Abstract:
Provided are a linearized variable-capacitance module for a voltage-controlled oscillator (VCO) and an LC resonance circuit using the same. The VCO is a circuit for outputting a certain frequency in response to an input control signal (voltage or current). The VCO includes an inductor, a variable capacitor (or a varactor), and an active device for compensating for loss of energy caused by the inductor and varactor. The frequency of the VCO is varied by changing inductance or capacitance. In general, the VCO includes a variable-capacitance device (i.e., the varactor) so that the frequency of the VCO may be varies by changing the capacitance via a control voltage. In most cases, the frequency of the VCO varies nonlinearly with respect to the control voltage. The nonlinear variation in the frequency of the VCO results in a great variation in a VCO gain within a certain control voltage range. When a phase locked loop (PLL) includes the VCO, the variation in the VCO gain leads to a variation in the entire loop gain, thus causing a variation in output phase noise. To solve this problem, a varactor designed to have a capacitance that varies linearly with a control voltage is provided so that a VCO gain can be held constant. The variable-capacitance module includes a plurality of variable-capacitance devices with respectively different linear variation regions on an application voltage axis. Also, the variable-capacitance devices are coupled in common and receive a control voltage at one end while each receiving a different fixed voltage at the other end.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 2006-0066409, filed Jul. 14, 2006, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a variable capacitor (hereinafter, varactor) applicable to a voltage-controlled oscillator (VCO), which generates a signal at a frequency that varies linearly with a control voltage. 
         [0004]    2. Discussion of Related Art 
         [0005]      FIG. 1  is a block diagram of a conventional voltage-controlled oscillator (VCO). The VCO is a circuit for generating an output signal at a specific frequency in response to a control signal. The VCO includes an inductor-capacitor (LC) resonance circuit, which is comprised of an inductor and a capacitor, and an active device for compensating for undesirable energy loss caused by the LC resonance circuit. In the LC resonance circuit, the frequency is varied by changing inductance (L) or, in most cases, capacitance (C). 
         [0006]      FIG. 2  is a graph showing characteristics of a conventional varactor. Specifically,  FIG. 2  is a graph of capacitance versus control voltage in the conventional varactor. As can be seen from  FIG. 2 , the capacitance of the varactor varies nonlinearly with respect to the control voltage. Thus, when this conventional varactor is applied to an oscillator, the gain of the oscillator, which is defined as change in frequency per change in control voltage (i.e., K VCO =Δf VCO /ΔV), varies greatly over the entire control voltage range. 
         [0007]    The VCO is located in a negative loop of a phase locked loop (PLL) in order to output a signal at a precise frequency. In this case, variation in the gain of the VCO leads to variation in the characteristic of the entire negative loop. That is, output phase noise is changed by varying the gain of the entire negative loop.  FIG. 3  is a circuit diagram of a single-ended LC resonance circuit of a conventional resonator, and  FIG. 4  is a circuit diagram of a differential-ended LC resonance circuit of a conventional resonator. As can be seen from  FIGS. 3 and 4 , one node of a varactor is coupled to an oscillation node, and the other node of the varactor is coupled to a control voltage for varying capacitance. In this case, since the capacitance varies nonlinearly with respect to control voltage as described above, it is impossible to ensure precise control of oscillation frequency. 
         [0008]    In order to solve this problem, a VCO may include a plurality of varactors for different control voltage ranges that can be switched between according to the control voltage. However, in this case, the VCO may suffer from disturbance due to switching operations and needs complicated control circuits for the switching operations. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to a variable-capacitance module having a linear frequency variance characteristic, and an LC resonance circuit using the same. 
         [0010]    The present invention is also directed to a variable-capacitance module capable of outputting a linear frequency variance characteristic without switching a varactor, and an LC resonance circuit using the same. 
         [0011]    One aspect of the present invention provides a variable-capacitance module including a plurality of variable-capacitance devices having different linear variation regions on a voltage axis. Herein, the variable-capacitance devices are coupled in common and receive a control voltage at one end while each receiving a different fixed voltage at the other end. 
