Abstract:
A single-pole-double-throw switch is provided, which is configured to be integrated with a bandpass filtering function and includes four quarter-wavelength transmission lines connected in series, five resonators connected in parallel to each other, and four transistors connected in parallel to four of the five resonators. When two of the four transistors are turned on and the others are turned off, the single-pole-double-throw switch is equivalent to a third-order quarter-wavelength short-circuited stub bandpass filter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to switches, and more particularly, to a single-pole-double-throw switch integrated with a bandpass filtering function. 
       BACKGROUND OF THE INVENTION 
       [0002]    The quality of a time-division-duplex wireless communication system is greatly influenced by a radio frequency (RF) switch. In order to compensate for the undesirable characteristics of the switch (e.g. the on-state resistance and off-state capacitance), prior art adopts a parallel-resonator configuration to enable resonant of inductance and parasitic capacitance, as disclosed in, for example, “A high performance V-band monolithic FET transmit-receive switch” in 1988  IEEE Microwave and Millimeter - wave Monolithic Circuits Symp. Dig.,  New York, N.Y./USA, June 1988, pp. 99-101; “W-band SPST transistor switches”,  IEEE Microwave and Guided Wave Lett.,  vol. 6, pp. 315-316, September 1996; “A sub-nanosecond resonant-type monolithic T/R switch for millimeter-wave systems applications”, IEEE  Trans. On Microwave Theory and Tech.,  vol. 46, no. 7, pp. 1016-1019, July 1998; and U.S. Pat. No. 7,239,858, entitled “Integrated Switching Device For Routing Radio Frequency Signals”, or adopts an impedance transformation network to switch the resistance and capacitance of the switch, as disclosed in, for example, “Millimeter-wave MMIC single-pole-double-throw passive HEMT switches using impedance transformation networks”,  IEEE Trans. Microwave Theory Tech.,  vol. 51, pp. 1076-1085, April 2003; and U.S. Pat. No. 6,801,108, entitled “A Millimeter-wave Switch Using Impedance Transformation Networks”. However, the above conventional techniques can only compensate for the resistance and capacitance of particular frequencies, but they fail to consider the frequency response of the overall system. 
         [0003]    In “Millimeter-wave MMIC passive HEMT switches using traveling-wave concept” (referring to  IEEE Trans. Microwave Theory and Tech.,  vol. 52, no. 8, pp. 1798-1808, August 2004), a traveling-wave switch configuration is proposed, which integrates additional inductance into an artificial transmission line. This configuration allows integration of the undesirable characteristics into the transmission line, and thus the switch may have a wideband frequency response and good switching characteristics. 
         [0004]    Since the undesirable characteristics of the switch are equivalent to lumped elements, U.S. Pat. No. 7,106,146 (entitled “RF Switch”) performs effective impedance matching with these equivalent lumped elements. Accordingly, other techniques have been proposed to replace the elements in a filter with switching elements, so that the filter may assume the characteristic of a single-pole-single-throw switch, as can be found in, for example, “Theoretical and Experimental Investigation of Novel Varactor-Tuned Switchable Microstrip Ring Resonator Circuits”,  IEEE Trans. Microwave Theory and Tech.,  vol. 36, no. 12, December 1988, pp. 1733-1739; “A band-pass filter-integrated switch using field-effect transistors and its power analysis”, in 2006  IEEE MTT - S Int. Microwave Symp. Dig.,  San Francisco, Calif./USA, 2006; and “New millimeter-wave MMIC switch design using the image-filter synthesis method”,  IEEE Microwave and Wireless Component Lett.,  vol. 14, pp. 103-105, March 2004. 
         [0005]    For example, in the above prior art, “A band-pass filter-integrated switch using field-effect transistors and its power analysis”, a quarter-wavelength impedance transformer  12  is used to integrate two single-pole-single-throw traveling-wave switches  14  and  16  into a single-pole-double-throw switch  10 , as shown in  FIG. 1 . Similar integration can be applied to single-pole-five-throw switches, for example, in U.S. Pat. No. 7,106,146, entitled “High Frequency Switch”. However, as limited by the quarter-wavelength impedance transformer  12 , the frequency response of the single-pole-double-throw switch  10  cannot be synthesized. This is because the single-pole-double-throw switch  10  must include two single-pole-single-throw switches  14  and  16 , and the impedances and frequency responses of the two single-pole-single-throw switches  14  and  16  may affect each other. The impedance transformer  12  may alleviate this influence. Nonetheless, the frequency response of the impedance transformer  12  itself may still influence the frequency responses of the single-pole-single-throw switches  14  and  16 . Therefore, the filter function cannot be effectively integrated into the single-pole-double-throw switch  10 . 
