JP4838154B2 - Automatic gain control circuit - Google Patents

Description

    The present invention relates to an automatic gain control circuit for receiving an optical signal, converting it into an electrical signal, and outputting it at a desired signal level.

  An optical receiving circuit that converts a received optical signal into an electric signal, amplifies the electric signal to a predetermined level, and outputs the electric signal includes an automatic gain control circuit for stabilizing the output level. In the case of processing a high-speed signal, an avalanche photodiode (hereinafter abbreviated as APD (Avalanche Photodiode)) using an InGaAs ternary semiconductor compound is used as a light receiving element that converts an optical signal into an electrical signal. For example, a configuration shown in FIG. 5 is known as an automatic gain control circuit of an optical receiving circuit using this APD as a light receiving element. In this figure, 38 is an APD, 39 is a preamplifier, 40 is a variable gain amplifier, 41 is a limiter, 42 is a DC / DC converter, 43 is an amplitude detection circuit, 44 is a comparator, and 45 is a gain control unit. Show.

  The APD 38 for inputting an optical signal shows a case where a predetermined constant voltage is applied as a reverse bias voltage from the DC / DC converter 42. The output signal of the APD 38 is amplified by the preamplifier 39 and amplified by the variable gain amplifier 40. The limiter 41 limits the amplitude to a predetermined amplitude to obtain a reception output signal. The amplitude detection circuit 43 detects the output signal amplitude of the variable gain amplifier 40, compares it with a reference value in the comparator 44, inputs the comparison output signal to the gain control unit 45, and changes the gain control unit 45. The gain of the gain amplifier 40 is controlled so that even if the level of the input optical signal fluctuates, a predetermined output signal amplitude is obtained.

  Further, since the APD 38 as a light receiving element can control a current multiplication factor by a reverse bias voltage, a configuration shown in FIG. 6 is known. In FIG. 5, the same reference numerals as those in FIG. 5 denote the same name parts, and an APD reverse bias voltage control unit 46 is provided to compare the output signal amplitude value by the comparator 44 with the reference value. The reverse bias voltage applied to the APD 38 from the DC / DC converter 42 by controlling the DC / DC converter 42 by the APD reverse bias voltage control unit 46. , And the current multiplication factor of the APD 38 and the amplification gain of the variable gain amplifier 40 are controlled so as to obtain a predetermined output signal amplitude.

  Further, in the configuration shown in FIG. 6, a configuration for controlling the amplification gain of the preamplifier 39 together with the control of the current multiplication factor of the APD 38 and the amplification gain of the variable gain amplifier 40 is provided, and according to the peak detection value of the output signal, There is also a variable gain control circuit that controls the APD 38 and the variable gain amplifier 40, controls the preamplifier 39 in accordance with the DC component detection value of the output signal, and makes the response speed of each control loop differ by selecting the sampling frequency. It is known (see, for example, Patent Document 1).

  In addition, when the current multiplication factor is reduced to a predetermined value or less, the band of the APD is drastically reduced, and a high-speed optical signal cannot be converted into an electrical signal. Therefore, the peak value of the high frequency component in the output electric signal is detected and the reverse bias voltage applied to the APD is controlled, and the current multiplication factor is reduced to a predetermined value or less from the relationship between the reverse bias voltage and the current multiplication factor. An optical receiver circuit that limits the reverse bias voltage so as not to decrease is also known (see, for example, Patent Document 2). A circuit configuration is also known in which the output signal level is made constant by a variable gain amplifier together with the control of the reverse bias voltage of the APD (see, for example, Patent Document 3).

