Patent Publication Number: US-2007105506-A1

Title: Switch circuit and high-frequency composite part

Description:
FIELD OF THE INVENTION  
      The present invention relates to a switch circuit for a multi-band mobile phone, which has a capable of making communications in plural systems, and a high-frequency composite part comprising it.  
     BACKGROUND OF THE INVENTION  
      There are various systems for mobile phones, for instance, EGSM (extended global system for mobile communications) and DCS (digital cellular system) widely used mostly in Europe, GSM 850 (global system for mobile communications 850) and GSM 1900 (global system for mobile communications 1900) widely used in the U. S., and PDC (personal digital cellular system) used in Japan. According to recent rapid expansion of mobile phones, however, a frequency band allocated to each system cannot allow all users to use their mobile phones in major cities in advanced countries, resulting in difficulty in connection and thus causing such a problem that mobile phones are sometimes disconnected during communication. Thus, proposal was made to permit users to utilize a plurality of systems, thereby increasing substantially usable frequencies, and further to expand serviceable territories and to effectively use communications infrastructure of each system.  
      To utilize a plurality of systems, a user should conventionally have a mobile phone capable of communicating in plural systems. As a high-frequency part used in such a mobile phone, the inventors proposed a high-frequency switch module for switching transmitting circuits and receiving circuits in different communication systems (WO 00/55983).  
      The high-frequency switch module of WO 00/55983 comprises first and second filter circuits having different passbands, a switch circuit connected to the first filter circuit for switching a transmitting circuit and a receiving circuit of a communication system A, and a switch circuit connected to the second filter circuit for switching transmitting circuits of communication systems B, C, a receiving circuit of a communication system B and a receiving circuit of a communication system C.  
      The first and second filter circuits function as circuits for branching a received signal of the communication system A and received signals of the communication systems B, C. The switch circuit is a diode switch comprising a diode and a transmission line as main elements, and any one of pluralities of communication systems A, B, C is selected by controlling the diode in an ON or OFF state by applying voltage from a control circuit, thereby switching the antenna and transmitting circuits and receiving circuits of the communication systems A, B, C.  
      Specific examples of the communication systems A, B, C disclosed by WO 00/55983 are GSM, DCS  1800  and PCS, respectively. GSM corresponds to the EGSM, DCS  1800  corresponds to the GSM  1800 , and PCS corresponds to the GSM  1900 . Table 1 shows transmitting frequency and receiving frequency of each communication system.  
                               TABLE 1                                   Communication   Transmitting   Receiving Frequency           System   Frequency (MHz)   (MHz)                          EGSM   880 to 915   925 to 960           GSM 1800   1710 to 1785   1805 to 1880           GSM 1900   1850 to 1910   1930 to 1990                      
 
      JP 2000-165288 A, JP 2001-44885 A and JP 2002-171195 A disclose high-frequency composite parts used in pluralities of different communication systems.  
      With respect to GSM  1800  and GSM  1900  among communication systems handled by such high-frequency composite parts, it is appreciated that the transmitting frequency of GSM  1900  overlaps the receiving frequency of GSM  1800  in a range of 1850 MHz to 1880 MHz.  
      Problems in a case where the receiving frequency of the first communication system (GSM  1800 ) partially overlaps the transmitting frequency of the second communication system (GSM  1900 ) in the conventional high-frequency switch module for handling GSM  1800  and GSM  1900  described in WO 00/55983 will be explained using the equivalent circuit shown in  FIG. 21 .  
      This high-frequency switch module selects a transmitting mode of GSM  1800 /GSM  1900 , a receiving mode of GSM  1800 , and a receiving mode of GSM  1900 , by controlling voltage applied from control terminals as shown in Table 2.  
                               TABLE 2                                   Mode   VC2   VC3                          GSM 1800 TX   V+   0           (Transmitting)           GSM 1900 TX   V+   0           (Transmitting)           GSM 1800 RX   0   0           (Receiving)           GSM 1900 RX   0   V+           (Receiving)                      
 
 (A) GSM  1800 /GSM  1900  Transmitting Mode 
 
      In the transmitting mode of GSM  1800  or GSM  1900 , positive voltage (V+) is applied to a control terminal VC 2 , and zero voltage is applied to a control terminal VC 3 , to control diodes DD 1 , DD 2  in an ON state. A transmission line ld 3  has such proper length that its resonance frequency is in a frequency range (1710 MHz to 1910 MHz) of transmitting signals of GSM  1800  and GSM  1900 , and grounded through the diode DD 2  in an ON state and a capacitor cd 4  for resonance. As a result, impedance is large (ideally infinitive) when the receiving circuits of GSM  1800  and GSM  1900  are viewed from the connection point IP 2 . Accordingly, transmitting signals sent from the transmitting circuit of GSM  1800  and GSM  1900  are sent to an antenna via the second filter circuit without leaking to the receiving circuit. At this time, the diodes DP 1 , DP 2  are controlled in an OFF state.  
      However, because impedance is practically not sufficiently large in other frequencies than the resonance frequency when the receiving circuits of GSM  1800  and GSM  1900  are viewed from a connection point IP 2 , part of the transmitting signals of GSM  1800  and GSM  1900  (hereinafter referred to as “leak signals”) leak to the receiving circuits of GSM  1800  and GSM  1900  via the transmission line ld 3 . In addition, the resonance frequency may be changed by the unevenness of the capacitance of the capacitor cd 4  and a capacitance component parasitic to the transmission line ld 3 , resulting in further increase in a signal leaking to the receiving circuits of GSM  1800  and GSM  1900 .  
      Specifically, because the diodes DP 1 , DP 2  are in an OFF state in the case of transmitting mode in GSM  1800 , the leak signal does not appear in the receiving circuit of GSM  1900  by isolation when the diode DP 1  is in an OFF state. On the other hand, the leak signal appearing in the receiving circuit of GSM  1800  via the transmission line lp 2  is removed by a filter circuit (not shown) disposed upstream of the receiving circuit, resulting in substantially no leakage to the receiving circuit of GSM  1800 . In the case of transmitting mode in GSM  1900 , however, a leak signal of 1850 MHz to 1880 MHz overlapping the receiving frequency of GSM  1800  among those in a transmitting frequency of GSM  1900  appearing in the receiving circuit of GSM  1800  is supplied to the receiving circuit of GSM  1800  without being removed by the filter circuit, entering into analog-processing ICs constituting an LNA (low-noise amplifier)and a mixer in the receiving circuit and a modulator/demodulator, and thus causing the malfunction of these circuit parts.  
      (B) GSM  1800  Receiving Mode  
      In a receiving mode in GSM  1800 , the diodes DP 1 , DP 2 , DD 1  and DD 2  are controlled in an OFF state by applying zero voltage to the control terminals VC 2  and VC 3 . With the diode DD 1  in an OFF state, impedance is large between the connection point IP 2  and the transmitting circuit of GSM  1800 /GSM  1900 . With the diode DP 1  in an OFF state, impedance is large between the connection point IP 3  and the receiving circuit of GSM  1900 . The connection point IP 2  is thus connected to the receiving circuit of GSM  1800  via transmission lines ld 3  and lp 2 .  
      (C) GSM  1900  Receiving Mode  
      In the receiving mode in GSM  1900 , positive voltage is applied to the control terminal VC 3 , and zero voltage is applied to the control terminal VC 2 , to control the diodes DD 1 , DD 2  in an OFF state and the diodes DP 1 , DP 2  in an ON state. With the diode DD 1  in an OFF state, impedance is large between the connection point IP 2  and the transmitting circuit of GSM  1800 /GSM  1900 . The transmission line lp 2  has such length that it is resonated at 1930 MHz to 1990 MHz in a frequency range of the received signal of GSM  1900 . Accordingly, it is grounded through the diode DP 2  in an ON state and the capacitor CP 1  for resonance, resulting in large impedance when the receiving circuit of GSM  1800  is viewed from the connection point IP 3 . The connection point IP 2  is thus connected to the receiving circuit of GSM  1900 .  
      The diodes DP 1 , DP 2  for switching the receiving circuits of GSM  1800  and GSM  1900  usually have low power consumption and small insertion loss. Such diodes are generally more likely subjected to distortion than diodes with large power consumption in an OFF state. Accordingly, the diodes DP 1 , DP 2  in an OFF state are likely to distort the transmitting signals of GSM  1800 , GSM  1900  leaking via the transmission line ld 3 , thereby generating harmonics having frequencies corresponding to integral multiples of those of these transmitting signals. These harmonics, which are added to the transmitting signals of GSM  1800 , GSM  1900 , are radiated from the antenna. With respect to such problems, none of WO 00/55983 JP 2000-165288 A, JP 2001-44885 A and JP 2002-171195 A provides any solutions.  
     OBJECTS OF THE INVENTION  
      Accordingly, an object of the present invention is to provide a switch circuit for a high-frequency composite part for use in a mobile phone with a capable of handling pluralities of communication systems, which suffers from extremely small leakage of transmitting signals to receiving circuits, thus having extremely large isolation, when handling communication systems having partially overlapping transmitting/receiving signals.  
      Another object of the present invention is to provide a switch circuit with suppressed generation of harmonics without suffering from increase in power consumption.  
     DISCLOSURE OF THE INVENTION  
      The first switch circuit of the present invention selectively switches the connection of a receiving circuit or a transmitting circuit in two communication systems, in which a receiving frequency bandwidth in a first communication system partially overlaps a transmitting frequency bandwidth in a second communication system, and an antenna circuit; 
      (a) it comprising two switches, a first switch switching the connection of the antenna circuit to transmitting circuit of first and second communication systems and the connection of the antenna circuit to receiving circuits of first and second communication systems, and a second switch being connected between the first switch and the receiving circuit of the first and second communication systems to switch the connection of the antenna circuit to the receiving circuit of the first communication system via the first switch, and the connection of the antenna circuit to the receiving circuit of the second communication system via the first switch;     (b) a transmitting circuit common to the first and second communication systems being connected to the transmitting circuit of the first and second communication systems in the first switch; and     (c) the second switch disconnecting the receiving circuit of the first communication system from the first switch, while the transmitting circuit of the first and second communication systems is connected to the antenna circuit.    

      The switch circuit preferably comprises switching elements, inductors and capacitors. There is preferably a capacitor between the first switch and the second switch.  
      The switching element of the present invention is constituted by semiconductor elements performing switching operations by changing impedance, such as field-effect transistors, bipolar transistors, PIN diodes, etc. The field-effect transistor is made conductive or non-conductive by changing impedance between a source and a drain by control voltage applied from a gate. The PIN diode is made conductive or non-conductive by changing impedance between an anode and a cathode by control voltage. The inductor is, for instance, a transmission line such as a strip line electrode, a microstrip line electrode, etc., a coil, a chip inductor, etc. The capacitor is, for instance, a laminate capacitor constituted by capacitor electrodes, a chip capacitor, etc. These elements may properly be selected depending on demanded requirements.  
      The second switch circuit of the present invention selectively switches the connection of a receiving circuit or a transmitting circuit for two communication systems, in which a receiving frequency bandwidth in the first communication system partially overlaps a transmitting frequency bandwidth in the second communication system, and an antenna circuit; 
      (a) the switch circuit, which comprises switching elements, inductors and capacitors, being constituted by a first switch and a second switch, the first switch comprising a first port connected to the antenna circuit, a second port connected to the transmitting circuit of the first and second communication systems, and a third port connected to the second switch, and the second switch comprising a fourth port connected to the first switch via a capacitor, a fifth port connected to the receiving circuit of the first communication system, and a sixth port connected to the receiving circuit of the second communication system;     (b) a first inductor being disposed between the fourth port and the fifth port;     (c) a first switching element being disposed between the fifth port and the ground;     (d) a second switching element being disposed between the fourth port and the sixth port; and     (e) the first and second switching elements being controlled in an ON state while the transmitting circuit of the first and second communication systems is connected to the antenna circuit.    

