JP5651266B1 - Optical receiver AGC circuit and optical receiver with AGC circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control an RF output level of an optical receiver to be constant even in a system in which a level difference of an optical signal input to the optical receiver is large. An optical receiver includes a light receiving element, a control circuit, a variable attenuator, and an amplifier. The variable attenuator and the amplifier are cascade-connected in two or more stages, and the first stage variable attenuator does not attenuate the RF signal. It has a through line to pass through, an attenuation line with an attenuator, and a switch that can switch between both lines. The changeover switch is switched between the through line and the attenuation line by the control voltage from the control circuit, and the optical input level is predetermined. When the level is lower than the level, the changeover switch is switched to the through line to pass the RF signal without attenuation, and when the optical input level is higher than the predetermined level, it is switched to the attenuation line with an attenuator. This is a switchable attenuator that can output a signal after attenuation by the attenuation amount of the attenuator of the attenuation line. [Selection] Figure 1

Description

  The present invention relates to an AGC circuit of an optical receiver used in the field of optical communication of a CATV system, and an optical receiver including the AGC circuit, and there is a level fluctuation in an optical signal received by the optical receiver. The level of the RF signal output from the receiver can be controlled (controlled) to be constant (including substantially constant, the same shall apply hereinafter), and any of the CATV system center, relay device, and subscriber terminal side It can be used.

  In the optical transmission system, when the light reception level of the optical receiver changes, the level of the RF signal output from the optical receiver also changes. In the optical receiver, it is desirable that the output level of the RF signal is constant even if the received light level changes.

  Conventionally, Patent Documents 1 and 2 disclose AGC circuits for keeping the output level of an RF signal constant even when the light reception level changes. As shown in FIGS. 1 and 2 of Patent Document 1 and FIGS. 1 and 5 of Patent Document 2, these AGC circuits are all converted from optical input signals by light receiving elements (photoelectric conversion elements). In this method, AGC is applied after the RF signal is amplified by the first stage amplifier.

JP-A-8-56200 JP 2007-31282 A

  Conventional optical receivers have two problems. One is that if the optical input level range of the optical receiver becomes wide, the performance as an optical receiver may not be satisfied, and the other is that the attenuation range of the variable attenuation circuit is widened (the input level of the optical signal) Is attenuated over a wide range).

  The first problem will be described in detail. As the main performance of the optical receiver, there is a so-called CNR (Carrier to Noise Ratio) or composite distortion. Composite distortion includes CSO (Composite Second Order) and CTB (Composite Triple Beat). In general, the CNR worsens when the optical input level of the optical receiver is low, and the distortion worsens when the optical input level is high. These performances are largely determined by the performance of the amplifier in the optical receiver. CNR is greatly influenced by NF (Noise Figure) of the amplifier, and distortion performance represented by CSO and CTB is greatly influenced by OIP2 (Output 2nd order Intercept Point) and OIP3 (Output 3rd order Intercept Point). NF and distortion performance are contradictory, and low noise devices generally have poor distortion performance, and low distortion devices generally have poor noise characteristics.

  In the case of the circuit configuration of Patent Document 1 (see FIG. 7 of the present application), an optical signal input from an optical fiber is converted into an electric signal (RF signal) by a photoelectric conversion element A, and the RF signal is amplified by an amplifier B. The attenuation amount of the variable amplifier D is adjusted by the control voltage from the control circuit C, is attenuated to a predetermined level by the attenuation amount, and is output after being amplified by the amplifier E. The control voltage is set by the control circuit C based on a DC voltage (including a low frequency component) output from the photoelectric conversion element A.

  In the optical receiver of FIG. 7, since the RF signal input from the optical fiber and photoelectrically converted is amplified before AGC, the RF output level increases as the optical input level increases, as described above. Strain performance deteriorates. Therefore, it is desirable that the amplifier B is a low distortion device that can satisfy the distortion performance as an optical receiver even when the optical input level is high. On the other hand, when the optical input level is lowered, the input level of the RF signal to the amplifier B is also lowered, and the CNR is deteriorated as described above. Therefore, it is desirable that the amplifier B is a low noise device that can satisfy the CNR of the optical receiver even when the optical input is low.

