Control of linear transimpedance amplifier (TIA) during stabilization after recovery from signal loss in receiver

Abstract

The invention relates to control of a linear transimpedance amplifier (TIA) during stabilization after recovery from signal loss in a receiver. An optical device for an optical signal includes: a photodiode configured to receive the optical signal; and a linear transimpedance amplifier (TIA) having an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage. The optical device further includes an automatic gain control loop configured to rectify an output of the at least one VGA and compare the rectified output to a threshold gain setting to generate an automatic gain control voltage; and detection circuitry configured to detect a rate of change of the automatic gain control voltage and configured to determine a first state indicative of the absence of the optical signal at the photodiode. In response to at least the determined first state, the detection circuitry is configured to disable the output stage of the linear TIA.

Description

Technical Field

[0001] The subject matter of the present disclosure relates generally to optical receivers and, in some non-limiting embodiments, to systems and methods for accurately detecting when a linear transimpedance amplifier (TIA) of an optical receiver has stabilized after a signal recovery event occurs at the input of the TIA (e.g., after a signal loss). Background Art

[0002] For the purposes of this disclosure, consider the fiber optic cable physical interface. Figure 1A As shown in FIG. 1 , data is transmitted from a laser transmitter 100 driven by a laser driver through an optical fiber 102. An optical signal 130 from the optical fiber 102 is received at a receiver 104 through a combination of a photodetector 110 followed by a linear transimpedance amplifier (TIA) 120, the output of which is electrical differential data 140, as shown in FIG. Figure 1B as shown in .

[0003] In a data communication receiver 104 such as a small form-factor pluggable (SFP) module, one requirement is to squelch the output of the receiver 104 when an input optical signal loss event is detected. The signal loss may be caused by unplugging the receiver, reconfiguring the system, or other reasons. For example, Figure 1B The loss of input 132 of the optical signal 130 is shown and muting 142 of the TIA output data 140 is shown. For PAM4 data, the amplification of the TIA must be linear and is referred to as a linear TIA. When the input optical signal 130 recovers, the receiver's TIA 120 requires time to readapt to the change in input optical power level, during which time the TIA 120 will not operate correctly. While adaptation is still in progress, the TIA 120 preferably does not send out invalid data. Therefore, it is desirable to stop outputting data 140 from the TIA 120 until valid data can be sent, and it is desirable to start sending valid data 140 from the TIA 120 as soon as possible once it is operating correctly.

[0004] Some existing solutions immediately turn on the output data 140 of the TIA and send invalid data D11 when the optical signal is restored, such as Figure 1B, until the TIA 120 stabilizes to a valid operating condition again. Other solutions use fixed delays to keep the output of the TIA muted 142 to avoid sending such invalid data D11. Because the stabilization time of the TIA depends on the input optical signal level, these fixed delays may be less than or greater than the actual stabilization time required by the TIA 120. If the delay is shorter than the actual TIA stabilization time, invalid data D11 is sent. If the delay is longer than the TIA stabilization time, the response time until the optical signal is restored is unnecessarily increased. Not only do existing solutions not adequately arrange when to restore the output data 140 for the TIA 120, but the prior art does not have a mechanism to speed up the time to output valid data (140) during the optical input signal recovery event to meet the desired shorter stabilization time. U.S. Patent No. 11,381,318, entitled “Control of Trans-Impedance Amplifier (TIA) during Settlement after Recovery from Loss of Signal in Receiver,” which has been assigned to the assignee of the present application and is incorporated herein by reference in its entirety, discloses a system and method for accurately detecting TIA stabilization after a signal recovery event to avoid sending invalid data, which patent does not anticipate the particularities of such a system when utilizing a linear TIA. For example, upon recovery of an input signal to a linear TIA, the automatic gain control (AGC) loop of the linear TIA takes time to stabilize, and the stabilization time is proportional to the input step size. Before the linear TIA control loop stabilizes to the transient step size input, the linear TIA will not be able to send out valid data. There is currently no method to accurately sense the settling moment of the linear TIA AGC feedback loop to avoid sending invalid data.

[0005] The presently disclosed subject matter is directed to overcoming one or more of the problems set forth above, or at least reducing the effects thereof. Summary of the invention

[0006] It is therefore an object of the presently disclosed subject matter to provide a system that overcomes some or all of the disadvantages of the prior art.

[0007] According to a non-limiting embodiment, an optical device for an optical signal is provided. The optical device includes: a photodiode configured to receive an optical signal; and a linear transimpedance amplifier (TIA) having an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage. The input stage is coupled to the photodiode, and the output stage is coupled to the input stage and is controllable to enable and disable the output at the output of the linear TIA. The optical device also includes: an automatic gain control loop configured to rectify the output of at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage; and a detection circuit system configured to detect a rate of change of the automatic gain control voltage and configured to determine a first state indicating that the optical signal is not present at the photodiode. At least in response to the determined first state, the detection circuit system is configured to disable the output stage of the linear TIA.

[0008] The detection circuit system can be configured to determine a second state indicating the presence of the optical signal at the photodiode, and in response to at least the second state determined after the first state and in response to the detected rate of change, the detection circuit system can be configured to disable the output stage of the linear TIA. The detection circuit system can include a received signal strength indicator coupled to the photodiode and configured to indicate the first and second states of the optical signal at the photodiode. In response to at least the second state and in response to the absence of the rate of change in the feedback signal of the feedback amplifier, the detection circuit system can be configured to enable the output stage of the linear TIA. The detection circuit system can include a digital logic circuit configured to process a logical combination of the first state, the second state, the presence of the rate of change, and the absence of the rate of change and provide enable and disable signals to the output stage of the linear TIA based on the processed logical combination.

[0009] The automatic gain control loop may include a rectifier connected to the output of at least one VGA and a threshold circuit system configured to generate a threshold gain setting. The detection circuit system may include a slope detection circuit system configured to detect the rate of change of the automatic gain control voltage. In a non-limiting embodiment, the slope detection circuit system includes: an operational amplifier; a first comparator configured to compare the output of the operational amplifier with the positive automatic gain control voltage and thereby provide a first comparison state; a second comparator configured to compare the output of the operational amplifier with the negative automatic gain control voltage and thereby provide a second comparison state; and an "OR" logic gate receiving the first and second comparison states and configured to output a signal indicating the rate of change of the automatic gain control voltage. The optical device may be at least a portion of an optical transceiver or an optical receiver.

