Abstract:
A phase controlling apparatus is disclosed. The phase controlling apparatus controls phases of signals which are output from a plurality of signal sources corresponding to first phase information which indicates a phase of a predetermined signal. The phase controlling apparatus includes a phase information storing section and a phase controlling section. The phase information storing section stores second phase information which indicates a phase of a signal which is output from each of the plurality of signal sources. The phase controlling section changes a phase of a signal which is output from at least one of the plurality of signal sources corresponding to the second phase information stored in the phase information storing means to control the difference of phases of signals which are output from the plurality of signal sources.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to control the difference of phases of a plurality of signal sources and to control changes of frequencies thereof.  
         [0003]     2. Description of the Related Art  
         [0004]     When an impedance measurement instrument or the like is used, it may be necessary to set it so that signals that a plurality of signal sources generate have different phases. Recently, as such signal sources, a Direct Digital Synthesizer (abbreviated as a DDS) has been widely used. A DDS has a table of phases and signal output values and a Digital-Analog converter (hereinafter referred to as a D/A converter). The DDS can output a signal of a sine wave.  
         [0005]     When the phase of the DDS is controlled, the phase shifting is affected by the bit resolution of a register that adjusts the phase offset. Thus, it was difficult to finely control the phase of the DDS. When an impedance is measured, it is necessary to accurately measure the gain of an amplifier of a voltage measurement section. To do that, it is necessary to improve the phase resolution.  
         [0006]     To control to change the phase of the DDS that is operating, since a Band Pass Filter (hereinafter abbreviated as a BPF) is disposed at an output of the DDS, a transient takes place in the waveform that is output from the DDS. Thus, it takes a time until the waveform becomes stable.  
         [0007]     In addition, when an impedance is measured, it is necessary to accurately measure the gain of the amplifier of the voltage measurement section as disclosed in a related art reference for example Japanese Patent Application Unexamined Publication No. 2004-317345.  
       SUMMARY OF THE INVENTION  
       [0008]     As was described above, when the phase of the DDS is controlled, since the phase shifting is affected by the bit resolution of the register that adjusts the phase offset, it is difficult to finely control the phase of the DDS. When the phase of the signal is controlled and the frequency thereof is changed, a transient takes place in the waveform of the signal. As a result, it takes a time until the waveform becomes stable.  
         [0009]     The present invention is made from the foregoing point of view. It would be desirable to provide a DDS controlling apparatus and so forth that allow the phase or frequency of a signal to be changed in various manners without an influence of the bit resolution of a register of the DDS, which adjusts a phase offset, and a transient to be short in the waveform of a signal when the phase or frequency thereof is changed.  
         [0010]     According to an embodiment of the present invention, there is provided a phase controlling apparatus which controls phases of signals which are output from a plurality of signal sources corresponding to first phase information which indicates a phase of a predetermined signal. The phase controlling apparatus includes a phase information storing section and a phase controlling section. The phase information storing section stores second phase information which indicates a phase of a signal which is output from each of the plurality of signal sources. The phase controlling section changes a phase of a signal which is output from at least one of the plurality of signal sources corresponding to the second phase information stored in the phase information storing section to control the difference of phases of signals which are output from the plurality of signal sources. The “first phase information” corresponds to phase information stored in a phase accumulator of a DDS. The “second phase information” corresponds to phase information stored in a phase counter of a DDS controlling section. The “signal source” corresponds to a DDS. The phase controlling apparatus corresponds to a DDS controlling section and a computer section.  
         [0011]     Thus, besides phase information used for oscillation of a DDS, phase information for the DDS is provided. Thus, the phase of the DDS can be flexibly controlled corresponding to the phase information for the DDS.  
         [0012]     When the second phase information has a higher phase resolution than the phase offset adjusting section does, the phase difference can be more accurately controlled.  
         [0013]     When a value of a signal which is output from a signal source under control becomes zero, the phase controlling section controls the signal source to stop outputting the signal and after a predetermined period of time has elapsed, the phase controlling section controls the signal source to start outputting the signal.  
         [0014]     The phase controlling section starts controlling the signal source early for a predetermined period of time necessary after a control signal is input to the signal source until it outputs a signal corresponding to the control signal. The “predetermined period of time” corresponds to the latency of the internal circuit of the signal source.  
