Abstract:
Systems and methods are disclosed for sharing a network with two switching devices forming a single pole double throw (SPDT) switch. Two SPDT switches can also be used in a double pole double throw (DPDT) configuration that shares a network, thus reducing component count and die size.

Description:
BACKGROUND  
       [0001]     Radio frequency (RF) signals often must be switched between two destinations, such as when switching an RF signal between a first antenna and a second antenna. Switches that support this configuration are classified as single pole, double throw (SPDT) switches.  
         [0002]     SPDT switches known in the art are either solid-state devices or mechanical relays. Mechanical switches are generally quite large, compared to other RF components, and consume significant amounts of power. In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. However, for high frequency RF signals in the gigahertz range and above, these devices could suffer from several shortcomings. For example, the device has high insertion loss, which is the loss across the switch when the switch is closed. On the other hand, the device has low isolation, which means signal ‘leakage’ across the switch when the switch is open. In addition, the insertion losses and isolation values for these switches varies depending on the frequency of the signal passing through the switches, which cause unevenness in frequency response and is unsuitable for broadband wireless communications. Isolation between the two output ports of the SPDT switch is of another concern, since coupling of the signal from one output port to the other output port limits the effectiveness of the switch as a dual output port device. Without proper circuitry to overcome the above-mentioned characteristics, semiconductor transistors and pin diodes would not function properly as switches in microwave applications.  
         [0003]      FIG. 1A  shows a basic prior art SPDT circuit, while  FIG. 2A  shows an improved prior art SPDT circuit with a parallel inductor. In  FIG. 1A , a first switching device T 101  such as a FET or PHEMT (Pseudomorphic High Electron Mobility Transistor) is positioned between the first port P 1  and the second port P 2 . Correspondingly, a second switching device T 102  is positioned between the second port P 2  and the third port P 3 .  
         [0004]     A simplified equivalent circuit with transistor off can be represented by a parasitic capacitor between the drain terminal and the source terminal. The insertion loss of the SPDT switch is determined by the resistance value between drain and source of each transistor. As shown in  FIG. 1B , when a semiconductor switching device is turned on, the device can be modeled with a resistor R. When a switching device is turned off, the device can be modeled with a capacitor C. Thus, when the first switching device is turned on and the second switching device is turned off, an equivalent circuit between P 2  port and P 3  port can be modeled as a resistance in series with a capacitance, with P 1  port at the junction between the resistance and capacitance. The resistance results in loss of signal strength, which is commonly known as insertion loss. In addition, the signal can leak through the capacitance of the “off state” switching device, especially at high frequency. For instance, as shown in  FIGS. 1C and 1D , using R=4 ohms and C=0.4 pF, there is a −1.7 dB insertion loss in transmission at 5 GHz between P 1  and P 2  and −6.5 dB of isolation at 5 GHz between P 1  and P 3 .  
         [0005]     To reduce the insertion loss, inductors can be added in parallel to the switching devices. The inductance and the capacitance of the switching device becomes a LC resonator with high impedance at the resonating frequency. For example, two inductors L are added in parallel to the source and the drain ends or terminals of the switching device T 201  and T 202  of  FIG. 2A . When the first switching device T 201  is turned on and the second switching device T 202  is turned off, the circuit of  FIG. 2A  can be modeled by an equivalent circuit as a resistor R in a parallel with the inductor L and a capacitor C in parallel with inductor L, as shown in  FIG. 2B . Assuming that L=2.5 nH, the insertion loss in transmission between P 1  and P 2  has been improved to −0.5 dB at 5 GHz, as shown in  FIG. 2C . Moreover, the isolation between P 1  and P 3  has been improved to −27 dB at 5 GHz as shown in  FIG. 2D .  
         [0006]     U.S. Pat. No. 5,774,792 uses such an approach where an SPDT switch includes a plurality of FETs, the FET on the receiver side through which a received signal passes and the shunt FET on the transmitter side are each formed of series-connected FETs, and a capacitor is connected between the first gate and the source and between the second gate and the drain. An inductance is connected in parallel with a series connection of FETs. Similarly, U.S. Pat. No. 6,693,498 uses a plurality of FETs in the SPDT switch and a first inductor connected in parallel to the first FET, and a second inductor connected in parallel to the second FET. The SPDT switch may further include a first inductor connected in series to the first FET, a first capacitor connected in parallel to the series connection of the first FET and the first inductor, a second inductor connected in series to the second FET, and a second capacitor connected in parallel to the series connection of the second FET and the second inductor.  
