Abstract:
This invention provides a voltage-controlled oscillator, comprising a first voltage-controlled oscillator circuit and a second voltage-controlled oscillator circuit. The first voltage-controlled oscillator circuit comprises a plurality of inductors, a plurality of variable capacitors, and a plurality of MOS transistors. The circuit configuration of the second voltage-controlled oscillator circuit is symmetrical to that of the first voltage-controlled oscillator circuit. The inductors of the first voltage-controlled oscillator circuit are cross-coupled to the inductors of the second voltage-controlled oscillator circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a voltage-controlled oscillator, and more particularly, to a quadrature-phase voltage-controlled oscillator(VCO) used in the receivers and transmitters of wideband wired/wireless transmission systems/networks, products for ultra-wideband communication systems, or wireless network adapters. 
         [0003]    2. Description of the Prior Art 
         [0004]    In recent years, as the wireless communication technology has rapidly developed and the semiconductor technology has been improved, wireless communication products are facing higher standards. As such, RF circuits are required to operate at higher frequencies and larger amplitudes, with low voltages and low power consumptions.  FIG. 1  shows a conventional quadrature-phase VCO using series coupling transistors. The structure of such a quadrature-phase VCO has the following disadvantages:
       1. The VCO cannot operate at a low voltage.   2. The flicker noise from the coupling transistors may be up-converted to a frequency near the operation frequency.   3. The coupling transistors require larger sizes, which limits the operation frequency.   4. The VCO is of the source degeneration type, and the transconductance may decrease at high frequencies; thus, high frequency operation is rather limited.       
 
         [0009]    On the other hand, there are some different quadrature-phase VCOs using coupling coils to overcome the aforementioned disadvantages; however, no conventional quadrature-phase VCO using coupling coils can overcome both the disadvantage that the flicker noise from the coupling transistors may be up-converted to a frequency near the operation frequency and the disadvantage of source degeneration at a high frequency. 
         [0010]    In view of the above, the present invention provides a voltage-controlled oscillator operating at higher operation frequencies and larger amplitudes but with low voltages and low power consumptions. The present invention is thus industrially applicable. 
       SUMMARY OF THE INVENTION 
       [0011]    An object of the present invention is to provide a voltage-controlled oscillator operating at higher operation frequencies and larger amplitudes but with low voltages and low power consumptions. 
         [0012]    To achieve the aforementioned object, the present invention provides a voltage-controlled oscillator comprising a first voltage-controlled oscillator circuit and a second voltage-controlled oscillator circuit. The first voltage-controlled oscillator circuit comprises: a first inductor having a first end and a second end, the first end of the first inductor being coupled to a voltage source; a second inductor having a first end and a second end, the first end of the second inductor being coupled to the voltage source; a first variable capacitor having a first end and a second end, the first end of the first variable capacitor being coupled to a first control voltage, and the second end of the first variable capacitor being coupled to the second end of the first inductor; a second variable capacitor having a first end and a second end, the first end of the second variable capacitor being coupled to the first control voltage, and the second end of the second variable capacitor being coupled to the second end of the second inductor; a third inductor having a first end and a second end, the first end of the third inductor being coupled to the second end of the first inductor; a fourth inductor having a first end and a second end, the first end of the fourth inductor being coupled to the second end of the second inductor; a first NMOS transistor having a drain terminal coupled to the second end of the third inductor, a gate terminal coupled to the second end of the second inductor, and a source terminal being grounded; and a second NMOS transistor having a drain terminal coupled to the second end of the fourth inductor, a gate terminal coupled to the second end of the first inductor, and a source terminal being grounded. The second voltage-controlled oscillator circuit comprises: a fifth inductor having a first end and a second end, the first end of the fifth inductor being coupled to the voltage source; a sixth inductor having a first end and a second end, the first end of the sixth inductor being coupled to the voltage source; a third variable capacitor having a first end and a second end, the first end of the third variable capacitor being coupled to a second control voltage, and the second end of the third variable capacitor being coupled to the second end of the fifth inductor; a fourth variable capacitor having a first end and a second end, the first end of the fourth variable capacitor being coupled to the second control voltage, and the second end of the fourth variable capacitor being coupled to the second end of the sixth inductor; a seventh inductor having a first end and a second end, the first end of the seventh inductor being coupled to the second end of the fifth inductor; an eighth inductor having a first end and a second end, the first end of the eighth inductor being coupled to the second end of the sixth inductor; a third NMOS transistor, having a drain terminal coupled to the second end of the seventh inductor, a gate terminal coupled to the second end of the sixth inductor, and a source terminal being grounded; and a fourth MOS transistor, having a drain terminal coupled to the second end of the eighth inductor, a gate terminal coupled to the second end of the fifth inductor, and a source terminal being grounded. 
         [0013]    In the aforementioned voltage-controlled oscillator of the present invention, the first inductor of the first voltage-controlled oscillator circuit is cross-coupled to the seventh inductor of the second voltage-controlled oscillator circuit, the second inductor of the first voltage-controlled oscillator circuit is cross-coupled to the eighth inductor of the second voltage-controlled oscillator circuit, the fifth inductor of the second voltage-controlled oscillator circuit is cross-coupled to the fourth inductor of the first voltage-controlled oscillator circuit, and the sixth inductor of the second voltage-controlled oscillator circuit is cross-coupled to the third inductor of the first voltage-controlled oscillator circuit. 
         [0014]    By specific ways of connection of the aforementioned elements (i.e., inductors, capacitors, and transistors) and cross-couplings of the aforementioned inductors, the voltage-controlled oscillator of the present invention obtains the advantages that it can operate at higher operation frequencies and larger amplitudes but with low voltage and low power consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a conventional quadrature-phase VCO using series coupling transistors. 
           [0016]      FIG. 2  is a schematic view showing a voltage-controlled oscillator according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    The present invention discloses a voltage-controlled oscillator with a new configuration and now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. 
         [0018]    Referring to  FIG. 2 ,  FIG. 2  is a schematic view showing a voltage-controlled oscillator according to an embodiment of the present invention. A voltage-controlled oscillator  200  of the present invention comprises a first voltage-controlled oscillator circuit having a first output IP and a second output IM, and a second voltage-controlled oscillator circuit having a third output QP and a fourth output QM. The first voltage-controlled oscillator circuit and the second voltage-controlled oscillator circuit are coupled to a voltage source in parallel. The circuit configuration of the second voltage-controlled oscillator circuit is identical to that of the first voltage-controlled oscillator circuit, and the third output QP and the fourth output QM are arranged at the nodes corresponding to the nodes of the first output IP and the second output IM. In the embodiment of the present invention, the first voltage-controlled oscillator circuit comprises a first inductor  211 , a second inductor  212 , a third inductor  213 , a fourth inductor  214 , a first variable capacitor  221 , a second variable capacitor  222 , a first NMOS transistor  231 , and a second NMOS transistor  232 . The second voltage-controlled oscillator circuit comprises a fifth inductor  215 , a sixth inductor  216 , a seventh inductor  217 , an eighth inductor  218 , a third variable capacitor  223 , a fourth variable capacitor  224 , a third NMOS transistor  233 , and a fourth NMOS transistor  234 . The voltage-controlled oscillator  200  further comprises PMOS transistors  241 ,  242 ,  243  and a current source  251  to provide the current needed for the voltage-controlled oscillator  200  and limit its current consumption. 
         [0019]    Referring to the first voltage-controlled oscillator on the left side of  FIG. 2 , connections among the elements (i.e., inductors, capacitors, and transistors) of the first voltage-controlled oscillator will be described more fully hereinafter. The first inductor  211  has a first end and a second end. The first end of the first inductor  211  is coupled to a voltage source VDD, and the second end of the first inductor  211  is the first output IP. The second inductor  212  has a first end and a second end. The first end of the second inductor  212  is coupled to the voltage source VDD, and the second end of the second inductor  212  is the second output IM. The first variable capacitor  221  has a first end and a second end. The first end of the first variable capacitor  221  is coupled to a first control voltage V TUNE , and the second end of the first variable capacitor  221  is coupled to the second end of the first inductor  211 . The second variable capacitor  222  has a first end and a second end. The first end of the second variable capacitor  222  is coupled to the first control voltage V TUNE , and the second end of the second variable capacitor  222  is coupled to the second end of the second inductor  212 . The third inductor  213  has a first end and a second end. The first end of the third inductor  213  is coupled to the second end of the first inductor  211 . The fourth inductor  214  has a first end and a second end. The first end of the fourth inductor  214  is coupled to the second end of the second inductor  212 . The drain terminal of the first NMOS transistor  231  is coupled to the second end of the third inductor  213 . The gate terminal of the first NMOS transistor  231  is coupled to the second end of the second inductor  212 . The source terminal of the first NMOS transistor  231  is grounded or coupled to the negative terminal of the voltage source VSS. The drain terminal of the second 
         [0020]    NMOS transistor  232  is coupled to the second end of the fourth inductor  214 . The gate terminal of the second NMOS transistor  232  is coupled to the second end of the first inductor  211 . The source terminal of the second NMOS transistor  232  is grounded or coupled to the negative terminal of the voltage source VSS. In one embodiment of the present invention, the inductors  211 ,  212 ,  213 , and  214  may be coils, transmission lines, waveguides, or other elements that can function as inductors; the capacitors  221  and  222  may be diodes or other elements that can function as variable capacitors. In addition, the first inductor  211  and the second inductor  212  may form the primary side of a first transformer, and the third inductor  213  and the fourth inductor  214  may form the secondary side of the first transformer. 
