JP2013005327A - Optical receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the adjustment time of an AGC amplification factor in an optical receiver having a function (AGC) of amplifying the amplitude of an electric signal output from a photodetector (photodiode).SOLUTION: The amplification factor control (AGC amplification factor control) of a variable gain amplifier 33 is performed only when receiving a preamble added to communication data from a transmission side, instead of continuously performing amplification factor control of the variable gain amplifier 33. While the preamble is not being received, the amplification factor of the variable gain amplifier 33 is fixed. With such processing, the AGC amplification factor is not increased during an optical modulation OFF period (period when a signal level is zero), and the AGC amplification factor in the previous preamble period can be retained and continued till the next preamble period. By this, it is possible to reduce the adjustment time (setup time) of the AGC amplification factor at the next preamble reception time.

Description

  The present invention relates to an optical receiver that receives an optical signal transmitted from a transmission side in an optical communication system using visible light or the like.

  In recent years, a visible light communication system has been developed that performs data communication using visible light output from a light source such as an LED (light emitting diode).

  As an example of a visible light communication system, an optical transmitter that superimposes and outputs a light modulation signal on illumination light (visible light) by controlling blinking of a light source such as an illumination device according to transmission data, and the optical transmitter There is an optical receiver that receives a visible light signal (optical modulation signal) transmitted from the receiver and demodulates the received optical modulation signal to obtain communication data (see, for example, Patent Document 1).

  In such a visible light communication system or the like, as an optical receiver that receives an optical modulation signal, for example, a light receiving element (for example, a photodiode) that converts the optical modulation signal into an electric signal, and an output signal of the light receiving element are amplified. The first-stage amplifier, a band-pass filter that extracts a frequency component corresponding to the optical modulation signal (communication data) from the electric signal amplified by the first-stage amplifier, and the gain of the electric signal extracted by the band-pass filter is adjusted. For example, there is an AGC (Auto Gain Control) circuit that keeps the signal amplitude constant (see, for example, Patent Document 2 or Patent Document 3). The AGC circuit includes, for example, a variable gain amplifier and a control unit that controls the gain (amplification factor).

JP 2008-252465 A JP 2001-168661 A Japanese Patent Application Laid-Open No. 08-018510

  By the way, in an optical communication system that uses a synchronization preamble, for example, as shown in FIG. 5, the preamble and communication data are transferred in a set state, and the optical modulation ON period during which the communication data is transferred, A period of light modulation OFF between communication data transfers is set. Here, in the normal AGC circuit, since the AGC gain control is always performed, the AGC circuit gain increases during the optical modulation OFF period (period in which there is no signal level). Therefore, next, when the optical modulation signal is turned on (when preamble), the amplification factor of the AGC circuit (hereinafter also referred to as AGC amplification factor) is exceeded. In such a situation, it takes a long time for the AGC gain to become stable, so it takes time to achieve synchronization, which is the purpose of the preamble. Further, in a normal AGC circuit, when the signal level of the optical modulation signal received on the optical receiver side changes when the communication distance between the optical transmitter and the optical receiver changes, an appropriate AGC circuit There are cases where the AGC gain cannot be set.

  The present invention has been made in view of such circumstances, and in an optical receiver having a function (AGC) for amplifying the amplitude of an electric signal output from a light receiving element, the adjustment time of the AGC amplification factor can be shortened. The aim is to realize a possible configuration.

  The present invention is directed to an optical receiver that receives an optical signal transmitted from a transmission side in an optical communication system that uses a preamble for synchronization. In such an optical receiver, an optical signal from the transmission side is provided. A light receiving element that receives the signal and converts it into an electric signal, a variable gain amplifier that amplifies the amplitude of the electric signal output from the light receiving element, and a control means that controls the amplification factor of the variable gain amplifier. The control means performs amplification factor control of the variable gain amplifier when receiving the preamble added to the communication data from the transmitting side, and controls the variable gain amplifier when not receiving the preamble. The technical feature is to fix the amplification factor.

