Patent Publication Number: US-7911242-B2

Title: Signal generating apparatus, test apparatus and circuit device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation application of PCT/JP2007/60783 filed on May 28, 2007 which claims priority from a Non-provisional patent application Ser. No. 11/509,307 filed on Aug. 24, 2006, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a signal generating apparatus, a test apparatus and a circuit device. The present invention particularly relates to a signal generating apparatus that can freely correct the waveform of an output signal. 
     2. Related Art 
     A typical method of testing a device under test (DUT) such as a semiconductor circuit is to input a predetermined signal into the DUT, and measure an output signal from the DUT to judge whether the DUT is good or bad. For example, the signal input into the DUT has a predetermined logical pattern, and the output signal from the DUT is judged whether to have a logical pattern matching an expected value pattern so as to determine whether the DUT operates normally or not. 
     According to the above-described testing method, the test apparatus inputs the predetermined signal into the DUT. Here, this signal may attenuate while being transmitted on the path from the test apparatus to the DUT. If such attenuation occurs, the signal input into the DUT may have a different logical pattern from a logical pattern that is designated to be input into the DUT. 
     To solve this problem, the typical test apparatus has a function to correct the waveform of the test signal in advance based on the potential signal attenuation on the transmission path. For example, the test apparatus generates a plurality of pulse signals having different pulse widths based on the timing of the edge of the test signal, and adds the waveforms of these pulse signals to the waveform of the test signal. Thus, the test apparatus sharpens the edge portion of the test signal, for example, as disclosed in Japanese Patent Application Publication No. 2002-40112. 
     Here, such a typical test apparatus can sharpen the edge of the test signal, but cannot freely correct the waveform of the test signal. To be specific, since the test apparatus corrects the test signal by adding the pulse widths of the pulse signals generated based on the timing of the edge of the test signal, the test apparatus cannot make a correction to compensate a reflected wave and the like which may occur in a phase that is temporally distant from the edge, for example. In addition, the typical test apparatus can sharpen the edge, but cannot blunt the edge. 
     For the reasons stated above, the typical test apparatus cannot always test the DUT accurately. Furthermore, it has not been possible to test quantitatively a waveform equalizing function of the DUT when the device has a function of restoring an attenuated test signal, that is to say, a waveform equalizing circuit. 
     SUMMARY 
     Therefore, it is an object of an aspect of the innovations herein to provide a signal generating apparatus, a test apparatus, and a circuit device which are capable of overcoming the above drawbacks accompanying the related art. The above and other objects can be achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the innovations herein. 
     According to the first aspect related to the innovations herein, one exemplary signal generating apparatus may include a signal generating apparatus for generating an output signal corresponding to pattern data supplied thereto. The signal generating apparatus includes a timing generating section that generates a periodic signal, a shift register section including a plurality of flip-flops in a cascade arrangement through which each piece of data of the pattern data is propagated sequentially in response to the periodic signal, a waveform generating section that generates the output signal whose value varies in accordance with a cycle of the periodic signal, based on data values output from the plurality of flip-flops, and an analog circuit that enhances a predetermined frequency component in a waveform of the output signal generated by the waveform generating section. 
     According to the second aspect related to the innovations herein, one exemplary test apparatus may include a test apparatus for testing a device under test. The test apparatus includes a pattern generator that generates a test pattern to test the device, a signal generating apparatus that generates a test signal to be input into the device, based on the test pattern, and a judging section that judges whether the device is good or bad based on a signal output from the device. The signal generating apparatus includes a timing generating section that generates a periodic signal, a shift register section including a plurality of flip-flops in a cascade arrangement through which each piece of data of the test pattern is propagated sequentially in response to the periodic signal, a waveform generating section that generates the test signal whose value varies in accordance with a cycle of the periodic signal, based on data values output from the plurality of flip-flops, and an analog circuit that enhances a predetermined frequency component in a waveform of the test signal generated by the waveform generating section. 
     According to the third aspect related to the innovations herein, one exemplary circuit device may include a circuit device for outputting a signal having a desired waveform. The circuit device includes a pattern generator that generates a waveform pattern for the signal to be output from the circuit device, and a signal generating apparatus that generates the output signal based on the waveform pattern. The signal generating apparatus includes a timing generating section that generates a periodic signal, a shift register section including a plurality of flip-flops in a cascade arrangement through which each piece of data of the waveform pattern is propagated sequentially in response to the periodic signal, a waveform generating section that generates the output signal whose value varies in accordance with a cycle of the periodic signal, based on data values output from the plurality of flip-flops, and an analog circuit that enhances a predetermined frequency component in a waveform of the output signal generated by the waveform generating section. 
