Abstract:
In a wideband variable frequency voltage controlled oscillator, an LC resonance circuit determines an oscillation frequency based on an externally inputted voltage signal. A negative resistance generating circuit generates a signal having the oscillation frequency determined by the LC resonance circuit. A buffer circuit transfers the oscillation frequency generated by the negative resistance generating circuit to a load. A variable capacitor connecting circuit connects the negative resistance generating circuit and the buffer circuit to transfer the signal having the oscillation frequency from the negative resistance generating circuit to the buffer circuit.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a wideband variable frequency voltage controlled oscillator for a mobile communication system; and, more particularly, to the wideband variable frequency voltage controlled oscillator of the mobile communication system, which is able to achieve a high integration thereof by adopting a variable capacitor connecting circuit.  
       BACKGROUND OF THE INVENTION  
       [0002]     A conventional voltage controlled oscillator is one of the important devices that have been widely used in a communication system. Especially, an LC voltage controlled oscillator is an indispensable device in a recent wireless mobile communication because of its original characteristics of low noise.  FIG. 1  shows a circuit of the conventional LC differential voltage controlled oscillator.  
         [0003]     In a recent wireless mobile communication field, especially, in a next generation of a second generation thereof, a voltage controlled oscillator that is usable in a wide bandwidth and a multiple bandwidth is required. A conventional method for expanding a bandwidth of a voltage controlled oscillator includes a method using a Yttrium Iron Garnet(YIG)-tuned oscillator, a method using a switching device including multiple inductors or capacitors, a method using an off-chip variable capacitor that operates with a high voltage applied thereto, and the like.  
         [0004]     However, the YIG-tuned oscillator capable of being used in a wide bandwidth requires a high voltage and is not appropriate for a wireless mobile communication terminal due to a considerably large size of a circuit.  
         [0005]     Further, the method using a switching device including multiple inductors or capacitors has a problem in that multiple inductors are inserted in one chip or multiple input signals are needed to control the switching device. And, in case of the method using an off-chip variable capacitor, a high voltage is required and, particularly, a high integration thereof is difficult to achieve.  
       SUMMARY OF THE INVENTION  
       [0006]     It is, therefore, an object of the present invention to provide a wideband variable frequency voltage controlled oscillator for a mobile communication system, which is able to achieve a high integration thereof by adopting a simple capacitor connecting circuit.  
         [0007]     In accordance with a preferred embodiment of the present invention, there is provided a wideband variable frequency voltage controlled oscillator comprising: an LC resonance circuit for determining an oscillation frequency based on an externally inputted voltage signal; a negative resistance generating circuit for generating a signal having an oscillation frequency determined by the LC resonance circuit; a buffer circuit for transferring the signal having the oscillation frequency generated by the negative resistance generating circuit to a load; and a variable capacitor connecting circuit for connecting the negative resistance generating circuit and the buffer circuit to transfer the signal having the oscillation frequency from the negative resistance generating circuit to the buffer circuit.  
         [0008]     In accordance with another preferred embodiment of the present invention, there is provided a Colpitts wideband variable frequency voltage controlled oscillator comprising: an LC resonance circuit for determining an oscillation frequency based on an externally inputted voltage signal; a negative resistance generating circuit for generating a signal having the oscillation frequency determined by the LC resonance circuit; and a variable capacitor connecting circuit for connecting the LC resonance circuit and the negative resistance generating circuit.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments, given in conjunction with the accompanying drawings, in which:  
         [0010]      FIG. 1  shows a circuit diagram of a conventional LC differential voltage controlled oscillator;  
         [0011]      FIG. 2  illustrates a circuit diagram of an LC differential voltage controlled oscillator in accordance with a preferred embodiment of the present invention;  
         [0012]      FIG. 3  depicts an exemplary circuit diagram of a variable capacitor in accordance with a preferred embodiment of the present invention;  
         [0013]      FIG. 4  describes a circuit diagram of a small signal-equivalent model of the voltage controlled oscillator of  FIG. 2 ;  
         [0014]      FIG. 5  provides a circuit diagram of a small signal-equivalent model of a negative resistance generating circuit of  FIG. 4 ;  
         [0015]      FIG. 6  presents an exemplary graph for showing a variation of a capacitance of a variable capacitor in accordance with a preferred embodiment of the present invention;  
         [0016]      FIG. 7  represents a circuit diagram of a conventional Colpitts voltage controlled oscillator; and  
         [0017]      FIG. 8  offers a circuit diagram of a Colpitts voltage controlled oscillator in accordance with another preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0019]      FIG. 2  illustrates a circuit diagram of an LC differential voltage controlled oscillator in accordance with a preferred embodiment of the present invention. As can be seen from  FIG. 2 , a circuit of the present invention is composed of an LC-resonance circuit  201 , a negative resistance generating circuit  202 , a buffer circuit  204 , and a variable capacitor connecting circuity  203  for connecting the negative resistance generating circuit  202  and the buffer circuit unit  204 .  
