Abstract:
A transmission circuit includes: a first switch configured to select one of a first baseband signal and an oscillation signal; a second switch configured to select one of a second baseband signal and the oscillation signal; a first multiplier configured to multiply a first local frequency signal based on the oscillation signal by the signal selected by the first switch; a second multiplier configured to multiply a second local frequency signal based on the oscillation signal by the signal selected by the second switch; an adder configured to add an output from the first multiplier to an output from the second multiplier; and a correction circuit configured to correct one of the first baseband signal and the second baseband signal based on an output from the adder when the first switch and the second switch select the oscillation signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority from Japanese Patent Application No. 2009-197120 filed on Aug. 27, 2009, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. FIELD 
         [0003]    Embodiments discussed herein relate to a transmission circuit. 
         [0004]    2. DESCRIPTION OF RELATED ART 
         [0005]    A transmission circuit in a communication system includes a quadrature modulation circuit. The quadrature modulation circuit multiplies baseband signals, which are an I component and a Q component, by local frequency signals, the phase difference between the local frequency signals being π/2. In addition, the quadrature modulation circuit adds the multiplication results and outputs a high-frequency signal that is an intermediate frequency IF or a high frequency RF. 
         [0006]    Related art is disclosed in Japanese Laid-Open Patent Publication No. 2006-41631, Japanese Laid-Open Patent Publication No. 2006-50331, and Japanese Laid-Open Patent Publication No. 2003-125014 or the like. 
       SUMMARY 
       [0007]    According to one aspect of the embodiments, a transmission circuit includes: a first switch configured to select one of a first baseband signal and an oscillation signal; a second switch configured to select one of a second baseband signal and the oscillation signal; a first multiplier configured to multiply a first local frequency signal based on the oscillation signal by the signal selected by the first switch; a second multiplier configured to multiply a second local frequency signal based on the oscillation signal by the signal selected by the second switch; an adder configured to add an output from the first multiplier to an output from the second multiplier; and a correction circuit configured to correct one of the first baseband signal and the second baseband signal based on an output from the adder when the first switch and the second switch select the oscillation signal. 
         [0008]    The object and advantages of the invention will be realized and achieved by at least those features, elements and combinations particularly pointed out in the claims. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates an exemplary transmission circuit and an exemplary reception circuit; 
           [0011]      FIG. 2  illustrates an exemplary transmission circuit; 
           [0012]      FIG. 3  illustrates an exemplary transmission circuit; 
           [0013]      FIG. 4  illustrates an exemplary correction circuit; 
           [0014]      FIG. 5  illustrates an exemplary transmission circuit; 
           [0015]      FIG. 6  illustrates exemplary signal waveforms in a transmission circuit; and 
           [0016]      FIG. 7  illustrates exemplary transmission circuit. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0017]    In the figures, dimensions and/or proportions may be exaggerated for clarity of illustration. It will also be understood that when an element is referred to as being “connected to” another element, it may be directly connected or indirectly connected, i.e., intervening elements may also be present. Further, it will be understood that when an element is referred to as being “between” two elements, it may be the only element layer between the two elements, or one or more intervening elements may also be present. 
         [0018]    A carrier leak may occur in an output signal from a quadrature modulation circuit due to voltage variations among circuit devices in a transmission circuit or a characteristic change of a multiplier based on a temperature change. Consequently, a bit error rate (BER) on a reception side may increase due to an occurrence of the carrier leak. Because the carrier leak corresponds to a DC offset component included in the output signal from the quadrature modulation circuit, input signals to the quadrature modulation circuit may be corrected based on a detection of such a DC offset component. However, the carrier leak additionally may occur in the frequency band of a local frequency existing in the frequency band of the output signal from the quadrature modulation circuit. Hence, a DC offset component that does not factor in the effect of the frequency band of the local frequency may not provide correction of the input signals to the quadrature modulation circuit as accurate as desired. 
