Abstract:
A PIN diode antenna RF switch especially suited for a multi-mode transceiver that includes a full duplex mode such as an AMPS analog mobile telephone. Six PIN diodes ( 30, 32, 34, 36, 42, 46 ) are configured as RF switches which are controlled by an arrangement of four DC switches ( 60,64,68,72 ) to produce a high degree of isolation in a path parallel to a duplexer ( 52 ) and low insertion loss in transmitting modes while optimizing current drain.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of antenna switching circuitry. More particularly, this invention relates to an antenna switching circuit arrangement for multi-mode transceivers including a full duplex mode of operation. 
     BACKGROUND 
     Antenna switches are circuits which are commonly used in radio communication devices to direct RF signals along their proper signal paths during receive and transmit functions of the radio communication device. In devices such as cellular telephones and other full duplex transceivers, transmitter and receiver circuits can be active simultaneously while sharing the same antenna. In such transceivers, the transmitted power from the transmitter power amplifier should generally be isolated with a high level of attenuation from the receiver circuitry in order to prevent the transmitted power from damaging the receiver circuitry. This is commonly implemented using a duplexer to isolate the signal paths. Those having ordinary skill in the art will appreciate that a duplexer is normally a device made up of two series band pass filtering devices with a center tap, but any other component configuration which provides the functionality of a duplexer can be used equivalently. 
     The advent of multi-mode transceivers substantially complicates the design requirements for antenna switching circuits while marketplace factors demand long battery life, low cost and high levels of performance. In some designs, it is particularly important to provide high linearity to effect a high adjacent channel coupled power ratio (ACCPR), even under high voltage standing wave ratio (VSWR) conditions. 
     It is desirable to provide an antenna switching circuit which can be used in a variety of applications thereby increasing economies of manufacturing scale while providing the required functionality across multiple configurations of multi-mode transceivers. For example, Motorola, Inc., the Assignee of the present invention, manufactures a series of radios conforming to the iDEN (Integrated Digital Enhanced Network) specification which provides two way “push to talk” type simplex communication in combination with AMPS (Advanced Mobile Phone Service) cellular telephone service. AMPS cellular telephone service is the conventional analog cellular in the United States. Other multi-mode transceiver configurations which can share this common design include iDEN/CDMA (Code Division Multiple Access), TDMA (Time Division Multiple Access)/AMPS, TETRA (Trans-European Trunk Radio) AMPS, and TETRA/CDMA. Other multi-mode transceiver configurations may also be able to adapt use of the antenna switching circuitry disclosed herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with objects and advantages thereof, may be best understood by reference to the following detailed description of the invention, which describes certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a functional block diagram of an antenna switch in accordance with an embodiment of the present invention. 
     FIG. 2 is a functional block diagram of an antenna switch in accordance with an embodiment of the present invention showing only closed RF switches in a first mode of operation. 
     FIG. 3 is a functional block diagram of an antenna switch in accordance with an embodiment of the present invention showing only closed RF switches in a second mode of operation. 
     FIG. 4 is a functional block diagram of an antenna switch in accordance with an embodiment of the present invention showing only closed RF switches in a third mode of operation. 
     FIG. 5 is a functional block diagram of an antenna switch in accordance with an embodiment of the present invention showing only closed RF switches in a fourth mode of operation. 
     FIG. 6 is a schematic diagram of a PIN diode RF switch implementation of an embodiment of an antenna switch in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of the drawings. 
     For purposes of the present discussion, consider a multi-mode transceiver which utilizes AMPS conventional analog telephone service combined with a two way (push to talk) style simplex transceiver in which the user activates a “push to talk” switch in order to talk and releases the “push to talk” switch to listen. In one implementation of such a transceiver, four modes of transceiver operation can be defined. These modes of operation are best understood with reference to FIG.  1 . 
     In a first mode of operation which herein will be referred to as Mode 1, only the duplex receiver is operatively coupled to the antenna. This mode corresponds to, for example in an AMPS cellular telephone, the state wherein the telephone receiver is in standby awaiting receipt of a telephone call. 
     Mode 2 is full transmitter/receiver duplex operation. In this mode of operation, both a transmitter and a receiver are sharing use of the antenna. This mode generally places very high demands on the design constraints of an antenna switch. 
     In Mode 3, only the simplex receiver  14  is operating. This mode corresponds to receipt of signals from a user carrying out a “push to talk” type simplex transmission. 
     In Mode 4, the transmitter is coupled to the antenna for the “push to talk” or simplex type communication method. 
     For ease of explanation, the present invention is being described in terms of an antenna switching device which, in part, switches signals from an antenna to either a simplex receiver or a duplex receiver. However, those of ordinary skill in the art will recognize that only one receiver is generally active at any given time. Accordingly, while the description refers to a simplex receiver and a duplex receiver, both of these receivers may share most or all of the same components. For example, a single receiver may serve the purpose of both the simplex receiver and the duplex receiver by, for example, operating at different frequencies for each mode of operation. This might involve only a change in oscillator frequency and/or input filter frequency. That notwithstanding, the switching will be described as though there are two separate receivers, even though they may be the same physical device. 
