Abstract:
A method for producing an oscillating signal comprises: generating an oscillating signal by discharging after charging to a high trigger level and charging after discharging to a low trigger level; and turbo-charging at the initial of a change-over from charging to discharging while resuming a normal charging/discharging thereafter, and vice versa. The present invention makes use of the turbo-charging/discharging for a linear compensation, such that the produced oscillating signal has the features of concurrently eliminating phase noises and jitters as well as maintaining the modulation linearity.

Description:
1. FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and apparatus for producing an oscillating signal, and more particularly to a method and an apparatus for producing an oscillating signal by turbo compensation.  
       2. BACKGROUND OF THE INVENTION  
       [0002]     A common relaxation oscillator, such as a relaxation voltage/current controlled oscillator, is an oscillator circuit as seen in  FIG. 1 . In  FIG. 1 , the relaxation oscillator  100  mainly uses current sources  110 ,  120  to alternate the charge and discharge of a capacitor  130  so as to produce an oscillating signal at a node  140 . That is, the voltage at the node  140  is inputted to the two comparators  150  and  160  while independent reference voltages, namely V H  and V L , are coupled to each of the remaining inputs of the comparators  150 ,  160 . The outputs of each of the two comparators  150 ,  160  are respectively coupled to the inputs of a “Set-Reset type” flip-flop, that is, respectively to the S terminal and R terminal of the SR latch  170 , which enables the SR latch  170  to output a control signal  180  from the Q terminal thereof for controlling the switch-over at node  140  between the power source  110  and the power source  120  and serving to alternately charge and discharge the capacitor  130 .  
         [0003]     As such, if the voltage at the node  140  is higher than the potential of the reference voltage V H , then the node  140  will coupled to the input terminal of the power source  120  for enabling the discharging of the capacitor  130 ; and if the voltage across the node  140  is lower than the potential of the low voltage V L , then the node  140  will coupled to the output terminal of the power source  110  for enabling the charging of the capacitor  130 .  
         [0004]     Please refer to  FIG. 2  for an oscillating signal outputted from the relaxation oscillator  100 . In  FIG. 2 , the voltage of an oscillating signal rises with the time during a charging process, and, as the voltage reaches a high trigger level (V H ), a discharging process will be triggered and the voltage of the oscillating signal starts to drop with the time. However, since each component in the relaxation oscillator  100  has a so-called response time, particularly the comparator  150 ,  160 , therefore the discharging process cannot be activated immediately when the voltage of the oscillating signal reaches the high trigger level (V H ), and the charging continues for a period of time enabling the voltage of the oscillating signal to exceed the high trigger level (V H ).  
         [0005]     Therefore, the waveform of the oscillating signal outputted by the relaxation oscillator  100  will exceed the high trigger level (V H ) and delay for some time before the waveform starts dropping. Similarly, it will drop beyond the low trigger level (V L ) and delay for some time before the waveform starts rising.  
         [0006]     Please refer to  FIG. 3  for an enlarged diagram of a section of the waveform of an oscillating signal produced by a prior-art relaxation oscillator. In  FIG. 3 , the oscillating signal rises with a slope R 1  and has a time delay td after passing the high trigger level (V H ) before reaching its highest peak. Similarly, as the waveform falls from its maximum value at a slope which may or may not equal −R 1 , there is e another time delay t d ′ (which may or may not equal td) before it reaches the high trigger level (V H ) again.  
         [0007]     As the waveform of the oscillating signal with a time delay for the capacitor  130  in the relaxation oscillator  100  can average out some of the noises produced by the output of the comparators  150 ,  160 , the phase noise and jitter of the oscillating signal is reduced. However, the existence of the time delay will make the frequency of the oscillating signal not directly proportional to the current supplied by the current sources  110 ,  120 , which will adversely affect the modulation linearity.  
         [0008]     Therefore, the oscillating signal produced by the relaxation oscillator cannot have a waveform capable of concurrently eliminating the phase noise and jitter and solving the modulation linearity problem.  
         [0009]     In view of the description above, the present invention discloses a method and an apparatus for producing an oscillating signal to effectively overcome the shortcomings of the oscillating signal outputted by a prior-art relaxation oscillator that has phase noises, jitters and modulation linearity problem.  
       SUMMARY OF THE INVENTION  
       [0010]     The primary object of the invention is to provide a method for producing an oscillating signal that comprises: generating an oscillating signal by discharging after charging to a high trigger level and charging after discharging to a low trigger level; and turbo-charging at the initial of a change-over from charging to discharging while resuming a normal discharging thereafter, and vice versa  
         [0011]     In a preferred embodiment of the present invention, the oscillating signal has a waveform with a fixed ratio of the slope of a normal discharge to the slope of an turbo-discharging, and the fixed ratio is determined by the waveform of a simulated oscillating signal during a normal charging and discharging without going through a time delay.  