  The control of the reverse bias voltage applied to the APD and the gain control of the variable gain amplifier are performed when the detected output signal level is below a predetermined value, the gain control of the variable gain amplifier is performed, and the output signal level is greater than the predetermined value. There is also known a configuration in which the input optical signal is cut off when the control voltage for controlling the reverse bias voltage of the APD and the gain of the variable gain amplifier becomes larger than a predetermined determination value (for example, (See Patent Document 4). Since the current multiplication factor of the APD changes depending on the temperature, the temperature of the APD is detected, the relationship between the reverse bias voltage and the current multiplication factor with respect to the temperature of the APD is read from the memory, and the reverse bias voltage is controlled according to the relationship. There is also known an optical receiving circuit provided with a control loop for controlling and a control loop for controlling the gain of a variable gain amplifier (see, for example, Patent Document 5).

  In addition, an automatic gain control circuit capable of converting an optical signal into an electric signal by APD and outputting it as an electric signal of a desired level by controlling the APD, the preamplifier, and the variable gain amplifier according to the temperature detection value. Previously proposed (see Patent Document 6). FIG. 7 shows an example of this automatic gain control circuit. For example, 51 is an APD as a light receiving element, 52 is a preamplifier, 53 is a variable gain amplifier, 54 is a limiter, 55 is an amplitude detection circuit, 56, 61 is an A / D converter, 57 is a comparator, 58 is a digital filter, 59 is a register, 60 is a memory (EEPROM), 62 is a temperature detection circuit, 63, 64, 65 and 67 are D / A converters, and 66 is a light A signal input interruption detection circuit 68 is a sampling frequency generation circuit.

  The APD 51 for inputting an optical signal, the preamplifier 52, the variable gain amplifier 53, and the limiter 54 are the same as those shown in FIGS. Further, the control of the reverse bias voltage of the APD 51, the current-voltage conversion gain control of the preamplifier 52, and the amplification gain control of the variable gain amplifier 53 are respectively performed by digital processing, and the D / A converters 63, 64, 65 are respectively performed. Thus, it is configured to convert it into an analog value. The preamplifier 52 can be applied with a transimpedance type amplifier. The transimpedance type amplifier has an advantage of having a wide band characteristic, and various known configurations can be applied.

  The amplitude detection circuit 55 detects the amplitude level of the output signal of the variable gain amplifier 53, converts it to a digital value by the A / D converter 56, and inputs it to the comparator 7. The A / D converter 56 samples the output signal of the amplitude detection circuit 55 in accordance with the sampling signal from the sampling frequency generation circuit 68 and converts it into a digital value. By increasing the sampling frequency in this case, the amplitude is increased. Since the detection speed of the change in the output signal of the detection circuit 55 can be increased, the response speed of the gain control loop can be improved. However, it is preferable to select in consideration of each characteristic. The comparator 57 compares the digital value of the amplitude detection signal from the A / D converter 56 with the reference value read from the register 59, and uses this comparison output as the address of the register 59 via the digital filter 58. The digital filter 58 generally has a low-pass filter characteristic, and can be configured by applying the function of a digital integrating circuit, for example. Therefore, when the comparison output of the same polarity when the amplitude detection signal tends to increase or decrease continues for a predetermined number of times, the address input to the register 59 can be changed.

  Further, the temperature detection circuit 62 detects the temperature of the APD 51 or its surrounding temperature, and the A / D converter 61 converts it to a digital value to obtain an address of the memory 60. The memory 60 shows a case where an electrically erasable programmable memory (EEPROM) is used. In this memory 60, a plurality of types of reverse bias voltages of the temperature-compatible APD 51, a plurality of types of current-voltage conversion gain values (transimpedance) of the preamplifier 52, a plurality of types of amplification gain values of the variable gain amplifier 53, and A gain table including the data of is stored. The temperature-corresponding data read from the gain table is written in the register 59, the reverse bias voltage, the current-voltage conversion gain value, and the amplification gain value are read out using the output signal of the digital filter 58 as an address, and each D / A converter. The analog value is converted by 63, 64, and 65, and the reverse bias voltage applied to the APD 51, the control voltage applied to the preamplifier 52, and the gain control voltage applied to the variable gain amplifier 53 are used.

  The optical signal input interruption detection circuit 66 detects an optical signal input interruption based on an address signal for the register 59. Since the optical signal input interruption corresponds to a decrease in the optical signal level, the reverse bias of the APD 51 is detected. The address signal for the register 59 increases the voltage, the transimpedance of the preamplifier 52, and the amplification gain of the variable gain amplifier 53. The optical signal disconnection is detected by the address signal and the comparison address corresponding to the detected temperature. Since the address signal for the register 59 changes according to the input level of the optical signal and environmental information such as temperature, the address signal is used as a digital output signal for monitoring and converted to an analog value by the D / A converter 67. Thus, the analog signal for monitoring is used.

An APD 51 for inputting an optical signal generally employs an avalanche photodiode of a GaInAs ternary compound, and has characteristics as shown in FIGS. 8A and 8B. (A) of the figure shows the relationship between the reverse bias voltage V _R [V] and the current multiplication factor M using temperature as a parameter. Increasing the reverse bias voltage increases the current multiplication factor M. However, even with the same reverse bias voltage, the current multiplication factor M decreases as the temperature increases. Therefore, when the temperature rises, it is necessary to raise the reverse bias voltage. FIG. 5B shows the relationship between the current multiplication factor M and the cutoff frequency fc [GHz]. When the current multiplication factor M is in the range of about 2 to 10, the range of about 2 to 5 GHz. It becomes possible to operate in the band.

Therefore, the gain table stored in the in the memory 60 in FIG. 7, for example, as shown a part in FIG. 8, the temperature corresponding, a reverse bias voltage [V] for APD51, before current to the preamplifier 52 - The combination of the transimpedance [kΩ] as the voltage conversion gain value and the voltage gain [dB] as the amplification gain control value for the variable gain amplifier 53 is held in the address 1, 2, 3,... The temperature detected by the temperature detection circuit 62 is converted into a digital value by the A / D converter 61 to obtain an address corresponding to the temperature. Therefore, when the detected temperature is, for example, 25 ° C., all data in the 25 ° C. region is written into the register 59. When the temperature changes in the gain control process, all the data in the temperature region of the gain table corresponding to the temperature is read from the memory 60 and set in the register 59. In addition, although the addresses of the respective temperature regions are shown in the case of 1 to 10, the addresses can be set to 1 to 256 , for example, when more finely set.

  If the reverse bias voltage applied to the APD as the light receiving element is a fixed value as shown in FIG. 5 of the conventional example, the control circuit becomes simple, but the light reception output signal is caused by the change in the current multiplication factor of the APD due to temperature fluctuation. The level changes, and the setting of the reverse bias voltage becomes a trade-off between the minimum reception sensitivity and the dynamic range, and there is a problem that it is difficult to optimize to satisfy both. Therefore, as shown in FIG. 6 of the conventional example, the configuration for controlling the reverse bias voltage of the APD can control the current multiplication factor due to temperature fluctuations to some extent, and can control the dynamic range. Although it is easy to enlarge, there is a problem that it is not easy to optimize the distribution ratio between the control for the gain control unit 45 and the control for the APD reverse bias voltage control unit 46, and the high-speed response is not easy.

  In addition, the configuration of the conventional example in which the control speed for controlling the APD, the preamplifier, and the variable gain amplifier is controlled independently by selecting the sampling frequency (see Patent Document 1) There is a problem that adjustment between loops is not easy and a problem that the characteristics of the APD greatly change depending on the temperature. Therefore, a conventional example is proposed in which a memory in which the relationship between the reverse bias voltage of the APD and the current multiplication factor is stored corresponding to the temperature is provided, and the reverse bias voltage corresponding to the detected temperature is applied to the APD (see Patent Document 5). The high frequency component peak value of the output signal of the preamplifier is detected and the reverse bias voltage control is performed to prevent the lowering of the high frequency side of the APD. There is a problem in stability because cooperative control is not performed.

