Narrowband antenna amplifier with tunable frequency response. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination

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The proposed antenna amplifier is designed to improve the sensitivity of television receivers that receive transmissions in the meter range. The amplifier gain is 22 ... 24 dB, a bandwidth of about 8 MHz can be easily tuned to one of the television channels using a tuned capacitor.

The construction of a narrow-band tunable amplifier in the frequency range from 1 to 12 television channels is associated with great difficulties due to the fact that there is a gap between 5 and 6 television channels. Therefore, an amplifier is proposed that operates in one of two subbands - from 1 to 5 or from 6 to 12 television channels. The use of low-noise transistors in the amplifier in combination with the optimization of their DC modes made it possible to ensure a low level of intrinsic noise of the amplifier at a sufficiently high gain.

The use of the proposed antenna amplifier is most effective in cases where the television receiver does not have sufficient gain margin for stable reception of television programs outside the coverage area. It also seems reasonable to use an amplifier to improve gain-limited sensitivity when receiving transmissions on televisions that have lost their gain margin as a result of long-term operation. The described amplifier can also be used to receive TV programs in areas remote from television centers and repeaters, for example, in rural areas, where in most cases there are no centralized receiving antenna systems (collective antennas). In this case, it is possible to use an amplifier when several television receivers are connected to one receiving antenna.

In practice, the implementation of this entire device is possible provided that the antenna inputs of the television receivers are matched with the output of the amplifier. The amplifier also allows, when using narrow-band and highly directional antennas and antenna systems with a high gain, to conduct confident long-range reception of television programs from television centers that lie outside the zone of reliable reception. In this case, it is recommended to place the amplifier on the mast, in the immediate vicinity of the antenna, in order to compensate for the signal attenuation in the antenna feeder, which will be greater, the longer the drop cable length. In this case, the use of the proposed amplifier will improve the signal-to-noise ratio at the input of the television receiver.

The developed two-stage amplifier is characterized by a much simpler design and circuit implementation compared to the three-stage amplifier described in [1]. The amplifier from [1] does not provide the possibility of tuning the amplitude-frequency characteristic (AFC) to one of the television channels of the range and has only a limited frequency adjustment relative to the average frequency, to which it is fixed, within ±3 MHz, which allows compensating only for temperature drift amplifier bandwidth. The negative power supply of the specified amplifier with respect to the case excludes the possibility of its operation from the power supply of a tube TV.

The main advantage of the proposed amplifier is the possibility of smooth tuning of its frequency response to various television channels in the meter range, which makes it possible to obtain a high gain on the desired channel with relatively simple circuit solutions. The amplitude-frequency characteristic of the first stage of the amplifier is uniform in a wide frequency range, which includes five - from the 1st to the 5th (48,5 ... 100 MHz) - or seven - from the 6th to the 12th (174 ... 230 MHz) - television channels.

The amplifier has two amplification stages, assembled on transistors VT1 and VT2 (Fig. 1), connected according to a common emitter circuit and a common base circuit, respectively. Such a cascode inclusion of transistors VT1 and VT2 is due to the desire to reduce the noise figure of the amplifier as a whole. The frequency tuning of the amplifier is carried out by smoothly adjusting the tuning capacitor C10 in the second amplification stage, the frequency response of which has the shape of a bell-shaped peak with a maximum gain in a narrow range with a width of about 8 MHz. The input circuit L1C1L2C2 is a high-pass filter with a cutoff frequency of about 48,5 MHz for the first subband and about 160 MHz for the second. The mode of operation of the transistor VT1 for direct current is set by resistors R1 and R2 in such a way as to obtain a voltage on its collector equal to +5 V at a collector current of about 5 mA. In this mode, the noise figure of the KT371A series transistor is 3,4...4,7 dB at a frequency of 400 MHz [2], and at frequencies below 400 MHz, the transistor noise will be less. The capacitance of the capacitor C3 together with the input capacitance of the transistor VT1 limit the gain of the first stage at the highest frequency of the subband. The measured gain of the first stage is 13...15 dB in both subbands.

Elements C5, L3, C6 are the input high-pass filter of the second stage and are used to suppress low-frequency signals. Transistor VT2, in the collector circuit of which the L4C10 circuit is included, is a resonant amplifier. The parameters of the L4C10 circuit elements determine the narrow frequency response of the second stage, and changing them makes it possible to tune the frequency response over a wide frequency range. The use of a transistor of the GT2A series as VT346, connected according to a common base circuit, is due to the fact that in this switching circuit the transistor has a small throughput capacitance. Resistors R3 - R5 provide the following DC mode for the transistor VT2: collector voltage 10 V, emitter current - about 1 mA. In this case, the noise factor of the GT346A transistor does not exceed 4 dB [2]. The gain of the second stage is 12 ... 14 dB with a bandwidth of 8 MHz.

Capacitors C4, C8 are necessary to smooth out the ripple of the supply voltage and prevent self-excitation of the amplifier.

Rational installation and optimized design ensure reliable and stable operation of the amplifier at high gain (Fig. 2).

The change in the gain over the range within each of the sub-bands, caused by the uneven frequency response, does not exceed 3 dB. Thus, when the amplifier is tuned to different channels within the same subband, the gain values ​​differ by no more than 3 dB; while the gain in the first subband is 2...3 dB higher than in the second.

