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
What are the options for transmitting pulses? The voltage pulses used by cable fault measuring instruments are generally rectangular, exponential, bell-shaped (raised cosine) and so on. Since it is easier to form a rectangular pulse, it has many applications, and we also use this method. What are the options for transmitting pulses? The voltage pulses used by cable fault measuring instruments are generally rectangular, exponential, bell-shaped (raised cosine) and so on.
Since the formation of rectangular pulses is relatively easy, there are many applications, and we also use this method. The transmitted pulse always has a certain time width, assuming that it is , then the reflected pulse that arrives within the time and the transmitted pulse overlap and cannot be distinguished, so the distance to the fault point cannot be measured, and a blind spot appears. Assuming that the transmission pulse width is 0.2uS, the cable radio speed is 160m/us, and the measurement blind spot is 16M. The wider the instrument's emission pulse, the larger the test blind spot.
From the point of view of reducing the blind spot, it is better to have a narrower width of the transmitted pulse, but the narrower the pulse width, the richer the high-frequency components it contains, while the high-frequency loss of the line is large, so that the amplitude of the reflected pulse is too small, the distortion is serious, and the far The distance test doesn't work well. To solve this problem, pulse reflection instruments divide the pulse width into two ranges. now that“far and near”key, the near key has a pulse width of 0.2us and an amplitude of 180V; the far key has a pulse width of 2us and an amplitude of 280V.
When the fault distance is relatively large (400M), select the far key. Otherwise, select the near key.
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