Abstract:
A low-pass filter eliminates a high-frequency component contained in an input signal. An inverter outputs a signal at a high level or a low level in response to an output of the low-pass filter that is larger or smaller than a threshold level. A one-shot pulse generating circuit outputs a pulse signal at a point of time when an output level of the inverter is changed. FETs receive the pulse signal output from the one-shot pulse generating circuit, and pulls in forcedly the output of the low-pass filter to the high level or the low level. According to this pulling-in operation, generation of the noise at an output terminal can be prevented.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a noise canceling circuit for canceling a noise that enters into a clock input terminal, or the like, irrespective of a variation in production. 
     FIG. 6  is a circuit diagram showing a configurative example of a noise canceling circuit using an RC filter in the prior art. In  FIG. 6 , reference numerals  1  to  4  are inverters, R 1  is a resistor, and C 1  is a capacitor. Now, when a signal IN containing a noise NZ shown in  FIG. 7A  is input into an input terminal, a signal ND 2  at a connection point between the resistor R 1  and the capacitor C 1  appears as shown in  FIG. 7B  and an output signal ND 3  of the inverter  2  and an output signal OUT of the inverter  4  appear as shown in  FIGS. 7C and 7D , respectively. As apparent from this Figure, if a width of the noise NZ exceeds a predetermined value, such noise cannot be absorbed by an RC filter and the noise appears on the output signal OUT. This can be improved by constructing the inverter  2  as a Schmidt circuit. 
     FIGS. 8A to 8D  are operating waveform diagrams when the Schmidt circuit is used. The output signal ND 2  of the RC filter is given as shown by a thin line in  FIGS. 8B to 8D  in response to the input signal IN containing the noise NZ shown in  FIG. 8A , and the output signal OUT is given as shown by a thick line in the same Figure in response to threshold levels VIL, VIH of the Schmidt circuit. In other words, if the threshold levels VIL is low, the output signal OUT rises later regardless of the noise NZ, as shown in  FIG. 8B . If both the threshold levels VIL, VIH are higher than those in the case of  FIG. 8B , the output signal OUT rises when the signal ND 2  crosses the threshold levels VIL, as shown in  FIG. 8C . In this manner, if the Schmidt circuit is used, the influence of the noise can also be suppressed by the Schmidt circuit. However, if the threshold levels VIL is relatively high and the threshold levels VIH is low, it is possible that the noise responding to the noise NZ appears in the output signal OUT, as shown in  FIG. 8D . 
   Patent Literature 1 discloses the circuit in which the hysteresis input circuit is implemented by the internal circuit without the external circuit and the noise is canceled by this hysteresis input circuit. However, even though the hysteresis characteristic is provided to the input circuit, in some cases the noise cannot be canceled according to the type of the noise. In the circuit disclosed in Patent Literature 2, the hysteresis characteristic is provided to the input circuit and also the delay characteristic is provided to the feedback loop by applying the positive feedback from the output end to the input end. However, this circuit can cancel the narrow noise, but such circuit has such a shortcoming that it cannot cancel the noise whose width is in excess of a predetermined value. 
   Patent Literature 3 discloses the noise canceling circuit in which the input stage is constructed by the Schmidt circuit with the hysteresis characteristic. However, this circuit has such a shortcoming that it does not operate when the input signal does not have a width that is in excess of a predetermined value. 
   Patent Literature 1 
   JP-B-3-30323 
   Patent Literature 2 
   JP-A-59-172826 
   Patent Literature 3 
   JP-B-1-29094 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances and an object of the present invention is to provide a noise canceling circuit capable of canceling a noise without fail in both cases a width of the noise is wide and the width of the noise is narrow, and capable of operating surely when a pulse width of an input signal is narrow. 
   In order to solve the aforesaid object, the invention is characterized by having the following arrangement.
     (1) A noise canceling circuit comprising:   

   a low-pass filter for eliminating a high-frequency component contained in an input signal; 
   an amplifying unit which outputs a signal at either high or low level in response to an output of the low-pass filter that is larger or smaller than a threshold level; 
   a pulse generating circuit for outputting a pulse signal at a point of time when an output level of the amplifying unit is changed; and 
   a pulling-in circuit for receiving the pulse signal output from the pulse generating circuit, and forcibly pulling the output of the low-pass filter in the high level or the low level.
     (2) The noise canceling circuit according to (1), wherein the pulling-in circuit includes a first transistor interposed between the output of the low-pass filter and a terminal for the high level and a second transistor interposed between the output of the low-pass filter and a terminal for the low level, and   

   an output of the pulse generating circuit is supplied to control terminals of the first and second transistors.
