GB2273423A

GB2273423A - An electrical switching circuit for a radio receiver

Info

Publication number: GB2273423A
Application number: GB9225802A
Authority: GB
Inventors: Bernhard Frerichs
Original assignee: Motorola GmbH
Current assignee: Motorola Solutions Germany GmbH
Priority date: 1992-12-10
Filing date: 1992-12-10
Publication date: 1994-06-15
Anticipated expiration: 2012-12-10
Also published as: DE4342249C2; GB2273423B; DE4342249A1; GB9225802D0

Description

2273423 An Electrical Switching Circuit and Method for Operating a Swite.

Back2round to the Invention.

This invention relates, in general, to electrical switching circuits and is particularly, but not exclusively, applicable to pin diode switches used in radio communications equipment.

10Summary of the Prior A.C.

A broadband pin diode switch may be used selectively to isolate a transmitter output from a receiver input and vice versa and simultaneously to switch an antenna, having low insertion loss, respectively between transmitter output circuitry and receiver input circuitry. Typically, the broadband pin diode switch comprises a pair of serially coupled pin diodes having their cathodes coupled together, with the antenna coupled between the cathodes. A forward bias selectively applied to one of the diodes determines in which one of the two circuits the antenna is operative, whilst the other pin diode isolates the antenna from the other circuit. Furthermore, in this type of pin diode switch, it is often necessary to apply a reverse voltage across the diode responsible for isolating the receiver circuitry if the communications device has high power output (greater than 50 watts). Typically, a broadband pin diode switch has a receiver isolation of greater than 40dB at a frequency of 15OMHz, and greater than 20dB at 50OMHz. Moreover, the transmitter and receiver insertion losses at a forward current of 5OmA (milliamps), and an output power level of up to 50 watts, are O.ldB and 0.2dB respectively. In general, the insertion loss/isolation of a receiver-transmitter path is primarily dependent on the physical state of the pin diodes employed in switch.

As will be appreciated, when pin diodes are used as switching elements, higher reliability, better mechanical ruggedness and faster switching speeds are achieved relative to comparable electro-mechanical designs. Unfortunately, the use of prior art broadband pin diode switches in communications devices, especially portable and mobile radio telephones, is undesirable because of the inherent current consumption, during receive and standby modes, associated with the use of such switches. More specifically, this current consumption, typically between 10-50mA, drains the internal power supply of the mobile radio telephone, such as a battery, and thereby considerably reduces the operational life of the power supply. It will be appreciated that there are alternative methodologies and circuits for achieving isolation of the antenna from either receiver C or transmitter circuitry in a communications device. For example, frequency dependent transformation networks, e.g. transformer pin diode switches utilising quarter-wave (7r/4) line transformers, achieve the desired isolation. However, these circuits can only be classified as narrowband or "conditional broadband" circuits, i.e. operational performance of components in these circuits, such as coils, capacitors and transmissions lines, are subject to operating frequency. Furthermore, these circuits are relatively expensive vis-a-vis broadband pin diode switches. In addition, the transformer pin diode switches are significantly larger than the corresponding broadband pin diode switches. Therefore, the use of transformer pin diode switches is prohibitive in electrical devices, such as portable radios, in which component space is limited.

It will be appreciated that there is a requirement in the art to provide a switch that can selectively isolate a transmitter output from a receiver input and vice versa and simultaneously to switch, at reduced current consumption during receive and standby modes, an antenna therebetween. Furthermore, it is desirable that such a switch has broadband application. It would also be desirable if the switch is small, inexpensive and easy to construct, thereby making the use of such a switch in portable communications devices particularly attractive.

