What Is an Impedance Bridge?

Impedance measurement is a measurement of the ratio of a sinusoidal voltage U applied to a system, circuit or component and the current I flowing through them. Impedance measurement belongs to the measurement of basic parameters of telecommunications.

Impedance measurement is a measurement of the ratio of a sinusoidal voltage U applied to a system, circuit or component and the current I flowing through them. Impedance measurement belongs to the measurement of basic parameters of telecommunications.

Chinese name

Impedance measurement

Foreign name

lmpedance measurement

Applied discipline

Power and communication

Introduction to impedance measurement

Telecommunication basic parameter measurement is to measure the parameters related to telecommunications. Impedance is a parameter related to the structure of the circuit. In a circuit with resistance, inductance, and capacitance, the blocking effect on the current in the circuit is called impedance. Impedance is often represented by Z, which is a complex number. The real part is called resistance and the imaginary part is called reactance. Impedance is the sum of resistance and reactance on a vector. The blocking effect of capacitors on AC power in circuits is called capacitive reactance, the blocking effect of inductors on AC power in circuits is called inductive reactance, and the blocking effect of capacitors and inductors on AC power in circuits is called reactance. The unit of impedance is Euro.

In current, the effect of an object on current obstruction is called resistance. Except for superconductors, all substances in the world have electrical resistance, just the difference in resistance value. In direct current and alternating current, resistance has a blocking effect on both types of currents. As common components, in addition to resistance, there is also capacitance and inductance. Both of them do not have the same blocking effect on alternating current and direct current as resistance. Capacitors are "blocked to AC", that is, they have a blocking effect on DC power, that is, DC cannot pass, and AC power can pass, and as the capacitance value increases or AC power increases, the smaller the blocking effect of capacitance on AC power, Obstruction can be understood as "resistance", but it is not the same as resistance. This is a kind of reactance. The reactance is the same as the unit of resistance and is collectively called "impedance".

Introduction to impedance measurement input and output impedance

2.1 Input impedance

Input impedance is the equivalent impedance at the input of a circuit. Add a voltage source U to the input and measure the current I at the input. The input impedance Rin is U / I. You can think of the input as two ends of a resistor. The resistance of this resistor is the input impedance.

The input impedance is no different from an ordinary reactance element, it reflects the magnitude of the current blocking effect. For voltage-driven circuits, the larger the input impedance, the lighter the load on the voltage source, and therefore the easier it is to drive without affecting the signal source. For current-driven circuits, the smaller the input impedance, the The lighter the load on the current source. Therefore, we can think that if it is driven by a voltage source, the larger the input impedance is, the better; if it is driven by a current source, the smaller the impedance is, the better (note: only suitable for low frequency circuits, at high frequencies In the circuit, we also need to consider the impedance matching problem.) In addition, if we want to obtain the maximum output power, we must also consider the impedance matching problem.

2.2 Output impedance

Regardless of the signal source, amplifier, or power supply, there are problems with output impedance. The output impedance is the internal resistance of a signal source. Originally, for an ideal voltage source (including power supply), the internal resistance should be 0, or the impedance of an ideal current source should be infinite. The output impedance needs special attention in circuit design.

But the voltage source in reality cannot do this. We usually use an ideal voltage source in series with a resistor r to be equivalent to an actual voltage source. This resistor r in series with the ideal voltage source is the internal resistance of (signal source / amplifier output / power supply). When the voltage source supplies power to the load, a current I flows through the load, and a voltage drop of I × r occurs on the resistor. This will cause the output voltage of the power supply to drop, thereby limiting the maximum output power (for why the maximum output power is limited, please see "Impedance Matching" later). Similarly, for an ideal current source, the output impedance should be infinite, but the actual circuit is impossible.

Classification and measurement of impedance

According to different frequencies and circuit forms, impedance is divided into lumped parameter impedance and distributed parameter impedance.

When the frequency is low, the size of the circuit and components is very small compared to the wavelength. The circuit can be considered to be composed of a single lumped parameter component such as resistance, capacitance, and inductance; as the frequency increases, at high frequencies, all circuit components Must be regarded as uniformly distributed at various points in the circuit, and the impedance appears as a distributed parameter impedance.

3.1 Measurement method of lumped parameter impedance

The lumped parameter impedance measurement methods are: voltammetry method, bridge method and resonance method.

Voltammetry

That is, the voltage Ux on the measured impedance is measured by a voltmeter, and the current _Ix flowing through it is measured by an ammeter, and then the modulus value of the measured impedance | Z _x | = U _x / I _x . The principle of voltammetry for measuring impedance is simple and convenient, but the accuracy is low.

Bridge method

The bridge method is a comparative method. The measurement accuracy is very high, and some AC bridges can be used for measurements in the hundreds of MHz band. The bridges used to measure impedance include: Wheatstone (or four-arm) bridges, differential bridges, etc. Their measurement principles are shown in Figures 1 (a), (b), and 2 respectively.

In Figure 1 (a), the measured impedance Z _x and the standard variable impedance Z _{0 are} placed on adjacent arms of the bridge, which is called an arm ratio bridge. When the bridge is balanced, you get:

.

Figure 1 Schematic of Wheatstone Bridge

In the formula, A = Z ₁ / Z _{2 is} called a proportional value, and is often used as a constant value (Z ₁ / Z ₂ = R ₁ / R ₂ ). Therefore, the impedance properties of Z _x and Z _{0 are the} same. In Figure 1 (b), Z _x and Z _{0 are} placed on the opposite side of the bridge arm in the bridge, which is called an arm-by-bridge. When the bridge is balanced, get

.

R ₁ R ₂ = Z _{1 When} Z ₂ remains constant, Z ₁ is directly proportional to Y ₀ , which is the opposite of Z ₀ impedance. Therefore, the standard inductance can be used to measure the unknown inductance instead of the standard inductance. Forms impedance bridges for various uses. In FIG. 2, the standard impedance Z ₀ and the measured impedance Z ₀ and the measured impedance Z _x and Z ₁ and Z ₂ composed of two well-shielded pairs of wires form a differential bridge. The indicator G is isolated from the power supply. This kind of bridge can be used in high frequency and very high frequency band because of good shielding. Generally Z ₁ = Z ₂ = jL ₁ = jL ₂ , when the bridge is balanced, Zx = Z ₀ .

Figure 2 Differential bridge circuit

There is also a double T-type bridge with a complicated structure and troublesome adjustment, but with high accuracy, which can be used in high frequency and very high frequency bands.

Resonance method

A measurement method based on the resonance characteristics of a tuned loop. Can be used to measure various parameters of the component such as: R, L, C, Q, etc. The operating frequency can reach several hundred MHz. The disadvantage is that the accuracy is not good (2% 5%), but it can be combined with the substitution method to eliminate the influence of parasitic parameters. The Q meter is a special instrument composed of the resonance method and is one of the important instruments for impedance parameter measurement.

Vector impedance method

Using the definition of impedance Z = U / I, and directly measuring the ratio of the complex voltage to the complex current to give the impedance, is called the vector impedance method. There are two types of meters made by this principle: automatic impedance bridge and vector impedance meter. The use of a microprocessor can make impedance measurement with higher accuracy, faster speed, and more functions, which is an important direction for the current development of impedance measurement.