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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Selective antenna amplifiers UHF. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers When receiving television signals in the UHF range, many TV owners are forced to use several different antennas, which, at times, can give rise to specific problems associated with signal summation. Antenna amplifiers will help to solve them, providing not only signal amplification, but also their filtering. One of the problems that viewers have to deal with when watching television programs is the need to receive signals from different directions and at different levels. This forces them to use two or more directional antennas, and with a low signal level - active antennas [1, 2] or antenna amplifiers [3-5], it is necessary to turn on adders or TV signal splitters [6]. Unfortunately, all this often does not provide the desired reception quality. The reason for this is not necessarily due to a bad feeder or poor matching. If, for example, you have several antennas designed to operate in the same range, then the reception of the same signal, especially powerful, will be possible with two or more antennas. However, in this case, due to the different signal propagation time in the feeders, a multi-contour or blurring of the image appears, although the signal level is quite sufficient for high-quality reception. This disadvantage can be eliminated by applying bandpass filters or selective amplifiers, which isolate one or more signals received by one of the antennas and suppress interfering signals. And so - after each antenna, while filtering different channels. Then all signals are summed. For the MB range, this problem is solved using the amplifiers and filters discussed in [7]. For the UHF range, there are almost no descriptions of such structures. Therefore, options for selective amplifiers specifically for the UHF range are described here. However, it should be noted that the use of filters is not always advisable (although it is permissible). The fact is that, firstly, filters introduce attenuation, and when receiving weak signals, this can affect the image quality. Secondly, the frequency response of filters, especially narrow-band ones, significantly depends on their matching with connecting cables. Therefore, even small changes in the load impedance can greatly change the frequency response and reduce the reception quality. To eliminate this undesirable effect, amplifier stages must be installed at the input and output of the filter. A schematic diagram of a selective amplifier for isolating one or more closely spaced signals is shown in fig. 1.
The device uses a bandpass filter of two coupled circuits L2C7 and L3C9. At the input of the filter, an amplifying stage is installed on a transistor VT1, and at the output there are two stages on transistors VT2 and VT3. The overall gain reaches 20...23 dB, and the bandwidth is determined by a bandpass filter. The signals received by the antenna are fed to the C1L1C2 filter, which suppresses signals with a frequency of less than 450 MHz. Diodes VD1, VD2 protect the transistor VT1 from powerful signals and electrical interference from lightning discharges. From the input stage, the signal passes into the first circuit L2C7. To obtain the required quality factor, a partial inclusion is applied (to the tap of the coil L2). To communicate with the L3C9 circuit, capacitor C8 is included (capacitive coupling). The signal from part of the turns of the coil L3 comes to the base of the transistor VT2, and after amplification - to the base of the transistor VT3. The frequency response of the output amplifier in order to further increase its selectivity can be adjusted by setting the L4C11 circuit in the feedback circuit. Diodes VD3, VD4 protect the amplifier from electrical discharges from the TV. They can occur due to the fact that the switching power supply of modern devices is connected to a 220 V network through small capacitors. The amplifier is powered by a stabilized voltage source of 12 V and consumes a current of about 25 mA. The VD5 diode will protect the amplifier when a power source is connected to it in the wrong polarity. If it is planned to be powered via a separate wire, then the voltage is applied directly to the VD5 diode, and if via a drop cable, decoupling elements L5, C16 are introduced into the amplifier. All parts of the amplifier are placed on one side of the printed circuit board made of double-sided foil fiberglass, shown in Fig. 2.
The second side of the board is left almost completely metallized. It only cut out areas for input, output and supply voltage (in the figure they are shown by a dashed line). The metallization of both sides is connected to one another along the board contour with soldered foil. After setting up the amplifier, the board from the side of the parts is closed with a metal cover, soldered to it. KT382A.B transistors can be used in the amplifier, and if high sensitivity is not required, KT371A is also suitable; diodes KD510A, KD521A. Capacitors C7, C9, C11 - KT4-25, the rest - K10-17, KM, KLS; resistors - MLT, S2-10, S2-33, R1-4. The conclusions of all parts must be of a minimum length. Coil L1 is wound with wire PEV-2 0,4 on a mandrel with a diameter of 2,5 mm and contains 2,8 turns. Coils L2, L3 are made with PEV-2 0,7 wire on a mandrel with a diameter of 3 mm. Winding length - 7 mm. They have three turns with a tap from the middle of the first turn. The L4 coil is wound with the same wire and contains two turns, and the L5 coil is wound with PEV-2 0,4 wire and has 15 turns, both on a mandrel with a diameter of 4 mm. The design of the capacitor C8 is shown in fig. 3. It is made of two sheets of tin or thick foil, which are soldered to the pads of the board. By changing the distance between the plates, the capacitance of the capacitor is changed.
The adjustment of the amplifier begins with the installation and verification of the necessary modes for direct current. By selecting the resistor R1, a voltage of 4 ... 5 V is achieved at the collector of the transistor VT1. The mode of transistors VT2, VT3 is obtained automatically. To adjust the frequency response of the amplifier, a panoramic indicator is used. Capacitors C7 and C9 tune the circuits to the desired frequencies. With the indicated ratings, the center frequency of the filter can be changed from 500 to 700 MHz. The bandwidth is set by adjusting the capacitance of the capacitor C8. At the same time, the gain of the amplifier also changes within a small range. By adjusting the capacitor C11, the maximum gain at the required frequency is obtained. By changing the capacitance of the capacitor C8, you can achieve a minimum amplifier bandwidth of 10 ... 12 MHz with a single-hump frequency response. This is necessary to highlight the signal of only one television channel. If it is necessary to select two adjacent channels, then the bandwidth is increased to 40 ... 50 MHz (the plates of the capacitor C8 are brought together) with a double-humped frequency response with a slight unevenness. In addition, the location of the taps of the coils L2, L3 also affects the frequency response of the filter. However, the ethereal environment can be complex. For example, in Kursk, in the UHF range, broadcasting is carried out on channels 31 and 33 from one place and with high power, and on channels 26 and 38 from another place and with less power. This option is quite typical for most cities in the country. Therefore, the already described amplifier can be used to receive and extract the signals of the 31st and 33rd channels. To receive the signals of the 26th and 38th channels (or two others with a large frequency separation), such an amplifier is not suitable. Here another one is needed, which has two bandwidths, i.e. contains two filters. A schematic diagram of such an amplifier is shown in Fig. four.
The signal from the antenna through the filter C1L1C2 is fed to the first amplifying stage on the transistor VT1. From its output, the signal is divided and comes to two independent stages on transistors VT2 and VT3, each of them is loaded on its own band-pass filter: L2C10-C12L3 and L4C13-C15L5. Amplifying stages on transistors V4 and VT5 are connected to the filters, the outputs of which operate on the same load. The overall gain of this device is 18 ... 20 dB, and the current consumption is approximately 40 mA. In such an amplifier, the same parts are used as in the one discussed above. A drawing of its printed circuit board with the placement of parts is shown in fig. 5.
Adjustment is carried out in a similar way. By selecting resistors R11 and R12, a constant voltage of about 5 V is set on the collectors of transistors VT4 and VT5. The filters are tuned to the desired frequencies. By adjusting the capacitors C6 and C7, the maximum gain is obtained at the selected frequencies. If it is necessary to narrow the bandwidth and increase the selectivity of the filter, increase the quality factor of the circuits by using thicker silver-plated wire in the coils and tuned capacitors with an air dielectric, or by increasing the number of circuits. Literature
Author: I. Nechaev, Kursk
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