4 J1 Slimmary of the - Invention.

This invention addresses at least some of the deficiencies that prevail in the prior art described above. In accordance with the present invention, there is provided an electrical switching circuit for selectively coupling and de-coupling a radio signal between receiver circuitry, in a communications device, and a circuit node for coupling to an antenna. The electrical circuit comprises: a semiconductor element, such as a diode, coupled between the receiver circuitry and the circuit node, having a conductive electrical state and a substantially non-conductive electrical state; a current source coupled to the receiver circuitry for controllably supplying electrical current to the receiver circuitry for the controlled electrical operation of the receiver circuitry, such that during operation of the receiver circuitry a drain current emanates from said receiver circuitry; and means for coupling the drain current to the diode to bias the semiconductor element, during operation of the receiver circuitry, from its non-conductive electrical state to its conductive electrical state, thereby coupling the radio signal between the receiver circuitry and the circuit node.

In a preferred embodiment, the electrical circuit comprises at least one pin diode and the diode selectively couples and de-couples an antenna from the transmitter and receiver circuitry. Furthermore, the electrical circuit of the preferred embodiment comprises a microprocessor, coupled to the current source for providing a control signal thereto, for regulating electrical current from the current source, wherein the microprocessor determines whether electrical current is supplied to the receiver circuitry. In addition, a choke is coupled between said receiver circuitry and the diode. The drain current is fed through the choke to produce a current suitable to bias the diode in its conductive electrical state. The present invention, the electrical circuit comprises at least one pin diode and the diode selectively couples and de-couples an antenna from the transmitter and receiver circuitry.

In an alternative aspect of the present invention, there is provided a method of operating a semiconductor element switch, such as a pin diode, operatively coupled between receiver circuitry and an antenna in a communications device, comprising the steps of: de-coupling a radio signal between the receiver circuitry and the antenna by making the semiconductor element switch substantially non-conductive; supplying current to the receiver circuitry for the selective operation thereof; routing a drain current, emanating from the receiver circuitry during operation thereof, to the semiconductor element switch in order to bias the semiconductor element switch, during operation of the receiver circuitry, from a substantially non-conductive electrical state to a conductive electrical state, thereby coupling the radio signal between the receiver circuitry and the antenna.

An exemplary embodiment of the present invention will now be described with reference to the accompanying drawings.

Brief Description of the - DrawiUs.

Fig. I illustrates a prior broadband pin diode switch for isolation of an antenna from either transmitter or receiver circuitry of a communications device.

Fig. 2 illustrates a broadband pin diode switch constructed in accordance with a preferred embodiment of the present invention.

Fig. 3 illustrates an alternative embodiment for the broadband pin diode switch of Fig. 2.

DCtailed - Description of a Preferred Embodiment.

With reference to Fig. 1, there is shown a prior art broadband pin diode switch, suitable for operation in a communications device 10. The switch selectively isolates a transmitter output from a receiver input and vice versa and simultaneously switches an antenna 11, having low insertion loss, respectively between transmitter output circuitry 12 and receiver input circuitry 14. The switch comprises two serially coupled pin diodes D1 and D2 coupled between the transmitter circuitry 12. and receiver circuitry 14, with the diodes D1 and D2 having their cathodes coupled together. The antenna 11 is coupled to a circuit node 16 located between the cathodes of the two diodes DI and D2. Circuit node 16 is coupled to ground through a series combination of a choke (inductor) Ch3 and a resistor Rl.

As has previously been discussed, the antenna 11 is selectively switched between either the transmitter circuitry 12 or the receiver circuitry 14 by the forward biasing of one of the two diodes DI and D2, i.e. by making one of the diodes conductive and the other substantially non-conductive. Therefore, dedicated biasing voltages VTx and VRx are selectively applied to the anodes of diodes D1 and D2 respectively. Furthermore, biasing voltages VTx and VRx are communicated to the anodes through chokes (inductors) Chl and Ch2 respectively. In high power applications (greater than 50 watts) it is often necessary to apply a reverse voltage to diode D2 during transmission from the communications device in order to isolate the receiver circuitry 14. Typically, this reverse voltage may either be derived by connecting a negative polarity power supply at biasing voltage VRx or by having the forward bias current, flowing through diode D1, flow to ground through resistor R1, thereby developing the required negative voltage at the cathode of diode D2.