[0010] According to a non-limiting embodiment, an optical receiver is provided, comprising: a photodiode; a linear transimpedance amplifier (TIA) including an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage, wherein the linear TIA is coupled to the photodiode; an automatic gain control loop configured to rectify an output of at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage signal; a slope detection circuit configured to receive the automatic gain control voltage signal and configured to monitor the automatic gain control voltage signal from the automatic gain control loop, wherein the slope detection circuit is configured to provide a first slope state signal indicating a detected slope in response to detecting a slope in the automatic gain control voltage; and a logic circuit coupled to the slope detection circuit and to the output stage of the linear TIA. The logic circuit is configured to mute the output stage of the linear TIA in response to the first slope state signal.

[0011] The optical receiver may further include a received signal strength indicator (RSSI) configured to provide a first indication signal of asserting a loss of signal (LOS) at the photodiode and a second indication signal of deasserting the LOS at the photodiode. The logic circuit may be configured to output a squelch signal from the logic circuit to the output stage of the linear TIA in response to asserting the first indication signal of the loss of signal (LOS). The logic circuit may be configured to output the squelch signal from the logic circuit to the output stage of the linear TIA in response to deasserting the second indication signal of the loss of signal (LOS) and the first slope state signal indicating that the slope is detected. The slope detection circuit may also be configured to provide a second slope state signal indicating that the slope is not detected to the logic circuit in response to not detecting the slope in the automatic gain control voltage signal. The logic circuit may be configured to output a squelch deassertion signal from the logic circuit to the output stage of the linear TIA in response to deasserting the second indication signal of the loss of signal (LOS) and the second slope state signal indicating that the slope is not detected. The output stage of the linear TIA may be activated in response to receiving the squelch deassertion signal.

[0012] The automatic gain control loop may include a rectifier connected to the output of at least one VGA and a threshold circuit system configured to generate a threshold gain setting. The slope detection circuit may include: an operational amplifier; a first comparator configured to compare the output of the operational amplifier with a positive automatic gain control voltage and thereby provide a first comparison state; a second comparator configured to compare the output of the operational amplifier with a negative automatic gain control voltage and thereby provide a second comparison state; and an OR logic gate receiving the first and second comparison states and configured to output a signal indicating a slope in the automatic gain control voltage.

[0013] According to a non-limiting embodiment, a method for controlling the output of a linear transimpedance amplifier (TIA) is provided. The method includes: sending a squelch signal to an output buffer of the linear TIA in response to asserting a first indication signal of a loss of signal (LOS) to squelch the output voltage of the linear TIA; rectifying the output of at least one variable gain amplifier VGA of the linear TIA and comparing the rectified output with a threshold gain setting to generate an automatic gain control voltage signal; monitoring the automatic gain control voltage signal to determine whether a slope is detected in the automatic gain control voltage; sending a first slope status signal indicating that a slope is detected in response to detecting a slope in the automatic gain control voltage signal; and sending a squelch signal to the output buffer of the linear TIA in response to deasserting a second indication signal of the LOS and the first slope status signal to keep squelching the output voltage of the linear TIA.

[0014] The method may also include: sending a second slope status signal indicating that no slope is detected in response to no slope being detected in the automatic gain control voltage signal; and sending a de-noising signal to an output buffer of the linear TIA to de-noise the output voltage of the linear TIA in response to the second indication signal and the second slope status signal of de-asserting LOS. The first indication signal asserting LOS and the second indication signal asserting LOS may be provided by a received signal strength indicator (RSSI) at a photodiode of an optical receiver. The automatic gain control voltage signal may be monitored by a slope detection circuit to determine whether a slope is detected in the automatic gain control voltage.

[0015] Further embodiments are set forth in the following numbered clauses:

[0016] Item 1: An optical device for an optical signal, the optical device comprising: a photodiode configured to receive an optical signal; a linear transimpedance amplifier (TIA) having an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage, the input stage coupled to the photodiode, the output stage coupled to the input stage and controllable to enable and disable an output at an output terminal of the linear transimpedance amplifier; an automatic gain control loop configured to rectify an output of at least one VGA and compare the rectified output to a threshold gain setting to generate an automatic gain control voltage; and a detection circuit system configured to detect a rate of change of the automatic gain control voltage and to determine a first state indicating that an optical signal is not present at the photodiode, wherein at least in response to the determined first state, the detection circuit system is configured to disable the output stage of the linear transimpedance amplifier.

[0017] Item 2: The optical device of Item 1, wherein the detection circuit system is configured to determine a second state indicating the presence of an optical signal at the photodiode; and wherein in response to at least the second state determined after the first state and in response to the detected rate of change, the detection circuit system is configured to disable the output stage of the linear TIA.

[0018] Clause 3: The optical device of clause 1 or clause 2, wherein the detection circuitry comprises a received signal strength indicator coupled to the photodiode and configured to indicate the first state and the second state of the optical signal at the photodiode.

[0019] Clause 4: The optical device of clause 1 or clause 2, wherein in response to at least the second state and in response to an absence of a rate of change in the feedback signal of the feedback amplifier, the detection circuitry is configured to enable an output stage of the linear TIA.

[0020] Item 5: The optical device of Item 4, wherein the detection circuit system includes a digital logic circuit configured to process logical combinations of the first state, the second state, the presence of the rate of change, and the absence of the rate of change and provide enable and disable signals to the output stage of the linear TIA based on the processed logical combinations.

[0021] Clause 6: The optical device of any of clauses 1-5, wherein the automatic gain control loop comprises a rectifier connected to an output of the at least one VGA and threshold circuitry configured to generate a threshold gain setting.