  The band of the video signal (video band) of the CATV system generally starts from 70 MHz and includes a wide band up to 2602 MHz when BS / CS-IF right / left rotation is included. Therefore, in the case of the circuit configuration of FIG. 7, the amplifier B is required to have performances such as wide band, low distortion, and low noise. However, devices with such characteristics are rare.

  Other issues will be described in detail. There are two problems with the variable attenuator used in the AGC of the optical receiver. One problem is the amount of attenuation of the variable attenuator. The variable attenuator is a circuit that attenuates the signal level, but the amount (attenuation amount) that can be attenuated is finite, and it cannot be attenuated beyond a certain amount. The other is the problem of the control voltage of the variable attenuator. There are many methods (circuits) for AGC, but a circuit using a PIN diode as a variable attenuator is common. FIG. 9 shows a π-type attenuator using a PIN diode. In the π-type attenuator shown in FIG. 9, when the attenuation between the input and the output is reduced, the high-frequency series resistance of D3 is reduced by flowing a forward DC current flowing through D3, and the forward direction current flowing through D1 and D2 is reduced. By eliminating the direct current, the high-frequency series resistances of D1 and D2 can be increased, and the minimum loss as an attenuator can be achieved. When increasing the amount of attenuation between input and output, the high-frequency series resistance of D3 is increased by preventing forward DC current from flowing through D3, and D1 by flowing forward DC current through D1 and D2. The high-frequency series resistance of D2 can be reduced, and the maximum loss can be achieved as an attenuator. By changing the Vagc voltage in FIG. 9, the forward currents flowing through D1, D2, and D3 can be changed, and the respective high-frequency series resistances can be changed, whereby the attenuation between the input and output can be continuously changed. . The setting of the point A voltage and the determination of the amount of change in the control voltage Vagc, etc. require that the high-frequency series resistance components D1 to D3 have a desired value.

  In the PIN diode, as the forward current decreases as shown in FIG. 8, the rate of change of the high-frequency forward resistance increases. Therefore, the control voltage Vagc must be reduced as the attenuation increases as shown in FIG. Therefore, in the case of a variable attenuator having an attenuation characteristic as shown in FIG. 10 (inclination decreases as the amount of attenuation increases: forward bias decreases), it is difficult to attenuate by 16 dB or more with one variable attenuator.

  Further, since the forward characteristics of the PIN diode itself are non-linear as shown in FIG. 8, it is known that when the forward bias current is small, waveform distortion is likely to occur, and the intermodulation and intermodulation characteristics deteriorate. Yes. When the attenuation amount of the variable attenuator is increased, the forward bias current (control voltage Vagc in the case of FIG. 9) decreases, so that the distortion performance deteriorates. For this reason, the upper limit of the amount of attenuation that can be attenuated by a variable attenuator using a PIN diode is considered to be about several ten dB. Many optical receivers having an AGC function have an AGC characteristic of about 6 dB at an optical input level, that is, about 12 dB in an electric signal. As a desirable performance of JCTEA STD-014-3.0 standardized by the Japan CATV Technology Association, the AGC range is defined as 6 dB (Table 1).

  As shown in Table 1, the 6 dB range varies depending on the RF signal transmission band and the intensity modulation method, and ranges from −6 to +0 dBm, −8 to −2 dBm, −12 to −6 dBm, and −14 to −8 dBm. There are various types. For this reason, it is necessary for optical receiver manufacturers to line up several types of optical receivers equipped with AGC circuits capable of AGC within the above-mentioned range, and manufacturing and management are difficult.

  An object of the present invention is to provide an AGC circuit for an optical receiver capable of controlling the RF output level of the optical receiver to be constant even in an optical system in which the optical input level difference (level fluctuation) of the optical receiver is large. An object of the present invention is to provide an optical receiver including the AGC circuit.

