Digital optical receiving apparatus

BACKGROUND OF THE INVENTION:

1. Field of the Invention

0001The present invention relates to a digital optical receiving apparatus. In particular, the present invention relates to a digital optical receiving apparatus applicable to a burst signal transmission in a passive optical network (PON), an optical transmission system or an optical EATHERNET communication system.

2. Description of Related Art

0002In general, in the PON optical transmission system, etc., a signal propagating along a transmission path takes in the form of burst. Therefore, an optical receiving apparatus which receives an arbitrary signal pattern is of a DC coupling type. Japanese Patent Application No. Hei 6-217404 (filed on September 12, 1994) discloses an example of a conventional DC coupling type optical receiving apparatus.

0003It is general that, when an optical signal including a constant offset is input to an opto-electric conversion element of an optical receiving apparatus, a current is supplied to the opto-electric conversion element in a direction for removing the offset. Japanese Patent Laid Open Publication No. Hei 5(1993)-227104 discloses an example of a circuit construction of the optical receiving apparatus with which a signal having no offset is output from an opto-electric conversion element.

0004In such conventional optical receiving apparatus, however, there may be a case where an optical signal having a waveform containing a light component having a level which is not completely zero level and a light component having a level not lower than a predetermined level is input. Such offset optical signal waveform whose its low level does not become zero level is produced when the optical extinction ratio of the signal is degraded for such reason that a semiconductor laser as a transmission light source is preliminarily biased in order to reduce a delay of light emission thereof. Emission of light whose optical extinction ratio is degraded is cut off on a transmitter side in time periods other than a burst signal period such that such light emission does not affect other burst signals.

0005When the optical signal waveform having offset due to degradation of extinction ratio is input to an optical receiving apparatus, there may be an error in duty cycle of its waveform. That is, in general, the optical receiving apparatus detects a high level and a low level of the received optical signal and obtains an optical receiving signal waveform using an intermediate level between the high and low levels of the received signal as a reference. However, when the optical receiving signal in low level is not completely in zero level, the above-mentioned reference is deviated. When the duty cycle of the waveform is determined by using the deviated reference, there may a case where the duty cycle detected is large in appearance.

0006In the optical receiving circuit disclosed in Japanese Patent Application No. Hei 6-217404, a positive output signal and an negative phase output signal of an automatic threshold level control (ATC) circuit thereof become unbalanced in level. As a result, an amplitude of the output signal from the ATC circuit is reduced and thus a discriminating value is changed. The change of discriminating value causes the duty cycle of an output signal waveform of a comparator of the ATC cifcuit to be degraded.

0007Further, the degradation of duty cycle causes a data discriminating margin to be substantially degraded particularly when the receiving signal level is low. The degradation of data discriminating margin causes a minimum optical receiving level of the receiving circuit to be degraded.

0008Further, the reduction of the output signal amplitude of the ATC circuit leads to degradation of the minimum optical receiving level and reduction of dynamic range of the receiving circuit. The reduction of dynamic range results in a reduction of a system margin.

0009In addition, it is general, in an optical receiving apparatus handling burst data, that an operation timing for achieving a high speed pull-in is extracted from an information of both a rising edge and a falling edge of the data. Therefore, when the duty cycle varies, a jitter of an extracted clock is increased and the pull-in characteristics is degraded.

SUMMARY OF THE INVENTION:

0010An aim of at least the preferred embodiments of the present invention is to provide a digital optical receiving circuit with which a duty cycle variation of an output waveform of the circuit due to offset of an input signal and a degradation of signal amplitude can be minimized.

0011A digital optical receiving circuit according to the present invention comprises: <ul id="ul0001" list-style="none" compact="compact"><li>opto-electric conversion means for converting an optical signal into an electric signal;</li><li>amplifier means for amplifying the electric signal output from said opto-electric conversion means to a predetermined level and outputting a first signal and a second signal;</li><li>threshold control means for producing a first logical decision signal and a second logical decision signal, the first and second logical decision signals having respective intermediate values between amplitude values of the first and second signals from said amplifier means;</li><li>comparator means for producing a logic signal on the basis of the first and second logical decision signals; and</li><li>reset means for resetting said threshold control means on the basis of the first or second signal.</li></ul>

0012Preferably, the amplifier means comprises a differential type pre-amplifier, the threshold control means comprises an automatic threshold control circuit and the reset means comprises a self-reset circuit for resetting a charge on a capacitance of said automatic threshold control circuit on the basis of the first or second signal.