[0022] Item 7: An optical device of any of items 1-6, wherein the detection circuit system includes a slope detection circuit system configured to detect the rate of change of an automatic gain control voltage, the slope detection circuit system including: an operational amplifier; a first comparator configured to compare the output of the operational amplifier with a positive automatic gain control voltage and thereby provide a first comparison state; a second comparator configured to compare the output of the operational amplifier with a negative automatic gain control voltage and thereby provide a second comparison state; and an "OR" logic gate receiving the first and second comparison states and configured to output a signal indicating the rate of change of the automatic gain control voltage.

[0023] Clause 8: The optical device of any of clauses 1-7, wherein the optical device is at least a portion of an optical transceiver or an optical receiver.

[0024] Item 9: An optical receiver comprising: a photodiode; a linear transimpedance amplifier (TIA) comprising an input stage, an output stage, and at least one variable gain amplifier (VGA) disposed between the input stage and the output stage, wherein the linear TIA is coupled to the photodiode; an automatic gain control loop configured to rectify an output of at least one VGA and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage signal; a slope detection circuit configured to receive the automatic gain control voltage signal and configured to monitor the automatic gain control voltage signal from the automatic gain control loop, wherein the slope detection circuit is configured to provide a first slope state signal indicating a detected slope in response to detecting a slope in the automatic gain control voltage; and a logic circuit coupled to the slope detection circuit and to the output stage of the linear TIA, the logic circuit configured to mute the output stage of the linear TIA in response to the first slope state signal.

[0025] Clause 10: The optical receiver of clause 9, further comprising: a received signal strength indicator (RSSI) configured to provide a first indication signal asserting a loss of signal (LOS) at the photodiode and a second indication signal de-asserting the LOS at the photodiode.

[0026] Clause 11: The optical receiver of clause 9 or clause 10, wherein the logic circuit is configured to output a squelch signal from the logic circuit to an output stage of the linear TIA in response to asserting a first indication signal of loss of signal (LOS).

[0027] Clause 12: The optical receiver of any of clauses 9-11, wherein the logic circuit is configured to output a squelch signal from the logic circuit to an output stage of the linear TIA in response to a second indication signal deasserting a loss of signal (LOS) and a first slope status signal indicating that a slope is detected.

[0028] Clause 13: The optical receiver of any of clauses 9-11, wherein the slope detection circuit is further configured to provide a second slope state signal indicating no slope was detected to the logic circuit in response to no slope being detected in the automatic gain control voltage signal.

[0029] Item 14: The optical receiver of Item 13, wherein the logic circuit is configured to output a squelch de-assertion signal from the logic circuit to an output stage of the linear TIA in response to a second indication signal of a loss of de-assertion signal (LOS) and a second slope status signal indicating that a slope is not detected, wherein the output stage of the linear TIA is activated in response to receiving the squelch de-assertion signal.

[0030] Clause 15: The optical receiver of any of clauses 9-14, wherein the automatic gain control loop comprises a rectifier connected to the output of the at least one VGA and threshold circuitry configured to generate a threshold gain setting.

[0031] Clause 16: An optical receiver of any of clauses 9-15, wherein the slope detection circuit comprises: an operational amplifier; a first comparator configured to compare the output of the operational amplifier with a positive automatic gain control voltage and thereby provide a first comparison state; a second comparator configured to compare the output of the operational amplifier with a negative automatic gain control voltage and thereby provide a second comparison state; and an OR logic gate receiving the first and second comparison states and configured to output a signal indicating a slope in the automatic gain control voltage.

[0032] Item 17: A method of controlling the output of a linear transimpedance amplifier (TIA), the method comprising: sending a squelch signal to an output buffer of the linear TIA in response to asserting a first indication signal of loss of signal (LOS) to squelch the output voltage of the linear TIA; rectifying the output of at least one variable gain amplifier (VGA) of the linear TIA and comparing the rectified output with a threshold gain setting to generate an automatic gain control voltage signal; monitoring the automatic gain control voltage signal to determine whether a slope is detected in the automatic gain control voltage; sending a first slope status signal indicating that a slope is detected in response to detecting a slope in the automatic gain control voltage signal; and sending a squelch signal to the output buffer of the linear TIA in response to deasserting a second indication signal of LOS and the first slope status signal to maintain squelching the output voltage of the linear TIA.

[0033] Clause 18: The method of clause 17, further comprising: sending a second slope status signal indicating that no slope is detected in response to no slope being detected in the automatic gain control voltage signal; and sending a de-noising signal to an output buffer of the linear TIA to de-noise an output voltage of the linear TIA in response to the second indication signal and the second slope status signal of de-asserting LOS.

[0034] Clause 19: The method of clause 17 or clause 18, wherein the first indication signal asserting LOS and the second indication signal asserting LOS are provided by a received signal strength indicator (RSSI) at a photodiode of the optical receiver.

[0035] Clause 20: The method of any of clauses 17-19, wherein the automatic gain control voltage signal is monitored by a slope detection circuit to determine whether a slope is detected in the automatic gain control voltage.

[0036] These and other features and characteristics of the subject matter of the present disclosure, as well as the methods of operation and function of the related elements of the structure and the economy of incorporation and manufacture of the parts, will become more apparent by considering the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification wherein like reference numerals designate corresponding parts in the various figures. It should be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as definitions of limitations of the disclosed subject matter. As used in the specification and claims, the singular forms of "a", "an", and "the" include plural references unless the context clearly dictates otherwise. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Additional advantages and details of the disclosed subject matter are explained in more detail below with reference to exemplary embodiments shown in the accompanying drawings, in which:

[0038] Figure 1A illustrates a conventional optical communication system having a transmitter that transmits an optical signal to a receiver via an optical fiber;

[0039] Figure 1B illustrates an optical signal received at a receiver compared to an invalid output signal output by the receiver due to recovery from a loss of signal (LOS) condition;

[0040] Figure 2 illustrates an example of the expected output of a linear transimpedance amplifier (TIA) operating in accordance with the present disclosure;

[0041] Figure 3 illustrates a simplified block diagram of an optical receiver with a linear TIA according to the present disclosure;