         [0012]    Another aspect of the present invention provides a single-ended LC resonance circuit including an inductor providing a resonance inductance; and a variable-capacitance module having one end coupled to one end of the inductor and the other end coupled to the other end of the inductor. Herein, the variable-capacitance module includes a plurality of variable-capacitance devices coupled in common and receiving a control voltage at one end while each receiving a different fixed voltage at the other end, respectively. 
         [0013]    Yet another aspect of the present invention provides a differential-ended LC resonance circuit including an inductor providing a resonance inductance; a first variable-capacitance module having one end coupled to one end of the inductor; and a second variable-capacitance module having one end coupled to the other end of the inductor and the other end coupled to the other end of the first variable-capacitance module and to which a control voltage is applied. Herein, each of the first and second variable-capacitance modules includes a plurality of variable-capacitance devices respectively coupled in common and receiving a control voltage at one end while each receiving a different fixed voltage at the other end. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0015]      FIG. 1  is a block diagram of a conventional voltage-controlled oscillator (VCO); 
           [0016]      FIG. 2  is a graph showing characteristics of a conventional varactor; 
           [0017]      FIG. 3  is a circuit diagram of a single-ended LC resonance circuit of a conventional resonator; 
           [0018]      FIG. 4  is a circuit diagram of a differential-ended LC resonance circuit of a conventional resonator; 
           [0019]      FIG. 5  is a conceptual diagram of a linearized variable-capacitance module including n varactors according to an exemplary embodiment of the present invention; 
           [0020]      FIG. 6  is a graph showing characteristics of the linearized variable-capacitance module of  FIG. 5 ; 
           [0021]      FIG. 7  is a conceptual diagram of a linearized variable-capacitance module including 3 varactors according to another exemplary embodiment of the present invention; 
           [0022]      FIG. 8  is a graph showing characteristics of the linearized variable-capacitance module of  FIG. 7 ; 
           [0023]      FIG. 9  is a circuit diagram of a single-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 7 ; 
           [0024]      FIG. 10  is a circuit diagram of a differential-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 7 ; 
           [0025]      FIG. 11  is a conceptual diagram of a linearized variable-capacitance module including n varactors and n switched capacitor blocks according to yet another exemplary embodiment of the present invention; 
           [0026]      FIG. 12  is a graph showing characteristics of the linearized variable-capacitance module of  FIG. 11 ; 
           [0027]      FIG. 13  is a conceptual diagram of a linearized variable-capacitance module including 3 varactors and 3 switched capacitor blocks according to yet another exemplary embodiment of the present invention; 
           [0028]      FIG. 14  is a graph showing characteristics of the linearized variable-capacitance module of  FIG. 13 ; 
           [0029]      FIG. 15  is a circuit diagram of a single-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 13 ; 
           [0030]      FIG. 16  is a circuit diagram of a differential-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 13 ; 
           [0031]      FIG. 17  is a conceptual diagram of a linearized variable-capacitance module including n switched varactors according to yet another exemplary embodiment of the present invention; and 
           [0032]      FIG. 18  is a circuit diagram of a differential-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 17 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0033]    Hereinafter, exemplary embodiments of the present invention will be described in detail. However, the present invention is not limited to the exemplary embodiments disclosed below and can be implemented in various forms. Therefore, the present exemplary embodiments are provided for complete disclosure of the present invention and to fully convey the scope of the present invention to those of ordinary skill in the art. 
         [0034]      FIG. 5  is a conceptual diagram of a linearized variable-capacitance module including n varactors according to an exemplary embodiment of the present invention, and  FIG. 6  is a graph of frequency versus control voltage in a voltage-controlled oscillator (VCO) using the linearized variable-capacitance module of  FIG. 5 . 
         [0035]    As can be seen from the lowermost graph of  FIG. 6 , one varactor is characterized such that the maximum capacitance Var-n and the minimum capacitance Var- 1  are sufficiently within the entire variation range of control voltage. In this case, as shown in  FIG. 5 , a variable-capacitance module includes a plurality of varactors coupled in common and receiving a control voltage at one end while each receiving a different fixed voltage V- 1  to V-n at the other end, in order that the entire variable-capacitance module may have a linear frequency-control voltage characteristic. 
