Abstract:
A voltage controlled oscillator with a microstrip line suitable for coarse adjustment of an oscillation frequency band, the voltage controlled oscillator includes a transistor for oscillation that outputs an oscillation signal; a microstrip line formed on a substrate together with the transistor for oscillation, having one end connected to the transistor for oscillation and the other end connected to a ground electrode formed on the substrate, and thus constituting a part of a frequency determining element that determines a frequency of the oscillation signal according to a line length from the one end of the microstrip line to the ground electrode; and a conductor for coarse adjustment that connects between the one end and the other end of the microstrip line to the ground electrode, thereby reducing the line length of the microstrip line in the connection state.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present invention contains subject matter related to and claims priority to Japanese Patent Application JP 2009-172988 filed in the Japanese Patent Office on Jul. 24, 2009, the entire content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to an oscillator with a microstrip line suitable for coarse adjustment of an oscillation frequency band. 
         [0004]    2. Related Art 
         [0005]    In regard to such oscillators, there is known a technology of performing coarse adjustment of an oscillation frequency band by partially trimming a microstrip line formed on the upper surface of a substrate (for example, refer to the third page and FIGS. 2 and 3 of Japanese Unexamined Patent Application Publication No. 2001-94346). According to the existing technology, first and second resonant circuits are formed on the upper surface of the substrate while being connected in parallel to each other, and have first and second microstrip lines, respectively. The first and second microstrip lines are arranged in parallel to each other on the upper surface of the substrate, and one end of the first microstrip line is connected to one end of the second microstrip line through a third microstrip line. Further, the substrate is formed with a plurality of through holes at positions thereof corresponding to the third microstrip line, and ground conductors located at the lower surface of the substrate are connected to the third microstrip line through the through holes. 
         [0006]    According to the existing technology, since a part of the plurality of through holes is sequentially subject to trimming (e.g., cutting using a drilling process) from the through holes located adjacent to the first microstrip line, a part of the third microstrip line is added to the first microstrip line, so that a line length up to a ground point is changed, resulting in a step-by-step change in an inductance value. Since the position of each through hole is determined in advance, when the holes are sequentially subject to trimming to some extent, the degree of coarse adjustment of an oscillation frequency can be understood in advance. Thus, according to the existing technology, coarse adjustment of the oscillation frequency can be quickly and easily performed according 
         [0007]    However, when the oscillation frequency is adjusted as described in the existing technology, if the through holes are cut together with the microstrip line, the line width of the microstrip line is thus narrowed, resulting in the deterioration of a Q factor. In general, since Q characteristics are easily affected by a part with the narrowest width on the microstrip line, when the width of the microstrip line is extremely narrowed by trimming as described in the existing technology, the deterioration of the Q factor may easily occur due to frequency adjustment and exchange. to the number of the through holes to be trimmed. 
       SUMMARY 
       [0008]    An oscillator includes a transistor for oscillation that outputs an oscillation signal; a microstrip line formed on a substrate together with the transistor for oscillation, having one end connected to the transistor for oscillation and the other end connected to a ground electrode formed on the substrate, and thus constituting a part of a frequency determining element that determines a frequency of the oscillation signal according to aline length from the one end of the microstrip line to the ground electrode; and a conductor for coarse adjustment that connects between the one end and the other end of the microstrip line to the ground electrode, thereby reducing the line length of the microstrip line in the connection state. 
         [0009]    According to the oscillator of the present invention, with respect to the microstrip line constituting a part of the element that determines an oscillation frequency, it is possible to perform adjustment of the oscillation frequency by using a conductor for coarse adjustment provided separately from the microstrip line. That is, the conductor for coarse adjustment connects between the one end and the other end of the microstrip line to the ground electrode on the substrate, and in such a state, the line length from the one end of the microstrip line to the ground electrode is reduced. Consequently, in the state in which the conductor for coarse adjustment is arranged (maintained) on the substrate, the frequency of the oscillation signal is determined based on the reduced line length (inductance is small). In such a case, the frequency band of the oscillation signal due to the transistor for oscillation is set to a relatively high value. 
         [0010]    Meanwhile, in the state in which the conductor for coarse adjustment is removed from the substrate, the line length of the microstrip line becomes the original length of the entire range from the one end to the other end (ground electrode) thereof. In such a case, since the frequency band of the oscillation signal is determined based on the original line length (inductance is large) of the microstrip line, the frequency band of the oscillation signal due to the transistor for oscillation is set to a relatively low value. 
         [0011]    According to the present invention as described above, since the oscillation frequency of the oscillator can be significantly changed by removing the conductor for coarse adjustment provided separately from the microstrip line from the substrate, it is possible to cope with a plurality of frequency bands on a substrate with the same configuration (the pattern of the microstrip line). In addition, since no change occurs in the line width of the microstrip line before and after the conductor for coarse adjustment is removed, deterioration of a Q factor does not occur due to the adjustment of the frequency band. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a circuit diagram schematically illustrating the configuration of a voltage controlled oscillator according to a first embodiment. 
