Background of the Invention Transmitters in wireless communications terminals for use in mobile communication systems commonly employ a power transistor circuit as the final amplification stage of the RFPA (Radio Frequency Power Amplifier) of the transmitter. As illustrated later, the functional blocks which carry the transmitted signal from the power transistor circuit of the transmitter to the antenna of the terminal usually comprise an impedance matching circuit, a directional coupler (to facilitate power control), an antenna switch and a harmonic filter.

One of the environmental requirements of such a transmitter is that it must survive ESD (Electro-Static Discharge) in use without sustaining any permanent damage. In practice, ESD is commonly caused by an electro-static charge that tends to accumulate on the human body and is discharged upon contact with the terminal, e. g. via the antenna. ESD immunity is tested by means of an"ESD gun"which simulates the human body by charging a capacitor with the required voltage and

then discharging it onto the terminal under test.

Generally speaking, providing adequate ESD immunity to a RF transmitter included in a wireless communications terminal, e. g. for mobile communications, is a difficult requirement to meet. Typical ESD specifications require testing at voltage levels of 8kV by contact discharge, and of 15kV by air discharge, using both positive and negative polarity discharges.

The final amplification stage of the transmitter RFPA is usually the part of the transmitter which is most susceptible to damage as a result of ESD. Some component technologies, such as LDMOS and other MOS devices, are particularly sensitive to ESD and therefore are difficult to protect. The final amplification stage is especially sensitive to ESD when the transmitter is turned off, owing to the fact that it is not conducting, during receive mode, thus allowing high voltages to develop across it. When the transmitter is operating, the final stage conducts, thereby possessing a low impedance, which effectively bleeds the unwanted electro-static charge to ground.

There are several known ways of protecting a RF transmitter, particularly its final RFPA amplification stage, against damage caused by ESD. None is entirely satisfactory. For example, one method is to bleed the collected charge to ground at the antenna by using either a resistor or an inductor. However, this method becomes difficult to implement when the transmitted power is high, owing to significant power dissipation on these protective components. Furthermore, in some cases, this known method is not successful in protecting the final RFPA stage reliably against ESD damage.

The antenna switch, which is designed primarily for switching the antenna between the transmitter and the receiver, may contain PIN diodes, which may help to mitigate ESD damage when the transmitter is turned off.

However, known antenna switch configurations will protect against only one of the two possible ESD polarities (positive or negative, depending on the particular implementation of the antenna switch). Thus, again, this known method does not give satisfactory ESD protection as explained later.

Summary of the invention In accordance with a first aspect of the present invention, there is provided an antenna switch circuit for use in a RF transceiver, the antenna switch circuit comprising: (i) a transmitter terminal for receiving an input RF signal from a transmitter; (ii) a receiver terminal for providing an output RF signal to a receiver; (iii) an antenna terminal for providing an output RF signal to an antenna and for receiving an input RF signal from an antenna; (iv) a transmission transfer path for transferring a RF signal from the transmitter terminal to the antenna terminal when the switch circuit is in a transmit mode; (v) a reception transfer path for transferring a RF signal from the antenna terminal to the receiver terminal when the switch circuit is in a receive mode; (vi) in a first of said transfer paths a first electrically controlled device having a conducting state

and a blocking state, the electrically controlled device in its blocking state in a first mode of the switch circuit isolating a second of the transfer paths; (vii) in the second of the transfer paths an isolator which isolates the first transfer path in a second mode of the switch circuit; and (viii) a second electrically controlled device having a conducting state and a blocking state and operable in its conducting state to connect one of the transfer paths to ground; and characterised in that the antenna switch circuit further includes: (ix) at least one further electrically controlled device having a conducting state and a blocking state; wherein the electrically controlled devices are arranged in the circuit to protect the transmitter terminal from discharge of both positive and negative electrostatic charge.

