Abstract:
A waveform generating circuit is provided which generates a modified triangular wave signal suitable for being input to a frequency modulation circuit such as a voltage-controlled oscillator (VCO). The waveform generating circuit includes a triangular generator, an offset generator for generating first and second offset component signals, a combiner for adding the triangular wave signal generated by the triangular wave generator and the offset component signals, and an output for delivering an output signal resulting from the addition by the combiner.

Description:
This application claims priority from Japanese Patent Application No. 2004-363094 filed Dec. 15, 2004, which is incorporated hereinto by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a waveform generating circuit for making certain modifications to a triangular wave signal, and to a spread spectrum clock generator using the waveform generating circuit. 
   The spread spectrum clock generator refers to a clock generator for reducing electromagnetic radiation by varying the clock frequency by the so-called spread spectrum clock technique. 
   2. Description of the Related Art 
   In the field of the digital circuit, it is known to vary the clock frequency slightly to reduce the electromagnetic radiation caused by the clock. 
   To obtain the frequency modulated clock, it is generally performed to input a triangular wave to a VCO (voltage-controlled oscillator). 
     FIG. 1  shows an example of a triangular wave generating circuit conventionally known. The input signal to the circuit is a rectangular wave, and complementary switch  22  and switch  23  which are driven through an inverter  20  repeat turning on and off alternately. Thus, a capacitor (CAP)  25  is supplied with the current from a current source  21  and the current from a source  24  alternately. In this way, the CAP  25  repeats charge and discharge, thereby outputting a triangular wave. 
   Japanese patent application laid-open No. 06-242851 discloses the following object and configuration (in abstract and  FIG. 1 ). Its object is “to make the interference visually inconspicuous on a screen caused by spurious radiation being fed to a TV tuner, which is generated by digital circuits such as a microcomputer and the like”. Its configuration is “a circuit for generating a clock pulse of a microcomputer, which includes pulse generating means for generating a pulse signal with a fixed frequency; and random modulation means for modulating the pulse signal at random the pulse generating means generates, wherein the random modulation means includes a triangular wave generator; a frequency modulator for carrying out frequency modulation of the triangular wave produced by the triangular wave generator; and a frequency converter for carrying out frequency conversion based on the output of the frequency modulator and a signal obtained from the pulse generator, and wherein the output of the frequency converting means is used as the clock pulse”. 
   However, when the frequency characteristic applied for the input to the modulation circuit or the oscillator is insufficient for achieving the frequency modulation by the triangular wave as described above, there arises a problem in that the vertices of the triangular wave round, and ideal spread spectrum is not achieved. 
   When the spread spectrum is brought about by the triangular wave with such a shape, a spectrum is observed which has higher level portions at both ends of the spread spectrum than in other regions, which presents a problem in that the effect of the spread spectrum is impaired. 
   On the other hand, even if the frequency modulation is carried out using a triangular wave with an ideal shape, since the EMI (electromagnetic interference) measurement is made based on the finite frequency resolution and measurement integral time, the frequency transition stays at frequencies near the vertices of the triangular wave longer than in other frequency ranges to be measured. As a result, a problem arises in that the spectrum is observed which has the higher level portions than in the other regions (see  FIG. 7A ). 
   To circumvent such a problem, a technique is conceived which carries out modulation after changing the waveform for the frequency modulation to a particular shape. The technique, however, must pay extra cost in both a circuit area and cost (see Electromagnetic Compatibility EMC, May, pp. 22-28). 
   In view of the foregoing problems, a first object of the present invention is to provide a waveform generating circuit for generating a modified triangular wave signal suitable for being input to a frequency modulation circuit such as a VCO (voltage-controlled oscillator). 
   A second object of the present invention is to provide a spread spectrum clock generator capable of improving the spread spectrum effect with little increase in the circuit cost by modifying the shape of the triangular wave used for the frequency modulation with a simple method. 
