic circuit diagram of a quadrature voltage controlled oscillator according to yet another exemplary embodiment of the present invention.  
         [0057]     The quadrature VCO of  FIG. 5  has the same structure as that of  FIG. 4  except for constant current sources  509   a  and  509   b.    
         [0058]     The constant current source  509   a  supplies constant current to the VCO  500   a  in response to the quadrature-phase output signals QP and QN, unlike the constant current source  409   a  of  FIG. 4 . Similarly, the constant current source  509   b  supplies constant current to the VCO  500   b  in response to the in-phase phase output signals IP and IN, unlike the constant current source  409   b  of  FIG. 4 .  
         [0059]     With the configuration as shown in  FIG. 5 , a minimum point of a swing of the in-phase output voltage signals IP and IN and a minimum point of a swing of a voltage at a node e are aligned to each other, and a minimum point of the swing of the quadrature-phase output voltage signals QP and QN and a minimum point of the swing of the voltage at the node f are aligned to each other. As a result, the swing width of output signals becomes greater than that in the exemplary embodiment of  FIG. 4 . Further, the quadrature VCO in  FIG. 5  has power consumption (for example, 1.8V, 4 mA) less than that of the quadrature VCO in  FIG. 4 .  
         [0060]      FIG. 6  is a comparative diagram of characteristic curves of phase noise of the conventional quadrature voltage controlled oscillator and the quadrature voltage controlled oscillators of  FIGS. 4 and 5 .  
         [0061]     In  FIG. 6 , line A indicates phase noise of the conventional VCO of  FIG. 1 , line B indicates phase noise of the VCO of  FIG. 4 , and line C indicates phase noise of the VCO of  FIG. 5 . The horizontal axis indicates an offset frequency representing a spacing from the central frequency, and the vertical axis indicates the size of power at the offset frequency. The power at the offset frequency has a value indicated in the case where the size of the power at the central frequency is referenced to 0 dB. Accordingly, the size of the phase noise becomes smaller as a value on the vertical axis is lowered.  
         [0062]     It can be seen from  FIG. 6  that the phase noise in the VCO of  FIG. 4  is 2 to 3 dB lower than that of the conventional VCO at the offset frequency of about 100 KHz, and the phase noise in the VCO of  FIG. 5  is 7 to 8 dB lower than the phase noise of the conventional VCO. Similarly, it can be seen that the phase noise is reduced even at offset frequencies of 1 KHz and 10 KHz which are close to the central frequency.  
         [0063]      FIG. 7A  is a graph illustrating a waveform produced by the quadrature voltage controlled oscillator of  FIG. 4 , and  FIG. 7B  is a graph illustrating a waveform produced by the quadrature voltage controlled oscillator of  FIG. 5 .  
         [0064]     In  FIG. 7A , a large swing waveform indicates the output voltages IP and IN from the VCO of  FIG. 4  and a small swing waveform indicates the voltage at the node e of the VCO of  FIG. 4 . In  FIG. 7B , a large swing waveform indicates the output voltages IP and IN of the VCO of  FIG. 5  and a small swing waveform indicates the voltage at the node e of the VCO of  FIG. 5 . In  FIGS. 7A and 7B , the vertical axis indicates a voltage and the horizontal axis indicates time.  
         [0065]     Referring to  FIGS. 7A and 7B , the VCO of  FIG. 5  has a even better alignment between the minimum points of the output voltages IP and IN and the minimum point of the voltage at the node e, compared to the VCO of  FIG. 4 . Accordingly, it can be seen that the quadrature VCO of  FIG. 5  has an excellent swing characteristic of the output voltages, compared to the quadrature VCO of  FIG. 4 .  
         [0066]     While the negative resistance part and the coupling circuit part are composed of MOS transistors in the foregoing exemplary embodiments, they may be composed of other active devices, such as BJT transistors. Any type of LC circuit may be used in place of the above-described LC circuit as long as it has the minimum impedance at resonant frequency. Further, resonant frequency may be adjusted by electrical, mechanical, optical or thermal means other than the above-described varactor.  
         [0067]     According to the present invention as described above, it is possible to provide in-phase signals and quadrature-phase signals having a low phase noise characteristic. It is also possible to provide in-phase signals and quadrature-phase signals having an excellent output swing characteristic.  
         [0068]     The foregoing exemplary embodiment and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.
A quadrature voltage controlled oscillator having low phase noise and excellent output swing characteristics includes a first voltage controlled oscillator for outputting a positive in-phase output signal and a negative in-phase output signal; a second voltage controlled oscillator for outputting a positive quadrature-phase output signal and a negative quadrature-phase output signal, the second voltage controlled oscillator having a symmetrical structure with the first voltage controlled oscillator and constituting a feedback loop together with the first voltage controlled oscillator; a first constant current source for supplying constant current to the first voltage controlled oscillator in response to the output signals; and a second constant current source for supplying constant current to the second voltage controlled oscillator in response to the output signals.