[0042] Figure 4 A non-limiting embodiment of a slope detection circuit and a non-limiting embodiment of a digital logic circuit are shown;

[0043] FIG5 illustrates the use of a fixed delay to “turn on” the output of a linear TIA from the point in time when LOS is de-asserted; and

[0044] Figure 6 The use of a dynamic delay determined by a non-limiting embodiment slope detection circuit to "turn on" the output of a linear TIA is shown. DETAILED DESCRIPTION

[0045] For purposes of the description below, the terms "end," "upper," "lower," "right," "left," "vertical," "horizontal," "top," "bottom," "lateral," "longitudinal," and their derivatives shall relate to the disclosed subject matter as it is oriented in the accompanying drawings. However, it should be understood that the disclosed subject matter may assume various alternative variations and step sequences unless expressly indicated to the contrary. It should also be understood that the specific devices and processes illustrated in the drawings and described in the following specification are merely exemplary embodiments of the disclosed subject matter. Therefore, unless otherwise indicated, specific dimensions and other physical characteristics associated with the embodiments disclosed herein should not be considered limiting.

[0046] Unless explicitly described as such, any aspect, component, element, structure, action, step, function, instruction, etc. used in this article should not be interpreted as critical or essential. Moreover, as used herein, the articles "one" and "an" are intended to include one or more items and can be used interchangeably with "one or more" and "at least one". In the case of only one item intended, the term "one" or similar language is used. Moreover, as used herein, the terms "having", "having", etc. are intended to be open terms. In addition, the phrase "based on" is intended to mean "based at least in part on", unless otherwise explicitly stated.

[0047] Some non-limiting embodiments are described herein in conjunction with threshold values. As used herein, satisfying a threshold value may refer to a value greater than a threshold value, more than a threshold value, above a threshold value, greater than or equal to a threshold value, less than a threshold value, less than a threshold value, below a threshold value, less than or equal to a threshold value, equal to a threshold value, etc.

[0048] The present disclosure relates to a slope detection circuit for accurately detecting when a linear transimpedance amplifier (TIA) of an optical receiver is stable after a signal recovery event occurs at the input of the linear TIA (e.g., after a signal loss). Accurate sensing of the linear TIA being stable is used to avoid sending invalid data by muting the output of the linear TIA until the control loop of the TIA readapts and stabilizes back to a correct operating condition.

[0049] refer to Figure 2, illustrates an example of the expected output of a linear TIA operating in accordance with the present disclosure. Data 200 of the output of the linear TIA continues until a signal loss is detected at the receiver. When an input optical signal loss event is detected and the LOS indication is asserted, the output of the linear TIA data will be muted for a time period P21. At point 202, the input optical signal is restored, the LOS indication is deasserted, but the muting of the TIA output has not been deasserted because the receiver (TIA) needs time to adapt or re-adapt. After point 202, there is a slope of the automatic gain control (AGC) loop voltage signal of the TIA, which can be detected by the slope detection circuit described in more detail below. Thus, the output of the receiver is further muted in time period P22 until the linear TIA has adapted. Finally, after time period P22, at point 204 when the adaptation or stabilization of the linear TIA of the receiver is completed, there is no slope in the AGC loop voltage signal of the linear TIA, and the muting of the output of the linear TIA is deasserted. Thus, the data 200 for the output of the receiver can continue without including invalid data. By using the slope detection circuit disclosed herein, the stabilization moment of the AGC loop of the linear TIA can be accurately sensed to avoid sending invalid data before the loop stabilizes; and valid data can be obtained in time at the output of the linear TIA. In a non-limiting configuration, the time period P22 from when the input signal is restored to when the squelch of the output of the linear TIA ends can be within 80 μs of the transient step input.

[0050] Having understood how the slope detection circuit of the present disclosure is intended to control the output of a linear TIA for a receiver, the discussion now turns to the circuitry and other features of the receiver and methods of controlling the output of a linear TIA for a receiver. In particular, Figure 3 A simplified block diagram of an optical receiver 310 according to the present disclosure is illustrated. The optical receiver 310 includes a photodiode 311, a received signal strength indicator (RSSI) 317, a linear TIA 312, a DC canceling feedback amplifier 316, a slope handling circuit 329, and an output buffer 315.

[0051] In general, and as will be described below, the optical receiver 310 receives an optical signal at a photodiode 311, and the linear TIA 312 converts the varying input current from the photodiode (which is also amplified) into a voltage to output as a differential output voltage 680 from an output buffer 315. In one non-limiting embodiment, the linear TIA 312 includes a low noise input stage 313 configured to amplify the signal from the photodiode 311, followed by a variable gain amplifier (VGA) stage 314 and an output buffer 315. Although Figure 3Only a single VGA stage 314 is illustrated in FIG, but this should not be construed as limiting the present disclosure as multiple VGA stages may exist based on the application. Based on the input signal amplitude, the VGA stage 314 increases or decreases its gain to maintain the output linearity of the TIA 312. The VGA output is rectified by a rectifier 318 and compared to a threshold gain setting generated by a threshold circuit system 320 to generate a control voltage (V agc ) to adjust the VGA gain. In one non-limiting embodiment, the threshold circuit system 320 provides a threshold current to achieve a specific VGA gain. The control voltage (V agc ) is set by the AGC loop so that the current output of the rectifier is equal to the threshold current.

[0052] The DC cancelling feedback amplifier 316 is used to reduce the DC component of the current of the varying input current using negative feedback. The DC cancelling feedback amplifier 316 is connected to the differential outputs 656, 658 from the VGA stage 314.

[0053] In brief, the photodiode 311 has a cathode connected to a received signal strength indicator (RSSI) 317 and has an anode connected to an input stage 313 of the linear TIA 312. The photodiode 311 is configured to receive an input optical signal and convert the input optical signal into an electrical signal 650, the current of which is proportional to the optical power. The low noise input stage 313 of the linear TIA 312 is configured to receive the electrical signal 650 from the photodiode 311 and provide an electrical signal 652 to the VGA stage 314 of the linear TIA 312. The input stage 313 is connected to feedback from a DC canceling feedback amplifier 316. That is, a feedback signal 670 from the feedback amplifier is connected to the input stage 313 to handle the DC offset in the output of the VGA stage 314.