An AGC circuit of an optical receiver according to the present invention is an AGC circuit of an optical receiver that photoelectrically converts an optical signal by a light receiving element of the optical receiver to obtain an RF signal and a DC voltage and adjusts the level of the RF signal by a variable attenuator. In the circuit, the optical receiver includes a light receiving element, one control circuit, a variable attenuator and an amplifier, and the variable attenuator and the amplifier are connected in two or more stages, and the control circuit controls the control voltage based on the DC voltage. The first stage variable attenuator is provided before the first stage amplifier, and the attenuation amount is adjusted by the control signal from the control circuit before the RF signal is input to the first stage amplifier. The level of the RF signal can be adjusted by the attenuation amount after the adjustment, and the second and subsequent variable attenuators are provided in front of the amplifiers of the respective stages, and the control circuit or a control circuit different from the control circuit is provided. Control signal output from Is adjusted more attenuation is, the attenuation after the adjustment, it is possible to adjust the level of the amplified RF signal at the preceding stage of the amplifier, through line first-stage variable attenuator to pass without attenuating the RF signal to switch between the, and the attenuation line equipped with attenuators, a switch capable of switching both lines, and through line attenuation line by the control voltage selector switch is set on the basis of the optical input level from the control circuit can be, when the optical input level is lower than the predetermined level, the changeover switch is connected to the through line is passed without attenuation RF signal, when the optical input level is higher than a predetermined level is switched to the damping line Te, a switchable attenuator that can output attenuates the RF signal attenuation of the attenuator in the attenuation lines, the switches Attenuator the attenuation line of the formula attenuator is adjustable attenuation by the control voltage from the control circuit, variable attenuator after the second stage is not provided with a through-line, be attenuator only attenuation line , One control circuit is shared by the first stage variable attenuator and the second and subsequent variable attenuators, and one control circuit controls the same or different voltage. A voltage can be output, and two or more different control voltages or the same control voltage output from one control circuit, the attenuation amount of the first stage variable attenuator and one or more variable attenuators after the second stage. Attenuation amount can be adjusted individually, and the first-stage variable attenuator changeover switch controls the control signal from an intermediate level (including almost the intermediate level) between the lowest optical input level and the highest optical input level to the optical receiver. Based on low light input level If it is based on a light input level higher than the intermediate level, it is switched to an attenuation line, and the variable attenuator in the next stage is connected to the optical receiver. Reset at the intermediate level (including almost the intermediate level) between the lowest optical input level and the highest optical input level, and the relationship between the optical input level before and after the reset and the control voltage becomes the same (including almost the same relationship) It is.

In the AGC circuit of the optical receiver according to the present invention, in the AGC circuit of the optical receiver, one control circuit is a CPU, and a correspondence between an input level of the monitor signal and an output level corresponding to the input level in the storage circuit of the CPU A table is stored, and an output signal corresponding to the level of the input monitor signal is selected from the table, and two or more variable attenuators are used as control signals for controlling the attenuation amount of the variable attenuator. It is also possible to input to each of the above.

An optical receiver of the present invention a light receiving element capable of photoelectrically converting an optical signal, an AGC circuit capable of adjusting the level of an RF signal photoelectrically converted by the light receiving element, and an RF signal whose level is adjusted by the AGC circuit In the optical receiver including an amplifier, the AGC circuit is the AGC circuit according to claim 1 or 2 , and the RF signal obtained by photoelectrically converting the optical input signal is level-adjusted by the AGC circuit.

The AGC circuit of the optical receiver of the present invention has the following effects.
(1) Since the AGC before amplifying the RF signal in the first-stage amplifier, when the input level of the RF signal is high, it is possible to attenuate the input level variable attenuator before the first-stage amplifier, the input to the first-stage amplifier Generation of RF signal distortion (composite secondary distortion or composite tertiary distortion) due to excess is reduced.
(2) When the input level of the RF signal is low, the RF signal can be input to the first-stage amplifier without being attenuated by passing through the through line of the first-stage variable attenuator, so that CNR can be prevented from being reduced.
(3) Since distortion generation and CNR reduction are improved, an optical receiver with low distortion and low noise can be realized even if the amplifier is not exceptionally wideband, low distortion and low noise.
(4) Since there is no need to use multiple types of optical receivers in the same system, inventory management becomes easy for CATV operators, and there is no need for optical receiver manufacturers to prepare multiple types of optical receivers. Therefore, a small number of types of optical receivers can be mass-produced, and cost can be reduced. In addition, a low noise device can be selected for the amplifier.

When the AGC of the RF signal is performed by a variable attenuator having two or more stages, the following effects can be obtained in addition to the above effects.
The AGC range can be widened by the number of AGC circuits (when there are two AGC circuits, the AGC range is doubled: for example, −14 to −2 dBm, which is a combination of −8 to −2 dBm and −14 to −8 dBm) A wide range of AGC is possible). For this reason, the optical signal input level range of the optical receiver can be doubled as compared with the conventional one without degrading the distortion performance.