         [0015]     The plurality of signal sources are direct digital synthesizers which output sine waves. The signal sources may generate square waves or triangular waves.  
         [0016]     The phase controlling apparatus includes a clock signal generating section that generates a clock signal to each section of the phase controlling apparatus and each of the plurality of signal sources which are operated based on the clock signal. The phase information storing section has a phase counter which performs a count operation with the clock signal. The phase controlling section has a first register, a second register, and a control signal sending section. The first register stores first phase timing information with which a control signal which causes a signal source under control to stop outputting the signal is sent. The second register stores second phase timing information with which a control signal which causes the signal source under control to start outputting the signal is sent. The control signal sending section compares phase information which is output from the phase counter of the phase information storing section corresponding to a reference signal source with the first phase timing information stored in the first register and sends the control signal to the signal source under control. The control signal causes the signal source under control to stop outputting the signal when they match. In addition, the control signal sending section compares phase information which is output from the phase counter with the second phase timing information stored in the second register and sends the control signal to the signal source under control. The control signal causes the signal source under control to start outputting the signal when they match. The “phase timing information” corresponds to update data. The “control signal sending section” corresponds to a DDS trigger controlling circuit.  
         [0017]     According to an embodiment of the present invention, there is provided an oscillating apparatus. The oscillating apparatus includes a plurality of signal sources, a phase information storing section, and a phase controlling section. The plurality of signal sources output signals corresponding to first phase information which indicates a phase of a predetermined signal. The phase information storing section stores second phase information which indicates a phase of a signal which is output from each of the plurality of signal sources. The phase controlling section changes a phase of a signal which is output from at least one of the plurality of signal sources corresponding to the second phase information stored in the phase information storing section to control the difference of phases of signals which are output from the plurality of signal sources. The “frequency controlling section” corresponds to the DDS controlling section and the computer section.  
         [0018]     Since the frequency is changed with a phase of 360 degrees, the transient period becomes short and a waveform can be quickly and stably obtained.  
         [0019]     According to an embodiment of the present invention, there is provided a phase controlling apparatus which controls phases of signals which are output from a plurality of signal sources based on a clock signal. The phase controlling apparatus includes a phase controlling section. The phase controlling section stops supplying the clock signal to a predetermined one of the plurality of signal sources for a predetermined period of time and then starts supplying the clock signal to the signal source so as to control the phases of the signals which are output from the plurality of signal sources. The “phase controlling section” corresponds to the DDS controlling section.  
         [0020]     As was described above, the phase of a signal can be controlled with high accuracy. In addition, the phase and frequency of a signal can be changed in a short transient period.  
         [0021]     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of best mode embodiments thereof, as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0022]      FIG. 1  is a block diagram showing the structure of an oscillating apparatus using a DDS according to an embodiment of the present invention;  
         [0023]      FIG. 2  is a block diagram showing the structure of a phase accumulator;  
         [0024]      FIG. 3  is a block diagram showing the structure of a phase counter section;  
         [0025]      FIG. 4  is a block diagram showing the structure of a DDS trigger controlling section;  
         [0026]      FIG. 5A  to  FIG. 5E  are schematic diagrams showing waveforms of signals that are output from individual sections when a phase control is performed;  
         [0027]      FIG. 6  is a block diagram showing a structure of circuitry that obtains the gain of an amplifier;  
         [0028]      FIG. 7A  to  FIG. 7E  are schematic diagrams showing waveforms of signals that are output from individual sections when an FSK terminal of a DDK according to an embodiment of the present invention is used;  
         [0029]      FIG. 8  is a block diagram showing a structure of circuitry that controls a clock signal supplied to a DDS and changes the phase of a signal according to an embodiment of the present invention;  
         [0030]      FIG. 9A  to  FIG. 9C  are schematic diagrams showing waveforms of signals that are output from individual sections of an oscillating apparatus shown in  FIG. 8 ; and  
         [0031]      FIG. 10A  to  FIG. 10C  are schematic diagrams showing waveforms of signals that are output from individual sections of the oscillating apparatus when the frequency of a sine wave of an output signal of a DDS is changed. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0032]     Next, with reference to the accompanying drawings, embodiments of the present invention will be described.  FIG. 1  is a block diagram showing the structure of an oscillating apparatus using two DDSs according to an embodiment of the present invention. As shown in  FIG. 1 , the oscillating apparatus according to this embodiment is composed of a DDS  100   a , a DDS  100   b , a DDS controlling section  200 , a computer section  300 , and a clock signal generating section  400 . The DDS controlling section  200  controls the DDS  100   a  and the DDS  100   b . The clock signal generating section  400  supplies a clock signal to each section of the oscillating apparatus.  
