Abstract:
The invention discloses a variable attenuator, comprising two or more resistors each resistor having its own effective resistance value, and means for simultaneously short circuiting at least a portion of two or more of said resistors, whereby simultaneously changing the effective resistance values. The variable attenuator of the invention is suitable for use in various high frequency and microwave circuits and systems, and has the features of a wide frequency band, small size, easy fabrication, low cost, and so on.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of International Patent Application No. PCT/CN2005/000872 with an international filing date of Jun. 17, 2005, designating the United States, now pending, which claims priority benefits to the Chinese Patent Application No. 200410051879.9 filed Oct. 13, 2004. This application further claims priority benefits pursuant to 35 U.S.C § 119 and the Paris Convention Treaty to the Chinese Patent Application No. 200610156824.3 filed Nov. 11, 2006 The contents of all of the above-mentioned specifications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to variable attenuators in the electronics and communication fields, and more particularly, to microstrip variable attenuators suitable for use in various high frequency and/or microwave circuits and systems.  
         [0004]     2. Description of the Related Art  
         [0005]     In the family of electronic components, the variable attenuator is one of the common and basic components in electrical circuits and systems. The existence of a variable attenuator makes the fabrication of electrical circuits and the debugging of systems more flexible and convenient. Currently, the variable attenuator is being widely used in circuits and systems with operating frequencies below a few hundred megahertz (MHz). For example, in CATV (Community Antenna Television) systems and microwave circuits, the variable attenuator is used for testing, regulating power levels, increasing isolation, etc. However, as the operating frequency is in a comparatively high frequency band, the current three-dimensional variable attenuator which is made of a contact spring, a slide block, a guide screw, and so on, has the drawbacks of large parasitic parameters and comparatively poor high frequency characteristics.  
       SUMMARY OF THE INVENTION  
       [0006]     In view of the above-described problems, it is one objective of the invention to provide a variable attenuator with good wide band characteristics that is suitable for use in high frequency and/or microwave circuits and systems.  
         [0007]     In accordance with one objective of the invention, provided is a variable attenuator comprising: a base  11 , a film resistor  1  located on the base  11 , and an input terminal  9  and an output terminal  10  connected to the two ends of the film resistor  1 , respectively; the two ends of the film resistor  1  are also electrically connected to one end of a film resistor  6  and one end of a film resistor  7 , respectively; the other ends  14  of the film resistor  6  and film resistor  7  are electrically connected to one end of the film resistor  2 , and the other end of the film resistor  2  is electrically connected to a ground terminal  13 ; the variable attenuator further comprises a conductive sheet  3  and a conductive sheet  4  that can be electrically connected to the film resistor  1  and the film resistor  2  for changing the resistance values thereof; the variable attenuator further comprises an insulator  12  for fixing the conductive sheet  3  and the conductive sheet  4  disposed thereon.  
         [0008]     In a class of this embodiment, the resistance value of the film resistor  6  is equal to that of the film resistor  7 .  
         [0009]     In a class of this embodiment, the position of the conductive sheet  3  and the conductive sheet  4  can be changed when moving the insulator  12  so as to change the contact area between the conductive sheet  3  and the film resistor  1  and that between the conductive sheet  4  and the film resistor  2 .  
         [0010]     In a class of this embodiment, the conductive sheet  3 , the conductive sheet  4 , the film resistor  1 , and the film resistor  2  can be in the shape of an arc or rectangular; and the conductive sheet  3  and the conductive sheet  4  are also film resistors.  
         [0011]     In a class of this embodiment, the common plane of the film resistor  1  and the conductive sheet  3  is without limitation in the same plane as that of the film resistor  2  and the conductive sheet  4 ; and the base  11  is a multi-layered base.  
         [0012]     In a class of this embodiment, the force to change the geometrical position of the conductive sheet  3  and the conductive sheet  4  is a mechanical manual force, an automatic controlled mechanical force, an electromagnetic force, a force produced by heat or temperature, a force produced by the flow, expansion, or contraction of a liquid, or a force initiated by an optoelectronic excitation process.  
         [0013]     In a class of this embodiment, the configuration of the variable attenuator is of a surface mount type, a pin leg lead type, or a patch cord type.  
         [0014]     In a class of this embodiment, a silicon rubber film conductive in the vertical direction is added between the base  11  and the insulator  12 .  
         [0015]     In a class of this embodiment, a groove is disposed on the insulator  12 ; the conductive sheet  3  and the conductive sheet  4  are located inside of the groove; and an elastic substance is added between the conductive sheet  3  and the conductive sheet  4  within the groove.  
