Transmission power control circuit

Abstract: A transmission power control circuit comprises a control data table (56) in which digital data of monitor voltage depending on a transmission power level for a plurality of values of transmission frequency and a plurality of values of transmission power are stored, and a D/A converter (62) for converting selected values of the digital data to an analog signal as a reference voltage. The transmission power is controlled so that the monitor voltage becomes equal to the reference voltage.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission power control circuit, more particularly, a transmission power control circuit which controls transmission power of a transmitter used in a mobile communication system such as an automobile telephone system, a portable telephone system, and the like. 2. Description of the Related Art In a mobile communication system, transmission power of the mobile terminal is controlled in response to electrical field strength which varies with the distance between the mobile terminal and a base station and other conditions, to prevent the mobile terminal from transmitting needlessly, and to reduce power consumption of the mobile terminal. In order to perform this function, a radio transmitter for the mobile terminal is provided with an automatic power control circuit (APC). The APC controls the amount of amplification of a power amplifier (PA) so that a monitor voltage corresponding to the transmission power of the PA becomes equal to a reference voltage corresponding to a required transmission power, and thus, the transmission power is controlled to be a required value. As the reference voltage can be discretely altered according to a digital control signal, the transmission power is controlled stepwise. An AM detector for detecting level of the transmission power is provided in the PA, in order to generate the monitor voltage signal. If there is a certain deviation in the characteristics of elements used for the AM detector from standard characteristics, a different monitor voltage can be generated, and thus the level of the transmission power becomes incorrect. Therefore, the transmission power of the products is adjusted before shipping. The adjustment of the transmission power is carried out by altering values of reference voltage V₁ and V₂ which correspond to the minimum and the maximum transmission power, respectively, with two variable resistors. In the aforementioned conventional transmission power control circuit, another problem to be solved arises in the mobile communication system where a carrier frequency of the transmission wave is frequently changed in accordance with an assigned frequency channel. In this case, though the values of the reference voltage V₁ and V₂ are correctly adjusted for one frequency channel, the transmission power can be changed for another frequency, because there is a frequency dependency in the characteristics of the elements constituting the AM detector. Additionally, there can be dispersion in the frequency dependency of the elements. Therefore, it is also desirable to overcome this problem. Moreover, in the aforementioned conventional transmission power control circuit, the level of the transmission power can be incorrect in a medium range even if the maximum and the minimum transmission power are correctly adjusted. JP-A-60244107 shows a transmission power control circuit with a D/A conversion circuit to produce an analog reference, and control means to maintain an output voltage equal to the reference. EP-A-0261967 shows digital control but doesn't show storing digital reference values. SUMMARY OF THE INVENTION It is an object of the present invention to provide a transmission power control circuit which is not affected by the frequency dependency of the elements. It is another object of the present invention to provide a transmission power control circuit which can be adjusted for all transmission power levels to be selected. In accordance with the present invention, there is provided a transmission power control circuit comprising: control data table means for storing digital values of monitor voltage which are to be output from a power amplifier depending on a transmission power level of the power amplifier, for a plurality of transmission frequencies and for a plurality of transmission powers, control means for selecting one of said digital values stored in said control data table means in accordance with a specified value of one of said transmission frequencies, and a specified value of one of said transmission powers, and for outputting a selected digital value, D/A conversion means for converting one of said digital values to an analog signal as a reference voltage, power control means for controlling an amplification factor of the power amplifier so that the monitor voltage which is output from the power amplifier becomes equal to said reference voltage, and A/D conversion means for converting the monitor voltage, which is output from the power amplifier, to a digital signal, wherein said control means sequentially alters the transmission powers and the transmission frequencies, receives said digital signal from said A/D conversion means for combination of each transmission power and each transmission frequency, and writes each digital data into said control data table means. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing an example of a mobile terminal provided with a conventional transmission power control circuit; Figure 2 is a schematic circuit diagram showing a more detailed construction of the PA and APC shown in Fig. 1; Figure 3 is a diagram representing relationship between output power of PA and monitor voltage; Figure 4 is a block diagram showing a mobile terminal provided with a conventional transmission power control circuit, Figure 5 is a schematic circuit diagram showing a more detailed construction of the PA and the APC shown in Fig. 4; Figure 6 is a timing chart representing data transferred from the control part to the APC shown in Fig. 4; Figure 7 is a diagram representing the relationship between PA and POUT at different carrier frequencies; Figure 8 is a block diagram showing a first embodiment of the present invention; and Figure 9 is a block diagram showing a second embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the preferred embodiments according to the invention, examples of aforementioned related art are given with