         [0012]     Another object of the present invention is to provide an apparatus for producing an oscillating signal, which comprises a relaxation oscillator and a charge/discharge current control circuit, wherein the relaxation oscillator outputs an oscillating signal by a alternating charging/discharging and a voltage comparison method and the charge/discharge current control circuit is provided for regulating the current provided to the relaxation oscillator for charging/discharging.  
         [0013]     In a preferred embodiment of the present invention, the relaxation oscillator comprises a latch logic gate, a first comparator, a second comparator, a capacitor, a first current source and a second current source; wherein the latch logic gate, the first comparator, the second comparator separately, each has a first input terminal, a second input terminal and an output terminal.  
         [0014]     The second input terminal of the first comparator is coupled to a high voltage, and the output terminal of the first comparator is coupled to the first input terminal of the latch logic gate. The first input terminal of the second comparator is coupled to a low voltage, and the second input terminal of the second comparator is coupled to a node at the first input terminal of the first comparator, and the output terminal of the second comparator is coupled to the second input terminal of the latch logic gate.  
         [0015]     The capacitor has a first terminal and a second terminal, and the first terminal of the capacitor is coupled to the node and the second terminal of the capacitor is coupled to a ground.  
         [0016]     The first current source and the second current source separately have an input terminal and an output terminal. The input terminal of the first current source is coupled to a reference voltage, and the output terminal of the second current source is coupled to a ground. A signal outputted from the output terminal of the latch logic gate controls the foregoing node to be coupled to the output terminal of the first current source and the input terminal of the second current source.  
         [0017]     The charge/discharge current control circuit comprises a third current source, a fourth current source and an electronic switch. The third current source and the fourth current source separately have an input terminal and an output terminal. The input terminal of the third current source is coupled to a reference voltage, and the output terminal of the third current source is coupled to the output terminal of the first current source. The output terminal of the fourth current source is coupled to a ground, and the input terminal of the fourth current source is coupled to the input terminal of the second current source.  
         [0018]     An electronic switch controls and selects an electric connection of the third current source and the fourth current source according to a signal outputted from the input terminals of the first comparator and the second comparator.  
         [0019]     In summation of the description above, the present invention provides a method and an apparatus for producing an oscillating signal and uses an accelerated charge/discharge compensation method to overcome the shortcomings of a prior-art relaxation oscillator that outputs an oscillating signal having phase noises and jitters and modulation linearity problem. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a schematic circuit diagram of a prior-art relaxation oscillator;  
         [0021]      FIG. 2  is a schematic waveform diagram of a prior-art relaxation oscillator;  
         [0022]      FIG. 3  is an enlarged view of a waveform of an oscillating signal produced by a prior-art oscillator;  
         [0023]      FIG. 4  is a schematic view of an apparatus for producing an oscillating signal according to a preferred embodiment of the present invention;  
         [0024]      FIG. 5  is a schematic view of a section of an oscillating signal outputted by an apparatus for producing an oscillating signal according to the present invention; and  
         [0025]      FIG. 6  is a schematic view of a waveform outputted from an 8 MHz oscillator according to a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]     Since the components inside a prior-art relaxation oscillator has a response time, therefore the waveform of an oscillating signal produced by a prior-art relaxation oscillator will go beyond the high trigger level and the low trigger level.  
         [0027]     If the delay time of an oscillating signal is shortened when the waveform still indicates a time delay and the simulated waveform of the oscillating signal occurs at the ideal path, then the invention will concurrently achieve the effects of eliminating the phase noise and jitter and maintaining good modulation linearity.  
         [0028]     If a larger charging/discharging current is provided at an initial charge/discharge stage of a relaxation oscillator according to the present invention, which accelerates the charge/discharge, the waveform of an ideal simulated oscillating signal follows the normal charge/discharge path after passing through the high trigger level and the low trigger level. After the normal charge/discharge is resumed, some part of the waveform produced by the oscillating signal at the delay time can be maintained.  
         [0029]     Therefore, the present invention can concurrently have the advantages of improving the modulation linearity and eliminating the phase noises and jitters.  
         [0030]     To further illustrate the objectives, functions and technical characteristics of the present invention, we use a preferred embodiment together with the attached drawings for the detailed description for the invention.  
         [0031]     Please refer to  FIG. 4  for the schematic view of an apparatus for producing an oscillating signal (or an oscillator signal generator) according to a preferred embodiment of the present invention. In  FIG. 4 , the oscillator signal generator  400  adopts the architecture of a traditional relaxation oscillator  100 , and adds a charge/discharge control circuit  410  for increasing the charging/discharging current at the initial charge/discharge stage of the relaxation oscillator  100  to achieve the accelerated charging/discharging effect at the initial charge/discharge stage.  
         [0032]     The charge/discharge control circuit comprises an electronic switch  420  and a current sources  430 ,  440 ; wherein the electronic switch  420  is similar to a latch logic gate  170  that uses comparators  150 ,  160  to output signals  157 ,  167  respectively as a basis for determining the output of the control signals  450 ,  460  for controlling the electric connection to the current sources  430 ,  440 .  