           [0013]      FIGS. 2A and 2B  are detailed plan views illustrating the configuration of a microstrip line applied to a voltage controlled oscillator according to a first embodiment. 
           [0014]      FIGS. 3A and 3B  are plan views illustrating a microstrip line in which a conductor for coarse adjustment is removed (after trimming). 
           [0015]      FIGS. 4A and 4B  are plan views illustrating a configuration example of a microstrip line applied to a voltage controlled oscillator of a second embodiment. 
           [0016]      FIGS. 5A and 5B  are plan views illustrating microstrip line in which one conductor for coarse adjustment is removed (after trimming of a first step). 
           [0017]      FIGS. 6A and 6B  are plan views illustrating a microstrip line in which the other conductor for coarse adjustment is removed (after trimming of a second step). 
           [0018]      FIGS. 7A and 4B  are plan views illustrating a configuration example of a microstrip line applied to a voltage controlled oscillator of a third embodiment. 
           [0019]      FIGS. 8A and 8B  are diagrams illustrating an example of trimming a conductor for coarse adjustment in a microstrip line. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0020]    Hereinafter, embodiments of the invention will be described with reference to the accompanying drawings. In the following description, an embodiment of a Voltage Controlled Oscillator (VCO) that voltage-controls an oscillation frequency by using a varactor diode is exemplified. However, the invention is not limited to the configuration of the voltage controlled oscillator. 
         [0021]      FIG. 1  is a circuit diagram schematically illustrating the configuration of the voltage controlled oscillator  1  according to a first embodiment. The voltage controlled oscillator  1 , for example, includes wiring patterns (not shown) formed on the upper surface of a substrate, and various circuit elements (transistors, resistors, capacitors and the like) mounted on the substrate. Further, although not shown in  FIG. 1 , a metal cover is provided on the substrate to cover the whole of the circuit. Hereinafter, the circuit configuration of the voltage controlled oscillator  1  will be described. 
         [0022]    The voltage controlled oscillator  1  mainly includes a transistor  3  for oscillation and a tank circuit  4 . Further, the tank circuit  4  includes two feedback capacitors  43  and  44 , vias resistors  52  and  53  and a resonant circuit  2 . Such a voltage controlled oscillator  1  basically includes a Colpitts oscillator circuit in which a base of the transistor  3  for oscillation is connected to one end of a resonant circuit  2  through a coupling capacitor  45 . 
         [0023]    That is, a collector of the transistor  3  for oscillation is connected to the supply voltage Vcc and is grounded in a high frequency manner through a bypass capacitor  4 . Further, an emitter of the transistor  3  for oscillation is grounded through an emitter vias resistor  51 . In addition, a vias voltage is applied to the base of the transistor  3  for oscillation from a connection point between the two base vias resistors  52  and  53  serially connected to each other. 
         [0024]    The feedback capacitor  43  of the two feedback capacitors  43  and  44  in the tank circuit  4  connects between the emitter and the base of the transistor  3  for oscillation. Further, the feedback capacitor  44  is connected between the collector (a connected state) and the emitter of the transistor  3  for oscillation on the circuit. In addition, the coupling capacitor  45  connected between the base of the transistor  3  for oscillation and the resonant circuit  2  adjusts impedance of the tank circuit  4 . 
         [0025]    The resonant circuit  2  includes a varactor diode  18  and a microstrip line  10 . The resonant circuit  2  receives a control voltage from a control terminal through a choke inductor  9 . In the resonant circuit  2 , a cathode of the varactor diode  18  is connected in parallel to one end of the microstrip line  10 . Further, an anode of the varactor diode  18  and the other end of the microstrip line  10  are grounded, respectively. The control terminal is grounded in a high frequency manner through a separate bypass capacitor  46 . 
         [0026]    If the control voltage is applied to the resonant circuit  2  through the control terminal, the capacitance of the varactor diode  18  is changed, resulting in a change in the resonant frequency of the resonant circuit  2 . Consequently, it is possible to control the oscillation frequency of the entire voltage controlled oscillator  1  by using the control voltage. At this time, the resonant frequency of the resonant circuit  2  is determined according to the inductance of the microstrip line  10  in addition to the capacitance of the varactor diode  18 . Therefore, the oscillation frequency of the voltage controlled oscillator  1  can be adjusted (coarsely adjusted) by changing the inductance (mainly, a line length) of the microstrip line  10  through trimming. 
         [0027]    The voltage controlled oscillator  1  according to the first embodiment includes the microstrip line  10  suitable for coarse adjustment of the oscillation frequency as described above, and particularly, the microstrip line  10  can significantly change the oscillation frequency band of the voltage controlled oscillator  1  before and after the trimming thereof. 
         [0028]      FIGS. 2A and 2B  are detailed plan views illustrating the configuration of the microstrip line  10  applied to the voltage controlled oscillator  1  according to the first embodiment. 