0013According to the digital optical receiving circuit of the present invention, when the self-reset circuit detects that a level of the first or second signal output from the differential type pre-amplifier becomes equal to or higher than a predetermined level above an optical zero level, the self-reset circuit outputs a reset signal to reset the peak holding capacitor of the automatic threshold control circuit, so that the zero level of the offset optical signal is exactly held to thereby obtain an exact logic signal.

0014Further, in the digital optical receiving circuit of the present invention, the automatic threshold control circuit comprises a first peak holding circuit for holding a peak value of the first signal of positive phase output from the differential type pre-amplifier, a second peak holding circuit for holding a peak value of the second signal of inverted phase output from the differential pre-amplifier, a first adder for producing a first logical decision signal by adding the first signal of positive phase from the differential pre-amplifier and an output signal of the second peak holding circuit and a second adder for producing a second logical decision signal by adding the second signal of inverted phase from the differential type pre-amplifier and an output signal of the first peak holding circuit.

0015The automatic threshold control circuit produces the first logical decision signal from the peak value of the signal from the second peak holding circuit and the first signal and the second logical decision signal from the peak value of the output signal of the first peak holding circuit and the second signal.

0016The self-reset circuit includes a level detector, a reset pulse generator and a reset circuit. The level detector detects at least one of the first and second signals output from the differential type pre-amplifier which is equal to or higher than a predetermined level. Further, the self-reset circuit generates the reset pulse on the basis of the output signal of the level detector. The self-reset circuit outputs the reset signal for resetting the second peak holding circuit in the automatic threshold control circuit in order to hold the zero level of the optical signal when the output signal of the differential type pre-amplifier is equal to or higher than a certain constant level. With the second peak holding circuit being reset, the latter circuit becomes ready for operation. The reset circuit discharges the capacitor of the second peak holding circuit in the automatic threshold control circuit on the basis of the output signal of the reset pulse generator.

0017In a preferred embodiment of the present invention, the digital optical receiving circuit comprises an opto-electric conversion element, a differential type pre-amplifier, a first peak holding circuit, a second peak holding circuit, a first adder, a second adder, an automatic threshold control circuit, a comparator, a level detector, a reset pulse generator and a self-reset circuit.

0018The differential type pre-amplifier amplifies an electric signal from the opto-electric conversion element to a predetermined level and outputting it as a first signal and a second signal. The first peak holding circuit holds a peak value of the first signal from the differential type pre-amplifier. The second peak holding circuit holds a peak value of the second signal from the differential pre-amplifier.

0019The first adder produces a first logical decision signal by adding the first signal from the differential pre-amplifier and an output signal of the second peak holding circuit. The second adder produces a second logical decision signal by adding the second signal from the differential type pre-amplifier and an output signal of the first peak holding circuit. The automatic threshold control circuit is composed of the first and second peak holding circuits and the first and second adders. The comparator produces a logic signal on the basis of the first and second logical decision signals from the automatic threshold control circuit. The level detector determines whether or not at least one of the first and second signals from the differential type pre-amplifier exceeds a predetermined value.

0020The reset pulse generator generates a reset pulse on the basis of an output of the level detector. The self-reset circuit includes a reset circuit for discharging the holding capacitor of the second peak holding circuit by the reset pulse from the reset pulse generator.

0021According to the digital optical receiving circuit of the present invention, the self-reset circuit detects that the first or second signal from the differential type pre-amplifier becomes equal to or higher than a certain constant level. When it is equal to or higher, the self-reset circuit generates the reset signal to reset the peak holding capacitor in the automatic threshold control circuit to thereby hold the zero level of the offset optical signal, resulting in an exact logic signal.

0022In the digital optical receiving circuit of the present invention, the first adder includes a first resistor having one end connected to the output of the second peak holding circuit and a second resistor having one end connected to one of the output terminals of the differential type pre-amplifier. The other ends of the first and second resistors are connected commonly to one of the input terminals of the comparator. The second adder includes a third resistor having one end connected to the output of the first peak holding circuit and a fourth resistor having one end conected to the other output terminal of the differential pre-amplifier. The other ends of the third and fourth resistors are commonly connected to the other input terminal of the comparator.

0023With the above construction of the first and second adders, two signals on each of the first discriminating signal side and the second discriminating signal side are added through the resistors.