         [0016]     In a second embodiment of the invention provided is a microstrip variable attenuator, comprising: a base  101 , a film resistor  105  located on the base, an input terminal  102  and an output terminal  103  connected to the two ends of the film resistor  105 ; the two ends of the film resistor  105  are further electrically connected to one end of a film resistor  106  and one end of a film resistor  107 , respectively; the other ends of the film resistor  106  and the film resistor  107  are electrically connected to a ground terminal  109 ; the variable attenuator of the invention further comprises a conductive sheet  110 , a conductive sheet  111 , and a conductive sheet  112  that can be electrically contacted by the film resistor  105 , the film resistor  106 , and the film resistor  107 , respectively, and are used to change the resistance values of the film resistor  105 , the film resistor  106 , and the film resistor  107 , respectively; the variable attenuator of the invention further comprises an insulator  113 , on which the conductive sheet  105 , the conductive sheet  106 , and the conductive sheet  106  are fixed.  
         [0017]     In a class of this embodiment, the resistance value of the film resistor  106  is equal or close to that of the film resistor  107 .  
         [0018]     In a class of this embodiment, the position of the conductive sheet  110 , the conductive sheet  111 , and the conductive sheet  112  can be changed when moving the insulator  113  so as to change the contact area between the conductive sheet  110  and the film resistor  105 , that between the conductive sheet  111  and the film resistor  106 , and that between the conductive sheet  112  and the film resistor  107 .  
         [0019]     In a class of this embodiment, the conductive sheet  110 , the conductive sheet  111 , the conductive sheet  112 , the film resistor  105 , the film resistor  106 , and the film resistor  107  are in the shape of an strip arc or rectangular; the conductive sheet  110 , the conductive sheet  111 , and the conductive sheet  112  are also film resistors; the insulator  113  is a PCB board with conductive sheets disposed thereon, wherein the PCB board can be in the shape of a circle with an arc mouth formed on its peripheral edge.  
         [0020]     In a class of this embodiment, the force to change the geometrical position of the conductive sheet  105 , the conductive sheet  106 , and the conductive sheet  107  is a mechanical manual force, an automatic controlled mechanical force, an electromagnetic force, a force produced by heat or temperature, a force produced by the flow, expansion, or contraction of a liquid, or a force initiated by an optoelectronic excitation process.  
         [0021]     In a third embodiment of the invention provided is a microstrip variable attenuator, comprising: a base  229 , a film resistor  219 , a film resistor  220 , a film resistor  221 , an input terminal  216  and an output terminal  217  located on the base; the input terminal  216  is connected to one end of the film resistor  219 , the other end of the film resistor  219  is connected to one end of the film resistor  220 , and is connected to one end of the film resistor  221 , the other end of the film resistor  221  is connected to the ground terminal  222 ; the other end of the film resistor  220  is connected to the output terminal  217 ; the variable attenuator of the invention further comprises a conductive sheet  223 , a conductive sheet  224 , and a conductive sheet  225  that can be electrically contacted by the film resistor  219 , the film resistor  220 , and the film resistor  221 , respectively, and are used to change the resistance values of the film resistor  219 , the film resistor  220 , and the film resistor  221 , respectively; the variable attenuator of the invention further comprises an insulator  227 , on which the conductive sheet  223 , the conductive sheet  224 , and the conductive sheet  225  are fixed.  
         [0022]     In a class of this embodiment, the resistance value of the film resistor  219  is equal or is close to that of the film resistor  220 .  
         [0023]     In a class of this embodiment, the position of the conductive sheet  223 , the conductive sheet  224 , and the conductive sheet  225  can be changed when moving the insulator  227  so as to change the contact area between the conductive sheet  223  and the film resistor  219 , that between the conductive sheet  224  and the film resistor  220 , and that between the conductive sheet  225  and the film resistor  221 .  
         [0024]     In a class of this embodiment, the conductive sheet  223 , the conductive sheet  224 , the conductive sheet  225 , the film resistor  219 , the film resistor  220 , and the film resistor  221  are in the shape of an strip arc or rectangular; the conductive sheet  223 , the conductive sheet  224 , the conductive sheet  225  are also film resistors; the insulator  227  is a PCB board with conductive sheets disposed thereon, wherein the PCB board is in the shape of a circle with an arc mouth formed on its peripheral edge.  
         [0025]     In a class of this embodiment, the force to change the geometrical position of the conductive sheet  223 , the conductive sheet  224 , and the conductive sheet  225  is a mechanical manual force, an automatic controlled mechanical force, an electromagnetic force, a force produced by heat or temperature, a force produced by the flow, expansion, or contraction of a liquid, or a force initiated by an optoelectronic excitation process.  
         [0026]     Therefore, the variable attenuator according to the invention provides the following advantages:  
         [0027]     (a) since a microstrip base structure is adopted, the range of useful frequencies for the variable attenuator is very wide; the continuous variable attenuation of a signal in the high frequency and microwave frequency range can be realized;  
         [0028]     (b) it has a small size, is easy to adjust, and is suitable for use in various miniaturized circuits and communication circuits;  
         [0029]     (c) it has a simple structure, and a low fabrication cost;  
         [0030]     (d) it is suitable for various equalization circuits;  
         [0031]     (e) it is suitable for various isolation circuits;  
         [0032]     (f) it is suitable for various regulating circuits, controlling circuits, stabilizing circuits, and circuits for adjusting the amount of coupling;  
         [0033]     (g) it is suitable for circuits where high attenuation is required, systematic error of an actual circuit is large, and regulation of all parts is needed to satisfy characteristics of overall circuits;  
         [0034]     (h) it has a low insertion loss; and  
         [0035]     (i) it can serve in adjusting and testing instruments for research and development work in laboratories. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     The invention is described hereinafter with reference to accompanying drawings, in which:  
         [0037]      FIG. 1  is a structural diagram of a variable attenuator in accordance with one embodiment of the invention;  
         [0038]      FIG. 2  is an exploded view thereof;  
         [0039]      FIG. 3  is an equivalent electric diagram thereof;  
         [0040]      FIG. 4  shows a theoretical characteristic variation curve of the resistance value of the film resistor  1  and the film resistor  2 , when the insulator drives the conductive sheet to rotate clockwise as the variable attenuator is adjusted by an external force, in accordance with one embodiment of the invention;  
         [0041]      FIG. 5  shows an attenuation variation curve of a variable attenuator in accordance with one embodiment of the invention when the insulator drives the conductive sheet to rotate clockwise as the variable attenuator is adjusted by an external force;  
         [0042]      FIG. 6  is a structural diagram of a variable attenuator in accordance with a second embodiment of the invention;  
         [0043]      FIG. 7  is a structural diagram of a conductive sheet thereof;  
         [0044]      FIG. 8  is an equivalent electric diagram thereof;  
         [0045]      FIG. 9  shows a theoretical characteristic variation curve of the resistance value of the film resistor  105 , the film resistor  106 , and the film resistor  107  when the insulator drives the conductive sheet to rotate clockwise as the variable attenuator is adjusted by an external force, in accordance with a second embodiment of the invention;  
         [0046]      FIG. 10  shows an attenuation variation curve of a variable attenuator in accordance with a second embodiment of the invention when the insulator drives the conductive sheet to rotate clockwise as the variable attenuator is adjusted by an external force;  
         [0047]      FIG. 11  is a structural diagram of a variable attenuator in accordance with a third embodiment of the invention;  
         [0048]      FIG. 12  is a structural diagram of a conductive sheet thereof;  
         [0049]      FIG. 13  is an equivalent electric diagram thereof;  
         [0050]      FIG. 14  shows a theoretical characteristic variation curve of the resistance value of the film resistor  219 , the film resistor  220 , and the film resistor  221  when the insulator drives the conductive sheet to rotate clockwise as the variable attenuator is adjusted by an external force, in accordance with a third embodiment of the invention; and  
         [0051]      FIG. 15  shows an attenuation variation curve of a variable attenuator in accordance with a third embodiment of the invention when the insulator drives the conductive sheet to rotate clockwise as the variable attenuator is adjusted by an external force. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0052]     With reference to  FIGS. 1-2 , a variable attenuator according to the first embodiment of the invention comprises a base  11 , an input terminal  9  located on the base  11 , an arc shaped microstrip signal line  5  with one end connected to the input terminal  9 , an arc shaped film resistor  1  with one end connected to the other end of the microstrip signal line  5 , and an output terminal  10  connected to the other end of the film resistor  1 . In addition, the two ends of the film resistor  1  are electrically connected to one end of a film resistor  6  and one end of a film resistor  7 , respectively; the other ends  14  of the film resistor  6  and film resistor  7  are both electrically connected to one end of the film resistor  2 , the other end of the film resistor  2  is connected to a ground terminal  13 , or is connected to the ground terminal  13  via a microstrip signal line  8 . In certain embodiments of the invention, the film resistor  1 , the film resistor  2 , the film resistor  6 , and the film resistor  7  are all printed film resistors with the bottom side connected to the base  11  and the top side made of conductive and non-insulated material.  