         [0029]    [Configuration Before Trimming] 
         [0030]    Referring to  FIG. 2A , as described above, one end of the microstrip line  10  is connected to the control terminal (not shown in  FIG. 2A ) and the other end of the microstrip line  10  is grounded on the circuit. On the substrate (not shown), for example, a plurality of vias  7  are formed at the other end position of the microstrip line  10 , and the other end of the microstrip line  10  is connected to a ground electrode (not shown) through the vias  7 . Further, the ground electrode, for example, is formed on the lower surface of the substrate (not shown). 
         [0031]    The microstrip line  10  is provided at one end thereof with a signal input terminal  6 , and a line from the signal input terminal  6  to the other end (ground) is formed in a spiral shape. In such an example, the one end of the microstrip line  10  is located at an outer side of the spiral, and the other end of the microstrip line  10  is located at the central side thereof. In more detail, in the microstrip line  10 , if a portion of one end located at the outer side of the spiral is employed as an input conductive portion  10   b  and a portion of the other end further located at the central side of the spiral is employed as a ground conductive portion  10   e , a configuration is obtained in which two intermediate conductive portions  10   c  and  10   d  are interposed between the conductive portions  10   b  and  10   e . These four conductive portions  10   b  to  10   e  are sequentially connected in series to each other from the one end (the signal input terminal  6 ) to the other end of the microstrip line  10 . 
         [0032]    Among the conductive portions, the input conductive portion  10   b  and the first intermediate conductive portion  10   c  are arranged approximately at a right angle to each other on the substrate, and the intermediate conductive portion  10   c  and the second intermediate conductive portion  10   d  are arranged approximately at a right angle to each other on the substrate. In addition, the second intermediate conductive portion  10   d  and the ground conductive portion  10   e  are arranged approximately at a right angle to each other on the substrate. As described above, these four conductive portions  10   b  to  10   e  are sequentially connected in series to each other in the state in which the conductive portions  10   b  and  10   c , the conductive portions  10   c  and  10   d  and the conductive portions  10   d  and  10   e  are sequentially arranged approximately at a right angle to each other, so that the microstrip line  10  has a rectangular spiral shape as a whole. 
         [0033]    All the conductive portions  10   b  to  10   e  constitute the line (conductive line) from the one end to the other end of the microstrip line  10 . In addition, a conductor  10   a  for coarse adjustment is connected to the microstrip line  10 . The conductor  10   a  for coarse adjustment is formed on the substrate as a conductive pattern similarly to the other conductive portions  10   b  to  10   e . Particularly, in the first embodiment, the conductor  10   a  for coarse adjustment is arranged to serve as a bridge between the ground, conductive portion  10   e  and the intermediate conductive portion  10   c . Thus, the intermediate conductive portion  10   c  located between the one end and the other end of the microstrip line  10  is connected to the ground electrode through the conductor  10   a  for coarse adjustment. In such an example, the conductor  10   a  for coarse adjustment is added to the four conductive portions  10   b  to  10   e , so that the entire microstrip line  10  has a shape like a figure “6”. 
         [0034]    [Line Length Before Trimming] 
         [0035]    Referring to  FIG. 2B , if a control voltage is input (applied) to the resonant circuit  2  in the state (before trimming) in which the conductor  10   a  for coarse adjustment is maintained on the substrate, as indicated by the arrow in  FIG. 2B , the shortest range from the signal input terminal  6  to the via  7  by way of the input conductive portion  10   b , a part of an upstream side of the intermediate conductive portion  10   c  when seen in the conduction direction, and the conductor  10   a  for coarse adjustment in the middle of the intermediate conductive portion  10   c  becomes a conductive line as the microstrip line  10 . In such a case, since the range from a part of the intermediate conductive portion  10   c , which is located at a downstream side as compared with the conductor  10   a  for coarse adjustment when seen in the conduction direction, to the ground conductive portion  10   e  in front of the via  7  by way of the next intermediate conductive portion  10   d  does not substantially contribute to conduction, the line length of the microstrip line  10  is reduced to that extent. 
         [0036]    In the state in which the conductor  10   a  for coarse adjustment is maintained as described above, the inductance of the microstrip line  10  is determined after the line length of the microstrip line  10  is reduced (shorter than the original entire length from the one end to the other end thereof). In the first embodiment, the resonant frequency (oscillation frequency of the voltage controlled oscillator  1 ) of the resonant circuit  2  at that time, for example, can be adjusted to a 2 GHz band. 
         [0037]    Next,  FIGS. 3A and 3B  are plan views illustrating the microstrip line  10  after removing the conductor  10   a  for coarse adjustment (after trimming). The conductor  10   a  for coarse adjustment, for example, can be easily removed from the substrate by using automatic machine (drilling machine, a laser irradiator and the like). 