     Referring now to FIG. 1 in greater detail, a transmitter circuit (not shown) provides signals to be transmitted to a radio frequency power amplifier  20  which is connected to a circulator  26  in order to provide a constant impedance load for the power amplifier  20 . The output of circulator  26  is coupled to a first RF switch  30  and a second RF switch  32  so that energy can be selectively routed from RF power amplifier  20  to one of two possible paths. A third RF switch  34  is connected in series to the output of RF switch  32  and a fourth RF switch  36  is connected from the junction of RF switch  32  and RF switch  34  to radio frequency ground. The output of RF switch  34  is coupled to two more RF switches, a fifth RF switch  42  and a sixth RF switch  46 . The fifth RF switch  42  is coupled further to an output of a duplexer  52 . The sixth RF switch  46  is further coupled to simplex receiver  14 . The junction of RF switches  34 ,  42  and  46  is coupled to antenna  50 . 
     The input of duplexer  52  is connected to the output of RF switch  30  and duplex receiver  10  is also connected to an output of duplexer  52 . In order to simplify the diagram of FIG. 1 (as well as FIGS. 2 through 5) it will be understood by those of ordinary skill in the art that control circuitry to selectively open and close RF switches  30 ,  32 ,  34 ,  36 ,  42  and  46  is not shown. This control can be implemented in any variety of ways including simple switch actuation by the user and more complex microcomputer or microcontroller control. A more detailed circuit arrangement that illustrates an embodiment of the control of these RF switches will be shown later. 
     In order to more fully appreciate the operation of the present invention in the four functional Modes described earlier, FIG. 1 has been rearranged to show only the active signal paths through RF switches in the ON position in FIGS. 2 through 5. FIGS. 2 through 5 correspond to operational Modes 1 through 4 respectively. 
     Referring now to FIG. 2, in Mode 1, the duplex receiver  10  is operationally coupled to antenna  50  via RF switch  42  and duplexer  52  so that radio frequency energy picked up by antenna  50  is passed through RF switch  42 , duplexer  52  and is received by duplex receiver  10 . In this configuration, in the context of an AMPS cellular telephone, the duplex receiver is receiving signals from antenna  50  to listen for receipt of a telephone call. 
     Referring now to FIG. 3, in Mode 2, duplex receiver  10  also remains active and is connected through duplexer  52  and RF switch  42  to antenna  50  to receive incoming signals. Simultaneously, however, full duplex transmissions may be taking place from the transmitter. In this mode (Mode 2) RF power amplifier  20  is supplying RF power through circulator  26  to RF switch  30 . RF switch  30  is turned ON and supplies this power through duplexer  52  and RF switch  42  to the antenna  50  where the energy is radiated. Due to the need to provide multi-mode switching, as will be appreciated upon consideration of FIGS. 4 and 5, there exists a leakage signal path  56 , shown by broken lines, in which RF energy from RF power amplifier  20  can bypass RF switch  30  and duplexer  52  directly to duplex receiver  10 . This is generally caused by leakages through RF switches  32  and  34 , which, like the other RF switches in common use, do not provide perfect RF isolation when switched in the OFF configuration. This leakage path  56  should provide less energy to duplex receiver  10  from the RF power amplifier  20  than would normally be provided by the duplexer  52 . Preferably, greater than 3 dB more isolation should be provided in the leakage path  56  than through the duplexer. Duplexer  52  may, for example, provide approximately 50 dB of isolation between the transmit and receive paths. Since RF switches such as PIN diodes may typically reach 20 to 25 dB of isolation in the 800 MHz frequency band (for example), RF switch  36  is also turned ON to shunt energy to radio frequency ground to thereby provide an additional measure of isolation in this leakage path  56 . 
     Referring now to FIG. 4, Mode 3 of the transceiver operation is illustrated. In this mode, the simplex receiver  14  is coupled through RF switch  46  directly to antenna  50 . In this mode of operation, the duplexer function  52  is not utilized. The insertion loss from the antenna switch circuitry in this mode is that of a single radio frequency switch  46  thus providing minimal loss of receiver sensitivity. 
     Referring now to FIG. 5, simplex or “push to talk” transmission is illustrated in which energy from the RF power amplifier  20  is coupled through circulator  26  to RF switch  32  and RF switch  34  to the antenna  50 . In this mode of operation, it is important that power be optimally transmitted from the RF power amplifier  20  to the antenna  50  with minimal losses. Therefore, it is important that the insertion loss of RF switches  32  and  34  be minimized. The method for accomplishing this will be described in greater detail later. Any of a number of RF switching devices may be used for RF switches in various antenna switch designs. For example, mechanical relays and Gallium Arsenide field effect transistors may be used. The present implementation preferably utilizes PIN diodes as switching elements for fabricating the RF switches  30 ,  32 ,  34 ,  36 ,  42  and  46 . PIN diodes can be turned ON by forward biasing the diodes and turned OFF by reverse biasing the diodes. DC switching circuits are utilized with various isolation techniques including choke inductors and bypass capacitors to separate the DC and radio frequency components in the PIN diode RF switch implementation. 
     The switched states for RF switches  30 ,  32 ,  34 ,  36 ,  42  and  46  are summarized in Table 1 below. 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 RF SWITCH STATE X = Don&#39;t Care 
               
