CLOCK GENERATOR

Abstract: The clock generator includes a quartz oscillator (1) which generates clock signals that pass through a band-pass filter (2) which form sinusoidal reference clock signals. The clock signals are fed to a multiplier (3), and delayed by one-fourth period through a one-fourth period delay unit (4) to form clock signals cos 2πf ct which are then applied to a multiplier (5). When a horizontal synchronizing signal is introduced from an external unit, an A/D converter (6) samples the reference clock signal with the horizontal synchronizing signal and sends a digital signal to an operation unit (7) which operates a phase difference between an absolute phase of the digital signal and a predetermined phase, and produces a sinusoidal wave components in (a-b) and a cosine wave component cos (a-b) that correspond to the phase difference. The multiplier multiplies the reference clock signal sin 2πf ct by the sinusoidal wave component sin (a-b), and the multiplier (S) multiplies the cosine clock signal cos 2πf ct by the cosine wave component cos (a-b). These multiplied results are added up together through an adder (9), and the sum is converted into a binary value through a comparator (12) which then produces an output clock pulse W.CK.

Description: The present invention relates generally to clock generating apparatuses. More specifically, the present invention relates to a clock signal generating apparatus for application in a writing clock generating circuit of a time base corrector or the like for generating a clock signal by synchronizing an externally applied horizontal synchronizing signal with a reference synchronizing signal. In recording a video signal on an optical disc or in a video tape recorder in the form of an analogue signal and reproducing the same, a time base corrector is used for removing time base fluctuation of the reproduced video signal. Fig. 1 is a schematic block diagram showing one example of a conventional time base corrector. Referring to Fig. 1, a terminal 26 receives a reproduced video signal (still picture signal) having a fluctuating time base. This reproduced video signal is applied to an A/D converter 21 and a writing clock generating circuit 24. Writing clock generating circuit 24 separates a horizontal synchronizing signal from the reproduced video signal and also generates a writing clock signal W·CK corresponding to a time base fluctuation of the reproduced video signal. In response to the writing clock signal W·CK, A/D converter 21 samples the reproduced video signal and digitalizes the same to be applied to a memory 22. Memory 22 writes the digitalized video signal in response to the writing clock signal W·CK having the same time base fluctuation. Meanwhile, an external reference synchronizing signal is applied to a reading clock generating circuit 25, which circuit generates a reading clock signal R·CK synchronized with the external reference synchronizing signal having a fixed time base, which clock signal is applied to memory 22 and a D/A converter 23. In response to the reading clock signal R·CK, memory 22 reads the stored digital video signal and applies the same to D/A converter 23. In response to this reading clock signal R·CK, D/A converter 23 converts the digital video signal to an analogue signal. Therefore, the video signal output from D/A converter 23 to an output terminal 27 has a fixed time base. The time base correcting capability of such a time base corrector as shown in Fig. 1 depends on whether the writing clock signal W·CK can be generated or not which precisely corresponds to a time base fluctuation of the reproduced video signal. A conventionally proposed common BCO (burst controlled oscillator) using such elements as a crystal oscillator, a coil and a capacitor is not satisfactory and there is required a circuit having a wide frequency response range and a high response speed. Such a BCO is essentially liable to become unstable, affected by noise, waveform distortion, drop out or skew. Thus, various improvements have been made in a burst gate circuit or a synchronization separating circuit included in a time base corrector in order to prevent attenuation of a time base fluctuation component of an input video signal while reducing an effect of noise or the like. For example, used as a horizontal synchronizing signal separating circuit are a delay circuit, a synchronization gate circuit employing a gate signal and comprising a flywheel oscillator, and such a circuit as detecting drop out to mute the drop out noise in a video signal and inhibiting synchronization separating or clamping. A wide-band circuit is used as a color burst gate circuit for amplifying and separating a synchronizing signal and a color burst signal. Fig. 2 shows one example of a case where a precise writing clock signal W·CK is generated by using the thus separated synchronizing signal and color burst signal. In a writing clock generating circuit 24 shown in Fig. 2, a horizontal synchronizing signal is applied to a phase comparator 31 through a terminal 40. Phase comparator 31 compares the phase of the horizontal synchronizing signal with that of a signal obtained by frequency-dividing an output of a variable voltage controlled oscillator (VCO) 33 by a frequency-dividing circuit 34 to apply an error voltage to VCO 33 through a loop filter 32. Phase comparator 31, loop filter 32, VCO 33 and frequency-dividing circuit 34 constitute a PLL loop. Loop filter 32 is provided in order to prevent oscillation of the PLL loop, by which loop filter 32, even at a sudden phase fluctuation of the video signal, the output of VCO 33 scarcely follows the phase fluctuation, resulting in being independent of a phase of a sub carrier. The output of VCO 33 is applied to a frequency-dividing circuit 35 wherein the frequency of the output is divided by four to output a 3f sc (f sc is a sub