JPS63178633A - Optical reception agc equipment - Google Patents

Abstract

PURPOSE:To exactly detect the disconnection of an optical signal when the optical signal is disconnected by switching an APD bias voltage in two stages when a control voltage reaches a prescribed value so as to eliminate almost the fluctuation of the peak detection voltage. CONSTITUTION:A signal of the 2nd output level is outputted from a hysteresis comparator 10 in the range where a photodetection level is high and a multiple factor of an APD (avalanche photo diode) 1 is ydB. When a photodetection level reaches Pb and the control voltage rises up to Vb, the hysteresis comparator 10 outputs a signal of a 1st output level and an APD bias supply circuit 7 outputs a 1st voltage as a bias voltage. Thus, the multiple factor of the APD 1 is changed from ydB to xdB. If the photodetection level is decreased below the limit of the increase in the gain of a variable gain amplifier 3, the photodetection level is reduced and also the peak detection voltage is decreased. Thus, the disconnection of the optical signal is detected by monitoring the peak detection voltage.

Description

Detailed Description of the Invention [Objective of the Invention] (Industrial Application Field) The present invention provides an optical reception system suitable for obtaining electrical signals with little level fluctuation regardless of fluctuations in the optical reception level in optical communication. This is related to AGC equipment.

(Prior art) Conventionally, on the receiving side of optical communication, APD (AV-alanch) is used to convert received optical signals into electrical signals.
e Photo Diode) is used. As an optical receiving AGC device, a device as shown in FIG. 3 is known which uses both APD multiplication factor control and electric AGC. The APDI converts the received optical signal into an electrical signal and sends it to the preamplifier 2. In this preamplifier 2, the electrical signal is amplified to a predetermined value, sent to a variable gain amplifier 3, gain controlled, and output terminal 11.
is output from. The output signal of the variable gain amplifier 3 is taken into a level detector (such as a peak detector) 4, and a signal at a corresponding level is output. The output signal of the level detector 4 is given to error signal amplifiers 5 and 6, which each have a reference voltage 8
, 9. In this case, the range where the received light level is high (
That is, when the output of the level detector 4 is higher than the voltage value of the reference voltage 8), the error signal amplifier 5 operates, the output of the level detector 4 is amplified by the error signal amplifier 5, and the output is used to generate a variable gain. The gain of amplifier 3 is controlled.

That is, the output signal of the error signal amplifier 5 is negatively fed back as the AGC control voltage of the variable gain amplifier 3.

When the received light level decreases to a predetermined level, the limit of amplification by the variable gain amplifier 3 is exceeded, and the output level of the variable gain amplifier 3 decreases. Therefore, the output of the level detector 4 also decreases. The output of the level detector 4 decreases, and the reference voltage 9
When the voltage drops below the voltage value of
Level detector 4 amplified by this error signal amplifier 6
The output is given to the APD bias supply circuit 7. As a result, in a range where the reception level is low, the APD bias supply circuit 7 is controlled based on the output of the level detector 4.1.
The bias voltages of A and PD1 are changed, and the multiplication factor of APD1 is controlled in accordance with the input signal level.

FIG. 4 shows a graph showing the operating characteristics of the optical receiving AGO device. As is clear from this graph, the gain of the variable gain amplifier 3 increases from the region where the light reception level is high until the light reception level reaches Pa (a), even though the control WJ voltage increases from Va ( d)
, in a range where the received light level is smaller than P, the gain does not increase and remains constant (a). On the other hand, the received light level is P
In the case below a, the APDI multiplication factor increases by controlling the bias voltage by the APD bias supply circuit 7 (b
). On the other hand, when the received light level exceeds Pa, the APD bias supply circuit 7 is no longer controlled by the error signal amplifier 6, and the multiplication factor of APDl becomes constant (b). In this way, after the received light level reaches Pa, in the range where the received light level is high, gain control is performed by the variable gain amplifier 3, while in the range where the received light level is low, the API) 1 is controlled by bias control of APDl. As a result of the multiplication factor control, a step occurs in the peak detection voltage detected by the level detector 4 (C).

In other words, when an analog signal is transmitted, the amplitude of the signal output from the output terminal 11 will vary depending on the received light level Pa.
This means that the signal changes significantly after the point where it becomes , and a normal received signal cannot be obtained. Further, when a digital signal is transmitted, even if a subsequent circuit performs identification based on amplitude, the identification level fluctuates, making appropriate identification difficult. In addition, in a range where the light reception level is low, the AP
Since the multiplication factor of Dl is controlled, when the optical signal is interrupted, the bias voltage of APDI becomes the breakdown voltage of APDl, the noise current of APDl is multiplied, and the peak detection voltage becomes It won't be any different than when you were there. Therefore, by monitoring the peak detection voltage,
This causes a problem in that it is not possible to detect a disconnection of the optical signal.