             
          
           
               
                 MODE 
                 switch30 
                 switch32 
                 switch34 
                 switch36 
                 switch42 
                 switch46 
               
               
                   
               
               
                 1 
                 X 
                 X 
                 Open 
                 X 
                 Closed 
                 Open 
               
               
                 2 
                 Closed 
                 Open 
                 Open 
                 Closed 
                 Closed 
                 Open 
               
               
                 3 
                 X 
                 X 
                 Open 
                 X 
                 Open 
                 Closed 
               
               
                 4 
                 Open 
                 Closed 
                 Closed 
                 Open 
                 Open 
                 Open 
               
               
                   
               
             
          
         
       
     
     The states shown in Table 1 as “X” are don&#39;t care states. That is, from an RF signal point of view, it does not matter what state the RF switches are in. However, from a practical point of view, the PIN diode implementation of the antenna switch to be disclosed in conjunction with FIG. 6, always selects these “don&#39;t care” states as OFF (the PIN diode reverse biased). This is to minimize current drain and thus maximize battery life in a battery powered transceiver; however, other switch configurations may be utilized. 
     Referring now to FIG. 6, a detailed schematic diagram showing a PIN diode implementation of the present invention is shown. In this implementation RF switches  30 ,  32 ,  34 ,  36 ,  42  and  46  are shown as PIN diodes  30 ,  32 ,  34 ,  36 ,  42  and  46 , respectively, for clarity. The switching of PIN diodes in this embodiment is accomplished by a plurality of DC switches  60 ,  64 ,  68  and  72 . These DC switches may be implemented as shown in DC switch  68  with a transistor  74  having a grounded emitter and a base coupled to a switching terminal  78  through a resistor  80 . Thus, the output node of DC switch  68  is either essentially grounded (by applying a forward bias to the base emitter junction of transistor  74  via application of a positive voltage to node  78  so that the collector output terminal  84  is essentially grounded), or at an open circuit (open collector). DC switches  60 ,  64  and  72  are shown schematically as being either a normally open-circuit position or shorted to ground for simplicity. 
     Table 2 below details the bias state of each of the PIN diodes for each of the respective modes of operation. Table 3 below shows the switch state of each of the DC switches  60 ,  64 ,  68  and  72  for each of the four modes of operation according to the present implementation. In Table 3, the “G” indication in mode 4 of DC switch  68  indicates that terminal  84  is coupled to ground via turned ON transistor  74 . Similarly, the G indications for modes 1 through 3 represent closed positions as the switches are shown schematically in FIG.  6 . 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 DIODE BIAS - F = forward, diode ON; R = reverse, diode OFF 
               
             
          
           
               
                 MODE 
                 Diode 30 
                 Diode 32 
                 Diode 34 
                 Diode 36 
                 Diode 42 
                 Diode 46 
               
               
                   
               
               
                 1 
                 R 
                 R 
                 R 
                 R 
                 F 
                 R 
               
               
                 2 
                 F 
                 R 
                 R 
                 F 
                 F 
                 R 
               
