Author: Noyafa–CCTV Tester
Internal Impedance Balancing Technique in Cable Fault Tester Internal Impedance Balancing Technique in Cable Fault Tester 1. Why use Internal Impedance Balancing technique? Internal impedance balance technology in fault tester Internal impedance balance technology in cable fault tester 1. Why use internal impedance balance technology? The pulse sent by the pulse reflection instrument has a certain width, because the output impedance of the instrument does not match the cable wave impedance , the transmission pulse obtained on the cable or sensed by the receiving circuit of the instrument drags a tail, and the reflected pulse at the fault point overlaps the transmitted pulse, which will cause a significant measurement blind area; the instrument receives and displays the transmitted pulse and 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 gain of the amplifier to achieve the purpose of increasing the amplitude of the reflected pulse at the fault point, the signal amplifier circuit will be saturated 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. 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 match the cable. If the wave impedance is the same, 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, there is no signal on the internal balance network, and all the reflected pulses are sent to the receiving circuit of the instrument go up.
The measurement waveform of the instrument with or without internal balance is shown in the figure. The function of the internal balance network has no internal balance network a. Without internal balance network, there is an internal balance network b. With internal balance network without internal balance network, the instrument gain is too large c. No internal balance Network, the instrument gain is too large. Comparison of the pulse reflection waveform with or without the internal balance network. The pulse reflection waveform using the internal balance network in b is measured by increasing the instrument gain after eliminating the transmitted pulse from the waveform. 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. A comparison of the waveforms of a core with a short-circuit fault near the head of a cable and an intact core is given.
Use the waveform comparison method to measure cable head faults.
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