JPS6041498B2 - Input signal disconnection detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a receiving device for pulsed optical transmission using an avalanche photodiode (also referred to as an APD) as an optical/electrical conversion element, in which the avalanche multiplication factor of the APD is controlled by negative feedback to achieve equal output power. The present invention relates to an input signal disconnection detection circuit for the above-mentioned receiving device which adopts a method of stabilizing the output amplitude of the b amplifier.

Generally, an APD is often used as an optical/electrical conversion element in an optical transmission receiving device.

This is because the reverse biased APD has a photocurrent multiplication effect using the avalanche effect, and high sensitivity can be easily obtained. However, since the APD's reverse bias voltage vs. avalanche amplification characteristic (hereinafter abbreviated as Vr-M characteristic) is in principle greatly affected by the ambient temperature, some avalanche amplification degree M (hereinafter amplification degree M or simply M) (abbreviated)) Stabilization measures are required. In the normal method, M stabilization stabilizes the amplification degree M by changing the reverse bias voltage Vr (hereinafter simply referred to as Vr) using negative feedback control so as to keep the output amplitude of the final stage of the receiving equalization amplifier circuit constant. The method is adopted. In the receiver for pulsed optical transmission as described above, in order to detect an input signal interruption, it is sufficient to detect the interruption of the signal component or the interruption of the clock component, but in the case of an interruption of the input signal, reverse bias Voltage Vr is APD
As will be explained later, the former method has not been taken up in the past because it is difficult to remove the noise, and the latter method has not been used. This has been put into practical use because the influence of noise can be almost completely eliminated by detecting the clock interruption in the middle of the operation of the IJ transmitter.

However, this latter method compresses the level difference between when the input signal is cut off and when the signal is input, so although it can be detected, there is a problem in terms of stability, and the pulse repetition frequency is
It could not be used at around 00MHZ. Note that detecting clock loss after the limiter is a problem because even if the signal is interrupted, the noise level from the APD will be 100% higher than the normal level.
or 2) Since a small clock component is extracted and amplified by the limiter circuit to a size comparable to that of a normal clock, this becomes completely impossible. Furthermore, if detection is to be performed in front of the limiter, a large gain amplifier will be required, resulting in an increase in circuit scale and power consumption, which is inconvenient. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a circuit that reliably detects input signal interruption even when the pulse repetition frequency is high.

In order to achieve the above object, the present invention reconsiders the method using signal component disconnection detection, which was not considered because it was difficult to remove the noise mentioned above, and performs special signal processing as explained later. On the contrary, this is achieved by taking advantage of the fact that the noise is quite large.

That is, according to the present invention, an avalanche photodiode is used as an optical/electrical conversion element, and negative feedback is applied to compensate for changes in the amplification degree of the avalanche photodiode due to temperature or other factors so as to keep the amplitude of the output of the receiving amplifier circuit constant. In a circuit for detecting a disconnection of an input signal in a controlled type of pulsed optical transmission receiver, a delay circuit that gives the output a delay time equal to a pulse repetition period, and either one of the sign of the delayed output and the sign of the driving force. means for inverting the signal and adding the inverted sign and the other sign to generate an addition signal; a peak amplitude circuit for detecting the peak value of the amplitude of the addition signal; There is obtained an input signal disconnection detection circuit characterized in that it has means for detecting input signal disconnection.

Next, the present invention will be explained in detail with reference to the drawings.