         [0053]     Particularly, the resistance value of the film resistor  6  is equal to that of the film resistor  7 . Generally, the film resistor  6  and the film resistor  7  are film resistors having the same resistance value, Zo, at the input and output terminals, for example, 50 Ohms. A conductive sheet  3  for contact short-circuiting, and having the same shape as the microstrip signal line  5 , is located above the top side of the microstrip signal line, and is fixed on the insulator  12 . The insulator  12  is a forced displacement board, and is further fixed with a conductive sheet  4 . The conductive sheet  3  and the conductive sheet  4  are fixed at the bottom side of the insulator  12  (namely the forced displacement board), respectively. The function of the conductive sheet  3  is to adjust the effective resistance value of the film resistor  1 , while that of the conductive sheet  4  is to adjust the effective resistance value of the film resistor  2 . The conductive sheet  3  does not contact with the conductive sheet  4 . The conductive sheet  3  and the conductive sheet  4  rotate with the rotation of the insulator  12 . For example, when the insulator  12  (the forced displacement board) rotates clockwise, the conductive sheet  3  rotates on and in contact with the microstrip signal line  5  and the film resistor  1  simultaneously. The contact area between the conductive sheet  3  and the film resistor  1  increases so that the resistance value of the film resistor  1  decreases. The conductive sheet  4  rotates on the microstrip signal line  8  and the film resistor  2  simultaneously. The contact area between the conductive sheet  4  and the film resistor  2  decreases so that the resistance value of the film resistor  2  increases. Through the change in the geometric area, namely the change in the contact area between the conductive sheet and the film resistor, the actual effective resistance values of the film resistor  1  and the film resistor  2  are changed.  
         [0054]     When the insulator  12  (forced displacement board) rotates clockwise, it is preferred that the maximum rotation angle of the insulator  12  be maintained so as to make the conductive sheet  3  nearly or totally short-circuit the film resistor  1 ; the length (arc length) of the conductive sheet  3  should cover or nearly cover the film resistor  1 , and should be prevented from contacting the film resistor  2 . When the conductive sheet  4  rotates clockwise, the conductive sheet  4  needs to be designed not to contact the microstrip signal line  5  and the input terminal  9 . Similarly, when the insulator  12  (forced displacement board) rotates counter-clockwise, the maximum rotation angle of the conductive sheet  4  needs to be maintained so as to avoid the conductive sheet  4  from contacting the output terminal  10 . When the conductive sheet  3  rotates counter-clockwise, the conductive sheet  3  needs to be designed not to contact the ground terminal  13 .  
         [0055]     The conductive sheet  3  and the conductive sheet  4  can also be film resistors, which overlap and are electrically connected, and can be regarded as two resistors in parallel. Similarly, the resistance value of the film resistor can be changed and the same effect can be achieved. However, it is required that the conductive sheet  3  can only be used to electrically contact the film resistor  1  to change the resistance value thereof, and does not directly contact other microstrip signal lines or film resistors. It is required that the conductive sheet  4  can only be used to electrically contact the film resistor  2  to change the resistance value thereof, and does not directly contact other microstrip signal lines or film resistors. Therefore, the film resistor  1  and the film resistor  2  can be fabricated on the base  11  in different layers from other microstrip signal lines, the input and output terminals, and other film resistors so as to keep the basic principle and structure of the variable attenuator.  
         [0056]     The co-plane of the film resistor  1  and the conductive sheet  3  is, without limitation, in the same plane as that of the film resistor  2  and the conductive sheet  4 .  
         [0057]      FIG. 3  illustrates the basic principle diagram of the variable attenuator of the invention. The operation principle of the variable attenuator is equivalent to a continuous variable bridge T-shaped attenuator, which is a symmetric wide band network with interchangeable input and output terminals.  
         [0058]      FIG. 4  illustrates an ideal theoretical variation curve of the film resistor  1  and the film resistor  2  when the insulator  12  (forced displacement board) rotates clockwise. The variation trend of the resistance value of the film resistor  1  is opposite to that of the film resistor  2 .  
