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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING VHF receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Despite the fact that at present there are a lot of various portable radio receivers on store shelves (mostly foreign, much less domestic), the interest of radio amateurs in the design of such products has not disappeared. They are very simple in their design, can be implemented quite quickly, literally within a few days. In addition, the work itself, even if it is simple, replenishes your treasury of experience in adjusting and solving other problems that often arise when creating and operating more complex equipment. On the pages of the Radio magazine, I have repeatedly read detailed descriptions of receivers for individual use. Their basis in recent times, as a rule, is the K174XA34 chip. It allowed to significantly simplify the circuitry of high-frequency cascades. However, for wearable structures, a serious drawback of most of the proposed options can be called the use of rather expensive and energy-intensive electronic tuning units in them, with program selection blocks. Such solutions, in my opinion, are not justified from the point of view of ease of use, since their main advantages (the possibility of fixed settings, remote control, etc.) are simply unnecessary in small devices and are more appropriate in stationary structures. A distinctive feature of the proposed VHF receiver is the use of a self-made, easy-to-manufacture and easy-to-use tuning unit, combined with structural elements. True, due to the simplicity of the design, radio reception of the same stations is possible in two positions of the regulator. However, this drawback can be eliminated quite simply - by setting the stop for the tuning knob. During the development of the receiver, the tasks of its maximum reduction in cost, simplification of operation, and expansion of functionality were also set. In this regard, only the monophonic reception mode was used, small-sized disk batteries were used for power supply, and an incandescent bulb was built in for illumination (it can also serve as a flashlight). The receiver is equipped with the simplest economical battery charging unit from the AC mains, the wire of the connected head phone works as an antenna. The main technical characteristics of the receiver are determined by the applied K174XA34 chip. The range of received frequencies is 65 ... 74 MHz. The output power of the receiver is 15 mW (determined by the supply voltage and the resistance of the phone) with a harmonic coefficient of no more than 2% (if the volume decreases, it also decreases). The reproducible frequency range is 100...12000 Hz depending on the type of telephone used, dimensions - 120x80x20 mm. The receiver is powered by two D-0,26 batteries and consumes no more than 20 mA at maximum output power. The housing from the BZ-04 microcalculator was used as the receiver housing. The circuit diagram of the receiver is shown in fig. 1. It is made on two microcircuits and two transistors. Tuning to the received station is carried out with a homemade variable capacitor. The high-frequency part A1 of the receiver, which fully corresponds to the typical switching circuit of the K174XA34 (DA1) microcircuit, has a simplified input circuit and is located on a separate circuit board, on which a variable capacitor C1 is also made.
In order to simplify and ensure guaranteed quality, the audio frequency amplifier is based on the KR1407UD2 (DA2) operational amplifier. To increase its output power, emitter followers on germanium transistors VT1 and VT2 are used. The amplifier is covered by direct and alternating current feedbacks, the latter being a depth-changing resistor R4 (volume control). The operating point of the amplifier is set by a divider on resistors R1, R2, and the operating mode for the current of the op-amp and emitter followers is set by resistor R3. Capacitors C14, C15 create an alternating current midpoint to connect the load VA1 of the output stage, and for direct current they are a filter in the power circuit. If it is necessary to expand the range of the reproduced signal towards lower frequencies, the values of these capacitors should be increased. The inductors L2 and L3, together with the capacitor C13, are used to decouple the radio signals received by the antenna (headphone wire in BA1). To switch the GB1 power circuit and turn on the HL1 lamp, a small-sized switch SA1 of type PD9 - 2 and a microswitch SA2 of type MP - 12 were used. The battery is charged through a rectifier on diodes VD1 - VD4, a protective resistor R5 and limiting elements C16 and R6 from an alternating current network of 50 Hz 127 or 220 V. Elements C16 and R6 are placed in a separate case, and all other elements of the charger are placed in the receiver . With this solution, it is possible to use any diodes with an operating current of at least 30 mA. The receiver uses details of the smallest possible sizes - resistors of the MLT-0,125 type, it is better to take a variable resistor from a hearing aid as R4, it is possible to use variable resistors of the SPZ-3 type from pocket radios. Capacitors - types KT, KD, KM, K50 - 6, K50 - 35, K40U - 9 (C16). The latter - for a voltage of at least 250 V. As VT1, VT2, any low-frequency germanium transistors with the highest