         [0019]      FIG. 1  illustrates an exemplary transmission circuit and an exemplary reception circuit. The transmission circuit includes an encoder  10 , a mapping circuit  11 , filters  12 A and  12 B, a D/A converter D/A, a quadrature modulation circuit  14 , an RF/IF circuit  15 , and a high power amplifier HPA. The encoder  10  encodes transmission data TX. The mapping circuit  11  maps encoded data to both a Q component and an I component. The filters  12 A and  12 B shape the waveforms of the I signal and the Q signal. The D/A converter D/A digital-analog-converts the waveform-shaped I signal and the waveform-shaped Q signal. The quadrature modulation circuit  14  multiplies the analog I signal and the analog Q signal by a local frequency signal and adds the multiplied I signal to the multiplied Q signal. The RF/IF circuit  15  up-converts the frequency of a quadrature-modulated transmission signal to a carrier frequency. The high power amplifier HPA amplifies an output from the RF/IF circuit  15 . The amplified transmission signal is transmitted from an antenna  17  to a communication medium through a duplexer  16 . 
         [0020]    The reception circuit includes a low noise amplifier LNA, an RF/IF circuit  25 , a quadrature demodulation circuit  24 , an A/D converter A/D, offset correction circuits  22 A and  22 B, a demodulation circuit  21 , and a decoder circuit  20 . The low noise amplifier LNA amplifies a reception signal that is received by the antenna  17  and input through the duplexer  16 . The RF/IF circuit  25  down-converts an output from the low noise amplifier LNA to an intermediate frequency. The quadrature demodulation circuit  24  quadrature-demodulates the down-converted signal using a local frequency signal. The A/D converter A/D analog-digital-converts the baseband signals of the demodulated I component and the demodulated Q component respectively. The demodulation circuit  21  demaps the baseband signals of the I component and the Q component. The decoder circuit  20  extracts reception data by decoding an output from the demodulation circuit. 
         [0021]      FIG. 2  illustrates an exemplary transmission circuit. The transmission circuit illustrated in  FIG. 2  may be the transmission circuit illustrated in  FIG. 1 . The baseband MODEM  100  includes a digital signal processing circuit. The digital processing circuit includes the encoder  10 , the mapping circuit  11 , the waveform shaping filters  12 A and  12 B, the decoder  20 , the demodulation circuit  21 , and the offset correction circuits  22 A and  22 B, which are illustrated in  FIG. 1 . The baseband signal of a digital I component and the baseband signal of a digital Q component are output from the digital signal processing circuit  100 . 
         [0022]    The baseband signal of the I component and the baseband signal of the Q component are converted to analog signals respectively by digital-analog converters D/A. High-frequency quantization noise, which is generated in the D/A conversion, is removed from the converted analog signals by low-pass filters LPF. 
         [0023]    The quadrature modulation circuit  14  includes a phase shifter  140  that generates a second local frequency signal LO (π/2) by shifting the phase of an oscillation signal LO, which has a local frequency generated by an oscillator OSC, by 90 degrees (π/2), first and second multipliers  141  and  142 , an adder  143 , and an amplifier  144 . The first multiplier  141  may include a mixer that multiplies the baseband signal of the I component by a first local frequency signal LO (0), the phase of which is substantially the same as that of the oscillation signal LO. The second multiplier  142  may include a mixer that multiplies the baseband signal of the Q component by the second local frequency signal LO (π/2) that is obtained by shifting the phase of the oscillation signal LO by (π/2). The adder  143  adds an output from the first multiplier  141  to an output from the second multiplier  142 , and the amplifier  144  amplifies the output from the adder  143  and outputs a high-frequency modulation signal IF/RF. 
         [0024]    When the local frequency of the oscillator OSC is an intermediate frequency, the modulation signal IF/RF may have an intermediate frequency. In addition, the modulation signal IF/RF is up-converted to the carrier frequency by a subsequent-stage circuit, not illustrated in  FIG. 2 , and transmitted from an antenna to a communication medium. When the local frequency of the oscillator OSC is the carrier frequency, the modulation signal IF/RF may have the carrier frequency. In addition, the modulation signal IF/RF is transmitted from the antenna to the communication medium. 
