RADIO FREQUENCY TRANSMITTERS

Abstract: There is disclosed a method of controlling a radio frequency power amplifier (1) in a digital communications device. The linearity of the amplifier (1) is adjusted such that during transmission of data from the device the amplifier operates in a non-linear mode, and before and after transmission of data from the device the amplifier operates in a linear mode.

Description: RADIO FREQUENCY TRANSMITTERS The present invention relates to radio frequency- transmitters, and in particular to transmitters suitable for use in a digital communications device. DESCRIPTION AND RELATED ART Radio frequency transmitters, particularly those used in battery-powered communications devices, should have the best efficiency possible because in such a device the transmitter power amplifier is the dominant user of battery power. However, the most efficient power amplifiers available are generally the most nonlinear, and can therefore generate unwanted spurious signals . In the GSM mobile telephone system, this is not necessarily a problem since the modulation format is not degraded by non-linear amplifiers. However, a very non-linear amplifier cannot be used since interference is generated at the beginning and end of a transmitted burst of data as the carrier, i.e. signal is switched on and off. This situation has led to a compromise in which amplifiers for GSM applications are designed to deliver less than optimum efficiency in order that the amplifier can retain enough linearity to meet the spurious signal generation specifications. Accordingly, it is desirable to provide a power amplifier which is as efficient as possible, but which can avoid the generation of unwanted spurious signals. SUMMARY OF THE PRESENT INVENTION The present invention is therefore intended to provide a system in which an amplifier operates efficiently during data transmission, but that operates to avoid generation of spurious signals during ramp-up and ramp-down phases of data transfer. According to one aspect of the present invention, there is provided a method of controlling a radio frequency power amplifier such that during data transmission from the amplifier, the- amplifier operates in an efficient non-linear operating mode. According to another aspect of the present invention there is provided a method of operating a radio frequency amplifier in a digital communications device, wherein the linearity of the amplifier is controlled such that the amplifier operates in a nonlinear mode during transmission of data from the device, and such that the amplifier operates in a linear mode before and after such data transmission. According to a further aspect of the present invention, there is provided a radio frequency transmitter comprising a power ammplifier which operates in a non-linear mode during transmission of data from the transmitter. According to another aspect of the present invention, there is provided a transmitter for a digital communications device, comprising a power amplifier having a linear operating mode and a nonlinear operating mode, and control means operable to control the power amplifier such that the amplifier operates in non-linear mode during transmission of data from the device, and in the linear mode before and after transmission of data from the device. BRIEF DESCRIPTION OF THE DRAWINGS. Figure 1 is a block diagram illustrating a first transmitter embodying the present invention; Figure 2 is a block diagram illustrating a second transmitter embodying the present invention; Figure 3 is a timing diagram illustrating operation of the transmitter of Figure 1 or Figure 2; Figure 4 is a block diagram illustrating a- further part of the transmitter of Figure 1 or Figure 2; Figure 5 is a circuit diagram illustrating part of the transmitter of Figure 1 or Figure 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a block diagram of a radio frequency transmitter, for example for use in a GSM mobile telephone. The transmitter includes a power amplifier 1, a modulated carrier signal source 2 , and a control unit 3. The control unit 3 includes a timing unit 4 and a bias control unit 5. The timing unit 4 is connected for transferring control information with the modulated carrier signal source 2 and with the bias control unit 5. The bias control unit 5 is connected to control the power amplifier 1. Figure 2 shows a second transmitter, which includes additional units which provide a feedback loop. The feedback loop units include a frequency conversion unit 6, a phase detector 7, a voltage control oscillator 8 and a reference signal generator 9. The reference signal generator 9 generates the required modulation signal, and this is applied via the phase detector 7 to the carrier signal generated by the voltage control oscillator 8. The phase difference detection provided by the frequency converter 6 and the phase detector 7 are used to ensure that the transmitted signal has its modulation accuracy unaffected when the amplifier" s characteristics are changed, as will be described in more detail below. Figure 3 shows timing diagram illustrating an example of the operation of the transmitters of Figures 1 and 2. The upper line 10,11 on the graph shows the level of the bias control signal output from the bias control unit 5, and the lower line 12,13 shows the transmitter output power level. As can be seen from the lower line, during an output burst of data between times B and C (13) , the amplifier output level is maximised. Whilst data is not being transmitted, the power level is minimised, i.e. before time A and after time D (12) . The transition between the minimised power level and maximised power level takes a period of time illustrated between times A and B (ramp-up) and times C and D (ramp-down) on the graph. During the ramp up of the amplifier output power level between times A and B, the bias control unit 5 operates to output a bias control signal which changes from a signal indicating linear operation of the amplifier to a signal indicating non-linear operation of the amplifier. The bias control unit adjusts the linearity of the power amplifier 1 during a transmission burst by adjusting the amplifier's bias point, such that at the beginning and end of the burst the amplifier is operating in a linear but inefficient mode, and at the middle of the burst the amplifier operates in a non-linear but efficient mode. The spurious signals generated as the transmitter ramps up and down in output power are minimised because the amplifier operates in a linear mode during these times, but once clear of this region the amplifier is adjusted to give the best efficiency for the rest of the burst. Since the ramp up and ramp down periods of the burst last only about 10% of the length of the whole burst, the average efficiency of the transmitter will approach the figure achieved for continuous efficient operation. The timing unit 4 of the control unit 3 generates a control signal in synchronism with the transmitter power burst. During ramp up or ramp down of the power level it is possible to adjust the linearity of- the power amplifier to the optimum value for the power level concerned. The optimum value can be determined by experiment and the required numbers programmed into a ROM, or by some other means . Data can be loaded into the lookup table 51 using an external connection to the telephone and suitable test software, or by the use of a test jig which allows programming of the ROM directly. The lookup table may be stored in separate memory or as part of the telephone's main software ROM. The bias control unit 5 can continue to change its output after the maximum power level has been reached and the ramp up period is over. This allows the transmitter to move between linear and non-linear states without a sudden shift in phase or amplitude in the output. Thus the transmitter operates in an inefficient, but linear, mode during ramp up and down, and operates in a non-linear but efficient mode for the majority of the data burst. Figure 4 is a block diagram illustrating one implementation of the control unit 3, including the timing unit 4 and the bias control unit 5. The timing unit comprises a first counter 41, rising and falling comparators 42 and 43 and a second counter 44. The first and second counters 41 and 44 are supplied with a system clock signal CLK. The output of the first counter 41 is supplied to each of the comparators 42 and 43, and the respective outputs of those comparators are supplied as inputs to the second counter 44. The output of the second counter 44 is supplied to the bias control unit 5, and in particular to a lookup table 51. The output of the lookup table 51 is converted to an analog signal by a digital to analog converter DAC 52 and supplied to the power amplifier. The lookup table also receives an indicated power level signal BWR. In operation, the control unit of Figure 4 is initiated by reception of an enable signal EN to the first counter 41. The enable signal signifies the start of a transmission of a data burst and in a GSM system would be issued approximately every 4.6ms. The source of this signal would normally be a microprocessor or similar device which has been programmed in accordance with the requirements of the particular system. In GSM systems, the transmitter output level is required to increase from 0 to a defined power level on receipt of such an enable or start transmission burst command. As an example, assume that the clock to both counters is running at 1MHz i.e. lus per cycle. For a GSM system the rising part of the burst lasts approximately 30us and the main part 577us. Therefore after the start of a TX burst the counter 44 will count to 30 while the power is ramped up, then count 31 to 607 during the burst and then 608 to 638 as power ramps down. For simplicity, assume that the bias is to be adjusted during the 30us periods only. The 'rising comparator' allows the second counter to operate when the first counter is in the region 0 to 30 (i.e. the rising comparator provides a 'count upwards' command if its input is less than 31) . The 'falling comparator' provides a 'count downwards' command to the second counter when the first counter has reached €08. This causes the ROM to be addressed in a falling sequence which exactly mirrors the rising sequence at the start of the burst. The number of clock cycles devoted to counting upwards must be the same as the number devoted to counting downwards . To ensure this, the second "counter might be reset at the start of each transmission burst. The contents of the lookup table equates to bias levels which set the efficiency of the power amplifier to the required level. The lookup table is simply a set of memory locations which can be addressed by the second counter. One value is output for each unique combination of input address data. Figure 5 is a diagram illustrating a simple power amplifier having power level and bias controls. The main supply voltage V s sets the available output power and the bias voltage signal N B is supplied by the bias control unit. In the example shown, a high level V B would cause the transistor 11 to operate in a linear mode, whilst a low V B would cause the transistor 11 to operate in a non-linear fashion. Impedances 12 and 13 are provided in the usual manner, and the amplified input signal is output at the output port O/P. A typical practical power amplifier would comprise two or three power transistors arranged in a cascade so that the output power level of the modulated carrier signal is amplified to the level of a few watts. It will thus be appreciated that a transmitter embodying the present invention enables efficient use of battery power in a portable device, whilst retaining the requirements for the prevention of generation of unwanted spurious signals.

