Abstract:
An analog-to-digital converter conventionally performing a detection of a differential offset is replaced by a comparator  20 . A reference voltage is input to a terminal on one side of the comparator  20  and each of a pair of differentials of a differential voltage signal is input to the other terminal one by one. Then, a setup of voltages of both of the pair of differentials to closer values to the reference voltage makes both voltages of the pair of differentials eventually the same, thereby making it possible to correct a differential offset.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-265601 filed on Sept. 28, 2006, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circuit for correcting a differential offset of a transmission apparatus sending out a transmission signal of a telecommunication system. 
     2. Description of the Related Art 
     When using a quadrature modulation such as a binary phase shift keying (BPSK)/quadrature phase shift keying (QPSK) and such in a digital wireless technique, a differential offset amount of a transmission output circuit inputting a transmission signal as a differential signal to a quadrature modulator has a great deal of influence to a transmission performance. Therefore, the conventional technique feeds back a voltage of a differential output signal of a transmission output circuit, applies an analog/digital (AD) conversion and obtains a value of the output voltage, thereby correcting the differential offset based on the obtained value. 
       FIG. 1  is a diagram exemplifying a configuration of a conventional differential offset correction circuit. 
     A transmission digital signal generated at a transmission digital signal generation unit  10  is input to a primary signal differential output digital/analog converter (DAC)  12  of a transmission analog signal output unit  11 - 1 . The transmission analog signal output unit  11 - 1  corresponds to an I channel-use when performing a quadrature modulation. When performing a quadrature modulation, it is also equipped in a Q channel-use transmission analog signal output unit  11 - 2 . The configuration of the transmission analog signal output unit  11 - 2  is the same as that of the transmission analog signal output unit  11 - 1  and therefore the drawing thereof is omitted herein. 
     The primary signal differential output DAC  12  outputs a primary signal that is a digital signal as an analog difference voltage signal. The difference voltage signal output from the primary signal differential output DAC  12  is amplified by a fully differential amplifier  13 . Respective signals of the differential voltage signal are input to a voltage value adjustment circuit comprising resistors R 1  and R 2  and a differential amplifier  14 - 1 , and resistors R 3  and R 4  and a differential amplifier  14 - 2 , followed by being set at a prescribed voltage and being output by way of an output control switch  15 . The output voltages are indicated by the OIP on one side and by the OIM on the other. Because it is a differential voltage signal, the quadrature modulator is operated on the basis of the voltage difference between the OIP and OIM if an output signal is given to the quadrature modulator. Therefore, if there is a differential offset in the differential voltage signals OIP and PIM, the operation of the quadrature modulator is adversely influenced. This accordingly requires a control of the voltage values of the OIP and OIM by controlling the differential amplifiers  14 - 1  and  14 - 2 . 
     The conventional technique shown in  FIG. 1  is configured to input the outputs of the differential amplifiers  14 - 1  and  14 - 2  respectively to an offset detection-use AD converter (ADC)  17  by way of a comparator input select switch  16 . The offset detection-use ADC  17  is an AD converter for converting the differential offset value of the analog differential voltage signal output from the differential amplifiers  14 - 1  and  14 - 2 . The output of the offset detection-use ADC  17  is input to a control logic unit  18  which then inputs a reference voltage value, as a digital value, to the offset correction-use DAC  19 - 1  and  19 - 2 , so as to minimize a differential offset amount as much as possible. The offset correction-use DACs  19 - 1  and  19 - 2  convert the digital voltage value obtained from the control logic unit  18  into analog voltage values and input to the respective terminals, on one side, of the differential amplifiers  14 - 1  and  14 - 2 , respectively, as a reference voltage. Such an operation results in the reference voltage which is input to the differential amplifiers  14 - 1  and  14 - 2  being adjusted so as to minimize the differential offset, and therefore the voltage values of the differential voltage signal output from the differential amplifiers  14 - 1  and  14 - 2  are adjusted so as to minimize the differential offset. 
     As another conventional technique for reducing an adverse effect of a differential offset, there is one noted in a reference patent document  1 , in which a method for adding correction data to transmission data in order to remove an adverse effect of a differential offset. 
     Patent document 1: Laid-Open Japanese Patent Application Publication No. H07-30596 
     In the case of the conventional technique shown in  FIG. 1 , there are various methods of an AD converter (i.e., the offset detection-use ADC  17 ), such as a consecutive conversion type, flash type, et cetera, in which a circuit area size, hence consumption current, generally increases with conversion speed. And, if a highly accurate offset adjustment is required, a minute adjustment is necessary after a correction because an error at an AD conversion and errors of a correction-use DAC (i.e., offset correction-use DAC  19 - 1  and  19 - 2 ) are accumulated. 