   SUMMARY OF THE INVENTION 
   To accomplish the object, according to one aspect of the present invention, there is provided a waveform generating circuit comprising: triangular wave generating means for generating a triangular wave signal having first slope sections and second slope sections; offset generating-means for generating first offset component signals corresponding to the first slope sections, and for generating second offset component signals corresponding to the second slope sections, the second offset component signals differing from the first offset component signals; combining means for adding the triangular wave signal generated by the triangular wave generating means and the offset component signals generated by the offset generating means; and output means for delivering an output signal resulting from the addition by the combining means. 
   The triangular wave generating means can include: a fixed capacitor; and charge and discharge means for causing the fixed capacitor to be charged and discharged via a pair of switches turning on and off complementarily in response to an input clock signal, wherein a voltage corresponding to the first slope sections and the second slope sections is obtained across the fixed capacitor. 
   The offset generating means can be a fixed resistor for generating as its terminal voltage the first offset component signals and the second offset component signals by causing a charging current and a discharging current of the fixed capacitor to flow through the fixed resistor; the combining means can be a wire connection that connects the fixed capacitor and the fixed resistor in series; and the output means can be a terminal to which the fixed resistor is not connected among the two terminals of the fixed capacitor. 
   According to another aspect of the present invention, the offset generating means can be an auxiliary capacitor; the combining means can be a wire connection that connects the auxiliary capacitor across the input terminal of the input clock signal and a terminal of the fixed capacitor; and the output means can be a common connection point of the auxiliary capacitor and the fixed capacitor. 
   According to still another aspect of the present invention, the first offset component signals and the second offset component signals can have opposite polarities to each other. 
   The waveform generating circuit can generate an asymmetric wave having an operation level that varies at a predetermined period, and that is asymmetric before and after a maximum point and a minimum point of the waveform. 
   The asymmetric wave can have a first slope section that makes a transition from a first low level to a first high level in a predetermined time period; a first level transition that makes an instant transition from the first high level to a second high level; a second slope section that makes a transition from the second high level to a second low level in a predetermined time period; and a second level transition that makes an instant transition from the second low level to the first low level. 
   The first slope section, the first level transition, the second slope section, and the second level transition are generated repeatedly in this order at a predetermined period. 
   According to another aspect of the present invention, there is provided a waveform generating circuit including: means for outputting data on a waveform of an asymmetric wave having an operation level that varies at a predetermined period, and that is asymmetric before and after a maximum point and a minimum point of the waveform; and a DAC for converting the data on the waveform to an analog signal, and for delivering an output signal. 
   The means for outputting can be one of a microprocessor, DSP, ROM and a programmable memory. 
   According to another aspect of the present invention, there is provided a spread spectrum clock generator including: the waveform generating circuit; and a variable frequency clock generating circuit that receives the output signal delivered by the waveform generating circuit, and generates a clock signal with a frequency corresponding to the output signal. 
   The means for outputting can be one of a microprocessor, DSP, ROM and a programmable memory. 
   The present invention can implement a waveform generating circuit for generating a modified triangular wave signal suitable for inputting to a frequency modulation circuit such as a VCO. In addition, according to the present invention, the shape of the triangular wave used for the frequency modulation can be modified by a simple method, which makes it possible to implement the spread spectrum clock generator capable of improving the spread spectrum effect without increasing the circuit cost so much. 