  According to the present invention, the gain control (AGC gain control) of the variable gain amplifier is not always performed, but the AGC gain control is performed when the preamble from the transmission side is received (preamble period). Since the AGC gain control is not performed when the optical signal is not received (period in which the optical modulation is OFF) (amplification ratio is fixed), the AGC gain is in the optical modulation OFF period (period in which the signal level is zero). The AGC gain of the previous preamble period can be held and continued until the next preamble period. As a result, the adjustment time (setup time) of the AGC gain at the next preamble reception can be shortened.

  In the present invention, when a preamble is not received (when optical modulation is OFF), a DC component included in an electrical signal output from the light receiving element is detected, and the variable gain amplifier at the time of preamble reception is detected based on the DC component. An initial value of the gain (AGC gain) may be set. If such a configuration is adopted, the signal level of the received signal (light modulation signal) at the light receiving element has changed due to a change in the communication distance between the optical transmitter and the optical receiver constituting the optical communication system. Even in this case, the AGC gain can always be set appropriately.

  Specifically, for example, the relationship between the direct current component (corresponding to the light amount of the optical signal received by the light receiving element) included in the electrical signal output from the light receiving element and the appropriate AGC gain is measured in advance and tabulated. When the preamble is not received (when the optical modulation is OFF), the DC component included in the electrical signal output from the light receiving element is detected, and the AGC gain is obtained by referring to the above table based on the DC component. . Then, after detecting the DC component (after the optical modulation OFF period), at the next preamble, the initial amplification factor (initial value) at the time of this preamble is the AGC amplification factor (amplification obtained from the DC component) The AGC gain is adjusted as the rate. By such processing, it is possible to cope with a change in the signal level of the received signal due to a change in communication distance or the like, and an appropriate AGC gain adjustment can be completed in a short time.

  As an example of DC component detection means for detecting a DC component included in the electrical signal output from the light receiving element, only a DC component included in the electrical signal output from the first-stage amplifier is extracted. A low-pass filter can be mentioned. When such a low-pass filter is used, a direct current component can be detected with a simple circuit configuration.

  According to the present invention, in an optical receiver having a function of amplifying the amplitude of an electric signal output from a light receiving element, amplification factor control of a variable gain amplifier is performed only when a preamble added to communication data is received. As a result, the AGC gain adjustment time can be shortened.

It is a block diagram which shows an example of the optical receiver to which this invention is applied. It is a figure which shows an example of a low-pass filter. It is a block diagram which shows an example of the optical transmitter which comprises an optical communication system. It is a wave form diagram of the output signal (signal containing a direct-current component) of the first stage amplifier which amplifies the output signal of a photodiode. It is a timing chart which shows an example of the data transfer in an optical communication system.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  In this embodiment, an example in which the present invention is applied to the optical receiver in an optical communication system including an optical transmitter and an optical receiver that perform data communication by visible light communication will be described.

-Optical transmitter-
First, an example of the optical transmitter 100 constituting the optical communication system will be described with reference to FIG. The optical transmitter 100 of this example includes a modulation circuit 101, an LED drive circuit 102, an LED (light emitting diode) 103 that is a light source of a lighting device, and the like.

  The modulation circuit 101 modulates communication data (data superimposed on the illumination light) and outputs the modulated communication data to the LED drive circuit 102. The signal modulation method of the modulation circuit 101 may be a modulation method generally used in optical communication or the like. For example, an ASK method (Amplitude Shift Keying) method or a PPM (Pulse Position Modulation) method is used. ) Method. In addition, an orthogonal modulation / demodulation method such as a QPSK (quadture phase shift keying) method may be used.

  The LED drive circuit 102 blinks the LED 103 at high speed based on the modulation data modulated by the modulation circuit 101. As a result, a light modulation signal (communication data) is output superimposed on the illumination light. The output optical modulation signal is received by the optical receiver 1.

  Here, in the optical transmitter 100 of this example, the preamble and communication data are set (transmitted) in a set state. When transmitting the communication data, the preamble is added to the head of the communication data (see FIG. 5).

-Optical receiver-
Next, the optical receiver 1 will be described with reference to FIG.

  The optical receiver 1 receives a light modulation signal (communication data) output from the optical transmitter 100 and converts the light modulation signal into an electric signal (current signal) 2. An amplification circuit 3 that amplifies the electrical signal photoelectrically converted by the diode 2 and a demodulation circuit 4 that demodulates the electrical signal (light modulation signal) that has passed through the amplification circuit 3 are provided.

