JPH01314423A - Waveform shaping circuit - Google Patents

Abstract

PURPOSE:To stabilize clip operation and limit operation and to suppress the waveform distortion of a receiving signal by providing a potential setting means and setting a clip potential with a diode and a limit potential to always be higher or lower than a prescribed potential or the potential of a power source only by a prescribed value. CONSTITUTION:For a limit potential V5 of a high level and a clip potential V6 of a low level in a signal in a receiver 4, when the potential in one edge side of a resistor R3 is defined as VA and the potential in the other edge side is defined as VB, the condition of V5=VA+VT and V6=VB-VT can be obtained. In such a condition, the VT is the joining potential of diodes D1-D6. For the potential VA in one edge side of the resistor R3 and the potential VB in the other edge side, the condition of VA=VD-3VT and VB=VT can be obtained. Then, the condition of V5=VD-2VT is obtained and V6=0 can be established. Accordingly, the V5 in the receiving edge 4 always goes to be equal to a high level potential V1 in a transmitting edge 2. The V6 is always equal to a low level potential V2 in the transmitting edge 2.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a waveform shaping circuit that suppresses waveform distortion due to reflected waves that cannot be suppressed by a matching circuit for the characteristic impedance of the circuit when a digital signal is transmitted by a digital transmission circuit. It is related to.

(Prior Art) FIG. 4 is a diagram showing an example of a digital transmission circuit using this type of waveform shaping circuit. In the figure, 1 is a line driver that outputs a TTL level signal. 2 is a transmitting end, 3 is a digital transmission line, 4 is a receiving end, and 5 is a Schmitt type line receiver, which inputs a TTL level signal. i70 The signal output from the line driver 1 is input to the line receiver 5 via the transmitting end 2, the transmission line 3, and the receiving @4.

FIG. 5 shows signal waveforms at the transmitting end 2 and receiving end 4 when no waveform shaping circuit is added to the digital transmission circuit. The high-level potential V1 of the transmission signal waveform at the transmitting end 2 is the potential of the power supply of the line driver 1, VD, and the m difference between the base and emitter of the transistor used inside the line driver 1. If the junction potential is VT, it will be approximately (VD - 2VT)V. The reason for this is that the output circuit of the line driver 1 is usually configured by a Darlington coupling of 2VT.
Due to the decline in rank. Further, the low level potential v2 of the transmission signal waveform at the transmission end 2 becomes Ov,
It is output from the transmitting end 2 as a rectangular wave. However, the received signal waveform at the receiving end 4 is distorted due to the superimposition of reflected waves, causing an overshoot at the front end and an undershoot at the rear end of the received signal waveform. Conventionally, a waveform shaping circuit as shown in FIG. 2 is provided at the receiving end 4 to suppress waveform distortion of the received signal.

In FIG. 2, the same components as those in FIG. 4 are represented by the same symbols. That is, 4 is the receiving end, 5 is the line receiver,
Dl and p2 are diodes whose junction potential is the same as the junction potential VT of the transistor in the line driver. Furthermore, R1 and R2 are fixed resistors, and C is a capacitor, which bypasses the current.

The anode of the diode D1 and the cathode of the diode D2 are connected to the receiving end 4, and the anode of the diode D2 is grounded. Further, the cathode of the diode D1 is connected to one end of the resistors R1, R2 and the capacitor C.

Further, the other end of the resistor R2 and the capacitor C are grounded, and the other end of the resistor R1 is connected to the power supply VD.

Further, the other end of the resistor R2 and the capacitor C are grounded, and the other end of the resistor R1 is connected to the power supply VD.

By adding this waveform shaping circuit, the signal waveform at the receiving end 4 becomes a waveform as shown in FIG. Third
In the figure, the high-level limit potential v3 of the signal is (
L) It can be expressed by the following formula.

V3- (R2/ (R1+R2)) -VD +VT - (1) Furthermore, the low level clip potential V4 of the signal becomes V4--VT due to the potential drop of the diode D2.

On the other hand, since the high level potential of the signal at the transmitting end 2 is (VD - 2VT) - C -, by selecting the resistors R1 and R2 whose resistance values satisfy the relationship of equation (2), the received signal waveform distortion can be suppressed.

(R2/ (R1+R2))-VD 10VT-VD
-2VT - (2) (Problem to be Solved by the Invention) However, in this type of conventional waveform shaping circuit, when a fluctuation occurs in the power supply potential VD, the relationship in equation (2) cannot be satisfied. As a result, stable high-level limit operation of the signal becomes impossible. Furthermore, since the low-level clipping potential v4 is -VT, a potential below 0 potential is generated, which causes a problem such as a reverse bias effect on the receiving circuit.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is to provide a waveform shaping circuit that equalizes potential fluctuations of a received signal and fluctuations in clip potential and limit potential when power supply fluctuations occur, and does not generate reverse bias. It's about doing.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present invention aims to improve the waveform of the digital signal due to the reflection generated at the receiving end of the digital transmission circuit when the digital signal is transmitted by the digital transmission circuit. In a waveform shaping circuit that suppresses distortion by the clipping action and limiting action of a diode, a potential setting circuit is provided that always sets the clipping potential and the limiting potential of the diode to a predetermined potential or a potential higher or lower than a power supply potential by a predetermined value. Ta.

