Prefix for receiving UHF. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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Sufficiently high sensitivity and selectivity is provided by an attachment for receiving decimeter waves (UHF), made according to the above scheme. It is a frequency converter with a separate local oscillator and oscillatory circuits at the input and output.

Rice. 1 (click to enlarge)

UHF television signals through the communication line L1 enter the input circuit L2C1 (it also includes capacitance and mounting inductance). The oscillations selected by him through the capacitor C2 come to the emitter circuit of the transistor VT1 of the mixer. The local oscillator voltage is also supplied here through the capacitor C5. Signals with meter wave frequencies (MB) are emitted in the output circuit of the mixer, formed by coil L4, as well as capacitance and mounting inductance. It is advisable to tune the circuit to the frequency of the first channel MB, since in this case its quality factor will be the highest (due to the maximum inductance of the coil L4). If on the first channel there are television broadcasts or there is systematic interference, the circuit is tuned to the nearest free (second or third) channel. Resistors R1 -R4 and inductor L3 provide the mode of operation of the transistor VT1 for direct current.

The local oscillator is assembled on a transistor VT2 according to the self-oscillator circuit with capacitive coupling. Its oscillatory circuit includes the line L5. capacitors C7, C8. C10, as well as the capacitance of the mounting, the transistor VT2 and the VDI varicap. The DC transistor operation mode is set by resistors R5--R8. The local oscillator is tuned to the required frequency with a variable resistor R10. changing the constant voltage that is supplied to the varicap through the elements R9, L6. With the ratings of the parts indicated on the diagram and the conversion of the UHF signal into the signal of the first channel, the local oscillator frequency is tuned in the frequency range of the 26-37th television channels. If it is necessary to cover another range, the effective inductance of the L5 line should be changed by soldering the terminals of the capacitors C9 and C11 in the direction of shortening or lengthening it. For the same purpose, you can change the capacitance of the capacitor C10 within 0 ... 6 pF and lengthen the terminals of the capacitor C11.

The prefix is ​​powered by a source of stabilized voltage, consisting of a transformer T1, a rectifier (VD3-VD6). parametric voltage stabilizer (VD2.R12) and filter elements (R11, C12. C15, C16). You can also use constant voltages (within 12 ... 150 V) available on the TV. In this case, the transformer and the rectifier bridge are excluded, and the resistance of the resistor R12 (in kiloohms) is determined from the ratio: R12 = Upit-7) / Ipit, where Upit is the voltage (in volts) taken from the TV power supply, and Ipit = 7 mA - the current consumed by the prefix. Naturally, it should be taken into account that the greater the voltage drop across the resistor R12, the greater should be its rated power dissipation.

The prefix used fixed resistors MLT, variable resistor SP-1, capacitors KM-5a (C2, Sat. C9, SI). KT-20 (C1), K10-7B (C3. C4. C13-C15), K.T-1 (C5, C7, C8. C10) and K50 6 (C12, C16). Transistors can be with any current transfer coefficient p;¦e (without selection). Instead of GT341A, it is permissible to use transistors of the KT329 series. GT362, they can also replace the KT306B transistor, but this will require the selection of capacitors C7 and C8.

Coils L1, L2, L5 are made in the form of printed lines. Inductor L3 - DM-0,1 with an inductance of at least 25 μH. Coils L4, L6 are frameless and must contain 22-25 turns of PEV-2 wire with a diameter of 0,35 ... 0,49 mm. They are wound coil to coil on a cylindrical mandrel with a diameter of 2.7 ... 3 mm. Coil trimmer L4 - ferrite (100НН) from the shortwave coil of a transistor radio receiver. The T1 mains transformer can be made on the basis of an output transformer from any tube radio: the plates of its magnetic circuit should be assembled into an overlap, and the secondary winding should be rewound to obtain an alternating voltage of 9 ... 12 V.

The appearance, design and arrangement of parts on the attachment board are shown in Fig. 2-3. All parts except connectors XS1, XS2. variable resistor R10 and capacitor C14 are mounted on a printed circuit board made of foil fiberglass by mechanical means. The contours of the conductors are applied with a thin needle to the foil and cut with a sharp knife along the ruler. Then, peeling off a small piece of foil with a knife, grab it with tweezers and open it. Thus, all unnecessary metal sections are removed.

Ris.2

The board is placed in a plastic case with dimensions 175X115X60 mm made of sheet vinyl plastic 2 mm thick (metal can also be used). It can be installed h inside the TV by fixing the variable resistor R10 on the case. In this case, the wires connecting the resistor to the board are placed in a shielding braid, and the capacitor C14 is soldered to its terminals.

Ris.3

The set-top box is adjusted with weak television signals (for example, when the antenna is turned off) according to the maximum image contrast. If you intend to receive several programs, the input circuit L2C1 is tuned by capacitor C1 to the highest frequency channel. The output circuit is adjusted by changing the inductance of the L4 coil (in the position found, the trimmer is fixed with ceresin or stearin).

Author: N. Katrichev, Khmelnitsky; Publication: N. Bolshakov, rf.atnn.ru

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