[0054] The VGA stage 314 is coupled to an output buffer 315 of the linear TIA 312, wherein the gain amplifier 314 provides differential outputs 656, 658 to the output buffer 315. The output buffer 315 in turn provides a differential output voltage 680 to the receiver 310. However, the differential output voltage 680 of the output buffer 315 may be muted and de-muted in response to a control signal 672 (e.g., a mute signal or a de-mute signal) received at the output buffer 315 from circuitry described below.

[0055] To mute the differential output voltage 680 of output buffer 315 in response to loss of signal, linear TIA 312 uses LOS state 600. To de-mute the differential output voltage 680 of output buffer 315 in response to restoration of signal, TIA 312 uses LOS state 600 and slope handling circuit 329.

[0056] On the cathode side of the photodiode 311, a received signal strength indicator (RSSI) 317 is configured to indicate received signal strength and provide a LOS state signal 600 indicating a loss of signal (LOS) via an output terminal of the RSSI 317 in response to a loss of the input optical signal. That is, in response to a loss of the input optical signal, the loss of signal (LOS) is asserted. Thus, the output stage 315 is turned off in the manner discussed below.

[0057] When the input optical signal is restored, LOS is de-asserted. That is, a LOS status signal 600 is generated or provided indicating that the optical signal is not lost (i.e., not LOS or LOS de-asserted). Thus, the linear TIA 312 uses an automatic gain control (AGC) loop 331 and a DC cancellation loop 333 to adapt or readapt to this change. The AGC loop 331 is configured to rectify the output of the VGA stage 314 and compare the rectified output with a threshold gain setting to generate an automatic gain control voltage signal 675. The linear TIA 312 needs to spend a certain stabilization time to adapt or readapt. During the stabilization time, the output data from the linear TIA 312 is invalid.

[0058] To facilitate recovery and transmission of valid data from output buffer 315, optical receiver 310 ( Figure 3 ) includes a slope handling circuit 329. As schematically shown here and discussed in more detail below, the slope handling circuit 329 includes a slope detection circuit 330 and a digital logic circuit 350. The slope handling circuit 329 receives the LOS status signal 600 from the RSSI 317 and receives an automatic gain control voltage signal (V agc ) 675. The slope detection circuit 330 may be configured to receive and monitor the automatic gain control voltage signal (V agc )675.

[0059] In response to the LOS status signal 600 indicating a loss of signal (LOS), the slope handling circuit 329 provides or sends a squelch signal 672 to the output buffer 315 to squelch the differential output voltage 680. For example, the squelch signal 672 is used to disable the output buffer 315. When the LOS status signal 600 indicates that the optical signal is not lost, the slope handling circuit 329 may provide or send ... agc )675 determines the control signal 672 from the slope handling circuit 329 to the output buffer 315, as discussed in more detail below.

[0060] The slope detection circuit 330 is configured to monitor the stabilization of the linear TIA 312 after the input optical signal is restored, as indicated by the LOS status signal 600. In some examples, after the linear TIA 312 stabilizes, the slope handling circuit 329 turns the output buffer 315 of the linear TIA "on" by sending a de-mute signal 672 (e.g., a mute de-assertion signal) to the output buffer 315. That is, in response to the linear TIA 312 stabilizing after the signal loss, the slope handling circuit 329 turns the output buffer 315 of the linear TIA on by sending a de-mute signal 672 (e.g., a mute de-assertion signal) from the slope handling circuit 329 to the output buffer 315. In some examples, the slope handling circuit 329 turns the output buffer 315 of the TIA "off" (e.g., mutes) by sending the mute signal 672 to the output buffer 315. As shown here only schematically, signal 672 is illustrated as enabling and disabling output buffer 315 by operating a switch for current source 335 to output buffer 315 (see Figure 4 ).

[0061] In response to the automatic gain control loop 331 being in a state with the automatic gain control voltage signal (V agc )675, the output from the slope detection circuit 330 is asserted, for example, at a high voltage level. In response to the automatic gain control loop 331 having stabilized, for example, the stabilization of the automatic gain control loop 331 is completed, the output from the slope detection circuit 330 is de-asserted, for example, at a low voltage level. Thus, the output of the slope detection circuit 330 can be used to keep the output of the linear TIA 312 muted by the muting signal 672 until the stabilization of the automatic gain control loop 331 of the linear TIA is completed, thereby preventing invalid data from appearing at the differential output voltage 680 of the output buffer 315. This accurate determination of the stabilization moment of the automatic gain control loop 331 of the linear TIA using the slope detection circuit 330 can prevent invalid data from being sent from the optical receiver 310. Referring to Figure 4 Further details of this slope detection circuit 330 are discussed.

[0062] With an understanding of the circuitry of receiver 310 and its slope handling circuit 329 (slope detection circuit 330 and digital logic circuit 350) used with linear TIA 312, the discussion turns to further details of slope detection circuit 330. Figure 4 , illustrates additional details of an example of a slope detection circuit 330 and an example of a digital logic circuit 350 for the slope handling circuit 329. In this configuration, the slope detection circuit 330 and the digital logic circuit 350 are configured to output a mute / undo mute signal 672.

[0063] The slope detection circuit 330 receives an automatic gain control voltage signal (V agc ) 675, and has an output terminal for outputting a slope state signal 671. The slope detection circuit 330 is configured to receive and monitor the slope state signal from the automatic gain control loop (331; Figure 3 ) of the automatic gain control voltage signal (V agc ) 675. Processing that signal 675, the slope detection circuit 330 is configured to determine or detect the received automatic gain control voltage signal (V agc )675. The received automatic gain control voltage signal (V agc ) The slope in 675 indicates that the signal changes over time and has not yet stabilized.

