GB2404797A

GB2404797A - Antenna switch circuit

Info

Publication number: GB2404797A
Application number: GB0318612A
Authority: GB
Inventors: Haim Friedlander; Ariel Luzzatto; Zalman Schwartzman; Gadi Shirazi
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2003-08-08
Filing date: 2003-08-08
Publication date: 2005-02-09
Anticipated expiration: 2023-08-08
Also published as: WO2005015761A1; GB0318612D0; EP1654811A1; GB2404797B

Abstract

An antenna switch circuit 300 for use in an RF transceiver is connected to a transmitter circuit 103, a receiver circuit 102, and an antenna 101. The switch circuit 300 selectively connects the transmitter 103 and the receiver 102 circuits to the antenna 101. Electrically-controlled devices, such as PIN diodes D1-4, are arranged to perform the switching and to prevent electrostatic discharges of either positive or negative polarity from reaching the transmitter 103 or receiver 102 circuits. Two pairs of PIN diodes D1,3, D2,4 may each be connected so that they have opposed unipolar conductivities. A dc voltage may be applied through an RF-isolating component RFC1, 2 to control the electronically-controlled devices D1-4. The antenna switch circuit 300 may be included in an RF communications apparatus, such as a mobile or fixed radio terminal, or in mobile telephones, mobile radios, or data communications terminals. In a further embodiment the antenna switch circuit may also protect from excessive RF power entering the antenna.

Description

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2404797

TITLE: Antenna switch circuit Field of the Invention

5 The present invention relates to an antenna switch circuit. In particular, the invention relates to an antenna switch circuit for use in a radio frequency (RF) transceiver, e.g. for use in a wireless terminal for use in mobile communications, to allow sharing of an antenna 10 between a transmitter and a receiver of the transceiver.

Background of the Invention

Transmitters in wireless communications terminals for 15 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 20 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 25 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 30 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

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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, 5 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 10 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 15 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 20 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 25 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 30 these protective components. Furthermore, in some cases, this known method is not successful in protecting the final RFPA stage reliably against ESD damage.

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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. 5 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). Again, this known method does not give satisfactory ESD 10 protection as explained later.

Summary of the invention

In accordance with a first aspect of the present 15 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;

20 (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;

25 (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

30 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

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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

5 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

10 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;

15 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 20 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 25 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 30 transmitter terminal of the antenna switch circuit from ESD of both positive and negative polarity.

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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 5 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 10 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 15 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 20 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 25 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.

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

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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 5 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 10 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 15 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 20 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 25 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 30 switch circuit according to the first aspect connected to the transmitter, optionally through one or more intermediate components such as an impedance matching

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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.

5 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 10 generally understood to mean frequencies of greater than lOKHz, e.g. up to 500GHz. In many cases the RF energy produced in the application will have a frequency of from lOOKHz to 100GHz.

Where the antenna switch circuit according to the 15 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 20 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 25 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 30 communications systems.

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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 5 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

10 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.

15 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

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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.

25 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.

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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 5 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 10 103 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 15 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 20 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 25 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 30 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

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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 Dl. The connection 203 includes a RF choke RFC1 in 5 series with a resistor Rl. 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. 10 Operation of the antenna switch circuit 200 is as follows.

In a receive mode, both PIN diodes Dl and D2 are biased off (in a blocking state), and can be regarded as open circuits. The receiver (not shown) presents a 15 matched load to the quarter-wave transmission line Tl,

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 20 terminal 203, turning both diodes Dl, 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 25 ground created by the diode D2 is transformed by the quarter-wave transmission line Tl to an open-circuit at the other end of the line Tl (the end connected to the transmission path 201). This effectively disconnects the reception path 202 and the receiver from the antenna. 30 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

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

The diodes Dl and D2 help to protect the transmitter from ESD when the transmitter is turned off, but only 5 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 10 conducts it helps to bleed some of the unwanted charge from ESD to ground. However, for negative polarity ESD, the diodes Dl 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 15 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 20 between the transmitter terminal 103 and the diode Dl. 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, 25 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 30 provide unipolar conduction in mutually opposite directions.

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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 5 of the transmitter (not shown) against negative polarity ESD in the same way that Dl 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 10 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 15 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 20 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 25 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 30 a common phenomenon where a RF terminal including the antenna is being used in mobile communications.

