Duplexer structure for coupling a transmitter and a receiver to a common antenna

[0001] The invention relates generally to radio frequency transceivers and, more particularly, to transceivers having a transmitter and a receiver that share a common antenna.

[0002] It is often desirable to have a radio frequency (RF) transmitter and receiver share a common antenna. This practice reduces the overall cost of implementing an RF system by eliminating one antenna. It also ensures that the transmit and receive beams of the system are identical. A number of challenges arise, however, in implementing a shared antenna arrangement. For example, during transmit operations, the relatively high transmit power generated by the transmitter cannot be permitted to overload the front end of the receiver. In addition, during a receive operation, the received signal must be directed from the antenna to the receiver input with little loss in the intervening circuitry. Furthermore, both the transmitter and the receiver must be appropriately matched to the antenna during transmit and receive operations, respectively, to enhance power transfer between these elements. Duplexer structures are normally employed to address these issues.

[0003] An ongoing trend in the electronics industry is to integrate entire systems or subsystems onto a single semiconductor chip using, for example, very large scale integration (VLSI) techniques. Because of its characteristic low power consumption, low cost, and high integration density, complementary metal-oxide-semiconductor (CMOS) technology is often used to achieve this integration. The integration of RF transceiver subsystems, however, has thus far presented a major challenge to circuit designers. This is because RF transceivers typically make extensive use of passive components (e.g., inductors and capacitors) which are not easily implemented on a semiconductor chip. Another reason is because RF transceivers are generally required to detect very small signals from an antenna which can be obscured by, for example, digital switching noise when the RF circuitry is implemented on the same chip as digital circuitry performing control and signal processing functions for the transceiver. This is especially true for RF transceiver subsystems that utilize antenna sharing.

[0004] FIG. 1 is a block diagram illustrating an RF transceiver in accordance with one embodiment of the present invention;

[0005] FIG. 2 is a block diagram illustrating the internal configuration of a duplexer within the RF transceiver of FIG. 1 in one embodiment of the present invention; and

[0006] FIG. 3 is a schematic diagram illustrating a differential duplexer circuit in accordance with one embodiment of the present invention.

[0007] In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the spirit and scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views.

[0008] The present invention relates to a duplexer structure that allows an RF transmitter and an RF receiver to share a common antenna. The duplexer includes impedance matching circuitry to match the receiver input impedance to the antenna impedance during a receive operation and impedance transformation circuitry to transform the antenna impedance to a lower impedance at the receiver input terminals during a transmit operation (to protect the receiver from damage). Significantly, the impedance matching circuitry and the impedance transformation circuitry of the duplexer share one or more passive components. This component sharing reduces the overall number of passive components within the corresponding RF transceiver, thus increasing the ease with which the RF transceiver is integrated onto a semiconductor chip and reducing the amount of chip area required to implement the duplexer. In one embodiment, the duplexer uses a differential topology to provide common mode noise rejection and even harmonic distortion cancellation. This is beneficial when the duplexer is located within a hostile digital environment (e.g., when the duplexer is implemented on the same chip with digital control circuitry). The differential topology also allows the duplexer structure to take advantage of the high integration capabilities of CMOS technology.

[0009] FIG. 1 is a block diagram illustrating an RF transceiver 10 in accordance with one embodiment of the present invention. As shown, a receiver 12 and a transmitter 14 are each coupled to a duplexer 16 that communicates with an external antenna 18. The duplexer 16 includes an input 20 for receiving a transmit/receive (TX/RX) signal that is indicative of the present operational state of the transceiver. If the TX/RX signal indicates that the transmitter 14 is currently active (i.e., a transmit operation is being performed), the duplexer 16 is configured so that the transmitter 14 is appropriately coupled to the antenna 18 to deliver a relatively high power transmit signal thereto. Although the voltage swing at the antenna port 22 will typically be relatively high during the transmit operation, the duplexer 16 is designed so that the voltage swing at the input port 24 of the receiver 12 remains relatively low during the transmit operation to prevent damage to the front end receiver circuitry. As will be described in greater detail, this is accomplished by implementing an impedance transformation within the duplexer 16 that transforms the relatively large antenna impedance to a much lower impedance at the input port 24 of the receiver 12.