         [0059]      FIG. 5  illustrates a line showing the attenuation amount of the variable attenuator fabricated according to the curve of  FIG. 4  when the insulator  12  (forced displacement board) rotates clockwise. During designing and fabricating, the film resistor  1  and the film resistor  2  are chosen according to the curve of  FIG. 4  so as to realize variation in the attenuation amount, which is required when the displacement of the variable attenuator is changed.  
         [0060]     When the resistance value of one of the film resistors increases, the resistance value of the other film resistor decreases, and vice versa. Based on the variation trend of  FIG. 3 , a continuous variable attenuator can be fabricated.  
         [0061]     The variable attenuator can be made into various package types, such as a surface mount type, a pin leg lead type, or a patch cord type.  
         [0062]     In addition, in accordance with the invention, a silicon rubber film that is conductive in the vertical direction can be added between the base  11  and the insulator  12  so as to stabilize the contact between the film resistor and the conductive sheet, and thereby, to avoid wear between the film resistor and the conductive sheet.  
         [0063]     Besides, in accordance with the invention, a groove can also be processed on the insulator  12 , and the conductive sheet  3  and the conductive sheet  4  are located inside of the groove. An elastic substance having a negligible influence on the high frequency and microwave characteristics is added between the conductive sheet  3  and the conductive sheet  4  acting for contact short-circuiting within the groove so as to stabilize the contact between the film resistor and the conductive sheet, and thereby, to avoid wear between the film resistor and the conductive sheet.  
         [0064]     The main feature of the variable attenuator of the invention is that in one plane (it can be multi layered), through the short-circuiting function of the conductive sheets, the resistance value of the film resistor  1  and the film resistor  2  can be simultaneously and flexibly changed in opposite directions. The conductive sheet  3 , the conductive sheet  4 , the film resistor  1 , and the film resistor  2  can be in the geometric shape of an arc, rectangular, or other shape. The variable attenuator of the invention is miniaturized and cost-effective, and is suitable for use in the upper microwave frequency band.  
         [0065]      FIGS. 6-7  illustrate the structural diagram of the microstrip variable attenuator and the structural diagram of the conductive sheet in accordance with the second embodiment of the invention, respectively, comprising: a base  101 , an input terminal  102  and an output terminal  103  located on the base  101 , an arc shaped strip film resistor  105 , an arc shaped strip film resistor  106 , an arc shaped strip film resistor  107 , a microstrip signal line  104 , a microstrip signal line  108 , a ground terminal  109  for the connection of microstrip signal lines. The microstrip variable attenuator further comprises a conductive sheet  110 , a conductive sheet  111 , and a conductive sheet  112  disposed on an insulator  113 . The insulator can also be a PCB board with conductive sheets disposed thereon. The PCB board can be in the shape of a circle for easy regulation. An arc mouth  115  is formed on the peripheral edge of the circle so as to limit the range of rotation regulation.  
         [0066]     The base can be a ceramic base or a PCB board that is convenient to use with microstrip resistors.  
         [0067]     One end of the microstrip signal line  104  is connected to the input terminal  102 , while the other end is connected to the film resistor  105 , and to one end of the film resistor  106  via the microstrip signal line  108 . The other end of the film resistor  105  is connected to the output terminal  103 . The other end of the film resistor  106  is connected to the ground terminal  109 . One end of the film resistor  107  is connected to the output terminal  103  via the microstrip signal line  108 , while the other end is connected to the ground terminal  109 .  
         [0068]     In certain embodiments of the invention, the film resistor  105 , the film resistor  106 , the film resistor  107  are all printed film resistors with the bottom side connected to the base  101  and the top side made of conductive and non-insulated material.  
         [0069]     Particularly, the resistance value of the film resistor  106  is equal or close to that of the film resistor  107 .  
         [0070]     The base can be multi-layered, the film resistors and the conductive sheets can be in the shape of a strip arc, rectangular, or other shape. Particularly, the shape of the conductive sheet is the same as or similar to that of the film resistor.  
         [0071]     The resistance value, Zo, is generally designed to be equal at the input and output terminals, for example, about 50 Ohms. The PCB board  113  and the base  101  share the same center  114 . The PCB board  113  is installed on the base according to the position of the arc mouth  115 , the side fixed with conductive sheets of the PCB board meets the base; a conductive sheet  110  for contact short-circuiting, and having the same shape as the microstrip signal line  104 , is located above the top side of the microstrip signal line  104 , and is fixed on the PCB board  113 , which is further fixed with a conductive sheet  111  and a conductive sheet  112 .  