carrier frequency) signal. Frequency-dividing circuit 35 is reset in response to one pulse of a color burst signal input to a terminal 41. Through the resetting operation, a phase difference between the 3f sc signal, the frequency-divided output of frequency-dividing circuit 35, and the color burst signal becomes 30° or less in color sub carrier phase. The 1/4 frequency-divided output of frequency-dividing circuit 35 is applied to a phase modulator 36 wherein the phase of the output is modulated, which modulated output is applied to a frequency-dividing circuit 37 wherein the frequency thereof is divided by 3 to attain the same frequency as the color sub carrier. The 1/3 frequency-divided output of frequency-dividing circuit 37 is applied to a phase comparator 38. Phase comparator 38, with the color burst signal applied thereto, compares phases of the 1/3 frequency-divided output signal and the color burst signal to apply an error voltage to phase modulator 36 through a loop filter 39. In response to the error voltage, phase modulator 36 modulates the phase of the 3f SC signal to output to a terminal 42 a writing clock signal W·CK following the input video signal and having a phase locked to the horizontal synchronizing signal. Since in time base corrector 20 using writing clock generating circuit 24 shown in Fig. 2, however, feedback control is employed wherein loop filters 32 and 39 are used in VCO 33 for generating a signal from which a writing clock signal is generated and in phase modulating circuit 36 for phase-locking the output of frequency-dividing circuit 35 to a color burst signal, respectively, even such improvements as described above do not allow a writing clock generating circuit having a high response speed to be achieved. In the case of an optical disc still picture file in particular, it is impossible to completely correct a time base at a high speed through feedback control because one frame of still picture is reproduced only for a short time period and a rotation jitter of the optical disc has a high frequency component. An object of the present invention is to provide a clock generating apparatus capable of generating a clock signal having an arbitrary frequency in a predetermined frequency range in response to a reference clock signal. The present invention provides the clock signal generating apparatus defined by claim 1. Claim 2 is directed to an embodiment of the invention. In accordance with the present invention, a clock signal having an arbitrary frequency can be obtained in response to a setting signal. Therefore, it is possible to easily obtain a clock signal having a frequency very close to that of a reference signal, whereby a clock signal can be easily obtained having a higher frequency than that of the reference signal. Fig. 1 is a schematic block diagram of a conventional time base corrector. Fig. 2 is a block diagram of a writing clock generating circuit for use in the time base corrector shown in Fig. 1. Fig. 3 is a block diagram showing one example which is useful for understanding the present invention. Fig. 4 is a waveform diagram of sine wave data and cosine wave data read from the ROM shown in Fig. 3. Fig. 5 is a block diagram showing another example which is useful for understanding the present invention. Fig. 6 is a waveform diagram of each reference clock signal output from the reference clock generating circuit shown in Fig. 5. Fig. 7 is a diagram showing a band characteristic of a band pass filter shown in Fig. 5. Fig. 8 is a diagram showing a phase characteristic of the same. Fig. 9 is a circuit diagram showing one example of the bit-binarizing circuit shown in Fig. 5. Fig. 10 is a table of truth values of the bit-binarizing circuit shown in Fig. 9. Fig. 11 is a circuit diagram showing one example of the digital multiplier shown in Fig. 5. Figs. 12A and 12B are tables showing truth values of the digital multiplier shown in Fig. 11. Fig. 13 is a block diagram showing a further example which is useful for understanding the present invention. Figs. 14A, 14B, 15A and 15B are frequency spectra of clock signals generated by the clock generating apparatus shown in Fig. 13. Fig. 16 is a block diagram showing a still further example which is useful for understanding the present invention. Fig. 17 is a block diagram showing an embodiment of the present invention. Fig. 3 is the block diagram showing one example of a clock generating circuit. The arrangement will be described with reference to Fig. 3. A crystal oscillator 1 generates a clock signal of rectangular waveform having a frequency of 4.05MHz (= f c ), for example. The reference clock signal is applied to a band pass filter 2. Band pass filter 2 converts the reference signal of rectangular waveform to a sine wave signal Sin(2πf c t) of 4.05MHz. π denotes a ratio of the circumference of a circle to its diameter and t denotes a time. The sine wave signal Sin(2πf c t) is applied to a first multiplier 3, a 1/4 cycle delayer 4 and an A/D converter 6. 1/4 cycle delayer 4 delays the input sine wave signal sin(2πf c t) by 1/4 cycle to output a cosine wave signal cos(2πf c t) as a second clock signal. The cosine wave signal cos(2πf c t) is applied to a second multiplier 5. A horizontal synchronizing signal is applied to A/D converter 6 through a terminal 11. The horizontal synchronizing signal is input to adjust a phase of a clock signal to that of a horizontal synchronizing signal. A/D converter 6 samples a voltage of the sine wave signal sin(2πf c t) in response to which a horizontal synchronizing signal with a time base fluctuated is obtained and applies the same to an operating device 7 as a digital signal. An initial phase b is set in operating device 7 by an initial phase setting device 8. Operating device 7 contains a ROM for storing sine wave data and a ROM for storing cosine wave data (not shown). Operating device 7 