Abstract:
A two-channel RF digital transmitter. The digital RF transmitter comprises a first and second digital modulator for receiving and modulating a first and second digital base-band signal from a first and second channel, a first and second local oscillator for generating a first and second digital carrier signal, a first and second mixer for receiving the first and second digital base-band and carrier signal and implementing multiplication of the first and second digital base-band and carrier signal to generate a first and second transmission signal, a first and second filter for band-pass filtering the first and second transmission signal respectively, and an adder for summing the filtered first and second transmission signal to generate a summation signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a digital RF transmitter, particularly to a digital RF transmitter with IQ modulator.  
           [0003]    2. Description of the Prior Art  
           [0004]    [0004]FIG. 1 illustrates the functional diagram and the basic elements of a conventional digital communication system. 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 sends 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, discrete in time and having 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]    [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 thereof. 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.  
           [0008]    [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, 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.  
         SUMMARY OF THE INVENTION  
         [0012]    Therefore, the object of the present invention is to provide a digital RF transmitter more easily designed 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.  
           [0013]    The present invention provides a digital RF digital transmitter. The transmitter comprises a first and second digital modulator for receiving and modulating a first and second digital base-band signal from a first and second channel, a first and second local oscillator for generating a first and second digital carrier signal, a first and second mixer for receiving the first and second digital base-band and carrier signal and implementing multiplication of the first and second digital base-band and carrier signal to generate a first and second transmission signal, a first and second filter for band-pass filtering the first and second transmission signal respectively, and an adder for summing the filtered first and second transmission signal to generate a summation signal.  
           [0014]    The present invention further provides a method for RF digital transmission. The method comprises the step of receiving and modulating a first and second digital base-band signal from a first and second channel, the step of generating a first and second digital carrier signal, the step of receiving the first and second digital base-band and carrier signal, and the step of implementing multiplication of the first and second digital base-band and carrier signal to generate a first and second transmission signal, the step of band-pass filtering the first and second transmission signal respectively, and the step of summing the filtered first and second transmission signal to generate a summation signal.  
           [0015]    The present invention also provides a digital RF transmitter. The transmitter comprises a first and second digital modulator for receiving and modulating a first and second N-bit digital base-band signal with a frequency f s  from a first and second channel, and generating a first and second 1-bit modulated digital base-band signal with a frequency N×f s , a first and second local oscillator for generating a first and second digital carrier signal, a first and second digital mixer for receiving the first and second 1-bit modulated digital base-band signal and digital carrier signal, and implementing multiplication thereof to generate a first and second transmission signal, a first and second filter for band-pass filtering the first and second transmission signal respectively, and an adder for summing the filtered first and second transmission signal to generate a summation signal.  
           [0016]    The present invention provides a method for RF digital transmission. The method comprises the step of receiving and modulating a first and second N-bit digital base-band signal with a frequency f s  from a first and second channel, and generating a first and second 1-bit modulated digital base-band signal with a frequency N×f s , the step of generating a first and second digital carrier signal, the step of receiving the first and second 1-bit modulated digital base-band signal and digital carrier signal, and implementing multiplication thereof to generate a first and second transmission signal, the step of band-pass filtering the first and second transmission signal respectively, and the step of summing the filtered first and second transmission signal to generate a summation signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    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:  
         [0018]    [0018]FIG. 1 illustrates the functional diagram and the basic elements of a digital communication system.  
         [0019]    [0019]FIG. 2 is a block diagram showing a conventional RF transmitter used in the transmitting end of the digital communication system.  
         [0020]    [0020]FIGS. 3 a ,  3   b  and  3   c  are diagrams showing the relation between the signals ABS, ACS and TS in frequency domain respectively.  
         [0021]    [0021]FIG. 4 a  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.  
         [0022]    [0022]FIG. 4 b  is a block diagram showing an RF transmitter used in the transmitting end of the digital communication system according to another embodiment of the present invention.  
         [0023]    [0023]FIGS. 5 a - 5   d  are diagrams showing the relation between the signals MDBS, DCS, STS and TS in frequency domain respectively.  
         [0024]    [0024]FIG. 6 is a flowchart of a method for RF transmission according to one embodiment of the present invention.  
         [0025]    [0025]FIG. 7 is a diagram showing one embodiment of the modulator of the digital RF transmitter in FIG. 4.  