In accordance with this invention, the problem of having to supply a dedicated biasing voltage VRx to a pin diode D2 during receive and standby modes of operation of the communications device, is overcome by implementation of the concept disclosed in 30 Fig. 2. Specifically, the biasing voltage VRx, for switching of diode D2, is replaced by a biasing voltage derived directly from component contained and operative in the receiver circuitry 14. Furthermore, it will be understood that the pin diode D2 may be substituted for an alternative form of semiconductor element having a suitable operating characteristic.

As will be appreciated, the receiver circuitry 14 comprises, amongst other things, a series combination of a blocking capacitor C I a pre-filter 20, a pre-amplifier 22, a mixer and local oscillator 24 and an intermediate frequency (IF) amplifier 26. An output signal provided by the intermediate frequency amplifier 26 will be routed through the back-end of the radio, in accordance with techniques known to one skilled in the art, whereby a signal received by the antenna 16 will be relayed, for example, to a user of the communications device through an external speaker or a C visual display. Furthermore, the pre-amplifier 22, the mixer and local oscillator 24 and the IF amplifier 26 are coupled to RF ground through capacitors C2, C3 and C4 respectively. A battery (not shown) supplies voltage VBatt to the pre-amplifier 22, the mixer and local oscillator 24 and the IF amplifier 26 during standby and receive modes. Electrical currents 11, 12, and 13 emanate from the pre-amplifier 22, the mixer and local oscillator 24 and the IF amplifier 26 respectively. These currents are summed together at circuit nodes 28, 30 and 32 located between the capacitors C2- C4 and the pre-amplifier 22, the mixer and local oscillator 24 and IF amplifier 26 respectively. The summation of the currents is coupled to the anode of diode D2, through choke (inductor) C112, and thereby provides the necessary forward bias to the diode during receive and standby modes of operation.

As will be appreciated, the choke (inductor) Ch2 is any suitable electrical device, such as a filter, that has both a high impedance at operating frequencies of the communications device and a low direct current (dc) impedance. Therefore, references to the choke Ch2 should be construed accordingly.

In summary, the current that is necessary to supply the receiver stages of the communications device during receive and standby modes is used to switch pin diode D2- More specifically, the summation of the DC currents emanating from each stage of the Rx circuitry 14 is fed back to the anode of pin diode D2 during receive and standby operation.

With reference to Fig. 3, there is shown a preferred embodiment of the present invention. In this preferred embodiment, a battery 40 supplies a voltage VBatt to a switchable current source 42. The current source 42 is controlled by a control voltage 44 administered by a microprocessor 45, 7 responsible for operation of the communications device, as will be appreciated. The switchable current source 42 supplies a current 46 of, in this case, 22mA to the pre-amplifier 22 of the receiver circuitry 14. As has been described, the pre-amplifier 22 is coupled through a capacitor C2 to RF ground. A current II emanating from the pre-amplifier is fed through a choke (inductor) Ch2 to provide a suitable current to render the diode (switch) D2 conductive, i. e. forward bias the pin diode D2Furthermore, the choke Ch2 (or high impedance) may be realised by internal components of the pre-filter 20.

In this example, the use of the pre-amplifier DC current for biasing pin diode D2 gives an antenna to Rx insertion loss of approximately 0.3 dB. As will be appreciated, the 22mA DC current passing through the preamplifier 22 is determined by intermodulation and noise figure requirements of the receiver circuitry. In addition, the microprocessor 45 may regulate the bias voltage VTx for the transmitter circuitry in an analogous way to which it controls the receiver circuitry 14, i.e. by controlling a switchable current source (not shown), responsible for supplying bias voltage VTx to the transmitter circuitry 12, with a control signal 47. Alternatively, the bias voltage VTx for the transmitter circuitry may be regulated by a hardware switch, such as a pushto-talk (PTT).

In an alternative embodiment to Fig. 3, the switchable current source 42 could be replaced by a standard current source, with control of the current 11, output from the pre-amplifier 22, regulated between RF ground and diode D2 by respectively closing and opening a switch located in parallel with capacitor C2. The switch could be a mechanical or transistorised switch.

