GB2401264A

GB2401264A - Method of controlling the bias and quiescent current of an RF transmitter

Info

Publication number: GB2401264A
Application number: GB0309995A
Authority: GB
Inventors: Hagen Roger Purschke
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2003-04-30
Filing date: 2003-04-30
Publication date: 2004-11-03

Abstract

An RF transmitter includes means 7, 11 for adjusting the transmitted RF power level, and means 7, 17 for adjusting the bias of an RF component to control the dc quiescent current in the component. The controller 7 adjusts the bias in a manner such that, as the transmitted power level is changed, the quiescent current is correspondingly changed (fig.3). The bias control circuit 17 also produces a signal which controls an adjustable gain device 13, which maintains substantially constant forward gain of the transmitter chain at lower power levels. It is also disclosed that the bias may be adjusted by a feedback control circuit. Savings in dc power consumption and battery life are obtained, as the quiescent current is not held at a single level corresponding to maximum output power, as in prior art. The invention may be applied to mobile stations in a trunked radio communication system such as TETRA or GSM.

Description

1 2401264 Title: RF transmitter and communication terminal and method

including the transmitter

Field of the invention

The present invention relates to an RF transmitter and a communication terminal and a method including the transmitter. The transmitter and terminal, e.g. mobile station (which may incorporate the transmitter), and method using the terminal, may be for use in a mobile radio communications network.

Background of the invention

Portable and mobile radio communication terminals in use in a mobile communication system are often arranged to communicate with one another through a fixed system infrastructure including amongst other things one or more base transceiver stations (BTSs).

The terminals and the infrastructure are often collectively referred to as a bunked radio system.

Examples of such systems are TETRA systems operating in accordance with the TETRA standard operating protocols defined by European Telecommunications Institute (ETSI) and APCO systems operating in accordance with standards defined by the US Telecommunication Industry Association (TIA).

The coverage area of a bunked radio communication system is often divided into 'cells'. Each cell is a region served by a particular base (transceiver) station. Several BTSs may be linked together to form a network. A terminal may communicate with another terminal in the same cell via the serving BTS, or another terminal in a different cell via the serving BTS and the network.

Henceforth the term 'mobile station' ('MS') will be used to describe a portable or mobile radio terminal that is capable of communication via a bunked or cellular communication system (and possibly also in a direct mode) with other such terminals. Thus, the term mobile station (MS) will cover portable or mobile radio terminals, portable or mobile radio telephones, 3G or 4G mobile communication devices, wirelessly linked laptop computers, wirelessly linked personal digital assistants (PDA) and the like.

TETRA systems employ a n/4 DQPSK (differential quadrature phase shift key) modulation procedure as is well known in the art. One feature of this particular procedure is the non-constant envelope of the RF carrier signal it produces. This requires the use of linear transmitter components in order to prevent disturbances due to intermodulations from being transmitted in neighbouring channels. In particular, in a RF transmitter in a TETRA terminal, it is necessary to keep the active device, e.g. transistor, used in the power amplifier circuit in a linear region of its transfer characteristic to meet the requirements for a linear transmitter design as defined in the TETRA operating standard.

In a transmitter for use in a TETRA or other bunked digital system, the quiescent current of the transmitter power amplifier determines the class of operation (A, AB or B) and hence its linearity. The setting of the quiescent current by appropriate biasing of the RF power amplifier is optimised for transmission at a given power output level. 3.'

Modern bunked radio communication systems such as TETRA and GSM systems use transmitter power control mechanisms in MSs to minimise interference with signals from other MSs. During a call, MSs which are near to a BTS set their transmitter output power level to a minimum. When a MS moves away from the BTS its output power is increased in predefined steps.

In a transmitter for use in a TETRA or other bunked digital system, the quiescent current of the transmitter power amplifier determines the class of operation (A, AB or B) and hence its linearity. In currently available transmitters, the setting of the quiescent current by biasing of the RF power amplifier is optimised for transmission at a given power output level which is the maximum power level.

For lower power levels obtained in steps as described earlier, this quiescent current setting is usually maintained.

The present inventor has recognized in connection with the present invention that this currently used method of operation is not ideal.