     The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. The above and other features and advantages of the present invention will become more apparent from the following description of the embodiments taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a configuration of a test apparatus  200  relating to an embodiment of the invention. 
         FIG. 2  is a timing chart showing an operation performed by a signal generating apparatus  100  as an example. 
         FIG. 3A  shows the edge timings of periodic signals as an example. 
         FIG. 3B  shows the edge timings of periodic signals as an example 
         FIG. 4  shows another example of the configuration of the signal generating apparatus  100 . 
         FIG. 5  shows another example of the configuration of the signal generating apparatus  100 . 
         FIG. 6  illustrates an example of an analog waveform output from an analog circuit  500 . 
         FIG. 7  illustrates another example of the configuration of the signal generating apparatus  100 . 
         FIG. 8  illustrates another example of the configuration of the signal generating apparatus  100 . 
         FIG. 9  illustrates an exemplary operation of the signal generating apparatus  100  illustrated in  FIG. 8 . 
         FIG. 10  illustrates an exemplary configuration of the analog circuit  500 . 
         FIG. 11  shows another example of the configuration of the test apparatus  200 . 
         FIG. 12  shows an operation performed by a calibrating section  180  as an example. 
         FIG. 13  shows an exemplary configuration of a circuit device  400  relating to another embodiment of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENT 
     Some aspects of the invention will now be described based on the embodiments, which do not intend to limit the scope of the present invention, but exemplify the invention. All of the features and the combinations thereof described in the embodiment are not necessarily essential to the invention. 
       FIG. 1  shows an example of the configuration of a test apparatus  200  relating to an embodiment of the invention. The test apparatus  200  tests a device under test (DUT)  300 , which is a semiconductor circuit for example. For example, the test apparatus  200  inputs a signal having a predetermined logical pattern into the DUT  300 , and compares the logical pattern of a signal output from the DUT  300  with an expected value pattern, to determine whether the DUT  300  is good or bad. The test apparatus  200  relating to the embodiment includes a signal generating apparatus  100 , a pattern generator  110 , a judging section  120 , and a transmission path  140 . 
     The pattern generator  110  generates a test pattern to test the DUT  300 . For example, the pattern generator  110  generates a test pattern including a logical pattern (pattern data) that should be included in a signal to be input into the DUT  300 . 
     The signal generating apparatus  100  generates a test signal to be input into the DUT  300 , on the basis of the test pattern generated by the pattern generator  110 . For example, the signal generating apparatus  100  generates a test signal indicating a level corresponding to the pattern data included in the test pattern. In addition, the signal generating apparatus  100  corrects the waveform of the test signal in advance. The configuration and operation of the signal generating apparatus  100  will be later described in detail. 
     The transmission path  140  transmits the test signal output from the amplifier  130  to an input terminal of the DUT  300 . The transmission path  140  may be a wiring such as a cable. The transmission path  140  may cause a predetermined degree of attenuation or a predetermined reflected wave, in the test signal. 
     The judging section  120  judges whether the DUT  300  is good or bad based on a signal output from the DUT  300 . For example, the judging section  120  may make such judgment by comparing the logical pattern of the output signal with an expected value pattern supplied from the pattern generator  110 . Here, the pattern generator  110  generates the expected value pattern based on the test pattern. 
     The signal generating apparatus  100  includes a timing generating section  10 , a shift register section  20 , a register section  40 , and a waveform generating section. In this embodiment, the waveform generating section includes a first calculating section  50 , a second calculating section  60 , an output section  70 , and an amplifier  130 . 
     The timing generating section  10  includes a plurality of timing generators ( 12 - 1  to  12 - n , hereinafter collectively referred to as timing generators  12 ) that use a supplied reference clock to generate a plurality of periodic signals each having a different phase with respect to the reference clock. In other words, the timing generators  12  generate periodic signals that all have substantially the same cycle but each have a different phase. Each of the timing generators  12  may be a PLL circuit. Alternatively, one of the timing generators  12  which is designated as a reference circuit may be a PLL circuit and the rest may be delay circuits. If this is the case, the reference timing generator  12  generates a first periodic signal, which then branches to be received by the rest of the timing generators  12 . Each of the rest of the timing generators  12  may delay the received first periodic signal by a different delay amount. 