[0010] If the TX/RX signal indicates that a receive operation is being performed, the duplexer 16 is configured so that the antenna 18 is coupled through to the receiver 12 in a relatively low loss manner. In previous transceiver designs, the receiver has typically been internally matched so that an input impedance of the receiver was the same as, or similar to, the antenna impedance. Thus, the intervening duplexer in these designs only needed to provide a through connection at substantially the same impedance to generate a low loss connection between the antenna and the receiver during receive mode. In one aspect of the present invention, however, the duplexer 16 includes matching circuitry for matching the input impedance of the receiver 12 to the impedance of the antenna 18 during receive operations, thus reducing or eliminating the need to provide internal matching within the receiver 12. In addition, in accordance with one embodiment of the invention, this matching circuitry shares one or more passive components with the impedance transformation circuitry used during transmit operations. As can be appreciated, this component sharing reduces the amount of chip space required to implement the duplexer and simplifies the integration process.

[0011] FIG. 2 is a block diagram illustrating an internal configuration of the duplexer 16 of FIG. 1 in one embodiment of the present invention. As shown, the duplexer 16 includes a reconfigurable network 28 that is configured as an impedance matching network during receive operations and as an impedance transformation network during transmit operations. In the illustrated embodiment, the input 24 of the receiver 12 is directly coupled to the output 32 of the transmitter 14 within the duplexer 16. In another embodiment, as will be described in greater detail below, additional switching functionality is provided within the duplexer 16 for isolating the transmitter 14 during receive operations. The network 28 receives the TX/RX signal at input 20 which configures the network 28 based on the current operational state of the transceiver 10. The impedance transformation of the network 28 during transmit operations transforms the antenna impedance at the antenna port 22 (e.g., 50 ohms) to a relatively low impedance (e.g., 10 ohms) at the input port 24 of the receiver 12. Thus, although the voltage swing at the antenna port 22 will typically be relatively high during a transmit operation to achieve a desired transmission range, the voltage swing at the input terminal 24 of the receiver 12 will remain low. The impedance transformation is designed so that the maximum anticipated voltage swing at the antenna port 22 during a transmit operation will require a voltage swing at circuit node 34 (and thus the input port 24 of the receiver 12) that is below a value that could potentially damage the receiver front end. The impedance transformation should also provide a good impedance match between the output 32 of the transmitter 14 and the antenna 18.

[0012] As described above, in one embodiment of the present invention, the impedance matching network and the impedance transformation network implemented within the duplexer 16 share one or more passive circuit elements. That is, the impedance matching network will use the circuit element(s) during receive operations and the impedance transformation network will use the circuit element(s) during transmit operations. By implementing element sharing within the duplexer 16, the physical size of the resulting circuit can be reduced. FIG. 3 is a schematic diagram illustrating a duplexer circuit 40 that employs element sharing in accordance with one embodiment of the present invention. The duplexer circuit 40 is a differential circuit and thus provides common mode noise isolation and even mode distortion cancellation. The duplexer circuit 40 includes first and second receiver terminals 42, 44 for connection to a low noise amplifier (LNA) 50 that is part of an RF receiver; first and second transmitter terminals 46, 48 for connection to a power amplifier (PA) that is part of an RF transmitter; and first and second antenna terminals 56, 58 for connection to an antenna assembly 54. As illustrated, the antenna assembly 54 includes an optional balun 60 for use in coupling the balanced antenna terminals 56, 58 to a single-ended antenna structure 62 (if this is the type of antenna used).

[0013] The duplexer 40 includes an input terminal 64 to receive a control signal PAon that is high whenever the power amplifier 52 is active and low at all other times. The input terminal 64 is connected to the gate terminals of three n-type metal oxide semiconductor (NMOS) transistor switches (M₁, M₂, and M₃) 66, 68, 70 within the duplexer 40. The input terminal 64 is also connected to the gate terminals of two p-type metal oxide semiconductor (PMOS) transistor switches (M₉ and M₁₀) 66, 68, 70 through an inverter 86. The PAon signal is also delivered to a disable transistor (M₈) 76 within the LNA 50 to deactivate the LNA 50 when the power amplifier 52 is active. In the illustrated embodiment, this is achieved by shorting the bias to the first stage current source within the LNA 50 when PAon is high. As illustrated, the first receiver terminal 42 is connected to the first antenna terminal 56 through a first inductor (L₁) 72 and the second receiver terminal 44 is connected to the second antenna terminal 58 through a second inductor (L₂) 74. The first NMOS transistor 66 is connected at one source/drain terminal to a first capacitor (C₁) 78 and at the other source/drain terminal to a second capacitor (C₂) 80. The first and second capacitors 78, 80 are each coupled at another end to the first and second antenna terminals 56, 58, respectively. The second NMOS transistor 68 and the first PMOS transistor 82 are each connected at one source/drain terminal to the first transmitter terminal 46 and at the other source/drain terminal to the first receiver terminal 42. Similarly, the third NMOS transistor 70 and the second PMOS transistor 84 are each connected at one source/drain terminal to the second transmitter terminal 48 and at the other source/drain terminal to the second receiver terminal 44.