  In addition, the automatic gain control circuit of Patent Document 6 previously proposed enables each part to be stably controlled by stable feedback control corresponding to a temperature change. In the optical signal transmission system, the ON / OFF of the optical signal is made to correspond to the continuous data “1” and “0”, and the optical signal level at the ON time and the optical signal level at the OFF time are determined. In the burst optical signal transmission system for transmitting data in a burst manner, the extinction ratio indicating the difference is controlled so that the optical signal level can be identified on the receiving side. A period in which the optical signal is not transmitted occurs, and an input optical signal is interrupted on the receiving side. By detecting the disconnection of the input optical signal, the amplification gain by the APD, the preamplifier, and the variable gain amplifier is abnormally increased, and only the noise light component is amplified and output. In order to avoid this, Means are known for controlling the current multiplication factor and the amplification factors of the preamplifier and the variable gain amplifier to a minimum value. In that case, the level of the burst optical signal that is input from the state where the input optical signal is interrupted rises from the zero level to a predetermined level to perform steady-state data transmission, but the response characteristic of the feedback control loop is usually In general, it is set so as to guarantee stable operation in a steady state. Therefore, for burst optical signal input, there is a problem that it takes a long time from the rising of the optical signal input until the stable operation is performed, and it is difficult to perform stable reception processing of the burst optical signal.

  The present invention solves the above-mentioned problems, and provides a configuration capable of obtaining high-speed response characteristics and stable operation characteristics for both continuous optical signal input and burst optical signal input. With the goal.

  An automatic gain control circuit of the present invention includes an APD as a light receiving element for inputting an optical signal, a preamplifier, a variable gain amplifier, and an amplitude detection circuit for detecting an output signal amplitude of the variable gain amplifier. In an automatic gain control circuit that controls the APD, preamplifier, and variable gain amplifier based on the detection value of the detection circuit, a current that flows according to the reverse bias voltage applied to the APD and the input optical signal is detected. An APD current detector, an analog switch connected in parallel with the APD current detector to control whether or not to bypass the APD current detector, and the analog switch is turned on by detecting the input of the optical signal; When the optical signal input interruption is detected, the analog switch is turned off, and the current multiplication factor of the APD when the analog switch is turned off becomes a predetermined initial response characteristic. A reverse bias voltage urchin selected, and a APD current detection control circuit for controlling the application of the APD through the APD current detector.

  Also, a gain table in which control data for controlling the reverse bias voltage of the temperature-compatible APD, the gain of the preamplifier, and the gain of the variable gain amplifier is set, and the control data in the temperature-corresponding area of the gain table is read according to the detected temperature. The output amplitude value of the register to be set and the variable gain amplifier is detected, the control data is read from the register so that the output amplitude value becomes a predetermined value, the reverse bias voltage of the APD, and the gain of the preamplifier Control data that controls the gain of the variable gain amplifier, corresponds to the temperature at the time of detection of optical signal input interruption, and is the lowest fixed value that enables the APD current multiplication factor to detect optical signal input. A control structure is provided that reads out from the register and applies a reverse bias voltage according to the control data via the APD current detector.

  The APD current detection control circuit is a gain table based on the detection signal obtained by detecting the current flowing through the APD by the APD current detector corresponding to the reverse bias voltage of the APD and the level of the optical signal when the input of the optical signal is started. To read and control the control data having the initial values of the current multiplication factor of the APD and the gains of the preamplifier and the variable gain amplifier, and the analog switch is turned on.

  The APD current detection control circuit is connected in parallel with the APD current detector after the APD current detector detects the current flowing through the APD due to the reverse bias voltage of the APD at the start of optical signal input, and after a predetermined time has elapsed. An analog switch is controlled to be turned on.

  In the optical signal cut-off state, the control data is read from the register so that the current multiplication factor M of the APD becomes the lowest predetermined value such as M = 2, and the reverse bias that becomes the current multiplication factor M is read. A voltage is applied via the APD current detector, and at the time of burst optical signal input, the current of the APD flowing through the APD current detector is detected, thereby enabling rapid detection of the burst optical signal input. As a result, the APD current detector is bypassed, a reverse bias voltage is applied to the APD, the current multiplication factor of the APD is controlled, and the gains of the preamplifier and the variable gain amplifier correspond to the burst optical signal input level. This makes it possible to perform stable burst optical signal reception processing.

  The automatic gain control circuit according to the present invention will be described with reference to FIG. 1. An APD 1 as a light receiving element for inputting an optical signal, a preamplifier 2, a variable gain amplifier 3, and an output signal of the variable gain amplifier 3 An automatic gain control circuit for controlling the APD 1, the preamplifier 2, and the variable gain amplifier 3 on the basis of the detection value by the amplitude detection circuit 5, which is applied to the APD 1. An APD current detector 19 that detects a current that flows in accordance with the reverse bias voltage and the input optical signal, and is connected in parallel to the APD current detector 19 to control whether or not the APD current detector 19 is bypassed. And the analog switch 21 is turned on by detecting the input of the optical signal, and the analog switch 21 is turned off by detecting the input interruption of the optical signal. An APD current detection control circuit that controls to apply a reverse bias voltage, which is selected so that the current multiplication factor of the APD 1 when the switch 21 is turned off to a predetermined initial response characteristic, to the APD via the APD current detector 19 22.

  FIG. 1 is an explanatory diagram of Embodiment 1 of the present invention, in which 1 is an APD as a light receiving element, 2 is a preamplifier, 3 is a variable gain amplifier, 4 is a limiter, 5 is an amplitude detection circuit, 6, 11, 20 is an A / D converter, 7 is a comparator, 8 is a digital filter, 9 is a register, 10 is a memory (EEPROM) forming a gain table, 12 is a temperature detection circuit, 13, 14, 15 and 17 are D / A A converter, 16 is a comparator for detecting an optical signal input interruption, 18 is a sampling frequency generation circuit, 19 is an APD current detector, 21 is an analog switch, and 22 is an APD current detection control circuit.

  The basic configuration for amplifying the output signal of the APD 1 for inputting the optical signal by the preamplifier 2 and the variable gain amplifier 3 and binarizing the output signal by the limiter 4 is the same as the conventional example shown in FIG. However, in the first embodiment, an APD current detector 19, an A / D converter 20, an analog switch 21, and an APD current detection control circuit 22 are provided, and an analog connected in parallel to the APD current detector 19. When the APD current detection control circuit 22 controls on / off of the switch 21 and the analog switch 21 is turned on, the APD current detector 19 is bypassed, and the APD 1 is connected to the APD 1 via the D / A converter 13 and the analog switch 21. When the reverse bias voltage is applied and the analog switch 21 is turned off, the reverse bias is applied to the APD 1 via the APD current detector 19. Applying a pressure, an operating state for detecting the current flowing in APD1. The current detection value of the APD current detector 19 is converted into a digital signal by the A / D converter 20 and input to the APD current detection control circuit 22.

  The APD current detection control circuit 22 turns off the analog switch 21 when an optical input interruption detection output signal from the optical signal input interruption detection comparator 16 or an optical input interruption detection output signal by a detection means (not shown) is input. The control data is read out from the register 10 so that the current multiplication factor of the APD 1 becomes a predetermined minimum value corresponding to the temperature at that time, and the control data is returned to the APD 1 via the D / A converter 13 and the APD current detector 19. Applied as a bias voltage. If the current multiplication factor of the APD 1 in this case is M, for example, control data with a predetermined minimum value M = 2 is read from the register 9 and used as the reverse bias voltage of the APD 1. This reverse bias voltage is selected to a value that allows the APD current detector 19 to detect the current flowing through the APD 1 when a burst optical signal is input. The gain of the preamplifier 2 and the variable gain amplifier 3 when the optical signal input is cut off is set so that the amplitude can be detected by the amplitude detection circuit 5 when the burst optical signal is input. 9 is controlled in accordance with the control data read from 9.

