Refinement of the instrument for measuring complex resistance. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Meters of complex resistance (impedance) based on a voltage divider and three voltmeters are known. In particular, they are used by radio amateurs to measure the electrical parameters of antennas [1]. A simplified diagram of such a device is shown in Fig. 1. The studied complex resistance (RH, CH) and the known (exemplary) reactive capacitive C0 or active resistance R0 are connected in series to the AC voltage source.

Fig. 1

Relationship between experimental data - measured values ​​of voltages U1 U2, U3, constants fBX, Ro. С0 and the desired parameters RH, Сн describe the equations [2]:

With known values ​​of the reference resistance R0 or capacitance C0, as well as the frequency of the input signal fBX, the error in measuring the complex resistance is determined by the error in measuring the voltages U1-U3. To improve accuracy, it is necessary to monitor the constancy of the voltage U1 and the signal frequency, and the resistance of the reference element (R0, Co) should not differ significantly from the expected load resistance. If the load impedance is not known in advance, a reference resistor R0 with a resistance of 50 ... 100 Ohms is installed and measurements are taken. If the voltages U2 and U3 differ by more than two times in the corresponding direction, change the resistance of the resistor R0 and repeat the measurements. According to formulas (3b) and (5), using (1) and (2), determine the real part of the resistance - RH. Replacing the resistor R0 with a capacitor C0 with a capacitance at the measurement frequency, approximately equal to the resistance of the resistor R0, measurements are taken and similarly from (Za) and (4) the reactive component of the unknown resistance Xn is determined. If the result has a plus sign, the reactive component is capacitive, and if it is negative, it is inductive. According to formula (6) or (7), the capacitance or inductance of the load is found.

Fig. 2

The voltages Ut and U3 can be measured with respect to the common wire with a standard AC voltmeter with a large input resistance, but it is impossible to measure the voltage U2 in this way. Therefore, to implement a meter in the radio frequency range, they resort to converting AC voltage to DC using a semiconductor diode rectifier. The rectified voltage is measured with a DC voltmeter. To unify the measurements, the voltages U1 and U3 are measured in the same way.

One of the sources of error is the asymmetry of the voltage of the generator, the source of the high-frequency signal. This feature must be taken into account, therefore, in meters with rectifiers, the amplitude of the same half-wave of alternating voltage must be measured. In addition, diode rectifiers, due to the non-linear transfer characteristic at a voltage of less than 1 V, introduce an additional error, which can be reduced by using calibration curves [3] or correction tables.

The scheme of the proposed meter is shown in fig. 2. Resistor R1 ensures that the device matches the output of the signal generator. The rectifier on the diode VD1, depending on the position of the contacts of the switches SA1 and SA2, can be connected to various points of the device. In the position of the switches indicated in the diagram, the voltage U1 is measured. In the lower position of the movable contact of the switch SA1 (SA2 in the upper) - U3, and in the lower position SA2 (SA1 in the upper) - U2. The rectifier output through the R2R3C2 low-pass filter is connected to a DC voltmeter, which can be used as a digital multimeter.

All parts are mounted in a plastic case with dimensions of 30x80x120 mm. The input RF socket XW1 (BNC-124) is placed on one of the sides, sockets for connecting the load (pressure terminal blocks RT-213-03, RT-224-01) - on the adjacent one, switches - P2K with return by repeated pressing and sockets for connecting exemplary elements (RT-213-03, RT-224-01) - on the top. All of these elements should be placed as close to each other as possible. Diode VD1 and capacitor C1 are mounted on the terminals of the switches. Sockets XS1, XS2 can be of any type, they are placed on the free wall of the case, they are equipped with a capacitor C2. Resistors R2 and R3 are soldered between the switch terminals and sockets XS1, XS2.

Calibrate the meter as follows. The input (jack XW1) is supplied with an alternating voltage of the generator (usually 1 V), resistors are connected to the contacts XT1 and XT2

C2-10 to 51 ohms, and to the sockets XS1, XS2 - a DC voltmeter. By selecting the resistor R3, the voltmeter readings are set to 1 V. Then, the correction factors are determined, which will improve the accuracy of the measurements. To do this, a constant voltage of 1 V is applied to the input, resistors with a resistance of 1... =) and XT2 (U10=). Applying an alternating voltage of 100 V with a frequency of 2 MHz to the input, they measure the voltages U50vch and U1B2 and find the correction factors for this frequency P2 = U3= - U1Vch and P1,6 = U2= -U3B4 Similarly, the correction factors are determined at other frequencies up to 2 MHz .

By connecting resistors with other resistances to the contacts XT1 and XT2, the measurements are repeated and correction factors are found for other voltage values ​​U2 and U3 at different frequencies. The results obtained are summarized in a table, which is used when making measurements of the load impedance.

Literature

1. Barsky A. A device for measuring the impedance of antennas. - Radio, 2001, No. 12, p. 59,60.
2. Korobeinikov V. Analysis of an instrument for measuring impedance. - Radio, 2003, p. 65,70.
3. Stepanov B. RF head to the multimeter. - Radio, 2006, No. 10, p. 58.

Author: V.Korobeinikov

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