Figure 1 shows the APD when the input signal to the receiver is cut off.
2 is a diagram showing the spectral characteristics of noise generated from the

This noise consists of two types of components. One noise component, indicated by A in the figure, is microplasma noise (MP It is a nearly trapezoidal pulse-like noise with a spectrum spread of several tens of MHz.
It is. Another noise component indicated by B is called shot noise, which is a multiplication of the floor current of the APD, and its spectrum is independent of frequency and exhibits substantially flat characteristics. C is the combined noise obtained by adding A and B. Furthermore, APD
The noise generated when the signal is inputted shows a small value of -3 old to -5 medium old or so, depending on the state of the APD, regardless of the frequency, and the intermediate value B of -4 is marked with a circle and marked with a dotted line in the same diagram. As you can easily see by comparing curve C and ○, even if the value of D is 1 3 MB, you can see how large the noise generated by the APD is when the input signal is disconnected. This makes it difficult to detect signal interruption. FIG. 2 is a block diagram showing the circuit configuration of an embodiment of the present invention.

In FIG. 2, 11 represents a delay circuit that provides a delay time td equal to the pulse repetition period T, 12 a sign inversion circuit, 13 an addition circuit, 14 a peak amplitude detection circuit, and 15 a comparison circuit. Note that a to d representing signals will be explained later. FIG. 3 is a time chart for explaining the operation of the circuit in FIG. 2,
The symbols al and a2 shown on the left side of the figure are a in Figure 2 and bl
and b2 correspond to b, and cl and c2 correspond to c, respectively. Here, al represents the waveform when the input signal is input, and in this case, the pulse repetition period T is 2.8 mm (40 MHz), the rise time is 1 ship, and the duty ratio is 0.0 amplitude Vp. , is a code string of "01lloo01" of 1 volt. bl and cl represent the outputs of the inverting circuit 12 and the adder 13, respectively. a2, b2, and c2 represent the noise generated when the input signal is disconnected and its This represents change.Next, the second
The operation of the circuit shown in the figure will be explained with reference to FIG.

When there is an input signal, that is, during normal operation, when the signal al is input, this signal is delayed by td in the delay circuit 11, and is inverted in the inverting circuit 12 to become a signal as shown by bl. This signal is added to the input signal al in the adder circuit 13, and becomes a large signal with an amplitude of 2Vp, that is, 2 volts, as shown by cl. In this case, as can be seen from the figure, where the original signal al has consecutive "1" codes, only one pair of upper and lower pulses separated from each other remains in cl for the 1-bit codes. This signal cl is applied to the comparator 15 by a reference voltage Vrd which is selected to be smaller than 2Vp and equal to, for example, Vp (described in more detail later).
The voltage of cl is compared with the reference voltage V as the comparison output d.
Outputs the code "1" indicating that it is higher than rer.
In this case, 2Vp is 2 volts and Vrd is 1 volt, so it is easy to send out the comparison output d. Note that the signal cl is also used separately for timing extraction. Next, to explain the case where the signal is disconnected, if the input signal al has the characteristics described above, the input noise a2 gradually reaches the stabilizing voltage Vp with a rise time of about 1 second, and the signal Once the cut is over, it will slowly return to zero in the same way.

Therefore, the signal b2 delayed by td and inverted becomes as shown in the figure, and by adding this b2 and bl, as shown in c2, a signal with a very small amplitude Vp2, a simple numerical value Z
According to the calculation, the signal has an amplitude of 0.5 volt, which is one-fourth of the amplitude 2Vp2 when the input signal is not interrupted. This signal c2 is compared with the reference voltage Vref in the comparator 15, but since the voltage Vref is selected to be Vp, that is, 1 volt, the rhombus of both voltages is about 0.5 volt J, and the comparator 1
5 outputs the code "0" indicating that the signal c2 is smaller than the reference voltage VM. Therefore, by knowing the content of the comparison output d, it is possible to easily detect an input signal disconnection.

Note that although specific numerical values have been used in the explanation above to facilitate understanding, it goes without saying that these numerical values are merely examples and are not limited to these. For example, the duty ratio may be 1. Regarding the circuit configuration, the inverter 12 may be provided on the line that does not delay the input signal, unlike the figure. FIG. 4 is a diagram showing a configuration circuit of another embodiment of the present invention.
I is a coaxial cable with terminal short circuit length L and characteristic impedance RT, and D and D2 are peak detection diodes.