  By inputting the burst optical signal, a current corresponding to the level of the burst optical signal and the reverse bias voltage flows through the APD 1, the current is detected by the APD current detector 19, and converted into a digital signal by the A / D converter 20. The converted signal is input to the APD current detection control circuit 22. The APD current detection circuit 22 turns on the analog switch 21 immediately after the APD current detection or after a predetermined time by an internal timer or the like, and from the register 9, the amplitude detection circuit 5, the A / D converter 6, and the comparator 7. And the control data is read out by address control through a path through the digital filter 8 and the current multiplication factor of the APD 1 and the gains of the preamplifier 2 and the variable gain amplifier 3 are controlled.

  The control of the reverse bias voltage of the APD 1, the current-voltage conversion gain control of the preamplifier 2, and the amplification gain control of the variable gain amplifier 3 are each performed by digital processing, and the control data read from the register 9 is D / A converter 3, 4, 5 converts the analog value into an analog value, and APD 1 can be an avalanche photodiode formed of, for example, an InGaAs ternary compound semiconductor. The preamplifier 2 may be a transimpedance amplifier, for example. Since this transimpedance amplifier has a wide band characteristic, various configurations are already known, and a desired configuration can be applied. Further, the APD current detector 19 can be constituted by a resistor, and the voltage across the resistor can be detected as a current value. The analog switch 21 can be constituted by a semiconductor switching element such as a transistor.

  Briefly explaining each part, the amplitude detection circuit 5 detects the amplitude level of the output signal of the variable gain amplifier 3, converts it to a digital value by the A / D converter 6, and inputs it to the comparator 7. This A / D converter 6 samples the output signal of the amplitude detection circuit 5 according to the sampling signal from the sampling frequency generation circuit 8 and converts it into a digital value. By increasing the sampling frequency, the gain control loop However, the sampling frequency is selected in consideration of the characteristics of other control loops. The comparator 7 compares the digital value of the amplitude detection signal from the A / D converter 6 with the reference value read from the register 9, and uses the comparison output as the address of the register 9 via the digital filter 8. In general, the digital filter 8 has a low-pass filter characteristic. For example, the digital filter 8 can be configured by applying the function of a digital integrating circuit. In such a configuration, when the amplitude detection signal continues to increase or decrease, the comparison output of the same polarity continues for a predetermined number of times. Therefore, the integrated value changes and the address input to the register 9 is changed. Will change.

  Further, the temperature detection circuit 12 detects the temperature of the APD 1 or its surrounding temperature, converts it into a digital value by the A / D converter 11 and uses it as the address of the gain table formed in the memory 10. The case where an erasable programmable memory (EEPROM) is used is shown. The gain table formed in the memory 10 includes a plurality of types of reverse bias voltages of the temperature-compatible APD 1, a plurality of types of current-voltage conversion gain values (transimpedances) of the preamplifier 2, and a plurality of types of variable gain amplifiers 3. It is a table containing control data with an amplification gain value. The temperature-corresponding data read from the gain table is written into the register 9, and the control data of the reverse bias voltage, the current-voltage conversion gain value, and the amplification gain value are read out using the output signal of the digital filter 8 as an address, respectively. / A converter 13, 14, 15 converts it into an analog value, which is used as a reverse bias voltage applied to APD 1, a control voltage applied to preamplifier 2, and a gain control voltage applied to variable gain amplifier 3.

  The optical signal input interruption detection comparator 16 inputs an optical signal based on a change in the address signal to the register 9 due to a continuous decrease in the sample value of the output signal of the amplitude detection circuit 5 as the optical signal level decreases. The case of the structure which detects disconnection is shown. In addition, it is also possible to apply the structure which detects an optical signal input interruption by another structure, and inputs an optical input interruption detection output signal also into the APD current detection control circuit 22. FIG. By this optical signal input interruption detection signal, the APD current detection control circuit 22 turns off the analog switch 21 and applies a reverse bias voltage corresponding to the temperature at that time from the register 9 and having a fixed APD current multiplication factor. The voltage is applied to the APD 1 via the D / A converter 13 and the APD current detector 19. In this case, the APD current multiplication factor is set to the lowest value that can be detected when an optical signal is input as described above.