operates a phase difference (a - b = c) between an absolute phase a of the digital signal supplied from A/D converter 6 and a phase b set by initial phase setting device 8 to output a sine wave component sin(a-b) and a cosine wave component cos(a-b) having the phase difference from the ROM. The sine wave component sin(a-b) output from operating device 7 is applied to first multiplier 3 and the cosine wave component cos(a-b) is applied to second multiplier 5. Multiplier 3 multiples the sine wave signal sin(2πf c t) by the sine wave component sin(a-b) which is a first detection signal and outputs the multiplication result as an analog signal. Multiplier 5 multiplies the cosine wave signal cos(2πf c t) which is a second reference signal by the cosine wave component cos(a-b) which is a second detection signal to output the multiplication result as an analog signal. For example, a D/A converter having a four quadrant multiplying function is used as multipliers 3 and 5. The multiplication results output from multipliers 3 and 5 are added by an adder 9, whose addition output is applied to a comparator 12. Comparator 12 binarizes the addition result to be output as a writing clock signal W·CK. Fig. 4 is the waveform diagram of the sine wave data and the cosine wave data read from the ROM contained in the operating device 7 shown in Fig. 3. As can be seen from Fig. 4, the phase of the sine wave component sin(a-b) and that of the cosine wave component cos(a-b) are orthogonal to each other. Referring to Figs. 3 and 4, a specific operation of one example of a clock generating apparatus useful for understanding the present invention will be described. The reference clock signal of rectangular waveform output from crystal oscillator 1 is converted by band pass filter 2 to a sine wave signal sin(2πf c t) of 4.05MHz which is applied to multiplier 3, 1/4 cycle delayer 4 and A/D converter 6. 1/4 cycle delayer 4 delays the sine wave signal sin(2πf c t) by 1/4 cycle to generate a cosine wave signal cos(2πf c t) which is a second clock signal, and applies the same to multiplier 5. Phase resolutions of the sine wave signal sin(2πf c t) and the cosine wave signal cos(2πf c t) input to multipliers 3 and 5 depend on the bit arrangements of multipliers 3 and 5, respectively. For example, assuming that both multipliers 3 and 5 utilise 5 bits, the phase resolution is 11.25°(= 360° ÷ 32). The phase resolution is determined based on a residual time base error based on a S/N which an analog circuit of the system has or a correction range of a time base corrector required by the system. On receiving the horizontal synchronizing signal having the time base fluctuated through terminal 11, A/D converter 6 samples the voltage of the sine wave signal sin(2πf ct ) to output the sampling result as a digital signal. Operating device 7 receives the digital signal and obtains the absolute phase a of the digital signal to operate a phase difference (a - b = c) between the absolute phase and the phase b previously set by initial phase setting device 8. Operating device 7 reads the sine wave component sin(a-b) and the cosine wave component cos(a-b) having the phase difference from the built-in ROM. That is, operating device 7 simultaneously accesses the ROM for sine wave and the ROM for cosine wave, using the phases a and b as addresses to simultaneously output the sine wave component and the cosine wave component corresponding to the phase difference (a-b) as digital signals. The sine wave component sin(a-b) output from the operating device 7 is applied to multiplier 3 and the cosine wave component cos(a-b) is applied to multiplier 5. Used as multipliers 3 and 5 are those whose output amplitude can be controlled by a reference signal when converting an input digital signal which is a detection signal to an analogue signal. Therefore, multiplier 3 multiplies the sine wave signal sin(2πf c t) which is the first reference signal by the sine wave component sin(a-b) which is the first detection signal to output the multiplication result sin(2πf c t) · sin(a-b) as an analogue signal. Multiplier 5 multiplies the cosine wave signal cos(2πf c t) which is the second reference signal by the cosine wave component cos(a-b) which is the second detection signal to output the multiplication result cos(2πf c t) · cos(a-b) as an analogue signal. The multiplication outputs from multipliers 3 and 5 are added by analogue adder 9 and the following output signal is applied to comparator 12 : sin(2πf c t) · sin(c) + cos(2πf c t) · cos(c) = cos(2πf c t-c) wherein c = (a-b). As is clear from equation (1), the first reference signal is output as a cosine wave signal cos(2πf c t-c) with a phase lag of c with respect to the cosine wave signal cos(2πf c t) out of phase by 90° from the first reference signal. The cosine wave signal cos(2πf c t-c) is binarized by comparator 12 and output as a writing clock signal W·CK. Thus, the writing clock signal W·CK is a clock with a phase synchronizing with that of a horizontal synchronizing signal. As the foregoing, the output cosine wave signal cos(2πf c t-c) includes a phase c corresponding to a phase difference of the horizontal synchronizing signal with respect to one clock of the reference signal. More specifically, the phase of the cosine wave signal cos(2πf c t), whose phase is the same as that of the sine wave signal, is changed instantly by a phase difference c between the sine wave signal which is the reference signal and the horizontal synchronizing signal which is the input signal. Besides, this cosine wave signal cos(2πf c t) is phase-locked to the horizontal synchronizing signal having a time base fluctuated. With the cosine wave signal cos(2πf c t) converted to a binary signal by comparator 12, a stable writing clock signal W·CK can be obtained. In the above-described circuit, a time base correction of a reproduced video signal is