Determination of the wave impedance of the line. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Determination of the wave impedance of the line. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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A simple method is proposed for determining the wave impedance of a coaxial cable or a balanced line. To do this, you need to make a meander generator on digital microcircuits of the K155, K555, K1533 or K1554 series and connect an oscilloscope. The circuit diagram of the generator, its frequency and stability do not play any role. It is only important that the fronts of the output signal of a rectangular shape are as steep as possible, and the load capacity of the microcircuit used is larger.

Measurements are made as follows. A measured line of arbitrary length is connected to the output of the digital generator through an isolation capacitor with a capacity of 0,1 - 1,0 μF. If it is a coaxial cable, then the braid is connected to a common power wire. A load is soldered to the far end of the line - a variable resistor made by a rheostat. The resistance of this resistor can be in the range of 300 Ohm ... 1 kOhm. The oscilloscope controls the waveform.

Determination of the wave impedance of the line. Measurement scheme
Measurement scheme

At first, we will most likely see an oscillogram that only remotely resembles a meander. By changing the value of the resistance of the variable resistor, it should be ensured that the signal shape is as rectangular as possible, without surges and blockages at the fronts. The amplitude of the signal will also change during manipulations with the load, but this is not important - only the shape of the fronts is important.

Now it remains only to unsolder the variable resistor and measure its resistance using a conventional ohmmeter. The resulting value will be equal to the desired wave impedance of the measured line.

Author: RX3AKT; Publication: N. Bolshakov, rf.atnn.ru

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