The at least one further electrically controlled device may be a third electrically controlled device included in the first transfer path in series with the first electrically controlled device. Alternatively, or in addition, at least one further electrically controlled device may be a fourth electrically controlled device in parallel with the second electrically controlled device.

The electrically controlled devices are such as to provide protection to a RF transmitter when connected (optionally through one or more other components) to the transmitter terminal of the antenna switch circuit from ESD of both positive and negative polarity.

The electrically controlled devices may comprise devices having a unipolar conductivity (conductivity in a single directional sense). These devices may comprise rectifying diodes such as PIN diodes. The isolator may include a quarter wave transmission line, or alternatively, depending mainly on the frequency band of operation, an LC (inductor-capacitor) equivalent of a quarter wave transmission line.

The first and third electrically controlled devices may be arranged so that they have opposing unipolar conductivities. Similarly, the second and fourth electrically controlled devices may have opposing unipolar conductivities. The second and fourth electrically controlled devices are connected in parallel to ground so that they provide a conducting path to ground one during a positive polarity ESD and the other during a negative polarity ESD. The first and third voltage controlled devices are connected in series, so that each provides a protective blocking state (or an open circuit), one during a positive polarity ESD and the other during a negative polarity ESD.

In the antenna switch circuit according to the invention, the first transfer path including the first voltage controlled device may be the transmission path.

In this case the first mode is a transmit mode.

Correspondingly, the second transfer path including the isolator may be the reception path. In this case, the second mode is a receive mode.

In the antenna switch circuit according to the invention, a voltage application connection may be connected to the first transfer path between the first

and third voltage controlled devices. The connection may include at least one RF isolating component to facilitate application of a d. c. voltage via the connection. The voltage, when applied, provides an electrical change to change the state of the first and third voltage controlled devices and the mode of the antenna switch circuit. Application of the voltage may be controlled by a controller, e. g. microprocessor, controlling functional operations of a transceiver in which the antenna switch circuit is included.

In accordance with a second aspect of the present invention, there is provided a RF transceiver including an antenna switch circuit according to the first aspect.

The transceiver may include an antenna, a transmitter and a receiver and the antenna switch circuit may be incorporated between the transmitter and the antenna and between the receiver and the antenna. The antenna terminal of the antenna switching circuit may be connected to the antenna, optionally through a harmonic filter. The transmitter terminal of the antenna switching circuit may be connected to the transmitter, e. g. to a RF power amplifier (RFPA) of the transmitter, optionally through a directional coupler and optionally with an impedance matching circuit incorporated between the directional coupler and the RFPA.

In accordance with a third aspect of the present invention there is provide a method of protecting a RF transmitter from damage caused by electrostatic discharge in use, which includes use of an antenna switch circuit according to the first aspect connected to the transmitter, optionally through one or more intermediate components such as an impedance matching

circuit and a directional coupler, and the antenna, optionally through one or more intermediate components, such as a harmonic filter, to provide a RF transmission path between the transmitter and the antenna.

The antenna switch circuit according to the present invention may find use in RF transceivers for a number of applications, particularly RF communications for communication of any one or more of voice, data, picture and video information. In this specification,'RF'is generally understood to mean frequencies of greater than 10KHz, e. g. up to 500GHz. In many cases the RF energy produced in the application will have a frequency of from 100KHz to 100GHz.

Where the antenna switch circuit according to the invention is employed in a RF communications transceiver, such a transceiver may be incorporated in a communications apparatus. For example, the apparatus may comprise a mobile or fixed radio terminal. Terminals including mobile radio transceivers are also often referred to as mobile stations (MSs). The term mobile station (MS)'is intended to include within its meaning apparatus such as mobile and portable telephones and mobile and portable radios, data communication terminals and the like which operate by RF communication. Systems which provide communications to or from MSs by fixed or base transceivers known in the art as base transceiver stations'or BTSs'may be arranged to give communications coverage in a network of regions known as cells and are referred to herein as cellular radio communications systems.