BRIEF DESCRIPTION OF THE DRAWINGS:

0024Preferred features of the present invention will now be described, purely by way of example only, with reference to the accompanying drawings, in which:- <ul id="ul0002" list-style="none" compact="compact"><li>Fig. 1 is a block diagram showing an example of the construction of a conventional digital optical receiving circuit;</li><li>Fig. 2 is a timing chart for explaining a basic function of the conventional digital optical receiving circuit;</li><li>Fig. 3 is a timing chart showing an operation of the conventional digital optical receiving circuit;</li><li>Fig. 4 is a block diagram showing an embodiment of a digital optical receiving circuit according to the present invention;</li><li>Fig. 5 is a timing chart showing an operation timing of the digital optical receiving circuit according to the present invention;</li><li>Fig. 6 is a circuit diagram showing an embodiment of a reset circuit in the digital optical receiving circuit according to the present invention; and</li><li>Fig. 7 is a timing chart showing waveforms at various portions of the reset circuit shown in Fig. 6.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

0025In order to facilitate understanding of the present invention, a construction of a conventional digital optical receiving circuit will be described first.

0026In a conventional optical receiving circuit of the DC coupling type shown in Fig. 1, an anode of an opto-electric conversion element 101 is connected to a pre-amplifier 103 and a cathode thereof is connected to a bias source. The amplifier 103 is constructed with a differential amplifier 131 and resistors 132 and 133. An output of the amplifier 103 is connected to an input terminal of an automatic threshold control (ATC) circuit 105. The automatic threshold control circuit 105 is constructed with a first peak holding circuit 151, a second peak holding circuit 152 and resistors 153, 154, 155 and 156. An output of the ATC citcuit 105 is connected to one of input terminals of a comparator 107.

0027An operation of this optical receiving circuit will be described with reference to Figs. 1 to 3.

0028Fig. 2 is a time chart showing a normal operation of the optical receiving circuit and Fig. 3 is a time chart showing an abnormal operation of the optical receiving circuit. In these figures, abscissa is a time axis and (a) shows a burst signal, (b) an output waveform of the pre-amplifier, (c) an output waveform of the peak holding circuit and (d) an output signal waveform of the comparator. First, it is assumed that the optical receiving circuit is in a signal waiting state in which peak holding capacitors of the peak holding circuits 151 and 152 are discharged. In such state, when a optical burst signal having a level in which there is completely no light and a level in which there is an optical level equal to or higher than a predetermined level is input to the opto-electric conversion element 101, the optical signal is converted into an electric signal S_a as shown in Fig. 2(a). This signal S_a is input to the pre-amplifier 103 of transimpedance type. The pre-amplifier 103 composed of the differential amplifier 131 and the resistors 132 and 133 outputs differential signals S_bp and S_bm having positive and negative polarities as shown in Fig. 2(b). The differential signals S_bp and S_bm are input to the ATC circuit 105 composed of the peak holding circuits 151 and 152 and the adders composed of the resistors 152 to 156. The signals S_bp and S_bm are produced such that a discriminating value L_s used in the ATC circuit 105 to discriminate between a logic "1" and a logic "0" is set to an intermediate value between the signals S_dp and S_dm. The output signals S_dp and S_dm of the ATC circuit 105 which are shown in Fig. 2(d) are supplied to the comparator 107. In the comparator 107, these signals are converted into a signal S_e composed of logic values "1" and "0" shown in Fig. 2(e).

0029Now, an operation of the conventional optical receiving circuit when an optical burst signal having a level in which there is completely no light and a level in which there is an optical level equal to or higher than a predetermined level is input to the conventional optical receiving circuit as shown in Fig. 3(a) will be described. This optical signal has a waveform having optical quenching ratio degraded for the reason of preliminary biasing of a semiconductor laser as a transmission light source to reduce delay of light emission of the semiconductor laser. Light emission due to degradation of optical quenching ratio is cut off on a transmitter side in time periods other than a burst signal period such that such light emission does not affect other burst signals.