(Problems to be Solved by the Invention) As described above, according to the conventional optical reception AGCgA device, the electrical AGC gA is
Since the APD multiplication factor control and the APD multiplication factor control are performed selectively, a step occurs in the peak detection voltage of the output of the variable gain amplifier, and when an analog signal is transmitted, it is difficult to obtain a normal received signal due to amplitude fluctuation. However, when a digital signal is transmitted, it is necessary to change the discrimination level, making it difficult to obtain the correct signal. Furthermore, in a range where the light reception level is low, the bias voltage of the APD becomes a breakdown voltage, and the noise current is multiplied, so there is a problem that it is not possible to detect the disconnection of the optical signal depending on the peak detection voltage. The present invention was made in view of the shortcomings of the conventional optical reception AGC device, and its purpose is to almost eliminate fluctuations in the peak detection voltage regardless of changes in the reception level within the range in which the AGC operates. Another object of the present invention is to provide an optical receiving AGC device that is capable of accurately detecting a disconnection of an optical signal by reducing the peak detection voltage when the optical signal is disconnected.

[Structure of the Invention] (Means for Solving the Problems) The present invention includes a photoelectric conversion element, a variable gain amplifier that amplifies the electrical signal obtained by the photoelectric conversion element, and an output signal of the variable gain amplifier. Control voltage based on 7! i- amplifier control means for controlling the gain of the variable gain amplifier based on the control voltage; and bias control means for switching between a first voltage and a second voltage based on the control voltage and applying it as a bias voltage to the photoelectric conversion element. An optical reception AGC device is constructed by including the following.

(Function) In the optical reception AGC device having the above configuration, when the control voltage becomes a predetermined value, that is, when the light reception level becomes a predetermined value,
By switching the bias voltage of the APD in two stages, the control voltage of the variable gain amplifier changes accordingly, expanding the range in which gain can be controlled by the variable gain amplifier, and performing gain control.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of one embodiment of the present invention. In this figure, the same components as in FIG. 3 are given the same reference numerals, and their explanations will be omitted.

In this embodiment, the output of the error signal amplifier 5 is provided to a hysteresis comparator 10. The hysteresis comparator 10 detects that when the output of the error signal amplifier 5, that is, the control voltage of the variable gain amplifier 3, transitions from a high level to a low level and reaches Vc (FIG. 2(d)), a second
A signal with an output level of is output to the APD bias supply circuit 7. As a result, the APD bias supply circuit 7
The second voltage is applied to APDI as a bias voltage so that the multiplication factor of DI becomes ydB (FIG. 2(b)).

On the other hand, when the control voltage of the variable gain amplifier 3 transitions from a low level to a high level and reaches ■b (FIG. 2(d)), the hysteresis comparator 10 converts the signal at the first output level to the APD bias level. Output to the supply circuit 7. As a result, the APD bias supply circuit 7 applies the first voltage to APDI as a bias voltage so that the multiplication factor of APDl becomes ydB (FIG. 2(b)). Due to the operation of the APD bias supply circuit 7, the multiplication factor of APDl changes as shown in FIG.
, level detector 4, and error signal amplifier 5 so that the gain of variable gain amplifier 3 compensates for the change in the multiplication factor of APDl (the magnitude is the same) as shown in FIG. (with the direction of change reversed).

The operation of the optical reception AGC device configured as described above will be explained with reference to FIG. First, in a range where the received light level is high, the hysteresis comparator 10 outputs the second signal.
Since a signal with an output level of is outputted, the second voltage is outputted from the APD bias supply circuit 7 as a bias voltage, and the multiplication factor of APDl is VdB. *As the light level decreases, the control voltage of the variable gain amplifier 3 changes as indicated by U in FIG. 1 output level signal, which causes the APD bias supply circuit 7 to output the first output level signal.
outputs the voltage as the bias voltage. As a result, A
The multiplication factor of PDl changes from yda to ydB (Fig. 2(b)), and the control voltage becomes 8 due to the increase in the peak detection voltage by the level detector 4 (Fig. 2(d)), and the variable gain amplifier 3 The gain of is reduced by 1χ-yldB. (Second
Figure (a)). After this, as the received light level decreases, the control voltage changes as shown in D in Figure 2 (d>), and the gain of the variable gain amplifier 3 correspondingly changes as shown in Figure 2 (a).On the other hand, In a range where the received light level is low, the hysteresis comparator 10 outputs a signal at the first output level, so the APD bias supply circuit 7 outputs the first voltage as a bias voltage, and the multiplication factor of APDl is ydB. As the received light level increases, the control voltage of the variable gain amplifier 3 increases to D in FIG. 2(d).
It changes like this. Then, the light reception level is P.

When the control voltage drops to vc, the hysteresis comparator 10 outputs a signal at the second output level,
As a result, the APD bias supply circuit 7 outputs the second voltage as a bias voltage.