       SUMMARY OF THE INVENTION 
       [0006]    In light of foregoing drawbacks, an objective of the present invention is to provide a single-pole-double-throw switch integrated with a bandpass filtering function, which integrates the bandpass filtering function into the switch by taking advantage of the undesirable characteristics of the switch. 
         [0007]    In accordance with the above and other objectives, the present invention provides a single-pole-double-throw switch integrated with a bandpass filtering function, comprising: a first transmission line; a second transmission line with a first end being coupled to a second end of the first transmission line; a third transmission line with a first end being coupled to a second end of the second transmission line; a fourth transmission line with a first end being coupled to a second end of the third transmission line; a first resonator with a first end being coupled to a first end of the first transmission line and an opposing second end being grounded; a first transistor having a drain being coupled to the first end of the first transmission line, a source being grounded, and a gate for receiving a first selection signal; a second resonator with a first end being coupled to the second end of the first transmission line and an opposing second end being grounded; a second transistor having a drain being coupled to the second end of the first transmission line, a source being grounded, and a gate for receiving the first selection signal; a third resonator with a first end being coupled to the first end of the fourth transmission line and an opposing second end being grounded; a third transistor having a drain being coupled to the first end of the fourth transmission line, a source being grounded, and a gate for receiving a second selection signal; a fourth resonator with a first end being coupled to a second end of the fourth transmission line and an opposing second end being grounded; a fourth transistor having a drain being coupled to the second end of the fourth transmission line, a source being grounded, and a gate for receiving the second selection signal; and a fifth resonator with a first end being coupled to the second end of the second transmission line and an opposing second end being grounded, wherein the first transmission line, the second transmission line, the third transmission line and the fourth transmission line are of length equal to a quarter of a wavelength of the RF signals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
           [0009]      FIG. 1  is a functional block diagram of a conventional single-pole-double-throw switch; 
           [0010]      FIG. 2  is a circuit diagram of a single-pole-double-throw switch according to an embodiment of the present invention; 
           [0011]      FIG. 3  is an equivalent functional block diagram of the single-pole-double-throw switch of  FIG. 2 ; 
           [0012]      FIG. 4  is an equivalent circuit diagram of the single-pole-double-throw switch of  FIG. 2 ; and 
           [0013]      FIG. 5  is a circuit diagram of an equivalent bandpass filter of the single-pole-double-throw switch of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0014]    The present invention is described by the following specific embodiments. Those with ordinary skills in the arts can readily understand the other advantages and functions of the present invention after reading the disclosure of this specification. The present invention can also be implemented with different embodiments. Various details described in this specification can be modified based on different viewpoints and applications without departing from the scope of the present invention. 
         [0015]    Referring to  FIGS. 2 and 3 ,  FIG. 2  is a circuit diagram illustrating a single-pole-double-throw switch  20  integrated with a bandpass filtering function according to an embodiment of the present invention, and  FIG. 3  is a functional block diagram of the single-pole-double-throw switch  20 . The single-pole-double-throw switch  20  is used to pass (receive/transmit (R/T)) radio frequency (RF) signals. For instance, when the single-pole-double-throw switch  20  is switched by connecting a first port Port 1  to a second port Port 2 , the first port Port 1  receives RF signals transmitted from the second port Port 2 . Conversely, when the single-pole-double-throw switch  20  is switched by connecting the first port Port 1  to a third port Port 3 , the first port Port 1  transmits RF signals to the third port Port 3 . 
