Cable television mini-network. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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In the apartments of many residents today there are two or three televisions, and in addition, there are VCRs and music centers with VHF tuners. This encourages radio amateurs (but not only them) to create small television networks in apartments or private houses. Their essence is to, having one MB and UHF antenna or one broadband antenna, ensure the distribution of all received signals to all consumers located in different rooms of an apartment (several apartments) or a house. Naturally, the signal quality must be good, regardless of the number of devices connected to the network and the length of the coaxial cables.

The passive home TV network has already been described in [1]. With a weak level of signals from the antenna or with a long cable length, it is not always possible to achieve good image quality on a TV screen in such a television network. Therefore, in this situation it is desirable to use broadband active signal splitters [2]. However, it is not at all necessary to manufacture a broadband transistor amplifier-splitter yourself. To amplify the signal, it is quite possible to use ready-made plate antenna amplifiers (PAH), for example, the SWA, GPS, PAE series, etc.

As noted in [3], with the exclusion of a balancing-matching transformer at the PAH input, it has an input impedance of about 75 Ohm. This allows you to include it in a coaxial cable break or connect it to various signal splitters. And given the low cost of PAHs and the presence of a wide range of different models that differ in gain, frequency response and noise characteristics, it is of interest to use such amplifiers as internal (house) amplifiers in the cable television network of an apartment (several apartments) or at home.

Schematic diagram of the main (basic) block U1 of the network is shown in fig. 1. It includes a power supply unit based on an integrated stabilizer DA1 and a network transformer TZ, two hybrid couplers based on transformers and resistors T1R2 and T2R3, as well as a plate amplifier A1. A cable from an active broadband antenna, for example, the well-known ASP-1WA (CX-8WA) [8-4], as well as a similar imported or domestic one, is connected to the XW6 socket of this unit.

The television signal received by the antenna and amplified by the amplifier built into it is fed through the capacitor C1 to two hybrid couplers. From the two outputs of the T1R2 coupler, the signal passes through the XW2 and XW3 sockets to consumers, for example, to a TV and VCR. From one output of the T2R3 coupler, the signal comes to the XW4 jack, to which, for example, a VHF tuner of a music center is connected. From the other output of this coupler, the signal is fed to the input of a broadband plate amplifier A1 (PAA). Once again, we note that the balun-matching transformer at the input to the amplifier is removed, and the signal from the coupler is fed directly to the isolation capacitor at this input [3].

The signal from the output of the amplifier goes to the socket XW5. A cable is connected to it, going to another room or apartment, where, if necessary, one of the blocks considered further is installed.

The power supply unit of the base unit U1 consists of a network transformer TZ, a rectifier VD1, an integral stabilizer DA1, two smoothing and one blocking capacitors C2 - C4. It generates a stabilized voltage of +12 V, which is used through the decoupling choke L1 and the antenna cable to power the antenna amplifier, as well as through the choke L2 to power the amplifier A1 of the main unit and amplifiers of other units (if installed). Additional units are powered through a cable connected to socket XW5. LED HL1 indicates the inclusion of the main unit in the network.

It should be borne in mind that if amplifiers are not installed in subsequent distribution blocks (passive splitters are used), then at the output of block U1, an isolation capacitor C5 must be turned on (drawn by a dashed line).

The block U2 can serve as an active block for further branching of the signal, the schematic diagram of which is shown in fig. 2.

The signal from the main unit U1 through the XW6 socket and the coupling capacitor C6 enters the T4R5 hybrid coupler, in which it is divided into two directions: the T5R4 hybrid coupler and the plate amplifier A2. From the two outputs of the T5R4 coupler, the signal comes to the sockets XW7 and XW8 for connecting the following signal consumers.

Amplifier A2 amplifies the signal to the level necessary for its further transmission through the cable through socket XW9. Amplifier A2 is powered by cable from unit U1 through decoupling circuit L3C8L4. If you plan to connect another similar active signal splitter to the output of the U2 block, then to ensure its power supply via the cable, you should remove the capacitor C7 and apply the signal directly to the XW9 socket. If it is required to connect a third consumer to this active block, then the inclusion of hybrid couplers must be done as in block U1.

