JPH07193474A - Waveform shaping circuit - Google Patents

Abstract

PURPOSE:To shape a waveform by generating a stable comparison signal while using a voltage with an input voltage used as a reference as a comparing voltage. CONSTITUTION:A select signal output means 2 is turned on and off corresponding to an output from a differential amplifier 3. Based on an input voltage VB, a comparison voltage setting means 1 sets a comparison voltage VC. The voltage VC is delayed to rise or fall of the input voltage VB because of the response characteristics of a photodetector, the threshold voltages of respective transistors inside the waveform forming circuit and the transient characteristics of switching or the like. Thus, when an input signal B is changed, the voltage VB of the input signal crosses the comparison voltage VC. The rise and fall of the input voltage VB are detected by utilizing this crossing of respective voltages VB and VC caused by the delay inside the circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveform shaping circuit that compares an input voltage with a comparison voltage and outputs the resulting waveform, and is used, for example, in a light receiving device for processing a voltage-converted optical signal. .

[0002]

2. Description of the Related Art FIG. 6 is a diagram showing a conventional configuration example, showing a configuration of an optical signal transmission device including a waveform shaping circuit.

The optical signal transmission device includes a transmitting device and a receiving device. The transmission device includes a light emitting element drive circuit 51 and a light emitting element 52, and the transmission signal a is converted into an optical signal by the light emitting element 52 and output. The receiving device includes a light receiving element (photodiode) 61 and an amplifier circuit 6.
2. A peak hold circuit 63 and a comparator 64 are provided, and the portion indicated by the alternate long and short dash line in the figure is the waveform shaping circuit. The light receiving element 61 converts the optical signal from the light emitting element 52 into a current, which is converted into a current by the amplifier circuit 62 (input signal b), and then input to the peak hold circuit 63. The peak hold circuit 63 holds the peak voltage of the input signal b and outputs a voltage that is ½ of the peak voltage as the comparison signal c. The comparator 64 compares the input waveform b with the comparison signal c and outputs the result (output signal d).

[0004]

However, the conventional waveform shaping circuit has the following problems. Figure 7
~ It demonstrates with reference to FIG. 7 to 9 show a in FIG.
Waveforms at points d to e are shown.

A cycle of the input signal b (transmission signal a),
The output signal d depends on the time constant of the peak hold circuit 63.
There is a case where distortion occurs (delay time changes). When the cycle of the input signal b is sufficiently smaller than the time constant of the peak hold circuit 63, the operation is normally performed as shown in FIG. 7A, but the cycle of the input signal b is the time constant of the peak hold circuit 63. The input signal b
The level of the comparison signal c fluctuates within the period of
As shown in (B), the waveform of the output signal d is distorted by tw1 as compared with the case where the normal comparison signal c ′ is output.

[0006] When the level of the input signal b has a solid difference due to manufacturing factors or the like, the peak voltage held by the peak hold circuit 63 fluctuates, so that the level of the comparison signal c changes for each waveform shaping circuit and the output Variation occurs in the delay time of the signal d. For example, comparing the case where the level of the input signal b is high as shown in FIG. 8A and the case where the level of the input signal b is low as shown in FIG. 8B, the delay times are tw2 and tw3. There are variations. Therefore, for example, when signals are transmitted by using a plurality of optical signal transmission devices at the same time, variations in delay time must be taken into consideration.

If the responsiveness of the transmitting device decreases during high-speed signal transmission, the peak signal of the input signal b becomes lower than in the normal case, and the comparison signal c fluctuates and the pulse of the output signal d changes. The width was sometimes distorted. This will be described with reference to FIG. In FIG. 9, e
Shows the waveform of the optical signal output from the transmitting device. When the transmission device is sufficiently following the transmission signal a as shown in FIG. 9A, the peak voltages of the optical signal e and the input signal b have reached the maximum values, and the comparison signal c
Also has a stable value of 1/2 of the signal amplitude. However, as shown in FIG. 9B, the output e of the transmission device
Becomes no longer following the transmission signal a, the high level of the optical signal e decreases and the low level of the optical signal e increases. As a result, the waveform of the input signal b also becomes similar, and the comparison signal c has a signal amplitude of 1 /
It doesn't become 2 (it becomes low). As a result, there is a problem that the pulse width of the output signal d is distorted by tw4 and tw5.

In view of the above problems, the present invention generates a stable comparison signal without being affected by the cycle of the input signal, fluctuations in peak voltage due to manufacturing reasons, speed of the input signal, and the like. It is an object of the present invention to provide a waveform shaping circuit that can perform waveform shaping by using the above method.

[0009]

According to the present invention, there is provided a selection signal output means for detecting a rising edge and a falling edge of an input voltage and outputting a selection signal according to the detected state, and a selection signal indicating a rising edge of the input voltage. After inputting, a voltage lower than the input voltage by a constant voltage is output as the comparison voltage, and after a selection signal indicating the falling edge of the input voltage is input, a voltage higher than the input voltage by a constant voltage is used as the comparison voltage. It is characterized by further comprising a comparison signal setting means for outputting, a comparison means for comparing the input voltage and the comparison voltage, and outputting the comparison result.