      The first inductor is preferably a transmission line having such length that resonance occurs in a receiving frequency range of the second communication system. With such structure, the first inductor is grounded at high frequencies through the first switching element in an ON state for resonance, and impedance is relatively large in a receiving frequency of the second communication system, when the fifth port connected to the receiving circuit of the first communication system is viewed from the fourth port. Because impedance is also high in a transmitting frequency of the second communication system, a leak signal from the first switch, which appears in the fourth port, is attenuated, thereby making it possible to suppress leakage to the receiving circuit of the first communication system.  
      Though a leak signal having a frequency close to the receiving frequency of the second communication system may appear in the sixth port via the second switching element, the leak signal is removed by a filter circuit disposed downstream between the sixth port and the receiving circuit of the second communication system. Accordingly, the leak signal substantially does not leak to the receiving circuit of the second communication system. Also, because the switching element of the second switch is in an ON state, the generation of harmonics can be prevented.  
      The third switch circuit of the present invention selectively switches the connection of a receiving circuit or a transmitting circuit for two communication systems, in which a receiving frequency bandwidth in the first communication system partially overlaps a transmitting frequency bandwidth in the second communication system, and an antenna circuit; 
      (a) the switch circuit, which comprises switching elements, inductors and capacitors,.being constituted by a first switch and a second switch, the first switch comprising a first port connected to the antenna circuit, a second port connected to the transmitting circuit of the first and second communication systems, and a third port connected to the second switch, and the second switch comprising a fourth port connected to the first switch via a capacitor, a sixth port connected to the receiving circuit of the first communication system, and a fifth port connected to the receiving circuit of the second communication system;     (b) a first inductor being disposed between the fourth port and the fifth port;     (c) a first switching element being disposed between the fifth port and the ground;     (d) a second switching element being disposed between the fourth port and the sixth port; and     (e) the first and second switching elements being controlled in an OFF state, while the transmitting circuit of the first and second communication systems is connected to the antenna circuit.    

      With such structure, the second switching element has isolation characteristics in an OFF state, preventing the leak signal from leaking to the receiving circuit of the first communication system. Though the first inductor is designed to resonate in a receiving frequency range of the first communication system, the leak signal may appear in the fifth port via the first inductor. However, the leak signal is removed by a downstream filter circuit disposed between the fifth port and the receiving circuit of the second communication system, so that it does not leak to the receiving circuit of the second communication system.  
      In the second and third switch circuits, the first switch comprises a third switching element disposed between the first port and the second port, a second inductor disposed between the first port and the third port, and a fourth switching element disposed between the third port and the ground, and when the transmitting circuit of the first and second communication systems is connected to the antenna circuit, the third and fourth switching elements are preferably controlled in an ON state.  
      More preferably, the fourth switching element comprises a diode, and a first capacitor is disposed between the diode and-the ground. The diode generally has an inductance component because of lead terminals, etc. It also has parasitic inductance because of line patterns for connecting the diode to other circuit elements, etc., so that a completely short-circuited state is not necessarily achieved even when the diode is turned on.  
      This makes impedance smaller when the third port is viewed from the first port, resulting in poor isolation and insertion loss characteristics. Accordingly, the first capacitor is disposed in series to the fourth switching element for series resonance in the present invention, such that impedance is large when the third port is viewed from the first port, thereby reducing a transmitting signal leaking to the second switch and thus improving isolation and insertion loss characteristics.  
      In the second and third switch circuits, the operation current of the switching element of the second switch is preferably lower than that of the switching element of the first switch. Using a switching element operable with current of 2.5 mA or less, particularly 1 mA or less at a temperature of 0° C. to +85° C., power consumption can be reduced at the time of transmitting and receiving, resulting in reduced battery consumption of mobile phones.  
      In the second and third switch circuits, the first switch and the second switch can be made composite by making the characteristic impedance of the first inductor higher than that of the second inductor, thereby making it easy to achieve impedance matching in the production of the switch circuit.  
      The high-frequency composite part of the present invention comprises either of the first to third switch circuits, switching elements, inductors and capacitors being contained in or mounted onto a multi-layered ceramic substrate obtained by laminating pluralities of ceramic sheets, and connected by connection means formed in or on the multi-layered substrate.  
      In the high-frequency composite part, transmission lines constituting the first and second inductors are preferably contained in the multi-layered substrate, and these transmission lines are preferably formed in horizontally different regions in the multi-layered substrate. Such structure not only prevents their interference, but also prevents the resonance frequency and characteristic impedance of these transmission lines from changing due to parasitic capacitance, etc., thereby improving isolation characteristics, and making it easy to achieve impedance matching between the first switch and second switch. Further, when the transmission lines are formed as strip line electrodes in a region sandwiched by ground electrodes formed in the multi-layered substrate, their interference with electrode patterns constituting other circuit elements can preferably be prevented. Impedance may be changed by forming the first inductor and/or the second inductor by connecting transmission lines formed on two layers or more through viaholes, and by making the width of a transmission line formed on one layer different from the width of transmission lines formed on other layers.. Such structure makes it easy to achieve impedance matching with circuits disposed upstream or downstream of the transmission lines.  
      It is preferable that the fourth switching element is a diode, that the first capacitor disposed between this diode and the ground is contained in the multi-layered substrate, and that a hot-side electrode constituting the first capacitor is disposed above an upper ground electrode among those sandwiching transmission lines constituting the first and second inductors. The capacitor electrodes are connected to diodes through viaholes formed in the multi-layered substrate, and the diodes are mounted onto an upper surface of the multi-layered substrate. Accordingly, the above structure can make the distances between the diodes and the capacitor electrodes smaller, thereby reducing parasitic inductance and thus obtaining high isolation characteristics.  
      In the second switch, too, the first switching element may be a diode, and the second capacitor may be disposed between this diode and the ground to obtain high isolation characteristics. If a hot-side electrode constituting the first capacitor and a hot-side electrode constituting the second capacitor are formed on the same layer via a common ground electrode and a ceramic layer, they are less affected by lamination discrepancy in a horizontal direction, thereby making it possible to produce capacitors with uniform capacitance and thus provide stable isolation characteristics.  