  When a burst optical signal is input to the APD 1 in this state, an APD current flows in accordance with an APD current multiplication factor corresponding to the reverse bias voltage at that time. Therefore, the APD current is detected by the APD current detector 19, and A / The signal is converted into a digital signal by the D converter 20 and input to the APD current detection control circuit 22. The APD current detection control circuit 22 controls the analog switch 21 to be turned on immediately or after a predetermined time by a timer or the like, and reads control data from the register 9 according to the address signal input through the digital filter 8 The analog signal is converted by the D / A converters 13, 14, and 15 to obtain the reverse bias voltage of the APD 1, the gain control voltage of the preamplifier 2, and the gain control voltage of the variable gain amplifier 3. That is, when the burst optical signal is input, the control loop of the APD 1 is in an OFF state, and the current multiplication factor of the APD 1 at that time is maintained at a predetermined minimum value at which the optical signal can be detected when the burst optical signal is input. It becomes possible to detect an optical signal quickly. After the optical signal is detected, the control loop of the APD 1 is turned on, so that stable optical signal reception processing can be performed.

FIG. 2 is an explanatory diagram of APD control characteristics, and FIG. 2A is an explanatory diagram of APD characteristics showing the reverse bias voltage and current multiplication factor of APD similar to FIG. axis current multiplication factor M, the horizontal axis represents the reverse bias voltage V _R, the APD current multiplication factor M when the optical signal loss for example, shows a case where the M = 2 for a predetermined minimum value. FIG. 2B shows an example of the relationship between the temperature and the reverse bias voltage when the current multiplication factor M = 2, and the reverse bias voltage data corresponding to the temperature detected by the temperature detection circuit 12 is shown in FIG. By reading out the gain table formed in the memory 10 and setting it in the register 9, the reverse bias voltage corresponding to the temperature at the time of the optical signal interruption is read out from the register 9 as the reverse bias voltage of the APD 1, and the current increase at that time The magnification can be set to a set minimum value M = 2.

  FIG. 3 shows only the main part of the control configuration of the APD. The APD 1, the D / A converter 13 for supplying the reverse bias voltage, and the APD current capable of detecting a current of 1 μA as a resistance of 100 kΩ, for example. A detector 19, an A / D converter 20 for detecting an APD current, and an analog switch 21 composed of a semiconductor switch element such as a transistor are shown. The A / D converter 20 for detecting an APD current includes an APD current composed of a resistor. The voltage across the detector 19 is detected as an APD current and input to the APD current detection control circuit 22 in FIG.

  FIG. 4 shows a main part of an example of a gain table formed in the memory 10 such as an EEPROM, and the reverse bias voltage [V] of the APD 1 and the preamplifier corresponding to the address (gain table address) of the memory 10. 2 shows a combination of a transimpedance [kΩ] as a current-voltage conversion gain of 2 and a voltage gain [dB] as an amplification gain value of the variable gain amplifier 3, and the temperature detected by the temperature detection circuit 12 is expressed as A / The signal is converted into a digital signal by the D converter 11 and used as the address of the memory 10 (gain table address), and the data in the temperature-corresponding region is read and set in the register 9. In this case, if the detected temperature is 25 degrees, the reverse bias voltages 30 to 39 in the range of addresses 1 to 10, the transimpedances 1 to 9, and the voltage gains 1 to 9 are read and set in the register 9. Thereby, for example, when the address through the digital filter 8 is 3, the reverse bias voltage 32, the transimpedance 3, and the voltage gain 3 of the address 3 are read out, respectively, and the D / A converters 13, 14, 15 are read out. Will be entered. In the gain table formed in the memory 10, it is also possible to set the control data corresponding to each address with a larger number of addresses, for example, 1 to 256.