         [0072]     The function of the conductive sheet  110  is to adjust the resistance value of the film resistor  105 . The function of the conductive sheet  111  is to adjust the resistance value of the film resistor  106 , while that of the conductive sheet  112  is to adjust the resistance value of the film resistor  107 . The conductive sheet  110 , the conductive sheet  111 , and the conductive sheet  112  rotate with the rotation of the PCB board  113 . For example, when the PCB board  113  rotates clockwise by an external force, the conductive sheet  110  rotates on and in contact with the microstrip signal line  104  towards the film resistor  105 , so that the contact area between the conductive sheet  110  and the film resistor  105  increases, and thus the resistance value of the film resistor  105  decreases. The conductive sheet  111  rotates on and in contact with the film resistor  106  towards the microstrip signal line  108 , so that the contact area between the conductive sheet  111  and the film resistor  106  decreases, and thus the resistance value of the film resistor  106  increases. The conductive sheet  112  rotates on and in contact with the film resistor  107  towards the microstrip signal line  108  so that the contact area between the conductive sheet  112  and the film resistor  107  decreases, and thus the resistance value of the film resistor  107  increases. Through the change in the geometric area, namely the change in the contact area between the conductive sheet and the film resistor, the actual effective resistance values of the film resistor  105 , the film resistor  106 , and the film resistor  107  can be changed.  
         [0073]     An arc mouth  115  is formed on the peripheral edge of the PCB board  113  to limit the range of the rotation regulation. When the PCB board  113  rotates clockwise, it is preferred that the maximum rotation angle of the PCB board  113  be maintained so as to make the conductive sheet  110  nearly or totally short-circuit the film resistor  105 , and the length (arc length) of the conductive sheet  110  should cover or nearly cover the film resistor  105 . It is preferred that the length (arc length) of the conductive sheet  111  should cover or nearly cover the film resistor  106 . It is preferred that the length (arc length) of the conductive sheet  112  should cover or nearly cover the film resistor  107 . Moreover, the spacing between the film resistor  106  and the film resistor  107  should be considered so that the conductive sheet  111  does not contact the film resistor  107  in the process of clockwise rotation.  
         [0074]     Similarly, when the PCB board is rotated counter-clockwise by an external force, it is restricted to rotate only within the range of the arc mouth  115  so as to ensure that the conductive sheet  112  does not contact the film resistor  106 . The design of the position of the conductive sheet  111  and the conductive sheet  112  at each of the maxima of rotational movement should account for the fact that that the effective resistance value of the film resistor  106  is equal or close to that of the film resistor  107 .  
         [0075]     The conductive sheet  110 , the conductive sheet  111 , and the conductive sheet  112  can also be film resistors, which overlap and are electrically connected, and so can be regarded as three resistors in parallel. Similarly, the resistance value of the film resistors can be changed and the same effect can be achieved. However, it is required that the conductive sheet  110  can only be used to electrically contact the film resistor  105  to change the resistance value thereof, and cannot directly contact other film resistors. It is required that the conductive sheet  111  can only be used to electrically contact the film resistor  106  to change the resistance value thereof, and cannot directly contact other film resistors. It is required that the conductive sheet  112  can only be used to electrically contact the film resistor  107  to change the resistance value thereof, and cannot directly contact other film resistors. This design can be realized by using multi-layered PCB board so as to keep the basic principle and structure of the microstrip variable attenuator.  
         [0076]     With reference to  FIG. 8 , the equivalent circuit diagram of the microstrip variable attenuator according to a second embodiment of the invention is equivalent to that of a continuous variable π-shaped attenuator being a symmetric wide band network with interchangeable input and output terminals.  
         [0077]      FIG. 9  illustrates an ideal theoretical variation curve of the film resistor  105 , the film resistor  106 , and the film resistor  107  when the PCB board  113  is rotated clockwise by an external force. The variation trend of the resistance value of the film resistor  105  is opposite to those of the film resistor  106  and the film resistor  107 .  
         [0078]      FIG. 10  illustrates a line showing the attenuation amount of the variable attenuator fabricated according to the curve of  FIG. 9  when the PCB board  113  is rotated clockwise. During designing and fabricating, the film resistor  105 , the film resistor  106 , and the film resistor  107  are chosen according to the curve of  FIG. 9  so as to realize variation in the attenuation amount, which is required when the displacement of the variable attenuator is changed.  