     (3) The noise canceling circuit according to (1) or (2), wherein the pulse generating circuit includes a delay circuit for delaying an output of the amplifying unit, an inverting circuit for inverting the output of the amplifying unit, an AND circuit for calculating a logical product between the delay circuit and the inverting circuit, and an OR circuit for calculating a logical sum between the delay circuit and the inverting circuit.   (4) The noise canceling circuit according to any one of (1) to (3), wherein the amplifying unit includes a Schmidt circuit.   

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a configuration of a noise canceling circuit according to an embodiment of the present invention. 
       FIGS. 2A to 2E  are waveform diagrams explaining an operation of the embodiment. 
       FIG. 3  is a circuit diagram showing a particular example of the embodiment shown in  FIG. 1 . 
       FIGS. 4A to 4H  are waveform diagrams explaining an operation of the same example. 
       FIG. 5  is a circuit diagram showing another configurative example of a delay circuit in the same example. 
       FIG. 6  is a circuit diagram showing a configurative example of a noise canceling circuit in the prior art. 
       FIGS. 7A to 7D  are waveform diagrams explaining an operation of the circuit shown in  FIG. 6 . 
       FIG. 8A to 8D  are waveform diagrams explaining an operation when an inverter  2  is constructed by a Schmidt circuit in the circuit shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   An embodiment of the present invention will be explained with reference to the drawings hereinafter.  FIG. 1  is a block diagram showing a configuration of a noise canceling circuit according to an embodiment of the present invention. In  FIG. 1 ,  11  is an input terminal into which an input signal IN is input,  12  is an inverter for inverting the input signal IN to output, and  13  is a low-pass filter for eliminating a high-frequency component of an output of the inverter  12 . An output of this low-pass filter  13  is supplied to a connection point between a drain of a P-channel FET (Field Effect Transistor)  14  and a drain of an N-channel FET  15  and an input end of an inverter  16 . A source of the FET  14  is connected to a power supply voltage and a source of the FET  15  is grounded. An output of the inverter  16  is supplied to an input end of a one-shot pulse generating circuit  17  and is supplied to an output terminal  18 . The one-shot pulse generating circuit  17  generates an “H”-level pulse signal NACC having a predetermined width in response to a leading edge of an output signal (i.e., the signal OUT of the output terminal  18 ) of the inverter  16  and outputs it to a gate of the FET  15 . The one-shot pulse generating circuit  17  generates an “L”-level pulse signal PACC having a predetermined width in response to a trailing edge of the output signal of the inverter  16  and outputs it to a gate of the FET  14 . 
   Next, an operation of the above circuit will be explained with reference to a timing chart shown in  FIG. 2  hereunder. 
   When the input signal IN at the input terminal  11  rises to the “H” level, as shown in  FIG. 2A , the output of the inverter  12  falls and accordingly an output ND 2  of the low-pass filter  13  falls gradually, as shown in  FIG. 2B . Then, when the output ND 2  of the low-pass filter  13  falls to an inversion level of the inverter  16 , the output of the inverter  16 , i.e., the output signal OUT of the output terminal  18  rises to the “H” level, as shown in  FIG. 2C . When the signal OUT rises to the “H” level, the “H”-level pulse signal NACC ( FIG. 2E ) output from the one-shot pulse generating circuit  17  is supplied to the gate of the FET  15 . As a result, the FET  15  is turned ON and thus the output signal ND 2  of the low-pass filter  13  is pulled down forcedly to the “L” level (ground level). At this time, the signal PACC ( FIG. 2D ) is at the “H” level, and the FET  14  is in its OFF state. The signal NACC returns to the “L” level after a predetermined time. Accordingly, the FET  15  is turned OFF, but the “L” level state of the signal ND 2  is still continued. 
   During above operations, even though the noise NZ shown in  FIG. 2A  is contained in the input signal IN, this noise NZ is absorbed by the pulse signal NACC and therefore the noise is in no means generated in the output signal OUT. 
   Then, when the input signal IN falls, the output ND 2  of the low-pass filter  13  rises gradually. Then, when such output ND 2  rises to the inversion level of the inverter  16 , the output signal OUT of the inverter  16  falls to the “L” level, as shown in  FIG. 2C . Then, when such signal OUT falls, the “L”-level pulse signal PACC ( FIG. 2D ) is output from the one-shot pulse generating circuit  17  and supplied to the gate of the FET  14 . Accordingly, the FET  14  is turned ON and the output signal ND 2  of the low-pass filter  13  is pulled up forcedly to the “H” level. 
   Next, a particular example of the above embodiment will be explained with reference to  FIG. 3  hereunder. In  FIG. 3 , the same symbols are affixed to the same portions as respective portions in  FIG. 1 . 
   In the example in  FIG. 3 , the low-pass filter  13  in  FIG. 1  consists of a resistor R 1  and a capacitor C 1 , inverters  21 ,  22  are inserted between the inverter  16  and the output terminal  18 , and the one-shot pulse generating circuit  17  is composed of inverters  24  to  26 , a resistor R 2 , a capacitor C 2 , a NAND gate  27  and a low-active AND gate  28 . In this case, the inverter  24  inverts an output signal ND 3  of the inverter  16  and supplies an inverted signal to a delay circuit consisting of the resistor R 2  and the capacitor C 2 . An output of the delay circuit is supplied to respective first input terminals of the NAND gate  27  and the low-active AND gate  28  via the inverter  26 . 
   The inverter  24 , the resistor R 2 , the capacitor C 2 , and the inverter  26 , mentioned above, constitute the delay circuit. The signal ND 3  is delayed by a predetermined time decided by the resistor R 2  and the capacitor C 2  and then is supplied to respective first input terminals of the NAND gate  27  and the low-active AND gate  28  as a signal ND 3 D. The inverter  25  inverts the signal ND 3 D and supplies the inverted signal to respective second input terminals of the NAND gate  27  and the low-active AND gate  28 . An output of the NAND gate  27  and an output of the low-active AND gate  28  are supplied to the gates of the FETs  14  and  15  as the pulse signals PACC and NACC respectively. 
   Next, an operation of the above circuit will be explained with reference to a timing chart shown in  FIG. 4  hereunder. 
   When the input signal IN of the input terminal  11  rises to the “H” level, as shown in  FIG. 4A , the output ND 2  of the low-pass filter  13  falls gradually, as shown in  FIG. 4B . Then, when the output ND 2  of the low-pass filter  13  falls to the inversion level of the inverter  16 , the output signal ND 3  of the inverter  16  rises to the “H” level, as shown in  FIG. 4C . Then, when the signal ND 3  rises to the “H” level, an output signal ND 3 N of the inverter  25  falls ( FIG. 4D ). The signal ND 3 D of the inverter  26  rises after such signal is delayed for a predetermined time from the leading edge of the signal ND 3  ( FIG. 4(E) ). 
   The output signal NACC of the low-active AND gate  28  ( FIG. 4F ) rises to the “H” level after the signal ND 3 N falls but before the signal ND 3 D rises, and then the output signal NACC returns to the “L” level when the signal ND 3 D rises. In other words, the pulse signal NACC is output from the one-shot pulse generating circuit  17  at the same time when the signal ND 3  rises, and then supplied to the gate of the FET  15 . Accordingly, the FET  15  is turned ON and thus the output signal ND 2  of the low-pass filter  13  is pulled down forcedly to the “L” level (ground level) side. 
   During above operations, even if the noise NZ shown in  FIG. 4A  is contained in the input signal IN, this noise NZ is absorbed by the pulse signal NACC and therefore the noise is in no way generated in the output signal OUT ( FIG. 4H ). Also, even if a noise NZ 1  is generated later, such noise is absorbed by the low-pass filter  13  and thus no noise is generated in the output signal OUT. 
   Then, when the input signal IN falls, the output ND 2  of the low-pass filter  13  rises gradually. Then, when the output ND 2  rises to the inversion level of the inverter  16 , the output signal ND 3  of the inverter  16  falls to the “L” level, as shown in  FIG. 4C . Then, when the signal ND 3  falls, the output signal ND 3 N of the inverter  25  rises ( FIG. 4D ). The output signal ND 3 D of the inverter  26  falls after such signal is delayed for a predetermined time from the trailing edge of the signal ND 3  ( FIG. 4E ). 
   The output signal PACC of the NAND gate  27  ( FIG. 4G ) falls to the “L” level after the signal ND 3 N rises but before the signal ND 3 D falls, and then the signal PACC returns to the “H” level when the signal ND 3 D falls. In other words, the pulse signal PACC is output from the one-shot pulse generating circuit  17  at the same time when the signal ND 3  falls, and then supplied to the gate of the FET  14 . Accordingly, the FET  14  is turned ON and thus the output signal ND 2  of the low-pass filter  13  is pulled up forcedly to the “H” level side. 
   In this case, in the above example, the delay circuit is constructed by the inverter  24 , the resister R 2 , the capacitor C 2 , and the inverter  26 . As shown in  FIG. 5 , the delay circuit may be constructed by a series-connected circuit of inverters  31  to  34  may be constructed in place of this circuit. 
   Bipolar transistors may be employed instead of the FETs  14 ,  15  in the above example. 
   In the circuit in  FIG. 1  and  FIG. 3 , the well-known Schmidt circuit may be employed in place of the inverter  16 . In such case, the circuit configuration is improved such that, even if the larger noise is applied and change in amplitude of the ND 2  is caused more largely, the noise is not transmitted to the ND 3 . 
   As described above, according to the present invention, the noise can be canceled without fail in both cases a width of the noise is wide and the width of the noise is narrow. For example, the noise having a very narrow width like 5 nsec can be canceled in contrast to a clock pulse whose period is 40 μsec. According to the present invention, there can be achieved the advantage that the noise canceling circuit can operate surely when a pulse width of an input signal is narrow.