[0064] The digital logic circuit 350 has a first input terminal for receiving the LOS status signal 600, and has a second input terminal for receiving the slope status signal 671 from the slope detection circuit 330. The digital logic circuit 350, which may include any suitable digital logic, provides a squelch signal 672. In response to receiving the LOS status signal 600 indicating a loss of signal (LOS), the digital logic circuit 350 is configured to send the squelch signal 672 to the output buffer 315. In response to the output buffer 315 receiving the squelch signal 672, the output of the output buffer 315 is squelched, as previously described.

[0065] In response to the automatic gain control voltage signal (V agc )675, the slope detection circuit 330 provides or outputs an indication of the automatic gain control voltage signal (V agc ) 675. In response to the slope detection circuit 330 generating a first slope state signal 671 (eg, an asserted signal having a predetermined voltage level) of the slope of the automatic gain control voltage signal (V agc )675, the slope detection circuit 330 provides or outputs an indication that the automatic gain control voltage signal (V agc )675 may include a second slope state signal 671 having a slope in the first embodiment (eg, a de-asserted signal having a different predetermined voltage level).

[0066] Thus, the output of the slope detection circuit 330 is coupled to the digital logic circuit 350, which is configured to receive the slope state signal 671, which may indicate the slope of the automatic gain control voltage signal (V agc )675 detects or does not detect the automatic gain control voltage signal (V agc )675.

[0067] In some examples, the first input terminal of the digital logic circuit 350 receives the LOS status signal 600 indicating that the optical signal is no longer lost (i.e., has been restored after being lost); and the digital logic circuit 350 is configured to respond to receiving the automatic gain control voltage signal (V agc )675 while still sending the mute signal 672 to the output buffer 315, and then keeping the differential output voltage 680 muted.

[0068] In another example, the first input terminal of the digital logic circuit 350 receives the LOS status signal 600 indicating that the optical signal is no longer lost (i.e., has been restored after being lost); and the digital logic circuit 350 is further configured to respond to receiving the automatic gain control voltage signal (V agc ) 675 detects no slope in the second slope state signal 671 and sends a de-mute (mute release assertion) signal 672 to the output buffer 315. Thus, the differential output voltage 680 of the output buffer 315 is de-mute.

[0069] Looking at the slope detection circuit 330 in more detail, the slope detection circuit 330 is configured to receive the automatic gain control voltage signal (V agc )675 as input. The operational amplifier 337 amplifies the automatic gain control voltage signal (V agc )675 to generate an output voltage. For example, the arrangement of the resistor and capacitor causes a time delay in the voltage at the first input terminal of the operational amplifier 337 compared to the second input terminal of the operational amplifier 337. Thus, when the automatic gain control voltage signal (V agc )675, the voltage difference between the input terminals of the operational amplifier 337 can represent the voltage difference due to the automatic gain control voltage signal (V agc )675 or the voltage difference at different time points caused by the slope thereof.

[0070] The output terminal of the operational amplifier 337 is coupled to the base of the transistor 339. The emitter of the transistor 339 is coupled to the first comparator 341 and the second comparator 342 of the window comparator 343. The first comparator 341 and the second comparator 342 have output terminals connected to the OR gate 345, from which the slope state signal 671 can be obtained. The first comparator 341 receives the automatic gain control voltage signal (V agc)675, and receives at the non-inverting terminal a signal from the emitter of the transistor 339. The output terminal of the first comparator 341 is configured to (a) output a high voltage level in response to the non-inverting terminal of the first comparator 341 having a higher voltage than the inverting terminal of the first comparator 341; and (b) output a low voltage level in response to the voltage of the non-inverting terminal of the first comparator 341 being lower than or equal to the voltage of the inverting terminal.

[0071] For its part, the second comparator 342 receives the automatic gain control voltage signal (V agc )675, and receives at the inverting terminal a signal from the emitter of the transistor 339. The output terminal of the second comparator 342 is configured to (a) output a high voltage level in response to the inverting terminal of the second comparator 342 having a lower voltage than the non-inverting terminal of the second comparator 342; and (b) output a low voltage level in response to the voltage of the inverting terminal of the second comparator 342 being higher than or equal to the voltage of the non-inverting terminal.

[0072] Using the outputs from comparators 341, 342, the OR gate 345 is configured to output a high voltage level in response to the first comparator 341 or the second comparator 342 having a high voltage level; and the OR gate 345 is configured to output a low voltage level in response to both comparators 341, 342 providing a low voltage level. Although the OR gate can output a high voltage level in response to its two input terminals receiving a high voltage level, Figure 4 The comparators 341 and 342 will not output high voltage levels at the same time. That is, when the comparator 341 outputs a high voltage level, the comparator 342 can only output a low voltage level; and when the comparator 341 outputs a high voltage level, the comparator 342 can only output a low voltage level.

[0073] The first comparator 341 detects the automatic gain control voltage signal (V agc )675 changes in the positive slope, and the second comparator 342 detects the automatic gain control voltage signal (V agc ) 675. In some examples, in response to the automatic gain control voltage signal (V agc )675 has a positive slope, the output of the first comparator 341 is at a high voltage level; and the output of the OR gate 345 is at a high voltage level because the first slope state signal 671 indicates that a slope is detected. Therefore, the slope detection circuit 330 can detect the automatic gain control voltage signal (V agc )675 and outputs a first slope state signal 671 at a high voltage level to indicate the detected slope.

[0074] In another example, in response to the automatic gain control voltage signal (V agc )675, the output terminal of the second comparator 342 is at a high voltage level, and the output terminal of the OR gate 345 is at a high voltage level because the first slope state signal 671 indicates that a slope is detected. Therefore, the slope detection circuit 330 can detect the automatic gain control voltage signal (V agc )675, and outputs a first slope state signal 671 at a high voltage level to indicate the detected slope.

[0075] In other examples, in response to the automatic gain control voltage signal (V agc )675, the output of the first comparator 341 is at a low voltage level; and the output of the second comparator 342 is at a low voltage level. Therefore, the output of the OR gate 345 is at a low voltage level because the second slope state signal 671 indicates that no slope is detected. Therefore, the slope detection circuit 330 can determine that the automatic gain control voltage signal (V agc )675 does not have a slope, and a second slope state signal 671 at a low voltage level may be output to indicate that the automatic gain control voltage signal (V agc )675 does not have a slope.