carried out only by adjusting an initial phase of a writing clock signal W·CK to that of a horizontal synchronizing signal, which achieves a sufficient effect of a time base corrector. This is because a time base fluctuation in one horizontal period is small both for motion picture and still picture in a component recording. When a more precise time base corrector effect is required, a phase of a sampling clock (reading clock signal R·CK) of D/A converter 23 for converting a digital signal to an analogue signal is phase-modulated in every horizontal period based on a horizontal synchronizing signal phase error stored in memory 22 shown in Fig. 1. Then, the same means as the above-described writing clock signal generating circuit can be used as a reading clock signal R·CK generating circuit. Since a sine wave signal and a cosine wave signal are completely the same signals except that they are out of phase by 1/4 cycle from each other, completely the same effect can be obtained even exchanging sine wave and cosine wave in the embodiment shown in Fig.3. A subtraction may be carried out by using an analogue subtractor without an addition by analogue adder 9. Fig. 5 is the block diagram of another example of a clock generating circuit and Fig. 6 is the timing chart of the reference clock signal generated from the reference clock generating circuit shown in Fig. 5. The circuit shown in Fig. 5 employs the signals each out of phase by 0, π/2, 2π/2 and 3π/2 as reference clock signals and digital multipliers 3a and 5a, which circuit is the same as that shown in Fig. 3 except for the following points. Reference clock generating circuit 10 includes a crystal oscillator la and a 4-bit shift register 51. Crystal oscillator la generates a clock signal 4CK corresponding to a sine wave signal sin(2πf c t) of a frequency four times 4.05 MHz. The clock signal 4CK is applied to 4-bit shift register 51 wherein the signal is frequency-divided by 4, so that reference clock signals CK 0 -CK 3 of rectangular waveform each having a frequency of 4.05MHz and each sequentially out of phase by π/2 from each other as shown in Fig. 6 are output. The reference clock signals CK 0 -CK 3 are applied to digital multiplier 3a, a bit-binarizing circuit 52 and a register 55. Register 55 delays the respective reference clock signals CK 0 -CK 3 by one clock. The delay amount corresponds to π/2 in phase, and by applying the reference clock signals CK 0 -CK 3 to register 55, a second clock signal CKc corresponding to the cosine wave signal cos 2πf c t is output from register 55, which signal is applied to digital multiplier 5a. The three reference clock signals CK 1 -CK 3 out of the reference clock signals CK 0 - CK 3 are converted to have 2 bits by bit-binarizing circuit 52, which signals are applied to D/A converter 53. D/A converter 53 converts the bit-binarized reference clock signals CK 1 -CK 3 to analogue signals in response to the clock signal 4CK. The analogue signals are applied to a band pass filter 54 wherein only fundamental wave components of the signals are extracted and applied to A/D converter 6. Operating device 7 includes a phase exchanger 71 and ROMs 72 and 73. Similarly to the description of the above-described circuit of Fig. 3, phase exchanger 71 operates a phase difference a - b = c between the absolute phase a of the output of A/D converter 6 and the phase b set by initial phase setting device 8. ROM 72 previously stores data cos(a-b) of the cosine wave component and ROM 73 previously stores data sin(a-b) of the sine wave component. When a phase difference is operated by phase exchanger 71, ROM 72 applies the cosine wave data cos(a-b) having the phase difference to digital multiplier 5a, while ROM 73 applies the sine wave data sin(a-b) having the phase difference to digital multiplier 3a. Digital multiplier 3a multiplies the sine wave clock signals CK 0 -CK 3 as references by the sine wave data sin(a-b) and digital multiplier 5a multiplies the second clock signal CK c of cosine wave by the cosine wave data cos(a-b). The multiplication result of digital multiplier 3a is stored in a register 56 in response to the clock signal 4CK and the multiplication result of digital multiplier 5a is stored in a register 57 in response to the clock signal 4CK. The multiplication results respectively stored in registers 56 and 57 are added by a digital adder 9a and the addition result is converted to an analogue signal by a D/A converter 58 in response to the clock signal 4CK. The analogue signal is applied to a band pass filter 59 wherein only a fundamental wave component of the signal is extracted, which is binarized by comparator 12 and output as a writing clock signal W·CK. Fig. 7 is the diagram showing the band characteristic of the band pass filter shown in Fig. 5 and Fig. 8 is the diagram showing the phase characteristic of the same. Band pass filter 59 shown in Fig. 5 is desirably selected to have a band characteristic allowing a frequency component in the range of ±1/2f 0 to fully pass and wherein an attenuation amount at ±4f 0 is more than 1/(2 n -1), centering on a carrier frequency f 0 . Furthermore, it is desirable that a phase characteristic of band pass filter 59 is selected such that a phase lag characteristic maintains a linear characteristic as shown in Fig. 8 with respect to a frequency in the frequency range of ±1/2f 0 , centering on the carrier frequency f 0 . Fig.9 is the circuit diagram showing one example of the bit-binarizing circuit shown in Fig. 5 and Fig. 10 is the table of truth values of the bit-binarizing circuit shown in Fig.9. Referring to Figs. 9 and 10, a description will be given of the bit-binarizing circuit 52. Bit-binarizing circuit 52 includes OR circuits 521 and 522, OR circuit 521 receiving the reference clock signals CK 2 and CK 3 and OR circuit 522 receiving the reference clock signals CK 1 and CK 3 . OR