         [0020]     Especially, as can be understood by comparing of  FIGS. 1 and 2 , the variable capacitor connecting circuit  203  is connected to a portion where a conventional capacitor connecting circuit  103  is connected in  FIG. 1 , in order to expand a frequency bandwidth. In other words, by using a simple variable capacitor circuit, e.g., the variable capacitor connecting circuit  203 , it is possible to considerably expand the variable frequency bandwidth of the LC differential voltage controlled oscillator.  
         [0021]      FIG. 3  depicts a specific circuit diagram of the variable capacitor connecting circuit. As illustrated in  FIG. 3 , the variable capacitor circuit  203  implements a variable capacitance by applying a reverse voltage to a P-N junction between a base and a collector of a transistor TR 301  or between the base and an emitter thereof, to thereby increase an integration ratio in one chip. The P-N junction may be realized by a bipolar junction transistor or an Schottky P-N junction.  
         [0022]     A method for analyzing the LC voltage controlled oscillator of  FIG. 2  in accordance with the present invention includes a feedback method and a negative resistance method. Hereinafter, an analysis using the negative resistance method will be described.  
         [0023]      FIG. 4  describes a small signal equivalent model of the LC voltage controlled oscillator illustrated in  FIG. 2  in accordance with the present invention. An operating principal and method of the LC voltage controlled oscillator will be described with reference to  FIG. 4 . First, an oscillation frequency is as follow:  
               oscillation   ⁢           ⁢     frequency   ⁡     (   ω   )         =     1     LC               Eq   .           ⁢   1               
         [0024]     As illustrated in  FIGS. 2 and 4 , the negative resistance generating circuit  202  generates a negative resistance −R R 401  and the resonance circuit  201  determines a resistance +R R 402  corresponding to the negative resistance.  
         [0025]      FIG. 5  depicts a small signal equivalent model of a variable capacitor circuit  401  of  FIG. 4 , for determining frequency characteristics.  
         [0026]     At this time, each value of variables of the equivalent model in  FIG. 5  is obtained as follows:  
               X   s     =     1     ω   ⁡     (     C402   //   C403     )                 Eq   .           ⁢   2                 X   p     =         RL401   2     +     X   s   2         X   s               Eq   .           ⁢   3                 R   p     =         RL401   2     +     X   s   2       RL401             Eq   .           ⁢   4             
 
         [0027]     Herein, a capacitance of the capacitor provides a certain amount of capacitance in the voltage controlled oscillator in accordance with the present invention and the amount of the capacitance is obtained as follow:  
               C   p     =     1     ω   ·     X   p                 Eq   .           ⁢   5             
 
         [0028]     Therefore, the oscillation frequency is determined depending on variation limits of variable capacitors CV 201  and CV 202  and C p  shown in Eq. 5.  
         [0029]     More specifically, though the oscillator in accordance with the present invention uses variable capacitors CV 203  and CV 204 , the variation limits of the variable capacitors CV 201  and CV 202  in the resonance circuit  201  are not changed. However, when the oscillator generates a maximum frequency, the variable capacitors CV 201  and CV 202  have a smallest capacitance and the variable capacitors CV 203  and CV 204  of a connecting circuit also have a smallest capacitance. On the other hand, when the oscillator generates a minimum frequency, the variable capacitors CV 201  and CV 202  have a maximum capacitance. And also, the variable capacitors CV 203  and CV 204  of the connecting circuit have a maximum capacitance. From a point of view of an overall circuit, the oscillator of the present invention has a greater capacitance variation range than a general capacitor connecting circuit has, so that an oscillation frequency bandwidth can be expanded.  
         [0030]     The variable capacitor  203  of the present invention utilizes a capacitance between a base and a collector of a transistor, as shown in  FIG. 3 . In this case, a variation of the capacitance in accordance with a reverse voltage applied to a P-N junction is described in  FIG. 6 . As shown in  FIG. 6 , the capacitance decreases as the reverse voltage applied to the P-N junction increases. Further, the variable capacitors CV 201 , CV 202 , CV 203 , and CV 204  illustrated in the circuit of  FIG. 2  react to the reverse voltage applied to the P-N junction.  
         [0031]      FIG. 8  depicts a voltage controlled oscillation circuit in accordance with a preferred embodiment of the present invention, which corresponds to a conventional Colpitts voltage controlled oscillator illustrated in  FIG. 7 . Unlike a general capacitor connecting circuit of  FIG. 7 , a capacitor connecting circuit  802  of  FIG. 8  is connected to a variable capacitor CV 803 . Therefore, as in the voltage controlled oscillator of  FIG. 2  in accordance with another preferred embodiment of the present invention, the capacitor connecting circuit  802  provides the greater capacitance variation range than the general capacitor connecting circuit, thereby expanding a bandwidth of the oscillation frequency. At this time, a circuit of a Colpitts voltage controlled oscillator of  FIG. 8  can be equally analyzed as in that of the LC-differential voltage controlled oscillator of  FIG. 2 .  
         [0032]     While a conventional voltage controlled oscillator has difficulties in achieving miniaturization, integration, and minimization thereof, as described above, the voltage controlled oscillator in accordance with the present invention can be used for implementing a microwave monolithic integrated circuit (MMIC) by employing a simple variable capacitor circuit for expanding a frequency bandwidth of a voltage controlled oscillator without adding additional input signals or changing a size of a circuit.  
         [0033]     While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.