It can be appreciated that an invention so designed and described produces the novel advantages of a broadband pin diode switch that requires no additional biasing other than through current biasing supplied by components coupled to that switch. This novel switch configuration has the desirable advantage that there is a reduction in current consumption during operation of a broadband pin diode switch in a receive or standby mode, and therefore an increase in battery life. In addition, the ability to utilise a broadband pin diode switch has the accompanying advantage that there the pin diode switch is small compared with that for a quarter-wave line transformer pin switch. Additionally, implementation of the switch in accordance with the preferred embodiment reduces circuit complexity since the requirement for a dedicated bias voltage has been circumvented.

It will be appreciated by one skilled in the art that the above description has been given by way of example only and that modifications in detail, such as the application of the invention to electrical devices, in general, may be made within the scope of the invention. More particularly, this invention is applicable to portable electrical devices, employing battery powered techniques, that require the selective isolation of circuitry that shares a common electrical component. Furthermore, the concept of reducing power consumption through the pin diode by biasing the diode with current supplied to and emanating from other electrical circuitry operative in the device is widely applicable to all electrical circuits.

Claims

1. An electrical switching circuit (D1, D2) for selectively coupling and de-coupling a radio signal between receiver circuitry, in a communications device, and a circuit node (16) for coupling to an antenna (11), the electrical circuit comprising:

a) a semiconductor element (D2), coupled between the receiver circuitry (14) and the circuit node (16), having a conductive electrical state and a substantially non-conductive electrical state; is b) a current source (42) coupled to the receiver circuitry (14) for controllably supplying electrical current (46) to the receiver circuitry (14) for the controlled electrical operation of the receiver circuitry, such that during operation of the receiver circuitry a drain current (11- 13) emanates from said receiver circuitry (14); and c) means for coupling the drain current (11-13) to the semiconductor element (D2) to bias the semiconductor element, during operation of the receiver circuitry (14), from its nonconductive electrical state to its conductive electrical state, thereby coupling the radio signal between the receiver circuitry (14) and the circuit node (16).

2. An electrical switching circuit in accordance with claim 1, further comprising a microprocessor (45), coupled to the current source (42) for providing a control signal (46) thereto, for regulating electrical current from the current source (42), wherein the microprocessor (45) determines whether electrical current is supplied to the receiver circuitry (14).

3. An electrical switching circuit in accordance with claim 1 or 2, wherein the circuit further comprises a choke (Ch2) coupled between said receiver circuitry (14) and the semiconductor element, wherein said drain current (11-13) is fed through said choke (Ch2) to produce a current suitable to bias the semiconductor element in its conductive electrical state.

- is

4. An electrical switching circuit in accordance with claim 1, 2 or 3, wherein the antenna (11) is further coupled to transmitter circuitry (12) and the semiconductor element selectively couples and de- couples the antenna (11) from the transmitter (12) and receiver circuitry (14).

5. An electrical switching circuit in accordance with any preceding claim, wherein the electrical circuit comprises at least one pin diode (D1, D2).

6. A method of operating a semiconductor element switch operatively coupled between receiver circuitry (14) and an antenna (11) in a communications device (10), comprising the steps of: a) de-coupling a radio signal between the receiver circuitry (14) and the antenna (11) by making the semiconductor element switch substantially non-conductive; b) supplying current (45) to the receiver circuitry (12) for the selective operation thereof; 20 c) routing a drain current, emanating from the receiver circuitry (14) during operation thereof, to the semiconductor element switch in order to bias the semiconductor element switch, during operation of the receiver circuitry (14), from a substantially nonconductive electrical state to a conductive 25 electrical state, thereby coupling the radio signal between the receiver circuitry (14) and the antenna (11).

7. An electrical switching circuit substantially as described herein with reference to Figs. 2 & 3 of the accompanying 30 drawings.

8. A method of operating a switch substantially as described herein with reference to Figs. 2 & 3 of the accompanying drawings.