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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 5 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 10 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 15 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.

20 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 25 300 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 30 discharge of electrostatic charge, particularly charge collected at the antenna 407, in the manner described earlier with reference to FIG. 3.

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Claims

Claims 1. An antenna switch circuit for use in a RF transceiver, the antenna switch circuit comprising: 5 (x) a transmitter terminal for receiving an input RF signal from a transmitter; (xi) a receiver terminal for providing an output RF signal to a receiver; (xii) an antenna terminal for providing an output RF 10 signal to an antenna and for receiving an input RF signal from an antenna; (xiii) 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; 15 (xiv) 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; (xv) in a first of said transfer paths a first electrically controlled device having a conducting state 20 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; (xvi) in the second of the transfer paths an isolator which isolates the first transfer path in a second mode 25 of the switch circuit; and (xvii) 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; 30 and characterised in that the antenna switch circuit further includes: 15 (xviii) 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 5 discharge of both positive and negative electrostatic charge. 2. An antenna switch circuit according to claim 1 wherein the at least one further electrically controlled device comprises a third electrically controlled device 10 included in the first transfer path in series with the first electrically controlled device. 3. An antenna switch circuit according to claim 1 or claim 2 wherein the first and third electrically controlled devices are arranged so that they have 15 opposing unipolar conductivities. 4. An antenna switch circuit according to claim 3 wherein the first and third electrically controlled devices are arranged and operable so that each provides a blocking state, one during a positive polarity 20 electrostatic discharge and the other during a negative polarity electrostatic discharge. 5. An antenna switch circuit according to any one of the preceding claims including a further electrically controlled device having a conducting state and a 25 blocking state connected in parallel with the second electrically controlled device. 6. An antenna switch circuit according to claim 4 wherein the second electrically controlled device and the further electrically controlled device in parallel 30 with it have opposing unipolar conductivities. 7. An antenna switch circuit according to claim 6 wherein the second electrically controlled device and 16 the further electrically controlled device are connected in parallel to ground and are operable to provide a conducting path to ground one during a positive polarity electro-static discharge and the other during a negative 5 polarity electro-static discharge. 8. An antenna switch circuit according to any one of the preceding claims wherein the electrically controlled devices comprise rectifying diodes. 9. An antenna switch circuit according to any one of the 10 preceding claims wherein the electrically controlled devices comprise PIN diodes. 10. An antenna switch circuit according to any one of the preceding claims wherein the isolator comprises a quarter wave transmission line or its equivalent LC 15 circuit. 11. An antenna switch circuit according to any one preceding claim wherein the first transfer path including the first electrically controlled device is the transmission path and the first mode is a transmit 20 mode and the second transfer path including the isolator is the reception path and the second mode is a receive mode. 12. An antenna switch circuit according to any one of claims 2 to 11 including a voltage application 25 connection connected to the first transfer path between the first and third electrically controlled devices. 13. An antenna switch circuit according to claim 12 wherein the voltage application connection includes at least one RF isolating component to allow application of 30 a state changing d.c. voltage to the first and third electrically controlled devices via the connection. 17 14. An antenna switch circuit according to any one of the preceding claims and substantially as described herein with reference to FIG. 3 of the accompanying drawings. 5 15. A RF transceiver including an antenna switch circuit according to any one of the preceding claims. 16. A transceiver according to claim 15 including an antenna, a transmitter and a receiver and the antenna switch circuit is incorporated between the transmitter 10 and the antenna and between the receiver and the antenna. 17. A transceiver according to claim 16 wherein the antenna terminal of the antenna switching circuit is connected to the antenna, optionally through a harmonic 15 filter. 18. A transceiver according to claim 15 or claim 16 wherein the transmitter terminal of the antenna switching circuit is connected to a RF power amplifier (RFPA) of the transmitter, optionally through a 20 directional coupler and optionally with an impedance matching circuit incorporated between the directional coupler and the RFPA. 19. A terminal for use in mobile communications which includes a transceiver according to any one of claims 15 25 to 18. 20. 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 any one of claims 1 to 14 connected to the transmitter, optionally 30 through one or more intermediate components such as an impedance matching circuit and a directional coupler, and to an antenna, optionally through one or more 18 intermediate components, such as a harmonic filter, to provide a protected RF transmission path between the transmitter and the antenna. Amendments to the claims have been filed as follows Claims