circuit 521 outputs MSB bits, while OR circuit 522 outputs LSB bits. The table of truth values of this bit-binarizing circuit 52 is shown in Fig. 10. Fig. 10 shows relations in each state (0, 1, 0, -1), wherein the 2-bit output is applied to the above-described D/A converter 53 wherein the output is converted to an analogue signal. Fig. 11 is the circuit diagram showing one example of the digital multiplier shown in Fig. 5 and Fig. 12 is the table of truth values of the digital multiplier shown in Fig. 11. Referring to Figs. 11 and 12, a description will be made of digital multiplier 3a. Digital multiplier 3a includes 3-input NAND circuit 301 of 10 bits and an EXOR circuit 302. NAND circuit 301 has one input receiving each of bits D0-D9 constituting a sine wave component (a-b) and the other inputs receiving the reference clock signals CK 0 and CK 2 . The output signal of NAND circuit 301 is applied to one input of its corresponding EXOR circuit 302 and the reference clock signal CK 3 is applied to each EXOR circuit 302 except for the EXOR circuit 302 of the most significant bit. The most significant bit D 9 is a sign bit and therefore the corresponding EXOR circuit 302 receives a clock signal which is the inversion of the reference clock signal CK 1 . Figs. 12A and 12B are tables of truth values of the thus constituted digital multiplier 3a. Fig. 12A shows the input/output relation of bits D0-D8, wherein the upper columns indicate the cases where the bits D0-D8 are at a "L" level and the lower columns indicate the cases where they are at a "H" level. In a 0 state, "L" level (this level is considered 0) is output, in a 1 state, the input is output without inversion and in a -1 state, the input is inverted to be output. Similarly, Fig. 12B is the table of truth values of bit D9, wherein "L" level denotes minus (-) and "H" level denotes plus (+). With respect to an analogue sine wave signal, assuming that the zero point is " 0 (= 1000000000)", the minimum value is "-512(= 0000000000)" and the maximum value is "+511(= 1111111111)", the multiplication output of the bit D9 is zero in the 0 state, it should be therefore expressed not as (0000000000) but as (1000000000). The multiplier shown in Fig. 11 is accordingly structured to have such logic. As is clear from Fig. 12B, in the 1 state, the sign bit D9 is output without inversion and in the -1 state, it is inverted to be output. Digital multiplier 5a shown in Fig. 5 is similarly structured as that of Fig. 11 and therefore no description will be made thereof. A specific operation of the circuit shown in Fig. 5 will be described. Crystal oscillator la generates the clock signal 4CK having the frequency quadruple of 4.05MHz. As shown in Fig. 6, 4-bit shift register 51 frequency-divides the clock signal 4CK to output the reference clock signals CK 0 -CK 3 each having a different phase. Bit-binarizing circuit 52 binarizes the bits of the reference clock signals CK1-CK3 to apply to D/A converter 53 such MSB bits and LSB bits as shown in Fig. 10. D/A converter 53 converts the two-bit digital signals to analogue signals, only the fundamental waves of which are extracted by band pass filter 54 to apply analogue sine wave signals to A/D converter 6. Similarly to the above-described circuit shown in Fig. 3, A/D converter 6 samples a voltage of an analogue sine wave signal sin(2πf c t) when a horizontal synchronizing signal with a time base fluctuated is applied and A/D converter applies the same to operating device 7 as a digital signal. In operating device 7, phase comparator 71 calculates a phase difference between an absolute phase a and an initial phase b of a digital signal to output a sine wave component sin(a-b) and a cosine wave component cos(a-b) corresponding to the phase difference a - b = c from ROMs 73 and 72, using the phase difference as an address signal. Digital multiplier 3a multiplies the four reference clock signals CK 0 -CK 3 by the sine wave component sin(a-b) and digital multiplier 5a multiplies the cosine clock signal CK c by the cosine wave component cos(a-b). As a result, digital multiplier 3a outputs the multiplication result of sin(2πf c t) · sin(a-b), which is stored in register 56, and digital multiplier 5a outputs the multiplication result of -cos(2πf c t) · cos(a-b), which is stored in register 57. The multiplication results stored in registers 56 and 57 are subtracted by digital adder 9a. That is, digital adder 9a outputs the subtraction result expressed as the following equation (2): sin(2πf c t) · sin(c) + cos(2πf c t) · cos(c) = cos(2πf c t-c) wherein c = a - b. As is clear from equation (2), the output is the cosine wave signal cos(2πf c t-c) with the phase lag of c with respect to the cosine wave signal cos(2πf c t). The cosine wave signal cos(2πf c t-c) is converted to an analogue signal by D/A converter 58, the fundamental wave of which signal is extracted by band pass filter 59 to output a writing clock signal W·CK binarized by comparator 12. Fig. 13 is the block diagram showing a further example of a clock generating circuit. The circuit shown in Fig. 13 is structured to obtain an output clock signal having a desired single frequency in response to an externally applied setting signal. The frequency range of the output clock signal is within the range of a predetermined frequency Δf. Referring to Fig. 13, the clock generating apparatus comprises an integration circuit 60 and a phase modulating circuit 61. A terminal 62 of integration circuit 60 receives an external setting signal. The setting signal is an 8-bit digital signal whose digital value determines a frequency of an output clock signal. The setting signal is applied to an adder 64. Adder 64 adds the set value of the last clock and the present set value. Adder 64 has a 2n bit (n is an integer) arrangement, n is 5 in this embodiment. Therefore, the 8-bit setting signal is supplied