  The amplifier circuit 3 includes a first stage amplifier (photodiode light receiving circuit) 31, a band pass filter 32, a variable gain amplifier 33, a low pass filter 34, an AD converter 35, a processing control unit 36, a bias circuit 37, and the like.

  The first stage amplifier 31 is an amplifier (IV converter) composed of an operational amplifier 31a and a feedback resistor 31b, and amplifies the electrical signal photoelectrically converted by the photodiode 2. The electric signal amplified by the first stage amplifier 31 is input to the band pass filter 32.

  Since the first stage amplifier (IV conversion circuit) 31 is an inverting amplifier circuit using the operational amplifier 31a, the output voltage V of the operational amplifier 31a is [V = −Ipd × R Ipd: the output current R of the photodiode 2 R: Resistance value of the feedback resistor 31b], and a direct current component (see FIG. 4) described later has a negative value.

  The bandpass filter 32 is, for example, a filter having a three-stage Butterworth characteristic (amplitude flatness characteristic), and removes a noise component included in the electric signal output from the first-stage amplifier 31 to provide an optical modulation signal (communication data). Extract only. The amplitude of the optical modulation signal extracted by the bandpass filter 32 is adjusted by the variable gain amplifier 33 (details will be described later). The variable gain amplifier 33 is A / D converted by the AD converter 35 and then input to the processing control unit 36.

  The processing control unit 36 performs predetermined signal processing on the optical modulation signal (communication data) extracted by the bandpass filter 32 and passed through the variable gain amplifier 33 and the AD converter 35 and outputs the result to the demodulation circuit 4. Specifically, the optical modulation signal is sampled by the AD converter 35 at a frequency higher than twice that of the optical modulation signal, and the optical modulation signal is converted into a digital signal (sampling theorem). Next, the converted data is passed through a moving average filter to remove noise and output to the demodulation circuit 4. The demodulating circuit 4 demodulates the optical modulation signal from the processing control unit 36 to obtain communication data.

  Next, “DC component removal processing” and “AGC gain control”, which are characteristic parts of the optical receiver 1 of the present embodiment, will be described.

-DC component removal treatment-
First, in the amplifier circuit 3 shown in FIG. 1, the output signal of the first stage amplifier 31 when “DC component removal” is not performed has a waveform as shown in FIG. As can be seen from FIG. 4, the output signal of the first-stage amplifier 31 includes an optical modulation signal (communication data) and two signal components such as illumination light. For this reason, the amplification factor of the first stage amplifier 31 cannot be increased. That is, when the amplification factor of the first stage amplifier 31 is increased, the signal component (substantially a direct current component) such as illumination light is also greatly amplified, and the output of the first stage amplifier 31 is saturated.
End up.

  In consideration of such points, in this embodiment, a DC component included in the output signal of the first-stage amplifier 31 is extracted, and a bias current is fed back to the input side of the first-stage amplifier 31 so as to cancel this DC component. The direct current component of the electric signal input to the first stage amplifier 31 is removed. The specific configuration will be described below.

  In the present embodiment, as shown in FIG. 1, a low-pass filter 34 that removes a light modulation signal (communication data) included in the output signal of the first-stage amplifier 31 and extracts a DC component (signal component such as illumination light); A bias circuit 37 including a DA converter 37a and a resistor 37b is provided, and an output terminal (resistor 37b) of the bias circuit 37 is provided on the input side of the first stage amplifier 31 (between the photodiode 2 and the first stage amplifier 31). Connected.

  The DC component extracted by the low-pass filter 34 is A / D converted by the AD converter 35 and then input to the processing control unit 36. The processing control unit 36 temporarily stores the DC component (digital signal after A / D conversion) extracted by the low-pass filter 34, and cancels the DC component (DC component becomes zero). Signal) to the bias circuit 37. In the bias circuit 37, the bias signal (digital signal) from the processing control unit 36 is converted into an analog signal (bias current) by the DA converter 37a, and is fed back to the upstream side of the first stage amplifier 31 through the resistor 37b.

  In this way, by supplying a bias current that cancels the DC component to the input side of the first-stage amplifier 31, the DC component (signal component such as illumination light) of the electrical signal input to the first-stage amplifier 31 can be removed. The amplification factor of the first stage amplifier 31 can be increased. Thereby, for example, a weak light modulation signal can be received, and the dynamic range of reception of the light modulation signal can be expanded.