         [0036]    The fixed voltages V- 1  to V-n allow the respective variation central points of varactors  421 ,  422 , . . . , and  42 N to shift to a specific voltage point with respect to the control voltage, so that the respective capacitances Var- 1 , Var- 2 , . . . , and Var-n of the varactors  421 ,  422 , . . . , and  42 N are aligned within the control voltage range, as can be seen from the lowermost graph of  FIG. 6 . In other words, once the capacitance Var- 1  of the leftmost first varactor  421  having one side to which a voltage V- 1  is applied varies due to the control voltage and then reaches the maximum value, the capacitance Var- 2  of the second varactor  422  having one side to which a voltage V- 2  is applied subsequently varies due to the control voltage. In this case, the voltages V- 1  and V- 2  are determined such that the capacitances of two varactors  421  and  422  with similar characteristics vary linearly due to the control voltage. 
         [0037]    Therefore, the entire capacitance of the variable-capacitance module, which is a result obtained by adding all the variable capacitances Var- 1 , Var- 2 , . . . , and Var-n of the varactors  421 ,  422 , . . . , and  42 N, may have a linear variation as shown in the intermediate graph of  FIG. 6  within the control voltage range shown in the uppermost graph of  FIG. 6 . In this case, when designing a real VCO, the respective fixed voltages V- 1  to V-n should be isolated and applied using isolation capacitors such that an alternating current (AC) signal can swing at a node. 
         [0038]    As a result, a gain of the VCO as shown in the uppermost graph of  FIG. 6  approximates a specific constant. In the configuration of  FIG. 5 , the fixed voltages V- 1  to V-n, which cause a voltage offset among the varactors  421 ,  422 , . . . , and  42 N, are arbitrarily selected such that the overall capacitance varies linearly, and the capacitances Var- 1 , Var- 2 , . . . , and Var-n of the varactors  421 ,  422 , . . . , and  42 N also should be selected such that the overall capacitance varies linearly. To this end, all the varactors  421 ,  422 , . . . , and  42 N may be within the same capacitance range or within differential capacitance ranges. 
         [0039]      FIG. 7  is a conceptual diagram of a linearized variable-capacitance module including 3 varactors according to another exemplary embodiment of the present invention, and  FIG. 8  is a graph showing characteristics of the linearized variable-capacitance module of  FIG. 7 . The variable-capacitance module of  FIG. 7  is the same as that of  FIG. 5  except that the variable-capacitance module includes 3 varactors  421 ,  422 , and  423 . When a phase locked loop (PLL) is designed and the entire block is designed using a single power supply, a real variable-capacitance module has a variation in power supply voltage similar to the varactor  422  corresponding to an intermediate region among the three varactors  421 ,  422 , and  423  shown in  FIG. 8 . In an integrated circuit (IC), a junction varactor or a MOS varactor may be typically used as each of the varactors  421 ,  422 , and  423 . As described above, the variable-capacitance module has the construction shown in  FIG. 7  so that a VCO can output a signal at a frequency that varies linearly within the entire control voltage range. The entire control voltage range shown in the uppermost graph of  FIG. 8  covers at least all control voltage regions of the respective varactors  421 ,  422 , and  423 . 
         [0040]    Like in the previous exemplary embodiment, the entire variable-capacitance module is comprised of three varactors  421 ,  422 , and  423  of which one end is commonly coupled to a control voltage and of which the other end is coupled to fixed voltages V-h, V-m, and V- 1 , respectively, to have a voltage offset, so that the VCO shows a linear frequency variation in the entire control voltage range as shown in  FIG. 8 . In the present exemplary embodiment, since variable-capacitance devices are varactors, each end to which a control voltage is applied corresponds to anodes of the varactors  421 ,  422 , and  423 , while each end to which the respective fixed voltages V-h, V-m, and V- 1  are applied corresponds to cathodes thereof. As a result, a constant VCO gain characteristic can be obtained as shown in  FIG. 8 . 
         [0041]      FIGS. 9 and 10  show examples of an LC resonance circuits. Specifically,  FIG. 9  is a circuit diagram of a single-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 7 , while  FIG. 10  is a circuit diagram of a differential-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 7 . In  FIGS. 9 and 10 , each LC resonance circuit includes an inductor, the variable-capacitance module shown in  FIG. 7 , and a direct current (DC)-blocking coupling capacitor, and the variable-capacitance module includes 3 varactors  421 ,  422 , and  423  in order to obtain a linear frequency variation with respect to a control voltage. 