  When the optical signal input interruption is detected, the register 9 is accessed from the APD current detection control circuit 22, and the control data corresponding to the temperature at that time and indicating the reverse bias voltage at which the current multiplication factor M = 2 of the APD 1 is read. Since the analog switch 21 is controlled to be turned off, the APD 1 does not become an excessive current multiplication factor when a burst optical signal is input, and is quickly detected by the APD current detector 19. The switch 21 is turned on immediately or after a predetermined time, and stable reception processing of an optical signal can be performed by a normal control loop. That is, when there is an optimum response time corresponding to the system at the start of optical signal input, it is possible to set the ON control timing of the analog switch 21 correspondingly.

It is explanatory drawing of Example 1 of this invention. It is APD control characteristic explanatory drawing of Example 1 of this invention. It is principal part explanatory drawing of Example 1 of this invention. It is principal part explanatory drawing of the gain table in the memory of Example 1 of this invention. It is explanatory drawing of a prior art example. It is explanatory drawing of a prior art example. It is explanatory drawing of a prior art example. It is APD characteristic explanatory drawing.

Explanation of symbols

1 APD
2 Preamplifier 3 Variable gain amplifier 4 Limiter 5 Amplitude detection circuit 6, 11, 20 A / D converter 7 Comparator 8 Digital filter 9 Register 10 Memory 11 A / D converter 12 Temperature detection circuit 13, 14, 15, 17 D / A converter 16 Optical signal input break detection comparator 18 Sampling frequency generation circuit 19 APD current detector 21 Analog switch 22 APD current detection control circuit

Claims (4)

APD as a light receiving element for inputting an optical signal, a preamplifier for amplifying the output signal of the APD, a variable gain amplifier for amplifying the output signal of the preamplifier , and detecting an output signal amplitude of the variable gain amplifier An automatic gain control circuit for controlling the APD, the preamplifier, and the variable gain amplifier based on a detection value by the amplitude detection circuit.
An APD current detector for detecting a current flowing through the APD in accordance with a reverse bias voltage applied to the APD and an input optical signal;
An analog switch connected in parallel with the APD current detector to control whether to bypass the APD current detector;
The analog switch is controlled to be turned on by detecting the input of the optical signal and turned off by detecting the input interruption, and the current multiplication factor of the APD when the analog switch is controlled to be turned off is selected to have a predetermined initial response characteristic. An automatic gain control circuit comprising: an APD current detection control circuit having a control configuration for applying a reverse bias voltage to the APD via the APD current detector. A gain table in which control data for controlling a reverse bias voltage applied to the temperature-corresponding APD , a gain of the preamplifier, and a gain of the variable gain amplifier is set; a temperature of the APD or a periphery of the APD detecting the temperature, the so a register for setting the control data of the temperature corresponding regions of the gain table is read in accordance with the temperature, the output amplitude value by detecting the output amplitude of the variable gain amplifier becomes a predetermined value The control data is read from the register to control the reverse bias voltage of the APD, the gain of the preamplifier, and the gain of the variable gain amplifier, and corresponds to the temperature at the time of detecting the input interruption of the optical signal. In addition, the control data in which the current multiplication factor of the APD becomes the lowest fixed value that can detect the optical signal input is read from the register, and the reverse bias voltage according to the control data is read. The automatic gain control circuit according to claim 1, wherein through the APD current detector characterized by comprising a control arrangement to be applied to the APD. The APD current detection control circuit, a detection signal of the current flowing corresponding to the reverse bias voltage of the APD due to start of input optical signal and the level of the optical signal to the APD was detected by the APD current detector for said APD From the gain table, the control data with the initial values of the current multiplication factor of the APD and the gains of the preamplifier and the variable gain amplifier are read out , and control according to the control data is performed, and 2. The automatic gain control circuit according to claim 1, further comprising a configuration for controlling the analog switch to be turned on. The APD current detection control circuit, when starting to input the optical signal to the APD, the current flowing through the APD by the reverse bias voltage of the APD, after a predetermined time has elapsed is detected by the APD current detector, the APD current 2. The automatic gain control circuit according to claim 1, further comprising a configuration in which the analog switch connected in parallel with the detector is turned on.

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