Video power splitters. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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In the technique of cable television (CATV), power dividers (splitters) of signals are widely used. They are used for external and internal wiring of CATV networks and have a different configuration. Typically, the power supplied to the input of the divider is evenly distributed among several outputs. However, there is a separate class of dividers, called taps, that tap off some of the power transmitted over the trunk cable.

The circuit in Fig. 1 is a broadband divider that evenly distributes the input signal between the N outputs. The attenuation coefficient of the signal K3 at any output is calculated by the formula

Kz \u20d 1 * lg (N) (dB) (XNUMX)

Fig.1. Broadband divider

As you can see from this formula, the signal at any output is attenuated. If we amplify the signal at the output of the divider to the level of the input, we get an active divider, or splitter. Structurally, the amplifier is connected to the divider, and its gain is chosen equal to the attenuation coefficient of the divider (Kz). Resistors R1...RN are equal and are calculated by the formula

   (2)

The input and output resistances must be equal to Zn (load matching condition).

Table 1 shows the data of dividers with N outputs operating on a load of 75 ohms.

The main advantage of these devices is their broadband and frequency response uniformity in the passband. Figure 2 shows the design of a divider with three outputs. All resistors have a resistance of 37,5 ohms. The divider is assembled in a brass or duralumin box. The input and output connectors are "F" or "SMA" type. The former are preferable, because provide for the connection of coaxial cables without the use of soldering.

Fig.2. Divider design with three outputs

Fig.3. Divider circuit board with three outputs

Theoretically, the bandwidth of such a scheme is not limited. However, when using the installation shown in Fig. 2, at high frequencies (above 800 MHz), the frequency response becomes uneven and has a decline (the influence of parasitic capacitances and inductances of the resistor leads affects).

To eliminate this undesirable phenomenon, leadless resistors mounted on the surface of printed circuit boards are used. The printed circuit board (Fig. 3) is made of double-sided foil fiberglass of the STNF brand with a thickness of 1,5 mm. Track width - 1,2 mm. Resistors are soldered into track breaks. The application of this method makes it possible to obtain excellent results from the operation of dividers at frequencies up to 3 GHz. When using dividers at higher frequencies, the printed circuit board is made of PTFE.

In practice, broadband dividers are used to distribute signals from a satellite TV receiver converter between several tuners (indoor units). A compensating amplifier is used to compensate for signal attenuation in the divider.

A schematic diagram of the first IF signal splitter in STV program receivers is shown in Fig. 4, and the wiring diagram made using SMD technology is shown in Fig. 5.

Fig.4. XNUMXst IF signal splitter in STV receivers

Fig.5. Splitter Wiring Diagram

The crosses on the board drawing indicate through holes through which the corresponding printed tracks are connected to the common bus (second side). The common bus has electrical contact with the body of the splitter. XS1.. .XS3 - "F"-connectors. All elements (including L1 and L3) are SMD-type (you can use ordinary elements by biting off their leads completely and soldering them directly to the printed tracks). Coil L2 - frameless, with an inner diameter of 3 mm, has 4 turns of PEVTL wire with a diameter of 0,47 mm.

As you can see from the diagram, the compensating amplifier is powered by direct voltage (simultaneously supplying the external converter), which comes from the tuner connected to "Output 1". The passage of the supply voltage from the second tuner and oscillations with a frequency of 22 kHz are blocked by the decoupling capacitance C5. Thus, the master tuner is the one connected to connector XS2 "Output 1".

Figure 6 shows a schematic diagram of a divider-coupler, which, unlike the circuit in Figure 1, has less attenuation. Taps are widely used in CATV networks for access wiring. The signal from the trunk cable is fed through the trunk coupler to the access cable (thinner than the trunk cable). On each floor, the taps shown in Fig. 6 are included in the cable break. It does not matter which of the connectors, XS1 or XS8, is the input (output).

Fig.6. Schematic diagram of the divider-coupler

On the last floor, where the access cable ends, either a coupler is installed, to the output of which a 75 Ohm plug ("terminator") is connected, or a splitter shown in Fig. 7.

Fig.7. Schematic diagram of the splitter

Access splitters are assembled in brass or duralumin cases of appropriate sizes. All inductors are frameless, diameter 5 mm. L1, L4 (Fig. 6) and L1, L2 (Fig. 7) - 2,5 turns; 12, L3 (Fig. 6) - 6 turns wound with PEVTL wire, diameter 0,8 mm, winding pitch - 1,5 mm. All connectors are "F" type.

