Patent Publication Number: US-2004047429-A1

Title: RF digital transmitter

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an RF transmitter, particularly to an RF digital transmitter with low power consumption.  
       [0003] 2. Description of the Prior Art  
       [0004]FIG. 1 illustrates the functional diagram and the basic elements of a digital communication system of a prior art. The digital communication system comprises an information source and input transducer  11 , source encoder  12 , channel encoder  13 , and digital modulator  14  at the transmitting end, and a digital demodulator  15 , channel decoder  16 , source decoder  17  and output transducer  18  at the receiving end. The signal is sent from the transmitting end to the receiving end through a channel  19 . The communication channel is the physical medium that is used to send the signal from the transmitter to the receiver. In wireless transmission, the channel  19  may be the atmosphere (free space). The information source and input transducer  11  may output an analog signal, such as an audio or video signal, or a digital signal, such as the output of a teletype machine, that is discrete in time and has a finite number of output characters. The source encoder  12  implements the process of efficiently converting the signals output from the information source and input transducer  11  into a sequence of binary digits, called an information sequence. The purpose of the channel encoder  13  is to introduce, in a controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overcome the effects of noise and interference encountered in the transmission of the signal through the channel  19 . The digital modulator  14  serves as the interface to the communication channel  19 . The primary purpose of the digital modulator  14  is to map the binary information sequence into signal waveforms.  
       [0005] At the receiving end of the digital communication system are a digital demodulator  15 , channel decoder  16  and source decoder  17  to reconstruct the original signal from the source.  
       [0006] In the channel  19 , low frequency signals cannot be transmitted through the atmosphere over a long distance. It is possible only when the low frequency signals are carried on RF carrier signals. Therefore, there must be an RF transmitter in the transmitting end of the digital communication system.  
       [0007]FIG. 2 is a diagram showing a conventional RF transmitter used in the transmitting end of the digital communication system. The RF transmitter comprises a D/A converter  21 , a local oscillator  13 , a mixer  25  and a power amplifier  27 . The D/A converter  21  receives the digital base-band signal DBS with a baseband frequency f BB , for example less than 10 MHz, and converts it into an analog base-band signal ABS. The local oscillator  23  generates an analog carrier signal ACS with a high Local oscillator frequency f LO , for example 2.4 GHz or 5 GHz. The mixer  25  receives the analog base-band signal ABS and the analog carrier signal ACS, and implements signal multiplication of them. This causes a frequency shift of the signal ABS in frequency domain and produces a semi-transmission signal STS. The semi-transmission signal STS is further amplified by the power amplifier  27  and then a transmission signal TS is transmitted by an antenna. The power amplifier  27  may be a class A, B or C linear power amplifier shown in FIG. 9.  
       [0008]FIGS. 3 a ,  3   b  and  3   c  are diagrams showing the relation between the signals ABS, ACS and TS in frequency domain respectively. The signal ABS has a bandwidth BW (lower than 10 MHz) and a central frequency 0. The signal ACS has a frequency RF (for example, it may be 2.4 GHz or 5 GHz). After being mixed with analog base-band signal ABS by the mixer  25 , the signals ABS and ACS are mixed into the signal TS with the central frequency RF and bandwidth BW. Thus, the analog base-band signal ABS is carried on the analog carrier signal ACS and can be transmitted through the channel over a long distance.  
       [0009] However, there are some drawbacks in the conventional RF transmitter.  
       [0010] 1. The operation of the D/A converter and mixer easily generates a lot of noise. Signal distortion also easily results from the nonlinear transformation of the converter  21  and mixer  25 .  
       [0011] 2. It requires much more effort for the circuit designers to design the layouts for the conventional RF transmitter composed of analog circuits.  
       [0012] 3. The conventional RF transmitter suffers high power consumption due to class A, B, AB or C power amplifier wherein a DC bias always exists in the output signal.  