         [0042]    As can be seen from  FIGS. 9 and 10 , cathodes of the varactors  421 ,  422 , and  423  are isolated from one another and anodes thereof are coupled to one another. In this state, a control voltage is applied to the anodes of the varactors  421 ,  422 , and  423 , while respective fixed voltages are applied to the cathodes thereof. 
         [0043]    In order to prevent application of the control voltage to an LC oscillation path, first coupling capacitors  490  are located between the anodes of the varactors  421 ,  422 , and  423  and one end of the inductor  410 , and second coupling capacitors  461 ,  462 , and  463  are located between the cathodes of the varactors  421 ,  422 , and  423  and the other end of the inductor  410 , respectively. Since the anodes of the varactors  421 ,  422 , and  423  are coupled to one another, the first coupling capacitors  490  may be embodied by one capacitor. However, since the cathodes of the varactors  421 ,  422 , and  423  are isolated from one another, the second coupling capacitors  461 ,  462 , and  463  should be embodied by three capacitors as shown in  FIG. 9 . 
         [0044]    Meanwhile, in order to prevent an oscillated AC signal from passing through a line for applying the fixed voltage, AC-blocking resistors  441 ,  442 , and  443  may be located on lines for applying the fixed voltages, respectively, as shown in  FIG. 9 . The AC-blocking resistors  441 ,  442 , and  443  may be replaced by other devices, such as inductors, which allow application of DC signals and block application of AC signals. Although not shown in the drawings, an AC-blocking resistor or inductor may be further located on a line for applying the control voltage in order to prevent the oscillated AC signal from passing through the line for applying the control voltage. 
         [0045]      FIG. 11  is a conceptual diagram of a linearized variable-capacitance module including n varactors and n switched capacitor blocks according to yet another exemplary embodiment of the present invention, and  FIG. 12  is a graph showing characteristics of the linearized variable-capacitance module of  FIG. 11 . Specifically, switched capacitor tuning is applied to the variable-capacitance module of  FIG. 11  so that a frequency can vary within a larger range. Unlike conventional switched capacitor tuning, DC coupling capacitors located between varactors and oscillation nodes are embodied by switched capacitor blocks  661 ,  662 , . . . , and  66 N. 
         [0046]    A variation in capacitance caused by use of a switched capacitor block, which results in a great variation in an oscillation frequency range, is referred to as “switch tuning,” and a frequency range defined by the switch tuning is referred to as a “frequency band.” In other words, a frequency band is changed due to the switch tuning of the switched capacitor block. 
         [0047]    When a frequency becomes low due to the switching of the switched capacitor block, the variation range of a variable-capacitance device due to an analog voltage should increase more so that the same VCO gain can be obtained even at a low frequency. Therefore, when the switch tuning is embodied using the coupling capacitor block as shown in  FIG. 11 , the switched capacitor blocks  661 ,  662 , . . . , and  66 N and varactors  621 ,  622 , . . . , and  62 N are coupled in series, the entire variation range due to the varactors  621 ,  622 , . . . , and  62 N is automatically changed. Specifically, when the capacitance of the switched capacitor blocks  661 ,  662 , . . . , and  66 N is great, the entire variation range due to the varactors  621 ,  622 , . . . , and  62 N increases; on the other hand, when the capacitance of the switched capacitor blocks  661 ,  662 , . . . , and  66 N is small, the entire variation range due to the varactors  621 ,  622 , . . . , and  62 N decreases. As a result, the construction shown in  FIG. 11  enables switched frequency tuning while reducing a variation in the VCO gain. 
         [0048]      FIG. 13  is a conceptual diagram of a linearized variable-capacitance module including 3 varactors and 3 switched capacitor blocks according to yet another exemplary embodiment of the present invention, and  FIG. 14  is a graph showing characteristics of the linearized variable-capacitance module of  FIG. 13 . In  FIG. 13 , three varactors  621 ,  622 , and  623  are coupled to switched capacitor blocks  661 ,  662 , and  663 , respectively. The variable-capacitance module shown in  FIG. 13  is a simple type with high feasibility. Since the variable-capacitance module shown in  FIG. 13  is analogous to the variable-capacitance module described with reference to  FIGS. 11 and 12 , a detailed description thereof will be omitted here. 