               
                 3 
                 R 
                 R 
                 R 
                 R 
                 R 
                 F 
               
               
                 4 
                 R 
                 F 
                 F 
                 R 
                 R 
                 R 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 DC SWITCH OUTPUT STATE - O = open; 
                   
               
               
                   
                 G = Grounded 
               
             
          
           
               
                 MODE 
                 Switch 60 
                 Switch 64 
                 Switch 68 
                 Switch 72 
               
               
                   
               
               
                 1 
                 O 
                 G 
                 O 
                 O 
               
               
                 2 
                 G 
                 G 
                 O 
                 O 
               
               
                 3 
                 O 
                 O 
                 O 
                 G 
               
               
                 4 
                 O 
                 O 
                 G 
                 O 
               
               
                   
               
             
          
         
       
     
     When PIN diodes are forward biased, from an AC/RF modeling point of view, they appear to be a small resistance, the value of which depends upon the amount of DC current flowing through the diode and the actual physical properties of the PIN diode. The higher the current flowing through the diode, the smaller the ON resistance (within the normal operational parameters of the PIN diode). When reverse biased, from a radio frequency modeling point of view, the diode looks predominately like a large resistor value in parallel with a very small capacitance. This small capacitance and large resistance model limits the amount of isolation which can be obtained by turning OFF a single PIN diode through application of reverse bias. High performance PIN diodes such as the BAR 63-02W PIN diodes available from Siemens are suitable for some embodiments of this invention due to their low ON resistance (less than about 1.0 Ohm), but other devices are also suitable. 
     In order to understand the operation of the switching circuit of FIG. 6, the circuitry will be considered operationally mode by mode. Consider first the operation of the antenna switch of FIG. 6 operating in Mode 1. In Mode 1, DC switch  72  is closed while all of the remaining DC switches are open. This applies DC ground to the top side of resistor  90 . DC current flows from V 2  through inductor  92  which serves as an RF choke, through diode  46 , through inductor  94 , to resistor  90  and then to DC ground. This forward biases diode  46  and in one embodiment provides approximately 1 mA of current through the diode turning it ON to an adequate degree to provide good sensitivity to the simplex receiver  14 . The value of resistor  90  can be adjusted to effect a compromise between the amount of forward bias and thus insertion loss of diode  46  and acceptable current drain in Mode 1. Capacitors  100  and  102  provide RF isolation in conjunction with inductors  94  and  92  from the power supply V 2  and V 1 . 
     In the second mode of operation (Mode 2—duplex transmission and reception), PIN diodes  30 ,  36  and  42  are forward biased while the remaining PIN diodes are reversed biased. DC switches  60  and  64  are closed. When DC switch  64  is closed, PIN diode  42  is forward biased by voltage from V 2  passing through inductor  92 , through PIN diode  42  and in turn to inductor  110  and resistor  112  before passing through DC switch  64  to ground. Capacitors  102  and  116  provide RF isolation for the power supplies. PIN diode  30  is forward biased by the closure of DC switch  60  which supplies a biasing current from V 2  through resistor  120  through inductor  124  to PIN diode  30  and in turn through inductor  130  to DC switch  60 . Capacitor  132  and capacitor  134  provide RF isolation. In addition, current from V 2  is supplied through resistor  140  and inductor  142  to PIN diode  36  and in turn through inductor  146  through DC switch  60  to ground to turn ON (forward bias) PIN diode  36 . Capacitors  150  and  152  provide RF isolation to the power supply and switch. In this mode of operation, power from the RF power amplifier  20  passes through circulator  26  and capacitor  160  to diode  30  and capacitor  162 . RF energy then passes through duplexer  52  and capacitor  166  to diode  42  which is forward biased to supply RF energy through capacitor  170  to the antenna  50 . In the receive path, RF energy from a signal received at antenna  50  passes through capacitor  170  to diode  42  and capacitor  166 . The signal is thus delivered to duplexer  52  which in turn supplies the signal to duplex receiver  10 . 
