Abstract:
A receiver for receiving an amplitude shift keying signal includes: a threshold signal generating circuit that has a first diode to which a received amplitude shift keying signal is inputted and generates a threshold signal which practically follows a peak value of the amplitude shift keying signal; an input circuit including a second diode to which a received amplitude shift keying signal is inputted; a first comparator to which the amplitude shift keying signal that is outputted from the input circuit through the second diode is inputted, the first comparator acting with the threshold signal that is outputted from the threshold signal generating circuit through the first diode as a first threshold; a detection circuit for detecting output from the first comparator; and a second comparator to which output from the detection circuit is inputted, the second comparator acting with a second threshold that is a predetermined fixed value.

Description:
BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates to a receiver for receiving an amplitude shift keying signal (ASK signal). 
   (2) Description of the Related Art 
   As a conventional technology for receiving an ASK signal, there has been proposed a technology, in which a clock extract circuit of data carrier for receiving a signal subjected to ASK modulation, that is, a resonance circuit for receiving an ASK signal is connected to a full-wave rectifying circuit, then its output is discriminated with a threshold (i.e. threshold value) near to a peak value by using a comparator, then the discriminated output is provided to charge and discharge circuit, a time constant of which is selected to be about a carrier frequency, and then the output from the charge and discharge circuit is discriminated by a comparator, thereby extracting a clock (for example, see Japanese Patent Application Laid-Open No. H5-327794). 
   Further, as another conventional technology for receiving an ASK signal, there has been proposed a technology, in which a received signal is amplified by an amplifier, then the amplified signal is inputted into a comparator having an adjustable threshold, thereby outputting high-frequency rectangular wave from the comparator (for example, see Japanese Patent Application Laid-Open No. 2001-36591). 
   However, in the former, upon providing the signal to the comparator the threshold is a fixed value, therefore when the level of the signal changes, its way of influencing the threshold changes, thereby causing a problem that the output from the comparator is changed. 
   In the latter, when the inputted signal is weak (for example, several tens of mV), if the frequency of the signal is high (for example, equal to or higher than 100 kHz), it causes a problem of insufficient gain of an amplifier, thereby not enabling amplification with an amplifier having a simple circuit construction, needing high-performance components or a complicated circuit construction, thereby causing a problem of an increase in electric current consumption. 
   SUMMARY OF THE INVENTION 
   It is therefore an objective of the present invention to solve the above problems and to provide a receiver for receiving an ASK signal, which receiver acts excellently even if the ASK signal to be received is weak. 
   In order to attain the above objective, the present invention is to provide a receiver for receiving an amplitude shift keying signal comprising: 
   a threshold signal generating circuit that includes a first diode to which a received amplitude shift keying signal is inputted and generates a threshold signal which practically follows a peak value of the amplitude shift keying signal; 
   an input circuit including a second diode to which a received amplitude shift keying signal is inputted; 
   a first comparator to which the amplitude shift keying signal that is outputted from the input circuit through the second diode is inputted, the first comparator acting with the threshold signal that is outputted from the threshold signal generating circuit through the first diode as a first threshold; 
   a detection circuit for detecting output from the first comparator; and 
   a second comparator to which output from the detection circuit is inputted, the second comparator acting with a second threshold that is a predetermined fixed value. 
   With the construction described above, since the receiver for receiving an ASK signal comprises: a threshold signal generating circuit that includes a first diode to which a received amplitude shift keying signal is inputted and generates a threshold signal which practically follows a peak value of the amplitude shift keying signal; an input circuit including a second diode to which a received amplitude shift keying signal is inputted; a first comparator to which the amplitude shift keying signal that is outputted from the input circuit through the second diode is inputted, the first comparator acting with the threshold signal that is outputted from the threshold signal generating circuit through the first diode as a first threshold; a detection circuit for detecting output from the first comparator; and a second comparator to which output from the detection circuit is inputted, the second comparator acting with a second threshold that is a predetermined fixed value, therefore demodulated output can be obtained with a simple circuit construction of low consumption of electric current even for a weak ASK signal, wherein its output waveform is stable with respect to a change in signal amplitude. 
   Preferably, the threshold signal generating circuit further includes a first resistance and a first capacitor, which are in parallel connected to between the first diode and one of power source electric potentials, and the input circuit further includes a second resistance that is connected to between the second diode and the one of power source electric potentials. 