[0014] As discussed above, when the power amplifier 52 is active, the PAon signal is logic high. Thus, the first, second, and third NMOS transistors 66, 68, 70 and the first and second PMOS transistors 82, 84 are turned on. The second NMOS transistor 68 and the first PMOS transistor 82 act collectively as a pass gate for coupling the first transmitter terminal 46 to the first receiver terminal 42 when the PAon signal is logic high. Similarly, the third NMOS transistor 70 and the second PMOS transistor 84 act collectively as a pass gate for coupling the second transmitter terminal 48 to the second receiver terminal 44 when the PAon signal is logic high. In addition, the first NMOS transistor 66 appears as a very low impedance (e.g., a short circuit) between the first and the second capacitors 76, 78 when the PAon signal is high. Therefore, during a transmit operation, the power amplifier 52 is coupled to the antenna 54 through an LC network consisting of the first and second series inductors 72, 74 and the first and second shunt capacitors 78, 80. The inductance value of the first and second inductors 72, 74 and capacitance value of the first and second capacitors 76, 78 are selected so that the resulting LC configuration matches the output impedance of the power amplifier 52 to the antenna impedance at the antenna terminals 56, 58 (i.e., using an L match arrangement). Importantly, the LC configuration also behaves as an impedance transformation between the antenna terminals 56, 58 and the receiver terminals 42, 44 to transform the antenna impedance at the antenna terminals 56, 58 to a relatively low impedance at the receiver terminals 42, 44. It should be appreciated that other circuit configurations for performing this matching/impedance transformation function are also possible, including, for example, the well known match circuit. Methods for designing such circuits are well known in the art.

[0015] Because the impedance is low at the receiver terminals 42, 44 during transmit, the transmit signal being generated by the power amplifier 52 will maintain a relatively low voltage swing at the receiver terminals 42, 44. In accordance with the invention, the transmit voltage swing at the receiver terminals 42, 44 will be kept below a value that could potentially damage the input devices M₄ and M₅ (or other circuitry) within the LNA 50. Because the impedance at the receiver terminals 42, 44 is low, the currents being developed at the first and second transmitter terminals 46, 48 will be relatively high. However, because the input impedance of the LNA 50 is high, this current will not flow into or damage the LNA 50. The second and third NMOS transistors 68, 70 and the first and second PMOS transistors 82, 84, on the other hand, must be designed to handle this current load. The transmit signal generated by the power amplifier 52 will be transformed by the action of the LC configuration into a relatively high voltage swing signal at the antenna terminals 56, 58. Thus, the signal transmitted by the antenna 62 will have sufficient power to reach a remote transceiver with adequate signal level. Although the voltage swing on the antenna terminals 56, 58 will be relatively high, the first transistor switch 66 will not be damaged because it is located at a low voltage virtual ground between the antenna terminals 56, 58.

[0016] When the PAon signal goes low, the first, second, and third NMOS transistors 66, 68, 70 and the first and second PMOS transistors 82, 84 within the duplexer 40 are turned off. In addition, the disable transistor 76 within the LNA 50 is turned off, thus enabling the LNA 50. The first and second transmitter terminals 46, 48 are now isolated from the first and second receiver terminals 42,44, respectively. Furthermore, the first NMOS transistor 66 now appears as an open circuit between the first and second capacitors 78, 80, effectively removing the capacitors 78, 80 from the circuit. In this mode, the first and second inductors 72, 74 are used as matching elements to match the antenna impedance at the antenna terminals 56, 58 to the input impedance of the LNA 50. In one approach, the parasitic capacitances of the second and third NMOS transistors 68, 70 and the first and second PMOS transistors 82, 84 (in the off state) are used as additional tuning elements to match the antenna 62 to the LNA 50. Transistor design methods for achieving a desired parasitic capacitance at the appropriate terminals of the second and third transistor switches 68, 70 for a given set of bias conditions are known in the art.