         [0033]     The computer section  300  has a ROM  301 , a CPU  302 , and a RAM  303 . The ROM  301  stores a program and data. The CPU  302  executes the program stored in the ROM  301 . The RAM  303  is used as a work area and so forth on which the CPU  302  executes processes. The computer section  300  sends data such as frequencies, phases, and so forth to the DDS controlling section  200 , the DDS  100   a , and the DDS  100   b  through a data bus  304 .  
         [0034]     The DDS  100   a  has an internal register  110   a a, a phase accumulator  120   a , a sine wave lookup table  130   a , and a D/A converter  140   a . The internal register  110   a  stores data of phases and frequencies sent from the computer section  300 . The phase accumulator  120   a  outputs step-shaped waveforms corresponding to phases and frequencies stored in the internal register  110   a . The sine wave lookup table  130   a  converts phase data that are output from the phase accumulator  120   a  into data that represent an output value of a sine wave corresponding to the phase. The D/A converter  140   a  converts data that are output from the sine wave lookup table  130   a  into an analog signal.  
         [0035]     The structure of the DDS  100   b  is the same as that of the DDS  100   a . In other words, the DDS  100   b  has an internal register  110   b , a phase accumulator  120   b , a sine wave lookup table  130   b , and a D/A converter  140   b  (not shown).  
         [0036]     The internal registers  110   a  and  110   b  have phase offset adjustment registers that shift the phases of the phase accumulators  120   a  and  120   b , respectively. The DDS  100   a  and the DDS  100   b  shift the phases of the phase accumulators  120   a  and  120   b  corresponding to data written in the phase offset adjustment registers, respectively.  
         [0037]      FIG. 2  is a block diagram showing the structure of the phase accumulator  120   a . As shown in  FIG. 2 , the phase accumulator  120   a  has a phase register  121   a  and an adding device  122   a . The phase register  121   a  stores phase data. The adding device  122   a  adds an output of the phase register  121   a  and phase data sent from the internal register  110   a  and outputs the result to the phase register  121   a.    
         [0038]     The DDS controlling section  200  has a phase counter section  210   a , a phase counter section  210   b , a phase counter controlling section  220 , a DDS trigger controlling section  230   a , and a DDS trigger controlling section  230   b . The phase counter section  210   a  indicates the same phase as the phase of the DDS  100   a  that the phase accumulator  120   a  indicates. The phase counter section  210   b  indicates the same phase as the phase of the DDS  100   b  that the phase accumulator  120   b  indicates. The phase counter controlling section  220  sets data to the phase counter section  210   a  when the computer section  300  issues frequency and phase setup requests. The DDS trigger controlling section  230   a  outputs a change trigger signal to the DDS  100   a  at timing considering the latency of a DDS when the phase counter controlling section  220  issues a frequency or phase change request for the DDS. The DDS trigger controlling section  230   b  outputs a change trigger signal to the DDS  100   b  at timing considering the latency of a DDS when the phase counter controlling section  220  issues a frequency or phase change request for the DDS. The latency of a DDS is a period of time after a control signal is input to the DDS until an output signal thereof is affected by the control signal. The latency takes place due to a pipeline delay and so forth of a DDS.  
         [0039]     The DDS controlling section  200  can be composed of a Field Programmable Gate Array (hereinafter abbreviated as an FPGA). Instead of an FPGA, the DDS controlling section  200  may be composed of a custom LSI, an IC, or a discrete part.  