         [0079]     The force to change the geometrical position of the conductive sheet  110 , the conductive sheet  111 , and the conductive sheet  112  can be a mechanical manual force, an automatic controlled mechanical force, an electromagnetic force, a force produced by heat or temperature, a force produced by the flow, expansion, or contraction of a liquid, or a force initiated by an optoelectronic excitation process.  
         [0080]     The microstrip variable attenuator can be made into various package types, such as a surface mount type, a pin leg lead type, or a patch cord type.  
         [0081]     The insulator and the conductive sheets of the present invention can be made of PCB board. The PCB board in the specified embodiments is a circle PCB board with an open arc mouth on its peripheral edge, and is concentric to the base for easy regulation. A block can be added on one end of the arc mouth to limit the rotation range of the PCB board so as to realize the optimal conformity, precise positioning, and precise regulation of the film resistors and the conductive sheets. Besides, an elastic film that is rigid in the rotation direction and is elastic in the vertical direction can be added between the PCB board  113  and the enclosure so as to keep the position of the PCB board after regulation and to stabilize the contact between the film resistor and the conductive sheet.  
         [0082]      FIGS. 11-12  illustrate the structural diagram of the microstrip variable attenuator and the structural diagram of the conductive sheet in accordance with the third embodiment of the invention, respectively, comprising: a base  229 , an input terminal  216  and an output terminal  217  located on the base, an arc shaped strip film resistor  219 , an arc shaped strip film resistor  220 , an arc shaped strip film resistor  221 , a microstrip signal line  218   a,  a microstrip signal line  218 , a ground terminal  222  for the connection of the microstrip signal lines. The microstrip variable attenuator further comprises a conductive sheet  223 , a conductive sheet  224 , and a conductive sheet  225  disposed on an insulator  227 .  
         [0083]     A PCB board can also replace the insulator with conductive sheets disposed thereon. The PCB board can be in a circular shape for easy regulation. An arc mouth  228  is formed on the peripheral edge of the circle so as to limit the range of rotation regulation.  
         [0084]     The base can be a ceramic base or a PCB board that is easy to process for microstrip resistors.  
         [0085]     The input terminal  216  is connected to one end of the film resistor  219  via the microstrip signal line  218   a , the other end of the film resistor  219  is connected to one end of the film resistor  220  via the microstrip signal line  218 , and is connected to one end of the film resistor  221 , the other end of the film resistor  221  is connected to the ground terminal  222 ; the other end of the film resistor  220  is connected to the output terminal  217  via the microstrip signal line.  
         [0086]     In certain embodiments of the invention, the film resistor  219 , the film resistor  220 , the film resistor  221  are all printed film resistors with the bottom side connected to the base  229  and the top side made of conductive and non-insulated material.  
         [0087]     Particularly, the resistance value of the film resistor  219  is equal or close to that of the film resistor  220 .  
         [0088]     The base can be multi-layered, the film resistors and the conductive sheets can be in the shape of a strip arc, rectangular, or other shape. Particularly, the shape of the conductive sheet is the same as or similar to that of the film resistor.  
         [0089]     The resistance value, Zo, is generally designed to be equal at the input and output terminals, for example, about 50 Ohms. The PCB board  227  and the base  229  share the same center  226 . The PCB board  227  is installed on the base according to the position of the arc mouth  228 , the side fixed with conductive sheets of the PCB board meets the base; a conductive sheet  223  for contact short-circuiting, and having the same shape as the microstrip signal line  218   a , is located above the top side of the microstrip signal line  218   a , and is fixed on the PCB board  227 , which is further fixed with a conductive sheet  224  and a conductive sheet  225 .  
         [0090]     The function of the conductive sheet  223  is to adjust the resistance value of the film resistor  219 . The function of the conductive sheet  224  is to adjust the resistance value of the film resistor  220 , while that of the conductive sheet  225  is to adjust the resistance value of the film resistor  221 . The conductive sheet  223 , the conductive sheet  224 , and the conductive sheet  225  rotate with the rotation of the PCB board  227 . For example, when the PCB board  227  is rotated clockwise by an external force, the conductive sheet  223  rotates in contact from the microstrip signal line toward the film resistor  219 , so that the contact area between the conductive sheet  223  and the film resistor  219  increases, and thus the resistance value of the film resistor  219  decreases. The conductive sheet  224  rotates in contact from the microstrip signal line toward the film resistor  220 , so that the contact area between the conductive sheet  224  and the film resistor  220  increases, and thus the resistance value of the film resistor  220  decreases. The conductive sheet  225  rotates in contact from the film resistor  221  towards the microstrip signal line so that the contact area between the conductive sheet  225  and the film resistor  221  decreases, and thus the resistance value of the film resistor  221  increases. Through the change in the geometric area, namely the change in the contact area between the conductive sheet and the film resistor, the actual effective resistance values of the film resistor  219 , the film resistor  220 , and the film resistor  221  can be changed.  