      If one of the ground electrodes sandwiching the transmission line is a common ground electrode, the step of forming ground electrodes can be decreased, thereby making the multi-layered substrate thinner. Also, the interference of the capacitor electrode with electrode patterns constituting other circuit elements, the switching elements such as diodes mounted onto the multi-layered substrate, etc. and other mounted parts is reduced by sandwiching the capacitor electrode on the hot side by the ground electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram showing a high-frequency circuit comprising a switch circuit according to one embodiment of the present invention;  
       FIG. 2  is a view showing an equivalent circuit of the switch circuit according to one embodiment of the present invention;  
       FIG. 3  is a view showing an equivalent circuit of a switch circuit according to another embodiment of the present invention;  
       FIG. 4  is a view showing an equivalent circuit of a switch circuit according to a further embodiment of the present invention;  
       FIG. 5  is a view showing an equivalent circuit of a switch circuit according to a still further embodiment of the present invention;  
       FIG. 6  is a graph showing isolation characteristics between TX 1 , TX 2  and RX 1  in a GSM  1800 /GSM  1900  transmitting mode in the switch circuit according to one embodiment of the present invention;  
       FIG. 7  is a graph showing isolation characteristics between TX 1 , TX 2  and RX 1  in a GSM  1800 /GSM  1900  transmitting mode in a comparative switch circuit;  
       FIG. 8  is a view showing an equivalent circuit of a switch circuit according to a still further embodiment of the present invention;  
       FIG. 9  is a view showing an equivalent circuit of a switch circuit according to a still further embodiment of the present invention;  
       FIG. 10  is a view showing an equivalent circuit of a switch circuit according to a still further embodiment of the present invention;  
       FIG. 11  is a block diagram showing another example of a high-frequency circuit comprising the switch circuit according to one embodiment of the present invention;  
       FIG. 12  is a block diagram showing a further example of a high-frequency circuit comprising the switch circuit according to one embodiment of the present invention;  
       FIG. 13  is a plan view showing a high-frequency composite part comprising the switch circuit according to one embodiment of the present invention;  
       FIG. 14  is a squint view showing a multi-layered substrate for use in the high-frequency composite part shown in  FIG. 13 ;  
       FIG. 15  is an exploded plan view showing a laminate structure of the multi-layered substrate for use in the high-frequency composite part shown in  FIG. 13 ;  
       FIG. 16  is a view showing an equivalent circuit of the high-frequency composite part shown in  FIG. 13 ;  
       FIG. 17  is a plan view showing another high-frequency composite part comprising the switch circuit according to one embodiment of the present invention;  
       FIG. 18  is a perspective view showing a multi-layered substrate for use in the high-frequency composite part shown in  FIG. 17 ;  
       FIG. 19  is an exploded plan view showing a laminate structure of the multi-layered substrate for use in the high-frequency composite part shown in  FIG. 17 ;  
       FIG. 20  is a view showing an equivalent circuit of the high-frequency composite part shown in  FIG. 17 ; and  
       FIG. 21  is a view showing an equivalent circuit of a conventional switch circuit. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     [1] First Embodiment  
       FIG. 1  shows a high-frequency circuit comprising a switch circuit according to one embodiment of the present invention, and  FIG. 2  shows the equivalent circuit of the switch circuit. It is assumed below for the simplification of explanation without intention of restricting the present invention that among pluralities of communication systems, a first communication system f 1  is GSM  1800  (transmitting frequency: 1710 to 1785 MHz, receiving frequency: 1805 to 1880 MHz), and a second communication system f 2  is GSM  1900  (transmitting frequency: 1850 to 1910 MHz, receiving frequency: 1930 to 1990 MHz).  
      This switch circuit is constituted by a first switch  100  and a second switch  105  both comprising switching elements, inductors and a capacitor. The first switch  100  comprises a first port  100   a  connected to an antenna circuit; a second port  100   b  connected to transmitting circuit of GSM  1800  and GSM  1900 , and a third port  100   c  connected to a second switch  105 . The second switch  105  comprises a fourth port  105   a  connected to the first switch  100  via a capacitor CP, a fifth port  105   b  connected to the receiving circuit of GSM  1800 , and a sixth port  105   c  connected to the receiving circuit of GSM  1900 . The second switch  105  comprises a transmission line lp 2  as a first inductor disposed between the fourth port  105   a  and the fifth port  105   b,  a diode DP 2  as a first switching element disposed between the fifth port  105   b  and the ground, a capacitor CP 1 , as a second capacitor disposed between the first diode DP 2  and the ground, a diode DP 1  as a second switching element disposed between the fourth port  105   a  and the sixth port  105   c,  and a transmission line or inductor lp 1  disposed between the sixth port  105   c  and the ground.  
      The second switch  105  switches the receiving circuit RX 1  of GSM  1800  and the receiving circuit RX 2  of GSM  1900 , and the first switch  100  switches the transmitting circuit TX 1 , TX 2  of GSM  1800 /GSM  1900  and the second switch circuit  105 . The second switch  105  has as main elements two diodes DP 1 , DP 2  as switching elements, and transmission lines lp 1 , lp 2  as inductors (or an inductor in place of the transmission line lp 1 ). The diode DP 1  having an anode connected to the fourth port  105   a  and a cathode connected to the receiving circuit RX 2  of GSM  1900 , and a grounded transmission line lp 1  is disposed on the cathode side.  
      A transmission line lp 2  is connected between the fourth port  105   a  and the fifth port  105   b,  and the fifth port  105   b  is connected to a diode DP 2  connected to the ground via the capacitor CP 1 . A control circuit VC 3  is connected between the diode DP 2  and the capacitor CP 1  via an inductor LP and a resistor RP 1 .  
      Disposed upstream of the second-switch  105  is the first switch  100  for switching the transmitting circuit TX 1 , TX 2  of GSM  1800 /GSM  1900  and the second switch. The first switch  100  has two diodes DD 1 , DD 2  and two transmission lines ld 2 , ld 3  (or an inductor in place of the transmission line ld 2 ) as main elements.  
      A diode DD 1  disposed between the second port  100   b  and the first port  100   a  has an anode connected to the first port  100   a  and a cathode connected to a transmission line ld 2  connected to the ground. A transmission line ld 3  is connected between the first port  100   a  and the third port  100   c,  and a diode DD 2  connected to the ground via a capacitor cd 4  is disposed on the side of the third port  100   c.  A control circuit VC 2  is connected between the diode DD 2  and the capacitor cd 4  via an inductor LD and a resistor RD.  