       SUMMARY OF THE INVENTION  
       [0013] Therefore, the object of the present invention is to provide a digital RF transmitter with low power consumption. The digital RF transmitter is easier for the circuit designers to design the layout for the corresponding digital circuit. A mixer in the digital RF transmitter simply implements multiplication of digital bits from signals and does not cause nonlinear transformation. A power amplifier in the digital RF transmitter may be a class D, E or F power amplifier, which eliminates the unnecessary power consumption resulting from the DC bias in the output signal.  
       [0014] The present invention provides an digital RF digital transmitter. The transmitter comprises a digital modulator for receiving and modulating a digital base-band signal, a local oscillator for generating a digital carrier signal, a mixer for receiving the digital base-band and carrier signal and implementing multiplication of the digital base-band and carrier signal to generate a transmission signal, and a switching power amplifier for amplifying the transmission signal.  
       [0015] The present invention further provides a method for RF digital transmission. The method comprises the step of receiving and modulating a digital base-band signal, the step of generating a digital carrier signal, the step of receiving the digital base-band and carrier signal, and the step of implementing multiplication of the digital base-band and carrier signal to generate a transmission signal, and the step of amplifying the transmission signal.  
       [0016] The present invention also provides an digital RF digital transmitter. The transmitter comprises a digital modulator for receiving and modulating a N-bit digital base-band signal with a frequency f s , and generating a 1-bit modulated digital base-band signal with a frequency N×f s , a local oscillator for generating a digital carrier signal, a digital mixer for receiving the 1-bit modulated digital base-band signal and digital carrier signal, and implementing multiplication thereof to generate a transmission signal, and a switching power amplifier for amplifying the transmission signal.  
       [0017] The present invention provides a method for RF digital transmission. The method comprises the step of receiving and modulating a N-bit digital base-band signal with a frequency f s , and generating a 1-bit modulated digital base-band signal with a frequency N×f s , the step of generating a digital carrier signal, the step of receiving the 1-bit modulated digital base-band signal and digital carrier signal, and implementing multiplication thereof to generate a transmission signal, and the step of amplifying the transmission signal.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0018] The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which:  
     [0019]FIG. 1 illustrates the functional diagram and the basic elements of a digital communication system.  
     [0020]FIG. 2 is a block diagram showing a conventional RF transmitter used in the transmitting end of the digital communication system.  
     [0021]FIGS. 3 a ,  3   b  and  3   c  are diagrams showing the relation between the signals ABS, ACS and TS in frequency domain respectively.  
     [0022]FIG. 4 is a block diagram showing an RF transmitter used in the transmitting end of the digital communication system according to one embodiment of the present invention.  
     [0023]FIG. 5 a ˜ 5   d  are diagrams showing the relation between the signals MDBS, DCS, STS and TS in frequency domain respectively.  
     [0024]FIG. 6 is a flowchart of a method for RF transmission according to one embodiment of the present invention.  
     [0025]FIG. 7 is a diagram showing one embodiment of the modulator of the digital RF transmitter in FIG. 4.  
     [0026]FIGS. 8A and 8B shows one embodiment of the mixer circuit of the digital RF transmitter in FIG. 4 and its truth table.  
     [0027]FIG. 9 is a liner power amplifier, for example, a class A, B or C power amplifier.  