         [0040]      FIG. 3  is a block diagram showing the structure of the phase counter section  210   a . As shown in  FIG. 3 , the phase counter section  210   a  has a phase counter  211   a , a register  212   a , a register  213   a , and a comparing circuit  214   a . The phase counter  211   a  stores data that indicate the phase of the DDS  100   a . The register  212   a  stores a current count upper limit value FC of the phase counter  211   a . The register  213   a  stores a count upper limit value FC_next for the next setup change of the phase counter  211   a . The comparing circuit  214   a  compares data stored in the register  212   a  with output data of the phase counter  211   a  and resets the phase counter  211   a  when they match. The values of the registers  212   a  and  213   a  are controlled through the phase counter controlling section  220  with commands issued from the computer section  300 .  
         [0041]     Since data FC stored in the register  212   a  designates the count upper limit value of the phase counter. When the phase counter reaches the count upper limit, the comparing circuit  214   a  resets the phase counter  211   a . Thus, the data FC becomes a factor with which the duration of one period about the phase, namely the frequency stored in the phase counter  211   a , is decided. When data that indicate a frequency of 0 Hz are loaded to the register  212   a , the comparing circuit  214   a  stops the count-up operation corresponding to the clock signal, not resets the phase counter  211   a . In other words, when data that indicate a frequency of 0 Hz are loaded to the register  212   a , data of the same value are output from the phase counter  211   a.    
         [0042]     The structure of the phase counter section  210   b  is the same as the structure of the phase counter section  210   a . In other words, the phase counter section  210   b  has a phase counter  211   b , register  212   b  and  213   b , and a comparing circuit  214   b  (not shown).  
         [0043]      FIG. 4  is a block diagram showing the structure of the DDS trigger controlling section  230   a . As shown in  FIG. 4 , the DDS trigger controlling section  230   a  has registers  231   a ,  232   a ,  233   a , and  234   a  and a DDS trigger controlling circuit  235   a . The register  231   a  stores data UT 1  that indicate update timing at which the frequencies of sine waves that the DDS  100   a  and the DDS  100   b  are outputting are set to 0 Hz. The register  232   a  stores data UT 2  that indicate update timing at which the sine waves are output after the frequencies became 0 Hz. The register  233   a  stores data UT 1 _next that indicate update timing for the next setup change. The register  234   a  stores data UT 2 _next that indicate update timing for the next setup change. The DDS trigger controlling circuit  235   a  controls the phase of the DDS  100   a  in such a manner that when data that indicate the phase and that are output from the phase counter section  210   b  match data UT 1  stored in the register  231   a , the DDS trigger controlling circuit  235   a  outputs a trigger signal that causes the output frequency of the DDS  100   a  to become 0 Hz and that when data that indicate the phase and that are output from the phase counter section  210   a  matches data UT 2  stored in the register  232   a , the DDS trigger controlling circuit  235   a  outputs a trigger signal that causes a sine wave to be output from the DDS  100   a . When the output frequency is caused to become 0 Hz, the output signal is stopped. The structure (not shown) of the DDS trigger controlling section  230   b  is the same as that of the DDS trigger controlling section  230   a . When the DDS trigger controlling section  230   b  controls the DDS  100   b  to change the phase thereof, the DDS trigger controlling section  230   b  sends a control signal to the DDS  100   b  like the DDS trigger controlling section  230   b.    
         [0044]     Next, the operations of the oscillating apparatus will be described. First of all, a phase change operation will be described. For example, the case of which the DDS  100   a  and the DDS  100   b  output sine waves having the same frequency and the same phase, the frequency of the DDS  100   a  is changed to f 2 , and the phase of the DDS  100   a  is delayed from that of the DDS  100   b  by 180 degrees will be described.  
         [0045]      FIG. 5A  to  FIG. 5E  show waveforms of signals that are output from individual sections of the oscillating apparatus.  FIG. 5A  shows a waveform of a signal that is output from the DDS  100   b .  FIG. 5B  shows a waveform of a signal that is output from the DDS  100   a .  FIG. 5C  shows a waveform of a phase change trigger signal that is output from the DDS controlling section  200  to the DDS  100   a .  FIG. 5D  shows a waveform of a signal that indicates a counter value of the phase counter section  210   b  of the DDS controlling section  200 .  FIG. 5E  shows a waveform of a signal that indicates a counter value of the phase counter section  210   a  of the DDS controlling section  200 . The values of the phase accumulators  120   a  and  120   b  of the DDS  100   a  and the DDS  100   b  are different from the values of the phase counter sections  210   a  and  210   b , respectively. However, the values of the phase accumulators  120   a  and  120   b  are the same as phase information of sine waves.  
