JPH0322745B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical data transmission device using an optical fiber, and particularly to a carrier detection circuit.

[Prior Art and Problems to be Solved by the Invention] Recently, data signals using optical fibers have become popular, but when the transmission distance is long or when a wide dynamic range of reception level is desired, it is necessary to use optical fibers. It is common to use a so-called AC coupling configuration in which a capacitor is inserted between each amplifier stage of the receiver.

FIG. 1 is a circuit diagram of a conventional optical receiver. When pin photodiode PD2 receives light, it generates a current,
The signal is amplified by the preamplifier 3 and stored in the capacitor 4a. The current filling this capacitor 4a is amplified by the main amplifier 5, and is then amplified by the capacitor 4b.
The current stored in this capacitor 4b is compared with a reference current in a comparator 6 and produces a digital signal output 7. That is, this optical receiver 1 generates a digital signal output 7 proportional to the amount of light received by the pin photodiode PD2.

This method is suitable for long transmission distances or when you want to widen the dynamic level of the reception level, but it has the disadvantage that only noise is amplified when there is no signal or when the optical input is at a constant DC level. I have it too.

Signal A shown at the lower right of FIG. 1 shows the output when there is no signal, and signal B shows the output when there is a signal.
Therefore, it has the disadvantage of being bothered by noise when there is no signal.

When such an AC coupling type optical receiver 1 is used, the following circuit is often used to determine the presence or absence of a carrier.

FIG. 2 is a circuit diagram of another conventional optical receiver.
In this case, after the optical input signal is amplified by the amplifier 8,
The wave is detected by the detector 9, and the magnitude of the DC level is detected by the comparator 1.
The presence or absence of a signal is determined by comparing with 0. However, this method causes a fairly long detection lag time, so a noise signal is output for a while after the optical signal level drops, and this noise signal can cause optical data transmission equipment to malfunction. It had a drawback.

In addition, as another circuit for determining the presence or absence of a carrier, Japanese Patent Laid-Open No. 71502/1987 describes an avalanche photodiode that receives an optical signal and converts it into an electrical signal, and amplifies its electrical output. a peak detection circuit that detects the peak value of the output of the amplifier, a bias voltage control circuit that controls the bias voltage of the avalanche photodiode using the output of the peak detection circuit, and a slicer circuit that slices the output of the amplifier. An optical receiving alarm circuit is proposed.

However, since the above circuit includes an average value detection circuit, which is an analog circuit, in the digital circuit, it has a complicated circuit configuration, resulting in an increase in the size of the entire optical receiver. It was hot.

Furthermore, since it is an analog circuit, it takes a lot of effort to adjust the average value detection circuit.

SUMMARY OF THE INVENTION An object of the present invention is to provide a carrier detection circuit capable of reliably detecting the presence or absence of a signal using an extremely simple circuit consisting only of digital circuits, by eliminating the drawbacks of the conventional techniques described above.

[Means for Solving the Problems] The present invention uses pulse width modulation (hereinafter referred to as pulse width modulation) as a communication method.
(PWM) method, and its gist is that a retriggerable monostable multivibrator is connected in a two-stage cascade to the digital output section of the photoelectric conversion circuit of the optical receiver, and the first stage The time constant of the first stage monostable multivibrator is set so that the pulse width of the output signal of the monostable multivibrator arranged in the second stage is narrower than the unit information period width of the optical signal. The time constant of the second-stage monostable multivibrator is set so that the pulse width of the output signal of the arranged monostable multivibrator is wider than the unit information period width of the optical signal.

[Operation] PWM represents information by pulse widths, and for example, pulse widths t1, t2, t3, and t4 shown at a1 in FIG. 4A represent information.

Assuming that if a signal with a pulse width of t1 is sent, it will be "0", and if a signal with a pulse width of t2 is sent, it will be "1", then this signal information will be "0100". It turns out.

Note that each signal has a fixed period (T in Figure 4)
It will be sent to you. (This period will be referred to as a unit information period.) According to the present invention, the pulse width of the output signal of the first stage monostable multivibrator is made narrower than the unit information period width of the optical signal. As a result, this output signal is always in the OFF state while the next optical signal pulse is sent, and as a result, when a signal is present, a pulse signal is always output for each unit information period. become.

Next, by making the pulse width of the output signal of the second stage monostable multivibrator wider than the unit information period width of the optical signal, the output signal of the first stage monostable multivibrator From the rising edge of the pulse width to the rising edge of the next pulse,
The output signal of the second-stage monostable multivibrator is always on, and as a result, the output signal of the second-stage monostable multivibrator remains on when a signal is present.