As a result, the multiplication factor of APDl changes from ydB to ydB (Fig. 2 (b)), and as the peak detection voltage by the level detector 4 falls, the control voltage becomes Vd (Fig. 2 (d)). The gain of amplifier 3 is I z-y I
dB increase (Fig. 2(a)).

Thereafter, as the received light level increases, the control voltage changes as shown by U in FIG. 2(d), and correspondingly, the gain of the variable gain amplifier 3 changes as shown in FIG. 2(a).

As described above, in this embodiment, when the received light level decreases to P, the multiplication factor of APDl increases and the gain of the variable gain amplifier 3 decreases by the amount of increase, and the curve of the control VAN pressure changes to the Since we move to D in Figure 2 (d>), even if the received light level decreases after that, the gain will be uniformly controlled by the gain control of the variable gain amplifier 3, as before the received light level reached Pb, and vice versa. The reception level increases and P
.

When , the multiplication factor of APDl decreases, the gain of variable gain amplifier 3 increases by the decreasing force, and the control voltage curve shifts to the curve shown in Fig. 2<d), so the subsequent light reception level is Even if the light rises, the received light level is P. As before, the gain is uniformly controlled by the gain control of the variable gain amplifier 3. Therefore, the peak detection voltage has little deviation and changes as shown in FIG. 2(C). Here, the slight slope of the characteristic is due to the compression residual of AGC.

By the way, when the received light level drops beyond the limit of gain increase of the variable gain amplifier 3 (when the characteristic curve in FIG. 2(a) enters a flat region), the APDl
Since the multiplication factor remains constant (FIG. 2(b)), the peak detection voltage decreases as the received light level decreases. Therefore, by monitoring this peak detection voltage (FIG. 2(C)), disconnection of the optical signal can be detected.

As described above, in this embodiment, the level detector 4, the error signal amplifier 5, and the reference voltage 8 obtain the control voltage S- based on the output signal of the variable gain amplifier 3 to control the gain of the variable gain amplifier 3. The hysteresis comparator 10 and the APD bias supply circuit 7 function as an amplifier control means 100 that switches between the first voltage and the second voltage based on the control voltage and provides the bias voltage of the APDl. Play as 200.

Thus, in this embodiment, the APDI multiplication factor is switched with hysteresis, and at the time of this switching, the APDI
The fluctuation of the gain of the variable gain amplifier 3 according to the multiplication factor of
Since they occur with the same magnitude in the opposite direction, there is no step in the peak detection voltage when AGC control is switched, and good compression characteristics are obtained, and the output level remains constant against fluctuations in the received light level.
becomes more or less constant.

By the way, the APD multiplication factor is related to the reception S/N,
It is known that the optimum multiplication factor is low in a range where the light reception level is high, and the optimum multiplication factor is high in a range where the light reception level is low. Therefore, the higher multiplication factor χdB of APDl generated by switching the bias voltage of APDl is set so that the S/N is optimal at the minimum light reception level, and the lower multiplication factor ydB is set to satisfy the required dynamic range of AGC. If this is taken into consideration and the S/N ratio is set to be good in the high light reception level region, it is possible to achieve a nearly optimal S/N ratio in the entire range of light reception levels.

In this embodiment, the hysteresis comparator 10 is used, but any other circuit may be used as long as it has the function of the bias control means 200 described above.

[Effects of the Invention] As explained above, according to the present invention, when the control voltage of the variable gain amplifier becomes a predetermined value, the bias voltage of the photoelectric conversion element is switched in two steps, and the gain of the variable gain amplifier is changed. Since the variable gain amplifier varies in the opposite direction, the range in which the gain can be controlled is expanded, and regardless of changes in the reception level, fluctuations in peak voltage are almost eliminated, and when the optical signal is interrupted, , AGCt
! Since the peak detection voltage decreases due to the stoppage of the function, it is possible to accurately detect the disconnection of the received optical signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of one embodiment of the present invention, and FIG. 2 is a block diagram of an embodiment of the present invention.
Figure 3 is a block diagram of a conventional optical receiving AGC device;
The figure is a diagram showing the characteristics of each part for explaining the operation of FIG. 3. 1...APD 2...Preamplifier death・
・Variable gain amplifier 4...Level detector 5.6...
・Error signal amplifier 7...APD bias supply circuit 8, 9...Reference voltage 10...Hysteresis comparator 100...Amplifier control means 200...Bias control means agent Patent attorney Book 1) Takashi No. 1 Fig. 3 Fig. 9ffiL to Saki d outside - 1 → Fig. 2 Note to Jbo dB) -- Fig. 4

Claims (1)

[Claims] a photoelectric conversion element, a variable gain amplifier that amplifies the electrical signal obtained by the photoelectric conversion element, and an amplifier control means that obtains a control voltage based on the output signal of the variable gain amplifier and controls the gain of the variable gain amplifier. and a bias control means for switching between a first voltage and a second voltage based on the control voltage and applying the same as a bias voltage to the photoelectric conversion element.