      The control logic of the control circuits VC 2 , VC 3  for causing the switch circuit in this embodiment to perform the predetermined operation is shown in Table 3. Voltage is applied from the control circuits to control the switching elements in an ON or OFF state, thereby selecting a transmitting mode of GSM  1800 /GSM  1900 , a receiving mode of GSM  1800 , and a receiving mode of GSM  1900 .  
                               TABLE 3                                   Mode   VC2   VC3                          GSM 1800 TX   V+   V+           (Transmitting)           GSM 1900 TX   V+   V+           (Transmitting)           GSM 1800 RX   0   0           (Receiving)           GSM 1900 RX   0   V+           (Receiving)                      
 
      The operation of the switch circuit will be explained below in detail.  
      (A) GSM  1800 /GSM  1900  Transmitting Mode  
      To connect the transmitting circuit TX 1 , TX 2  of GSM  1800  and GSM  1900  to the antenna circuit ANT, positive (high) voltage is applied from the control circuit VC 2  and the control circuit VC 3 . Positive voltage applied from the control circuit VC 2  is deprived of a DC component by the capacitors C 1 , C 2 , cd 4 , CP, and supplied to the first switch  100  including the diodes DD 1 , DD 2 . As a result, the diodes DD 1 , DD 2  are turned on. When the diode DD 1  is in an ON state, there is small impedance between the second port  100   b  and the first port  100   a.  In addition, the transmission line ld 3  is grounded at high frequencies through the diode DD 2  in an ON state and the capacitor cd 4 , causing resonance, and thus making impedance high when the third port  100   c  is viewed from the first port  100   a.    
      Further, positive voltage applied from the control circuit VC 3  is deprived of a DC component by capacitors CP, C 20 , C 21 , CP 1 , and supplied to the switch circuit  105  including the diodes DP 1 , DP 2 . As a result, the diodes DP 1 , DP 2  are turned on. The transmission line lp 2  grounded at high frequencies through the diode DP 2  in an ON state and the capacitor CP 1 , causing resonance, so that there is large impedance when the fifth port  105   b  is viewed from the fourth port  105   a.    
      With the above structure, there is extremely large impedance in a line from the first port  100   a  to the receiving circuit RX 1  of GSM  1800  when the transmitting signal of GSM  1900  is sent to the antenna circuit. Accordingly, the leakage of the transmitting signal of GSM  1900  to the receiving circuit RX 1  of GSM  1800  can be reduced.  
       FIG. 6  shows isolation characteristic of TX 1 , TX 2  to RX 1  in a transmitting mode of GSM  1800 /GSM  1900 , when voltage (V+) of 2.6 V is applied from the control circuit VC 2  and the control circuit VC 3  in the switch circuit in this embodiment.  FIG. 7  shows isolation characteristic of TX 1 , TX 2  to RX 1  in the same switch circuit as in  FIG. 6  in a transmitting mode of GSM  1800 /GSM  1900 , when voltage (V+) of 2.6 V is applied from the control circuit VC 2 , and when voltage (0) of 0 V is applied from the control circuit VC 3  (Comparative Example).  
      As shown in  FIG. 6 , the receiving circuit of the first communication system is disconnected from the first switch to obtain excellent isolation characteristics in a desired frequency bandwidth in the second switch, in a case where the transmitting circuit of the first and second communication systems are connected to the antenna circuit. Improvement in the isolation characteristics resulted in improved insertion loss characteristics between the transmitting circuit TX 1 , TX 2  of GSM  1800  and GSM  1900  and the antenna circuit ANT. In addition, because the second switch  105  is operated with low operation current, the operation current of the first switch  100  and the second switch  105  was a little higher 8.8 mA than 8.0 mA in Comparative Example. It was further confirmed that by operating the second switch  105 , harmonics radiated from the antenna were reduced by about 2 dB to 5 dB.  
      (B) GSM  1800  Receiving Mode  
      When the receiving circuit RX 1  of GSM  1800  is connected to the antenna circuit ANT, zero voltage is applied from the control circuits VC 2  and VC 3 , resulting in the diodes DP 1 , DP 2 , DD 1 , DD 2  in an OFF state. With the diode DD 1  in an OFF state, there is large impedance between the first port  100   a  and the second port  100   b.  Also, with the diode DP 1  in an OFF state, there is large impedance between the fourth port  105   a  and the sixth port  105   c.  As a result, a receiving signal of GSM  1800  taken through the antenna is transmitted to the receiving circuit RX 1  of GSM  1800  via the transmission lines ld 3 , lp 2  with low loss without leaking to the transmitting circuit TX 1 , TX 2  of GSM  1800 /GSM  1900  and the receiving circuit RX 2  of GSM  1900 .  
      (C) GSM  1900  Receiving Mode  
      When the receiving circuit RX 2  of GSM  1900  is connected to the antenna circuit ANT, zero voltage is applied from the control circuit VC 2 , and positive voltage is applied from the control circuit VC 3 . The positive voltage applied from control circuit VC 3  is deprived of a DC component by capacitors, and supplied to the second switch  105  including the diodes DP 1 , DP 2 . As a result, the diodes DP 1  and DP 2  are turned on. With the diode DP 1  in an ON state, there is low impedance between the fourth port  105 a and the sixth port  105   c.  The diode DP 2  in an ON state and the capacitor CP 1  have the transmission line lp 2  grounded at high frequencies, causing resonance in a frequency bandwidth of the receiving signal of GSM  1900 , so that there is extremely large impedance in the bandwidth of the receiving signal of GSM  1900  when the fifth port  105   b  is viewed from the fourth port  105   a.  Further, with the diode DD 1  in an OFF state, there is large impedance between the first port  100   a  and the second port  100   b.  As a result, the receiving signal of GSM  1900  taken through the antenna is transmitted to the receiving circuit RX 2  of GSM  1900  with low loss without leaking to the transmitting circuit TX 1 , TX 2  of GSM  1800 /GSM  1900  and the receiving circuit RX 1  of GSM  100 .  