On the other hand, when there is no signal, the noise signal continuously triggers the first stage monostable multivibrator, so the output signal of the first stage monostable multivibrator remains on. become.

Therefore, no trigger is applied to the second-stage monostable multivibrator, and its output signal remains off.

[Embodiment] An embodiment of the carrier detection circuit according to the present invention will be described with reference to FIGS. 3 and 4.

In FIG. 3, the same parts as in FIG. 1 are designated by the same reference numerals. This figure 3 is similar to the normal one shown in figure 1.
Two stages of retriggerable monostable multivibrators 11 and 12 are connected to the output side of the AC-coupled optical receiver 1, and the output signal of the second stage monostable multivibrator 12, which is a carrier detection circuit, and the optical The output signal of the receiver 1 is connected with an AND circuit.

Therefore, when the carrier detection circuit is on,
That is, only when a signal is present, the output signal of the optical receiver 1 becomes the digital output signal 7 as it is.

FIG. 4 is a diagram showing output signals at points a, b, and c of the optical receiver of FIG. 3.

FIG. 4A shows output signals a1, b2, and c3 at points a, b, and c when a regular modulation signal is input, that is, when a signal is present.

When the PWM signal a1 is received with the carrier on, the first stage monostable multivibrator 11
Accordingly, a signal b1 having a pulse width narrower than the unit information cycle width of the optical signal is output in synchronization with the rising edge of the PWM signal a1. This signal b1 is input as is to the second stage monostable multivibrator 12.

The second-stage monostable multivibrator 12 is set to output a signal c1 with a pulse width wider than the unit information period width of the optical signal, and the pulse output does not end when regular data is input. The next trigger signal is input. At this time, since the second-stage monostable multivibrator 12 can be retriggered, its output signal c1 becomes "1" and the carrier-on state is maintained.

FIG. 4B is a diagram showing output signals a2, b2, c2 at points a, b, and c when there is no signal. When there is no signal, high frequency noise a2 is normally generated from the optical receiver 1, so the first stage monostable multivibrator 11 is retriggered one after another and the signal b2 is held in the "1" state. become. When this signal b2 is input to the second-stage monostable multivibrator 12, no trigger is applied, and the signal c2 becomes "0" and the carrier detection signal is turned off.

That is, in a signal state such as signal a1, the output signal of the second stage monostable multivibrator becomes an on signal such as signal c1, and in a non-signal state such as signal a2, the output signal of the second stage monostable multivibrator becomes an on signal such as signal c1. The output signal of the monostable multivibrator in the third stage is signal c
2, the presence or absence of an input signal can be clearly determined.

[Effects of the Invention] As explained above, according to the carrier detection circuit of the present invention, two stages of monostable multivibrators are connected to the output side of the optical receiver, and when the first stage monostable multivibrator The constant is set so that the pulse width of the output signal is narrower than the unit information period width of the optical signal, and the time constant of the second stage monostable multivibrator is set so that the pulse width of the output signal is narrower than the unit information period width of the optical signal. Since the presence or absence of a carrier can be reliably detected using only an extremely simple digital circuit that is set to be wider than the unit information cycle width, the carrier detection circuit can be extremely miniaturized, which in turn leads to the miniaturization of optical data transmission equipment. It also leads to

Additionally, compared to analog circuits, it takes less time and effort to adjust each circuit element, leading to improved maintenance.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional optical receiver, FIG. 2 is a circuit diagram of another conventional optical receiver, FIG. 3 is a circuit diagram of an optical receiver that is an embodiment of the present invention, and FIG. 4 is a circuit diagram of a conventional optical receiver. 4 is a diagram illustrating output signals at points a, b, and c of the optical receiver of FIG. 3. FIG. 1: Optical receiver, 2: Pin photodiode PD,
3: Preamplifier, 4: Capacitor, 5, 5': Main amplifier, 6, 10: Comparator, 7: Digital signal output, 8: Amplifier, 9: Detector, 11, 1
2: Monostable multivibrator.

Claims (1)

[Claims] 1. In a carrier detection circuit used in an optical data transmission device whose communication method is a pulse width modulation method, a retriggerable monostable multivibrator is connected in a two-stage cascade to the digital output section of the photoelectric conversion circuit of the optical receiver, and The time constant of the monostable multivibrator in the first stage is set so that the pulse width of the output signal of the monostable multivibrator arranged in the first stage is narrower than the unit information period width of the optical signal, and The time constant of the monostable multivibrator in the second stage is set so that the pulse width of the output signal of the monostable multivibrator arranged in the second stage is wider than the unit information period width of the optical signal. A carrier detection circuit featuring: