NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
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NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
Author: Noyafa–CCTV Tester
Question: Internal impedance balance technology Answer: Why use internal impedance balance technology? The pulse sent by the pulse reflection instrument has a certain width. Since the output impedance of the instrument does not match the wave impedance of the cable, what is obtained on the cable or the receiving circuit of the instrument is a problem: Internal Impedance Balance Technology Answer: Why use internal impedance balance technology? The pulse sent by the pulse reflection instrument has a certain width. Since the output impedance of the instrument does not match the wave impedance of the cable, the transmitted pulse obtained on the cable or felt by the receiving circuit of the instrument If a tail is dragged, the reflected pulse at the fault point overlaps with the transmitted pulse, which will cause a significant measurement blind area; the instrument receives and displays the transmitted pulse and the reflected pulse on the display at the same time. When the fault point is far away, the amplitude of the transmitted pulse is much larger than The reflected pulse at the fault point, for example, by increasing the amplifier gain to achieve the purpose of increasing the amplitude of the reflected pulse at the fault point, it will cause the saturation of the signal amplifying circuit, and the so-called so-called“block”Phenomenon. The purpose of using internal impedance balance technology is to compress or even eliminate the transmitted pulse received (and displayed) by the instrument, thereby reducing or eliminating the measurement dead zone, and can maximize the gain of the amplifier circuit and increase the amplitude of the reflected pulse at the fault point. without making the amplifier circuit“block”Causes the pulse reflection waveform to be distorted. 2. As shown in the internal impedance balance technology, the instrument transmits pulses to the cable under test and the internal balance network at the same time, and the signal received by the instrument is the difference between the signal on the cable under test and the internal balance network. Adjust the parameters of the internal balance network to make it. Consistent with the wave impedance of the cable, the signal generated by the sending pulse on the cable and the internal balance network is the same, the signal received by the instrument is zero, and when the reflected pulse arrives, no signal appears on the internal balance network, and all the reflected pulses are sent to The instrument receiving circuit goes up.
Instrument measurement waveforms with or without internal balance are shown. The function of the internal balance network There is no internal balance network, and the instrument gain is too large. The pulse reflection waveform of the internal balance network is compared. The pulse reflection waveform using the internal balance network is measured by eliminating the transmitted pulse from the waveform and increasing the gain of the instrument. . It can be seen that when the transmitted pulses appear on the waveform at the same time, because the amplitude of the transmitted pulses is much larger than that of the reflected pulses at the fault point, if the amplification gain of the instrument is increased to achieve the purpose of increasing the amplitude of the reflected pulses at the fault point, the input signal of the instrument will be too high. large, causing the signal amplification circuit to saturate, resulting in the so-called“block”phenomenon, waveform.
To simplify operation, practical low-voltage pulse reflection instruments tend to employ a fixed balance network rather than being adjusted by the operator. After the instrument adopts the internal balance technology to compress and display the transmitted pulse of the waveform, it can easily detect the fault of the cable outlet by using the waveform comparison method. First measure the pulse reflection waveform of an intact cable core wire, store it, and then measure the faulty core wire, and compare the two measured waveforms.
Figure 3.13 shows a comparison of the waveforms of a core with a short-circuit fault near the head of a cable and an intact core.
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