On fig. 3 shows a block diagram of the passive signal distribution U3 for two consumers. It contains only one T6R6 hybrid coupler. Such a block can be connected to the output of any of the blocks U1 and U2 described above with a sufficient signal level on them.

With a high signal level, you can install a passive unit U4 for four consumers. Its schematic diagram is shown in Fig. 4. It contains two hybrid couplers T7R7 and T8R8, which distribute the signal in four directions through sockets XW14-XW17.

In all the blocks described, you can use any capacitors and resistors. LED HL1 - any domestic or imported. Integral stabilizer DA1 - 78M12, KR142EN8B, K142EN8D, KR1180EN12A, KR1180EN12B. Anode bridge VD1 - any of the series KTs405, KTs412, etc. Network transformer TZ - any power 3 ... 5 W, having an output alternating voltage of about 13 ... 15 V at a current of 0,3 ... 0,5 A .

It should be borne in mind that if the U1 unit is connected to the network around the clock, for example, when using a cable network for several apartments, then you need to use a transformer from a series of increased electrical safety, for example, TP-121-8, TP-121-9, TP- 112-10, TP-112-11. Power switch SA1 - any, for example, PT-73.

Transformers T1, T2, T4-T8 hybrid couplers are the same. They are wound with a double-folded and twisted PEV-2 wire with a diameter of 0,2 ... 0,3 mm on a ferrite ring with a permeability of 600 ... 2000 and an outer diameter of 7 ... 10 mm. The windings contain 10 turns of wire, evenly distributed around the ring. When connecting transformers, the correct phasing of the windings must be observed. Inductors L1-L4 - standard, for example, DM-0,1, with an inductance of 25 ... 45 μH.

As broadband amplifiers A1 and A2, PAHs of the SWA, PAE, GPS, PA, etc. series can be used, as indicated above. Information on them can be found in [4-6]. In [4], in addition to the main parameters, their frequency response is also presented, which will help you choose an amplifier depending on the required gain in a particular frequency range. It should be remembered that with a large transmission coefficient, overload and excitation of the amplifier are possible. In addition, it must be borne in mind that the frequency response of PAHs is rather uneven, and when they are connected in series, the resulting frequency response of the path is obtained by multiplying the frequency response of its constituent nodes and the unevenness increases. Therefore, in the proposed blocks U1 and U2, it is best to use single-stage amplifiers, for example. SWA-1, SWA-1 LUX, PA-2, having a gain of 10 ... 15 dB, or two-stage with a gain of not more than 30 dB, for example, SWA-4 LUX, SWA-455 LUX, PA-10. RAE-14. WA-031,WA-032.

All described blocks U1-U4 are each mounted in a suitable size case made of plastic or fiberglass and must be shielded. Details are mounted in a hinged way. Their conclusions should have a minimum length. LED HL1 and switch SA1 block U1 is placed on the front panel of its housing. The blocks are connected to each other and to consumers with a coaxial cable with a characteristic impedance of 75 ohms through standard plugs and sockets. However, it is better to use special threaded cable connectors that do not require cable soldering and have a greater connection reliability.

Using the described blocks in various combinations, you can make a small TV network that allows you to connect several signal consumers located in different places to one antenna without a noticeable deterioration in signal quality. The main condition for high-quality network operation is a good signal from a passive or active antenna (or several antennas), so, first of all, you should pay attention to this.

Literature

1. Nechaev I. Home television network. - Radio, 1996, No. 10, p. 12, 13.
2. Nechaev I. Active TV signal splitter. - Radio, 2000, No. 4, p. 12.
3. Pakhomov A. On the matching of plate antenna amplifiers. - Radio, 2000, No. 11, p. 11, 12,32.
4. Nesterenko N., Zhuzhevich A. Choose the antenna yourself. - M.: Solon-R, 1999.
5. Pakhomov A. Antenna amplifiers SWA. - Radio, 1999, No. 1, pp. 10-12.
6. Pakhomov A. New antenna amplifiers. - Radio, 2000, No. 7, p. 6-8.

Author: I.Potachin, Fokino, Bryansk region

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