         [0021]    Referring to the second voltage-controlled oscillator on the right side of  FIG. 2 , connections among the elements (i.e., inductors, capacitors, and transistors) of the second voltage-controlled oscillator will be described more fully hereinafter: The fifth inductor  215  has a first end and a second end. The first end of the fifth inductor  215  is coupled to a voltage source VDD, and the second end of the fifth inductor  215  is the third output QP. The sixth inductor  216  has a first end and a second end. The first end of the sixth inductor  216  is coupled to the voltage source VDD, and the second end of the sixth inductor  216  is the fourth output QM. The third variable capacitor  223  has a first end and a second end. The first end of the third variable capacitor  223  is coupled to a second control voltage V TUNE , and the second end of the third variable capacitor  223  is coupled to the second end of the fifth inductor  215 . The fourth variable capacitor  224  has a first end and a second end. The first end of the fourth variable capacitor  224  is coupled to the second control voltage V TUNE , and the second end of the fourth variable capacitor  224  is coupled to the second end of the sixth inductor  216 . The seventh inductor  217  has a first end and a second end. The first end of the seventh inductor  217  is coupled to the second end of the fifth inductor  215 . The eighth inductor  218  has a first end and a second end. The first end of the eighth inductor  218  is coupled to the second end of the sixth inductor  216 . The drain terminal of the third NMOS transistor  233  is coupled to the second end of the seventh inductor  217 . The gate terminal of the third NMOS transistor  233  is coupled to the second end of the sixth inductor  216 . The source terminal of the third NMOS transistor  233  is grounded or coupled to the negative terminal of the voltage source VSS. The drain terminal of the fourth NMOS transistor  234  is coupled to the second end of the eighth inductor  218 . The gate terminal of the fourth NMOS transistor  234  is coupled to the second end of the fifth inductor  215 . The source terminal of the fourth NMOS transistor  234  is grounded or coupled to the negative terminal of the voltage source VSS. In one embodiment of the present invention, the inductors  215 ,  216 ,  217 , and  218  may be coils, transmission lines, waveguides, or other elements that can function, as inductors; the capacitors  223  and  2 , 24  may be diodes or other elements that can function as variable capacitors. In addition, the fifth inductor  215  and the sixth inductor  216  may form the primary side of a second transformer, and the seventh inductor  217  and the eighth inductor  218  may form the secondary side of the second transformer. 
         [0022]    As shown in  FIG. 5 , the first inductor  211  of the first voltage-controlled oscillator circuit is cross-coupled to the seventh inductor  217  of the second voltage-controlled oscillator circuit; the second inductor  212  of the first voltage-controlled oscillator circuit is cross-coupled to the eighth inductor  218  of the second voltage-controlled oscillator circuit; the fifth inductor  215  of the second voltage-controlled oscillator circuit is cross-coupled to the fourth inductor  214  of the first voltage-controlled oscillator circuit; and the sixth inductor  216  of the second voltage-controlled oscillator circuit is cross-coupled to the third inductor  213  of the first voltage-controlled oscillator circuit. Through this configuration, the present invention allows the first voltage-controlled oscillator. circuit and the second voltage-controlled oscillator circuit to be cross-connected to each other so as to generate quadrature-phase oscillating signals at the nodes of the first output IP, the second output IM, the third output QP, and the fourth output QM. Moreover, there is no coupling transistor used in the present invention, and hence, the disadvantage that the flicker noise from the coupling transistors may be up-converted to a frequency near the operation frequency does not exist. The source terminals of the NMOS transistors  231 ,  232 ,  233 , and  234  are grounded or coupled to the negative terminal of the voltage source VSS, and hence, the disadvantage of source degeneration at a high frequency does not exist. Accordingly, by specific ways of connection of the aforementioned elements (i.e., inductors, capacitors, and transistors) and cross-couplings of the aforementioned inductors, the voltage-controlled oscillator of the present invention obtains the advantages that it can operate at higher operation frequencies and larger amplitudes but with low voltage and low power consumption. 
         [0023]    While this invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that this invention is not limited hereto. For example, it will be apparent to those skilled in the art that the aforementioned NMOS transistors may be replaced by the PMOS transistors. Similarly, it will be apparent to those skilled in the art that the aforementioned PMOS transistors may be replaced by the NMOS transistors. It is to be understood that various changes and modifications can be made by those skilled in the art without departing from the spirit and scope of this invention. It is intended that the scope of the invention be defined by the claims appended hereto.