     The shift register section  20  includes a plurality of flip-flops in a cascade arrangement ( 22 - 1  to  22 - m , hereinafter collectively referred to as flip-flops  22 ). Through the flip-flops  22 , each piece of data of the pattern data output from the pattern generator  110  is propagated sequentially. Each of the flip-flops  22  receives the first periodic signal which is output from a first timing generator  12 - 1  as a clock frequency, and propagates each piece of data of the pattern data to the next-stage flip-flop  22  in response to the first periodic signal. 
     The second calculating section  60  includes a plurality of sign control circuits ( 62 - 1  to  62 - m , hereinafter collectively referred to as sign control circuits  62 ) and a plurality of calculating circuits ( 64 - 1  to  64 - m , hereinafter collectively referred to as calculating circuits  64 ), provided in a one-to-one correspondence with the flip-flops  22 . Each of the sign control circuits  62  determines the sign of a data value output from a corresponding one of the flip-flops  22 . To be specific, each sign control circuit  62  selects a positive or negative sign for the data value output from the corresponding flip-flop  22 , and then outputs the data value. Here, a user may designate, in advance, which one of positive and negative signs is to be selected by each of the sign control circuits  62 . While the signal generating apparatus  100  is operating, the sign selected by each of the sign control circuits  62  may or may not be varied. 
     Each of the calculating circuits  64  receives a data value output from a corresponding one of the flip-flops  22  via a corresponding one of the sign control circuits  62 . Each calculating circuit  64  multiplies the received data value by a predetermined coefficient, and outputs a signal indicating a level set in accordance with the multiplication result. Each calculating circuit  64  may be an amplifier having an amplification ratio corresponding to the coefficient. While the signal generating apparatus  100  is operating, the coefficient of each calculating circuit  64  may or may not be varied. 
     The output section  70  adds together the waveforms of the signals output from the calculating circuits  64 , and outputs the result of the addition. The amplifier  130  amplifies the test signal generated by the output section  70  at a predetermined amplification ratio, and outputs the amplified test signal. Also, the amplifier  130  may output the test signal by setting a predetermined signal level as a reference level. For example, the amplifier  130  may amplify the test signal at a predetermined amplification ratio, add a predetermined offset voltage to the test signal, and output the resulting test signal. With the above-described configuration, the signal generating apparatus  100  can correct the waveform of the output signal based on the edge of the first periodic signal by using the pattern data. 
     The register section  40  includes a plurality of registers ( 42 - 2  to  42 - n , hereinafter collectively referred to as registers  42 ) provided in a one-to-one correspondence with the timing generators  12 - 2  to  12 - n  which do not include the first timing generator  12 - 1 . The registers  42  are in a cascade arrangement, that is to say, output data from each of the registers  42  is input into the next-stage register  42 . Each of the registers  42  receives the input data in response to a periodic signal output from a corresponding one of the timing generators  12 , and outputs the received data. In this embodiment of the invention, the first-stage register  42  receives data output from a pre-selected one of the flip-flops  22 . The data is propagated sequentially in response to the periodic signals output from the timing generators  12 . 
     The first calculating section  50  includes a plurality of sign control circuits ( 52 - 2  to  52 - n , hereinafter collectively referred to as sign control circuits  52 ) and a plurality of calculating circuits ( 54 - 2  to  54 - n , hereinafter collectively referred to as calculating circuits  54 ), provided in a one-to-one correspondence with the registers  42 . Each of the sign control circuits  52  determines the sign of a data value output from a corresponding one of the registers  42 . To be specific, each sign control circuit  52  selects a positive or negative sign for the data value output from the corresponding register  42 , and then outputs the data value. Here, the user may designate, in advance, which one of positive and negative signs is to be selected by each of the sign control circuits  52 . While the signal generating apparatus  100  is operating, the sign selected by each of the sign control circuits  52  may or may not be varied. 
     Each of the calculating circuits  54  receives a data value output from a corresponding one of the registers  42  via a corresponding one of the sign control circuits  52 . Each calculating circuit  54  multiplies the received data value by a predetermined coefficient, and outputs a signal indicating a level set in accordance with the multiplication result. Each calculating circuit  54  may be an amplifier having an amplification ratio corresponding to the coefficient. While the signal generating apparatus  100  is operating, the coefficient of each calculating circuit  54  may or may not be varied. 
