JPH01121771A - Characteristic impedance measuring circuit - Google Patents

Abstract

PURPOSE:To measure the characteristic impedance of a transmission line with high accuracy by driving the transmission line with low output impedance to a large amplitude, and making the output impedance high and increasing a reflected wave when a reflected wave returns from the distant terminal of the line. CONSTITUTION:The circuit consists of a driving circuit 1 which inputs a pulse signal from an input terminal 5, a measurement terminal 2 connected to its output, the transmission line 3 to be measured, and a variable resistance circuit 4 provided between its distant terminal and a positive electrode voltage. In this constitution, the circuit 1 has output impedance larger than the characteristic impedance of the line 3 at the time of low output level, but sufficiently large output impedance is provided at the time of high output level. The line 3 is an unbalanced type and both terminals of its return wire are grounded. An inverter 11, a resistance 12 and a transistor(TR) 13 are provided between the input terminal 10 and output terminal 14 of the circuit 1 and the output terminal 14 is placed in the low-output or high-output impedance state according to the output of the inverter 11.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a characteristic impedance measuring circuit, and particularly to a measuring circuit that measures the characteristic impedance of a transmission line using pulses.

[Conventional technology]

Conventional characteristic impedance measuring circuits of this type use a drive circuit with a constant output impedance (generally 50l) regardless of the output level to transmit the signal to be measured using step pulses or pulses with a pulse width of t. A circuit that drives a transmission line, adjusts a variable resistance circuit connected to the far end of the transmission line, and sets the resistance value of the variable resistance circuit when there is no reflected wave at the far end of the transmission line as the characteristic impedance of the transmission line. is commonly used. Observation of reflected waves is performed using the output of the drive circuit.

[Problem that the invention seeks to solve]

However, in the nine conventional characteristic impedance measurement circuits mentioned above, the output impedance of the drive circuit is equal to or smaller than the characteristic impedance of the transmission line, so the voltage observed at the output end of the drive circuit is equal to or smaller than the incoming reflected voltage. becomes smaller. This is because the reflection coefficient at the output end of the drive circuit is close to zero or negative. As a result,
It has the disadvantage that it is difficult to determine the presence or absence of reflected waves and the accuracy of measuring characteristic impedance is poor.

- If the output impedance of the drive circuit is increased in order to make the reflection coefficient positive at the output end and increase the reflected wave, the drive voltage of the drive circuit will be divided by the output impedance and the characteristic impedance, so the transmission The driving voltage of the line becomes small, and therefore the reflected voltage also becomes small, and measurement accuracy does not improve.

[Means for solving problems]

The characteristic impedance measurement circuit of the present invention has an output impedance smaller than the characteristic impedance of the transmission line at low output levels, an output impedance sufficiently larger than the 1c% characteristic impedance at high output levels, and A drive circuit that drives a negative polarity pulse signal with a pulse width smaller than the delay time onto the transmission line, a variable resistance circuit connected to the far end of the transmission line, and a positive voltage applied to one end, and a nine variable resistance circuit connected to the output of the drive circuit. It has a measurement terminal.

〔Example〕

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

FIG. 1 is a circuit diagram showing an embodiment of the present invention, showing a drive circuit 1 that receives a pulse signal from an input terminal 5, a measurement terminal 2 connected to the output of the drive circuit 1, and a transmission line to be measured. 3, and a variable resistance circuit 4 connected between the far end of the transmission line 3 and the positive voltage VC.

The drive circuit 1 has an output impedance smaller than the characteristic impedance of the transmission line 3 at a low output level, and has an output impedance sufficiently larger than the characteristic impedance at a high output level. The transmission line 3 is an unbalanced type, and both ends of the return line are grounded.

FIG. 2 is a detailed circuit diagram of one embodiment of the drive circuit 1.

The input terminal 10 is connected to the input of the inverter circuit 11,
The output of the inverter circuit 11 is connected through a resistor 12 to an NPN transistor 130 pace.

The emitter of transistor 13 is grounded, and the collector is connected to output terminal 14.

When the input voltage is at a low level, the output of the inverter circuit 11 is at a high level, and the transistor 13 is turned ON.
The output terminal 14 becomes low level and has a low output impedance. When the input voltage is at a high level, the output of the inverter circuit 11 is at a low level, and the transistor 13
becomes OF'FlC, and the output terminal 14 becomes in a high output impedance state.