0030When the signal waveform (Fig. 3(a)) having an offset due to degradation of optical quenching ratio is input to the opto-electric conversion element 101, it is converted into an electric signal S_a which is input to the pre-amplifier 103. In the pre-amplifier 103, a constant bias voltage V_L is added in both positive and negative directions by offset as shown in Fig. 3(b), resulting in the differential signals S_bp and S_bm which have values (V_L + V_S) in both positive and negative directions when the logic "1". When this signal is input to the ATC circuit 105, the peak holding circuit 152 can not hold a level (-V_L) corresponding to the logic "0" of the second signal S_bm of the pre-amplifier 103 exactly. Therefore, the peak holding circuit 152 continues to hold the value in the signal waiting state before the burst signal arrives thereat. The output of the ATC circuit 105 becomes a signal S_dp whose level on the positive phase side varies between (V_L + V_S)/2 and (V_L/2) and a signal S_dm whose level on the inverted phase side varies between (V_s)/2 and (0) as shown in Fig. 3(d) and thus the positive phase output S_dp and the inverted phase output S_dm become unbalanced in level.

0031The comparator 107 outputs the signal S_e having a different duty cycle and composed of logic values "0" and logic values "1". As a result, the amplitude of the output signal (logical decision signal) of the ATC circuit 105 is reduced.

0032In general, when an optical signal having an offset such as shown in Fig. 3(a), a current flows through the opto-electric conversion element in a direction in which the offset is cancelled. However, in the conventional optical receiving circuit, the offset shown in Fig. 3(a) with which the quenching ratio is degraded can not be cancelled as shown in Fig. 3(d) and the positive phase output signal S_dp and the inverted phase signal S_dm of the output of the ATC circuit 105 are unbalanced in level. As a result, the duty cycle of the waveform of the output signal S of the comparator 109 is degraded. Thus, the amplitudes of the output signals S_dp and S_dm of the ATC circuit 105 are reduced and the discriminating value L_S is changed.

0033This degradation of the duty cycle causes the data discriminating margin to be degraded particularly when the receiving signal level is low. With such degraded margin of the data discrimination, the minimum light receiving level of the receiving circuit is degraded. In addition thereto, the conventional optical receiving apparatus handling a burst data generally extracts, in order to achieve the high speed pull-in operation, the operation timing on the basis of information of both the rising edge and the falling edge of the data. Therefore, when the duty cycle is varied, the jitter of the extracted clock may be increased and the pull-in characteristics may be degraded.

0034The reduction of the output signal amplitude leads to a reduction of the minimum light receiving level and a reduction of the dynamic range. The reduction of synamic range leads to a reduction of the system margin.

0035A digital optical receiving circuit of the present invention, which solves the above-mentioned problems, will be described in detail.

0036Fig. 4 shows, in a block form, a construction of an embodiment of the digital optical receiving circuit of the present invention. In order to receive an optical signal having an arbitrary signal pattern, respective components of the digital optical receiving circuit of the present invention are DC-coupled to each other.

0037The digital optical receiving circuit of the present invention is generally constructed with an opto-electric conversion element 1, a differential type pre-amplifier 3, an automatic threshold control (ATC) circuit 5, a comparator 7 and a self-reset circuit 9.

0038In detail, an anode of the opto-electric conversion element 1 is connected to the pre-amplifier 3 and a cathode thereof is connected to a bias source which is not shown. The pre-amplifier 3 is a transimpedance type differential amplifier composed of a differential amplifier 31 and feedback resistors 32 and 33. A positive phase output of the differential amplifier 31 is feedback to an inverted phase input thereof through the feedback resistor 33. On the other hand, an inverted phase output of the differential amplifier 31 is feedback to a positive phase input thereof through the feedback resistor 32. A first and second signals S_bp and S_bm from the pre-amplifier 3 are supplied to respective input terminals of the ATC circuit 5. The first signal is also supplied to the self-reset circuit 9.

0039The ATC circuit 5 is constructed with a first peak holding circuit 51, a second peak holding circuit 52, a first adder circuit 58 composed of resistors 53 and 56 and a second adder circuit 57 composed of resistors 54 and 55. In the ATC circuit 5, a logic decision signal for deciding a logic "1" or "0" can be set to an intermediate value of the amplitudes of the first and second output signals of the pre-amplifier 3. A positive phase output (first logical decision signal) of the ATC circuit 5 is obtained as a sum of the positive phase output of the pre-amplifier 3 and an output of the second peak holding circuit 52 which holds a peak value of the inverted phase output of the pre-amplifier 3. An inverted phase output of the ATC circuit 5 is obtained as a sum of the inverted phase output of the pre-amplifier 3 and an output of the first peak holding circuit 51 which holds a peak value of the positive phase output of the pre-amplifier 3.