         [0025]    High-frequency components may be extracted from outputs from the multipliers  141  and  142  by high-pass filters not illustrated in  FIG. 2 . 
         [0026]    In the transmission circuit illustrated in  FIG. 2 , when the baseband signal processing circuit  100  and the quadrature modulation circuit  14  are included in different LSIs respectively, a DC offset component in a modulation signal may be generated in response to a difference between the reference voltages of the LSIs. When the transmission circuit is housed in the small chassis of a mobile terminal device, the temperature in the chassis may increase during an operation, the characteristic of a mixer such as a multiplier widely may fluctuate, and a DC offset component may be generated. The DC offset component may turn out to be a carrier leak, and a bit error rate (BER) in the reception circuit may increase. 
         [0027]    A temperature sensor is provided adjacent to the quadrature modulation circuit  14  in the chassis in order to reduce the carrier leak. The correction of temperature fluctuation components for the baseband signals of the I component and the Q component may cause the carrier leak to be reduced. An uniform correction may not cause carrier leaks, which vary among individual devices, to be reduced. 
         [0028]    By monitoring the DC component of the quadrature modulation signal IF/RF output from the quadrature modulator  14 , the baseband signal may be corrected in response to the detected DC component. When the baseband signals of the I component and the Q component are set to no-signal states, the DC level of an output signal from the quadrature modulation circuit may turn out to be “ 0 ”. 
         [0029]    When the baseband signals of the I component and the Q component are set to no-signal states, a carrier leak due to the high-frequency characteristic of the multiplier  141  or the multiplier  142  or the like may not be detected. 
         [0030]    When the level detection circuit  18  illustrated in  FIG. 2  includes an A/D converter that analog-digital-converts the quadrature modulation signal IF/RF having a high-frequency, a carrier leak may be detected while the baseband signals of the I component and the Q component are input. 
         [0031]      FIG. 3  illustrates an exemplary transmission circuit. The transmission circuit illustrated in  FIG. 3  includes a baseband signal processing circuit  100 , digital-analog converters D/A, low-pass filters  30  and  31 , and a quadrature modulation circuit  14 . The baseband signal processing circuit  100  generates the baseband signals of the digital I component and the digital Q component. The digital-analog converters D/A convert the baseband signals to analog signals. The low-pass filters  30  and  31  remove high-frequency components from outputs from the digital-analog converters D/A. The quadrature modulation circuit  14  includes a phase shifter  140  that generates a second local frequency signal LO (π/2) by shifting the phase of an oscillation signal LO by 90 degrees (π/2), a first multiplier or a first mixer  141 , a second multiplier or a second mixer  142 , an adder  143 , and an amplifier  144 . 
         [0032]    The transmission circuit illustrated in  FIG. 3  includes a first switch SWi, a second switch SWq, and a phase shifter  145  provided on a Q signal side. The first switch SWi selects one of the baseband signal of the analog I component, which is an output from the low-pass filter  30 , and the oscillation signal LO of the local oscillator OSC. The second switch SWq selects one of the baseband signal of the analog Q component, which is an output from the low-pass filter  31 , and the oscillation signal LO. 
         [0033]    In a normal operation mode, the first switch SWi and the second switch SWq select the baseband signals of the I component and the Q component, which are outputs from the low-pass filters  30  and  31 , respectively. In a correction operation mode, the first switch SWi and the second switch SWq select the oscillation signal LO. The first switch SWi and the second switch SWq are controlled based on a switch control signal SW_Ctrl supplied from the baseband processing circuit  100 . In the normal operation mode, the phase shifter  145  allows the baseband signal of the Q component to pass therethrough with the phase of the baseband signal not being shifted. In the correction operation mode, the phase shifter  145  allows the oscillation signal LO to pass therethrough with the phase of the oscillation signal LO being shifted by  90  degrees. The phase shifter  145  is controlled based on a phase shift control signal PS_Ctrl supplied from the baseband processing circuit  100 . 