@Figure 3 shows voltage waveforms for explaining the operation of this example, and Figure 3 (a) shows the waveform at measurement terminal 2,
FIG. 3 (bl indicates the reflected pulse at the far end of the transmission line 3, respectively).

A negative pulse signal with a pulse width TI smaller than the round-trip propagation delay time T2 of the transmission line 3 is input to the input terminal 5! When /c is input, the drive circuit 1 connects the transmission line 3 with low output impedance only when the output is at a low level, that is, during the pulse width T1.
At other times, it becomes a high output impedance,
Since the transmission line 3 is connected to the positive voltage VC through the variable resistance circuit 4, the output of the drive circuit 1 is at a high level.

The driving pulse VD shown in FIG. 3(a) propagates through the transmission line 3 and reaches the variable resistance circuit 4. Characteristic impedance t'Ze of transmission line 3, resistance value 1 of variable resistance circuit 4
1 (, V), and assuming that the transmission line 3 is lossless for ease of explanation, the reflected pulse at the far end of the transmission line 30 is expressed as Vault.

The reflected pulse VRI propagates in the opposite direction and reaches the measurement terminal 21.
Reach 1C. At the measuring terminal 2, a voltage appears due to the superposition of the reflected pulse VR,t and its re-reflected pulse at the output of the drive circuit 1. Assuming that the output impedance of the drive circuit 1 is set to 1, the measurement pulse Va2 measured at the measurement terminal 2 is expressed as follows.

When the reflected pulse vR1 returns to the output of the drive circuit 1, the output of the drive circuit 1 is already at a high level, so the output impedance RL is sufficiently larger than the characteristic impedance 2. According to this result, the second term of equation (2) becomes approximately the reflected pulse VB, 1, so VPLz ”:; zVRt
(31, which means that the measurement pulse VB,2 at the measurement terminal 2 can be measured at twice the value of the reflected pulse VRI.

The characteristic impedance Z is the variable resistance circuit 4f: resistance value RV1 when one reflected pulse vR1 disappears after being varied.
<Determined by measurement. At that time, the resistance value kL
v becomes equal to the characteristic impedance Ze, and from equation (1), the reflected pulse V1 becomes zero.

From equation (3), the measurement pulse V2 is also zero.

Reflected pulse due to the difference between the resistance value ⇠V and the characteristic impedance 2G ■The fact that a voltage value twice that of ⇠1 can be measured at the measurement terminal 2 means that the measurement accuracy of the characteristic impedance 2 is also 2.
This means that the improvement will be doubled.

Since the output impedance of the drive circuit 1 at a low output level is small, the drive voltage of the transmission line 3 can be increased. This is because the drop in drive voltage due to voltage division between the output impedance and the characteristic impedance is small.

〔Effect of the invention〕

As explained above, in the present invention, when driving a transmission line, the transmission line is driven with a low output impedance and with a large amplitude, and when the reflected wave from the far end of the transmission line returns, it is set to a high output impedance and the reflected wave is increased. By using a drive circuit, the characteristic impedance of the transmission line can be measured with high precision.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention. Figure 2 is a circuit diagram showing one embodiment of the present invention.
A detailed circuit diagram of the drive circuit 1 shown in the figure, and FIG. 3 shows voltage waveforms for explaining the operation. l...Drive circuit, 2...Measurement terminal, 3
...Transmission line, 4...Variable resistance circuit,
5.10... Input terminal, 11... Inverter circuit, 12... Resistor, 13...
Transistor, 14... Output terminal. Agent Patent Attorney Uchihara Otodai 1 Meishi 2 Zuishi 3 Diagram

Claims (1)

[Scope of Claims] It has an output impedance that is smaller than the characteristic impedance of the transmission line at low output levels, and has an output impedance that is sufficiently larger than the characteristic impedance at high output levels, and has an output impedance that is sufficiently larger than the characteristic impedance at high output levels, and is smaller than the round-trip propagation delay time of the transmission line. a drive circuit that drives a negative polarity pulse signal with a small pulse width to the transmission line; a variable resistance circuit connected to the far end of the transmission line and having a positive voltage applied to one end; and a variable resistance circuit connected to the output of the drive circuit. A characteristic impedance measuring circuit comprising: a measuring terminal for measuring impedance;