     The output section  70  adds together the waveforms of the signals output from the calculating circuits  54 , and outputs the result of the addition. In other words, the output section  70  outputs a signal indicating a result of adding together the waveforms of the signals output from the calculating circuits  54  and  64 . With the above-described configuration, the signal generating apparatus  100  can correct the waveform of the output signal based on a timing other than the first periodic signal. 
     The user may freely set the phase of the periodic signal output from each of the timing generators  12 , with respect to the first periodic signal. With this configuration, the signal generating apparatus  100  can correct the waveform of the output signal based on a desired timing. For example, the signal generating apparatus  100  can generate a waveform corresponding to the signal edge of the output signal (the edge timing of the first periodic signal) in a phase (the edge timing of a different periodic signal) which is temporally distant from the signal edge. Therefore, the signal generating apparatus  100  can generate, in advance, in the output signal, a waveform to offset a reflected wave which may occur on the transmission path  140 . Thus, the signal generating apparatus  100  can accurately input a desired signal into the DUT  300 . 
     A tap control section  30  selects one of the data values output from the flip-flops  22 , and inputs the selected data value into the first-stage register  42 . Having this configuration, the signal generating apparatus  100  can select which one of the data values output from the flip-flops  22  is to be used as a reference to correct the waveform of the output signal. The user may designate in advance which one of the flip-flops  22  is to be selected by the tap control section  30 . 
     Also, the tap control section  30  is configured to input the data value output from each of the flip-flops  22  into a corresponding one of the sign control circuits  62 . The user may designate in advance how the flip-flops  22  and sign control circuits  62  are related to each other. While the signal generating apparatus  100  is operating, the settings for the tap control section  30  may not be changed. 
       FIG. 2  is a timing chart showing the operation performed by the signal generating apparatus  100  as an example.  FIG. 2  is mainly used to explain the waveform correction performed by the first calculating section  50 . In this embodiment, the number of timing generators  12  is set to five, and the tap control section  30  selects the data output from the flip-flop  22 - 1 , and inputs the selected data into the first-stage register  42 - 2 . 
     The flip-flop  22 - 1  propagates the data value output from the pattern generator  110  in response to the first periodic signal. As shown in  FIG. 2 , when the flip-flop  22 - 1  outputs a data value “1”, the register  42 - 2  receives the data value “1” in response to a second periodic signal output from the corresponding timing generator  12 - 2 , and outputs the received data value. In a similar manner, each of the subsequent-stage registers  42  receives the data value output from the previous-stage register  42  in response to a periodic signal output from a corresponding one of the timing generators  12 , and outputs the received data value. 
     Each of the calculating circuits  54  outputs a signal generated based on the data value output from a corresponding one of the registers  42  as shown in  FIG. 2 . As described above, each calculating circuit  54  multiplies the data value output from the corresponding register  42  by a predetermined coefficient, and outputs a signal indicating a level set in accordance with the multiplication result. Each of the sign control circuits  52  selects the sign for the signal output from a corresponding one of the calculating circuits  54 . 
     The output section  70  adds together the waveforms of the signals output from the calculating circuits  54 , to correct the waveform of the output signal. To this calculation, the output section  70  also adds a waveform in units of unit interval (UI) which is generated by the second calculating section  60 . This waveform can be generated by using a typical method, which is therefore not illustrated herein. Here, the UI may indicate the duration of one bit in the test signal. 
     Referring to  FIG. 2 , the shaded regions are the regions corrected by the first and second calculating sections  50  and  60 . As shown in  FIG. 2 , the signal generating apparatus  100  can correct the waveform of the output signal by using a plurality of periodic signals having different phases, thereby realizing highly variable waveform correction. 
     As described above, the signal generating apparatus  100  relating to the present embodiment can perform waveform correction, by using the pattern data to generate the output signal, in units of UI of the output signal and based on a desired timing. Having such a configuration, the embodiment can accurately correct the waveform of the output signal, thereby enabling the test apparatus  200  to accurately test the DUT  300 . 
       FIGS. 3A and 3B  each show the edge timings of the periodic signals as an example. The timing generating section  10  may output the periodic signals in such a manner that the edge timings of the periodic signals excluding the first periodic signal are more densely distributed in the vicinity of the edge timing of the first periodic signal, as shown in  FIG. 3A . With this configuration, the signal generating apparatus  100  can correct a portion of the waveform of the output signal which is in the vicinity of the signal edge in a finer manner. 