         [0034]    The transmission circuit includes a low-pass filter  32 , which removes a high-frequency component from the output IF/RF of the quadrature modulation circuit  14 , and an A/D converter  33  that analog-digital-converts an output from the low-pass filter  32 . A digital output S 33  from the A/D converter  33  is fed back to the baseband processing circuit  100  and used for correcting the DC offset component. 
         [0035]    In the normal operation mode, the first switch SWi and the second switch SWq select the baseband signals of the I component and the Q component, which are outputs from the low-pass filters  30  and  31  respectively, based on the switch control signal SW_Ctrl. The phase shifter  145  may not perform a phase-shift operation. The baseband signals of the I component and the Q component are input to the first multiplier  141  and the second multiplier  142 , respectively. The phase shifter  140  outputs to the first multiplier  141  the first local frequency signal LO (0) obtained by not shifting the phase of the oscillation signal LO. The phase shifter  140  outputs to the second multiplier  142  the second local frequency signal LO (π/2) obtained by shifting the phase of the oscillation signal LO by 90degrees. The adder  143  adds an output from the first multiplier  141  to an output from the second multiplier  142 , the amplifier  144  amplifies the addition signal, and the quadrature modulation output signal IF/RF is output. In the normal operation mode, a normal quadrature modulation may be performed. 
         [0036]    In the correction operation mode, the switches SWi and SWq select the oscillation signal LO based on the switch control signal SW_Ctrl. The phase shifter  145  may shift the phase of the oscillation signal LO by 90 degrees based on the phase shift control signal PS_Ctrl. The oscillation signal LO (0) and the phase-shifted oscillation signal LO (π/2) are input to the first multiplier  141  and the second multiplier  142 , respectively. The phase shifter  140  may be in the normal operation mode. 
         [0037]    For example, when the oscillation signal LO is “sin (X)”, the LO (0) is “sin (X)” and the LO (π/2) is “cos (X)”. Therefore, the output from the first multiplier  141  turns out to be “sin 2  (X)” and the output from the second multiplier  142  turns out to be “cos 2  (X)”. The output signal IF/RF from the quadrature modulation circuit, obtained by adding the output from the first multiplier  141  to the output from the second multiplier  142 , may turn out to be “cos 2  (X)+sin 2  (X)=1”. 
         [0038]    A value “1” in the output signal IF/RF may correspond to the amplitude “1” of the oscillation signal LO when the oscillation signal LO is “sin (X)”, or correspond to a DC component signal without high-frequency component. The A/D converter  33  may convert the DC component to the digital signal S 33  and supply the digital signal S 33  to the baseband signal processing circuit  100 . Since the first multiplier  141  and the second multiplier  142  in the quadrature modulation circuit  14  perform multiplication operations of the high-frequency signals LO (0) and LO (π/2) in the normal operation mode respectively, a highly accurate DC component may be output. 
         [0039]      FIG. 4  illustrates an exemplary correction circuit. The correction circuit illustrated in  FIG. 4  may be included in the transmission circuit illustrated in  FIG. 3 . In the transmission circuit illustrated in  FIG. 4 , the correction circuit in the baseband signal processing circuit  100  is illustrated. Other elements may be substantially the same as or similar to those illustrated in  FIG. 3 . 
         [0040]    The output signal IF/RF from the quadrature modulation circuit  14  may include a DC component signal in the correction operation mode. The DC component data S 33 , which is a digital output signal from the A/D converter  33 , is input to the correction circuit  101 . The correction circuit  101  compares the detected DC component data S 33  with a value corresponding to the amplitude “1”. A DC correction component S 34  is supplied to adders  102  and  103  based on the comparison result. Accordingly, the DC components of the digital I component and digital Q component, which are generated by the baseband signal processing circuit  100 , are corrected. The correction circuit  101  corrects the DC correction component S 34  to be added so that the detected DC component data S 33  substantially matches the value, for example, a difference between the detected DC component data S 33  and the value corresponding to the amplitude “1” becomes zero. 