     SUMMARY OF THE INVENTION 
     According to an aspect of an embodiment, a differential offset correction circuit comprising: a differential digital-to-analog conversion unit for converting a digital signal into differential analog signals; a comparator for detecting the differential analog signals; and a differential offset correction unit for correcting a differential offset based on the detection result of the comparator. 
     The present invention is contrived not to use an AD converter for detecting a differential offset, thereby making it possible to make a circuit area size and power consumption small, and also correct the differential offset highly accurately. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram exemplifying a configuration of a conventional differential offset correction circuit; 
         FIG. 2  is a diagram exemplifying a configuration of a differential offset correction circuit according to a first embodiment of the present invention; 
         FIG. 3  is a diagram describing an operation of a control logic unit; 
         FIG. 4  is a diagram exemplifying a configuration of a differential offset correction circuit according to a second embodiment of the present invention; and 
         FIG. 5  is a diagram exemplifying a configuration of a differential offset correction circuit according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  is a diagram exemplifying a configuration of a differential offset correction circuit according to a first embodiment of the present invention. 
     In the showing of  FIG. 2 , the same component number is assigned to the same constituent component as that of  FIG. 1  and it is briefly described. 
     In a common operation, a digital signal generated at a transmission digital signal generation unit  10  is digital-to-analog (DA)-converted at a primary signal differential output DAC  12  of transmission analog signal output units  11 - 1  and  11 - 2  and an analog signals is output. A differential offset defined as a problem here refers to a signal output from the transmission digital signal generation unit  10  being a voltage between the OIP and OIM (or between the OQP and OQM) in the state of the aforementioned signal being absent. In order to make the voltage between the OIP and OIM close to zero (“0”), a comparator  20  is equipped for detecting the voltage across the OIP and OIM. A reference voltage and a voltage of the OIP (or OIM) terminal are input to the comparator  20 , and the comparison result is input to a control logic unit  18  as a digital signal. 
     A comparator input select switch  16  of the comparator  20  is switched over by the control logic unit  18 . A closure of one switch of the two at a time compares the voltage of the OIP, or OIM, with the reference voltage, for either one at a time. Based on the comparison result of the comparator  20 , the control logic unit  18  changes a digital value (i.e., a setup code) to be given to each of offset correction-use DACs  19 - 1  and  19 - 2 . A repetition of the comparison by the comparator  20  and a setup of the offset correction-use DACs  19 - 1  and  19 - 2  based on the comparison result makes it possible to obtain an output voltage close to the reference voltage. Then, an execution of the same work on each terminal of the OIP and OIM with the two switches of the comparator input select switch  16  being closed in sequence makes it possible to make a differential offset amount approximately close to zero (“0”). 
     The comparator  20  usually possesses a self-offset, sometimes resulting in the output value of the comparator  20  being “0” even if the input and output voltages are not truly identical when they are compared with each other. The adjustment of the voltages of the OIP and OIM, respectively, as a result of comparing them with the reference voltage, respectively, makes the differential offset between the OIP and OIM nearly zero (“0”) even in the case of the comparator  20  setting an output value at “0” when the input voltage is higher than the reference voltage by “A” millivolts for example because both of the voltages of OIP and OIM are set at “A” millivolts higher than the reference voltage. 
     An output control switch  15  has the function of cutting off a line to an outside so as to prevent a voltage value of the differential amplifiers  14 - 1  and  14 - 2  during an adjustment of a differential offset. During the adjustment of a differential offset, the output voltages of the differential amplifiers  14 - 1  and  14 - 2  vary, and therefore, if the voltages are input to a circuit such as a quadrature modulator connected to the OIP and OIM terminals, the circuit is adversely affected. An avoidance of such problem is the purpose of the aforementioned function. 
     As such, a large scale circuit such as an AD converter has conventionally been required for correcting a differential offset; a preferred embodiment of the present invention, however, is configured to replace it with a single comparator, thereby enabling a large reduction of a circuit scale and also an accurate correction of a differential offset. 
       FIG. 3  is a diagram describing an operation of the control logic unit. 