         [0046]     First, the case of which data that indicate frequency f 1  are sent from the computer section  300  to the internal register  110   a  of the DDS  100   a , the internal register  110   b  of the DDS  100   b , and the phase counter section  210   a  of the DDS controlling section  200  and thereby the phase of the DDS  100   a  is delayed from that of the DDS  100   b  by 180 degrees will be described.  
         [0047]     While the DDS  100   a  is outputting a sine wave having frequency f 1 , the computer section  300  loads data FC_next that indicate a frequency of 0 Hz to the internal register  110   a  of the DDS  100   a  and the register  213   a  of the phase counter section  210   a  of the DDS controlling section  200 . At this point, the values of the phase counter section  210   a  and the phase accumulator  120   a  and the output frequency of the DDS  100   a  have not been changed to 0 Hz. Next, considering latency L intrinsic to the DDS  100   a , the computer section  300  loads update timings UT 1 _next and UT 2 _next to the registers  233   a  and  234   a  of the DDS trigger controlling section  230   a  so that the phases of the sine waves are inverted. Thereafter, the computer section  300  sends the phase change trigger that causes the phase of the DDS  100   a  to be changed to the phase counter controlling section  220  at timing T 1  shown in  FIG. 5B . When the phase counter controlling section  220  receives the phase change trigger, the phase counter controlling section  220  loads data stored in the registers  233   a  and  234   a  of the DDS trigger controlling section  230   a  to the registers  231   a  and  232   a , respectively.  
         [0048]     The DDS trigger controlling section  230   a  reads the counter value of the phase counter  211   b  and sends frequency change signal P 1  that causes the frequency to be changed to 0 Hz to the DDS  100   a  at update timing UT 1  after the DDS trigger controlling section  230  has read the phase change trigger and the phase counter  211   b  has changed from a position with a phase of 0 degree first time. After the DDS trigger controlling section  230   a  has sent the frequency change signal P 1  and a period of time for latency L has elapsed, namely, when the phase of the output sine wave of the DDS  100   a  becomes 180 degrees, the phase counter controlling section  220  loads data FC_next that indicate a frequency of 0 Hz stored in the register  213   a  of the phase counter section  210   a  to the register  212   a . In addition, data that indicate frequency f 1  are loaded to the register  213   a . Since latency L of the DDS  100   a  is considered at update timing UT 1 , when the value of the output signal of the DDS  100   a  is 0 (at timing T 2  shown in  FIG. 5B ), the frequency is changed to 0 Hz.  
         [0049]     Thereafter, when the sine wave that is output from the DDS  100   b  is inverted, namely the phase counter  211   b  becomes a value that indicates that the phase is inverted by 180 degrees at update timing UT 2  considering latency L, the frequency change signal P 2  is sent to the DDS  100   a . After the period of time for latency L has elapsed, the phase counter controlling section  220  loads data FC_next that indicate frequency f 1  stored in the register  213   a  as data FC to the register  212   a . While the frequency is 0 Hz (the output is stopped), the value of the phase counter section  210   a  as phase information of the DDS  100   a  and the value of the phase accumulator  120   a  of the DDS  100   a  do not change (P 3  in  FIG. 5E ). In consideration of latency L, at timing T 3 , the sine wave is output with a phase of 180 degrees. Thus, the DDS  100   a  outputs a sine wave whose phase is inverted from the phase of the output signal of the DDS  100   b.    
         [0050]     In the foregoing example, by counting the number of clock pulses, considering the latency, the phases of sine waves are inverted, namely the phases of sine waves are caused to have a difference of 180 degrees. Instead, when update data UT 1  and UT 2  cause the DDS trigger controlling section  230   a  to output sine waves having a frequency of 0 Hz (outputs of sine waves are stopped) for any number of clock pulses, the phases of sine waves can be changed for any amounts. To stop the outputs of sine waves at 0 V, by changing only update timing UT 2  rather than update timing UT 1 , the sine waves have a difference of phases.  