      This embodiment provides the switch circuit with extremely small leakage of the transmitting signal to the receiving circuit (extremely large isolation). In this embodiment, transmission lines, capacitors, etc. may be constituted in a multi-layered substrate made of dielectric materials, etc. by a low-temperature cofirable ceramic (LTCC) technology. The. transmission lines ld 2  and lp 1  may be mounted onto the laminate as inductors such as chip inductors, coils, etc. together with other circuit elements (diodes, resistors, etc.).  
      FIGS.  3  to  5  shows other examples of the first switch  100  and the second switch  105 . The embodiment shown in  FIG. 3  uses diodes as switching elements, and the embodiments shown in  FIGS. 4 and 5  use transistors as switching elements.  
      These embodiments will be explained below taking use in the second switch  105  for example, though the first switch  100  may also-be constituted by a similar equivalent circuit. The second switch  105  comprises a diode DP 1  between the fourth port  105   a  and the sixth port  105   c.  The cathode of the diode DP 1  is connected to the fourth port  105   a,  and the transmission line lp 1  connected to the ground via a capacitor CP 5  is disposed on the rode side of the diode DP 1 . Because the transmission line lp 1  functions as a choke coil, an inductor or a coil may be used instead. The transmission line lp 2  is connected between the fourth port  105   a  and the fifth port  105   b,  and a diode DP 2  connected to the ground via a capacitor CP 1  is disposed on the side of the fifth port  105   b.  A resistor R is connected in parallel with the capacitor CP 1 . In place of this resistor R, an inductor (coil) with sufficiently large impedance may be used. A control circuit VC 3  is connected between the transmission line lp 1  and the capacitor CP 5 . Such switch circuit can also exhibit excellent effects as above.  
      The second switch shown in  FIG. 4  comprises a transistor FET 1  between the sixth port  105   c  and the fourth port  105   a,  the transistor FET 1  having a drain connected to the fourth port  105   a  and a source connected to the sixth port  105   c.  The gate of the transistor FET 1  is connected to a control terminal VC 3  via a resistor R. A transmission line lp 2  is disposed between the fourth port  105   a  and the fifth port  105   b.  A transistor FET 2  is disposed on the side of the fifth port  105   b,  a drain is connected to the fifth port  105   b  and a source connected to the ground electrode. The gate of the transistor FET 2  is connected to the control terminal VC 3  via a resistor R.  
      This switch circuit can switch signal lines by voltage applied to the control terminal VC 3 , like other switch circuits. Incidentally, a control logic is different between a depression type and an enhancement type in the transistors FET 1 , FET 2 . Used in the operation according to the control logic shown in Table 3 is the enhancement type FET, in which impedance between the source and the drain becomes low when voltage is applied to the gate. Even with such switch circuit, excellent effects can be obtained like above.  
      The second switch shown in  FIG. 5  comprises a transistor FET 1  between the sixth port  105   c  and the fourth port  105   a,  and a transistor FET 2  between the fourth port  105   a  and the fifth port  105   b,  the gates of the transistors FET 1 , FET 2  being connected to the control terminals VC 3 , VC 4 , respectively, via resistors R.  
      This switch circuit can switch signal lines by voltage applied from the control terminals VC 3 , VC 4 , like other switch circuits. The series connection of pluralities of transistors can preferably suppress the generation of distortion even in an OFF state. Even with such switch circuit, excellent effects can be obtained like above.  
     [2] Second Embodiment  
      With respect to a switch circuit according to a second embodiment of the present invention, detailed explanation will be made below in a case where a first communication system is GSM  1800  (transmitting frequency: 1710 to 1785 MHz, receiving frequency: 1805 to 1880 MHz), and a second communication system is GSM  1900  (transmitting frequency: 1850 to 1910 MHz, receiving frequency: 1930 to 1990 MHz), like the first embodiment. Incidentally, because the equivalent circuit- of the switch circuit in this embodiment shares many common parts with that of the first embodiment, explanation will be concentrated on different parts for simplification.  
      The equivalent circuit of this switch circuit is shown in  FIG. 8 . Though it does not differ from the switch circuit of the first embodiment in an equivalent circuit, it is opposite to the first embodiment in the connection of the receiving circuits of GSM  1900  and GSM  1800  and the fifth and sixth ports in the second switch; the receiving circuit RX 2  of GSM  1900  being connected to the fifth port  105   b,  and the receiving circuit RX 1  of GSM  1800  being connected to the sixth port  105   c.  To have such circuit structure, the constants of the circuit elements, the length of the transmission lines, etc. are properly set in each switch circuit, and they may of course be different from those in the first embodiment.  
      The control logic of the control circuits VC 2 , VC 3  for subjecting the switch circuit in this embodiment to the predetermined operation is shown in Table 4.  
                               TABLE 4                                   Mode   VC2   VC3                          GSM 1800 TX   V+   0           (Transmitting)           GSM 1900 TX   V+   0           (Transmitting)           GSM 1800 RX   0   V+           (Receiving)           GSM 1900 RX   0   0           (Receiving)                      
 
 (A) GSM  1   800 /GSM  1900  Transmitting Mode 
 
      To connect the transmitting circuit TX 1 , TX 2  of GSM  1800 /GSM  1900  to the antenna circuit ANT, positive voltage (V+) is applied from the control circuit VC 2 , and zero voltage is applied from the control circuit VC 3 . With zero voltage applied from the control circuit VC 3 , the diode DP 1  is in an OFF state, resulting in large impedance between the fourth port  105   a  and the sixth port  105   c.  As a result, as in the first embodiment, the transmitting signals of GSM  1800 /GSM  1900  are sent to the first port  100   a  with low loss without leaking to the receiving circuit RX 1  of GSM  1800 , and radiated from the antenna.  