     Alternatively, the timing generating section  10  may output the periodic signals in such a manner that a difference in phase between a periodic signal output from one of the timing generators  12  and the first periodic signal is set larger than the UI (the UI of the first periodic signal), as shown in  FIG. 3B . With this configuration, the signal generating apparatus  100  can generate a waveform to offset a reflected wave that may occur in a phase temporally distant from a pulse of the output signal by a time equal to the UI or longer, for example. Here, the cycle of each periodic signal may be substantially equal to the cycle of the test signal (the UI). 
       FIG. 4  shows another example of the configuration of the signal generating apparatus  100 . The signal generating apparatus  100  relating to this embodiment is different in that the register section  40  is replaced with a set-reset latch section  80 . The other constituents indicated by the same reference numerals as in  FIG. 1  have the same or similar functions and configurations as/to the corresponding constituents of the previous embodiment. 
     The set-reset latch section  80  includes a plurality of set-reset latches ( 82 - 2  to  82 -( n - 1 ), hereinafter collectively referred to as set-reset latches  82 ) in a one-to-one correspondence with the timing generators ( 12 - 2  to  12 -( n - 1 )), which do not include the first timing generator  12 - 1  and the last-stage timing generator  12 - n . Each of the set-reset latches  82  receives a periodic signal from a corresponding one of the timing generators  12  and a periodic signal from the next-stage timing generator  12 . Here, the next-stage timing generator  12  may output a periodic signal having a phase delayed by the smallest amount with respect to the phase of the periodic signal output from the corresponding timing generator  12 . 
     Each set-reset latch  82  maintains the output of a signal indicating a logical value “1” during a time period defined by the edge of the periodic signal received from the corresponding timing generator  12  and the edge of the periodic signal received from the next-stage timing generator  12 . The tap control section  30  inputs the data value output from a selected one of the flip-flops  22  into each of the sign control circuits  52 . Each sign control circuit  52  selects a sign for the received data value and outputs the data value, when a corresponding one of the set-reset latches  82  outputs the logical value “1”. 
     According to this embodiment, the signal generating apparatus  100  can correct the waveform of the output signal at a desired timing based on the edge of each of the periodic signals, and by using a desired pulse width based on the difference in phase between the periodic signals. With this configuration, the signal generating apparatus  100  can perform very fine waveform correction, for example, by reducing the difference in phase between the periodic signals output from adjacent two of the timing generators  12 . 
       FIG. 5  shows another example of the configuration of the signal generating apparatus  100 . The signal generating apparatus  100  relating to the present example enhances a predetermined frequency component of a discrete waveform formed by combining rectangular waves, for example the waveform shown in  FIG. 2 , to generate a continuous waveform. For example, the signal generating apparatus  100  may enhance the waveform in units of the UI shown in  FIG. 2  or a predetermined frequency component of the output signal. In the latter case, an analog circuit  500  may be added to the configuration of the signal generating apparatus  100  shown in  FIG. 1  or  4  as a following stage of the amplifier  130 , for example. The analog circuit  500  enhances a predetermined frequency component of the output waveform from the amplifier  130 . The analog circuit  500  may be an analog peaking circuit that enhances a predetermined high frequency component, for example. The analog circuit  500  may be a circuit that enhances a high frequency component of an input waveform by superposing a differential waveform of the input waveform onto the input waveform, for example. Alternatively, the analog circuit  500  may be a circuit that smoothes an input waveform. With the above configuration, the signal generating apparatus  100  can change a discrete waveform of the output signal shown in  FIG. 2  into a continuous waveform in which a predetermined frequency component is enhanced. 
       FIG. 5  illustrates an exemplary configuration of the signal generating apparatus  100  that is utilized when the waveform in units of the UI, shown in  FIG. 2 , is enhanced. The signal generating apparatus  100  relating to the present example is different in configuration from the signal generating apparatus  100  described with reference to  FIG. 1  in terms of that the register section  40  and the first calculating section  50  are omitted and the analog circuit  500  is added. Furthermore, according to the present example, the timing generating section  10  is different in terms of having a single timing generator  12 - 1 . Except for these differences, the remaining constituents may be the same as the corresponding constituents shown in  FIG. 1  that are assigned the same reference numerals. 
     In the shift register section  20 , each piece of data of the pattern data is propagated sequentially through the flip-flops  22  in accordance with a periodic signal generated by the timing generator  12 - 1 . For example, the timing generator  12 - 1  may generate a periodic signal whose cycle is substantially the same as the cycle of the test signal to be generated by the signal generating apparatus  100  (the UI). The tap control section  30  may have the same functions and configuration as the tap control section  30  described with reference to  FIGS. 1 to 4 . 
