Abstract:
Disclosed is a digital attenuator, and more particularly, a digital attenuator for improving a performance in various respects, and also provided is a structure for reducing a phase difference of the digital attenuator by equipping an inductor as a phase retardation element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of Korean Patent Application No. 10-2012-0014312, filed on Feb. 13, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a digital attenuator, and more particularly, to a digital attenuator for enhancing a performance in various respects. 
         [0004]    2. Description of the Related Art 
         [0005]    In general, a digital attenuator is used to generate change in a signal magnitude in a microwave band. More particularly, the digital attenuator may have a specific attenuation value based on an operation state of a switching element used in a circuit configuration. A Monolithic Microwave Integrated Circuit (MMIC) attenuator may obtain an attenuation value of greater than 8 decibels (dB) in an S-band that is in a range of 2 to 4 gigahertz (GHz), without a phase difference based on a state transition. 
         [0006]    In general, achieving an attenuation quantity corresponding to a standard may be necessary for the digital attenuator, without a difference. More particularly, the digital attenuator may be utilized for adjusting a relative magnitude of input/output signals of an array element of an antenna in a system requiring a control of an antenna beam direction such as a phase array antenna system or a radar system. A high accuracy in the attenuating quantity of the digital attenuator being present, and an absence of an occurrence of a phase difference during attenuating operation may also be important since the digital attenuator controls the signal magnitude and a phase simultaneously. With current developments of minimizing the phase array antenna system or the radar system, use of an MMIC attenuator that is easy to control and relatively accurate in attenuating is widespread. 
         [0007]    Conventionally, a digital MMIC attenuator may be embodied in a structure using a switching element, a Pi-type attenuator circuit, or a T-type attenuator circuit. The digital MMIC attenuator may have a switched path structure for achieving a desired attenuation value by changing a signal path. 
         [0008]      FIGS. 1 and 2  are diagrams illustrating a conventional digital attenuator. 
         [0009]    Referring to  FIG. 1 , the conventional digital attenuator, that is, a switched T-type attenuator may include a plurality of field effect transistor (FET) switching elements  103  and  104  and a plurality of resistance elements  105  to  107  of a T-type attenuator. The plurality of the resistance elements  105  to  107  may be resistance elements that configure the T-type attenuator. The plurality of FET switching elements  103  and  104  may perform complementary operation of ON/OFF states. 
         [0010]    In the conventional digital attenuator of  FIG. 1 , when a first FET switching element  103  is switched ON, and a second FET switching element  104  is switched OFF, a signal path may be in an arrangement of an input  101 , the first switching element  103 , and an output  102 . Such a state may be referred to as a reference state. When the first FET switching element  103  is switched OFF, and the second FET switching element is switched ON, the signal path may be in an arrangement of the input  101 , the resistance elements  105  to  107  of the T-type attenuator, and the output  102 . Such a state may be referred to as an attenuation state. 
         [0011]    In such a conventional digital attenuator, a phase difference between the reference state and the attenuation state may occur, and the phase difference may increase in proportion to an attenuation value of the T-type attenuator. 
         [0012]    Referring to  FIG. 2 , a conventional digital attenuator, that is, a low phase variation switched T-type attenuator, to which a low phase variation is applied, is provided. The conventional digital attenuator of  FIG. 2  may decrease a phase difference by inserting a different length of transmission lines  211  to  213  in order to remove the phase difference of the conventional digital attenuator of  FIG. 1 . However, in a case of an attenuator that operates in a relatively low frequency band in a range of 2 to 4 GHz, such as in an S-band, a considerably long transmission line may be required for reducing the phase difference. Accordingly, problems may arise in embodying such an attenuator, and a performance of the attenuator may be degraded, for example, an insertion loss. 
       SUMMARY 
       [0013]    An aspect of the present invention provides a structure for reducing a phase difference of a digital attenuator by equipping an inductor as a phase retardation element. 
         [0014]    Another aspect of the present invention also provides a digital attenuator that is able to attenuate in a relatively low frequency band such as in an S-band, and has a small phase difference. 
