Watching television with the family. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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"There are no comrades for taste and color." Watching television in the family can cause contention. To view various programs at the same time, several TVs in the apartment must work simultaneously. However, the operation of TVs connected to the same feeder of a common antenna (individual or collective), without a splitter, causes mutual interference due to the beats of the frequencies of the receivers' local oscillators (modern receivers are all superheterodyne). This interference appears as standing or slowly moving oblique dark lines across the screen. The article below will help you find a way out in such a situation.

In the collective antenna of a multi-storey building, interference is eliminated by installing storey splitters in the feeder break of the coaxial cable (Fig. 1a), which in fact are T1 high-frequency transformers with each inductance and capacitance distributed along the combined coaxial lines. Moreover, the outputs and inputs of each line are loaded with a resistance equal to the wave resistance (usually 75 ohms). The latter ensures the mode of a traveling electromagnetic wave in the lines, and the absence of galvanic coupling of individual internal conductors 1 with a common braid 2 provides mutual decoupling of signals (there are no common non-linear elements in the circuit that are required in each superheterodyne).

Rice. 1a. Scheme of the collective wiring of the TV feeder

In passing, it should be noted that the one who connects the coaxial cable to the "barrel" 3 (vein of a thick coaxial cable connecting the floors), interferes with all televisions connected to this collective antenna. Therefore, if such interference appeared on your screen, and the signal weakened, then a certain "craftsman" connected to the "trunk". Contact the owner of the community antenna (the company that maintains the antenna) immediately. He will call the violator to order, seal the floor splitters. In no case do not look for the offender yourself, if you do not want to make an enemy in the house.

Connecting additional (intra-family) TVs to the free outputs of the floor/th splitter/s with separate coaxial cables will ensure against mutual interference and will not cause inconvenience to any of the neighbors, but, for sure, inside the apartment, additional coaxial cables through the entire apartment to each TV is a great inconvenience (as exception - rooms on both sides of the main entrance to the apartment).

Radio amateurs offered [1,2] simple TV signal splitters, both similar to [1] floor splitters (Fig. 1b), but essentially resonant (not broadband, for 1,2 channels), and frequency-independent on resistors (Fig. 2a ), where

a Zv (usually 75 ohms) is the characteristic impedance of all cables used, n is the number of connected TVs.

Rice. 1b. Resonant floor splitter

Rice. 2a. Frequency independent splitter

To shorten the connections (reduce parasitic capacitances and inductances), it is convenient to use not individual resistors, but B20 resistive blocks [3], connecting the resistors in them in pairs-parallel (Fig. 2b, c) or three (Fig. 2d). It is good to use such splitters if the signal is strong (TVC antenna is nearby or there is a powerful amplifier, converter).

Rice. 2b. A splitter based on a B20 resistive block with a pair-parallel connection of resistors for output to two TVs

Rice. 2c. A splitter based on a B20 resistive block with a pair-parallel connection of resistors for output to three TVs

Rice. 2y. A splitter based on a B20 resistive block with a connection of three resistors for output to two TVs

Without violating copyright, for their own use, radio amateurs can repeat the industrial television device of the UTR-2.N2 splitter [2] (Fig. 1a), the obvious advantage of which is the absence of losses and the presence of electrical isolation (crosstalk) between the outputs, the signals on which are antiphase, and broadband (12 ... 21 channels) in the direction of 60 ... 2 channels can be continued by installing capacitors C3, C1 KD-5,1 18 pF Crosstalk attenuation between the outputs is at least XNUMX dB.

Rice. 3a. Scheme of the splitter UTR-2.N2

Transformers T1 and T2 are identical and are wound with two wires of 5 turns with PEV-2 wire with a diameter of 0,47 mm directly on rings with rounded edges (turned with a round needle file) from ferrite 30VN K7x4x2 (used in BESM-4 computers, in transformers of blocking divider generators frequencies) [4].

The braids of all cables (input and output) of the splitter must be securely connected to each other and the metal case of the splitter, and their central cores directly to the terminals of the B20 block. Resistor R1 type MLT-0,125 150 Ohm.

If the TV has two separate antenna inputs MB and UHF, then separate UTR outputs are connected to them with independent cables with a characteristic impedance of 75 ohms.

The disadvantage of UTR-2.N2 is that a 3-way splitter requires two UTR splitters; 4-3; 8-7 (Fig. 3b)

Rice. 3b. Splitter circuits for 3, 4, 8 outputs

Note: capacitances are not used in B20-4-4 blocks, but with a very weak signal they can be connected, as indicated by the dotted line (Fig. 2b). The B20-4-4 circuit is identical for all resistor values. Ferrite rings can be of the 7BH type of any diameter, the diameter of the wire is not critical, it’s just that it’s more convenient for installation and winding. As a housing for the UTR splitter, you can use a part of the capacitor housing [5] KBG-MN 8 μF 200 V or similar in size. Naturally, 2 and 3 circuits can be placed in one case (Fig. 3a), when a splitter for 4, XNUMX outputs is needed and it is convenient to branch cables from one place.

Literature

1. Borkunyaev A.A. and others. Reference book of a radio amateur designer. - M.: Radio and communication, 1990, p. 602.
2. The use of integrated MS in electronic computing. - M.: Radio and communication, 1986, p. 186...188.
3. Handbook of circuitry for the radio amateur. - Kyiv, Technique, 1987, p. 33.
4. Ivashin N. Secondary use of capacitors. - Radio amateur, 2003, No. 6, p. 37.

Author: N. Ivashin, Minsk; Publication: cxem.net

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