      (B) GSM  1800  Receiving Mode  
      To connect the receiving circuit RX 1  of GSM  1800  to the antenna circuit ANT, zero voltage is applied from the control circuit VC 2 , and positive voltage is applied from the control circuit VC 3 . The positive voltage applied from the control circuit VC 3  is deprived of a DC component by capacitors C 20 , C 21 , CP, CP 1 , and sent to the second switch  105  including the diodes DP 1 , DP 2 . As a result, the diodes DP 1  and DP 2  are turned on. With the diode DP 1  in an ON state, impedance is low between the sixth port  105   c  and the fourth port  105   a.  Also, with the diode DP 2  in an ON state and the capacitor CP 1 , the transmission line lp 2  is grounded at high frequencies, resulting in resonance in a frequency bandwidth of the receiving signal of GSM  1800 , and thus extremely large impedance in the bandwidth of the receiving signal of GSM  1800  when the fifth port  105   b  is viewed from the fourth port  105   a.  Further, with the diode DD 1  in an OFF state, impedance is large between the first port  100   a  and the second port  100   b.  As a result, the receiving signal of GSM  1800  taken through the antenna is transmitted to the receiving circuit RX 1  of GSM  1800  with low loss without leaking to the transmitting circuit TX 1 , TX 2  of GSM  1800 /GSM  1900  and the receiving circuit RX 2  of GSM  1900 .  
      (C) GSM  1900  Receiving Mode  
      To connect the receiving circuit RX 2  of GSM  1900  to the antenna circuit ANT, zero voltage is applied from the control circuits VC 2  and VC 3  to turnoff the diodes DP 1 , DP 2 , DD 1 , DP 2 . With the diode DD 1  in an OFF state, impedance is large between the first port  100   a  and the second port  100   b.  Also, with the diode DP 1  in an OFF state, impedance is large between the fourth port  105   a  and the sixth port  105   c.  As a result, the receiving signal of GSM  1900  taken through the antenna is transmitted to the receiving circuit RX 2  of GSM  1900  via the transmission lines ld 3 , lp 2  with low loss without leaking to the transmitting circuit TX 1 , TX 2  of GSM  1800 /GSM  1900  and the receiving circuit RX 1  of GSM  1800 .  
      This embodiment can also provide the switch circuit with extremely small leakage of the transmitting signal to the receiving circuit (extremely large isolation).  
     [3] Third Embodiment  
      A switch circuit according to a third embodiment of the present invention will be explained below. Because the equivalent circuit of the switch circuit in this embodiment shares many parts with that of the second embodiment, explanation will be concentrated on different parts for simplification.  
      The equivalent circuit of this switch circuit is shown in  FIG. 9 . The equivalent circuit of this switch circuit differs from that of the second embodiment in that a capacitor CP 5  is disposed between the transmission line lp 1  of the second switch  105  and the ground, and that a control circuit VC 2  is connected between this transmission line lp 1  and the capacitor CP 5  via an inductor and a resistor.  
      When the switch circuit in this embodiment is operated according to the same control logic of the control circuits VC 2 , VC 3  as in the second embodiment, reverse voltage is applied to the diodes DP 1 , DP 2  of the second switch  105  in the transmitting mode of GSM  1800 /GSM  1900 , resulting in excellent isolation characteristics and attenuated harmonics radiated from the antenna. Incidentally, the control circuit VC 2  may be connected between a diode and a capacitor in place of the resistor R, such that reverse voltage is applied to the diodes DP 1 , DP 2  by the switch circuit as in  FIG. 3  as the second switch  105 , thereby obtaining the same effects.  
     [4] Forth Embodiment  
      A switch circuit according to a fourth embodiment of the present invention will be explained below. Because the equivalent circuit of the switch circuit in this embodiment shares many parts with that of the first embodiment, explanation will be concentrated on different parts for simplification.  
      The equivalent circuit of this switch circuit is shown in  FIG. 10 . The equivalent circuit of this switch circuit differs from that of the first embodiment in that the second switch  105  is a GaAs switch comprising transistors. The control logic is different between the depression-type transistor and the enhancement-type transistor. For instance, these control logics are shown in Table 5 below. This embodiment can also provide the switch circuit with extremely small leakage of the transmitting signal to the receiving circuit (extremely large isolation). In this embodiment, it is possible to suppress the generation of distortion by connecting pluralities of transistors in series, even when there is leakage in a transmitting signal from the first switch to the second switch.  
                                   TABLE 5                                   Mode   VC2   VC3   VC4                                        Depression Type                                     GSM 1800 TX   V+   0   V−           (Transmitting)           GSM 1900 TX   V+   0   V−           (Transmitting)           GSM 1800 RX   0   V−   0           (Receiving)           GSM 1900 RX   0   0   V−           (Receiving)                 Enhancement Type                                     GSM 1800 TX   V+   V+   0           (Transmitting)           GSM 1900 TX   V+   V+   0           (Transmitting)           GSM 1800 RX   0   0   V+           (Receiving)           GSM 1900 RX   0   V+   0           (Receiving)                      
 
     [5] Fifth Embodiment  
      The fifth embodiment provides a high-frequency composite part (multiband antenna switch module) comprising a branching circuit (diplexer)  300  and filter circuits  120 ,  125  integrated in a multi-layered substrate, and the switch circuit  10  of the present invention in a high-frequency circuit handling three communication systems shown in  FIG. 11 .  FIG. 13  is its plan view,  FIG. 14  is a squint schematic view showing the multi-layered substrate,  FIG. 15  is a development view showing each layer constituting the multi-layered substrate shown in  FIG. 14 , and  FIG. 16  shows the equivalent circuit of the high-frequency composite part.  
      In this embodiment, the inductors, capacitors and switching elements in the switch circuit  10  are formed in the multi-layered substrate, together with the inductors, capacitors and switching elements constituting the diplexer  300  comprising the first and second filter circuits, the lowpass filter circuits  120 ,  125  and the switch circuit  15  in the high-frequency circuit shown in  FIG. 11 . Transmission lines are formed as the inductors in the multi-layered substrate, and diodes as the switching elements and high-capacitance capacitors that cannot be contained in the multi-layered substrate as chip capacitors are mounted onto the multi-layered substrate, thereby constituting a one-chip, triple-band, high-frequency composite part.  
      The multi-layered substrate constituting this high-frequency composite part may be produced from a low-temperature-cofirable, dielectric ceramic material, by forming green sheets of 20 μm to 26 μm in thickness, printing an Ag-based conductive paste on each green sheet to form a desired electrode pattern, integrally laminating pluralities of green sheets with desired electrode patterns, and firing the multi-layered substrate. The width of most line electrodes is preferably 100 μm to 400 μm. The low-temperature-cofirable, dielectric ceramic materials may be, for instance, (a) Al 2 O 3 -based ceramics containing at least one of SiO 2 , SrO, CaO, PbO, Na 2 O and K 2 O as an additional component, (b) Al 2 O 3 -based ceramics containing at least one of MgO, SiO 2  and GdO as an additional component, or (c) ceramics based on Al 2 O 3, SiO   2 , SrO, Bi 2 O 3 , TiO 2 , etc.  