     According to the present example, a waveform generating section includes the second calculating section  60 , the output section  70 , and the amplifier  130 . The waveform generating section generates an output signal whose value varies at an interval equal to the cycle of the periodic signal generated by the timing generator  12 - 1 , based on the data values output from the flip-flops  22  of the shift register section  20 . Since the signal generating apparatus  100  relating to the present example does not include the register section  40  and the first calculating section  50 , the waveform of the output signal output from the amplifier  130  is equivalent to the waveform in units of the UI, for example, shown in  FIG. 2 . 
     The analog circuit  500  enhances a predetermined frequency component of the waveform of the output signal generated by the amplifier  130  of the waveform generating section, and inputs the resulting signal into the DUT  300  via the transmission path  140 . For example, the analog circuit  500  may be an analog peaking circuit that enhances a predetermined high frequency component, to enhance the edge portion of the output signal. For example, the analog circuit  500  may be configured, as later described with reference to  FIG. 10 , in such a manner that an RC high pass filter is provided in parallel with a transmission line and the signals from the RC high pass filter and the transmission line are combined together. With such a configuration, the analog circuit  500  may generate a waveform in which a predetermined high frequency component is enhanced. The time constant of the analog circuit  500  may be defined in accordance with the time constant of the transmission path  140 , which is measured in advance. 
       FIG. 6  illustrates an example of an analog waveform output from the analog circuit  500 . As described above, the analog circuit  500  receives a discrete waveform in units of the UI and generates an analog waveform in which a high frequency component of the received waveform is enhanced. The signal generating apparatus  100  relating to the present example can generate a test signal whose value varies at the interval smaller than the UI, as shown in  FIG. 6 , with a simple configuration illustrated in  FIG. 5 . 
       FIG. 7  illustrates another example of the configuration of the signal generating apparatus  100 . The signal generating apparatus  100  relating to the present example is obtained by adding an analog circuit  500  to the signal generating apparatus  100  having the configuration shown in  FIG. 1 . According to the present example, the timing generating section  10  includes a first timing generator  12 - 1  and a second timing generator  12 - 2 , the register section  40  includes one register  42 - 2 , and the first calculating section  50  includes one sign control circuit  52 - 2  and one calculating circuit  54 - 2 . Except for these differences, the remaining constituents may have the same functions and configurations as the corresponding constituents shown in  FIG. 1  that are assigned the same reference numerals. 
     The second timing generator  12 - 2  may generate a second periodic signal that has a phase different from the phase of a first periodic signal generated by the first timing generator  12 - 1 . The second periodic signal may have substantially the same cycle as the first periodic signal. The register  42 - 2  sequentially acquires the data output from one of the flip-flops  22  that is selected in advance by the tap control section  30 , in accordance with the second periodic signal supplied from the second timing generator  12 - 2 , and outputs the acquired data. 
     The waveform generating section according to the present example includes the first calculating section  50 , the second calculating section  60 , the output section  70 , and the amplifier  130 . The waveform generating section generates an output signal whose value varies in accordance with the phases of the first and second periodic signals, based on the data values output from the flip-flops  22  and the register  24 . 
     Specifically speaking, the sign control circuit  52 - 2  and the calculating circuit  54 - 2  in the first calculating section  50  generate a waveform whose value varies in accordance with the phase of the second periodic signal, based on the data value output from the register  24 . Also, the second calculating section  60  generates a waveform whose value varies in accordance with the phase of the first periodic signal, based on the data values output from the flip-flops  22 . The output section  70  then combines the waveforms output from the first and second calculating sections  50  and  60 , thereby generating an output signal whose value varies in accordance with the phases of the first and second periodic signals. 
     The amplifier  130  and the analog circuit  500  may have the same functions and configurations as the amplifier  130  and the analog circuit  500  described with reference to  FIG. 5 . With such a configuration, the signal generating apparatus  100  relating to the present example can compensate the waveform of the test signal more accurately. For example, the signal generating apparatus  100  can generate a waveform that has been corrected to compensate a reflected wave or the like that may occur at a given timing different from the edge of the first periodic signal. 