Thus, the invention may find particular use in a MS or in a BTS of a mobile or cellular communications system.

The antenna switch circuit according to the invention allows improved, reliable protection against damage from ESD to be provided to a RF transmitter to which the switch circuit is connected without excessive cost or performance degradation. Furthermore, the inclusion of the fourth electrically controlled device can provide an additional benefit by creating a clamping circuit which helps to protect the RF receiver from becoming damaged if excessive RF power enters the antenna during receive mode. This is an undesirable phenomenon which tends to occur often in the field of mobile communications.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which: Brief Description of the accompanying drawings FIG. 1 is a schematic representation of a known antenna switch.

FIG. 2 is a circuit diagram of a known form of antenna switch circuit.

FIG. 3 is a circuit diagram of a form of antenna switch circuit embodying the present invention.

FIG. 4 is a block circuit diagram of a RF transceiver including the antenna switch circuit shown in FIG. 3.

Detailed description of embodiments of the invention FIG. 1 illustrates the known functional operation of an antenna switch for use in a RF transceiver of a communications terminal. The switch, indicated by reference numeral 100, is an RF switch of the single pole, double throw (SPDT) type. An antenna terminal 101 is provided for connection to an antenna (not shown in FIG. 1). The switch 100 also has a transmitter terminal 103 for connection to a transmitter (not shown in FIG.

1) and a receiver terminal 102 (not shown in FIG. 1) for connection to a receiver (not shown in FIG. 1). The switch 100 includes a connector 104 to the antenna terminal 101 which can be connected alternatively to the receiver terminal 101 or to the transmitter terminal 102 by operation of the switch (to be described later).

Thus, when a receive mode is selected by a transceiver (not shown) connected to the switch 100, the connector 104 is connected to the receiver terminal 102. When a transmit mode is selected by the transceiver, the connector 104 is connected to the transmitter terminal 103. The switch 101 thereby allows sharing of a single antenna by a transmitter and receiver of a RF transceiver. As is known in the art, parts of the transmitter and receiver may in practice be formed as common components of the transceiver.

FIG. 2 shows a known antenna switch circuit 200 which is a known implementation of the antenna switch 100. The terminals 101,102 and 103 are the same as those shown in FIG. 1. A transmission path 201 exists between the terminals 103 and 101. A reception path 202 exists between the terminal 101 and the terminal 102. A PIN

diode D1 is included in the transmission path 201. A connection 203 exists between a terminal 204 and the transmission path 201 between the terminal 103 and the diode D1. The connection 203 includes a RF choke RFC1 in series with a resistor R1. A quarter wave transmission line T1 is included in the reception path 202. A connection 205 exists between the reception path 202, between the transmission line T1 and the terminal 102, and ground. The connection 205 includes a PIN diode D2.

Operation of the antenna switch circuit 200 is as follows.

In a receive mode, both PIN diodes D1 and D2 are biased off (in a blocking state), and can be regarded as open circuits. The receiver (not shown) presents a matched load to the quarter-wave transmission line T1, allowing RF energy to flow from the antenna terminal 101 to the receiver terminal 102 and thence to the receiver, with minimal insertion loss.

In a transmit mode, a bias voltage is applied at the terminal 203, turning both diodes D1, D2 on (into a conducting state). The RF choke RFC1 provides RF isolation between the DC and RF portions of the circuit 200. Since both diodes Dl, D2 are conducting, they can be regarded as short circuits. The short circuit to ground created by the diode D2 is transformed by the quarter-wave transmission line T1 to an open-circuit at the other end of the line T1 (the end connected to the transmission path 201). This effectively disconnects the reception path 202 and the receiver from the antenna.

The diode Dl in a conducting state provides a low-loss connection between the terminal 103 connected to the transmitter (not shown) and the terminal 101 connected

to the antenna (not shown). This allows RF energy to flow from the transmitter to the antenna.