  Next, an example of the low-pass filter 34 used in the present embodiment will be described with reference to FIG. The low-pass filter 34 shown in FIG. 2 is a filter having a simple circuit configuration including one resistor 34a and one capacitor 34b.

  The low-pass filter 34 used in the present embodiment is intended to extract only the DC component of the electrical signal output from the first stage amplifier 3 as described above, and therefore the cut-off frequency fc is set to a small value. For example, in this example, the resistance value R of the resistor 34a shown in FIG. ] Is set.

  If such a low-pass filter 34 is used, the light modulation signal (communication data) included in the output signal of the first-stage amplifier 31 is removed with a simple circuit configuration, and a direct current component (disturbance light signal component such as fluorescent light) is removed. It becomes possible to extract (detect) only.

-AGC gain control-
First, in the optical communication system to which this embodiment is applied, as shown in FIG. 5, the preamble and communication data are transferred in a set state, the optical modulation ON period during which the communication data is transferred, and the communication An optical modulation OFF period between data transfers is set.

  Here, in a normal AGC circuit, the signal that has passed through the bandpass filter 32 is always amplified to a constant amplitude. In such constant AGC gain control, since the AGC gain is adjusted even when the optical modulation between the communication data transfers is OFF, the amplitude of the variable gain amplifier 33 is shaken off at the next preamble reception, and AD The AD value of the converter 35 overflows. In such a situation, it takes a lot of time to keep the output value of the variable gain amplifier 33 (the AD value of the AD converter 35) within the specified value (the AGC gain is stabilized) at the time of preamble reception. For this reason, it is necessary to lengthen the preamble period, and it takes time until synchronization, which is the purpose of the preamble, can be achieved.

  In a normal AGC circuit, when the signal level of the optical modulation signal received on the optical receiver 1 side changes, such as when the communication distance between the optical transmitter 100 and the optical receiver 1 changes. In some cases, an appropriate AGC gain cannot be set.

  In consideration of such points, in the present embodiment, adjustment control (also referred to as AGC gain control) of the gain of the variable gain amplifier 33 is not always performed, but is performed only at the time of preamble reception. The technical feature is to shorten the adjustment time of the AGC gain. An example of the specific AGC gain control (control executed by the processing control unit 36) will be described.

  First, the processing control unit 36 receives a preamble based on a signal (after A / D conversion) received by the photodiode 2 and passed through the band pass filter 32 (hereinafter also referred to as a preamble period). It is determined whether or not there is. When it is a preamble period, AGC gain control is performed. Specifically, the processing control unit 36 determines that the output value (AD value) of the AD converter 35 is within a predetermined range (for example, when the 8-bit AD converter 35 is used) during the preamble period. The gain of the variable gain amplifier 33 is adjusted so that it falls within the range of 235 to 245. By such AGC gain control, the amplitude of the signal that has passed through the bandpass filter 32 can be made uniform.

  Then, when the AGC gain control is completed (when the AD value of the AD converter 35 falls within the specified range), the gain of the variable gain amplifier 33 is fixed, and this AGC gain is used until the next preamble period. Hold and continue. By such processing, the AGC gain control is not performed in the optical modulation OFF period (period in which the signal level is zero) shown in FIG. 5, and the AGC gain is not increased. Therefore, the AGC in the next preamble period is not performed. The adjustment time of the amplification factor can be shortened.

  Here, as described above, the signal level of the optical modulation signal received by the photodiode 2 may change (decrease) due to a change in the communication distance between the optical transmitter 100 and the optical receiver 1. In order to solve this problem, in the present embodiment, the initial AGC amplification factor at the time of preamble is set using the DC component (corresponding to the light amount of the optical signal received by the photodiode 2) extracted by the low-pass filter 34 described above. By doing so, the AGC gain is set appropriately. A specific example will be described.

  First, in the present embodiment, the relationship between the DC component (DC component extracted by the low-pass filter 34) included in the electrical signal output from the photodiode 2 and the appropriate AGC gain is acquired in advance by measurement and calculation. Then, it is made into a table and the table is stored in the processing control unit 36. Further, as described above, the processing control unit 36 of the present embodiment recognizes and stores the DC component extracted by the low-pass filter 34 and passed through the AD converter 35.

