TV signal amplifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Television antennas

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Currently, the delivery of a television signal to the consumer is carried out both through the air and using cable television systems. If the signal level is insufficient, television amplifiers have to be used to increase it. Previously, they were called antennas, since they were most often located near the antenna.

A description of this design is given in the article "Television antenna amplifier with a large dynamic range" ("Radio", 2005, No. 9, pp. 11, 12). But with the development of cable networks, this name - antenna amplifier - has become not entirely accurate. For example, it may be required if there are several consumers in the apartment and the television signal has to be divided “equally” among all, and its level may not be enough.

The amplifier must not only provide the required amplification, but also not degrade the signal quality, i.e., have a small noise figure. Since the number of simultaneously amplified signals can reach several tens, it must also have high linearity, i.e., low intermodulation distortion. In addition, it is desirable to install it at the signal input to the apartment and therefore feed it through the signal cable.

 
Rice. 1 (click to enlarge)

A diagram of such an amplifier, assembled on the basis of a specialized microcircuit, is shown in Fig. 1. One of the features of the applied microcircuit is the absence of a separate output for supplying the supply voltage that goes directly to the output, which simplifies the power supply of the amplifier via the signal cable. installed with a selection of current-limiting resistors.

At the input of the amplifier, a tuning resistor R1 is provided - the input signal regulator, it may be required to obtain the optimal signal level at the output. This resistor can be omitted by connecting the left terminal of the capacitor C1 according to the diagram directly to the center pin of the high-frequency socket XW1. The C1L1C2 high pass filter suppresses signals below 30 MHz. Diodes VD1, VD2 protect the input of the microcircuit, and VD3 - its output. Power is supplied to the microcircuit via a signal cable through current-limiting resistors R2 R3, R5, R6 At the same time, R4 and R5, R6 provide division of the power of amplified signals into two outputs, so the gain of the entire amplifier will be less than the gain of the applied microcircuit by 3 .. 4 dB

Various microcircuits can be used without changing the circuit and design, while in some cases it will be necessary to change only the values ​​of some resistors. Currently, the choice of microcircuits is very wide. The parameters of some of them are given in the table, where K is the gain; F-3 dB - the upper frequency of the passband at a level of 3 dB from the maximum value (the lower frequency of the passband is determined by the capacitance of the isolation capacitors and the HPF parameters); Ksh - noise figure.

The amplitude characteristics linearity indicators are: P, - output power at which the gain is reduced by 1 dB (Output Power at 1 dB Gain Compression); IP3 is a hypothetical point on the amplitude characteristic of the microcircuit, in which the power of intermodulation distortion is equal to the power of the main signal. In foreign sources, it is called the Third Order Intercept Point. All of the above parameters are normalized for a 50-ohm path, and the output power is measured in decibels relative to a milliwatt (dBmW). Other parameters: Ipotr - rated current consumption; Imax - the maximum allowable current consumption; Unp - constant voltage at the output of the microcircuit.

Fig. 2

Since the supply voltage is supplied to the amplifier via a signal cable, a decoupling device is used to supply it, the circuit of which is shown in Fig. 2. Diode VD1 protects the amplifier from reverse voltage polarity, through the inductor L1, the supply voltage is supplied to the signal cable, while the filter L1C2C3 does not pass the television signal to the power supply. The XW1 plug is inserted into the antenna jack of the TV, and the signal cable is inserted into the XW2 jack, which goes to the XW3 jack of the amplifier and supply voltage from the output of the mains stabilized power supply, which provides the required current consumption. If you plan to further divide the amplifier signal into several consumers, then you can connect any splitter to the XW2 socket or XW1 plug of the decoupling device.

Fig. 3

All elements of the amplifier are mounted on a printed circuit board made of fiberglass foiled on both sides with a thickness of 1,5 mm, the drawing of which is shown in Fig. 3. On the side of the board not shown in the figure, the foil is completely left. The device uses fixed resistors PH1-12, trimmer - 3303 from BOURNS, capacitors for surface mounting of size 0805. The inductor is wound with wire PEV-2 0,2 on a mandrel with a diameter of 2 mm and contains 10-12 turns. Coaxial sockets XW1 -XW3 - series F, they are mounted on metal corners made of tin. The corners are pre-soldered to the board on both sides. The appearance of the mounted amplifier is shown in fig. 4.

Fig. 4

The total resistance of the circuit of resistors R2, R3, R5, R6 (Rz) depends on the supply voltage 11pit, which should be several volts more ipr, and the current consumed by the microcircuit Ipot: Rsum = (Upit-Upr) / lpot. Since the amplifier is designed to connect two loads - RF cables with a resistance of 75 ohms, the values ​​​​of the resistors R5 and R6 should not be changed, i.e. R5 = R6 = 75 ohms. For example, for the SGA-6489 microcircuit (Ipot = 75 mA) with a supply voltage Upit = 12 V, we get Rsum = (12 - 5) / 75 - 93 Ohms The total resistance of resistors R5 and R6 is approximately 38 Ohms, therefore the resistance of resistors R2 connected in parallel and R3 is: 93 - 38 = 55 ohms For our case, choose R2 = R3 = 100 ohms. Power dissipated by one resistor R2 (or R3) Rpacc = (lcont/2)²-R2 = 0,14 W, so resistors with a nominal power dissipation of 0,25 W are selected.

Fig. 5

A drawing of the decoupler board is shown in fig. 5. It is also made of 1,5 mm thick fiberglass foiled on both sides, the foil on one side is also left. An EC-24 choke with an inductance of several microhenries (1 - 10 μH) is used, high-frequency connectors are of the CAT series (САТ-Ш, CAT-Г) or similar imported ones. Socket XW2 is installed on the board in the same way as sockets on the amplifier board. Pieces of tinned wire are inserted into the holes of both boards and soldered on both sides.

If the amplifier is planned to be used as an antenna and placed near it, then only one output (XW3) is left, and preference should be given to a microcircuit with the lowest noise figure, a gain of at least 15 dB and maximum values ​​P1 and IP3. In this case, the resistors R2, R3, R5, R6 are excluded from the amplifier, replacing them with a foil jumper 3 wide. 4 mm, and include them in series with the inductor L1 in the decoupling device. The amplifier housing must be sealed or all its elements must be protected in some way from environmental influences.

In conclusion, it should be noted that on the basis of some of the microcircuits shown in the table, it is possible to build an IF amplifier (= 0,95 ... 2,4 GHz) of satellite television systems. Such an amplifier is installed between the converter (LNB - Low Noise Block) and the receiver. It may be required if the cable is long or the signal is strongly attenuated in it.

Author: I. Nechaev

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