         [0016]    As shown in  FIG. 2 , the single-pole-double-throw switch  20  includes a first transmission line  22 ; a second transmission line  24  with a first end  242  being coupled to a second end  224  of the first transmission line  22 ; a third transmission line  26  with a first end  262  being coupled to a second end  244  of the second transmission line  24 ; a fourth transmission line  28  with a first end  282  being coupled to a second end  264  of the third transmission line  26 ; a first resonator  30  with an end being coupled to a first end  222  of the first transmission line  22  and an opposing end being grounded; a first transistor  32  having a drain  322  being coupled to the first end  222  of the first transmission line  22 , a source  324  being grounded, and a gate  326  for receiving a first selection signal V c1  via a first resistor R 1 ; a second resonator  34  with an end being coupled to the second end  224  of the first transmission line  22  and an opposing end being grounded; a second transistor  36  having a drain  362  being coupled to the second end  224  of the first transmission line  22 , a source  364  being grounded, and a gate  366  for receiving the first selection signal V c1  via a second resistor R 2 ; a third resonator  38  with an end being coupled to the first end  282  of the fourth transmission line  28  and an opposing end being grounded; a third transistor  40  having a drain  402  being coupled to the first end  282  of the fourth transmission line  28 , a source  404  being grounded, and a gate  406  for receiving a second selection signal V c2  via a third resistor R 3 ; a fourth resonator  42  with an end being coupled to a second end  284  of the fourth transmission line  28  and an opposing end being grounded; a fourth transistor  44  having a drain  442  being coupled to the second end  284  of the fourth transmission line  28 , a source  444  being grounded, and a gate  446  for receiving the second selection signal V c2  via a fourth resistor R 4 ; and a fifth resonator  46  with an end being coupled to the second end  244  of the second transmission line  24  and an opposing end being grounded, wherein the first transmission line  22 , the second transmission line  24 , the third transmission line  26  and the fourth transmission line  28  are of length equal to a quarter of a wavelength λ of the RF signals (i.e. ¼λ). 
         [0017]    When the first selection signal V c1  is lower than the threshold voltages of the first transistor  32  and the second transistor  36 , and when the second selection signal V c2  is higher than the threshold voltages of the third transistor  40  and the fourth transistor  44 , the first transistor  32  and the second transistor  36  are turned off, and the third transistor  40  and the fourth transistor  44  are turned on. Thus, the first transistor  32  and the second transistor  36  are equivalent to a first capacitance C off1  and a second capacitance C off2 , respectively, while the third transistor  40  and the fourth transistor  44  are equivalent to a second on-state resistance G on2  and a first on-state resistance G on1 , respectively, as shown in  FIG. 4 . 
         [0018]    The RF signal from the first port Port 1  to the second on-state resistance G on2  via the third transmission line  26  would be reflected by ground, and returned to the first port Port 1  via the third transmission line  26 , which cancels another RF signal subsequently coming from the first port Port 1  to the second on-state resistance G on2  via the third transmission line  26 . As such, RF signals would equivalently be transmitted between the first port Port 1  and the second port Port 2 , rather than between the first port Port 1  and the third port Port 3 . Thus, the single-pole-double-throw switch  20  is equivalent to a third-order quarter-wavelength short-circuited stub bandpass filter  20 ′ shown in  FIG. 5 . 
         [0019]    In  FIG. 5 , the third-order quarter-wavelength short-circuited stub bandpass filter  20 ′ includes the first transmission line  22 ; the second transmission line  24  with the first end  242  being coupled to the second end  224  of the first transmission line  22 ; a sixth resonator  48  with an end being coupled to the first end  222  of the first transmission line  22  and an opposing end being grounded; a seventh resonator  50  with an end being coupled to the first end  242  of the second transmission line  24  and an opposing end being grounded; and an eighth resonator  52  with an end being coupled to the second end  244  of the second transmission line  24  and an opposing end being grounded. 
         [0020]    Since the third-order quarter-wavelength short-circuited stub bandpass filter  20 ′ shown in  FIG. 5  is equivalent to the single-pole-double-throw switch  20  shown in  FIG. 4 , the susceptances Y Rf1 , Y Rf2  and Y Rf3  of the respective sixth, seventh and eighth resonators  48 ,  50  and  52  and the differential values of the susceptances Y Rf1 , Y Rf2  and Y Rf3  at the central frequency ω 0  are equal to the susceptances Y R1 , Y R2  and Y R3  of the respective first, second and fifth resonators  30 ,  34  and  46  and the differential values of the susceptances Y Rf1 , Y Rf2  and Y Rf3  at the central frequency ω 0 , respectively. 