To branch signals from the main cables, couplers are used, assembled according to similar schemes (Fig. 8,9). Due to the fact that passive components transmit more power in this case, the terminating resistors must have a power dissipation rating of at least 2 W. Accordingly, the type of connectors through which the tap is connected to the main cable has been changed. As XS1, XS2, microwave connectors of the SR-75-66FV type are used. Coils L1, L2 are wound with PEVTL wire with a diameter of 1,2 mm (when setting, the pitch of the turns is specified).

Fig.8,9. Main taps

In principle, it is possible to manufacture main taps with an arbitrarily large number of outlets, but in practice it is sufficient to have two outlets. At the end of the main cable, either a coupler (Fig. 8) is installed, to the output of which a 75-ohm terminator is connected, or a splitter (Fig. 7). The described couplers work well at frequencies up to 300 MHz and decently enough - in the range of 300 ... 800 MHz. If the access coupler is used to distribute the signal from the collective UHF or MMDS antenna, which have an external amplifier and converter, the couplers shown in Fig. 6 are installed on the floors, and a power splitter-injector is installed at the end of the cable (Fig. 10). Inductances L1 ... L4 are identical to those used in the circuit in Fig.6. L5 and L6 - type D-0,1. As T1, any small-sized transformer with an output voltage of 15 V and a permissible current of 0,5 ... 0,7 A is used. The device is assembled in a duralumin case; the elements of the power supply are separated from the splitter circuit by a partition. DA1 is attached directly to the case, which plays the role of a heat sink.

Fig.10. Power splitter-injector

Figure 11 shows a typical signal wiring diagram from one MMDS antenna (2,5...2,7 GHz) [1]. RG-6U is used as an access cable, RG-6 is used as a subscriber cable. When debugging the system, it is necessary to clarify the required supply voltage of the MMDS converter. If it differs from 12 V, it is necessary to replace DA1 (Fig. 10) with the corresponding one (for example, for Up \u15d 142 V, KR8ENXNUMXV is used).

Fig.11. Signal wiring diagram from one MMDS antenna

It is impossible to ignore the class of devices called "combiners-splitters" of STV/TV signals. The principle of their work is explained in Fig.12. The combiner combines the IF1 STV signals from the converter (the frequency band occupied by the signal is 950 ... 2050 MHz) and the signals of MB and UHF TV programs amplified by the antenna amplifier (48 ... 800 MHz). The resulting signal is fed through the drop cable to the splitter-splitter, where the IF1 STV signals (supplied to the STV tuner) and MV / UHF TV signals (supplied to the antenna input of the TV receiver) are again selected. Figure 13 shows a diagram of a combiner. XS1...XS3 - "F"-connectors. The circuit is mounted in a duralumin case. Inductances - frameless, d2,5 mm. They are wound with silver-plated wire d0,31 mm and have: L1 - 2 turns, L2 - 3 turns, and L3 - 2,5 turns.

Fig.12. The principle of operation of combiners-splitters

Fig.13. Combiner scheme

The antenna amplifier MV / UHF is powered by a constant voltage supplied from the STV tuner. The current consumption of the amplifier should not exceed 50...70 mA.

Figure 14 shows a diagram of an active splitter, which separates the signals combined by the combiner, and also compensates for the attenuation introduced by the splitter, which is part of the splitter. The equalizing amplifier is fed from the CTB tuner via a drop cable. L2 and L3 - frameless, d3 mm, wound with silver-plated wire d0,31 mm and have: L2 - 3,5 turns, and L3 - 3 turns, respectively. The splitter is mounted using the SMD method and is enclosed in a brass or duralumin case.

Fig.14. Schematic of an active splitter

In conclusion, it should be noted that when tuning the devices described above, it is desirable to use a GKCh with a swing band from 30 to 3000 MHz. After setting up the devices, it is necessary to take their exact frequency response and put them on the top covers of the devices, for a visual representation of the characteristics of the circuits used.

In order to avoid damage by floating potentials, it is necessary to provide grounding of the cases of all described devices.

Literature

1. Fedorov V. Microwave distribution systems for TV programs. - Radio amateur, 1999, N11, S.5-7

Author: V. Fedorov, 398046, Lipetsk-46, PO Box 1341; Publication: radioradar.net

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