   The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing an example of a conventional triangular wave generating circuit; 
       FIGS. 2A-2C  are diagrams showing a spread spectrum clock generator in accordance with the present invention, and signal waveforms of various portions; 
       FIG. 3  is a diagram showing a method of generating a modulation waveform represented by solid lines in  FIG. 2A ; 
       FIG. 4  is a circuit diagram showing a concrete configuration of the modulation waveform generating circuit  10  as shown in  FIG. 2A ; 
       FIG. 5  is a circuit diagram showing another concrete configuration of the modulation waveform generating circuit  10  as shown in  FIG. 2A ; 
       FIGS. 6A-6C  are diagrams illustrating comparison between the effect of the spread spectrum by the conventional modulation wave generating circuit ( FIG. 1 ) and the effect of the spread spectrum by embodiments ( FIGS. 2A-5 ) in accordance with the present invention; 
       FIGS. 7A and 7B  are diagrams illustrating in more detail a reduction effect or flattening of the spectrum as shown in  FIG. 6C ; 
       FIG. 8  is a circuit diagram showing another concrete configuration of the modulation waveform generating circuit  10  as shown in  FIG. 2A ; 
       FIG. 9  is a circuit diagram showing still another concrete configuration of the modulation waveform generating circuit  10  as shown in  FIG. 2A ; 
       FIG. 10A  is a circuit diagram showing a concrete configuration of the VCO  11 , and  FIG. 10B  is a diagram illustrating relationships between an ordinary input voltage and an output frequency of the VCO  11 ; 
       FIG. 11  is a diagram illustrating a method of generating the modulation waveform represented by solid lines in  FIG. 2A ; and 
       FIG. 12  is a diagram illustrating another method of generating the modulation waveform represented by solid lines in  FIG. 2A . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 2A-2C  show a spread spectrum clock generator of an embodiment in accordance with the present invention, and signal waveforms of various portions. In these figures, the reference numeral  10  designates a modulation waveform generating circuit whose output signal has a modulation waveform as illustrated by solid lines in  FIG. 2A . The modulation waveform is input to a VCO (voltage-controlled oscillator)  11 . Thus, the oscillation frequency of the VCO  11  is modulated in response to the modulation waveform, providing the output clock with frequency variations as illustrated in  FIG. 2B . The frequency transition of the output clock has temporal variations as indicated by solid lines of  FIG. 2C . 
     FIG. 3  shows an example of a method of generating the modulation waveform represented by solid lines in  FIG. 2A .  FIG. 3(   a ) shows a pure triangular wave, and  FIG. 3(   b ) shows a rectangular wave for an offset, which has transition points at the same timing as the vertices of the triangular wave. Adding the triangular wave of  FIG. 3(   a ) and the rectangular wave of  FIG. 3(   b ) results in the waveform as illustrated in  FIG. 3(   c ), which shifts sides from vertices to the next vertices of the triangular wave upward or downward. 
     FIG. 11  illustrates another method of generating the modulation waveform indicated by the solid lines in  FIG. 2A .  FIG. 11(   a ) shows a pure triangular wave as shown in  FIG. 3(   a ).  FIG. 11(   b ) shows a rectangular wave for an offset, which has transition points at the same timing as the vertices of the triangular wave. Although the rectangular wave of  FIG. 3(   b ) has a fixed signal level with its polarization reversed at the transition points, the rectangular wave of  FIG. 11(   b ) has its signal level changed at the transition points without the polarization reversal. Accordingly, adding the triangular wave of  FIG. 11(   a ) and the rectangular wave of  FIG. 11(   b ) results in the waveform as illustrated in  FIG. 11(   c ), which shifts sides from vertices to the next vertices of the triangular wave upward or downward in accordance with the signal levels. 
     FIG. 12  illustrates still another method of generating the modulation waveform indicated by the solid lines in  FIG. 2A .  FIG. 12(   a ) shows a pure triangular wave as  FIG. 3(   a ).  FIG. 11(   b ) shows a rectangular wave for an offset, which has the transition points at the same timing as the vertices of the triangular wave, but has the opposite polarity to that of  FIG. 3(   b ). Specifically, the rectangular wave in  FIG. 12(   b ) has the negative polarity in regions in which the rectangular wave in  FIG. 3(   b ) has the positive polarity, and has the positive polarity in regions in which the rectangular wave in  FIG. 3(   b ) has the negative polarity. Thus, adding the triangular wave of  FIG. 12(   a ) and the rectangular wave of  FIG. 12(   b ) results  15  in the waveform which slopes up toward the right, has a step at the top, slopes down toward the right and has a step at the bottom. 