[0017] The selection of the inductance value of the first and second inductors 72, 74 and the capacitance value of the first and second capacitances 78, 80 (and, if used, the parasitic capacitance value of the second and third NMOS transistors 68, 70 and the first and second PMOS transistors 82, 84 in the off state) is preferably performed as part of a single design process to achieve the dual goals of providing transformer action during transmit and impedance matching during receive. Techniques for accomplishing this are well known in the art and normally involve the use of computer based circuit design and analysis tools. The design may require that some additional matching circuitry be implemented within the LNA 50 and/or the power amplifier 52 to interact with the circuit elements within the duplexer 40 during receive and/or transmit mode to adequately match the antenna 62 to the LNA 50 and the power amplifier 52. The actual design that is implemented will depend upon the intrinsic input impedances of the LNA 50 and the power amplifier 52 before impedance matching/transformation is undertaken. It should be appreciated that the particular arrangement of inductances and capacitances within the duplexer 40 of FIG. 3 is merely an illustration of one possible approach. As will be apparent to a person of ordinary skill in the art, many alternative circuit element configurations can be implemented to achieve the dual goals of providing transformer action during transmit and impedance matching during receive.

[0018] In the illustrated embodiment, the three NMOS transistors 66, 68, 70 and the two PMOS transistors 82, 84 are metal-oxide-semiconductor field effect transistors (MOSFETs). This allows the duplexer 40 to be easily integrated within a CMOS environment. It should be appreciated, however, that other transistor types can also be used in accordance with the present invention, such as other insulated-gate FET structures, bipolar junction transistors, junction FETs, and others. In one embodiment of the invention, the duplexer 40 of FIG. 3 is implemented without the second and third NMOS transistors 68, 70 and the first and second PMOS transistors 82, 84. In this embodiment, the first and second transmitter terminals 46, 48 are connected directly to the first and second receiver terminals 42, 44, respectively. Using well known RF techniques, the length of the lines coupling the transmitter terminals 46, 48 to the receiver terminals 42, 44 can be adapted to provide a minimal impedance effect (e.g., an open circuit) at the receiver input terminals 42, 44 during receive mode.

[0019] Using the duplexer 40 of FIG. 3, it is possible to connect multiple transceivers to a single antenna 62, as long as only one transceiver is allowed to operate at any given time. In one approach, multiple power amplifiers 52 are connected to the transmitter terminals 46, 48 in parallel and multiple LNAs 50 are connected to the receiver terminals 42, 44 in parallel. The PAon signal is high if any of the power amplifiers 52 are enabled. As long as only a single power amplifier 52 or a single LNA 50 is operative at a time, the duplexer 40 will operate in substantially the same manner described above. Alternatively, additional transistor switches can be provided at the transmitter terminals 46, 48 and/or the receiver terminals 42, 44 to switch between the multiple transceivers in response to a control signal. This technique provides greater isolation between individual transceivers, but adds complexity to the duplexer 40. Other multi-transceiver topologies are also possible. In particular, simultaneous operation of multiple transceivers can be enabled if the transceivers each operate at frequency multiples of one another such that the transmission lines connecting each transceiver to the antenna appear as open circuits to the other transceivers.

[0020] With reference to FIG. 3, in one aspect of the present invention, the duplexer 40, the LNA 50, and the power amplifier 52 are all integrated on the same semiconductor chip. In one embodiment, very large scale integration (VLSI) techniques are used to integrate the elements. Preferably, the elements will be integrated using well known CMOS techniques, although other processes are also possible. Additional elements within the receiver and transmitter circuits (i.e., other than the LNA 50 and power amp 52, respectively) can also be integrated on the chip. In one embodiment, for example, an entire transceiver subsystem including receiver, transmitter, duplexer, and digital circuitry for control and/or signal processing is integrated onto a single chip. The chip will preferably be housed within an integrated circuit package for easy mounting on a circuit board or the like. The circuit board may then be coupled to an antenna, either with or without an intervening balun structure (depending on the type of antenna used). In one approach, a balun is implemented on the semiconductor chip with the other circuitry.

[0021] Although the present invention has been described in conjunction with certain embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.