[0076] As previously referenced Figure 3 As discussed, the slope detection circuit 330 and the digital circuit 350 may be coupled to the linear TIA 312. In other examples, the slope detection circuit 330 and the digital logic circuit 350 may be integrated into the linear TIA 312 or considered part of the linear TIA 312. For example, an integrated circuit for the linear TIA 312 may include components of the slope detection circuit 330 and the digital logic circuit 350.

[0077] To further illustrate the teachings of the present disclosure, FIG. 5 and Figure 6 5 graphically illustrates the output of a de-quieting operation using a fixed delay according to the prior art, and Figure 6 The output of a de-squelching operation using the slope detection circuit of the present disclosure is graphically represented.

[0078] 5, curve 501 shows the TIA input voltage at different time points. The TIA input voltage 501 can be measured and is equivalent to the electrical signal 650 from the photodiode 311. Curve 502 illustrates the automatic gain control voltage signal (V agc)675. The stability of the automatic gain control loop 331 of the linear TIA can be explained from the slope in the curve 502. The curve 503 illustrates the LOS state at different time points. The LOS state in the curve 503 switches from the LOS asserted state to the LOS deasserted state at the time point T53.

[0079] Curve 504 illustrates that the conventional squelch signal 672 sent to the output buffer 315 after the LOS asserted state is de-asserted at time point T53 using a fixed delay, and curve 505 illustrates the output of the linear TIA 312 from the output buffer 315. Curves 504 and 505 of FIG. 5 show that the output of the linear TIA is “turned on” at time point T53 using a fixed delay D51 from time point T51 when the LOS state curve 503 switches from the LOS asserted state to the LOS de-asserted state. The stability of the automatic gain control loop 331 of the linear TIA can be seen from the automatic gain control voltage signal (V agc )675. Between time points T51 and T52, the linear TIA (e.g., the automatic gain control loop of the linear TIA) re-adapts, as indicated by the slope between T51 and T52 in curve 502. Starting from time point T52, the automatic gain control loop of the linear TIA has stabilized. That is, starting from time point T52, the linear TIA enters a valid operating condition. However, in FIG. 5, the output of the linear TIA shown in curve 505 remains muted between time points T52 and T53, even after the automatic gain control loop of the linear TIA has stabilized from T52, as can be explained based on curve 502. Thus, this fixed delay D51 produces undesirable behavior because the linear TIA does not restore output data after it has generated valid data. Although not shown, the fixed delay D51 may also cause the linear TIA to restore output data prematurely before the automatic gain control loop of the linear TIA stabilizes, and this will cause the linear TIA to output invalid data.

[0080] In contrast, Figure 6 , the output of the TIA is “turned on” using a dynamic time span (between T61 and T62) determined by a slope detection circuit consistent with the present disclosure. Figure 6 , curve 601 shows the TIA input voltage at different time points. The TIA input voltage 601 can be measured and is equivalent to the electrical signal 650 from the photodiode 311. Curve 602 illustrates the automatic gain control voltage signal (V agc)675. The stability of the automatic gain control loop 331 of the linear TIA can be explained from the slope in curve 602. Curve 604 illustrates the LOS state at different time points. The LOS state in curve 603 switches from the LOS asserted state to the LOS deasserted state at time point T61.

[0081] Curve 604 illustrates the squelch signal 672 from the slope handling circuit 329 to the output buffer 315, and curve 605 illustrates the output of the linear TIA 312 from the output buffer 315. Curve 606 illustrates the slope state signal 671 over time. agc )675, the slope detection circuit 330 provides or outputs an indication of the automatic gain control voltage signal (V agc ) 675. In response to the slope detection circuit 330 generating a first slope state signal 671 (eg, an asserted signal having a predetermined voltage level) of the slope in the automatic gain control voltage signal (V agc )675, the slope detection circuit 330 provides or outputs a signal indicating that the automatic gain control voltage signal (V agc )675 in which no slope is detected, a second slope state signal 671 (eg, a de-asserted signal having a different predetermined voltage level).

[0082] Figure 6 Curves 604 and 605 show the use of a slope detection circuit (such as Figure 3 The dynamic delay determined by the circuit 330 in FIG60 turns on the output of the linear TIA at time T62. The stability of the automatic gain control loop 331 of the linear TIA can be determined from the automatic gain control voltage signal (V agc )675. Between time points T61 and T62, the linear TIA (e.g., the automatic gain control loop of the linear TIA) re-adapts, as indicated by the slope between T61 and T62 in curve 602. Starting from time point T62, the automatic gain control loop of the linear TIA has stabilized. That is, starting from time point T62, the linear TIA enters or switches to an effective or normal operating mode. In response to the linear TIA entering the normal operating mode, the output of the TIA shown in curve 605 is immediately "turned on" due to the use of the slope detection circuit. Therefore, once the linear TIA completes adapting to the recovery of the input optical signal, the output of the linear TIA is "turned on" (at time point T62) and enters the normal operating mode (at time point T62).

[0083] Although the disclosed subject matter has been described in detail for purposes of illustration based on what are presently considered to be the most practical and preferred embodiments, it should be understood that such detail is provided for that purpose only and that the disclosed subject matter is not limited to the disclosed embodiments, but rather is intended to cover modifications and equivalent arrangements within the spirit and scope of the appended claims. For example, it should be understood that the presently disclosed subject matter contemplates that, to the extent possible, one or more features of any embodiment may be combined with one or more features of any other embodiment.

Claims

1. An optical device for an optical signal, the optical device comprising: a photodiode configured to receive an optical signal; A linear transimpedance amplifier TIA having an input stage coupled to the photodiode, an output stage, and at least one variable gain amplifier VGA disposed between the input stage and the output stage, the input stage being coupled to the input stage and being controllable to enable and disable output at an output terminal of the linear transimpedance amplifier; an automatic gain control loop configured to rectify an output of at least one VGA and compare the rectified output to a threshold gain setting to generate an automatic gain control voltage; as well as detection circuitry configured to detect a rate of change of the automatic gain control voltage and configured to determine a first state indicative of the absence of an optical signal at the photodiode, Wherein, in response to at least the determined first state, the detection circuit system is configured to disable an output stage of the linear transimpedance amplifier.