[0010]

Now, assume that the input voltage is VB and the comparison voltage is VC. Then, after the rising edge of the input voltage VB is detected, "VB-α (constant voltage)" is output as a comparison voltage, and after the falling edge of the input voltage VB is detected, "VB-α (constant voltage)" is output.
“B + α” is output as a comparison voltage.

FIG. 1 is a diagram for explaining the operation of the present invention, showing the waveforms of the input signal B, the comparison signal C, and the output signal D. The voltage of the input signal B is the input voltage VB, and the voltage of the comparison signal C is the comparison voltage VC.

Now, it is assumed that when the input voltage VB is low (when no signal is input), it is stable at “VC = VB + α”. In this state, when the input voltage VB rises (when a signal is input), it switches to the state of “VC = VB−α” and stabilizes in this state while the input voltage VB is high.
In the figure, the delay of VC is shown.

Next, when the input voltage VB falls (when the signal input is turned off), the state is switched to "VC = VB + α" again. Then, while the input voltage is low, it is stable in this state.

As a waveform shaping output signal D, the comparison result of the input signal B and the comparison signal C is output.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a diagram showing a circuit configuration of an optical signal transmission apparatus having a receiving device including a waveform shaping circuit according to an embodiment of the present invention and a transmitting device. In the figure,
Light emitting element drive circuit 51 and light emitting element 5 of the transmitting device
The configuration of No. 2 and the configurations of the light receiving element 61, the amplifier circuit 62, and the comparator 64 of the receiving device are the same as the conventional ones, and thus the description thereof will be omitted.

This circuit comprises comparison voltage setting means 1, selection signal output means 2, differential amplifier 3, and constant current circuits 4 and 5.
Is included. The comparison means of the present invention corresponds to the comparator 64.

The differential amplifier 3 amplifies the difference between the voltage (input voltage VB) input from the amplifier circuit 62 and the comparison voltage VC generated by the comparison voltage setting means 1, and the comparator 64
And output to the selection signal output means 2. The selection signal output means 2 has transistors Q ₁₃ and Q ₁₄ , and a differential amplifier 3
Turns on and off according to the output from. Now, VB <VC
The state of No. 2 is a state without an optical signal input, and the state of VB> VC is a state with an optical signal input. Then, when no optical signal is input, the transistors Q ₁₃ and Q ₁₄ are turned off and on, respectively, and when an optical signal is input, the transistors Q ₁₃ and Q ₁₄ are turned on and off, respectively. By turning on and off, the current I ₁₃ flowing through the transistor Q ₁₃ becomes I ₁₃ = 0 when no optical signal is input, and I ₁₃ = I ₀ when an optical signal is input. Note that I ₀ is a constant current set by the constant current circuit 4. The current I ₁₃
Corresponds to the selection signal of the present invention.

The comparison voltage setting means 1 sets the comparison voltage VC based on the input voltage VB. The comparison voltage setting means 1 has transistors Q ₁₁ and Q ₁₂ . Transistor Q
Both emitters of ₁₁ and Q ₁₂ are connected to one current control transistor (base area is 3 times) in the constant current circuit 4,
The total sum of the currents I ₁₁ and I ₁₂ flowing in both transistors Q ₁₁ and Q ₁₂ is set as follows. I ₁₁ + I ₁₂ = 3 · I ₀ (1) Further, the transistor Q ₁₂ is connected to the constant current control circuit 5, and the transistor Q ₁₃ is connected to the collector, and the transistor Q ₁₂ flows to the transistor Q ₁₂ . The current I ₁₂ is 2 in the transistor on the left side of the constant current control circuit 5.
Since I ₀ , I ₁₂ = 2 · I ₀ −I ₁₃ (2) Therefore, the currents I ₁₂ and I ₁₃ depend on whether or not an optical signal is input (transistor Q ₁₃ is turned on / off).
Changes to

When no optical signal is input, I ₁₂ = 2 · I ₀ I ₁₁ = 3 · I ₀ -2 · I ₀ = I ₀ , and when an optical signal is input, I ₁₂ = 2 · I ₀ -I ₀ = I ₀ I ₁₁ = 3 · I ₀ −I ₀ = 2 · I ₀ .

Due to such a change in current, VC changes. That is, when the optical signal input without the base voltage V _BE11 of the transistor Q _11, the base voltage V of the transistor Q _{12 BE12,} and its base voltage V _BE11, VC obtained by the base voltage V _BE12 is as follows.

[0022]

[Formula 1]

That is, at Ta = 25 ° C., VC = VB +
It becomes 18 mV.

[0024] Similarly, the base voltage when there optical signal input _BE11, the base voltage V _BE12, and when seeking VC,

[0025]

[Formula 2]

[0026]

That is, at Ta = 25 ° C., VC = VB-
It becomes 18 mV.