1. An antenna switch circuit for use in a RF transceiver, the antenna switch circuit comprising:

(x) a transmitter terminal for receiving an input RF signal from a transmitter;

(xi) a receiver terminal for providing an output RF signal to a receiver;

(xii) an antenna terminal for providing an output RF signal to an antenna and for receiving an input RF signal from an antenna;

(xiii) 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;

(xiv) 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;

(xv) 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;

(xvi) in the second of the transfer paths an isolator which isolates the first transfer path in a second mode of the switch circuit; and

(xvii) 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:

(xviii) 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.

2. An antenna switch circuit according to claim 1 wherein the at least one further electrically controlled device comprises a third electrically controlled device included in the first transfer path in series with the first electrically controlled device.

3. An antenna switch circuit according to claim 1 or claim 2 wherein the first and third electrically controlled devices are arranged so that they have opposing unipolar conductivities.

4. An antenna switch circuit according to claim 3 wherein the first and third electrically controlled devices are arranged and operable so that each provides a blocking state, one during a positive polarity electrostatic discharge and the other during a negative polarity electrostatic discharge.

5. An antenna switch circuit according to any one of the preceding claims including a further electrically controlled device having a conducting state and a blocking state connected in parallel with the second electrically controlled device.

6. An antenna switch circuit according to claim 4 wherein the second electrically controlled device and the further electrically controlled device in parallel with it have opposing unipolar conductivities.

7. An antenna switch circuit according to claim 6 wherein the second electrically controlled device and

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the further electrically controlled device are connected in parallel to ground and are operable to provide a conducting path to ground one during a positive polarity electro-static discharge and the other during a negative polarity electro-static discharge.

8. An antenna switch circuit according to any one of the preceding claims wherein the electrically controlled devices comprise rectifying diodes.

9. An antenna switch circuit according to any one of the preceding claims wherein the electrically controlled devices comprise PIN diodes.

10. An antenna switch circuit according to any one of the preceding claims wherein the isolator comprises a quarter wave transmission line or its equivalent LC circuit.

11. An antenna switch circuit according to any one preceding claim wherein the first transfer path including the first electrically controlled device is the transmission path and the first mode is a transmit mode and the second transfer path including the isolator is the reception path and the second mode is a receive mode.

12. An antenna switch circuit according to any one of claims 2 to 11 including a voltage application connection connected to the first transfer path between the first and third electrically controlled devices.

13. An antenna switch circuit according to claim 12 wherein the voltage application connection includes at least one RF isolating component to allow application of a state changing d.c. voltage to the first and third electrically controlled devices via the connection.

XL

14. An antenna switch circuit according to any one of the preceding claims and substantially as described herein with reference to FIG. 3 of the accompanying drawings.

15. A RF transceiver including an antenna switch circuit according to any one of the preceding claims.

16. A transceiver according to claim 15 including an antenna, a transmitter and a receiver and the antenna switch circuit is incorporated between the transmitter and the antenna and between the receiver and the antenna.

17. A transceiver according to claim 16 wherein the antenna terminal of the antenna switching circuit is connected to the antenna.

18. A transceiver according to claim 17 wherein the antenna terminal of the antenna switching circuit is connected to the antenna through a harmonic filter.

19. A transceiver according to claim 15, claim 16, claim 17 or claim 18 wherein the transmitter terminal of the antenna switching circuit is connected to a RF power amplifier (RFPA) of the transmitter.

20. A transceiver according to claim _l9 wherein the transmitter terminal of the antenna switching circuit is connected to a RF power amplifier (RFPA)through a directional coupler and with an impedance matching circuit incorporated between the directional coupler and the RFPA.

21. A terminal for use in mobile communicatior.;, which includes a transceiver according to any one of claims 15 to 20.

22. A method of protecting a RF transmitter from damage caused by electrostatic discharge in use, which includes

as use of an antenna switch circuit according to any one of claims 1 to 14 connected to the transmitter and to an antenna.

23. A method according to claim 22 wherein the antenna 5 switch circuit is connected to the transmitter through an impedance matching circuit and a directional coupler.

24. A method according to claim 22 or claim 23 wherein the antenna switch circuit is connected to the antenna through a harmonic filter.

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