   With the construction described above, since the threshold signal generating circuit further includes a first resistance and a first capacitor, which are in parallel connected to between the first diode and one of power source electric potentials, and the input circuit further includes a second resistance, which is connected to between the second diode and the one of power source electric potentials, therefore an appropriate received ASK signal and appropriate threshold signal that practically follows the ASK signal can be provided to both of the input terminals of the first comparator. 
   Preferably, the first and second diodes have the same temperature characteristic. 
   With the construction described above, since the first and second diodes have the same temperature characteristic, therefore a demodulated output waveform that is stable with respect to a change in circumferential temperature can be obtained. 
   Preferably, the first and second diodes have the same forward voltage. 
   With the construction described above, since the first and second diodes have the same forward voltage, therefore even a weak ASK signal of the order of several tens of mV can be demodulated. 
   Preferably, a time constant of the threshold signal generating circuit is set so that the threshold signal is applied only on a modulated portion of an amplitude shift keying signal to be received. 
   With the construction described above, since a time constant of the threshold signal generating circuit is set so that the threshold signal is applied only on a modulated portion of an amplitude shift keying signal to be received, therefore a demodulated output waveform without a time delay can be obtained. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram illustrating a preferred embodiment of a receiver for receiving an ASK signal according to the present invention; 
       FIGS. 2A-2E  illustrate signal waveforms on respective sections in the circuit diagram shown in  FIG. 1 , wherein (A) an output waveform on a resonance circuit  1 , (B) an input waveform for both of input terminals on a comparator  4 , (C) an output waveform on the comparator  4 , (D) an output waveform on a detection/smoothing circuit  5 , and (E) an output waveform on the comparator  6 ; 
       FIGS. 3A and 3B  illustrate actions of the receiver for receiving an ASK signal shown in  FIG. 1 , wherein (A) input and output waveforms on a conventional comparator having a fixing-type threshold and (B) input and output waveforms on the comparator  4  in the receiver for receiving an ASK signal shown in  FIG. 1 ; 
       FIGS. 4A and 4B  illustrate actions of the receiver for receiving an ASK signal shown in  FIG. 1 , wherein (A) input and output waveforms on a conventional comparator having a fixing-type threshold and (B) input and output waveforms on the comparator  4  in the receiver for receiving an ASK signal shown in  FIG. 1 ; 
       FIGS. 5A and 5B  illustrate actions of the receiver for receiving an ASK signal shown in  FIG. 1 , wherein (A) input and output waveforms on a conventional comparator having a fixing-type threshold and (B) input and output waveforms on the comparator  4  in the receiver for receiving an ASK signal shown in  FIG. 1 ; 
       FIG. 6  illustrates a method of setting a time constant in a threshold signal generating circuit  2  in the receiver for receiving an ASK signal shown in  FIG. 1 ; 
       FIG. 7  illustrates a method of setting a time constant in a threshold signal generating circuit  2  in the receiver for receiving an ASK signal shown in  FIG. 1 ; 
       FIG. 8  illustrates a method of setting a time constant in a threshold signal generating circuit  2  in the receiver for receiving an ASK signal shown in  FIG. 1 ; and 
       FIG. 9  is a circuit diagram illustrating another preferred embodiment of a receiver for receiving an ASK signal according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, the preferred embodiments of the present invention will be explained with reference to the attached drawings.  FIG. 1  is a circuit diagram illustrating a preferred embodiment of a receiver for receiving an ASK signal according to the present invention. The receiver  1  for receiving an ASK signal includes a resonance circuit  1 , threshold signal generating circuit  2 , input circuit  3 , comparator  4 , detection circuit  5 , comparator  6 , and control circuit  7 . 
   The resonance circuit  1  includes a coil L 1  and capacitor C 1 , which are connected to each other in parallel, and receives a weak ASK signal transmitted from a transmitting side being ASK-modulated with predetermined transmission data. 
   The threshold signal generating circuit  2  includes a diode D 1 , resistance R 1 , and capacitor C 2 , wherein a cathode of the diode D 1  is connected to the resonance circuit  1 , while the resistance R 1  and capacitor C 2  are in parallel connected to between an anode of the diode D 1  and one of power source electric potentials of a +B power source, that is, a plus electric potential of the +B power source. 