         [0041]      FIG. 5  illustrates an exemplary transmission circuit. In the transmission circuit illustrated in  FIG. 5 , the phase shifter  140  in the quadrature modulation circuit  14  may be controlled based on the phase shift control signal PS_Ctrl. In the correction operation mode, the phase shifter  140  may output the oscillation signal LO to the first multiplier  141  and the second multiplier  142  without the phase of the oscillation signal LO being shifted. In the normal operation mode, the phase shifter  140  may shift the phase of the oscillation signal LO by 90 degrees to generate the local frequency signal LO (π/2). 
         [0042]    In the transmission circuit illustrated in  FIG. 5 , the phase shifters  140  and  145  may not shift the phases of input signals in the correction operation mode. In the correction operation mode, the first switch SWi and the second switch SWq may select the oscillation signal LO. 
         [0043]    For example, when the oscillation signal LO is “sin (X)”, the oscillation signal LO (0) that is “sin (X)” turns out to be used as the multiplier value and the multiplicand value in the first multiplier  141  and the second multiplier  142 . Therefore, the outputs of the first multiplier  141  and the second multiplier  142  may turn out to be “sin 2  (X)”. The output signal IF/RF from the quadrature modulation circuit in which the output of the first multiplier  141  is added to the output of the second multiplier  142  turns out to be “sin 2  (X)+sin 2  (X)=1−cos(2*X)”. 
         [0044]    When the output signal IF/RF passes through the low-pass filter  32 , “cos (2*X)”, which is a high-frequency component, is removed and a DC component corresponding to the amplitude “1” is extracted. The correction circuit in the digital signal processing circuit  100  corrects the DC components in the baseband signals of the I component and the Q component based on the DC component data S 33  from the A/D converter  33 . 
         [0045]    In the transmission circuit illustrated in  FIG. 5 , the phase-shift operations performed in the phase shifters  140  and  145  are controlled by the phase shift control signal PS_Ctrl. Accordingly, the correction operation mode illustrated in  FIG. 3 , for example, a first correction operation mode and the correction operation mode illustrated in  FIG. 4 , for example, a second correction operation mode are set. 
         [0046]    In the first correction operation mode, the output signal IF/RF from the quadrature modulation circuit is set to “cos 2  (X)+sin 2  (X)=1”. In the second correction operation mode, the output signal IF/RF from the quadrature modulation circuit is set to “sin 2  (X)+sin 2  (X)=1−cos(2*X)”. 
         [0047]    The correction circuit in the digital signal processing circuit  100  reduces the DC offset of the output signal IF/RF based on the average value of the DC component data S 33  in the output signal IF/RF, which is detected in the first correction operation mode or the second correction operation mode. The DC component data, which is generated when the quadrature modulation circuit  14  performs modulation processing for different signals, is corrected based on the average value of DC component data, which is detected in the first correction operation mode and/or the second correction operation mode. Therefore, a carrier leak component generated in the normal operation mode may be suitably removed. 
         [0048]    The correction circuit in the digital signal processing circuit  100  reduces the DC offset of the output signal IF/RF based on the DC component data S 33  in the output signal IF/RF, which is detected in the first correction operation mode or the second correction operation mode. 
         [0049]      FIG. 6  illustrates exemplary signal waveforms in a transmission circuit. The signal waveforms illustrated in  FIG. 6  may be the signal waveforms of the transmission circuit illustrated in  FIG. 5  or  3 . A horizontal axis in  FIG. 6  indicates a time scale. A vertical axis in  FIG. 6  indicates a voltage. A value “1” on the vertical axis may correspond to the amplitude “1” of the oscillation signal LO. In  FIG. 6 , when the oscillation signal LO is equal to “sin (X)”, signals “cos (X)”, “cos 2  (X)”, “sin 2  (X)”, “cos 2  (X)+sin 2  (X)=1”, and “sin 2  (X)+sin 2  (X)=1−cos (2* X)” are Illustrated. The output signals “cos 2  (X)+sin 2  (X)=1” and “sin 2  (X)+sin 2  (X)=1−cos (2* X)” may have amplitudes “1” as DC components. 