      These laminate green sheets are integrally pressure-bonded, and fired at a temperature of about 900° C., to obtain a multi-layered substrate having an outer size of 6.7 mm×5.0 mm×1.0 mm, for instance. Terminal electrodes are formed on the side surfaces of this multi-layered substrate. Incidentally, the terminal electrodes may be formed on the bottom surface of the multi-layered substrate, and the positions of the terminal electrodes may be properly selected.  
      The internal structure of the multi-layered substrate is shown in  FIG. 15 . The reference numerals in the figure are the same as in the equivalent circuit shown in  FIG. 16 . The second transmission line ld 3  and the first transmission line lp 2  constituting the inductors of the switch, circuit  10  of the present invention are formed in a region sandwiched by the ground electrode G formed on the tenth layer and the ground electrode  15  formed on the 15-th layer, together with other transmission lines lp 1 , ld 2  constituting the switch circuit  10  and transmission lines lg 2 , lg 3  constituting the switch circuit  15  of SPDT. Electrode patterns constituting the second transmission line ld 3  and the first transmission line lp 2  are respectively formed on the 12-th to 14-th layers and connected through viaholes (shown by black circles in the figure). Transmission lines are formed in horizontally different regions, such that they do not overlap in a lamination direction. Such structure can prevent interference between electrode patterns constituting the other circuit elements and transmission lines, thereby improving isolation characteristics.  
      Impedance matching can be achieved by making the electrode pattern constituting the second transmission line ld 3  wider than the electrode pattern constituting the first transmission line lp 2 , and by making the characteristic impedance of the second transmission line ld 3  lower than that of the first transmission line lp 2 , when the first switch  100  and the second switch  105  are integrated into one multi-layered substrate. In this embodiment, the width of the second transmission line ld 3  is 0.25 mm, about two times that of the first transmission line lp 2 .  
      The first capacitor cd 4  arranged between the diode DD 2 , the fourth switching element, and the ground is constituted by the ground electrode G formed on the eighth layer and an opposing capacitor electrode formed on the seventh layer, above the ground electrode G formed on the tenth layer. The capacitor electrode preferably does not overlap the other electrode patterns (particularly capacitor electrode patterns) to prevent interference. However, in the case of forming a multi-layered composite part, it is sometimes difficult to prevent the capacitor electrodes from overlapping other electrode patterns. In this embodiment, accordingly, the capacitor electrodes are separate from the other electrode patterns (a line connected to the control terminal VC 2  formed on the third layer, and a line connected to the receiving terminal RX 1 ) by at least 100 μm in a lamination direction to prevent interference. The second capacitor CP 1  of the second switch  105  is similarly formed on the same layer as the first capacitor cd 4 , sharing the ground layer G on the eighth layer. With such structure, the high-frequency composite part with excellent isolation and insertion loss characteristics were obtained.  
     [6] Sixth Embodiment  
      In the sixth embodiment, the switch circuit  10  of the present invention is integrated in a multi-layered substrate together with the diplexer  300  and the filter circuits  120 ,  125 ,  130 ,  140  in a high-frequency circuit handling three communication systems shown in  FIG. 11 , to constitute a high-frequency composite part (multi-band antenna switch module).  FIG. 17  is a plan view showing the high-frequency composite part,  FIG. 18  is a squint schematic view showing a multi-layered substrate portion of the high-frequency composite part,  FIG. 19  is a development view showing the structure of each layer constituting the multi-layered substrate of  FIG. 18 , and  FIG. 20  is a view showing the equivalent circuit of the high-frequency composite part.  
      In this embodiment, a second transmission line ld 3  is formed in a region sandwiched by the ground electrode G formed on the 12-th layer and the ground electrode G formed on the fifth layer, and part of the ground electrode G formed on the fifth layer that overlaps the second transmission line ld 3  is cut off. Accordingly, the second transmission line ld 3  has higher characteristic impedance than when the ground electrode G is not cut off, thereby making it possible to achieve impedance matching when the first switch  100  and the second switch  105  are integrated into one multi-layered substrate. Also, an electrode pattern formed on the eighth layer to constitute the second transmission line ld 3  does not overlap electrode patterns constituting other circuit elements in a lamination direction. In this embodiment, the electrode pattern is separate from connection lines formed on the second layer by at least 100 μm, to prevent interference.  
      Further, the first capacitor cd 4  disposed between the diode DD 2 , the fourth switching element, and the ground is constituted by the capacitor electrode patterns formed on the fourth layer, in a region sandwiched by the ground electrode G formed on the fifth layer and the ground electrode G formed on the third layer. Such structure prevents interference with the connection lines on the second layer. This embodiment provides the high-frequency composite part with excellent isolation and insertion loss characteristics.  
      The switch circuit of the present invention has been explained in detail without intention of restricting the present invention thereto, and various modifications may be added unless they deviate from the scope of the present invention. Also, communication systems used in the switch circuit of the present invention are not restricted to those in the above embodiments, and the present invention is applicable to combinations of different communication systems having partially overlapping transmitting frequencies and receiving frequencies [for instance, GSM  850  (transmitting frequency: 824 MHz to 849 MHz, receiving frequency: 869 MHz to 894 MHz) and EGSM (transmitting frequency: 880 MHz to 915 MHz, receiving frequency: 925 MHz to 960 MHz)]. For instance, the present invention is applicable to high-frequency circuit blocks handling four different communication systems shown in  FIG. 12 .  
      The present invention provides the switch circuit which has a capable of switching transmitting circuits and receiving circuits with extremely small leakage of transmitting signals to the receiving circuits (extremely large isolation) in pluralities of communication systems which the frequency bandwidth of transmitting signals and the frequency bandwidth of receiving signals partially overlap, and the high-frequency composite part comprising such switch circuit.