   The input circuit  3  includes a diode D 2  and resistance R 2 , wherein a cathode of the diode D 2  is connected to the resonance circuit  1 , while the resistance R 2  is connected to between an anode of the diode D 2  and one of power source electric potentials of a +B power source, that is, a plus electric potential of the +B power source. The diode D 2  is situated within the same package with the diode D 1  of the threshold signal generating circuit  2 , having the same forward voltage and the same temperature characteristic as those of the diode D 1 . 
   The comparator  4  includes an operational amplifier, an inversion input terminal of which is connected to an anode of the diode D 1 , while a non-inversion input terminal of which is connected to an anode of the diode D 2 . When the received ASK signal is not inputted from the resonance circuit  1 , the inversion and non-inversion input terminals of the comparator  4  are set so that a slight offset voltage (for example, 10 mV) is generated between the two terminals. 
   The detection circuit  5  includes a diode D 3 , resistance R 3  and capacitor C 3 , wherein a cathode of the diode D 3  is connected to an output terminal of the comparator  4 , while an anode of the diode D 3  is connected to an input terminal of the comparator  6 , and the resistance R 3  and capacitor C 3  are in parallel connected to between an anode of the diode D 3  and a +B power source. 
   In the following, an action of the receiver for receiving an ASK signal having the construction described above will be explained. 
   When a weak ASK signal that is ASK-modulated as shown in  FIG. 2A  is received, it is provided to the threshold signal generating circuit  2  and to the input circuit  3 . 
   As shown in  FIG. 2B , the input circuit  3  provides an ASK signal S 1 , which is level-shifted by a factor of a voltage drop occurred in the diode D 2 , to the non-inversion input terminal of the comparator  4 . 
   As shown in  FIG. 2B , the threshold signal generating circuit  2  generates a threshold signal Vref 1 , which practically follows a peak value located on the lower side of the ASK signal S 1  that is provided to the non-inversion input terminal of the comparator  4 , then provides the threshold signal Vref 1  to the inversion input terminal of the comparator  4  as a threshold of the comparator  4 . 
   The comparator  4  compares the ASK signal S 1  that is provided to the non-inversion input terminal thereof with the threshold signal Vref 1  that is provided to the inversion input terminal thereof, then outputs a rectangular wave signal as shown in  FIG. 2C . 
   That is, a signal outputted from the comparator  4  is a signal, in which a carrier part of the received ASK signal is transformed to a rectangular wave, and this rectangular wave signal is provided to the detection circuit  5 . The detection circuit  5  detects and smoothes the rectangular wave, then outputs a signal shown in  FIG. 2D  to the comparator  6 . 
   The comparator  6  has a fixed threshold Vref 2  as shown in  FIG. 2D  with respect to its received signal outputted from the detection circuit  5  and outputs a demodulated signal of a rectangular wave as shown in  FIG. 2E  providing it to the control circuit  6  as received data. 
   The control circuit  6  performs a specific control on the basis of the received data provided from the comparator  6 . 
   The receiver for receiving an ASK signal according to the present invention acts as described above. A reason why the threshold signal generating circuit  2  is used to input a threshold to the comparator  4  is that when a fixed threshold is used, there might be a case that an input signal is not applied on the threshold upon a change in a level of the input signal. Therefore, a waveform that follows a peak value on the lower side of the input signal is generated by using the threshold signal generating circuit  2  and the waveform is set as a threshold of the comparator  4  so that the threshold always can follow the input signal even when the level of the input signal changes. 
   That is, for example, with respect to an input waveform of an ASK signal having an appropriate level (i.e. normal voltage amplitude), as shown in  FIG. 3A , a rectangular wave signal is obtained as an output from a comparator having a threshold Vref 0  of a conventional fixing-type, and as shown in  FIG. 3B , a rectangular wave signal is obtained as an output from the comparator  4  having a threshold signal Vref 1  that follows an inputted ASK signal of the present invention. 