     In this mode of operation (Mode 2) PIN diodes  32  and  34  are reverse biased. The path for DC current providing the reverse bias is from V 1  through resistor  180  and inductor  182  to PIN diode  32 , PIN diode  34  and forward biased diode  42 , inductor  110 , resistor  112  and DC switch  64  which is closed and grounded. V 1 , in this case is selected to be large enough so that under the worse case voltage standing wave ratio conditions (VSWR), the RF energy from the RF power amplifier will not effectively forward bias diodes  32  and  34  to turn them ON. PIN diode  36  effectively shorts out, from a RF point of view, the junction of diodes  32  and  34  through capacitor  190  to RF ground through capacitor  152  so that any RF energy passing from capacitor  160  through capacitor  192  and reaching diode  32 , and leaking through the stray capacitance of diode  32  is shunted to ground through forward biased PIN diode  36 . Any remaining energy at node  196  is blocked by open circuited reverse biased diode  34 . 
     This combination results in an isolation using PIN diodes that is greater than the isolation which duplexer  52  provides between receive and transmit paths. As a result, good receiver sensitivity and high ACCPR is maintained. Capacitor  200  and inductor  182  provide isolation to voltage supply V 1 . All inductors are utilized to choke off RF energy from the DC signal paths. Resistors  120  and  140  are selected for optimal forward bias current. Generally speaking, the current through diode  36  may be much less than the current through diode  30  to achieve the desired isolation in this mode of operation. 
     In Mode 3, only diode  46  is forward biased by closure of DC switch  72 . This produces a forward biasing current from V 2  through inductor  92 , PIN diode  46 , inductor  94  and resistor  90  to DC switch  72  and ground. Received RF energy is coupled from antenna  50  through capacitor  170  to PIN diode  46  and then to simplex receiver  14 . The value of resistor  90  is selected to determine and optimize the forward bias current in diode  46 . Inductor  94  and capacitor  100  provide RF isolation to DC voltage source V 1 . Inductor  92  and capacitor  102  provide RF isolation to DC voltage source V 2 . 
     In Mode 4, only DC switch  68  is closed to produce a DC ground at node  84  and forward bias PIN diodes  32  and  34 . The DC circuit path for providing this forward bias is from V 2  through inductor  92  then to PIN diode  34  and PIN diode  32  through inductor  182 , resistor  210  and DC switch  68  to ground. In this configuration, a single current path is utilized to forward bias both of PIN diodes  32  and  34  with the amount of forward bias current being dependent upon the selection of resistor  210 . In this case, it is desirable to provide a very low level of insertion loss at diodes  32  and  34  so that power emanating from RF power amplifier  20  is not ineffectively dissipated by the insertion loss of PIN diodes  32  and  34  prior to reaching antenna  50 . Accordingly, a significant amount of forward bias should be applied to PIN diodes  32  and  34  in order to minimize the insertion loss to, for example, less than 0.5 dB. More stringent designs may require that the forward bias current through these diodes produce an insertion loss of less than, for example, 0.3 dB or 0.2 dB. In any event, since the PIN diodes are in series, the amount of current overall required to forward bias diodes  32  and  34  is supplied in a single path in order to minimize the overall current drain on the radio&#39;s battery. 
     In the reverse bias configuration, only a minimal amount of current flows. Resistors  180 ,  220 ,  222  and  224  are generally selected to be large resistors such as 330 K ohms to minimize current drain while providing adequate reverse bias. The value of V 1 , as previously stated, is selected to be large enough to prevent RF energy from forward biasing any of the PIN diodes during transmission under worst case VSWR conditions. In the current embodiment 35 V is adequate to insure that such forward biasing by RF energy does not occur. Each of the nodes illustrated as V 1 , is diode isolated from a 35 V DC source in order to provide isolation of reverse power supply between different parts of the circuit. Resistors in series with each of the DC switches can be adjusted to determine the amount of forward bias current used to forward bias the various PIN diodes under each of the various operational modes. Thus the design is readily optimized to provide minimal current drain in receive modes while providing minimum insertion loss in the various transmit modes and high ACCPR. In Mode 4, inductor  92  and capacitor  102  provide RF isolation to DC voltage source V 2 , while inductor  182  and capacitor  200  provide RF isolation to DC voltage source V 1 . 
     While the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, permutations and variations will become apparent to those of ordinary skill in the art in light of the foregoing description without departing from the spirit and scope of the invention. By way of example, and not limitation, the PIN diodes of the present invention may be replaced by equivalent mechanical or solid state switching devices including hot carrier diodes, GasFETs or relays which have suitable properties for the particular design constraints of the implementation of interest. Moreover, while the particular DC switching arrangement shown effects the desired switching of the DC bias of the PIN diodes, similar arrangements can often be devised which reverse the polarity of the PIN diodes with complementary changes to the DC biasing and DC switching. Such changes are equivalent and contemplated. Accordingly, it is intended that the present invention embrace all such alternatives, modifications and variations as fall within the scope of the appended claims.