(Function) According to the present invention, the clip potential and limit potential of the diode are always set to a predetermined potential or a potential higher or lower than the power supply potential by a predetermined value, so that even if the power supply potential fluctuates, the potential of the received signal remains constant. The variation becomes equal to the variation in the clip potential and limit potential.

(Example) FIG. 1 is a circuit diagram showing an embodiment of the present invention.

In the figures, the same components as those of the conventional example are represented by the same reference numerals. That is, Dl to D6 are diodes whose junction potential is the same as the junction potential VT between the base and emitter of the transistor used in the line driver 1. R3 is a resistor and C is a capacitor. The anode of the diode D1 and the cathode of the diode D2 are connected to the receiving end 4. In addition, the diode D
The cathode of No. 1 is connected to the cathode of the diode D5, one end of the resistor R3, and one end of the capacitor C. The other end of the capacitor is grounded, the anode of the diode D5 is connected to the cathode of the diode D4, the anode of the diode D4 is connected to the cathode of the diode D3, and the anode of the diode D3 is connected to the power supply VD. It is connected to the. Further, the anode of the diode D2 is connected to the other end of the resistor R3 and the anode of the diode D6, and the cathode of the diode D6 is grounded.

By the waveform shaping circuit of the present invention, the signal waveform at the receiving end 4 becomes a waveform as shown in FIG. In FIG. 6, the high level limit potential 5 of the signal and the low level clip potential v6 are set to the potential of one end of the resistor R3 by VA.
, the potential at the other end is VB, it can be expressed by the following equation.

V5-VA +VT
・(3)VB-VB-VT
(4) Furthermore, the potential V^ on one end side and the potential VB on the other end side of the resistor R3 can be expressed by the following equation.

VA -VD -3VT -(5)V
B-VT ... (6) From the above equations (3) and (5), the high level limit) potential V51; tV5 - VD 2VT is obtained, and the above (
From equations (4) and (6), the low-level clipping potential V
6 becomes VB-0.

Therefore, the high level limit potential v5 at the receiving end 4 is always equal to the high level potential Vl (-VD -2VT) of the signal waveform at the transmitting end 2. Furthermore, the low-level clipping potential v6 at the receiving end 4 is always equal to the low-level potential V2 (-0) of the signal waveform at the transmitting end 2.

Although this embodiment has been described with respect to a TTL level digital transmission circuit, the waveform shaping circuit of the present invention can also be applied to digital transmission circuits using other signal levels.

(Effects of the Invention) As explained above, according to the present invention, when a digital signal is transmitted by a digital transmission circuit, the waveform distortion of the digital signal due to reflection occurring at the receiving end of the digital transmission circuit is reduced by clipping the diode. In the waveform shaping circuit that suppresses by action and limit action, a potential setting circuit is provided that always sets the clip potential and limit potential by the diode to a predetermined potential or a potential higher or lower than the power supply potential by a predetermined value. Even when a potential fluctuation occurs, the potential fluctuation of the received signal is equal to the fluctuation of the clipping potential and the limit potential, so stable clipping and limiting operations are possible, and waveform distortion of the received signal can be suppressed. Furthermore, it has the advantage that reverse bias and the like will not occur.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing an example of a conventional waveform shaping circuit, Fig. 3 is a received signal waveform diagram of the conventional example, and Fig. 4 is a conventional digital transmission. FIG. 5 is a diagram showing an example of a circuit, FIG. 5 is a waveform diagram of a transmitted/received signal, and FIG. 6 is a diagram of a received signal waveform of an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Line driver, 2... Transmission end, 3... Digital transmission line, 4... Receiving end, 5... Line receiver, D1-D6... Diode, R1-R3...
・Resistor, C...Capacitor・ Patent applicant: Oki Electric Industry Co., Ltd. Agent Patent attorney: Yoshi 1) Takashi Sei The fruit of the present invention An example of an exit path Σ is shown in Figure 1. A. Next signal signal for the next generation. Fig. 3. Transmission line. Detour - 1 piece of ice Figure 4

Claims (1)

[Scope of Claims] A waveform shaping circuit that suppresses waveform distortion of the digital signal due to reflection occurring at the receiving end of the digital transmission circuit when the digital signal is transmitted by the digital transmission circuit, using the clipping action and limiting action of a diode. A waveform shaping circuit comprising: a potential setting circuit that always sets the clip potential and limit potential of the diode to a predetermined potential or a potential higher or lower than a power supply potential by a predetermined value.