         [0051]     Thus, when the phases of sine waves that are output from the DDS  100   a  and the DDS  100   b  are changed, in consideration of the latency, the outputs of the sine waves are stopped with a phase of 180 degrees or 360 degrees, namely the values of the sine waves become 0. After a predetermined period of time has elapse, sine waves whose values are 0 (with a phase of 0 degree or 180 degrees) are output.  
         [0052]     When a BPF is disposed on the output side, if sine waves having positive values or negative values are stopped, it takes a time until the output signals become stable. However, according to this embodiment, when the output values of signals are 0, the outputs of the signals are stopped for a predetermined period of time. Thus, sine waves can be accurately output in a short transient period.  
         [0053]     When the phase resolutions of the phase counter sections  210   a  and  210   b  of the DDS controlling section  200  are higher than those of the phase offset adjustment registers of the internal registers  110   a  and  110   b  of the DDS  100   a  and the DDS  100   b , for example the bit resolutions of the phase offset adjustment registers are 12 bits and the bit resolutions of the phase counter sections  210   a  and  210   b  are 48 bits, the phases of sine waves can be more accurately controlled than the DDS  100   a  and the DDS  100   b.    
         [0054]     It may be necessary for a measurement instrument or the like to accurately measure the gain of its amplifier.  
         [0055]      FIG. 6  is a block diagram showing the structure of circuitry that obtains the gain of an amplifier. As shown in  FIG. 6 , an output of an adding device  2  is connected to an input of an amplifier  1  to be measured for a gain. Outputs of signal sources  3  and  4  that generate sine waves are connected to two inputs of the adding device  2 . In addition, an input of an A/D converter  5  is connected to an output of the amplifier  1 .  
         [0056]     When the gain of the amplifier  1  is 1, the signal sources  3  and  4  output signals S 1 (t) and S 2 (t) of sine waves whose amplitudes are A, frequencies are f, and phase difference is 0. In other words, the following relationship is satisfied. 
 
 S   1 ( t )= S   2 ( t )= A ·sin(2π ft ) 
 
 Since the outputs of the signal sources  3  and  4  are added by the adding device  2 , the input of the A/D converter can be expressed as follows. 
 
 S   1 ( t )+ S   2 ( t )=2 A ·sin(2π ft ) 
 
 In other words, the amplitude of the sine wave that is input to the A/D converter  5  is 2A. 
 
         [0057]     Next, S 3 (t) and S 4 (t) whose amplitudes are A, frequencies are f, and phase difference is 0 are output from the signal sources  3  and  4 . The gain of the amplifier  1  is denoted by G. In other words, the following relationship is satisfied. 
 
 S   3 ( t )= A ·sin(2π ft ),  S   4 ( t )= A ·sin(2π ft +θ) 
 
 The signals that are output from the signal sources  3  and  4  are added by the adding device  2 . The amplifier  1  amplifies the amplitudes of the signals at amplification factor G. Thus, the following relationship is satisfied. 
 
{ S   3 ( t )+ S   4 ( t )}· G= 2 A ·cos(θ/2)·sin(2π ft+θ/ 2)· G  
 
 Thus, the amplitude of the signal that is input to the A/D converter  5  can be expressed as follows. 
 
2A·cos(θ/2)·G 
 
         [0058]     The amplitude of a signal whose gain is 1 needs to be the same as the amplitude of a signal whose gain is G as much as possible to prevent the obtained gain from being adversely affected by nonlinearity of the A/D converter  5 .  
         [0059]     In other words, the following relationship is satisfied. 
 
2 A= 2 A ·cos(θ/2)· G  
 
Thus, cos(θ/2)=1/ G  
 
         [0060]     For example, using the bisection method or the like, the set value of θ is varied. Phase θ′ at which the amplitude of the input of the A/D converter  5  with a gain of 1 becomes the same as the amplitude of that with gain G is obtained. At this point, gain G is calculated by 1/cos(θ′/2). Using this method, gain G of the amplifier  1  can be obtained in the state that the amplitude value of the signal that is input to the A/D converter  5  with a gain of 1 is the same as the amplitude value of that with gain G in the range of the phase resolutions of the signal sources  3  and  4 . Thus, the gain of the A/D converter  5  is almost not affected by nonlinearity of the A/D converter  5 . In other words, the higher the resolution of the phase difference of output signals of the signal sources  3  and  4 , the more accurately the gain of the amplifier  1  can be measured.  