         [0091]     An arc mouth  228  is formed on the peripheral edge of the PCB board  227  to limit the range of the rotation regulation. When the PCB board  227  rotates clockwise, it is preferred that the maximum rotation angle of the PCB board  227  be maintained so as to make the conductive sheet  223  nearly or totally short-circuit the film resistor  219 , and the length (arc length) of the conductive sheet  223  should cover or nearly cover the film resistor  219 . Moreover, the spacing between the film resistor  219  and the film resistor  220  should be taken into account so that the conductive sheet  223  does not contact the film resistor  220  in the process of clockwise rotation. Similarly, when the PCB board  227  is rotated counter-clockwise by an external force, it can only rotate within the range of the arc mouth  228  so as to ensure that the conductive sheet  224  does not contact the film resistor  219 .  
         [0092]     The design of position of the conductive sheet  223  and the conductive sheet  224  at each of the maxima of the rotational movement, respectively, should account for the fact that the effective resistance value of the film resistor  219  is equal or close to that of the film resistor  220 .  
         [0093]     The conductive sheets can also be film resistors, which overlap and are electrically connected, and so can be regarded as three resistors in parallel. Similarly, the resistance value of the film resistor can be changed and the same effect can be achieved. However, it is required that the conductive sheet  223  can only be used to electrically contact the film resistor  219  to change the resistance value thereof, and does not directly contact other film resistors. It is required that the conductive sheet  224  can only be used to electrically contact the film resistor  220  to change the resistance value thereof, and does not directly contact other film resistors. It is required that the conductive sheet  225  can only be used to electrically contact the film resistor  221  to change the resistance value thereof, and does not directly contact other film resistors. This design can be realized by using multi-layered PCB board so as to keep the basic principle and structure of the microstrip variable attenuator.  
         [0094]     With reference to  FIG. 13 , the equivalent circuit diagram of the microstrip variable attenuator according to the third embodiment of the invention is equivalent to that of a continuous variable T-shaped attenuator being a symmetric wide band network with interchangeable input and output terminals.  
         [0095]      FIG. 14  illustrates an ideal theoretical variation curve of the film resistor  219 , the film resistor  220 , and the film resistor  221  when the PCB board  227  is rotated clockwise by an external force. The variation trend of the resistance value of the film resistor  219  and of the film resistor  220  is opposite to that of the film resistor  221 .  
         [0096]      FIG. 15  illustrates a line showing the attenuation amount of the variable attenuator fabricated according to the curve of  FIG. 14  when the PCB board  227  is rotated clockwise. During designing and fabricating, the film resistor  219 , the film resistor  220 , and the film resistor  221  are chosen according to the curve of  FIG. 14  so as to realize variation in the attenuation amount, which is required when the displacement of the variable attenuator is changed.  
         [0097]     The force to change the geometrical position of the conductive sheet  223 , the conductive sheet  224 , and the conductive sheet  225  is a mechanical manual force, an automatic controlled mechanical force, an electromagnetic force, a force produced by heat or temperature, a force produced by the flow, expansion, or contraction of a liquid, or a force initiated by an optoelectronic excitation process.  
         [0098]     The microstrip variable attenuator can be made into various package types, such as a surface mount type, a pin leg lead type, or a patch cord type.  
         [0099]     The insulator and the conductive sheets of the present invention can be made of PCB board. The PCB board in the specified embodiments is a circle PCB board with an open arc mouth on its peripheral edge, and is concentric to the base for easy regulation. A block can be added on one end of the arc mouth to limit the rotation range of the PCB board so as to realize the optimal conformity, precise positioning, and precise regulation of the film resistors and the conductive sheets. Besides, an elastic film that is rigid in the rotation direction and is elastic in the vertical direction can be added between the PCB board  227  and the enclosure so as to keep the position of the PCB board after regulation and to stabilize the contact between the film resistor and the conductive sheet.  
         [0100]     This invention is not to be limited to the specific embodiments disclosed herein and modifications for various applications and other embodiments are intended to be included within the scope of the appended claims. While this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.  
         [0101]     All publications and patent applications mentioned in this specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application mentioned in this specification was specifically and individually indicated to be incorporated by reference.