     In this case, the tap control section  30  may select one of the flip-flops  22  that is to be connected to the register  42 - 2 , depending on a unit interval in which reflection of a rectangular wave in a certain unit interval may occur. The selection of one of the flip-flops  22  made by the tap control section  30  can lead to selection of a unit interval in which a reflected-wave compensating waveform is generated. Furthermore, a phase in the selected unit interval at which the reflected-wave compensating waveform is generated can be adjusted in accordance with the phase of the second periodic signal generated by the second timing generator  12 - 2 . The second timing generator  12 - 2  may generate the second periodic signal having a phase difference, with respect to the first periodic signal, that is determined in accordance with the phase at which the reflected-wave compensating waveform is to be generated. 
       FIG. 8  illustrates another example of the configuration of the signal generating apparatus  100 . The signal generating apparatus  100  relating to the present example is obtained by replacing the register section  40  in the signal generating apparatus  100  shown in  FIG. 7  with a set-reset latch section  80 . The set-reset latch section  80  has one set-reset latch  82 , which is described with reference to  FIG. 4 . The timing generating section  10  further includes a third timing generator  12 - 3 . Except for these differences, the remaining constituents may have the same functions and configurations as the corresponding constituents shown in  FIG. 7  that are assigned the same reference numerals. 
     The third timing generator  12 - 3  generates a third periodic signal. The third periodic signal may have a phase different from the phase of the second periodic signal. The set-reset latch  82  receives the second and third periodic signals, and outputs a pulse whose width is determined in accordance with the phase difference between the second periodic signal and the third periodic signal, as described with reference to  FIG. 4 . 
     The sign control circuit  52 - 2  determines the sign of the logic value supplied from the tap control section  30  and outputs the logic value with the determined sign during a time period for which the signal received from the set-reset latch  82  indicates the H logic, as described with reference to  FIG. 4 . The processing performed by the calculating circuit  54  and the subsequent constituents may be the same as in the signal generating apparatus  100  described with reference to  FIG. 7 . 
     Having the above-described configuration, the signal generating apparatus  100  relating to the present example can generate a waveform that is corrected to compensate a reflected wave or the like and that has a pulse width different from the cycles of the respective periodic signals. In other words, the signal generating apparatus  100  can generate a waveform that is corrected to compensate a reflected wave or the like and that has a desired pulse width, by adjusting the phase difference between the second periodic signal and the third periodic signal. 
       FIG. 9  illustrates an exemplary operation of the signal generating apparatus  100  illustrated in  FIG. 8 . In  FIG. 9 , a referential mark T 1  indicates the phase of the first periodic signal and a referential mark T 2  indicates the phase of the second periodic signal, for example. As described above, the signal generating apparatus  100  can generate a waveform in which a pulse with a desired pulse width is positioned at a desired position, by adjusting the phases of the first and second periodic signals. As a result, the signal generating apparatus  100  can compensate a reflected wave or the like that has any pulse width and occurs at any position. 
       FIG. 10  illustrates an exemplary configuration of the analog circuit  500 . According to the present example, the analog circuit  500  includes a plurality of resistances  502 ,  512 ,  522  and  532 , a plurality of capacitors  514 ,  524 , and  534 , and a plurality of switches  526  and  528 . The resistances  502 ,  512 ,  522  and  532  are provided in parallel with each other. The capacitors  514 ,  524  and  534  are provided in a one-to-one correspondence with the resistances  512 ,  522 , and  532 , excluding the resistance  502  on a transmission path, and respectively connected in series with the corresponding resistances  512 ,  522  and  532 . The switch  526  switches whether or not to connect the resistances and capacitors of the second and subsequent stages in parallel to the resistance  502  on the transmission path. 
     For example, when the switch  526  is off, the analog circuit  500  generates a waveform by superposing a signal that has passed through a first-order CR high pass FIR filter onto an original signal. On the other hand, when all of the switches are on, the analog circuit  500  generates a waveform by superposing a signal that has passed through a third-order CR high pass FIR filter onto an original signal. The constants of the respective resistances and the respective capacitors may be adjustable in accordance with a desired time constant. With the above-described configuration, the analog circuit  500  can generate a waveform in which a predetermined high frequency component of an input signal is enhanced. It should be noted, however, that the configuration of the analog circuit  500  is not limited to the configuration shown in  FIG. 10 . The analog circuit  500  can be configured by using a known high-frequency component enhancing circuit. 
       FIG. 11  shows another example of the configuration of the test apparatus  200 . According to this embodiment, the test apparatus  200  includes a calibrating section  180  in addition to the constituents shown in  FIGS. 1 to 10 . The other constituents indicated by the same reference numerals as in  FIGS. 1 to 10  may have the same or similar functions and configurations as the corresponding constituents in the previous embodiment. 