     FIG. 4  shows a concrete circuit configuration of the modulation waveform generating circuit  10  as shown in  FIG. 2A . The input to the circuit is a rectangular wave as in the conventional example of  FIG. 1 , and complementary switch  42  and switch  43  which are driven through an inverter  40  repeat turning on and off alternately. Thus, a capacitor (CAP)  45  is supplied with the current from a current source  41  and the current from a current source  44  alternately. As a result, the CAP  45  repeats charge and discharge. When the charge and discharge are repeated, an offset voltage is generated across a resistor  46  connected between the CAP  45  and a ground. Thus, while the switch  42  is in the ON state and the switch  43  is in the OFF state, the offset voltage, which is generated with an amount of (resistance of the resistor  46 )×(supply current of the current source  41 ), is added to the output. On the other hand, while the switch  42  is in the OFF state and the switch  43  is in the ON state, the offset voltage, which is generated with an amount of (resistance of the resistor  46 )×(supply current of the current source  44 ), is subtracted from the output. The addition and subtraction can provide as the output waveform the waveform that shifts the sides from vertices to the next vertices of the triangular wave upward or downward. 
     FIG. 5  shows another concrete circuit configuration of the modulation waveform generating circuit  10 . The input to the circuit is also a rectangular wave, and repeating alternate turning on and of f of the complementary switches  32  and  33  driven by the inverter  30  causes the currents from the current source  31  and current source  34  to flow into a capacitor (CAP)  35 . As a result, the charge and discharge of the CAP  35  are repeated. In addition, a capacitor (CAP)  36  is connected across an input terminal and an output terminal in the present circuit. Accordingly, as long as the input rectangular wave is at a high level, the extra charges determined by the capacitance of the CAP  36  and by the capacitance of the CAP  35  are stored in the CAP  35 . As a result, as long as the input rectangular wave is at the high level, the upward slope voltage across the CAP  35  is shifted upward. On the other hand, as long as the input rectangular waveform is at a low level (zero volt), the charge of the CAP  35  flows into the CAP  36 . Thus, the downward slope voltage across the CAP  35  is shifted downward. 
     FIG. 8  shows another concrete circuit configuration of the modulation waveform generating circuit  10 . The input terminal is connected to the inverter  30 , and the output terminal of the inverter  30  is divided into three branches. Two of the three branches are connected to the switches  32  and  33  to turn on and off the switches in response to the high and low of the signal output from the inverter  30 . The current sources  31  and  34  and the switches  32  and  33  are connected in series in the order of the current source  31 , switch  32 , switch  33  and current source  34 . The current source  34  has its end, which is not connected to the switch  33 , connected to the ground. The output terminal is connected to the connecting point of the switch  32  and switch  33 . The CAP  35  is connected across the output terminal and the ground. The CAP  36  is connected across the output terminal and the output of the inverter  30 . 
   Although the CAP  36  of the modulation waveform generating circuit shown in  FIG. 5  is connected across the input terminal and the output terminal, the CAP  36  of the modulation waveform generating circuit of  FIG. 8  is connected across the output of the inverter  30  and the output terminal. Changing the connecting position of the CAP  36  can reverse the directions of the steps of the output waveform because the CAP  36  is supplied with the voltage opposite in polarity to the voltage supplied to the CAP  36  shown in  FIG. 5 . 
   The directions of the steps of the output waveform can also be changed by reversing the polarity of the switches  32  and  33 . Specifically, the directions of the steps of the output waveform can be altered by changing whether the switches are closed when the output of the inverter  30  is high or low. 