         [0038]    [After Trimming] 
         [0039]    Referring to  FIG. 3A , if the conductor  10   a  for coarse adjustment is removed from the substrate by trimming, the microstrip line  10  includes only the input conductive portion  10   b , the first intermediate conductive portion  10   c , the second intermediate conductive portion  10   d  and the ground conductive portion  10   e , starting from the signal input terminal  6 . 
         [0040]    [Line Length after Trimming] 
         [0041]    Referring to  FIG. 3B , if a control voltage is input (applied) to the resonant circuit  2  after the trimming, as indicated by the arrow in  FIG. 3B , the range from the signal input terminal  6  to the via  7  by way of the input conductive portion  10   b , the first intermediate conductive portion  10   c , the second intermediate conductive portion  10   d  and the ground conductive portion  10   e  becomes a conductive line as the microstrip line  10 . In such a case, since the range including the whole of the first intermediate conductive portion  10   c , the next intermediate conductive portion  10   d , and the ground conductive portion  10   e  in front of the via  7  contributes to the conduction, the line length of the microstrip line  10  extends to that extent as compared with the case in which the trimming is not performed. 
         [0042]    In the state in which the conductor  10   a  for coarse adjustment has been removed as described above, the inductance of the microstrip line  10  is determined after the line length of the microstrip line  10  extends (the original entire length) as compared with the case in which the trimming is not performed. In addition, in the first embodiment, the resonant frequency (oscillation frequency of the voltage controlled oscillator  1 ) of the resonant circuit  2  at that time, for example, can be adjusted to a 1.5 GHz band. 
         [0043]    In the microstrip line  10  as described above, the conductor  10   a  for coarse adjustment is formed in advance at a place where the intermediate conductive portion  10   c  located between the one end (the signal input terminal  6 ) and the other end of the microstrip line  10  is connected (short-circuited) to the ground electrode, so that the line length as a whole can be reduced before the trimming, as compared with the original length. Consequently, the oscillation frequency band of the voltage controlled oscillator  1  can be set to a relatively high value (e.g., a 2 GHz band) in the state in which the trimming is not performed. Meanwhile, if the conductor  10   a  for coarse adjustment is removed by the trimming, the entire line length of the microstrip line  10  increases (becomes the original length) as compared with the case in which the trimming is not performed, so that coarse adjustment to a low oscillation frequency band can be easily performed as compared with the case in which the trimming is not performed. 
         [0044]    In addition, the line width of the microstrip line  10  is not narrowed even after the trimming is performed as shown in  FIGS. 3A and 3B , so that extreme deterioration of the Q factor can be prevented even if the oscillation frequency is coarsely adjusted by the trimming. 
       Second Embodiment 
       [0045]    Next, the voltage controlled oscillator  1  of the second embodiment will be described.  FIGS. 4A and 4B  are plan views illustrating a configuration example of a microstrip line  11  applied to the voltage controlled oscillator  1  of the second embodiment. In addition, the voltage controlled oscillator  1  of the second embodiment has the same circuit configuration as that of the first embodiment, except for the microstrip line  11  with a configuration different from that of the first embodiment. Hereinafter, the microstrip line  11  applied to the second embodiment will be described. 
         [0046]    [Configuration Before Trimming] 
         [0047]    Referring to  FIG. 4A , one end of the microstrip line  11  used for the second embodiment is also connected to the control terminal (not shown in  FIG. 4A ) and the other end of the microstrip line  11  is connected to the ground electrode on the circuit. As indicated by a two-dot chain line in  FIG. 4A , the microstrip line  11  has a configuration in which two conductors  11   a  and  11   b  for coarse adjustment are integrally formed with each other in an internal area thereof. 
         [0048]    That is, the microstrip line  11  is provided at one end thereof with an input conductive portion  11   c  and at the other end thereof with a ground conductive portion  11   f . Between them, the input conductive portion  11   c  is continuous to the signal input terminal  6 , and the ground conductive portion  11   f  is connected to the ground electrode (not shown) through a plurality of vias  7 . Further, two intermediate conductive portions  11   d  and  11   e  are arranged between the input conductive portion  11   c  and the ground conductive portion  11   f . However, before trimming is performed, the conductor  11   a  for coarse adjustment is interposed between the input conductive portion  11   c  and the ground conductive portion  11   f  while being integrally formed with them, and the other conductor  11   b  for coarse adjustment is interposed between the two intermediate conductive portions  11   d  and  11   e  and the ground conductive portion  11   f  while being integrally formed with them. For this reason, it appears that the input conductive portion  11   c  and the ground conductive portion  11   f , and further the intermediate conductive portions  11   d  and  11   e  and the ground conductive portion  11   f  are continuously formed as the same conductive patterns. 