         [0061]     A Frequency Shift Keying terminal (hereinafter abbreviated as an FSK terminal) may be disposed in a DDS. The FSK terminal allows data of one of two frequencies stored in an DDS to be selected when an “H” signal or an “L” signal is input. Next, an embodiment using such DDSs will be described.  
         [0062]      FIG. 7A  to  FIG. 7E  are schematic diagrams showing waveforms of signals that are output from individual sections of an oscillating apparatus according to this embodiment.  FIG. 7A  shows a waveform of a signal that is output from a DDS  100   b .  FIG. 7B  shows a waveform of a signal that is output from a DDS  100   a .  FIG. 7C  shows a waveform of a phase change trigger signal that is output from a DDS controlling section  200  to the DDS  100   a .  FIG. 7D  shows a waveform of a signal that indicates a counter value of a phase counter section  210   b  of the DDS controlling section  200 .  FIG. 7E  shows a waveform of a signal that indicates a counter value of a phase counter section  210   a  of the DDS controlling section  200 .  
         [0063]     The oscillating apparatus according to this embodiment is the same as that shown in  FIG. 1  to  FIG. 4  except for the following points. When signal “L” is input to the FSK terminal of the DDS  100   a , a computer section  300  causes the DDS  100   a  to output a sine wave having frequency f 1 . When signal “H” is input to the FSK terminal of the DDS  100   a , the computer section  300  causes the DDS  100   a  to output a sine wave having a frequency of 0 Hz (namely, no output). When the count value of a phase counter  211   a  becomes UT 1 , a DDS trigger controlling circuit  235   a  inputs signal “H” to the FSK terminal of the DDS  100   a . When the value of the phase counter section  210   a  becomes UT 2 , the DDS trigger controlling circuit  235   a  outputs signal “L” to the FSK terminal of the DDS  100   a.    
         [0064]     In the foregoing embodiments, the case of which two DDSs were disposed was described. Instead, when three or more DDSs are disposed, the phases of signals can be controlled in the same manner as the foregoing embodiment. In other words, when N DDSs (where N is 3 or greater) are disposed, the phases of signals are controlled in such a manner that based on one DDS the phases of signals of the other DDSs have differences.  
         [0065]      FIG. 8  is a block diagram showing an oscillating apparatus having two DDSs whose phases are changed according to another embodiment of the present invention. Similar portions to those in  FIG. 1  are denoted by similar reference numerals and their redundant description will be omitted. A DDS controlling section  500  supplies a clock signal that is output from a clock generating section  400  to a DDS  100   b . The DDS controlling section  500  stops supplying the clock signal to the DDS  100   a  for a predetermined period of time.  
         [0066]      FIG. 9A  to  FIG. 9C  are schematic diagrams showing waveforms of signals that are output from or input to individual sections of the oscillating apparatus shown in  FIG. 8 .  FIG. 9A  shows a waveform of a signal that is output from the DDS  100   b .  FIG. 9B  shows a waveform of a signal that is output from the DDS  100   a .  FIG. 9C  shows a waveform of a clock signal supplied to the DDS  100   a.    
         [0067]     As shown in  FIG. 9A  to  FIG. 9C , the clock signal is directly supplied to the DDS  100   b  and it continuously outputs a signal of a sine wave, whereas the clock signal supplied to the DDS  100   a  is stopped at intervals of a predetermine period of time under the control of the DDS controlling section  500 . Thus, when the stop period of the clock signal is controlled, the output signals of the two DDS  100   a  and  100   b  can have a difference of phases. When the clock signal is stopped for 180 degrees of the sign wave, the DDS  100   a  outputs a sine wave whose phase is inverted from a sine wave that the DDS  100   b  outputs.  