The block diagram which showed an example of the AGC circuit of the optical receiver of this invention. The block diagram which showed the other example of the AGC circuit of the optical receiver of this invention. (A), (b) is explanatory drawing of the extraction method of RF signal and DC signal by a light receiving element. (A) is explanatory drawing of the control circuit using CPU, (b) is explanatory drawing of the control circuit using a comparator, (c) is explanatory drawing of the control circuit using an operational amplifier. Explanatory drawing which shows an example of a variable attenuator. Explanatory drawing which shows an example of the relationship between the optical input level and control voltage by the AGC circuit of the optical receiver of this invention. Explanatory drawing of the conventional optical receiver provided with the AGC circuit. The graph which shows the relationship between a control voltage and attenuation amount. Explanatory drawing which shows an example of the variable attenuation circuit using a PIN diode. Explanatory drawing which shows the relationship between the attenuation amount of a variable attenuation circuit, and a control voltage.

(Embodiment 1)
An example of an optical receiver including the AGC circuit of the present invention will be described with reference to the drawings. The AGC circuit of the optical receiver of the present invention includes a variable attenuator, an amplifier, and a control circuit. The variable attenuator and the amplifier have two or more stages. As an example of an optical receiver provided with the AGC circuit of the present invention, the optical receiver shown in FIG. 1 is an example in which a variable attenuator and an amplifier are two stages.

  The optical receiver shown in FIG. 1 includes a light receiving element (photoelectric conversion element) 1, a first stage variable attenuator 2, a first stage amplifier 3, a next stage variable attenuator 4, a next stage amplifier 5, a first stage control circuit 6, and a next stage control circuit 7. Prepare. Among these, the first stage AGC circuit is configured by the first stage variable attenuator 2 and the first stage control circuit 6, and the next stage AGC circuit is configured by the next stage variable attenuator 4 and the next stage control circuit 7.

(Light receiving element)
Various light receiving elements can be used as the light receiving element 1, but the light receiving element 1 shown in FIGS. 3A and 3B is a photodiode as an example. The cathode side of FIGS. 3 (a) In the photodiode is connected to the bias voltage Vcc via a choke coil L, the anode side is grounded via a resistor R and a capacitor C ₁ (ground). RF signals photoelectrically converted by the photodiode is taken out from the cathode side (choke coil L) through a capacitor C _2, it is as DC signal including a low frequency (monitor signal) is taken out from the anode side.

(RF signal, monitor current)
The extraction of the RF signal and the DC signal can also be performed as shown in FIG. Cathode shown in FIG. 3 (b) the photodiode is connected to the bias voltage Vcc via the resistor R, is grounded via a capacitor C _3, the anode is grounded through a choke coil L, it is photoelectrically converted by the photodiode RF signal is taken out from the anode side (the choke coil L) through a capacitor C _4, it is as DC signal including a low frequency (monitor signal) is taken out from the cathode side.

(amplifier)
A general-purpose high-frequency amplifier can be used as the first-stage amplifier 3 and the next-stage amplifier 5 in FIG.

(Control circuit)
A general-purpose AGC control circuit can be used for the first stage control circuit 6 and the next stage control circuit 7 of FIG. Both control circuits 6 and 7 may have the same configuration or different configurations, and the output voltages may be the same or different. Various examples of the first stage control circuit 6 and the next stage control circuit 7 are shown in FIGS.

  The control circuits 6 and 7 shown in FIG. 4A are general-purpose AGC control circuits using a CPU. In the control circuit 6 (7), the input level of the monitor signal and the output bell correspondence table corresponding to the input level are stored in the CPU (storage circuit). An output signal is selected from the table based on the level, set as a control signal, and input to the variable attenuator 2 (4).

  A control circuit 6 (7) shown in FIG. 4B is a general-purpose AGC control circuit using a comparator. In this control circuit 6 (7), the input level of the monitor signal and the reference level preset in the control circuit 6 (7) are compared by a comparator to set an appropriate control signal, and the control signal is output. (Input to the variable attenuator 2 (4)).