     The calibrating section  180  calibrates the signal generating apparatus  100  before the test apparatus  200  tests the DUT  300 . The calibrating section  180  includes a reference generating section  150 , a control section  160 , and a reference measuring section  170 . 
     The reference generating section  150  causes the signal generating apparatus  100  to output a reference signal having a predetermined waveform. In this embodiment, the reference generating section  150  causes the pattern generator  110  to output predetermined pattern data. 
     The reference measuring section  170  measures the waveform of the reference signal at a point when the reference signal is input into the input terminal of the DUT  300 . The control section  160  determines the settings for the first and second calculating sections  50  and  60  based on the waveform of the reference signal measured by the reference measuring section  170 . In detail, the control section  160  may set the signs selected by the sign control circuits  52  and  62 , and set the weight coefficients for the calculating circuits  54  and  64 . Furthermore, the control section  160  may set the phases of the periodic signals output from the timing generators  12 . 
       FIG. 12  shows the operation performed by the calibrating section  180  as an example. As mentioned above, the reference generating section  150  causes the signal generating apparatus  100  to output a predetermined reference signal, and the reference measuring section  170  measures the waveform of the reference signal at a point when the reference signal is input into the input terminal of the DUT  300 . 
     The control section  160  quantizes the waveform measured by the reference measuring section  170  as shown in  FIG. 12 . Based on the quantized waveform, the control section  160  then detects attenuation and the like of the reference signal which may have occurred on the transmission path  140 , and calibrates the signal generating apparatus  100  based on the detection result. 
     For example, the control section  160  approximates the quantized waveform by a plurality of pulses. By using the phases and pulse widths of the pulses, the control section  160  may control the phases of the periodic signals output from the timing generators  12 . Also, the control section  160  may control the weight coefficients for the calculating circuits  54  and  64  based on the levels of the rectangular waves. In addition, the control section  160  may compare the waveform of the reference signal with the quantized waveform, in order to judge whether to superimpose or reduce the components of the rectangular waves of the quantized waveform on/from the reference signal in advance. Based on the judgment, the control section  160  may control the signs to be selected by the sign control circuits  52  and  62 . 
     According to the above description with reference to  FIGS. 1 to 12 , the signal generating apparatus  100  corrects the waveform of the output signal in advance to compensate the attenuation, reflection or the like that may occur on the transmission path  140 . Additionally, the signal generating apparatus  100  has a different function. For example, the signal generating apparatus  100  may degrade the waveform of the output signal, and input the resulting signal into the DUT  300 . This can detect the maximum level of degradation in the waveform of the output signal which allows the DUT  300  to operate normally. 
       FIG. 13  shows an exemplary configuration of a circuit device  400  relating to another embodiment of the invention. The circuit device  400  may include a semiconductor circuit, for example. The circuit device  400  includes a substrate  410 , the pattern generator  110 , the signal generating apparatus  100 , and the control section  160 . The substrate  410  may be a semiconductor substrate, for example. The signal generating apparatus  100 , pattern generator  110 , and control section  160  may be circuits formed in the substrate  410 . 
     The signal generating apparatus  100 , pattern generator  110 , and control section  160  have the same or similar configurations and functions as/to the constituents identified by the same reference numerals in  FIGS. 1 to 12 . In this embodiment, the control section  160  may prestore therein information relating to the settings of the signal generating apparatus  100  such as the signs, weight coefficients and phases of the periodic signals. Alternatively, the control section  160  may adjust the signal generating apparatus  100  based on settings data provided from outside. Having the above-described configuration, the circuit device  400  can output a signal having a desired waveform. 
     Although some aspects of the present invention have been described by way of exemplary embodiments, it should be understood that those skilled in the art might make many changes and substitutions without departing from the spirit and the scope of the present invention which is defined only by the appended claims. 
     The claims, specification and drawings describe the processes of an apparatus, a system, a program and a method by using the terms such as operations, procedures, steps and stages. When a reference is made to the execution order of the processes, wording such as “before” or “prior to” is not explicitly used. The processes may be performed in any order unless an output of a particular process is used by the following process. In the claims, specification and drawings, a flow of operations may be explained by using the terms such as “first” and “next” for the sake of convenience. This, however, does not necessarily indicate that the operations should be performed in the explained order. 
     As clearly indicated above, some exemplary embodiments of the present invention can realize a signal generating apparatus that can correct the waveform of an output signal based on a desired phase. In addition, the embodiments of the present invention can realize a test apparatus that can accurately test a DUT.