         [0068]     According to the foregoing embodiments, the phase counter section  210   a  uses the phase counter  211   a . Instead, the phase counter section  210   a  may use a phase accumulator like the DDS  100   a . Alternatively, the DDS  100   a  may use a phase counter instead of the phase accumulator  120   a .  
         [0069]     In addition, according to the foregoing embodiments, the DDSs output sine waves. Instead, the DDSs may output square waves, triangular waves, or the like.  
         [0070]     Next, a frequency changing operation for sine waves that are output from the DDSs will be described.  
         [0071]      FIG. 10A  to  FIG. 10C  are schematic diagrams showing waveforms of signals that are output from individual sections of the oscillating apparatus when the frequencies of sine waves are changed.  FIG. 10A  shows a waveform of a sine wave that is output from the DDS  100   a .  FIG. 10B  shows a waveform of a sine wave that is output from the DDS  100   b .  FIG. 10C  shows a waveform of a signal that indicates a count value of each of the phase counter  211   a  and the phase counter  211   b.    
         [0072]     Next, the case of which the frequencies of signals that are output from the DDS  100   a  and the DDS  100   b  are changed from f 2  to f 3  will be described. When the two DDS  100   a  and DDS  100   b  output sine waves having frequency f 2 , data that indicate frequency f 2  are stored in registers  212   a  and  212   b.    
         [0073]     Thereafter, a computer section  300  loads data that indicate frequency f 3  to the DDS  100   a  and the DDS  100   b . In addition, the computer section  300  loads data that indicate frequency f 3  to a register  213   a  and a register  213   b  of a phase counter section  210   a  and a phase counter section  210   b  and data that indicate update timing UT 3  to a register  234   a  and a register  234   b  of a DDS trigger controlling section  230   a  and a DDS trigger controlling section  230   b , respectively. In addition, the computer section  300  loads data that indicate latency L to a phase counter controlling section  220 .  
         [0074]     The computer section  300  sends a frequency change notification signal to a DDS controlling section  200  at timing T 4  shown in  FIG. 10 . When the DDS trigger controlling sections  230   a  and  230   b  receive the frequency change notification signal, they compare values stored in the phase counter section  210   a  and the phase counter section  210   b  with data stored in the register  232   a  and the register  232   b , respectively. When they match, the DDS trigger controlling sections  230   a  and  230   b  output frequency change triggers to the DDS  100   a  and the DDS  100   b , respectively. In other words, the DDS trigger controlling sections  230   a  and  230   b  output the frequency change triggers both to the DDS  100   a  and DDS  100   b  with a phase of 360 degrees of sine waves having frequency f 2  earlier by latency L at timing T 5  shown in  FIG. 10 . Thereafter, data FC_next that indicate frequency f 3  stored in the registers  213   a  and  213   b  of the phase counter sections  210   a  and  210   b  of the phase counter controlling section  220  are loaded to the registers  212   a  and  212   b  with a phase of 360 degrees. In other words, the phase counter sections  210   a  and  210   b  perform count operations with sine waves having frequency f 3 .  
         [0075]     The DDS controlling section  200  sends frequency change triggers to the DDS  100   a  and DDS  100   b , considering latency L. Thus, the DDS  100   a  and DDS  100   b  output sine waves having frequency f 3  when the phases of sine waves having frequency f 2  become 360 degrees. Thus, the transient period necessary for changing frequencies is short and unnecessary signals are prevent from occurring.  
         [0076]     In the foregoing examples, the frequencies of the DDS  100   a  and the DDS  100   b  are changed. Instead, the frequency of one DDS may be changed. In this case, a frequency change trigger may be sent to a desired DDS instead of the DDS controlling section  200 .  
         [0077]     Although the present invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention.  
         [0078]     For example, in the oscillating apparatus according to the embodiment shown in  FIG. 1 , when two DDSs are controlled, the DDS controlling section  200  has two phase counters and two DDS trigger controlling sections. Instead, the phase counter and the DDS trigger controlling section may be disposed in a DDS for which the phase of a signal is not changed. In other words, only the phase counter section  210   b  and the DDS trigger controlling section  230   b  for the DDS  100   b  may be used. Based on the phase difference, the phase count of the DDS  100   a  may be calculated.