         [0021]    Accordingly, the design parameters of the third-order quarter-wavelength short-circuited stub bandpass filter  20 ′ and the single-pole-double-throw switch  20  should satisfy the following equations: 
         [0000]        Im ( Y   R1 )= Im ( Y   Rf1 )=ω 0   C   off1   −-Y   1  cot θ 1 =0   (1) 
         [0000]        Im ( Y   R2 )= Im ( Y   Rf2 )=ω 0   C   off2   −-Y   2  cot θ 2 =0   (2) 
         [0000]        Im ( Y   R3 )= Im ( Y   Rf3 )= Im ( Y   12   2   /Y   iso   −jY   3  cot θ 3 )=0   (3) 
         [0000]    wherein Y 12  is the admittance of the third transmission line  26 ; Y 1 , Y 2  and Y 3  are the admittances of the first, second and fifth resonators  30 ,  34  and  46 , respectively; θ 1 , θ 2 , and θ 3  are the phase shifts of the first, second and fifth resonators  30 ,  34  and  46 , respectively; and Y iso  in Equation (3) is the admittance from the on-state third transistor  40  to the isolated second port Port 2 . Since the third and fourth transistors  40  and  44  are turned on, the second and first on-state resistances G on2  and G on1  have very large conductance. Thus, 
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         [0022]    Since the differential values of these susceptances should be equal to each other, therefore, 
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         [0023]    By adopting a filter synthesis technique, the third-order quarter-wavelength short-circuited stub bandpass filter  20 ′ can be designed to have design parameters Y 12 , Y Rf1 , Y Rf2  and Y Rf3 , etc. When the device size is determined, C off1 , C off2 , G off3 , and G on2  can be calculated. Next, the design parameters Y 12 , Y 1 , Y 2 , Y 3 , θ 1 , θ 2  and θ 3  can then be calculated from Equations (1) to (7). 
         [0024]    When calculating insertion loss S 21  from the first port Port 1  via the second transmission line  24  and the first transmission line  22  to the second port Port 2 , only the second on-state resistance G on2  is considered. As can be seen from Equations (1) to (3), Y R1 , Y R2 , and Y R3  are all zero at ω 0 . Thus, the insertion loss S 21  can be expressed as: 
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         [0025]    Similarly, insertion loss S 31  from the first port Port 1  to the third port Port 3  can be calculated. Since 
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         [0026]    As can be seen from Equations (8) and (9), increasing the second on-state resistance G on2  and the first on-state resistance G on1  improves the insertion losses S 21  and S 31  as well as the degree of isolation. 
         [0027]    It should be noted that in order for Equations (1) to (7) to have a solution, the first capacitance C off1  and the second capacitance C off2  should fall within a reasonable range. Moreover, since the first capacitance C off1  and the second capacitance C off2  are the off-state channel resistances of the first transistor  32  and the second transistor  36 , respectively, the second on-state resistance G on2  and the first on-state resistance G on1  are the on-state channel resistances of the third transistor  40  and the fourth transistor  44 , respectively, and the first capacitance C Off1 , the second capacitance C Off2 , the second on-state resistance G on2  and the first on-state resistance G on1  are proportional to the widths of the gates  326 ,  366 ,  406  and  446  of the first, second, third and fourth transistors  32 ,  36 ,  40  and  44 , respectively, the first, second, third and fourth transistors  32 ,  36 ,  40  and  44  have to be selected properly in order for Equations (1) to (7) to be solvable. 
         [0028]    In the single-pole-double-throw switch  20  shown in  FIG. 2 , the first resonator  30  is identical to the fourth resonator  42 , and the second resonator  34  is identical to the third resonator  38 . In other words, the equivalent bandpass filters in the cases where the single-pole-double-throw switch  20  is receiving (Port 1  connected to Port 2 ) or transmitting (Port 1  connected to Port 3 ) RF signals have exactly identical bandpass filtering characteristics. However, it should be appreciated that, in the single-pole-double-throw switch of the present invention, different first resonator  30  and fourth resonator  42  and/or different second resonator  34  and third resonator  38  can be selected, depending on the bandpass filtering characteristics required for receiving/transmitting RF signals. 
         [0029]    Compared to the prior art, the single-pole-double-throw switch of the present invention has been integrated with a bandpass filtering function, so that the addition of a bandpass filter is no longer required. In addition, since the undesirable characteristics of the switch have been integrated as part of the bandpass filter, the single-pole-double-throw switch of the present invention does not require additional circuitry (e.g. the impedance transformer  12  of  FIG. 1 ) to compensate for the undesirable characteristics of the switch. Furthermore, since the undesirable characteristics of the switch have been integrated as part of the bandpass filter, the synthesizing steps of the filter can be used to design the switch of the present invention, thereby greatly reducing the steps and complexity of the switch. 
         [0030]    The above embodiments are only used to illustrate the principles of the present invention, and they should not be construed as to limit the present invention in any way. The above embodiments can be modified by those with ordinary skills in the arts without departing from the scope of the present invention as defined in the following appended claims.