Summary of the Invention

According to the present invention in a first aspect there is provided a RF transmitter including: at least one RF component in which in operation a quiescent current setting controls the performance of the component in the transmitter, means for providing an adjustable bias to the component to control setting of the DC quiescent current in the component, and means for adjusting the power level of RF signals transmitted by the transmitter, the transmitter being characterized in that it further includes a control circuit for adjusting the bias to the component in a manner such that as the power level of the transmitter is changed the DC quiescent current in the component is correspondingly changed.

The transmitter may include means for changing the output RF power level of the transmitter in a known manner, e.g. as the apparent distance of the transmitter to a serving base station transmitter changes. For, example the change may be an increase in received signal power level (e.g. based on a known transmitted power level from a BTS) detected when the apparent distance from the base station is increased and vice versa. The change in power level may be applied under control of a controller. The change may be made in steps as the distance changes in pre-defined steps.

A signal indicating a change in the output power level, and the extent of change, e.g. the number of change steps made, may be provided by a power level controller to the control circuit in the RF transmitter according to the invention. The control circuit may produce an output bias which reduces the quiescent current in the component accordingly. The control circuit may include a digital-to-analogue converter to convert digital control signals into bias voltages to apply to the said device to set the quiescent current thereof.

The signal indicating a change in the output power level provided to the said control circuit may be produced by a controller comprising a digital signal processor operable to apply digital control signals to a power level control circuit to apply changes in output power level.

The said component may comprise an RF power amplifier although it may alternatively or in addition be another DC current consuming component in the transmitter in which the quiescent current is adjustable, e.g. a mixer. The invention is however most useful in the high power component(s) , i.e. the power amplifier and its driver.

Where the said component comprises a power amplifier, the transmitter may include a variable gain device, such as an adjustable amplifier/attenuator, e.g. in the transmitter signal path prior to the power amplifier, which compensates for any change in gain of the power amplifier caused by the reduction in DC quiescent current level. The adjustable gain device may be controlled by a signal from the said means for adjusting output power level.

Where the said component comprises a power amplifier, the power amplifier may be included in an amplifier control circuit which is operable to derive an error control signal to provide bias adjustment of the power amplifier. The amplifier control circuit may include a detector for measuring an electrical property of an output from the power amplifier, e. g. the current through a resistor included in an output circuit, and a feedback loop including a comparator for comparing an output of the detector with a predetermined reference value to produce an error control signal. In this case, the level of the threshold value may be adjusted by applying as the threshold the signal mentioned earlier indicating a change in the output power level, e.g. as produced by a signal processor operable to apply signals to a power level control circuit.

Where the power amplifier circuit includes a feedback control loop, the loop may include, in the manner described in copending GB0307368.1, a processor, coupled to an output of the comparator, which is operable to apply an algorithm to detect when an output of the comparator indicates minimum deviation of the output of the detector from a predetermined reference value. The processor may comprise a digital signal processor and the processor may comprise a digital signal processor and the feedback loop may include, coupled to the output of the processor, a digital-to-analog converter. The said processor may comprise a successive approximation register. The comparator may have an output feeding the comparison result produced thereby selectively to the increment or decrement inputs of the successive approximation register. The output of the successive approximation register may be a digital signal which indicates a change in the amplifier bias needed to adjust the comparator output signal to a minimum. The successive approximation register may be connected to the input of a digital-to-analogue converter. The digital-toanalogue converter may be connected to produce from the digital output of the successive approximation register a bias voltage which is applied to adjust the potential at a control electrode of the power amplifier.

The RF transmitter according to the first aspect of the invention may comprise power amplifier which comprises a solid state amplifying device such as a transistor which may be in bipolar form or in field effect (FEET or MOSFET) form. For example, where a

MOSFET (metal oxide semiconductor field effect

transistor) is employed, the input signal to be amplified may be applied at a gate electrode of the transistor. The output signal from the transistor may for example be extracted from the drain electrode. Where the transistor is in the form of a bipolar junction transistor, the input signal may applied at the base of the transistor and the output signal may be extracted from the collector of the transistor.