   When a waveform of the inputted ASK signal is weak (i.e. when the voltage amplitude being small), as shown in  FIG. 4A , the inputted signal is not applied on the threshold Vref 0  with a conventional comparator, causing not enabling to obtain a rectangular wave signal as an output. On the other hand, as shown in  FIG. 4B , with the comparator  4  of the present invention, since the threshold signal Vref 1  follows an ASK signal, therefore the ASK signal is applied on the threshold signal Vref 1 , thereby obtaining a rectangular wave signal as an output from the comparator  4 . 
   When a waveform of the inputted ASK signal is large (i.e. when the voltage amplitude being large), as shown in  FIG. 5A , a rectangular wave signal can be obtained as an output from a conventional comparator, however, since the rectangular wave is outputted on a condition that the number of the waves is increased from the modulated portion to the modulation-attenuated portion to be attenuated, therefore it becomes difficult to be demodulated properly. However, as shown in  FIG. 5B , in the comparator  4  of the present invention, since the threshold Vref 1  follows so that it is applied on the modulated portion of the ASK signal but not on the modulation-attenuated portion of the ASK signal, therefore an appropriate demodulation can be attained practically without changing the number of the rectangular waves to be outputted. 
   As is described above, in order to carry out an appropriate demodulation such that the threshold Vref 1  follows so that it is applied on the modulated portion of the ASK signal but not on the modulation-attenuated portion of the ASK signal, a time constant in the threshold signal generating circuit  2  is set to be an suitable value in the present invention. In the following, a method of setting such a suitable value will be explained. 
   As an output from the comparator  4 , for example, if only the modulated portion of the ASK signal is shaped into a rectangular wave, when the modulated portion possesses 50% of the signal of one period=100% of the inputted ASK signal, an objective would be to shape this modulated 50% portion into a rectangular wave. However, because of a LC-resonance characteristic of the resonance circuit  1  that receives the ASK signal, even after the modulation is finished, the modulated portion of the ASK signal is gradually attenuated as shown in  FIGS. 6-8 . Here, the attenuated portion except for the modulated portion is called the modulation-attenuated portion. 
   In order to shape only the original modulated portion except for this modulation-attenuated portion into a rectangular wave, the setting of the time constant, which is determined by a resistance value of the resistance R 1  and a capacity value of the capacitor C 1  in the threshold signal generating circuit  2  that determines the threshold of the comparator  4 , is delayed compared to the inputted ASK signal, thereby enabling that the threshold is not applied on the modulation-attenuated portion. 
   Such examples of setting the time constant are shown below: 
   the time constant of the threshold signal generating circuit  2  in FIG.  6 =700 pF (capacity value of C 1 )×200 kΩ (resistance value of R 1 )=0.14 msec; 
   the time constant of the threshold signal generating circuit  2  in FIG.  7 =1000 pF (capacity value of C 1 )×200 kΩ (resistance value of R 1 )=0.2 msec; and 
   the time constant of the threshold signal generating circuit  2  in FIG.  8 =0.04 μF (capacity value of C 1 )×200 kΩ (resistance value of R 1 )=8 msec. 
   In the setting of the time constant in  FIG. 6 , the threshold signal Vref 1  is applied on the first period of the carrier of the modulated portion while it follows the signal avoiding the modulation-attenuated portion practically, therefore enabling to output the rectangular wave having an amount of nearly the objective 50% (being over by 10%). 
   In the setting of the time constant in  FIG. 7 , since the setting of the time constant is short, therefore the threshold signal Vref 1  follows so that it is even applied on the modulation-attenuated portion, thereby outputting a rectangular wave with regard to the modulation-attenuated portion besides to the modulated portion that is necessary originally and exceeding considerably the objective output (exceeding by about 45%). Consequently, the demodulated output obtained from the comparator  6  becomes a waveform having a time delay. 
   In the setting of the time constant in  FIG. 8 , the delay of setting of the time constant is long, therefore a discharge period of time of the capacitor C 1  becomes long. Consequently, when the inputted ASK signal changes from a large signal amplitude to a small signal amplitude, it takes a time for the threshold signal Vref 1  to recover back to the original offset voltage value, thereby causing not enabling to follow the subsequent signal having a small amplitude. That is, it becomes impossible to obtain a demodulated output form the comparator  6 . 
   Accordingly, an appropriate method of setting the time constant of the threshold signal Vref 1  is the setting as shown in  FIG. 6 , in which the threshold signal Vref 1  is applied only on the modulated portion and not on the modulation-attenuated portion, and the threshold signal Vref 1  can be recovered back to the offset voltage value until a modulated waveform of the subsequent period is inputted. 