         [0049]    Hereinafter, the microstrip line  11  will be described while focusing on each portion thereof. First, the input conductive portion  11   c  is arranged approximately perpendicular to the first intermediate conductive portion  11   d , and the first intermediate conductive portion  11   d  is arranged approximately perpendicular to the second intermediate conductive portion  11   e . The ground conductive portion  11   f  extends approximately at a right angle toward the input conductive portion  11   c  from a terminal end of the second intermediate conductive portion  11   e , and is spread to a rectangular shape with a certain size from there. In an area spreading to the rectangular shape of the ground conductive portion  11   f , the plurality (herein,  12 ) of vias  7  are arranged in a matrix shape. When the input conductive portion  11   c , the intermediate conductive portions  11   d  and  11   e , and the ground conductive portion  11   f  are seen in a continuous manner, it can be understood that the line from one end (the signal input terminal  6 ) to the other end (ground) of the microstrip line  11  is formed in a spiral shape. 
         [0050]    Further, the two conductors  11   a  and  11   b  for coarse adjustment are serially arranged to surround the ground conductive portion  11   f  in the microstrip line  11 . Between them, the conductor  11   a  for coarse adjustment is arranged at an inner side of the spiral along the input conductive portion  11   c  and has an external appearance with a strip shape as indicated by a two-dot chain line in  FIG. 4A . At this position, the conductor  11   a  for coarse adjustment employs the outer side of the spiral as a starting end and extends in parallel to the input conductive portion  11   c , and a terminal end of the conductor  11   a  for coarse adjustment reaches a connection point between the input conductive portion  11   c  and the first intermediate conductive portion  11   d . The other intermediate conductor  11   b  for coarse adjustment is arranged inside the spiral along the two intermediate conductive portions  11   d  and  11   e , and has an external appearance with a substantially L shape (key shape) as indicated by a two-dot chain line in  FIG. 4A . At this position, the conductor  11   b  for coarse adjustment employs the connection point between the input conductive portion  11   c  and the first intermediate conductive portion  11   d  as a starting end and extends in the spiral direction, and a terminal end of the conductor  11   b  for coarse adjustment reaches a connection point between the second intermediate conductive portion lie and the ground conductive portion  11   f.    
         [0051]    [Line Length Before Trimming] 
         [0052]    Referring to  FIG. 4B , if a control voltage is input (applied) to the resonant circuit  2  in the state (before trimming) in which the two conductors  11   a  and  11   b  for coarse adjustment are maintained on the substrate, as indicated by the arrow in  FIG. 4B , the shortest range from the signal input terminal  6  to the via  7  by way of a part of an upstream side of the input conductive portion  11   c  when seen in the conduction direction, and the conductor  11   a  for coarse adjustment becomes a conductive line as the microstrip line  11 . In such a case, since the range from a part of the input conductive portion  11   c , which is adjacent to the conductor  11   a  for coarse adjustment when seen in the width direction, to the intermediate conductive portions  11   d  and  11   e  (including the other conductor  11   b  for coarse adjustment) after the part does not substantially contribute to conduction, the line length of the microstrip line  11  is minimized in such a state. 
         [0053]    In the state in which the two conductors  11   a  and  11   b  for coarse adjustment are maintained as described above, the inductance of the microstrip line  11  is determined after the line length of the microstrip line  11  is minimized. In the second embodiment, the resonant frequency (oscillation frequency of the voltage controlled oscillator  1 ) of the resonant circuit  2  at that time, for example, can be adjusted to a 2 GHz band. 
         [0054]    Next,  FIGS. 5A and 5B  are plan views illustrating the microstrip line  11  after removing the conductor  11   a  for coarse adjustment of the two conductors (after trimming of a first step). In the second embodiment, the conductor  11   a  for coarse adjustment can be easily removed from the substrate by using automatic machine similarly to the first embodiment. 
         [0055]    [After Trimming of First Step] 
         [0056]    Referring to  FIG. 5A , the trimming of the first step is performed with respect to the microstrip line  11 , so that the input conductive portion  11   c  is separated from the ground conductive portion  11   f  on the substrate. In addition, the other conductor  11   b  for coarse adjustment remains on the substrate. 
         [0057]    [Line Length After Trimming of First Step] 
         [0058]    Referring to  FIG. 5B , if a control voltage is input (applied) to the resonant circuit  2  after the trimming of the first step, as indicated by the arrow in  FIG. 5B , the shortest range from the signal input terminal  6  to the vias  7  by way of the whole of the input conductive portion  11   c , a part of the first intermediate conductive portion  11   d , and the remaining conductor  11   b  for coarse adjustment becomes a conductive line as the microstrip line  11 . In such a case, since the whole of the input conductive portion  11   c  contributes to the conduction differently from the case in which the trimming is not performed, the range from the part of the first intermediate conductive portion  11   d  and the remaining conductor  11   b  for coarse adjustment to the front of the vias  7  of the ground conductive portion  11   f  contributes to the conduction. Consequently, in such a case, the line length of the microstrip line  11  extends by one step as compared with the case in which the trimming is not performed. 
         [0059]    In the state in which the conductor  11   a  for coarse adjustment is removed by the trimming of the first step as described above, the inductance of the microstrip line  11  is determined after the line length of the microstrip line  11  extends by one step as compared with the case in which the trimming is not performed. In the second embodiment, the resonant frequency (oscillation frequency of the voltage controlled oscillator  1 ) of the resonant circuit  2  at that time, for example, can be adjusted to a 1.5 GHz band. 