The RF transmitter according to the first aspect of the invention may include two or more amplifying devices. Such devices may be mutually connected in a parallel or a series configuration in a known manner to give a greater output for a given input.

The RF transmitter according to the first aspect of the present invention may find use in a number of applications in which there is a need to change output power level to suit operational requirements.

Such applications include transmitters for RF

communications, RF smartcards, RF near field

excitation devices, radio and television broadcasting and many others. The invention is likely to find greatest use in a wireless communication system.

In this specification, 'RF' is 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 lOOGHz.

The transmitter according to the first aspect of the invention may, for example, be included in a mobile station or handset for use in a RF mobile communication system, e.g. a system operating in accordance with TETRA or GSM standard protocols.

The invention beneficially allows the quiescent current level of the power amplifier or other component to be adaptively adjusted according to adjustments made to the output power level. This adjustment may be carried out without substantially prejudicing the linearity of the transmitter. As a result, savings in overall consumed DC power are obtained and these savings are observed as prolonged battery life compared with that obtained from a

prior art transmitter. This is because the setting

of the quiescent current at a single level corresponding to maximum output power in the prior art, as described earlier, results in an unduly high ratio of DC current to RF current for lower power level values giving an undesirably high power consumption and a corresponding reduction of stand by time especially for small cell systems.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

Brief description of the drawings

FIG. l is a schematic block circuit diagram of an RF transceiver incorporating a power amplifier circuit embodying the invention.

FIG. 2 is a schematic circuit diagram illustrating operation of an adjustable gain device as used in the circuit of FIG.1.

FIG. 3 is a graph of bias current versus RF output power for the power amplifier in the circuit shown in FIG.l.

Detailed description of embodiments of the invention In FIG. 1, a transceiver 1 for use in a mobile station of a mobile communications system is shown. A carrier frequency generator 3 produces a carrier frequency signal which is applied to a modulator 5 (which may be realised in the same component (a so-called Gibert cell). The carrier frequency signal is modulated in the modulator 5 by applying thereto digital data from a controller 7 in the form of a DSP (digital signal processor) which may optionally control other signal processing functions within the transceiver 1. The modulated RF signal is applied as an input signal to a power level control circuit 11 which is controlled by a control signal from the controller 7. An output RF signal from the circuit 11 is applied to an adjustable gain device 13 providing a controllable attenuation or amplification as appropriate of the RF signal. An output RF signal from the device 13 is applied as an input signal to a power amplifier 15. A digital signal from the controller 7 is also applied to a bias control circuit 17 which in practice may be a digital signal processor. A first output digital signal from the control circuit 17 is applied as a control signal to the device 13. A second output digital signal from the control circuit 17 is applied to a digital-to-analogue converter 21. An analogue voltage from the converter 21 is applied as a bias to control the quiescent current in the power amplifier 15 in a known manner. An amplified output RF signal is produced by the power amplifier 15 and is filtered by a LPF (low pass filter) 23 which extracts from the output signal harmonics other than the first harmonic. The amplified and filtered RF output signal from the amplifier 15 and filter 23 is delivered via a switch or circulator 25 to an antenna 27 which transmits the signal over the air as an RF signal to a remote receiver (not shown).

Incoming RF signals may be received by the antenna 27 and diverted by the circulator 23 to be processed by a receiver 29 in a known manner.

The transceiver 1 operates in the following manner.

Information to be transmitted as an RF communication is applied at baseband as modulation signals by the modulator 5 to the RF carrier signal generated by the generator 3. The modulated signal is unconverted to RF by the upconverter 9.

The controller 7 issues control signals which control the power level at which output RF signals are to be transmitted. The control signals may be generated in one of the ways known in the art. For example, the received signal strength may be measured in the mobile station in which the transceiver 1 is incorporated and the measured value may be compared with a set of values held in a look up table in a memory to estimate and classify apparent distance from a serving BTS.

Alternatively, as in a TETRA system, the MS's signal strength may be measured by a BTS serving the MS and the power control parameters to be used by the MS may be calculated by the BTS or system infrastructure and may be transmitted by the BTS to the MS on the logical channel BNCH (Broadcast Network CHannel) available in the TETRA communication protocol.