reference to the accompanying drawings. Figure 1 shows a block diagram of an example of a mobile terminal provided with a conventional transmission power control circuit. A control part 10 comprises a central processing unit (CPU) 12, a random access memory (RAM) 14, and a read only memory (ROM) 16. Transmission data TDATA, which is output from the control part 10 to a modulator 18, and received data RDATA, which is input from a demodulator 20 to the control part 10, are used as control signals to perform operations of "call", "receive", and "registration of place", and they are processed in the control part 10. During transmission to a base station, an audio signal picked up by a mic 22 is processed in a transmission audio processing circuit 24, is frequency modulated together with the transmission data TDATA in the modulator 18, and is introduced into a transmitter 26. An output signal of the transmitter 26 is amplified in a power amplifier (PA) 28, whose output signal POUT is transmitted through a duplexer 30 and an antenna 32. During receiving from the base station, a receiver 34 receives a signal through the antenna 32 and the duplexer 30, and the output signal of the receiver 34 is demodulated in the demodulator 20. An output signal of the demodulator 20 is separated into an audio signal and the received data RDATA, and the audio signal is processed in a receiving audio processing circuit 36 and drives a speaker 38. The base station issues an instruction to control the transmission power of the mobile terminal in accordance with measured electrical field strength. When the control part 10 in the mobile station receives the instruction included in the received data RDATA, the control part 10 outputs a three bit parallel signal PC1 to PC3 to an automatic power control circuit (APC) 40. The APC 40 outputs a control voltage V c to control an amount of amplification of the PA 28 in accordance with the parallel signal PC1 to PC3 and a monitor voltage MON from the PA 28. Additionally the APC 40 supplies source voltage V d which is switched ON/OFF according to a carrier on signal CAR-ON from the control part 10, to the PA 28 for ON/OFF control of the transmission carrier. Figure 2 shows a more detailed construction of the PA 28 and the APC 40. In this example, the PA 28 comprises three serially connected amplifiers 42, 44, and 46. Amplification factors of the amplifiers 42 and 44 are controlled by the control voltage V c and source voltage V d is supplied to the third amplifier 46. A diode D1 detects a level of an output signal of the amplifier 46 through a capacitor C1 and the voltage of the detected signal is divided by resistors R1 and R2 to output the monitor voltage MON to APC 40. Resistors R3 to R10 divide the constant-voltage V to derive various voltages. An analog switch 41 selects one of the input terminals to which the various voltages are applied, in accordance with the parallel signal PC1 to PC3. A selected voltage is amplified in an amplifier 42 and input to one input terminal of a differential amplifier 46, as the reference voltage. The minimum value and the maximum value of the reference voltage can be adjusted with variable resistors VR1 and VR2, respectively. The monitor voltage MON is input to another input terminal of the differential amplifier 46 through a buffer amplifier 44. The output voltage of the differential amplifier 46 is applied to a base terminal of a transistor 48, and the control voltage V c is produced in a collector terminal of the transistor 48. The carrier on signal CAR-ON is applied to a base terminal of a transistor 50 through a buffer amplifier 52. Therefore, while the carrier on signal CAR-ON is active, the source voltage V d and the control voltage V c are supplied to the PA 28. A constant-voltage circuit 54 is provided to derive the constant-voltage V from source voltage V DD . Figure 3 shows the relationship between output power of the PA 28 (POUT) and the monitor voltage (MON), wherein the broken line A denotes standard characteristics and the solid line B denotes characteristics in a certain case. As shown in Figure 3, the relationship between POUT and MON is non-linear. If the relationship between POUT and MON in a product coincides with the standard relationship A, P max results by setting the reference voltage to V₂ , P min results by setting the reference voltage to V₁ , and P i results by setting the reference voltage to V i , because the APC 40 operates so that the monitor voltage MON becomes equal to the reference voltage. On the other hand, if the relationship between POUT and MON is as denoted by B, the reference voltage must be V₂′ when P max is required, and the reference voltage must be V₁′ when P min is required. Therefore, the reference voltage is adjusted with the variable resistor VR1 and VR2 so as to be V₂′ when V₂ is selected and to be V₁′ when V₁ is selected. In the aforementioned conventional transmission power control circuit, the maximum transmission power P max and the minimum transmission power P min can be exactly adjusted for a specific carrier frequency. However, if the carrier frequency is changed, incorrect transmission power can be transmitted due to frequency dependency of the elements constituting the PA 28, especially, the diode D1. The preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Figure 4 shows a mobile terminal provided with a transmission power control circuit according to the present invention, and Figure 5 shows a more detailed construction of an APC 40′ shown in Fig. 4. The same reference numerals as used in Figs. 1 and 2 are used for constituents which have the same construction as those in Figs. 1 and 2, and thus descriptions thereof are left out. Referring to Fig. 4, the control part 10′ outputs signals TFREQ and RFREQ to change the carrier frequency of transmission and receiving, respectively, according to an assigned channel. An area of a control data table 56 (explained later) is reserved in a ROM 16′. The control part 10′ outputs data stored in the control data table in the form of serial data (DATA) together with a clock (CLK) and strobe signal (STB), as shown in Figure 6. Referring to Fig. 5, the APC 40′ comprises a shift register 58 for storing the serial data synchronizing with the clock, flip-flops 60 for retaining data stored in the shift register 58 when the strobe signal is active, and an D/A converter 62 for converting the data stored in the flip-flops 60 to an analog signal