  Then, the processing control unit 36 obtains the AGC gain by referring to the table using the DC component (after A / D conversion) extracted by the low-pass filter 34 in the light modulation OFF period shown in FIG. The AGC gain obtained when the light modulation is OFF is set as the initial AGC gain at the next preamble. By such processing, it is possible to cope with a change in the signal level of the received signal (optical modulation signal) due to a change in communication distance or the like, and an appropriate AGC gain adjustment can be completed in a short time.

  When there is no change in the communication distance (when there is no difference between the direct current component when the previous optical modulation is OFF and the direct current component when the current optical modulation is OFF), the AGC amplification set at the previous preamble is used. The rate is directly used as the initial AGC gain at the time of this preamble.

-Other embodiments-
In the above embodiment, the low-pass filter 34 is used as means for detecting the DC component included in the output signal of the first-stage amplifier 31, but the present invention is not limited to this and is included in the output signal of the first-stage amplifier 31. Any other circuit configuration detection means may be used as long as it can detect the direct current component.

  In the above embodiment, an optical communication system for one-way communication including the optical transmitter 100 having the modulation circuit 101, the LED 103, and the like, and the optical receiver 1 having the photodiode 2, the amplification circuit 3, the demodulation circuit 4, and the like. Although described as an example, the present invention is not limited to this, and bidirectional communication is provided with an LED drive circuit system (modulation circuit, etc.) and a light receiving circuit system (demodulation circuit, etc.) on both the transmission side and the reception side. The present invention can also be applied to an amplifier circuit used in an optical communication system.

  In the above embodiment, the visible light communication system that performs data communication using the visible light output from the LED has been described as an example. However, the present invention is not limited thereto, for example, a discharge lamp such as a fluorescent lamp, The present invention is also applicable to an amplifier circuit used in a visible light communication system that performs data communication using visible light output from a light source such as an organic EL or an inorganic EL.

  INDUSTRIAL APPLICABILITY The present invention can be used for an optical receiver that receives an optical modulation signal in visible light communication or the like, and more specifically, in an optical receiver having a function of amplifying the amplitude of an electric signal output from a light receiving element. It can be effectively used for a technique for shortening the rate adjustment time.

1 Optical receiver 2 Photodiode (light receiving element)
DESCRIPTION OF SYMBOLS 3 Amplifier circuit 31 First stage amplifier 31a Operational amplifier 31b Feedback resistor 32 Band pass filter 33 Variable gain amplifier 34 Low pass filter (DC component detection means)
35 AD converter 36 Processing control unit (control means)
37 Bias circuit 37a DA converter 37b Resistor 4 Demodulation circuit 100 Optical transmitter 101 Modulation circuit 102 LED drive circuit 103 LED

Claims (4)

In an optical communication system that uses a preamble for synchronization, an optical receiver that receives an optical signal transmitted from a transmission side,
A light receiving element that receives an optical signal from the transmission side and converts it into an electric signal, a variable gain amplifier that amplifies the amplitude of the electric signal output from the light receiving element, and a control means that controls the amplification factor of the variable gain amplifier And
The control means performs gain control of the variable gain amplifier when receiving a preamble added to communication data from the transmitting side, and performs gain control of the variable gain amplifier when not receiving the preamble. An optical receiver characterized by fixing. The optical receiver according to claim 1.
When the preamble is not received, the DC component included in the electrical signal output by the light receiving element is detected by the DC component detecting means, and based on the DC component, the variable gain amplifier at the time of receiving the preamble is detected. An optical receiver characterized by setting an initial value of an amplification factor. The optical receiver according to claim 2.
Using a table in which the relationship between the DC component and the gain of the variable gain amplifier is set in advance, the gain of the variable gain amplifier at the time of receiving the preamble is based on the DC component when the preamble is not received. An optical receiver characterized by setting an initial value. The optical receiver according to claim 2 or 3,
2. The optical receiver according to claim 1, wherein the direct current component detecting means is a low-pass filter that includes a resistor and a capacitor and extracts only a direct current component included in an electric signal output from the light receiving element.

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