0040The respective outputs of the ATC circuit 5 are supplied to respective input terminals of the comparator circuit 7.

0041The self-reset circuit 9 is constructed with a level detector 91 for detecting a signal in the output of the pre-amplifier 3, a reset pulse generator circuit 92 for generating a single pulse, reset signal in response to the signal detected by the level detector circuit 91 and a reset circuit 93 for discharging a hold capacitor of the second peak holding circuit 52 in response to the reset signal.

0042Now, an operation of the above-mentioned digital optical receiving circuit according to the present invention will be described with reference to Fig. 4 and a timing chart of the operation shown in Figs. 5(a) to 5(g) having abscissa commonly showing time, in which Fig. 5(a) shows a burst signal, Fig. 5(b) shows an output waveform of the pre-amplifier, Fig. 5(c) shows an output waveform of the peak holding circuit and Fig. 5(d) shows an output waveform of the comparator.

0043First, it is assumed that the burst signal shown in Fig. 5(a) is input to the digital optical receiving circuit which is in a waiting state with the holding capacitors of the peak holding circuits being discharged. Further, it should be remembered that the quenching ratio of the burst signal is degraded for the reasons of such as preliminary biasing of a semiconductor laser as a transmission light source to reduce the delay of light emission of the semiconductor laser. The assumption corresponds to a case where, in order to prevent the light emission with degraded quenching ratio from affecting other burst signals, the light emission is cut off on a transmitter side in other time than a time in which the burst signal exists.

0044When such optical signal S_a is input to the opto-electric conversion element 1, the optical signal is amplified to a predetermined level by the pre-amplifier 3. The first signal S_bp of the pre-amplifier 3 which is of a positive phase has a level (+V_L) as zero level and a level [+(V_L+V_S)] as peak level and the second signal S_bm of the pre-amplifier 3 which is of an inverted phase has a level (-V_L) as zero level and a level [-(V_L+V_S)] as peak level, as shown in Fig. 5(b).

0045These signals S_bp and S_bm are held in the first peak holding circuit 51 and the second peak holding circuit 52 of the ATC circuit 5, respectively, resulting in a signal S_e shown in Fig. 5(e). Incidentally, when the optical receiving circuit is changed in state from the waiting state to a signal receiving state, the holding capacitor of the second peak holding circuit 52 is discharged once. In this case, it is necessary to hold the inverted phase voltage (-V_L) of the pre-amplifier 3 which corresponds to the zero level of the signal waveform but has a different value. In order to hold such inverted phase voltage, the holding capacitor of the second peak holding circuit 52 of the ATC circuit 5 must be discharged.

0046The reset signal S_d which is used to discharge the holding capacitor of the second peak holding circuit 52 is generated in a manner described below. The output signal S_c of the level detector 91 becomes logic "1" simultaneously with the reception of the signal S_a, as shown in Fig. 5(c). This signal S_c is supplied to the reset pulse generator 92 from which a single pulse R_d shown in Fig. 5(d) is obtained. When the single pulse R_d is input to the reset circuit 93, the latter circuit causes the holding capacitor of the second peak holding circuit 52 to be discharged. Thus, an output signal S_ep of the first peak holding circuit 51 is continuously held at the peak level of the signal S_bp which is the level +(V_L+V_S) and an output signal S_em of the second peak holding circuit 52 is held at the zero level (-V_L) of the signal S_bm.

0047The peak hbld values S_ep and S_em and the positive and inverted phase signals S_bp and S_bm from the pre-amplifier 3 are added in the first adder 58 and the second adder 57, respectively. As a result, logical decision signals S_fp and S_fm shown in Fig. 5(f) are produced as the output voltages of the ATC circuit 5. When the logical decision signals S_fp and S_fm are the same in amplitude, the discriminating value L_s becomes a half value of the signal amplitude. The logical decision signals S_fp and S_fm are compared with the discriminating value by the comparator 7, resulting in a normalized output signal S_g which takes logic "1" or "0" level as shown in Fig. 5(g).

0048Returning to Fig. 4, constructions and operations of the self-reset circuit 9 for generating the reset signal and the peak holding circuit 52 will be described with reference to Fig. 6 which shows embodiments of these circuits and Fig. 7 which shows signal waveforms at various portions thereof.

0049As mentioned previously, the self-reset circuit 9 is constructed with the level detector 91, the reset pulse generator 92 and the reset circuit 93. The level detector 91 is composed of a comparator 201 and an RS flip-flop 202. The output signal of the pre-amplifier 31 shown in Fig. 4 is input to a plus (+) terminal of the comparator 201 and a reference voltage V₁ is supplied to a minus (-) terminal thereof.

0050Assuming that the pre-amplifier 31 outputs a signal S_bp having an offset such as shown in Fig. 7, the comparator 201 outputs a signal waveform S1. The latter signal is supplied to one input of the RS flip-flop 202 and a reset signal S2 is supplied to the other input of the flip-flop 202. The flip-flop 202 outputs a signal waveform S3.

0051The signal S3 is supplied to the reset pulse generator 92 which composed of a delay circuit 203 and an Exclusive-OR circuit 204. The signal S3 is directly supplied to one input of the Exclusive-OR gate 204 and indirectly to the other input thereof through the delay circuit 203 as a signal S4 which is delayed from the signal S3 by a constant time. In response to the signals S3 and S4, the Exclusive-OR gate 204 outputs a pulse signal S5.

0052The pulse signals S2 and S5 are ORed by an OR circuit 205 of the reset circuit 93, resulting in a signal S6 which is supplied to a base of a transistor 206 of the reset circuit 93. An output of the transistor 206, that is, an output of the reset circuit 93, is supplied to the peak holding circuit 52 and held in a capacitor 210 thereof by charging the latter with the signal S_bm from the pre-amplifier 3. As shown in Fig. 6, the peak holding circuit 52 is composed of a comparator 207 and the capacitor 210, etc., and the charge of the capacitor 210 is discharged in response to the reset signal S6. Of course, the self-reset circuit 9 and the peak holding circuit 52 may be constructed in any of other known circuit constructions, respectively.

0053According to this embodiment, the peak holding circuits hold the level of logic "1" or "0" of the signal exactly. Therefore, the degradation of duty cycle of the output waveform and the reduction of the output signal amplitude due to offset of the input signal and/or the pre-amplifier are minimized. Further, there are no degradation of system margin, no increase of jitter of the extracted clock and no degradation of the pull-in characteristics. Further, since the reduction of the output signal amplitude is minimized, there is substantially no degradation of the minimum receiving level and no reduction of the dynamic range in which optical signals are receivable.

0054As described hereinbefore, according to the digital optical receiving circuit of the present invention, in which the peak hold circuits can hold the level of logic "1" and "0" of the signal exactly, the degradation of duty cycle of the output waveform and the reduction of the output signal amplitude due to offset of the input signal and/or the pre-amplifier are minimized. Further, there are no degradation of system margin, no increase of jitter of the extracted clock and no degradation of the pull-in characteristics.

0055Further, since the reduction of the output signal amplitude is minimized, there is substantially no degradation of the minimum receiving level and no reduction of the dynamic range in which optical signals are receivable. Further, the peak value of the signal from the pre-amplifier is reliably held in the second peak holding circuit of the automatic threshold control circuit and can reliably discharge the holding capacitor of the second peak holding circuit, resulting in an exact peak holding characteristics.

0056Since the reduction of the output signal amplitude, there is neither the degradation of minimum receiving level nor the reduction of the dynamic range.

0057While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the present invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.

0058Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

0059The text of the abstract filed herewith is repeated here as part of the specification.

0060An output signal of an opto-electric conversion element 1 is amplified by a differential type pre-amplifier 3 to a predetermined level and input to an ATC circuit 5 as a positive phase signal and an inverted phase signal. These signals are held in respective peak holding circuits 51 and 52 of the ATC circuit. The positive phase signal held in the peak holding circuit 51 is added by an adder 57 to the inverted phase signal from the pre-amplifier 3 and the inverted phase signal held in the peak holding circuit 52 is added by an adder 58 to the positive phase signal from the pre-amplifier 3. Resultant sum signals are compared by a comparator with a reference and discriminated as logical "1" and "0". The positive phase signal from the pre-amplifier is input to a self-reset circuit 9 including a level detector 91, a reset pulse generator and a reset circuit 93. The level detector 91 detects a signal from the pre-amplifier 3. The reset pulse generator 92 generates a reset signal in the form of a single pulse by using an output of the level detector 91. The reset circuit 93 uses the reset signal to discharge a holding capacitor of the peak holding circuit 52. Degradation of duty cycle and of signal amplitude due to offset of an input signal can be minimized.