2. The optical device according to claim 1, wherein The detection circuitry is configured to determine a second state indicating the presence of an optical signal at the photodiode; and wherein in response to at least the second state determined after the first state and in response to the detected rate of change, the detection circuitry is configured to disable an output stage of the linear TIA.

3. The optical device according to claim 2, wherein: The detection circuitry includes a received signal strength indicator coupled to the photodiode and configured to indicate a first state and a second state of an optical signal at the photodiode.

4. The optical device according to claim 2, wherein: Responsive to at least the second state and to the absence of a rate of change in the feedback signal of the feedback amplifier, the detection circuitry is configured to enable an output stage of the linear TIA.

5. The optical device according to claim 4, wherein: The detection circuitry includes a digital logic circuit configured to process logical combinations of the first state, the second state, the presence of the rate of change, and the absence of the rate of change and to provide enable and disable signals to an output stage of the linear TIA based on the processed logical combinations.

6. The optical device according to claim 1, wherein: The automatic gain control loop includes a rectifier connected to the output of at least one VGA and threshold circuitry configured to generate a threshold gain setting.

7. The optical device according to claim 1, wherein: The detection circuitry includes slope detection circuitry configured to detect a rate of change of an automatic gain control voltage, the slope detection circuitry comprising: Operational amplifier; a first comparator configured to compare the output of the operational amplifier to a positive automatic gain control voltage and thereby provide a first comparison state; a second comparator configured to compare the output of the operational amplifier to the negative automatic gain control voltage and thereby provide a second comparison state; and An OR logic gate receives the first comparison state and the second comparison state and is configured to output a signal indicating a rate of change of the automatic gain control voltage.

8. The optical device according to claim 1, wherein: The optical device is at least a part of an optical transceiver or an optical receiver.

9. An optical receiver comprising: Photodiode; A linear transimpedance amplifier TIA, comprising an input stage, an output stage and at least one variable gain amplifier VGA arranged between the input stage and the output stage, wherein the linear TIA is coupled to a photodiode; an automatic gain control loop configured to rectify an output of at least one VGA and compare the rectified output to a threshold gain setting to generate an automatic gain control voltage signal; a slope detection circuit configured to receive the automatic gain control voltage signal and configured to monitor the automatic gain control voltage signal from the automatic gain control loop, wherein the slope detection circuit is configured to provide a first slope state signal indicating detection of the slope in response to detecting a slope in the automatic gain control voltage; as well as A logic circuit is coupled to the slope detection circuit and to the output stage of the linear TIA, the logic circuit being configured to mute the output stage of the linear TIA in response to the first slope state signal.

10. The optical receiver according to claim 9, further comprising: A received signal strength indicator (RSSI) is configured to provide a first indication signal asserting a loss of signal LOS at the photodiode and a second indication signal de-asserting the LOS at the photodiode.

11. The optical receiver according to claim 10, wherein: The logic circuit is configured to output a squelch signal from the logic circuit to an output stage of the linear TIA in response to asserting the first indication signal of a loss of signal LOS.

12. The optical receiver according to claim 11, wherein: The logic circuit is configured to output a squelch signal from the logic circuit to an output stage of the linear TIA in response to deasserting the second indication signal of the loss of signal LOS and the first slope status signal indicating that a slope is detected.

13. The optical receiver according to claim 11, wherein: The slope detection circuit is further configured to provide a second slope state signal indicating no slope was detected to the logic circuit in response to no slope being detected in the automatic gain control voltage signal.

14. The optical receiver according to claim 13, wherein: The logic circuit is configured to output a squelch de-assertion signal from the logic circuit to an output stage of the linear TIA in response to a second indication signal of a loss of de-assertion signal LOS and a second slope status signal indicating that a slope is not detected, wherein the output stage of the linear TIA is activated in response to receiving the squelch de-assertion signal.

15. The optical receiver according to claim 9, wherein: The automatic gain control loop includes a rectifier connected to the output of at least one VGA and threshold circuitry configured to generate a threshold gain setting.

16. The optical receiver according to claim 9, wherein: The slope detection circuit includes: Operational amplifier; a first comparator configured to compare the output of the operational amplifier to a positive automatic gain control voltage and thereby provide a first comparison state; a second comparator configured to compare the output of the operational amplifier to the negative automatic gain control voltage and thereby provide a second comparison state; and An OR logic gate receives the first comparison state and the second comparison state and is configured to output a signal indicative of a slope in the automatic gain control voltage.

17. A method for controlling an output of a linear transimpedance amplifier (TIA), the method comprising: sending a mute signal to an output buffer of the linear TIA to mute an output voltage of the linear TIA in response to asserting a first indication signal of a loss of signal LOS; rectifying the output of at least one variable gain amplifier VGA of the linear TIA and comparing the rectified output to a threshold gain setting to generate an automatic gain control voltage signal; monitoring the automatic gain control voltage signal to determine whether a slope is detected in the automatic gain control voltage; responsive to detecting a slope in the automatic gain control voltage signal, sending a first slope state signal indicating detection of the slope; as well as A mute signal is sent to an output buffer of the linear TIA to maintain an output voltage of the mute linear TIA in response to deasserting the second indication signal of the LOS and the first slope state signal.

18. The method according to claim 17, further comprising: sending a second slope state signal indicating no slope was detected in response to no slope being detected in the automatic gain control voltage signal; as well as A de-mute signal is sent to an output buffer of the linear TIA to de-mute an output voltage of the linear TIA in response to de-asserting the second indication signal of the LOS and the second slope state signal.

19. The method according to claim 17, wherein: The first indication signal asserting LOS and the second indication signal asserting LOS are provided by a received signal strength indicator (RSSI) at a photodiode of the optical receiver.

20. The method according to claim 17, wherein: The automatic gain control voltage signal is monitored by a slope detection circuit to determine whether a slope is detected in the automatic gain control voltage.

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