as the reference signal. The reference signal is supplied to one input terminal of the differential amplifier 46 through the amplifier 47. The D/A converter 62 has a well-known construction comprising a buffer 64, a current switch 66, an equivalent resistor network 68, and a constant-voltage circuit 70. Figure 7 shows the relationship between monitor voltage MON (V) and power level of POUT (dBm) in various conditions. The curve V MH represents standard characteristics in the case where the carrier frequency of a transmission wave has a higher typical value, and the curve V ML represents standard characteristics in the case where the carrier frequency of the transmission wave has a lower typical value. Black dots on the curves correspond to eight transmission power levels one of which is selected as an adequate transmission power level. Table I shows numerical values of the POUT and MON denoted by the black dots. The values of V MH and V ML as shown in Table I are converted to corresponding digital values, beforehand, and the digital values are stored in the control data table 56 shown in Fig. 4. When the control part 10′ receives an instruction to alter the transmission power or the carrier frequency, the CPU 12 reads out an applicable digital value from the control data table 56 in accordance with the values of carrier frequency and the transmission power, and output to the APC 40′ in the form of serial data as shown in Fig. 6. As mentioned previously, the serial data received by the APC 40′ is retained in the flip-flops 60 in the form of parallel data, and converted into an analog signal in the D/A converter 62 to be supplied to the differential amplifier 46 as a reference signal. If dispersion among characteristics of the individual elements can be neglected, standard values of V MH and V ML are stored in the control data table 56. If the dispersion cannot be neglected, the values of V MH and V ML are measured for individual products and digital values corresponding to the measured V MH and V ML are stored in the control data table 56, before shipping. The control data table 56 is made for two typical frequencies in the aforementioned example, but three or more than three frequencies may be applicable. However, a control data table for two or three typical frequencies is preferable. Figure 8 shows a first embodiment of the present invention. An A/D converter 72 is provided for automatically measuring the monitor voltage MON, at each transmission power level and each frequency value. The control part 10′ sequentially alters the transmission power and the carrier frequency, receives digital value of the monitor voltage in combination of each transmission power level and each frequency value, and writes the data into the control data table 56 according to a predetermined writing sequence. Figure 9 shows a second embodiment of the present invention. A temperature sensing element 74 such as a thermo-couple and a thermistor is provided for sensing a temperature of the elements within the PA 28. An output signal of the temperature sensing element 74 is also converted in the A/D converter 72, and supplied to the control part 10′. The control part 10′ sequentially alters the transmission power and the carrier frequency under several temperature conditions recognized by the digital temperature signal, receives a digital value of the monitor voltage, and writes the data into the control data table 56. Using this control data table 56, compensation for temperature dependency of the characteristics of the elements is also performed. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A transmission power control circuit comprising: control data table means (56) for storing digital values of monitor voltage which are to be output from a power amplifier (28) depending on a transmission power level of the power amplifier (28), for a plurality of transmission frequencies and for a plurality of transmission powers, control means (10′) for selecting one of said digital values stored in said control data table means (56) in accordance with a specified value of one of said transmission frequencies, and a specified value of one of said transmission powers, and for outputting a selected digital value, D/A conversion means (62) for converting one of said digital values to an analog signal as a reference voltage, power control means (46, 48) for controlling an amplification factor of the power amplifier (28) so that the monitor voltage which is output from the power amplifier (28) becomes equal to said reference voltage, and A/D conversion means (72) for converting the monitor voltage, which is output from the power amplifier (28), to a digital signal, wherein said control means (10′) sequentially alters the transmission powers and the transmission frequencies, receives said digital signal from said A/D conversion means (72) for combination of each transmission power and each transmission frequency, and writes each digital data into said control data table means (56). A transmission power control circuit as claimed in claim 1, further comprising temperature sensing means (74) for sensing the temperature of elements within the power amplifier (28), wherein an output signal of said temperature sensing means (74) is input to said A/D conversion means (72) to obtain a digital temperature signal and said control means (10′) sequentially alters said transmission powers and said transmission frequencies under a plurality of temperature conditions recognised by said digital temperature signal, receives said digital signal from said A/D conversion means for said combination with respect to individual temperature conditions, and writes each digital data to said control data table and said control means selects one of said digital values in accordance with the values of a transmission frequency and a temperature condition recognised by the digital temperature signal, using the digital data written to said control data table means. A transmission power control circuit as claimed in claim 1, further comprising a serial/parallel conversion means (58, 60), wherein said control means (10′) serially outputs said selected digital value, and said serial/parallel conversion means (58, 60) converts said serial digital value to a parallel signal to be supplied to said D/A conversion means (62).

Inventors: TODA YOSHIFUMI, TODA, YOSHIFUMI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: H04B7
IPC codes: H04B7, H04B1, H03G3