         [0007]     In integrated circuits, size and space constraints are of primary importance. One problem with adding multiple inductors or other passive RF components to microcircuits is that the increase of the die size and thus increasing the cost of a switch.  
       SUMMARY  
       [0008]     Systems and methods are disclosed for sharing a network with two switching devices forming a single pole double throw (SPDT) switch. Two SPDT switches can also be used in a double pole double throw (DPDT) configuration that shares a network, thus reducing component count and die size.  
         [0009]     In a first aspect, a multi-port circuit to switch radio frequency (RF) signals among one or more RF signal paths includes a first RF switching device; a second RF switching device coupled to the first RF switching device; a third RF switching device coupled to the second RF switching device; a fourth RF switching device coupled to the third and first RF switching devices; and a shared circuit coupled to the first and third RF switching devices.  
         [0010]     Implementations of the aspect may include one or more of the following. Each switching device can be a transistor. The shared circuit can be a frequency selecting circuit, a matching network, a resonator, a filter, a micro-strip, or a transmission line. The shared circuit can also be a resistor, a capacitor or an inductor.  
         [0011]     In yet another aspect, an SPDT switch having first, second and third ports includes first and second switching devices each having first and second terminals, wherein the first terminals are coupled to the first port and the second terminals are coupled to the second and third ports respectively; and a shared network positioned between the second and third ports.  
         [0012]     The shared network can be a frequency selecting circuit, a matching network, a resonator, a filter, a micro-strip, or a transmission line. The shared network can also be a resistor, a capacitor or an inductor. The ports can be connected to a transmitter, a receiver or an antenna.  
         [0013]     In yet another aspect, a double pole double throw (DPDT) switch is constructed by using two SPDT switches. The DPDT switch has first, second, third and fourth ports and includes first and second switching devices each having first and second terminals, wherein the first terminals are coupled to the first port, and the second terminals are coupled to the second port and the third port, respectively; third and fourth switching devices each having third and fourth terminals, wherein the third terminals are coupled to the fourth port, and the fourth terminals are coupled to the second port and third port, respectively; and a shared network positioned between the second and third ports.  
         [0014]     Advantages of the invention may include one or more of the following. The switch circuits provide low transmission loss, high isolation, miniaturization and low cost. The system provides higher performance and increased miniaturization for multi port, multi frequency switches. These features are ideal for RF applications that operate with multiple band communication systems such as GSM, DCS, CDMA, W-CDMA, Wireless LAN, among others.  
         [0015]     Instead of multiple sets of passive component as required by the multiple switching transistors in a typical SPDT or DPDT switches, the entire SPDT or DPDT switch requires only one shared set of passive circuit to operate, even for multi frequency signals. Due to the reduced number of passive LC components employed in the invention, the entire switch circuitry can be fabricated to fit in a small substrate space with lower cost. Furthermore, the frequency selecting circuitry can include filter, impedance matching, and other RF frequency shaping function using inductor, capacitor and RF micro strip, transmission line, and etc. Thus, the shared frequency selecting circuit can also function as impedance match for the transmission path. Moreover, certain embodiments of the switch are capable of switching signals within multiple frequencies by changing the operating frequency of the frequency selecting circuit.  
         [0016]     Consequently, the switch can be especially useful for microwave and millimeter wave signal switching applications. Additional advantages are that the switch exhibits low insertion loss and mismatch loss along the “ON” RF path, thus creating very little power loss over the bandwidths of interest. Also, the switch exhibits high RF isolation along the “OFF” RF path.  
         [0017]     The single SPDT switch as well as the dual SPDT switch can be easily controlled and reduced in size and cost. Furthermore, a communication unit using such an SPDT or dual SPDT switch can also be reduced in size and cost.  
         [0018]     Additional features and advantages of the invention will be set forth in the description, which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. This invention is not limited to any specific semiconductor technology. For example, the invention can be implemented with various semiconductor materials such as PHEMT, MESFET, and CMOS, among others. Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings, in which like references denote like elements and parts.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     In order that the manner in which the above recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated, in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0020]      FIGS. 1A-1D  show a basic prior art SPDT circuit, an equivalent circuit, a simulated transmission characteristic plot and a simulated isolation characteristic plot, respectively.  
         [0021]      FIGS. 2A-2D  show an improved prior art SPDT circuit, an equivalent circuit, a simulated transmission characteristic plot and a simulated isolation characteristic plot, respectively.  
         [0022]      FIGS. 3A-3D  show a first embodiment of an SPDT circuit, an equivalent circuit, a simulated transmission characteristic plot and a simulated isolation characteristic plot, respectively.  
         [0023]      FIGS. 4A-4D  show a second embodiment of a dual SPDT circuit, an equivalent circuit, a simulated transmission characteristic plot and a simulated isolation characteristic plot, respectively.  
         [0024]      FIGS. 5A-5D  show a third embodiment of a dual SPDT circuit for two frequency bands, an equivalent circuit, a simulated transmission characteristic plot and a simulated isolation characteristic plot, respectively.  
         [0025]      FIG. 6  shows a fourth embodiment of a circuit of multiple SPDT switches that shares one or more common devices.  
     
    
     DESCRIPTION  
       [0026]     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.  
         [0027]      FIG. 3A  shows a first embodiment of an SPDT circuit in accordance with one aspect of the invention. In this embodiment, an SPDT switch includes a first port P 1 ; a second port P 2 ; and a third port P 3 . The switching devices T 301  and T 302  have gate terminals that control signal flow across the source and drain terminals. In one embodiment, ports P 2  and P 3  correspond to the source terminals of the transistors T 301  and T 302 , respectively, and port P 1  coupled to the drain terminals of the transistors. In another embodiment, P 2  and P 3  correspond to the drain terminals of the transistors T 301  and T 302 , respectively, and port P 1  coupled to the source terminals of the transistors. An inductor L 300  is connected between P 2  and P 3 .  
         [0028]     During operation, one of the transistors is ON, while the other is OFF. To illustrate, if transistor T 301  is ON, a positive voltage is applied to the gate of T 301 , while a ground voltage is applied to the gate of T 302  to turn T 302  OFF. In such an example,  FIG. 3B  shows an equivalent circuit where the resistor R is in series with a capacitor C, with the inductor L 300  in parallel.  
         [0029]     In a simulation with R=4 ohms, C=0.4 pF, and L 300 =2.4 nH, the transmission characteristic of  FIG. 3A  is charted in  FIG. 3C , and the isolation characteristic of  FIG. 3A  is charted in  FIG. 3D . The charts show that at 5 GHz, the insertion loss in transmission between P 1  and P 2  is at −0.6 dB, and the isolation between P 1  and P 3  is at −23 dB. In comparison to prior art as shown in  FIG. 2A , prior art needs two inductors, one inductor for each transistor, while the invention in  FIG. 3A  needs only one inductor.  
         [0030]     In one implementation, a single pole double throw (SPDT) switch includes a first port, a second port, a third port. A first transistor with its source terminal and the drain terminal is connected to the first port and the second port. Corresponding, a second transistor with its source terminal and the drain terminal is connected to the first port and the third port. An inductor is positioned across to the second and third ports. In an application as a transmit/receive (T/R) switch, the first port can be connected to an antenna, the second and third ports can be connected to the RF output of a transmitter and the RF input of a receiver. In an application for antenna diversity, the first port can be connected to either transmit or receive signal, while the second and third ports can be connected to a plurality of antennas.  
         [0031]     The inductor sharing can be expanded into multiple SPDT configurations for multiple bands.  FIG. 4A  shows a second embodiment for a set of two SPDT switches connected together, which is generally known as a DPDT switch. Each SPDT consists of two transistors with the same arrangement as shown in  FIG. 3A . As shown in  FIG. 4A , the first SPDT consists of T 401  and T 402 , and the second SPDT consists of T 403  and T 404 . The two SPDT switches are connected to each other at the P 2  and P 3  ports. An inductor L 400  is connected between P 2  and P 3 . As a comparison, the embodiment of invention as shown in  FIG. 4A  needs only one inductor while the prior art technique requires one inductor for each transistor or a total of four inductors.  
         [0032]     During a typical operation, one of the transistors T 401  -T 402  is OFF, while the other is ON. Similarly, one of the transistors T 403 -T 404  is OFF, while the other is ON. As shown  FIG. 4A , T 402  and T 403  are turned OFF by applying a ground voltage to their gates, and T 201  and T 204  are turned ON by applying a positive voltage to their gates. In such an example,  FIG. 4B  shows an equivalent circuit for the DPDT switch. The equivalent circuit is two series RC connected in parallel with the inductor L 400  connected between the RC junctions P 2  and P 3 .  
         [0033]     In a simulation with R=4 ohms, C=0.4 pF, and L=1.3 nH, the transmission characteristic of  FIG. 4A  is charted in  FIG. 4C , and the isolation characteristic of  FIG. 4A  is charted in  FIG. 4D . The simulated charts show that at 5 GHz, the insertion loss of transmission is at −0.9 dB, and the isolation is at −24 dB.  