The diodes D1 and D2 help to protect the transmitter from ESD when the transmitter is turned off, but only for positive discharge. When positive discharge occurs, the diode Dl does not conduct in a reverse direction, providing some isolation between the antenna and the transmitter in a reverse direction along the transmission path. Additionally, when the diode D2 conducts it helps to bleed some of the unwanted charge from ESD to ground. However, for negative polarity ESD, the diodes D1 and D2 do not provide any protection.

FIG. 3 shows an antenna circuit 300 which is an implementation of the antenna switch 100 in accordance with an embodiment of the present invention. Components in FIG. 3 having the same reference numerals as corresponding components in FIG. 2 have the same function as those components. In FIG. 3 a further PIN diode D3 is in the transmission path 201 being connected between the transmitter terminal 103 and the diode D1.

The diodes Dl and D3 are arranged to provide unipolar conduction in mutually opposite directions. A further connection 301 including a RF choke RFC2 in series with a resistor R2 extends between the transmission path 201, between the terminal 103 and the diode D3. The connection 301 is grounded at its end remote from the transmission path 201. A further PIN diode D4 is connected in parallel with the diode D2 in the connection 205. The diodes D2 and D4 are arranged to provide unipolar conduction in mutually opposite directions.

The antenna switch circuit 300 shown in FIG. 3 operates in the same general manner as the circuit 200 shown in FIG. 2. However, the additional PIN diodes, D3 and D4, in the circuit 300 provide additional protection of the transmitter (not shown) against negative polarity ESD in the same way that D1 and D2 protect against positive polarity ESD. The additional connection 301 including the choke RF2 and the resistor R2 provide a DC conduction path for the diode D3, thus enabling it to conduct in transmit mode, which is necessary to ensure low insertion loss of the transmission path 201. The additional diode D4 has no effect on the RF transfer operation of the antenna switch 300 but provides a discharge to ground of charge caused by negative polarity ESD as mentioned earlier.

The antenna switch circuit 300 has been implemented in practice, for example by incorporation in a transceiver of a mobile communications terminal having a 50W transmitter, and has been proven to be successful in protecting the RFPA of the transmitter from damage during both positive polarity and negative polarity ESD at the antenna, measured according to the standard environmental tests.

The antenna switch circuit 300 has an additional benefit as follows. The addition of the diode D4 creates a clamping circuit which helps to protect the receiver to which the switch is connected from becoming damaged owing to excessive RF power entering the antenna during receive mode. Such undesirable excessive power entry is a common phenomenon where a RF terminal including the antenna is being used in mobile communications.

FIG. 4 shows a RF transceiver 400 in which the antenna switch circuit 300 may be used. The transceiver 400 includes a transmitter 401 having an output applied in turn to an impedance matching circuit 402 and a directional coupler 403. The direction coupler is connected to a power control circuit 404, which detects the sampled RF power level provided by the coupler 403 and employs a closed-loop control circuit to stabilize the transmitted power in a known manner. An output from the directional coupler 403 is applied to the antenna switch circuit 300 at the terminal 103 (FIG. 3). An output from the switch circuit 300 is connected at the terminal 101 through a harmonic filter, which ensures sufficiently low harmonic content of the transmitted spectrum, to an antenna 407 to allow RF transmission signals to be sent over the air to a distant receiver (not shown). Another output from the antenna switch circuit 300 is connected at the terminal 102 (FIG. 3) to a receiver 405.

In operation, with the antenna switch circuit 300 in a transmit mode, RF transmission signals generated by the transmitter 401 are delivered to the antenna 407 and are sent over the air by the antenna 407 to a distant receiver (not shown). When the antenna switch circuit 300 is in a receive mode, RF signals received by the antenna 407 are delivered via the harmonic filter 406 and antenna switch 300 to the receiver 405 where they are processed in a known manner. The transmitter 401 is protected against damage by both positive and negative discharge of electrostatic charge, particularly charge collected at the antenna 407, in the manner described earlier with reference to FIG. 3.