Impulse searcher for the place of damage to the communication line. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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The principle of operation of the proposed device is similar to that used in pulse radars. A short probing voltage pulse is applied to the tested line (cable or air). Spreading along the line, it reaches the place of its inhomogeneity (malfunction), is reflected from it and returns back. By measuring the delay between the probing and reflected pulses and knowing the speed of propagation of an electromagnetic wave in the line, it is easy to determine the distance from the device to the fault location.
Today, many communication lines are of considerable length and are laid on the roofs of buildings, walled up in walls or buried in the ground. Therefore, access to them for repair, if the location of the malfunction is not exactly known, is very difficult. However, there are devices that allow you to accurately determine the distance from the beginning of the cable to the point of damage that is still inaccessible. Knowing this distance, you can find the right place, excavate here and repair the cable.
Such devices include what is proposed in this article.
Main Specifications
- Display screen resolution, px ....... 128x64
- Probing pulse duration, µs ......0,25
- Sweep speed, µs/px ....... 0,17
- Supply voltage, V ....... 5
- Current consumption, mA.......115
- Dimensions, mm ....... 80x62x30
- Weight, g.......110
The maximum length of the tested cable, m ...... 2500
The scheme of the device is shown in fig. 1. The probing pulse generated by the DD1 PIC18F4550-I/P [1, 2] microcontroller at the RA5 output enters the tested line through the capacitor C1 and propagates along it. If this pulse encounters an open or short circuit between the cable wires on its way, it is reflected from this place, comes back along the line and enters the voltage divider from resistors R1 and R2. Then, through capacitor C2 and resistor R4, the signal enters the ADC input of the DA1 TDA8708A video interface chip, where it is converted into a digital code.
Rice. 1. Scheme of an impulse searcher for a fault location in a communication line (click to enlarge)
The digitized values are stored in the RAM of the microcontroller DD1, and then displayed on the graphic LCD HG1 (MT-12864J-2FLA) [3] in the form of an oscillogram. Variable resistor R5 shift the scan line vertically.
The farther the reflected pulse returned from a greater distance, the farther from the beginning of the sweep is its image on the screen. To calibrate the range scale, it is enough to connect a piece of a known-good cable of known length to the device and observe the reflection from its opposite end. Please note that this cable must be of the same type as the one being tested, since the speed of propagation of electromagnetic waves in cables of different types varies markedly.
On fig. 2, a cable break is visible at a distance of 600 m. If the fault is a short circuit of the cable wires, then the pulse reflected from it has the opposite polarity (Fig. 3).
Rice. 2. Data display on the graphical LCD in the form of an oscillogram
Rice. 3. Data display on the graphical LCD in the form of an oscillogram
The circuit board of the device is shown in fig. four.
Rice. 4. Instrument circuit board
Literature
- PIC18F2455/2550/4455/4550 28/ 40/44-Pin, High-Performance, Enhanced Flash, USB Microcontrollers with nanoWatt Technology/. - URL: www1.microchip.com/downloads/en/DeviceDoc/39632e. pdf.
- Yatsenkov VS Microchip microcontrollers with USB hardware support. - M.: Hot line - Telecom, 2008.
- Liquid crystal module MT-12864J. - URL: melt.com.ru/files/file2150172.5.pdf.
The microcontroller program can be found at ftp://ftp.radio.ru/pub/2015/04/lokator.zip.
Author: A. Pichugov
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