         [0060]      FIGS. 6A and 6B  are plan views illustrating the microstrip line  11  after removing the other conductor  11   b  for coarse adjustment (after trimming of a second step). In the second embodiment, the conductor  11   b  for coarse adjustment can be easily removed from the substrate by using automatic machine similarly to the above. 
         [0061]    [After Trimming of Second Step] 
         [0062]    Referring to  FIG. 6A , the trimming of the second step is performed with respect to the microstrip line  11 , so that the intermediate conductive portion  11   d  is separated from the ground conductive portion  11   f  on the substrate in addition to the trimming of the first step. Therefore, it is apparent that the microstrip line  11  has a spiral shape as a whole. 
         [0063]    [Line Length After Trimming of Second Step] 
         [0064]    Referring to  FIG. 6B , if a control voltage is input (applied) to the resonant circuit  2  after the trimming of the second step, as indicated by the arrow in  FIG. 6B , the range from the signal input terminal  6  to the vias  7  by way of the whole of the input conductive portion  11   c , the whole of the first intermediate conductive portion  11   d , the whole of the second intermediate conductive portion  11   e , and the ground conductive portion  11   f  becomes a conductive line as the microstrip line  11 . In such a case, since the entire range from one end to the other end of the microstrip line  11  contributes to the conduction differently from the cases in which the trimming is not performed and the trimming of the first step is performed, the line length of the microstrip line  11  is maximized. 
         [0065]    In the state in which the other conductor  11   b  for coarse adjustment is removed by the trimming of the second step as described above, the inductance of the microstrip line  11  is determined after the line length of the microstrip line  11  is maximized. In the second embodiment, the resonant frequency (oscillation frequency of the voltage controlled oscillator  1 ) of the resonant circuit  2  at that time, for example, can be adjusted to a 1 GHz band. 
         [0066]    In the microstrip line  11  applied to the voltage controlled oscillator  1  according to the second embodiment, since the conductor  11   a  for coarse adjustment is formed in advance at a place where the input conductive portion  11   c  located between the one end (the signal input terminal  6 ) and the other end (vias  7 ) of the microstrip line  11  is connected (short-circuited) to the ground electrode, and the other conductor  11   b  for coarse adjustment is formed in advance at a place where the intermediate conductive portions  11   d  and  11   e  are connected (short-circuited) to the ground electrode, for example, the following coarse adjustment is possible. 
         [0000]    (1) That is, before the trimming is performed with respect to the microstrip line  11 , the line length as a whole can be minimized, and in such a state, the oscillation frequency band of the voltage controlled oscillator  1  can be set to a relatively high value (e.g., a 2 GHz band).
 
(2) Next, if the conductor  11   a  for coarse adjustment is removed by the trimming of the first step, the line length of the microstrip line  11  as a whole increases by one step as compared with the case in which the trimming is not performed, so that coarse adjustment to a low oscillation frequency band by one step can be easily performed as compared with the case in which the trimming is not performed.
 
(3) In addition, if the other conductor  11   b  for coarse adjustment is removed by the trimming of the second step, the line length of the microstrip line  11  as a whole is maximized, so that coarse adjustment to the lowest oscillation frequency band can be easily performed.
 
         [0067]    Moreover, in the second embodiment, the line width (minimum width) of the microstrip line  11  is not narrowed after the trimming of both the first step and the second step. Consequently, even in the second embodiment, extreme deterioration of the Q factor can be prevented even if the oscillation frequency band is coarsely adjusted by trimming the microstrip line  11 . 
         [0068]    [Modification of Second Embodiment] 
         [0069]    In addition, in the second embodiment, the conductors for coarse adjustment are divided into two conductors  11   a  and  11   b  and are subject to the trimming. However, the conductors  11   a  and  11   b  for coarse adjustment can be treated as a continuous semiconductor area for coarse adjustment. In such a case, the conductors (reference numerals  11   a  and  11   b ) for coarse adjustment have a certain length as a whole, a part of the conductors is removed by the trimming step by step, and the length of a remaining area is continuously reduced, so that coarse adjustment of an oscillation frequency band can be continuously performed. 
         [0070]    In this regard, for example, in a conventional general coarse adjustment, when an oscillation frequency band is significantly changed, there exists a technique for changing a conductive pattern of a microstrip line. In such a case, since a difference exists in an area (size, surface edge) of a substrate, which is occupied by the conductive pattern of the microstrip&#39;line, for each oscillation frequency band, it is necessary to change a substrate layout according to the change in the conductive pattern of the microstrip line. However, when employing the first and second embodiments, no change occurs in the maximum area of the substrate, which is occupied by the microstrip lines  10  and  11  before and after the coarse adjustment. Consequently, even if the conductive patterns of the microstrip lines  10  and  11  are equal to each other on the same substrate, it is possible to easily realize coarse adjustment corresponding to various oscillation frequency bands. 