to the less significant 8 bits of adder 64 and the remaining 2 bits receive no input. The addition output is again supplied to register 65. By sequentially adding a setting signal of a last clock in this way, an integrated digital setting signal can be obtained from register 65. Crystal oscillator 1b is for generating a reference clock signal CK having a frequency of 2.5 MHz, for example, and which reference clock signal CK is applied to a terminal 63 of integration circuit 60 and through a band pass filter 2 also to phase modulating circuit 61. Phase modulating circuit 61 includes multipliers 3 and 5, 1/4 cycle delayer 4, adder 9, ROMs 72 and 73, band pass filter 59 and comparator 12. Multipliers 3 and 5, 1/4 cycle delayer 4, adder 9 and comparator 12 are the same as those of the circuit shown in Fig. 3 and ROMs 72 and 73 and band pass filter 59 are the same as those of the circuit shown in Fig. 5. The integrated digital setting signal is applied to ROMs 72 and 73, so that ROM 72 outputs a cosine setting signal cos(c) having an amplitude value corresponding to the contents of the bit data of the integrated digital setting signal and which cosine signal is applied to multiplier 3, and ROM 73 outputs a sine digital setting signal sin(c) having an amplitude value corresponding to the contents of the bit data of the integrated digital setting signal is read and which sine signal is applied to multiplier 5. Multiplier 3 receives a sine wave reference clock signal sin(2πf c t) and multiplier 5 receives a cosine clock signal cos(2πf c t) obtained by inverting the sine wave reference clock signal sin(2πf c t) by 1/4 cycle delaying circuit 4. Multiplier 3 multiplies the sine wave signal sin(2πf c t) by the cosine digital setting signal cos(c) read from ROM 72 and the multiplier 5 multiplies the cosine wave signal cos(2πf c t) by the sine digital setting signal sin(c). Multipliers 3 and 5 output the multiplication results as analogue signals, which signals are applied to adder 9. Adder 9, which is an analogue adder, adds the multiplication results of multipliers 3 and 5 and applies the addition result to band pass filter 59. Band pass filter 59 extracts only the fundamental wave component from the addition output and applies the same to comparator 12. Comparator 12 binarizes the analogue value of the fundamental wave component to output the binarized value from a terminal 40. Figs. 14A, 14B, 15A and 15B are frequency spectra of clock signals generated by the clock generating apparatus shown in Fig. 13. Referring to Figs. 13, 14A, 14B, 15A and 15B, an operation of the clock generating apparatus according to the present embodiment will be described. Crystal oscillator 1b, band pass filter 2 and 1/4 cycle delayer 4 are the same as those described above of Fig. 3 and therefore no description will be made thereof. When a setting signal is applied to terminal 62 of integration circuit 60, adder 64 adds the set value of a last clock stored in register 65 and the set value of this time, which addition result is stored in register 65. Then, register 65 applies the integrated digital setting signal to phase modulator 61. ROM72 reads a cosine wave digital setting signal cos(c) having an amplitude value corresponding to the contents of the bit data of the integrated digital setting signal and applies the same to multiplier 3. Similarly, ROM73 reads a sine wave digital setting signal sin(c) having an amplitude value corresponding to the contents of the bit data of the input digital setting signal and applies the same to multiplier 5. Multiplier 3 multiplies the reference clock signal sin(2πf c t) by the cosine wave digital setting signal cos(c) to apply the multiplication result sin(2πf c t) · cos(c) to analogue adder 9. Multiplier 5 multiplies the cosine clock signal cos(2πf c t) by the sine digital setting signal sin(c) and applies the multiplication result cos(2πf c t) · sin(c) to analogue adder 9. Analogue adder 9 adds the two multiplication results and applies the output expressed as the following equation (3) to band pass filter 59: sin(2πf c t) · cos(c) + cos(2πf c t) · sin(c) = sin(2πf c t+c) That is, adder 9 outputs the sine reference signal sin(2πf c t+c) leading the sine reference signal sin(2πf c t) by c and applies the same to band pass filter 59. Band pass filter 59 limits a band width of the sine wave reference signal sin(2πf c t+c) and applies the band width-limited signal to comparator 12. Comparator 12 binarizes the sine wave reference signal sin(2πf c t+c) to output an output clock signal having a frequency corresponding to the setting signal from terminal 40. As the foregoing, since the phase of the output clock signal obtained at output terminal 40 can be changed corresponding to an input setting signal at a high speed (for 2/f c time period) with respect to a reference signal for every cycle of the reference signal, which results in the phase modulation of the reference signal, as a result, the frequency of the output clock signal thus output is controlled by the input setting signal. Assuming that both multipliers 3 and 5 comprise 10 bits, the phase resolution is 0.35° (= 360° ÷ 1023). The following equation (4) represents a relation between a minimum phase change dc per unit time t and a frequency change df: df = (1/2π) (dc/dt) Therefore, the relation between the minimum phase change dc per unit time and a maximum frequency displacement Δf will be expressed as: Δf = df (2 8 - 1) Taking into consideration that either for the phase c, positive or negative polarity can be selected every cycle, an oscillating frequency f is expressed as the following equation: f = f c ± Δf More specifically, it is possible to output a frequency in the range of ±Δf, with the reference frequency f c from crystal oscillator 1b as a center frequency. Therefore, when the following equations are obtained: dc = 6.14 x 10 -3 rad dt = 