     FIG. 9  shows another concrete circuit configuration of the modulation waveform generating circuit  10 . The modulation waveform generating circuit  10  has a ROM  91  and a digital-to-analog converter (DAC)  92 . The ROM  91  stores data on the waveform in advance which has offsets in the slopes. The ROM  91  supplies its output data string to the DAC  92 , and the DAC  92  converts the data to an analog signal and outputs it as the output signal. Thus, the waveform having the offsets in the slopes can be produced as in the foregoing embodiment. Instead of the ROM  91 , it is also possible for the present embodiment to use a processing unit such as a programmable memory, microprocessor or DSP (digital signal processor) (not shown) to generate a prescribed data string, thereby being able to produce the waveform having the offsets in the slopes just as in the case of using the ROM  91 . 
     FIG. 10A  shows a concrete circuit configuration of the VCO  11 . The VOC  11  includes inverters  104 - 106  whose speed can be varied by a current, current sources  101 - 103  for supplying currents to the inverters  104 - 106 , and a V/I (voltage/current) converter  100  for determining the currents to be supplied to the inverters  104 - 106  by controlling the current sources  101 - 103  in response to the voltage. The voltage supplied via the input terminal is converted to the current corresponding to the voltage by the V/I converter  100 , and the current is supplied to the inverters  104 - 106 .  FIG. 10B  illustrates relationships between the ordinary input voltage to the VCO  11  and the output frequency. Generally, the frequency of the output signal increases with the current supplied to the inverters  104 - 106 . Thus, the output frequency increases with the input voltage. However, it is also possible to decrease the output frequency with the input voltage by changing the polarity of the V/I converter  100 . 
     FIGS. 6A and 6B  are graphs illustrating for comparison the effects of the spread spectrum by the conventional modulation wave generating circuit (see  FIG. 1 ) and the spread spectrum by the embodiments in accordance with the present invention (see  FIG. 2A  to  FIG. 5 ).  FIG. 6A  shows a spectrum when a sine wave or rounded triangular wave is used as the frequency modulation waveform. Since the frequency modulation becomes mild near the top and bottom vertices of the sine wave, the spread spectrum has a shape with peaks at both ends.  FIG. 6B  shows a spectrum when a triangular wave is used as the frequency modulation waveform. When the triangular wave is used as the frequency modulation waveform, although the temporal change of the frequency modulation is constant, the EMI measurement is made at a finite frequency resolution. Accordingly, the frequency transition stays in the frequency resolution at the vertices of the triangular wave for a time longer than other times, resulting in the spread spectrum having peaks at both ends as well. In contrast with the two examples, the embodiments in accordance with the present invention employ the waveform as shown in  FIG. 6C , which shifts the sides from the vertices to the next vertices of the triangular wave upward or downward. Thus, considering the vertices of the waveform, the stay duration at a fixed frequency band becomes constant. As a result, the peaks at both ends of the spectrum can be reduced or flattened. 
     FIGS. 7A and 7B  are diagrams illustrating the reduction effect or flattening of the spectrum shown in  FIG. 6C  in more detail.  FIG. 7A  is a diagram illustrating the case where the modulation waveform is a triangular wave as in the conventional example, and the frequency resolution of the EMI measurement is Δf. As is clear from  FIG. 7A , assuming that the time passing through the frequency region Δf is Δt in portions other than the vertices of the triangular wave, then the time passing through the frequency region Δf at the vertices is 2Δt including the time of a turn. Thus, in this frequency region, the spectrum is observed which is higher than in the other portions. On the other hand,  FIG. 7B  is a diagram illustrating the embodiments in accordance with the present invention, which employs the waveform that shifts the sides from the vertices to the next vertices of the triangular wave upward or downward as the modulation waveform. As is clear from  FIG. 7B , as for the waveform of the present embodiment, the time for passing through the frequency region with the frequency resolution Δf is identical for the portions at the vertices and for the other portions, the two ends of the spread spectrum can be flattened. 
   The present invention has been described in detail is with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspect, and it is the intention, therefore, in the apparent claims to cover all such changes and modifications as fall within the true spirit of the invention.