     [0028]FIG. 10 is a switching power amplifier, foe example, a class D, E or F power amplifier.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0029]FIG. 4 is a block diagram showing an RF transmitter used in the transmitting end of the digital communication system according to one embodiment of the present invention. The RF transmitter comprises a digital modulator  41 , a local oscillator  43 , a digital mixer  45 , a switching power amplifier  47  and a band-pass filter  49 . The modulator  41  receives the digital base-band signal DBS with a bandwidth baseband frequency f BB , for example lower than 10 MHz, and modulates it into a modulated digital base-band signal MDBS. The modulator  41  may comprise a noise shaping quantization circuit or over-sampling circuit disclosed in U.S. Pat. No. 5,068,661, which provides a substantial improvement in S/N ratio, implements bit compression resulting in a digital signal having a high resolution being converted to a digital signal having much lower resolution and reduces quantization noise level. The local oscillator  43  generates a digital carrier signal DCS with a Local oscillator frequency f LO , for example 2.4 GHz or 5 GHz. The mixer  45  receives the modulated digital base-band signal MDBS and the digital carrier signal DCS, and then implements multiplication of the digital bits thereof. This causes a frequency shift of the signal MDBS in frequency domain and produces a semi-transmission signal STS. The semi-transmission signal STS is further amplified by the switching power amplifier  47  and filtered by the band-pass filter  49 . Then, a transmission signal TS is transmitted by an antenna. The power amplifier  47  may be a class D, E or F power amplifier as shown in FIG.  10 . The linear power amplifiers, for example, class A, class B and class C power amplifier etc. as shown in FIG. 9, are not suitable for amplifying the digital signal due to the lower efficiency. Power consumption is an important issue in the mobile RF transmitter application.  
     [0030]FIG. 7 is a block diagram showing one embodiment of the modulator  41  of the present invention. In this embodiment, the modulator  41  is a Sigma-Delta modulator, the Sigma-Delta modulator includes an adder  72 , an accumulator  73  and a quantizer  74 . The N-bit signal DBS into a one-bit signal with a frequency N×f s  is input to the adder  72 , wherein f s  is the baseband sampling frequency of the signal DBS. Thus the frequency of output signal MDBS is also N×f s . The circuit of the quantizer  74  can be an AND gate circuit wherein a high logic level is output when the voltage output from the accumulator  73  to the gate is lower than 0V and a low logic level is output when the voltage output from the accumulator  73  to the gate is higher than 0V.  
     [0031]FIG. 8A is a block diagram showing one embodiment of the mixer  45  of the present invention. The mixer  45  may be an AND gate receiving bits A and B respectively from the signals MBDS and DCS. The output of the AND gate is a multiplication of A and B, as shown in the truth table of FIG. 8B.  
     [0032]FIG. 5 a ˜ 5   d  are diagrams showing the relation between the signals MDBS, DCS, STS and TS in frequency domain respectively. The signal MDBS has a bandwidth BW (lower than 10 MHz) and a central frequency 0. Additionally, the signal MDBS also has signal components at higher frequencies. The signal DCS has a frequency RF (2.4 GHz or 5 GHz). After being mixed by the mixer  25 , the signals MDBS and DCS are integrated into the signal STS with the central frequency RF and bandwidth BW. The band-pass filter  49  filters the signal MDBS and eliminates the signal components at the higher frequencies. Thus, the digital base-band signal DBS is carried on the digital carrier signal DCS and can be transmitted over a long distance.  
     [0033]FIG. 6 is a flowchart of a method for RF transmission according to one embodiment of the invention.  
     [0034] In step S 1 , a N-bit digital base-band signal with a frequency f s  is received and modulated, and accordingly a 1-bit modulated digital base-band signal with a frequency N×f s  is generated. The modulation of the N-bit digital base-band signal may be Sigma-Delta modulation.  
     [0035] In step S 2 , a digital carrier signal is generated.  
     [0036] In step S 3 , the 1-bit modulated digital base-band signal and digital carrier signal are received, and multiplication of the two received signals is implemented to generate a semi-transmission signal.  
     [0037] In step S 4 , the semi-transmission signal is amplified by a class D, E or F power amplifier.  
     [0038] Finally, in step S 5 , the amplified semi-transmission signal is received and band-pass filtered, and then transmitted through an antenna.  
     [0039] In conclusion, the present invention provides a digital RF transmitter with low power consumption. The digital RF transmitter is easier for circuit designers to work on. A mixer in the digital RF transmitter simply implements multiplication of digital bits from signals and does not cause nonlinear transformation. A power amplifier in the digital RF transmitter may be a class D, E or F power amplifier, which eliminates the unnecessary power consumption resulting from the DC bias in the output signal.  
     [0040] While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.