400nsec (= 1/f c = 2.5MHz) then the frequency range and interval can be expressed as follows: Δf = 0.623MHz df = 2443Hz so that a frequency in the range of the above-described equation (6) can be obtained at an interval of df. A value of df is determined by the resolutions of multipliers 3 and 5. From the foregoing description, the clock signal is a frequency in the range of ± Δf, with a frequency fc of a reference signal as a center frequency, as shown in Figs. 14A and 14B. A single frequency such as a, b, c, d .... can be output at an interval of df, with the frequency fc of the reference signal as a reference. The interval of df is determined by the number of bits that multipliers 3 and 5 can employ, wherein when the decreased number of bits results in a long interval of df the increased number results in a short interval. A value and polarity of the phase c, that is, the contents of bit data of an input setting signal, determine which output clock signal of which frequency is to be output. When the bit data is small, a clock signal having a frequency close to that of a reference signal and when the bit data is large, a clock signal having a frequency away from the reference signal, are selected. When all the bit data of the above-described 8-bit setting signal is "0", then c = 0, whereby the reference signal itself is output. One example of an output clock signal when the bit data is small is shown by each solid line of Figs. 14A, 14B, 15A and 15B. As shown in Figs. 14A and 15A, when the polarity of the phase c is positive, a signal having a higher frequency than that of a reference signal is output as an output clock signal and when the polarity is negative, a signal having a lower frequency than that of the reference signal is output as shown in Figs. 14B and 15B. The polarity of the phase c is changed to positive or negative by reversing for example, the sine wave and cosine wave digital setting signals sin(c) and cos(c) input to multipliers 3 and 5. As is clear from the above-described equations, a completely linear relation is established between an input voltage of an input setting signal and an output frequency of a reference signal. That is, a linear characteristic is obtained. A frequency range fc± Δ f which can be set will be expressed as the following equation: fc(1-1/2) < fc ± Δ f < fc(1+1/2) Therefore, an output clock signal having a wide range of frequency can be generated depending on a selected center frequency fc. As the foregoing, in this circuit a clock signal CK generated by crystal oscillator 1b has a frequency not fluctuated through a digital processing, the frequency fluctuation accordingly depends only on a temperature characteristic, thereby achieving a clock generating apparatus having an excellent temperature characteristic. Fig. 16 is the block diagram showing a still further example of a clock generating circuit. The circuit shown in Fig. 16 employs low-cost 5-bit multipliers 3b, 3c, 5b and 5c in place of the multipliers 3 and 5 in the abovedescribed circuit shown in Fig. 13. The sine wave reference clock signal sin(2πf c t) is applied to multiplier 3b and an attenuator 67 wherein the input level of the signal is attenuated to 1/(2 n -1), which attenuated signal is applied to multiplier 3c. The more significant 5 bits of the cosine digital setting signal cos(c) output from ROM72 are applied to multiplier 3b and the less significant 5 bits are applied to multiplier 3c. Furthermore, the cosine clock signal cos(2πf c t) delayed by 1/4 cycle delayer 4 is applied to an attenuator 68 wherein an input level of the signal is attenuated to 1/(2 n -1), which attenuated signal is applied to multiplier 5c. The more significant 5 bits of the sine digital setting signal sin(c) read from ROM 73 are applied to multiplier 5b and the less significant 5 bits are applied to multiplier 5c. Multiplier 3b multiplies the reference clock signal sin(2πf c ( by the more significant 5 bits of the cosine digital setting signal cos(c) and applies the multiplication output to an adder 9b. Multiplier 3c multiplies the attenuated reference clock signal by the less significant 5 bits of the cosine digital setting signal cos(c) and applies the multiplication result to adder 9b. Similarly, multiplier 5b multiplies the cosine clock signal cos(2πf c t) by the more significant 5 bits of the sine digital setting signal sin(c) and applies the multiplication result to adder 9b. Multiplier 5c multiplies the attenuated cosine clock signal cos(2πf c t) by the less significant 5 bits of the sine digital setting signal sin(c) and applies the multiplication result to adder 9b. Adder 9b adds the input multiplication results and applies the output clock signal sin(2πf c t+c) through band pass filter 59 to comparator 12. In the circuit shown in Fig. 16, when the maximum amplitude of the reference clock signal is resolved into n bits, that is, 5 bits, the amplitude per bit becomes 1/(2 5 -1) of the maximum amplitude of the reference clock signal. Therefore the minimum resolved amplitude of multiplier 3b is further resolved into 5 bits by attenuator 67 and multiplier 3c. As a result, a pair of multipliers 3b and 3c and attenuator 67 function as a multiplier of 2n bits. As the foregoing, multipliers 3b, 3c, 5b and 5c each comprising 5-bit arrangement reduce a manufacturing cost. Since a sine wave signal and a cosine wave signal, for example, are completely the same signals except that they are out of phase by 1/4 cycle, completely the same effect can be obtained even when the sine wave signal and a cosine wave signal are exchanged in the above-described examples. Sine waves or cosine waves may be multiplied by each other in multipliers 3b, 3c, 5b and 5c. Furthermore, analogue adder 9b can perform not an addition but a subtraction. Fig. 17 is a block diagram showing an embodiment of the present invention. Similar to the above-described