The control signals from the controller 7 indicate a required setting value of the output RF power level.

The control signals are applied to the power level control circuit 11 to adjust the gain thereof to correspond to a required output power level. Power control by the circuit 11 may be achieved in a manner known per se, e.g. by setting an attenuator in a feedback loop included in the power control circuit 11.

The RF output of the control circuit 11 is applied to the adjustable gain device 13 and the RF output of the device 13 is applied to and amplified by the power amplifier 15. The quiescent current in an active device (e.g. FET) of the amplifier 15 is adjusted in the following manner to correspond with the output power level applied by the controller 7 and power level control circuit 11. A digital control signal indicating the output power level setting value is provided by the controller 7 to the bias control circuit 17. The circuit 17 produces accordingly a digital signal which indicates a corresponding bias needed at the amplifier to produce a corresponding change in the quiescent current of the active device of the power amplifier 15.

The digital signal is converted by the converter 21 to provide a suitable voltage to adjust the bias voltage of the amplifier 15. The circuit 17 also produces a control signal which is applied to adjust the gain of the device 13. The device 13 is provided to maintain substantially constant forward gain of the transmitter chain at lower power levels. The gain of the device 13 is set by the control signal from the controller 7 to compensate for any change of gain of the power amplifier 15 when the quiescent current of the power amplifier is reduced. The device 13 may apply a compensating attenuation or amplification depending on whether the quiescent current of the power amplifier is increased or reduced. A variable attenuation depending on the amplifier output is preferred. The device 13 may comprise a step attenuator, e.g. consisting of one or more resistors, or for continuous control, a continuously adjustable device such as a pin diode.

FIG. 2 is a simple circuit diagram of use of a pin diode in this way. The signal input from the circuit 11 is passed to the power amplifier 15 via a PIN diode 31 and a capacitor 33. The junction between the output of the pin diode 31 and the capacitor 33 is earthed via a resistor 35. A variable control input U control obtained from a signal having an input from the controller 7 in the manner described below is applied through a resistor 35 to adjust the gain of the pin diode 31 to which an input signal of power level Pin is applied. The power amplifier 15 has a bias current Ibias adjusted in the manner decribed herein in accordance with the output power level and an output power level Pout. Operation of the circuit shown in FIG. 2 can be appreciated from the following examples.

1) For a first power output level (full power) the bias current of the amplifier 15 is Ibiasl; and the output power level is Poutl = 30dBm; In this case Pin = 0dBm.

The Overall Gain = Gain of PIN (pin diode 31) + Gain of PA (power amplifier 15) = -6dB + 36dB = 30dB 2) For a second output power level the bias current of the amplifier 15 is Ibias2; and the output power level Pout2 = 25dBm. In this case, Pin = - 5dBm. This gives Overall Gain = Gain PIN + Gain PA = -4dB + 34dB = 3OdB Thus the overall gain in the two examples, of different output power levels, is constant.

FIG. 3 is a graph of bias (quiescent) current versus output power level for the amplifier 15. For a maximum power level P0 the power amplifier 15 current level is a maximum value I0. As the power level is reduced in steps to be levels respectively more than OdB, 5dB, 10dB and 15dB below the maximum value I0, the current is adaptively reduced in steps to be respectively values It, I2, IS and I4. Similarly, as the power level is increased to be less than 15dB below the maximum level PO, the current level is adaptively raised to I3 and so on. Changing the current level in the amplifier 15 adaptively in this manner in accordance with an embodiment of the invention is in contrast to the prior art whereby the current level for a given maximum power is set at IO for all power level values. The current level is adjusted in the prior art to maintain the setting of IO, but it is not adjusted in accordance with step changes in the output power level as described.

In an alternative embodiment of the invention, the amplifier 15 may be incorporated in a feedback loop in the manner described in GB0307368.1 (CM00160B). In this case the output of the bias control circuit 17 is applied as an adjustable threshold to a comparator also receiving via a control loop a detected signal measuring an electrical output of the amplifier. The comparator provides an error control signal output to a processor (e.g. a digital successive approximation register) which in turn applies a digital signal to the digital to analogue converter 21 for use as described earlier with reference to FIG. 1.