         [0026]    [0026]FIGS. 8A and 8B shows one embodiment of the mixer circuit of the digital RF transmitter in FIG. 4 and its truth table. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    [0027]FIG. 4 a  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 digital modulators  41   a  and  41   b , local oscillator  43   a  and  43   b , digital mixers  45   a  and  45   b , switches  47   a  and  47   b , band-pass filters  49   a  and  49   b , an adder  42 , and a power amplifier  44 . The modulator  41   a  receives the digital base-band signal DBS 1  from an I channel with a bandwidth baseband frequency f BB , for example lower than 10 MHz, and modulates it into a modulated digital base-band signal MDBS 1 . The modulator  41   a  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 converted to a digital signal having much lower resolution and reduced quantization noise level. Similarly, the modulator  41   b  receives the digital base-band signal DBS 2  from a Q channel with a bandwidth baseband frequency f BB , for example lower than 10 MHz, and modulates it into a modulated digital base-band signal MDBS 2 . The modulator  41   b  may also comprise a noise-shaping quantization circuit or over-sampling circuit disclosed in U.S. Pat. No. 5,068,661. There is a phase difference of 90° between the signals from the I and Q channel. The local oscillator  43   a  generates a digital carrier signal DCS 1  with a Local oscillator frequency f LO , for example 2.4 GHz or 5 GHz. Similarly, the local oscillator  43   b  generates a digital carrier signal DCS 2  with a Local oscillator frequency f LO . The mixer  45   a  receives the modulated digital base-band signal MDBS 1  and the digital carrier signal DCS 1 , and then implements multiplication of the digital bits thereof. This causes a frequency shift of the signal MDBS 1  in frequency domain and produces a semi-transmission signal STS 1 . Similarly, the mixer  45   b  receives the modulated digital base-band signal MDBS 2  and the digital carrier signal DCS 2 , and then implements multiplication of the digital bits thereof. This also causes a frequency shift of the signal MDBS 2  in frequency domain and produces a semi-transmission signal STS 2 . The semi-transmission signals STS 1  and STS 2  are sent to the switches  47   a  and  47   b , and then filtered by the band-pass filters  49   a  and  49   b , respectively. The filtered semi-transmission signals TS 1  and TS 2  are added by the digital adder  42 . The adder  42  generates a summation signal SS sent to the power amplifier  44 . The summation signal SS is amplified by the power amplifier  44  and then transmitted by an antenna.  
         [0028]    [0028]FIG. 4 b  is a block diagram showing an RF transmitter used in the transmitting end of the digital communication system according to another embodiment of the present invention. The same elements in FIGS. 4 a  and  4   b  refer to the same symbols for clarity. It is noted that, in the RF digital transmitter shown in FIG. 4 b , there is only one local oscillator  43   c . The oscillator  43   c  generates the two digital carrier signals DCS 1  and DCS 2  with a phase difference of 90° respectively for I and Q channel.  
         [0029]    [0029]FIG. 7 is a block diagram showing one embodiment of the modulator  41   a  or  41   b  of the present invention. In this embodiment, the modulator  41   a  or  41   b  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 1  or DBS 2  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 10 R DBS 2 . Thus the frequency of output signal MDBS 1  or MDBS 2  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.  
         [0030]    [0030]FIG. 8A is a block diagram showing one embodiment of the mixer  45   a  or  45   b  of the present invention. The mixer  45   a  or  45   b  may be an AND gate receiving bits A and B respectively from the signals MBDS 1  or MDBS 2 , and DCS 1  or DCS 2 . The output of the AND gate is a multiplication of A and B, as shown in the truth table of FIG. 8B.  
         [0031]    [0031]FIGS. 5 a - 5   d  are diagrams showing the relation between the signals MDBS 1 /MDBS 2  DCS 1 /DCS 2 , STS 1 /STS 2  and TS 1 /TS 2  in frequency domain respectively. The signal MDBS 1 /MDBS 2  has a bandwidth BW (lower than 10 MHz) and a central frequency 0. Additionally, the signal MDBS 1 /MDBS 2  also has signal components at higher frequencies. The signal DCS 1 /DCS 2  has a frequency RF (2.4 GHz or 5 GHz). After being mixed by the mixer  45   a / 45   b , the signals MDBS 1 /MDBS 2  and DCS 1 /DCS 2  are integrated into the signal STS 1 /STS 2  with the central frequency RF and bandwidth BW. The band-pass filter  49   a / 49   b  filters the signal STS 1 /STS 2  and eliminates the signal components at the higher frequencies. Thus, the digital base-band signal DBS 1 /DBS 2  is carried on the digital carrier signal DCS 1 /DCS 2  and can be transmitted over a long distance.  
         [0032]    [0032]FIG. 6 is a flowchart of a method for RF transmission according to one embodiment of the invention.  
         [0033]    In step S 1 , N-bit digital base-band signals from the I and Q channel with a frequency f s  are received and modulated, and accordingly two 1-bit modulated digital base-band signals with a frequency N×f s  are respectively generated. The modulation of the N-bit digital base-band signals may be Sigma-Delta modulation.  
         [0034]    In step S 2 , digital carrier signals for the signals from the I and Q channel are generated.  
         [0035]    In step S 3 , the 1-bit modulated digital base-band signals and digital carrier signals are received, and multiplication of the two received signals for each channel is implemented to respectively generate two semi-transmission signals.  
         [0036]    In step S 4 , the semi-transmission signals are band-pass filtered.  
         [0037]    In step S 5 , the two semi-transmission signals from the I and Q channel are added to generate a summation signal.  
         [0038]    Finally, in step S 6 , the summation signal is amplified and transmitted through an antenna.  
         [0039]    In conclusion, the present invention provides a two-channel digital RF transmitter. 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.  
         [0040]    While the invention has been described by way of example and in terms of the preferred embodiments, 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.