  A control circuit 6 (7) shown in FIG. 4C is a general-purpose AGC control circuit using an operational amplifier. In the control circuit 6 (7), the control voltage is set by adjusting the voltage gain of the operational amplifier based on the input level of the monitor signal, and the control voltage can be input to the variable attenuator 2 (4).

(Variable attenuator)
A general-purpose variable attenuator (ATT) can be used for the first stage variable attenuator 2 and the next stage variable attenuator 4 in FIG. A variable range of at least 6 dB is used. Both variable attenuators 2 and 4 may have the same configuration or different configurations.

The variable attenuators 2 and 4 may be any circuit as long as the amount of attenuation can be varied, such as a circuit using a PIN diode or a circuit that switches between the through line SL and the attenuation line AL by a switch. . The circuit shown in FIG. 5 is configured to switch the switches (switch SW ₁ and switch SW ₂ ) when an AGC voltage is input from the control circuit 6 (7). For example, in the case of an optical receiver having a minimum optical input level of −14 dBm and a maximum optical input level of −2 dBm, the switches SW ₁ and SW ₂ are connected to AB and B′-A ′ when less than −8 dBm. AC and C′-A ′ are connected at −8 dBm or more. When the optical input level becomes -8 dBm or more by switching the switches SW ₁ and SW ₂ , the signal level is attenuated by 12 dB because a 12 dB attenuation circuit is connected. As a result, the distortion performance of the amplifiers 3 and 5 is reset and improved.

Similarly, the control voltage of the variable attenuator 4 needs to be reset at less than −8 dBm and at −8 dBm or more. This is because there is a limit to the amount of attenuation with one variable attenuator. The control voltage to the variable attenuator 4 is preferably as shown in FIG. By performing such AGC control, the RF output level of the optical receiver can output a constant value from −14 dBm to −2 dBm, and a wide range of level adjustment can be performed.

(AGC control)
In the optical receiver of FIG. 1, an optical signal input to the light receiving element 1 is converted into an electric signal, and an RF signal and a DC voltage are output. Before the RF signal is input to the first-stage amplifier 3, the AGC voltage is supplied from the first-stage control circuit 6 to the first-stage variable attenuator 2, and the attenuation amount of the first-stage variable attenuator 2 is adjusted. AGC is performed so that is constant. As the control voltage, the DC voltage output from the light receiving element 1 is input to the first stage control circuit 6 and set based on the level of the DC voltage.

  The AGC RF signal is amplified by the first stage amplifier 3 and input to the next stage variable attenuator 4. At this time, the AGC voltage is supplied from the next-stage control circuit 7 to the next-stage variable attenuator 4 to adjust the attenuation amount of the next-stage variable attenuator 4, and the AGC level is set so that the level of the RF signal becomes constant with the attenuation amount. Then, it is sent to the next stage amplifier 5. Also in this case, the DC voltage is input to the first stage control circuit 7 and set based on the level of the monitor voltage.

  The RF signal amplified to a predetermined level by the next stage amplifier 5 is output to a receiving terminal (not shown) ahead.

(Embodiment 2)
An example of an optical receiver including the AGC circuit of the present invention is shown in FIG. The basic configuration of FIG. 2 is the same as that of FIG. 1, and the difference is that the first-stage control circuit 6 and the next-stage control circuit 7 are also used. Also in this case, the DC voltage output from the light receiving element 1 is input to the control circuit 6 (7), and the AGC voltage is set based on the level of the DC voltage. It is sent to the stage variable attenuator 4, and the attenuation amount of each variable attenuator 2, 4 is adjusted.

  For the control circuit 6 (7), a CPU, a microcomputer or the like can be used. Specifically, as shown in FIG. 2, a control circuit 6 (7) using a CPU, a microcomputer or the like is arranged at a DC voltage extraction terminal (monitoring terminal) 8, and the control circuit 6 (7) is variable at the first stage. Both the attenuator 2 and the next-stage variable attenuator 4 can be connected.

  In the above embodiment, AGC is performed by the first-stage variable attenuator 2 provided before the first-stage amplifier 3. However, when two or more AGCs are performed, the AGC is necessarily performed by the first-stage variable attenuator 2 before the first-stage amplifier 3. It is not necessary, and AGC can be performed with any two or more stages of variable attenuators. The AGC in this case can be performed in the same manner as the AGC in the above embodiment.