possible base current transfer coefficient are suitable, for example, the GT108, GT109, GT115, MP20, MP41, MP42, PZO and MP38, MP37, MP35 series. Headphone BA1 is one of the elements of the H-23C-1 stereo telephone (Vega company). The L1 coil is frameless, wound with PEV-20,31 wire on a mandrel with a diameter of 3,5 mm and has 15 turns. Inductors L2 and L3 are wound simultaneously in two wires on one ferrite rod with a diameter of 2,8 mm (trimmer from the coils of broadcasting receivers) with a winding wire with a diameter of 0,19 mm and have 30 turns each. As already noted, the tuning capacitor C1 is made directly on the circuit board (dimensions 68x36 mm) of node A1. The center of its axis of rotation has coordinates of 17 and 10 mm, respectively, along the larger and smaller sides of the board. On the rest of the board, made of ABC plastic (in particular, the body parts of household refrigerators are made from it), the elements of the A1 assembly are placed. Board thickness 2...2,5 mm. The elements are fixed to the board with Moment glue or by soldering their leads with a soldering iron. The DA1 chip is glued to the board by the upper side of the case, the installation is carried out with a single-core tinned wire with a diameter of 0,15 ... 0,2 mm (cores from a wire of the MGSHV type or similar) using insulating tubes in the right places. The design of the capacitor C1 is shown in fig. 2 (section along the axis of rotation). Tuning disk 1 is cut out with a measuring compass from ABC plastic, its side face is corrugated with a needle file or by rolling over a hot file. From the inside, on half of the disk, with the help of a compass, arcs are cut according to the dimensions of the rotor plate 5. The recess for it in disk 1 is performed by scraping with a knife. Similarly, the stator plate 7 is made (placed in the board 5 of the A1 node). Both plates are in the form of semicircles. For conclusions 4, holes with a diameter of 1 ... 6 mm are drilled in parts 2 and 3. The entire capacitor structure is assembled using a screw connection (parts 2, 3, 8, 9). Screw and nut with M2,6 or M3 thread are used. Parts 2, 8, 9 are pre-tinned at the soldering points.
The assembly of the capacitor is carried out in the following order. Part 1 is attached to part 2 with BF-6 or BF-2 glue, the rotor and stator plates 5 and 7 are installed and also glued, on which the places for soldering leads 4 are pre-marked. nuts 1. In this case, it is necessary to ensure that the spring 8 is securely locked. After the final adjustment, protecting from excessive overheating, lock the nut 9 with soldering and solder the output 8 to the bearing 4. The output 2 from the part 4 in the receiver design is soldered to the positive power bus in the immediate vicinity of the location of the coil L9. On dial 1, during the final adjustment, you can mark the received radio stations with multi-colored dots. Charger A2 is made in the form of a case into which the body of the receiver is inserted (without disassembly) for the time of recharging the batteries. At the bottom of the case, foil plates are glued, which form the elements of the XZ connector. Capacitor C16 and resistor R6 are fixed in its lower part, a cable is also brought out from there, ending with a power plug X2. All current-carrying circuits should be securely isolated. The elements of the charger are made of plastic using adhesive joints. If necessary, it is easy to introduce into it a light indication that signals charging, for example, based on back-to-back LEDs or a neon lamp. The receiver is equipped with mating parts of the HZ connector, made in the form of pins fused into its housings, protruding 0,5 ... 1 mm above the surface. When charging, the receiver is inserted from above into the charger, as in a bag. Instead of ABC, other thermoplastic materials are suitable, such as polystyrene. In case of self-manufacturing of the receiver housing, its dimensions can be selected depending on the specific operating conditions of this design. Setting up the receiver with serviceable parts is easy. After checking the correct installation, unsolder the amplifier input from pin 14 of the DA1 chip and turn on the power (the initial adjustment is best done when powered by freshly charged batteries). Then measure the voltage at the emitter VT1 - it should be equal to half the supply voltage. Otherwise, the divider R1, R2, the health of VT1, VT2 and DA2 are checked again. With a working amplifier in the phone, the background should be heard if you touch its input with your finger. Then proceed to adjust the range of received frequencies. Listening to the range, as a rule, they detect working stations. They are identified using an auxiliary receiver and adjusted by stretching or compressing the winding of the L1 coil. After tuning, the coil turns must be fixed with paraffin. The receiver steadily receives most of the radio stations operating in Samara, the most powerful ones are well audible in the vicinity of the city. It is convenient to manage the receiver settings, the need for recharging is easily detected by its operation. If necessary, you can increase the sensitivity of the receiver by introducing a broadband high-frequency amplifier on a bipolar transistor into it, and a third battery will not interfere. Author: V.Guskov, Samara
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