circuit shown in Fig. 5, the present invention employs four reference clock signals CK 0 -CK 3 and integration circuit 60 and ROMs 72 and 73 are similarly structured to those of Fig. 16. Crystal oscillator 1b generates a clock signal 4CK having a frequency of 2.5MHz x 4 = 10.0MHz to generate the four reference clock signals CK 0 -CK 3 . The clock signal 4CK is applied to 4-bit shift register 51 to output the reference clock signals CK 0 -CK 3 each out of phase by π/2 one after another as shown in Fig. 6. Assuming that a reference clock signal having a reference phase is CK 0 , the four reference clock signals CK 0 - CK 3 out of phase by π/2, 2π/2, 3π/2, respectively can correspond to a signal repeatedly changing from state 1 → state 0 → state -1 → state 0. Such repeatedly changing signal is a reference signal obtained by analog-converting a digital reference clock signal, each state corresponding to each amplitude value, which is a phase of 0, π/2, 2π/2, 3π/2 of the sine wave signal sin(2πf c f) having the same frequency as that of the reference clock signal. The sine wave signal sin(2πf c t) can be therefore expressed by the four reference clock signals CK 0 -CK 3 and the amplitude values are 0, 1, 0 and -1, respectively. The four reference clock signals CK 0 - CK 3 are applied to a one-clock delayer 55 comprising a register wherein each signal is delayed by one clock. The delay amount corresponds to π/2 in phase, whereby a cosine reference clock signal CK c (= -cos(2πf c t)) is output through one-clock delayer 55. The sine wave reference clock signal sin(2πf c t) and the cosine digital setting signal cos(c) output from ROM 72 are applied to digital multiplier 3a and the cosine clock signal -cos(2πf c t) and the sine digital setting signal sin(c) read from ROM 73 are applied to digital multiplier 5a. The following operation is the same as that of Fig. 5 and therefore no description will be made thereof. While in the above described embodiment, sine and cosine digital setting signals are obtained by ROMS 72 and 73, the signals can be generated by using either one of the ROMs because a sine signal and a cosine signal have an orthogonal phase to each other. In addition, since a sine wave signal and a cosine wave signal are completely the same signals except that they are out of phase by 1/4 cycle, the same result can be obtained by changing the sine wave signal to/from the cosine wave signal in each circuit described above.

Claims: A clock signal generating apparatus for generating a clock signal having an arbitrary frequency in a predetermined frequency range in response to a reference clock signal, comprising: first clock signal generating means (10) for generating said reference clock signal as a first clock signal; second clock signal generating means (55) responsive to the first clock signal for generating a second clock signal having a phase orthogonal to the phase of the first clock signal; an input terminal (62) receiving a setting signal for setting said arbitrary frequency; integrating means (60), responsive to the reference clock signal generated by said first clock signal generating means, for integrating the setting signal supplied to said input terminal; storing means (72, 73) for previously storing sine wave data and cosine wave data, and reading sine wave data and cosine wave data each having an amplitude value related to the integrated setting signal in response to the integrated setting signal supplied by said integrating means; and output clock signal generating means (3a, 5a, 56, 57, 9, 58, 59), responsive to the first clock signal, the second clock signal, and the sine wave data and the cosine wave data read from said storing means, for generating an output clock signal having a phase synchronized with that of said reference clock signal and a frequency set by said setting signal; wherein said first clock signal generating means comprises: clock signal generating means (la) for generating a clock signal having a frequency four times the frequency of said first clock signal, and frequency-dividing means (51) for frequency-dividing the clock signal generated by said clock signal generating means by 4 to output four clock signals, each having a different phase, as four first clock signal components; wherein said second clock signal generating means comprises delaying means (55) for delaying the four first clock signal components and outputting the delayed four first clock signal components as four second clock signal components; and wherein said output clock signal generating means comprises: first multiplying means (3a) for multiplying the four first clock signal components by said cosine wave data read from said storing means, second multiplying means (5a) for multiplying the four second clock signal components by said sine wave data read from said storing means, means (9) for combining the output signals of said first and second multiplying means, a digital-to-analog converter (58) for converting the signal output by said means for combining into an analog signal, and a band pass filter (59) for extracting a fundamental wave component of said analog signal as said output clock signal. A clock signal generating apparatus according to claim 1, wherein said integrating means comprises: temporary storing means (65) for temporarily storing the setting signal supplied to said input terminal (62) in response to said first clock signal and outputting the same to said storing means (72, 73), and adding means (64) for adding the output of said temporary storing means and the setting signal every time said setting signal is applied to said input terminal (62) and temporarily storing the addition in said temporary storing means.

Inventors: HIRAMATSU YONEJIROU, SATOU SHUN-ICHI, HIRAMATSU, YONEJIROU, SATOU, SHUN-ICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: H04N5
IPC codes: H04N5, G11B20, H04N9