Active splitter of TV signals. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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Many users of various video equipment - televisions, VCRs, video cameras - are faced with the problem of distributing television signals in the apartment. For this purpose, various filters, dividers, home networks, etc. are used. We hope that the material published here will be useful to our readers.

The number of television signal consumers in many apartments is constantly growing. In recent years, to the already traditional televisions (and more than one) and VCRs, computers with video cards with a TV tuner have also been added. At the same time, the number of television programs is increasing, mainly due to the opening of new commercial television stations in the UHF range. It is inconvenient to install your own antenna for each receiver.

Therefore, a typical situation has become when a large number of TV signals, which also have different levels, must be divided among many consumers. If the dividers are made passive, then reception of weak signals often becomes impossible. Therefore, an active divider is needed that provides signal distribution over the entire frequency range and operates without loss or with low gain. The obligatory parameters of such a divider must be a low level of intrinsic noise and a large dynamic range. This is necessary because the levels of received signals can differ from each other by an order of magnitude or more. If this is neglected, weak signals may be “lost” in noise or crosstalk distortion may occur, making reception virtually impossible.

To solve this problem, an active splitter is proposed, the circuit diagram of which is shown in Fig. 1. It consists of a two-stage broadband amplifier using VT1 transistors. VT2 and signal divider on hybrid taps T1. T2. The device bandwidth is 40...750 MHz. gain - 3...6 dB. The first stage of the amplifier is assembled on a low-noise transistor, and the second on a medium-power transistor, due to which the device has a low noise level and a relatively large dynamic range. The input resistance of transistor VT1 exceeds 75 Ohms and varies by 1.5...2 times over the entire frequency range. For the purpose of coordination for its alignment and obtaining values ​​close to the characteristic impedance of the cable, the resistance of resistor R1 is selected equal to 200 Ohms. The amplifier provides undistorted voltages of up to 400 mV at each of the four outputs. The used hybrid couplers make it possible to obtain an isolation between outputs of no worse than 10 dB over the entire frequency range, which reduces the mutual influence of signal consumers.

The splitter can be powered from a stabilized source via separate conductors or via a high-frequency cable. The first case is possible if there is a power outlet next to the device. If this is not the case, then power it via a high-frequency cable, changing the power circuits in accordance with the diagram in Fig. 2.

The supply voltage is supplied through a cable connected to output 1. At the same time, at the other end, a separating capacitor with a capacity of about 100 pF is connected to the cable and the supply voltage is supplied to the point of their connection through an inductor with an inductance of about 100 μH. The current consumption of the splitter is about 60 mA.

In addition to those indicated in the diagram, the device can use KT382A transistors. KT371A (VT1). KT640A 2. KT640V-2 (VT2); capacitors K10-17, KD. KLS: resistors MLT, S2-33, R1-4.

The transformers are wound with double folded and twisted PEV-2 0.2 wire on a ferrite ring with a permeability of 600...2000 and a diameter of 7...10 mm. The windings contain ten turns, they should be evenly distributed throughout the ring. Coil L1 is wound in the same way. but with a single wire. The leads of resistors, capacitors and transformers must have a minimum length.

All parts of the device are placed on one side of a printed circuit board made of double-sided foil fiberglass, the drawing of which is shown in Fig. 3.

The second side (fully metallized) serves as a common wire. The braids of the input and output coaxial cables are soldered to the foil of the common wire, and the leads of the parts are soldered through the holes in the board.

The DC mode is set automatically, and the setup comes down to adjusting the frequency response by selecting capacitor C4. It is desirable to obtain a slight increase in the frequency response in the UHF range. which will compensate for the increased losses in the coaxial cable at these frequencies.

The stability of the amplifier will increase if a resistor with a resistance of 7...5 Ohms is connected in series with capacitor C15.

Author: I. Nechaev, Kursk

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