   The following is a reason why the diodes D 1  and D 2  are connected with regard to signals which are inputted to the inversion input terminal and non-inversion input terminal of the comparator  4 . 
   That is, the diode D 1  used for the threshold signal generating circuit  2  on the side of the threshold input is an element which is necessary to generate a threshold signal that practically follows the peak value of the ASK signal to be received. However, in the diode D 1 , its forward voltage value changes being affected by a change in the circumferential temperature and as a result, the offset voltage of the threshold changes. Because of this change in the offset voltage, the threshold is not applied on the ASK signal inputted to the non-inversion input terminal of the comparator  4 , and a rectangular wave might not be outputted from the comparator  4 . Therefore, a diode D 2  that is packaged in the same package as the diode D 1  and has the same temperature characteristic as the diode D 1  is connected to the non-inversion input terminal of the comparator  4  so that both of the ASK signal provided to both input terminals of the comparator  4  and the threshold signal are changed similarly in response to the circumferential temperature, thereby the offset voltage between both input terminals of the comparator  4  is maintained constant and the circuit is obtained, in which the threshold always can follow the input signal. 
   Since the diode D 2  has the same forward voltage as the diode D 1 , even if the ASK signal inputted to the non-inversion input terminal of the comparator  4  is as weak as several tens of mV, it exceeds the offset voltage that is set to be ten mV between the inversion input terminal and the non-inversion input terminal, therefore an output of a rectangular wave can be obtained from the comparator  4 . 
   That is, if the forward voltage of the diode D 2  is different from that of the diode D 2  or if the diode D 2  itself is not connected to the diode D 1 , the offset voltage increases by a factor of the difference between both values of the forward voltage or a factor of the forward voltage of the diode D 1 , respectively, causing not enabling to demodulate a weak ASK signal of several tens of mV. However, there is no such an undesired possibility according to the present invention. 
   As is explained above, according to the present invention, even when the inputted ASK signal is weak, such a weak ASK signal can be subjected to comparison with a simple construction that the ASK signal is directly provided to the comparator without using an amplifier. 
   In the present invention, with regard to the ASK signal to be inputted to the comparator  4 , a threshold inputting section of the comparator  4  is set to be the threshold signal generating circuit  2  which generates a threshold signal that follows the peak value of the inputted ASK signal. Thereby, since the threshold follows a change in the inputted signal, a stable output of a rectangular wave always can be obtained from the comparator  4 . 
   Further, upon inputting to the comparator  4 , since the diode D 1  that is used for the threshold signal generating circuit  2  on the side of the threshold input is an element that is affected by a change in temperature, therefore the threshold is changed with a change in circumferential temperature, however, in the present invention, the diode D 2  packaged in the same package as the diode D 1  is also connected to the ASK signal-input side thereof, thereby both input signals are changed similarly with respect to temperature and the offset voltage between both input terminals of the comparator  4  can be maintained constant. 
   Therefore, according to the present invention, even when the ASK signal is weak, a demodulated output can be obtained by a simple circuit construction with low consumption of electric current and its output waveform is stable with respect to a change in circumferential temperature and a change in the signal amplitude. 
   The aforementioned preferred embodiments are described to aid in understanding the present invention and variations may be made by one skilled in the art without departing from the spirit and scope of the present invention. 
   For example, as shown in  FIG. 9 , the polarity of the diodes D 1  and D 2  may be set inverse of that shown in  FIG. 1 . In such a case, a direct current voltage source E as a electric source is connected to between the resonance circuit  1  and the ground, the anode of the diode D 1  for the threshold signal generating circuit  2  is connected to the resonance circuit  1  while the cathode is connected to the inversion input terminal of the comparator  4 , the resistance R 1  and the capacitor C 2  are in parallel connected to the cathode of the diode D 1  and the ground (i.e. power source electric potential on the minus side). Further, the anode of the diode D 2  for the input circuit  3  is connected to the resonance circuit  1  while the cathode is connected to the non-inversion input terminal of the comparator  4 , and the resistance R 2  is connected to between the cathode of the diode D 2  and the ground (i.e. power source electric potential on the minus side).