In this coaxial cable, the terminal short circuit length L is selected so that the official transmission time is td, and it has substantially the same function as the circuit of FIG. FIG. 5 is a block diagram showing an example of the configuration of a receiving device for pulsed light transmission using the input disconnection detection circuit according to the present invention.

The optical signal input to the receiving device is converted into an electrical signal by the APD 21, amplified to a required amplitude by the equalization amplifier circuit 22, and output as a signal a. This signal a branches into three directions,
The first signal is detected by the output amplitude detection circuit 23 and
By operating the PD bias DC booster circuit 24, the amplification degree of the APD is controlled in a feedback manner, and the output amplitude of the stabilizing amplifier 22 is kept constant. Next, the second signal enters the signal disconnection detection circuit 25 (corresponding to the circuit shown in FIG. 3 or 4), and by the operation described above, it generates a signal c, which can also be called a timing extraction signal, and a comparison output d. Output. The signal c with a repetition period of T is sent to the timing circuit 26, and although not shown in the figure, it is double-wave rectified here and has a repetition frequency of 1.
/T is extracted as a clock e, and the identification circuit 27 is operated. Comparison output d is switch circuit 2
Sent to 8th. Furthermore, the third signal is sent directly to the switch circuit 28, which is connected when the comparison output d is an output representing "peak amplitude>Vrer", which in the previous case is the sign "1", and " Peak amplitude > VM
Since the output representing "f", which in the previous case is "0", is set to be disconnected, the third signal is determined according to al, whether input signal a is disconnected or a2 is not. Either it ends at the switch circuit 28 or it passes through this switch circuit and is input to the identification circuit 27.The identification circuit is activated by the clock e, so the identification operation is performed. The operation of the receiving device is carried out.The device shown in Fig. 5 above corresponds to a pulse repetition frequency of 400 MHZ for the input signal a, but if the circuit characteristics are appropriately selected, it is possible to achieve a pulse repetition frequency of up to about 800 MHZ. According to the present invention, it is possible to obtain a signal detection circuit that has sufficient margin and operates at a high frequency by utilizing the difference between the characteristics of noise generated from an APD and the characteristics of a normal signal.

[Brief explanation of the drawing]

Figure 1 shows the APD when the input signal to the receiver is cut off.
2 is a block diagram showing the circuit configuration of an embodiment of the present invention, and FIG. 3 is a time chart for explaining the operation of the circuit in FIG. 2. FIG. 4 is a diagram showing a circuit configuration of another embodiment of the present invention, and FIG. 5 is a block diagram showing an example of the configuration of a pulsed light transmission receiving device using an input disconnection detection circuit according to the present invention. be. Explanation of symbols: 11 is a delay circuit, 12 is an inversion circuit, 13 is an adder, 14 is a peak amplitude detection circuit, 15 is a comparison circuit,
21 is an avalanche photodiode (APD), 22
is an equalization amplifier circuit, 23 is an output amplitude detection circuit, and 24 is an AP
DC booster circuit for D bias, 25 is a signal disconnection detection circuit, 2
6 represents a timing circuit, 27 represents an identification circuit, and 28 represents a switch circuit. Figure 1 Figure 2 Figure 3 Figure 5

Claims (1)

[Claims] 1 A type of device in which an avalanche photodiode is used as an optical/electrical conversion element, and negative feedback is applied to control changes in the amplification degree of the avalanche photodiode due to temperature, etc., so that the amplitude of the output of the receiving amplifier circuit is constant. A circuit for detecting disconnection of an input signal in a receiver for pulsed optical transmission includes a delay circuit that gives the output a delay time equal to a pulse repetition period, and a delay circuit that inverts either the sign of the delayed output or the sign of the original output. means for generating an addition signal by adding the inverted sign and the other sign; a peak amplitude detection circuit for detecting the peak value of the amplitude of the addition signal; An input signal stage detection circuit comprising means for detecting a disconnection.