An active TV signal divider is from a passive one. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television, video equipment

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Adding an RF amplifier to the passive divider of the TV signal allows you to increase the level of the signal coming to the TV or TV tuner. The supply voltage to the amplifier with such a refinement is supplied through the signal cable.

To connect several TV sets (TV tuners) to one antenna or a collective cable network, a signal power divider (splitter) is used, which in foreign literature is called a splitter. Domestic and foreign industry offers a wide range of these devices.

If the signal level is sufficient, passive power dividers can be used, which attenuate the signal, and the more outputs the divider has, the greater the attenuation. So, a divider for two outputs introduces an attenuation of 3 ... 4 dB, for three - 7 ... 8 dB.

To receive weak signals, you can use a home-made amplifier or an active divider with a built-in amplifier [1, 2] or purchase them in a store, but they cost several hundred rubles. Meanwhile, it is not so difficult to turn a passive TV signal divider into an active one. This will require a specialized microwave amplifier chip and a few other parts. And it will look like an ordinary passive one, which will reduce the desire of intruders to steal it if it is installed, for example, on a landing.

In some power dividers, there is enough space for an amplifier even in excess. This applies primarily to dividers assembled according to the simplest circuits and containing only transformers. These include, for example, a TV divider for three outputs of the PROconnect brand (one of the cheapest). It uses only two transformers, each of which contains two windings of one turn, wound on a tubular ferrite magnetic core. The electrical parameters of such a divider, frankly speaking, are "not very good". By the way, in the purchased copy, the common wire of the printed circuit board is not connected to the case! Although there are several pads on the board for this, and special pins on the case. Apparently, someone decided to dramatically increase labor productivity. But on the other hand, there are "extra" metallized areas on the printed circuit board that can be used to place amplifier elements. However, there are dividers and completely without a printed circuit board.

Fig. 1

A diagram of the option for refining such a divider is shown in fig. 1. All newly entered elements are highlighted in color, and their numbering begins with the prefix 1 (this also applies to the following diagram). The modified device uses the MSA-1105 chip, the scheme for switching it on is standard. It is powered by a signal cable connected to socket XW2 "Output 1". A feature of the microcircuit, as well as most of the others listed in [1, 2], is that it automatically maintains a constant voltage at its output (in our case, 5,5 V) when the current consumption changes within wide limits. This allows you to set the current consumed by the microcircuit by selecting a resistor 1R1. If, during adjustment, the resistance of this resistor turns out to be less than 75 Ohms, a choke should be connected in series with it - 5-10 turns of PEV-2 0,3 wire wound on a mandrel with a diameter of 2.3 mm. Choke 1L1 closes low-frequency signals, pickups and charges of static electricity to a common wire. The diode assembly 1VD1 limits the amplitude of the voltage of the RF signals, protecting the microcircuit from failure.

Fig. 2

In the divider itself, the common terminal of the transformer T1 is unsoldered from the printed conductor going to the XW1 connector and soldered to the 1C2 capacitor. The printed conductor connecting the XW2 socket and the T1 transformer is cut in the middle and a 1C4 capacitor is installed at the cut site. For the remaining capacitors, resistors and microcircuits, pads are cut out on the free area of ​​the metallized surface of the printed circuit board and, after tinning, elements are mounted on them (Fig. 2). The gain of the applied microcircuit is about 10 dB. Therefore, the gain at each of the three outputs will be about +7 dB instead of -3 dB. The current consumed by the microcircuit is set by selecting a resistor 1R1 at a supply voltage of 10 V.

Fig. 3

Better, in terms of electrical parameters, a three-output divider marked 19139R of the Evologe trademark. Its printed circuit board has a more complex topology and, in addition to two matching-balancing transformers (a divider for three outputs) on magnetic circuits in the form of "binoculars", the device includes separating and correcting capacitors, as well as terminating resistors. A fragment of the divider circuit, taking into account the refinement, is shown in fig. 3. There is less space for the amplifier, so the 1C2 capacitor (for surface mounting) is used as a support stand. With one contact pad, it is soldered to the "ground" foil, and a 1R1 resistor, a 1VD2 diode cathode and a 1L2 inductor are soldered to the second (Fig. 4). Diode 1VD1 and inductor 1L1 - to the central contact of socket XW1, capacitor C1 is removed. With the indicated microcircuit, the transfer coefficient at each output is 10 ... 12 dB.

Fig. 4

The device can use microcircuits of microwave amplifiers, the list and parameters of which are given in [1, 2]. Capacitors and resistor for surface mounting, sizes 0805, 1206, output resistor 1R1 - P1-4. The BAV99 diode assembly can be replaced with a similar one - TMPD7000, and the KD522B diode - with any low-power rectifier or pulse. All chokes are wound on a mandrel with a diameter of 2 mm and contain 15-20 turns of PEV-2 0,4 wire.

Fig. 5

The scheme of the power supply unit is shown in fig. 5. Connector XW2 is connected to the TV input, and a cable is connected to XW1, which is connected to connector XW2 of the divider (see Fig. 1 and Fig. 3). A resistor R2 is installed in the power line of the amplifier, and in parallel with it is an HL1 LED with a current-limiting resistor R1. By the glow of the LED, you can judge the health of the amplifier and the connecting cable. Resistor R2 is selected so that in normal mode the LED shines dimly. If any malfunction occurs, its glow will change. For example, if the coaxial cable breaks, it will go out, and if the power supply circuit of the microwave amplifier is shorted, the brightness of the glow will increase sharply. In addition, resistor R2 will limit the short circuit current. In the power node, you can use MLT, S2-23, P1-4 resistors, capacitors - K10-17, a choke - as in an amplifier. The LED should be of high brightness red glow with a voltage drop of not more than 1,8 V. Power supply - mains stabilized with an output voltage of 12 V and a current of up to 150 mA.

Literature

1. Nechaev I. Television antenna amplifier with a large dynamic range. - Radio, 2005, No. 9, p. 11, 12.
2. Nechaev I. Television signal amplifier. - Radio, 2013, No. 2, p. 11,12.

Author: I. Nechaev

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