Claims: CLAIMS 1. A method of controlling a radio f equency power amplifier in a digital communications device, wherein the linearity of the amplifier is adjusted such that during transmission of data from the device the amplifier operates in a non-linear mode, and before and after transmission of data from the device the amplifier operates in a linear mode. 2. A method as claimed in claim 1, wherein the transition between the non-linear and linear operating modes is made in a continuous manner. 3. A method as claimed in claim 1 or 2, wherein the linearity of the amplifier is controlled by varying the bias level of the amplifier. 4. A radio frequency transmitter for a digital communications device comprising a power amplifier having a linear operating mode and a non-linear operating mode, and control means operable to control the power amplifier such that the amplifier operates in the non-linear operating mode during transmission of data from the device, and in the linear mode before and after transmission of data from the device. 5. A transmitter as claimed in claim 4, wherein the transition of the power amplifier between the linear and non-linear operating modes is made in a continuous manner. 6. A transmitter as claimed in claim 4 or 5, wherein the control mean includes a timing unit for controlling the output of the control means in dependence upon the a data output timing, and a bias control means for controlling the bias level of te power amplifier so as to vary the linearity of the amplifier.

Inventors: BRISTOW ROBERT OWEN, WILSON ALEXANDER COLIN, BRISTOW, ROBERT, OWEN, WILSON, ALEXANDER, COLIN
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1999
Main classification: H04B1
IPC codes: H03F1, H04B1