         [0015]    According to an aspect of the present invention, there is provided an attenuator, including a switching element connected in parallel between a signal input end and a signal output end, at least one resistance element connected via a different path between the signal input end and the signal output end, and an inductor connected in series between at least one of the signal input end and the signal output end and the at least one resistance element. 
         [0016]    The inductor may include a first inductor connected in series between the signal input end and the at least one resistance element, and a second inductor connected in series between the at least one resistance element and the signal output end. 
         [0017]    Each of the at least one resistance element is a resistance element of a T-type attenuator, and the inductor is connected to each of both ends of the T-type attenuator. 
         [0018]    According to another aspect of the present invention, there is provided a Monolithic Microwave Integrated Circuit (MMIC) attenuating apparatus for processing an S-band signal, the attenuating apparatus including a switching element connected in parallel between a signal input end and a signal output end with respect to the S-band signal, at least one resistance element connected via a different path between the signal input end and the signal output end, and an inductor connected in series between at least one of the signal input end and the signal output end and the at least one resistance element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    These and/or other aspects, features, and advantages of the invention will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
           [0020]      FIGS. 1 and 2  are diagrams illustrating a conventional digital attenuator; 
           [0021]      FIG. 3  is a diagram illustrating a configuration of a digital attenuator according to an embodiment of the present invention; 
           [0022]      FIG. 4  is a diagram illustrating another digital attenuator to describe a performance of the digital attenuator of  FIG. 3 ; and 
           [0023]      FIG. 5  is a diagram illustrating an example of a result of simulating the digital attenuators of  FIGS. 3 and 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Exemplary embodiments are described below to explain the present invention by referring to the figures. 
         [0025]      FIG. 3  is a diagram illustrating a configuration of a digital attenuator according to an embodiment of the present invention. 
         [0026]    Referring to  FIG. 3 , as a first embodiment, the digital attenuator  300  may include at least four of field effect transistor (FET) switching elements  303  to  306 , a plurality of resistance elements  309  to  311  configuring a T-type attenuator, and a plurality of inductors  307  and  308 . Here, the plurality of inductors  307  and  308  may be inserted to correct a difference of a reference state path and a difference of an attenuation state path. 
         [0027]    Here, when a first FET switching element  303  and a second FET switching element  304  are switched ON, and a third FET switching element  305  and a fourth FET switching element  306  are switched OFF, a signal path may be in a sequence of an input  301 , the first FET switching element  303 , the second FET switching element  304 , and an output  302 . Such a state may be referred to as a reference state. Also, when the first FET switching element  303  and the second FET switching element  304  are switched OFF, and the third FET switching element  305  and the fourth FET switching element  306  are switched on, the signal path may be in a sequence of the input  301 , the third FET switching element  305 , a first inductor  307 , the plurality of resistance elements  309  to  311  configuring the T-type attenuator, a second inductor  308 , the fourth FET switching element  306 , and the output  302 . Such an instance may be referred to as an attenuation state. 
         [0028]    That is, the digital attenuator  300  may reduce a phase difference of a signal that is output in the attenuation state, by disposing at least one inductor in the attenuation state path. 
         [0029]    That is, the digital attenuator  300  may include at least one switching element connected in parallel between a signal input end and a signal output end, at least one resistance element connected via a different path between the signal input end and the signal output end, and an inductor connected in series between at least one of the signal input end and the signal output end and the at least one resistance element. 
         [0030]    More particularly, the inductor may include a first inductor connected in series between the signal input end and the at least one resistance element, and a second inductor connected in series between the at least one resistance element and the signal output end. 
         [0031]    Also, each of the at least one resistance element may be a resistance element of the T-type attenuator, and the inductor may be connected to each of both ends of the T-type attenuator. 
         [0032]    For example, when the inductors  307  and  308  are not disposed in the digital attenuator  300 , the digital attenuator  300  may be embodied as shown in  FIG. 4 . That is,  FIG. 4  is a diagram illustrating another digital attenuator to describe a performance of the digital attenuator  300  of  FIG. 3 . 