  In the above-described embodiment, the variable attenuators 2 and 4 are always provided in front of the amplifiers 3 and 5 in each stage. However, the present invention is not limited to this configuration. 2 or more are continuously arranged, and then the amplifier is arranged in one or more stages, or in any stage, two or more amplifiers are arranged in series, and the variable attenuator is arranged in one or two stages before the two or more amplifiers. The structure arrange | positioned above may be sufficient. The AGC in this case can be performed in the same manner as the AGC in the above embodiment.

1 Light receiving element (photoelectric conversion element)
2 First stage variable attenuator 3 First stage amplifier 4 Next stage variable attenuator 5 Next stage amplifier 6 First stage control circuit 7 Next stage control circuit 8 DC voltage extraction terminal (monitoring terminal)
SW ₁ switch SW ₂ switch A Light receiving element (photoelectric conversion element)
B Amplifier C Control circuit D Variable attenuator E Amplifier F Monitor terminal

Claims (3)

In an AGC circuit of an optical receiver that photoelectrically converts an optical signal with a light receiving element of an optical receiver to obtain an RF signal and a DC voltage and adjusts the level of the RF signal with a variable attenuator,
The optical receiver includes a light receiving element, one control circuit, a variable attenuator and an amplifier, and the variable attenuator and the amplifier are cascade-connected in two or more stages.
The control circuit can set a control voltage based on the DC voltage,
The first-stage variable attenuator is provided in front of the first-stage amplifier, and before the RF signal is input to the first-stage amplifier, the attenuation is adjusted by the control signal from the control circuit. The level of the RF signal can be adjusted,
The variable attenuators in the second and subsequent stages are provided in front of the amplifiers in the respective stages, and the attenuation is adjusted by a control signal output from the control circuit or a control circuit different from the control circuit. Can adjust the level of the RF signal amplified by the amplifier in the previous stage,
The first stage of the variable attenuator and through the line to pass without attenuating the RF signal, an attenuation line with an attenuator, a switch capable of switching both lines, the selector switch to the optical input level from the control circuit based can switch through line attenuation line by a control voltage which is set, when the optical input level is lower than the predetermined level is passed without attenuation RF signal the changeover switch is connected to the through line , when the optical input level is higher than a predetermined level by switching on the damping lines, a switchable attenuator that can output attenuates the RF signal attenuation of the attenuator in the attenuation lines,
The attenuator of the attenuation line of the switchable attenuator can adjust the amount of attenuation by a control voltage from a control circuit,
The variable attenuators in the second and subsequent stages are not equipped with through lines, but are only attenuators.
One control circuit is shared for the attenuation adjustment of the first stage variable attenuator and the attenuation adjustment of one or more variable attenuators after the second stage.
One control circuit can output the same or different control voltage,
With two or more different control voltages or the same control voltage output from one control circuit, the attenuation amount of the first stage variable attenuator and the attenuation amount of one or more variable attenuators after the second stage are individually set. Can be adjusted,
When the control signal of the first-stage variable attenuator is based on an optical input level lower than an intermediate level (including almost the intermediate level) between the lowest optical input level and the highest optical input level to the optical receiver. Is connected to the through line and switched to the attenuation line when it is based on a light input level higher than the intermediate level,
In the variable attenuator at the next stage, the control signal is reset at an intermediate level (including almost the intermediate level) between the lowest optical input level and the highest optical input level to the optical receiver. Is the same relationship (including almost the same relationship),
An AGC circuit for an optical receiver. The AGC circuit of the optical receiver according to claim 1,
One control circuit is a CPU, and a correspondence table (table) of the monitor signal input level and the output level corresponding to the input level is stored in the memory circuit of the CPU, and corresponds to the level of the input monitor signal. An output signal is selected from the table and input to each of the two or more variable attenuators as a control signal for controlling the attenuation amount of the variable attenuator.
An AGC circuit for an optical receiver. In an optical receiver comprising a light receiving element capable of photoelectrically converting an optical signal, an AGC circuit capable of adjusting the level of an RF signal photoelectrically converted by the light receiving element, and an amplifier for amplifying the RF signal level-adjusted by the AGC circuit ,
The AGC circuit is the AGC circuit according to claim 1 or claim 2 , and an RF signal obtained by photoelectrically converting an optical input signal is level-adjusted by the AGC circuit.
An optical receiver.

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