     To begin with, the assumption is that the OIP output voltage is higher than the target output voltage (i.e., the reference voltage) at the initial point in time ( 1 ), as shown in (B). Also assumed is that the offset correction DAC setup code is (A) and that the output of the comparator  20  is (C) in this event. The control logic unit  18  validates the output value of the comparator at the point of “a” in time and sets the offset correction-use DAC at the point of “b” in time. Assuming that the value of the offset correction DAC setup code is “B” at the point of ( 1 ), the output voltage of the OIP is apparently larger than the target output voltage, and therefore the next offset correction DAC code (i.e., a digital value) is defined as B−B/2. By this, the OIP output voltage becomes smaller as shown in ( 2 ). Validating the comparison value of the comparator  20  at the point ( 1 ), an OIP output voltage has apparently become the target output voltage, and therefore the control logic unit  18  sets an offset correction DAC setup code at B−B/2+B/4. Then, an OIP output voltage increases a little as shown in ( 3 ). Yet the OIP output voltage is lower than the target output voltage, and the control logic unit  18  accordingly sets an offset correction DAC setup code at B−B/2+B/4+B/8. Then, the OIP output voltage increases to become a little higher than the target output voltage as shown in ( 4 ). Now that the OIP output voltage is higher than the target output voltage, the control logic unit  18  sets an offset correction DAC setup code at B−B/2+B/4+B/8−B/16. This apparently makes the OIP output voltage approximately at the target output voltage as shown in ( 5 ). 
     As described above, the control logic unit  18  gives a correction value of B/2 n  to a setup code in the nth control where the B is defined as an offset correction DAC setup code at the time of starting the control operation. Whether a correction value is positive or negative is determined in a manner that it is negative if an OIP output voltage indicates higher than the target output voltage, and that it is positive if it is vice-versa. The number of controls “n” is n=N−1 where the N is defined as the number of bits of an offset correction DAC setup code. 
     A control of the above described for the OIM sets both of the OIP and OIM at a value close to the reference voltage, thereby making it possible to make the differential offset at nearly zero. 
       FIG. 4  is a diagram exemplifying a configuration of a differential offset correction circuit according to a second embodiment of the present invention. 
     In the showing of  FIG. 4 , the same component number is assigned to the same constituent component as that of  FIG. 2  and the description is omitted here. 
     In the configuration of  FIG. 4 , an offset correction-use DAC  19 - 1  is equipped only on the OIP side. A constant voltage is input to a differential amplifier  14 - 2 , and an output voltage value on the OIM side is set at a certain value. And, a reference voltage is not input to a comparator  20 , and instead an output voltage of the OIP side and that of the OIM side are input to the comparator  20 . This configuration aims at adjusting an output voltage value of the OIP side by using the output voltage of the OIM side as reference. Since the comparator  20  contains an error as described above, there may be a case of the output voltage of the OIP side being not identical with that of the OIM side. If the difference between the output voltage of the OIP side and that of the OIM side is within the allowable range of a circuit connected to the OIP and OIM terminals, however, the present configuration is also applicable. In this case, since there is no need of an offset correction-use DAC on the OIM side, a circuit scale can be small; and since there is no need to set the OIP side and OIM side in sequence, a benefit of a faster operation is obtained. 
     The comparator input select switch may merely be configured in a manner to close during an operation of correcting a differential offset and open during a normal operation. 
     An operation of the control logic unit  18  is similar to that of the first embodiment. 
       FIG. 5  is a diagram exemplifying a configuration of a differential offset correction circuit according to a third embodiment of the present invention. 
     In the showing of  FIG. 5 , the same component number is assigned to the same constituent component as that of  FIG. 2  and the description is omitted here. 
     In the configuration of  FIG. 5  an offset correction-use DAC is eliminated and a constant voltage is input to differential amplifiers  14 - 1  and  14 - 2 . A correction of a differential offset is carried out by outputting an offset correction-use digital value output from a control logic unit  18  and adding it to a primary signal of a transmission digital signal generation unit  10  by using an adder  21 . That is, a normal operation of a primary signal differential output DAC  12  is to output a zero volt as a differential voltage when a value of a digital primary signal of a prescribed number of bits is a half of the maximum value expressed by the aforementioned number of bits. If there is a differential offset, however, an input digital primary signal value at the time of outputting a zero volt as differential voltage is a value displaced from the half of the maximum value expressed by the number of bits of the primary signal. Therefore, it is possible to correct a differential offset by applying a correction to the primary signal for the displaced amount by using a digital value from the control logic unit  18 . The method for the correction is basically the same as the case described in  FIG. 3 , which gives a correction, to a digital value of the primary signal, by a large number at the beginning, followed by a smaller number as gradually proceeding with the number of times of controls so as to converge the control.