Instead of the KT371A transistor, transistors of the KT382A, KT382B, KT367A series can be used in the amplifier, and instead of the GT346A transistor, you can use the GT346B, but if the latter is used, the level of the amplifier's own noise increases. At the same time, the level of intrinsic noise can be reduced by using transistors KT371, KT372, KT3101, KT3115 with any letter index instead of the KT3132A transistor. In this case, the resistance of the resistor R1 must be reduced to 100 kOhm, and the resistance of the resistor R2 must be increased to 3,2 kOhm to provide a voltage on the collector of the transistor VT1 equal to 5 V at a collector current of about 3 mA. When replacing the transistor, the design of the printed circuit board must also be slightly changed so that the contact pads are located under the corresponding electrodes of the transistors. In the second stage of the amplifier, the GT346A transistor can be replaced with a KT3123A; in this case, the resistance of the resistor R3 should be reduced to 750 ohms in order to obtain a voltage at the emitter of the transistor equal to 10 V at a collector current of about 1 mA.

The inductors are made of silver-plated wire; coil winding is frameless. The wire diameter, the winding pitch and the inner diameter of the coils for each of the subranges are given in Table. 1.

The capacitances of the amplifier capacitors (in pF), depending on the subrange, are given in Table. 2.

The amplifier uses capacitors C4, C8 type KM-5, the rest KD-1, KD-2. Pass capacitor C7 - K.10-51; tuning capacitor C10 - KT4-23. All resistors in the amplifier are MLT-0,125.

The amplifier is assembled in a metal rectangular case with dimensions of 70x45x15 mm. The body is closed from above and below by easily removable covers, which are soldered to the body after final adjustment. Amplifier design details are made of 0,5 mm thick tinned copper; sheet brass or tin can also be used (Fig. 3, 4).

The basis of the amplifier is printed circuit board 1, made of one-sided foil fiberglass 1,5 mm thick, to which the rest of the structure is soldered. In the case of using double-sided fiberglass, the foil on the reverse side of the board is removed. The foil from the sections of the board indicated in the figure by dashed lines must be removed before assembly. Foil removal can be carried out both mechanically and by etching. On fig. 2 shows the arrangement of amplifier elements in the case, which should be installed in place only after the final assembly of the case. To ensure the minimum parasitic capacitance of the installation, the elements in the housing are installed, shortening the length of their leads as much as possible; when soldering elements, a heat sink must be used. The amplifier input is soldered to the antenna socket, which is attached to the side wall of the housing with two screws and nuts (detail 6 in Fig. 3). A piece of television cable with a characteristic impedance of 75 ohms, 0,7 ... 1 m long, is soldered to the amplifier output, with the help of which the amplifier output is connected to the antenna input of the television receiver.

Setting up the amplifier with proper installation and using known-good parts comes down to checking the modes of transistors VT1 and VT2 for direct current. The voltage deviation at the transistor terminals from those indicated in the diagram (see Fig. 1) should not exceed ± 5%. With the help of capacitor C 10, the amplifier is tuned to one of the television channels of the meter range for maximum contrast and image stability on the screen of the television receiver. Then, by stretching and compressing the turns of the coils L1, L2 (for the first stage high-pass filter) and the coils L3, L4 (for the second stage high-pass filter), the highest quality image is achieved, thus adjusting the cutoff frequencies of the high-pass filters. This compensates for the possible deviation of the cutoff frequency of both high-pass filters due to the spread of the parameters of the elements and capacitances of the installation. It is desirable to carry out the final adjustment by smooth adjustment of the capacitor C10 in the assembled amplifier with soldered top and bottom covers Using a contour screwdriver made of dielectric material.

The amplifier is usually located in close proximity to the television receiver. If the length of the television drop cable exceeds 15 ... 20 m, in order to improve the signal-to-noise ratio at the input of the television receiver, it is advisable to place the amplifier near the receiving antenna, taking measures for its moisture and heat insulation. When placing the amplifier on the mast of an outdoor antenna, in order to exclude the harmful effects of the atmosphere on it, the housing covers must be carefully soldered to the housing around the entire perimeter, and the tuning hole must be soldered to make the housing tight. It is also recommended to additionally protect the amplifier from moisture ingress by putting several plastic bags on it in such a way that one bag is put on the other, and the open side of each of them is directed down. In this case, the input and output cables connected to the amplifier must be bent so that they approach the amplifier from below. This will prevent the ingress of atmospheric precipitation into the cavity of the packages and will reliably protect the amplifier from moisture. In the case of significant air temperature fluctuations, it is advisable to place the amplifier in a simple passive thermostat, made, for example, from a suitable split foam box.

The amplifier can be operated from any power source that provides a constant voltage of +12 V at a load current of 10 mA; while the ripple level should not exceed 10 mV. The amplifier can be powered by +12 V, which feeds the decimeter channel selector (ACS unit) of the television receiver.

Literature

1. 1. Genshenza I., Kolomiets V., Savenko I. Antenna amplifier with remote tuning. - Radio, 1975, No. 4, p. 15 - 16.
2. 2. Transistors for equipment of wide application: Handbook / Ed. B. L. Perelman. - M.: Radio and communication, 1981, p. 272, 275, 243, 245.

Authors: O. Prystaiko, Yu. Pozdnyakov

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