         [0033]     If the voltage at the node  140  is higher than the high trigger level (V H ), the latch logic gate  170  will use a control signal  180  to control the node  140  to couple with the input terminal of the current source  120 , while the electronic switch  420  uses a control signal  460  to control the electric connection to the current source  440 . Therefore, if the voltage of the node  140  is higher than the high trigger level (V H ), the parallel connected current sources  430 ,  110  will supply current to the capacitor  130  for the discharge.  
         [0034]     Similarly, if the voltage of the node  140  is lower than the low trigger level (V L ), then the parallel connected current sources  430 ,  110  will simultaneously supply current to the capacitor  130  for the discharge.  
         [0035]     It should be noted that the control of the current sources  430 ,  440  by the electronic switch  420  is based on the signals  157 ,  167  respectively. In other words, the high/low potential of the signal  157  will determine whether or not the current source  430  is electrically connected and the high/low potential or the signal  167  will determine whether or not the current source  440  is electrically connected. Therefore, if the voltage of the node  140  higher than the high trigger level (V H ) drops to the high trigger level (V H ), the electronic switch  420  will stop the electric connection with the current source  440  while the current source  120  continues supplying current to the capacitor  130  for the charging. If the voltage of the node  140  lower than the low trigger level (V L ) rises to the low trigger level (V L ), the electronic switch  420  will stop the electric connection with the current source  430  while the current source  110  continues supplying current to the capacitor for the discharge.  
         [0036]     Therefore, the oscillator signal generator  440  can accelerate the charge/discharge at the initial charging/discharging stage, and resume the normal charge/discharge thereafter.  
         [0037]     Please refer to  FIG. 5  for the schematic view of a section of an oscillating signal outputted from the oscillating signal generator according to the present invention. In  FIG. 5 , if the waveform of an oscillating signal rises with a slope R 1  and its voltage potential is higher than the high trigger level (V H ), then the oscillating signal generator  400  will start accelerating the discharge. However, the oscillating signal generator  400  actually starts discharging at a delay time t d  caused by the response of the components inside the oscillating signal generator  400 . Therefore, the waveform of the oscillating signal will drop with a slope R 2  after a delay time t d .  
         [0038]     If the voltage of the oscillating signal is lower than the high trigger voltage (V H ) after the waveform of the oscillating signal has dropped with a slope R 2  and a time X has passed, the oscillating signal generator  400  will stop accelerating the discharge and switch to the normal discharge. However, the actual time for the oscillating signal generator  400  to stop accelerating the discharge and switch to the normal discharge takes place after a delay time t d ′. Therefore, the waveform of the oscillating signal drops with a slope R 2  and the time X plus the delay time t d ′ have passed before the waveform continues dropping with a slope R 1 ′. In general, t d ′ may not be equal to t d  and R 1 ′ may not be equal to −R 1 . In the following, as an illustrating but not restricting example, t d ′ is assumed to be equal to t d  and R 1 ′ is assumed to be equal to −R 1 .  
         [0039]     In  FIG. 5 , if the ratio of slope R 1  to slope R 2  is fixed, the dotted line of the oscillating signal at the area with a slope −R 1  will be intersected with the oscillating signal at the area with a slop R 1  at Point A of the high trigger level (V H ). Coincidentally, the waveform composed of the oscillating signals at the areas with slopes R 1 , −R 1  and the extended dotted line of the oscillating signal in the area with a slope −R 1  is an ideal path for the waveform of the prior-art oscillating signal without taking the delay time t d  into consideration.  
         [0040]     The frequency of the oscillating signal produced by the oscillating signal generator  400  according to the present invention is equivalent to that of the oscillating signal produced under the condition of having no delay time t d .  
         [0041]     Therefore, the oscillating signal produced according to the present invention still has the waveform produced at the high trigger level (V H ) with a delay time td so as to eliminate phase noises and jitters. The frequency of the oscillating signal produced according to the present invention is not related to the delay time td so as to achieve the best modulation linearity. Further, the result of the frequency of the oscillating signal being irrelevant with the delay time td can further lower the process sensitivity, which is very useful practically.  
         [0042]     The invention can be applied to an 8 MHz oscillator successfully. Please refer to  FIG. 6  for a schematic view of the waveform outputted by an 8 MHz oscillator according to a preferred embodiment of the present invention. In  FIG. 6 , the section of the accelerated oscillating signal can be observed easily.  
         [0043]     In summation of the description above, the present invention provides a method and an apparatus for producing an oscillating signal by accelerating the charge/discharge at the initial charging/discharge stage to achieve the linear compensation effect. Since the oscillating signal produced according to the present invention still has the waveform with a time delay, and its frequency is not related to the time delay, therefore the oscillating signal produced according to the present invention can simultaneously have low phase noises and jitters as well as the best modulation linearity. As a result, the oscillating signal generator of the present invention can further lower the process sensitivity, which is very useful practically.