         [0033]    Referring to  FIG. 4 , the digital attenuator  400  is not equipped with an inductor unlike the digital attenuator  300  of  FIG. 3 . That is, the digital attenuator  400  may include at least four of FET switching elements  403  to  406 , and a plurality of resistance elements  407  to  409  configuring a T-type attenuator. 
         [0034]    In a case of a reference state, a signal of the digital attenuator  400  may be transmitted as in a case of a signal of the digital attenuator  300 . However, in a case of an attenuation state, the signal of the digital attenuator  400  may be transmitted in a sequence of an input  401 , a third FET switching element  405 , the plurality of resistance elements  407  to  409  configuring the T-type attenuator, a fourth FET switching element  406 , and an output  402 , without passing through the inductor. 
         [0035]    Accordingly, in the digital attenuator  400 , a signal may be attenuated in the attenuation state, however, a phase of a signal passing through an attenuation state path may precede a phase of a signal passing through a reference state path. 
         [0036]      FIG. 5  is a diagram illustrating an example of a result of simulating the digital attenuators of  FIGS. 3 and 4 . According to an embodiment, the simulation of the digital attenuators is performed at an attenuation value of 16 dB. In graphs  501  to  503 , a result of the simulation may illustrate a difference of an attenuation value of a signal passing through a reference state path and an attenuation value of a signal passing through an attenuation state path. In graphs  502  to  504 , a phase difference of the signal passing through the reference state path and the signal passing through the attenuation state path is illustrated. 
         [0037]    Referring to the graph  501  of  FIG. 5 , the digital attenuator  400  of  FIG. 4  illustrates that an attenuation is performed accurately in an S-band frequency. However, referring to  502 , in the digital attenuator  400 , a phase difference of a signal passing through a reference state path and a signal passing through an attenuation state path is approximately 13 degrees in 2.6 GHz, that is, a center frequency. 
         [0038]    In the digital attenuator  400 , the transmission line of approximately 1500 micrometers (μm) may need to be inserted in order to reduce the difference phase, by using a method of a conventional digital attenuator in which a transmission line is inserted. However, such a structure may be difficult to embody in a minimized MMIC, and although embodied, an issue such as an insertion loss, a decreased bandwidth for use, and the like may arise. 
         [0039]    The digital attenuator  300  of  FIG. 3  according to embodiments of the present invention may equip at least one inductor to reduce a phase difference of a signal passing through an attenuation state path. More particularly, the digital attenuator  300  may insert an inductor for a phase retardation between a resistance element of a T-type attenuator and an FET switching element. That is, the inductor may reduce the phase difference between the signal passing through the attenuation state path and a signal passing through a reference state path at both ends of the T-type attenuator. 
         [0040]    Referring to a graph  503  of  FIG. 5 , the digital attenuator  300  illustrates an accurately performed attenuation in which approximately 16 dB of an attenuation value in an S-band frequency is obtained as shown in the graph  501 . 
         [0041]    However, referring to a graph  504 , the digital attenuator  300  may have the phase difference of ±0.15 degrees in a band in a range of 2.2 to 3.0 GHz. Accordingly, the digital attenuator  300  may have a small phase difference, when compared to the graph  503 . 
         [0042]    Such a digital attenuator may be provided in a relatively small size of 600 μm×400 μm as an embodiment. 
         [0043]    The above-described exemplary embodiments of the present invention may be recorded in computer-readable media including program instructions to implement various operations embodied by a computer. The media may also include, alone or in combination with the program instructions, data files, data structures, and the like. Examples of computer-readable media include magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD ROM discs and DVDs; magneto-optical media such as floptical discs; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory (ROM), random access memory (RAM), flash memory, and the like. Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter. The described hardware devices may be configured to act as one or more software modules in order to perform the operations of the above-described exemplary embodiments of the present invention, or vice versa. 
         [0044]    Although a few exemplary embodiments of the present invention have been shown and described, the present invention is not limited to the described exemplary embodiments. Instead, it would be appreciated by those skilled in the art that changes may be made to these exemplary embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.