         [0050]      FIG. 7  illustrates an exemplary transmission circuit. In the transmission circuit illustrated in  FIG. 7 , a phase shifter  146  is provided on an I component side. Other elements illustrated in  FIG. 7  may be substantially the same as or similar to those illustrated in  FIG. 5 . 
         [0051]    In the normal operation mode, the first switch SWi and the second switch SWq select the baseband signals of the I component and the Q component, which are outputs from the low-pass filters  30  and  31 , respectively. The phase shifter  140  outputs the oscillation signal LO to the first multiplier  141  without the phase of the oscillation signal LO being shifted. The phase shifter  140  outputs the oscillation signal LO to the second multiplier  142  with the phase of the oscillation signal LO being shifted by 90 degrees. 
         [0052]    In the correction operation mode, the first switch SWi and the second switch SWq select the oscillation signal LO. In the first correction operation mode, the phase shifter  146  outputs the oscillation signal LO to the first multiplier  141  with the phase of the oscillation signal LO being shifted by 90 degrees. The phase shifter  140  outputs the oscillation signal LO to the first multiplier  141  with the phase of the oscillation signal LO being shifted by 90 degrees, and the phase shifter  140  outputs the oscillation signal LO to the second multiplier  142  without the phase of the oscillation signal LO being shifted. When the oscillation signal LO is set to “sin (X)”, the output of the first multiplier  141  turns out to be “cos 2  (X)” and the output of the second multiplier  142  turns out to be “sin 2  (X)”. The output signal IF/RF from the quadrature modulation circuit, which is obtained by adding the output from the first multiplier  141  to the output from the second multiplier  142 , turns out to be “cos 2  (X)+sin 2  (X)=1”. 
         [0053]    In the second correction operation mode, the phase shifter  146  outputs the oscillation signal LO to the first multiplier  141  without the phase of the oscillation signal LO being shifted. The phase shifter  140  outputs the oscillation signal LO to the second multiplier  141  without the phase of the oscillation signal LO being shifted. When the oscillation signal LO is set to “sin (X)”, the outputs of the first multiplier  141  and the second multiplier  142  turn out to be “sin 2  (X)”. The output signal IF/RF from the quadrature modulation circuit, which is obtained by adding the output from the first multiplier  141  to the output from the second multiplier  142 , turns out to be “sin 2  (X)+sin 2  (X)=1−cos (2* X)”. 
         [0054]    The correction circuit in the digital signal processing circuit  100  may reduce the DC offset of the output signal IF/RF based on the average value of the DC component data S 33  in the output signal IF/RF, which is detected in the first correction operation mode or the second correction operation mode. 
         [0055]    The correction circuit in the digital signal processing circuit  100  may reduce the DC offset of the output signal IF/RF based on the DC component data S 33  in the output signal IF/RF, which is detected in the first correction operation mode or the second correction operation mode. 
         [0056]    As illustrated in  FIG. 5  or  7 , the phase shifters  145  and  146  may be provided on the Q component side or the I component side. In the correction operation mode, the phase shifter  140  in the quadrature modulation circuit  14  may perform a phase-shift operation different from that in the normal operation mode. In the phase shifter  140  in the quadrature modulation circuit  14  illustrated in  FIG. 3 , a phase-shift operation performed in the normal operation mode may be substantially the same as or similar to a phase-shift operation performed in the correction operation mode. 
         [0057]    As long as at least the first switch Swi, the second switch SWq, and the phase shifters  145  and  147  are provided, the DC offset of the output signal from the quadrature modulation circuit  14  in which a high-frequency operation is performed is detected. Therefore, a carrier leak may be reduced. 
         [0058]    All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.