         [0049]      FIG. 15  is a circuit diagram of a single-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 13 , and  FIG. 16  is a circuit diagram of a differential-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 13 . 
         [0050]    The variable-capacitance module shown in  FIG. 15  is the same as that of  FIG. 9  except that the second coupling capacitors  461 ,  462 , and  463  are replaced by switched capacitor blocks  661 ,  662 , and  663 , respectively. Although not shown in the drawings, the variable-capacitance module shown in  FIG. 15  may be constructed by replacing the first coupling capacitor  490  of  FIG. 9  by a switched capacitor block. In the latter case, since the variable-capacitance module may include only one switched capacitor block, the fabrication cost and an occupied area can be lessened, whereas a switching operation on one end of a varactor to which a control voltage is applied may deteriorate the stability of an oscillation operation of a VCO. Therefore, in the latter case, the variable-capacitance module should include three switched capacitor blocks considering the stability of the oscillation operation of the VCO. 
         [0051]    The variable-capacitance module shown in  FIG. 16  is the same as that of  FIG. 10  except that first coupling capacitors  571 ,  572 , and  573  are replaced by first switched capacitor blocks  771 ,  772 , and  773 , respectively, and second coupling capacitors  561 ,  562 , and  563  are replaced by second switched capacitor blocks  761 ,  762 , and  763 , respectively. 
         [0052]      FIG. 17  is a conceptual diagram of a linearized variable-capacitance module including n switched varactors according to yet another exemplary embodiment of the present invention. Specifically, another method for switched capacitor tuning is applied so that a frequency can vary within a larger range. In the present exemplary embodiment, a variable-capacitance device is embodied by a switched variable-capacitance block in order to control the switching of the variable-capacitance device. On switching the variable-capacitance device, when a frequency becomes high and low, a variation in frequency band due to switched tuning and a variation in variable capacitance due to a control voltage are caused by the switched variable-capacitance block, and thus a VCO gain is kept constant. 
         [0053]      FIG. 18  is a circuit diagram of a differential-ended LC resonance circuit of a resonator using the linearized variable-capacitance module of  FIG. 17 . Although only the differential-ended LC resonance circuit is illustrated, it would be apparent that the variable-capacitance module of  FIG. 17  may be applied likewise to a single-ended LC resonance circuit. Since both the differential-ended and single-ended LC resonance circuits are analogous to circuits explained above, a detailed description thereof will be omitted here. 
         [0054]    A variable-capacitance module according to the present invention is characterized by a linear frequency variation in a control voltage range for a variation in the frequency of a VCO, unlike conventional designs for varactors, so that a constant VCO gain can be obtained. 
         [0055]    Also, a conventional varactor leads a VCO gain to vary within a large range. When the variable-capacitance module according to the present invention is designed to have the same gain as the maximum gain of a VCO using the conventional varactor, the variable-capacitance module according to the present invention can have an even greater frequency variation range than the conventional varactor. 
         [0056]    Further, when the variable-capacitance module according to the present invention is designed to have the same gain as the average gain of a VCO, the variable-capacitance module according to the present invention can obtain a constantly low gain in the entire range while having a frequency variation range similar to that of a conventional varactor. A VCO with a relatively low gain is advantageous in lowering output phase noise of a PLL. 
         [0057]    Most importantly, a constant VCO gain can be achieved within the entire control voltage range. A conventional varactor can neither increase a frequency variation range because of a large variation in VCO gain nor obtain a constant VCO characteristic owing to a great change in output phase noise. In contrast, the variable-capacitance module according to the present invention can obtain a constant VCO gain within the entire control voltage range so that a frequency variation range can be increased and noise can be reduced. 
         [0058]    Considering that a VCO is an essential block for a PLL, which is broadly used in various circuits, such as data recoveries, clock recoveries, RF receivers, RF transmitters, and frequency synthesizers, it is important that the present invention should make a variation in VCO gain, which is regarded as a serious drawback to the VCO, constant. Therefore, by applying the present invention to the above-described circuits, the performance of the circuits can be improved clearly and simply, thus resulting in great marketability and economical efficiency. 
         [0059]    While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.