         [0034]      FIG. 5A  shows a third embodiment of a dual SPDT circuit optimized for two frequency bands, one at about 2.5 GHz and the other at about 5 GHz.  FIG. 5A  shows a second embodiment for a set of two SPDT switches connected together. Each one of the SPDT switches consists of two transistors with the same arrangement as illustrated in  FIG. 3A . As shown in  FIG. 5A , the first SPDT consists of T 501  and T 502 , and the second SPDT consists of T 503  and T 504 . P 2  and P 3  are connected by a frequency selecting circuit. The frequency selecting circuit consists of inductors L 500  and L 501  and a capacitor C 501 . C 501  is connected in parallel with L 501 , and the C 501 -L 501  parallel combination is connected in series with L 500 . Both C 501  and L 501  are coupled in series with the first inductor L 500 .  
         [0035]     During operation, one of the transistors T 501 -T 502  is OFF, while the other is ON. Similarly, one of the transistors T 503 -T 504  is OFF, while the other is ON. As shown  FIG. 5A , T 502  and T 503  are turned OFF by applying a ground voltage to their gates, and T 501  and T 504  are turned ON by applying a positive voltage to their gates. In such an example,  FIG. 5B  shows an equivalent circuit for the DPDT switch. The equivalent circuit is two series RC connected in parallel with the frequency selecting circuit connected to the RC junctions P 2  and P 3 . The frequency selecting circuit consists of the inductor L 500  connected to the RC junction P 3  and the other end of the inductor L 500  connected to a second capacitor C 501  and a second inductor L 501  in parallel. The other ends of the capacitor C 501  and second inductor L 501  are connected to the RC junction P 2 .  
         [0036]     In a simulation with R=4 ohms, C=0.4 pF, C 501 =2.7 pF, L 500 =1.7 nH, and L 501 =1.1 nH, the simulated transmission characteristic of  FIG. 5A  is charted in  FIG. 5C , and the isolation characteristic of  FIG. 5A  is charted in  FIG. 5D . The simulated charts show that at 2.5 GHz, the insertion loss is at −0.9 dB, and the isolation is at −25 dB. Additionally, at 5 GHz, the insertion loss is at −1.1 dB, while the isolation is at −22 dB.  
         [0037]      FIG. 6  shows a fourth embodiment, where a multi-port circuit for switch radio frequency (RF) signals among one or more RF paths. It includes two sets of SPDT switches T 601 -T 602  and T 603 -T 604 . Both SPDT switches share a common circuit network N 600 . The shared circuit can be a frequency selecting circuit, a matching network, a resonator, a filter, a micro-strip, or a transmission line, among others. The shared circuit can also be a resistor, a capacitor or an inductor, among others.  
         [0038]     The embodiment of  FIG. 6  switches RF signals by providing one or more SPDT switches coupled together at a plurality of ports and sharing a network between the ports. The count of the switches that share the network could also be a power of two, e.g., 4, 6, 8, or 16 SPDT switches sharing the same network.  
         [0039]     As illustrated above, conventional switching circuits need inductors at each of the switching semiconductor devices. Such circuits need the large area for the inductors. In the present invention, by the sharing the inductors or the frequency selecting circuit, the total switch circuit can be miniaturized. Further, the switch circuit is not used only for the SPDT type, but also for DPDT type. The switch circuit can operate at multiple operation frequency bands (for example, 2.5 GHz and 5 GHz). Further, the invented switch circuit keeps high performances but uses only one inductor or frequency selecting circuit in the multi-port applications. The shared frequency selecting circuit can also other components for impedance matching, filtering, micro strips, resonators, transmission lines circuits, which are known to those who are familiar to the art of microwave circuits.  
         [0040]     It is noted that the input and output port of a switch is symmetric and non-directional in general, and the external connection of P 2  and P 3  of  FIG. 3A  is not restricted to a first antenna and a second antenna. Using SPDT as an example, P 2  and P 3  can be connected to a pair of receiver and transmitter while P 1  is connected to an antenna at the same time. Similarly, in the case of DPDT switch of  FIG. 4A  and  FIG. 5A , P 2  and P 3  can be connected to a pair of receiver and transmitter, while P 1  and P 4  are connected to a first antenna and a second antenna, and vice versa.  
         [0041]     Although specific embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the particular embodiments described herein, but is capable of numerous rearrangements, modifications, and substitutions without departing from the scope of the invention. The following claims are intended to encompass all such modifications.