Here, the comparison voltage VC output from the comparison voltage setting means 1 is caused by the response characteristics of the light receiving element, the threshold voltage of each transistor in the waveform shaping circuit, the transient characteristics such as switching, etc. There is a delay with respect to the rising or falling of the input voltage VB. That is, there is a delay in the circuit. As a result, when the input signal B changes, the voltage VB of the input signal and the comparison voltage VC cross each other. In this embodiment, the rise and fall of the input voltage VB are detected by utilizing the crossing of the voltages VB and VC due to the delay in this circuit.

The above will be described in detail below with reference to the signal waveforms of this embodiment shown in FIGS.

FIGS. 3A and 3B show the conventional example shown in FIG.
Some correspond to the states of (A) and (B). First, FIG.
The operation will be described with reference to FIG. A is the waveform of the transmission signal, B is the waveform of the input signal after optical signal amplification, C is the waveform of the comparison signal, and D is the waveform of the output signal.

When no optical signal is input, VC = VB
It is stable at + kT / q · ln2. When an optical signal is input in this state, the input voltage VB rises, but the comparison voltage VB
Since C is delayed by the time t, it does not start up, and both VB and VC intersect. Immediately after this crossing of the two voltages, the transistors Q ₁₃ and Q ₁₄ are turned off → on and on → off, respectively, and switched to VC = VB−kT / q · ln 2. Then, while the input voltage VB is high, it is stable in this state. However, when the input voltage VB falls, the input voltage VB and the comparison voltage VC cross again, and immediately after that, the transistors Q ₁₃ and Q ₁₄ are switched ON → OFF and OFF → ON, respectively. As a result, VC becomes VC again
= VB + kT / q · ln2, which is stable in this state.

In this configuration, when the signal waveform changes or when there are variations in the light receiving element or the like due to manufacturing, the same operation is performed during the transmission of the high speed signal.

FIG. 3B shows the case where the cycle of the transmission signal is sufficiently large as in the conventional example. Even in this case, the delay time of the rising edge of the output signal becomes the delay time of the circuit,
It is the same as when the cycle is short. So in this configuration,
Since the comparison voltage VC is obtained in a state of following the input voltage VB, the output signal D without pulse width distortion can be obtained without considering the time constant as in the conventional example.

FIGS. 4A and 4B show the conventional example shown in FIG.
It corresponds to (A) and (B), and shows the waveforms when the level of the input voltage VB is high and low. As can be seen from the figure, the output signal D having a constant delay time can be obtained regardless of the voltage level of the input voltage VB.

FIG. 5 corresponds to FIG. 9B of the conventional example and shows a waveform when the response of the transmitting device does not follow. Even in this case, stable signal transmission can be performed without causing distortion in the output signal D.

As described above, the comparison voltage VC is equal to the input voltage V
It is determined by raising or lowering a certain voltage with respect to B, and the delay for the crossing of both voltages VB and VC is a delay in the circuit, so a stable delay can be generated. As a result, stable waveform shaping can be performed without being affected by the cycle of the input signal, fluctuations in peak voltage due to manufacturing reasons, speed of the input signal, and the like.

In this embodiment, the example applied to the transmission of the optical signal is shown, but the same can be applied to the transmission of other signals.

[0038]

According to the present invention, since the voltage (higher voltage or lower voltage by a constant voltage) with reference to the input voltage is used as the comparison voltage, the cycle of the input signal, the manufacturing reason, the input signal, and the like. It is possible to generate a stable comparison signal and perform waveform shaping without being affected by fluctuations in the peak voltage due to the speed and the like.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the operation of the present invention.

FIG. 2 is a diagram showing a configuration of an optical signal transmission device including a waveform shaping circuit according to an embodiment of the present invention.

FIG. 3 is a diagram showing a waveform of a signal of each unit in the same device.

FIG. 4 is a diagram showing a waveform of a signal of each unit in the same device.

FIG. 5 is a diagram showing waveforms of signals at various parts in the same device.

FIG. 6 is a diagram showing a configuration of an optical signal transmission device showing a conventional configuration example.

FIG. 7 is a diagram showing signal waveforms of respective parts in the apparatus of the conventional example.

FIG. 8 is a diagram showing signal waveforms of respective parts in the apparatus of the conventional example.

FIG. 9 is a diagram showing signal dispatch of each unit in the apparatus of the conventional example.

[Explanation of symbols]

 1 comparison voltage setting means 2 selection signal output means 3 differential amplifier 4,5 constant current circuit

Claims (1)

[Claims] 1. A selection signal output means for detecting a rising edge and a falling edge of an input voltage and outputting a selection signal according to the detection state, and an input voltage after a selection signal indicating a rising edge of the input voltage is input. A comparison signal setting means for outputting a voltage lower than the input voltage as a comparison voltage after outputting a voltage lower than the input voltage as a comparison voltage and inputting a selection signal indicating a fall of the input voltage; A waveform shaping circuit comprising: a comparison unit that compares the input voltage with the comparison voltage and outputs the comparison result.