       Third Embodiment 
       [0071]      FIGS. 7A and 4B  are plan views illustrating a configuration example of a microstrip line  12  applied to the voltage controlled oscillator  1  of the third embodiment. The voltage controlled oscillator  1  of the third embodiment also has the same circuit configuration as those of the first and second embodiments, except for the microstrip line  12  with a configuration different from those of the first and second embodiments. Hereinafter, the microstrip line  12  applied to the third embodiment will be described. 
         [0072]    [Configuration Before Trimming] 
         [0073]    Referring to  FIG. 7A , similarly to the first and second embodiments, one end (the single input terminal  6 ) of the microstrip line  12  used for the third embodiment is also connected to the control terminal (not shown in  FIG. 7A ) and the other end of the microstrip line  12  is connected to the ground electrode on the circuit. The microstrip line  12  used for the third embodiment is provided at an outer peripheral portion (an outer side of a spiral) with a conductor  12   a  for coarse adjustment. 
         [0074]    That is, the microstrip line  12  is provided at one end thereof with an input conductive portion  12   b  and at the other end thereof with a ground conductive portion  12   e . Between them, the input conductive portion  12   b  is continuous to the signal input terminal  6 , and the ground conductive portion  12   e  is connected to the ground electrode (not shown) through a plurality of vias  7 . Further, two intermediate conductive portions  12   c  and  12   d  are arranged between the input conductive portion  12   b  and the ground conductive portion  12   e . In this example, the one end of the microstrip line  12  is located at an outer side of the spiral and the other end thereof is located at a central side of the spiral. 
         [0075]    The input conductive portion  12   b  is arranged approximately perpendicular to the first intermediate conductive portion  12   c , and the first intermediate conductive portion  12   c  is arranged approximately perpendicular to the second intermediate conductive portion  12   d . In addition, the second intermediate conductive portion  12   d  is arranged approximately perpendicular to the ground conductive portion  12   e . These four conductive portions  12   b  to  12   e  are serially connected to one another while being sequentially arranged perpendicular to one another, so that the microstrip line  12  has a rectangular spiral shape as a whole. 
         [0076]    Thus, in the microstrip line  12 , the conductor  12   a  for coarse adjustment is located at an outer side of the spiral as described above. That is, the conductor  12   a  for coarse adjustment, for example, protrudes toward the outer side of the spiral from the vicinity of a connection point between the first intermediate conductive portion  12   c  and the second intermediate conductive portion  12   d  and extends to the second intermediate conductive portion  12   d  and an outer side of the ground conductive portion  12   e  continuous to the second intermediate conductive portion  12   d . In addition, the conductor  12   a  for coarse adjustment is formed with a plurality of vias  8  along the outer peripheral portion thereof. In this example, the conductor  12   a  for coarse adjustment is connected to the ground electrode (not shown) through the plurality of vias  8 . 
         [0077]    [Line Length Before Trimming] 
         [0078]    Referring to  FIG. 7B , if a control voltage is input (applied) to the resonant circuit  2  in the state (before trimming) in which the conductor  12   a  for coarse adjustment remains on the substrate, as indicated by the arrow in  FIG. 7B , the shortest range from the signal input terminal  6  to the via  8  of the outer peripheral portion by way of the input conductive portion  12   b , the first intermediate conductive portion  12   c , and the conductor  12   a  for coarse adjustment of the outer side from a starting end portion of the second intermediate conductive portion  12   d  becomes a conductive line as the microstrip line  12 . In such a case, since the range from a part of a downstream, as compared with the starting end portion of the second intermediate conductive portion  12   d  when seen in the width direction, to the ground conductive portion  12   e  continuous to the second intermediate conductive portion  12   d  does not substantially contribute to conduction, the line length of the microstrip line  12  is reduced to that extent. 
         [0079]    In the state in which the whole of the conductor  12   a  for coarse adjustment is maintained as described above, the inductance of the microstrip line  12  is determined after the line length of the microstrip line  12  is minimized. In the third embodiment, the resonant frequency (oscillation frequency of the voltage controlled oscillator  1 ) of the resonant circuit  2  at that time, for example, can be adjusted to a 2 GHz band. 
       Trimming Example 
       [0080]      FIGS. 8A and 8B  are diagrams illustrating an example of trimming the conductor  12   a  for coarse adjustment in the microstrip line  12 . 
         [0081]    Referring to  FIG. 8A , the microstrip line  12  used for the third embodiment is suitable when the conductor  12   a  for coarse adjustment is continuously trimmed as described in the modification of the second embodiment. In detail, as indicated in an arrow of a dashed dotted line of  FIG. 8A , the conductor  12   a  for coarse adjustment is continuously removed along the outer periphery of the second intermediate conductive portion  12   d  from the connection point between the two intermediate conductive portions  12   c  and  12   d  (i.e., the outer side of the starting end portion of the second intermediate conductive portion  12   d ), so that the line length of the microstrip line  12  as a whole can be continuously lengthened according to the length corresponding to the removed portion of the conductor  12   a  for coarse adjustment. 
         [0082]    Further, in the example shown in  FIG. 8A , it can be understood that the conductor  12   a  for coarse adjustment has been trimmed throughout the range from the starting end portion to the terminal end portion of the intermediate conductive portion  12   d , that is, the range to the vicinity of the connection point between the intermediate conductive portion  12   d  and the ground conductive portion  12   e . In such a case, in the range in which the conductor  12   a  for coarse adjustment has been removed by the trimming, the conductive line from the intermediate conductive portion  12   d  to the vias  8  of the conductor  12   a  for coarse adjustment is separated on the substrate. 
         [0083]    [Line Length after Trimming] 
         [0084]    Referring to  FIG. 8B , thus, if a control voltage is input (applied) to the resonant circuit  2  after the trimming, as indicated by the arrow in  FIG. 8B , the range from the signal input terminal  6  to the vias  8 , which is located at the outer periphery of the conductor  12   a  for coarse adjustment, by way of the input conductive portion  12   b , the first intermediate conductive portion  12   c , the second intermediate conductive portion  12   d , and the untrimmed portion of the conductor  12   a  for coarse adjustment becomes a conductive line as the microstrip line  12 . In such a case, since the range including the whole of the first intermediate conductive portion  12   c , the next intermediate conductive portion  12   d , and the conductor  12   a  for coarse adjustment in front of the vias  8  contributes to the conduction, the line length of the microstrip line  12  is reduced to that extent as compared with the case in which the trimming is not performed. 
         [0085]    In the state in which the conductor  12   a  for coarse adjustment has been partially removed as described above, since the line length of the microstrip line  12  extends according to the length of the trimmed range, the inductance is determined according to the line length after the extension. Consequently, in the third embodiment, the oscillation frequency band can be continuously adjusted according to the length of the range in which the conductor  12   a  for coarse adjustment is removed. 
         [0086]    For example, when seen in the conduction direction from the signal input terminal  6 , if it is assumed that the range up to the vias  8  located at the starting end portion of the conductor  12   a  for coarse adjustment serves as the shortest conductive line, the range up to the vias  8  located at the terminal end portion of the conductor  12   a  for coarse adjustment serves as the longest conductive line when seen in the conduction direction (here, vias  7  of the ground conductive portion  12   e  are excluded). Consequently, the trimming is continuously performed toward the terminal end from the starting end position of the conductor  12   a  for coarse adjustment, so that the line length in the microstrip line  12  extends according to the length corresponding to the trimmed portion, resulting in the continuous reduction of the oscillation frequency. 
         [0087]    Further, even in the third embodiment, the line width in the microstrip line  12  is not narrowed before and after the trimming, so that deterioration of the Q factor due to the coarse adjustment can be prevented. 
         [0088]    In addition, although not shown in  FIGS. 8A  and  8 B, in the third embodiment, the trimming can be performed throughout the whole of the conductor  12   a  for coarse adjustment. In such a case, since both the intermediate conductive portion  12   d  and the ground conductive portion  12   e  are completely separated from the conductor  12   a  for coarse adjustment on the substrate, the range from the one end (the signal input terminal  6 ) of the microstrip line  12  to the vias  7  of the ground conductive portion  12   e  directly becomes the line length, so that the oscillation frequency can be adjusted (e.g., adjusted to a 1 GHz band) according to the length. 
         [0089]    The present invention is not limited to the first to third embodiments as described above. That is, various modifications can be made. For example, the shapes of microstrip lines  10  to  12  are not limited to the examples shown in the drawings. That is, the microstrip lines  10  to  12  may have various shapes. Further, differently from the previous embodiments, the spiral may be formed in a reverse direction on the substrate, or the outer side and inner side of the spiral may be reversed. 
         [0090]    Further, in the first embodiment, only one conductor  10   a  for coarse adjustment is formed. However, two or more conductors for coarse adjustment may be formed between the intermediate conductive portion  10   c  and the ground conductive portion  10   e . In such a case, it is possible to perform the oscillation frequency band adjustment of three steps (high, intermediate and low) or more according to the number of conductors for coarse adjustment to be trimmed. 
         [0091]    In addition, in the second embodiment, the conductors for coarse adjustment are divided into two conductors  11   a  and  11   b . However, the conductors for coarse adjustment may be divided into three or more. 
         [0092]    Moreover, the circuit configuration of the voltage controlled oscillator  1  shown in  FIG. 1  is just one example. For example, it may be possible to form a circuit as an oscillator by adding other devices and circuit elements. Furthermore,  FIG. 1  exemplifies the circuit structure of base input and collector ground. However, other input and ground schemes may be employed. For example, it may be possible to employ a circuit structure of collector input and base ground. 
         [0093]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.