Claims

Claims 1. An RF transmitter including: at least one RF component in which

in operation a quiescent current setting controls the performance of the component in the transmitter, means for providing an adjustable bias to the component to control setting of the DC quiescent current in the component, and means for adjusting the power level of RF signals transmitted by the transmitter, the transmitter being characterized in that it further includes a control circuit for adjusting the bias to the component in a manner such that as the power level of the transmitter is changed the DC quiescent current in the component is correspondingly changed.
2. An RF transmitter according to claim 1 including means for changing the output RF power level of the transmitter.
3. An RF transmitter according to claim 2 and wherein the means for changing the output RF power level of the transmitter is operable to change the power level as the location of the transmitter changes.
4. An RF transmitter according to claim 3 and wherein the means for changing the output RF power level of the transmitter is operable to apply an increase in power level when the apparent distance of the transmitter from a serving base station is increased and a decrease in power level when the apparent distance of the transmitter from a serving base station is decreased.
5. An RF transmitter according to claim 3 or claim 4 and wherein the means for changing the output RF power level is operable to apply changes in power level in pre-defined steps.
6. An RF transmitter according to any one of claims 2 to 5 and wherein the means for changing the output power comprises a controller and, coupled to and under the control of the controller, a device operable to apply an adjustable gain to an RF signal to be transmitted by the transmitter.
7. A transmitter according to claim 6 and wherein the controller comprises a digital signal processor.
8. A transmitter according to claim 6 or claim 7 and wherein the controller is operably coupled to the control circuit and is operable to deliver thereto a signal indicating a change in the output power level.
9. A transmitter according to any one of the preceding claims and wherein the control circuit is operable to produce an output bias which reduces the quiescent current in the component when the output power level is indicated to be reduced and which increases the quiescent current when the output power level is indicated to be increased.
10. A transmitter according to claim 8 or claim 9 and wherein the control circuit include a digital to-analogue converter to convert digital control signals into bias voltages to apply to the said device to set the quiescent current thereof.
11. A transmitter according to any one of the preceding claims and wherein the said component comprises an RF signal amplifier.
12. A transmitter according to claim 11 and including a RF power control circuit for changing an output power level of a RF signal applied thereto and wherein the RF signal amplifier comprises a power amplifier operably coupled to receive an RF output signal from the RF power control circuit.
13. A transmitter according to claim 11 or claim 12 which includes an adjustable gain device in the transmitter signal path operable to compensate for any change in gain of the power amplifier caused by any change in DC quiescent current applied therein.
14. A transmitter according to claim 13 wherein the adjustable gain device is in operation controlled by a signal from the means for adjusting output power level.
15. A transmitter according to any one of claims 11 to 14 and wherein the RF signal amplifier is included in an amplifier control circuit which is operable to derive an error control signal to provide bias adjustment of the RF signal amplifier.
16. A transmitter according to claim 15 and wherein the amplifier control circuit comprises a detector for measuring an electrical property of an output from the RF signal amplifier and a feedback loop including a comparator for comparing an output of the detector with a predetermined reference value to produce an error control signal wherein the transmitter includes a controller to control output power level of the transmitter and wherein the controller is operably coupled to the comparator to apply thereto a signal which is adjusted according to the output power level and comprises a variable threshold value.
17. A transmitter according to claim 16 and wherein the feedback loop includes, operably coupled to an output of the comparator, a processor which is operable to apply an algorithm to detect when an output of the comparator indicates minimum deviation of the output of the detector from the predetermined reference value.
18. A transmitter according to claim 17 and wherein the processor comprises a digital signal processor and the feedback loop includes, coupled to the output of the processor, a digital-to-analogue converter having an output coupled to the RF signal amplifier to apply a bias thereto.
19. A transmitter according to claim 1 and substantially as described herein with reference to the accompanying drawings.
20. A mobile station operable in a mobile communication system and including a transmitter according to any one of the preceding claims.
21. A mobile station according to claim 20 and which is operable according to TETRA standard transmitter operating protocols.