Resonance method for measuring impedance

The resonance method is another basic method for measuring impedance. It is a measurement method based on the resonance characteristics of the tuned circuit. Although the measurement accuracy is not as high as the AC bridge method, the technical difficulty is smaller than that of the high-frequency bridge due to the simple and convenient measurement circuit (mainly due to the influence of stray coupling). In addition, most high-frequency circuit components are used as tuned circuit components, so the resonance method is more in line with the actual working situation.

The typical resonance measuring instrument is the q-meter, so the resonance method is also called the q-meter method, and its operating frequency range is quite wide.

The measurement principle of the resonance method is shown in the figure below. It consists of an oscillation source, a known component and a measured component, a resonant circuit, and a resonance indicator. When the circuit reaches resonance, there is and the total impedance of the circuit is zero, that is,

1. Resonance method to measure inductance

When measuring the inductance of small inductance, use the series substitution method, as shown in the following figure:

Then remove the short circuit between 1 and 2, press lx into the loop, keep the signal source frequency unchanged, and adjust c to c2. The loop resonates again.

The parallel substitution method is often used to measure larger inductance, as shown in Figure 5-1. Do not connect lx first, adjust variable c to the small capacity position, then c is c1, adjust the signal source frequency to make the loop resonate, then there is ,

The parallel substitution method is used to measure inductance, as shown in Figure 1:

Figure 1: Parallel substitution method for measuring inductance

2. Resonance method for measuring capacitance

(1) Direct method for measuring capacitance

Connect the capacitor cx to be measured, adjust the oscillation source frequency f, and make the voltmeter indicate the maximum. Then the measured capacitance is

The errors in measuring capacitance by direct method include: the error caused by distributed capacitance (distributed capacitance of coil and wiring); the error caused by lead inductance when the frequency is too high; when the loop q value is low, the resonance curve is very flat and it is not easy to accurately find the resonance point (the voltmeter indicates the maximum value), which will also cause errors.

(2) Substitution method for measuring capacitance

1) Parallel substitution method

It is suitable for measuring small capacitance. Its measurement error mainly depends on the scale error of the variable standard capacitance.

Using the substitution method to measure capacitance can eliminate the measurement error caused by distributed capacitance. The test circuit is shown in the figure below:

c is a variable capacitor that has been calibrated, and its capacity variation range is greater than the capacitance to be measured. Without connecting cx, adjust the variable capacitor c to a position with a larger capacity, set its capacity to c1, and adjust the signal source frequency to make the circuit resonate. Then connect the measured capacitor cx, and keep the signal source frequency unchanged. At this time, the circuit is detuned, and re-adjust c to make the circuit resonate again. At this time, c is c2, and the measured capacitance cx = c1-c2.

2) Series substitution method

When the capacitance to be measured is greater than the maximum capacitance of the standard capacitor, the series connection method must be used, as shown in the figure below. First, short-circuit the ends 1 and 2 in the figure, adjust to a smaller capacity position, and adjust the signal source frequency to make the loop resonate. At this time, the capacitance is c1. Then remove the short-circuit line, connect cx to the loop, keep the signal source frequency unchanged, and adjust c to make the loop resonate again. At this time, the variable capacitance c is c2. Obviously, cl is equal to the series value of c2 and cx.

3. Measurement of capacitance

(1) Measurement of capacitance less than 460pf

(2) Measurement of capacitance greater than 460pf

Capacitances greater than 460pf can be measured using the series substitution method: connect the standard inductor to the LX terminal, and adjust the main tuning capacitor dial to make the Q value meter indicate the maximum, and the dial reading is recorded as C1; remove the standard inductor, connect it in series with the measured capacitor, and then connect it to the LX terminal, readjust the main tuning capacitor dial to make the Q value meter indicate the maximum again, and the dial reading is recorded as C2. The measured capacitance cx is: