Shortwave receiver-observer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / radio reception

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A simple radio receiver, the circuit of which is shown in Fig. 1, is designed to receive signals from amateur radio stations operating in the HF bands of 10. 15, 20, 40 and 80 m. It is assembled according to a direct frequency conversion scheme and consists of a set of input bandpass filters tuned to the middle frequencies of the amateur bands, a broadband radio frequency amplifier ( RF) on the transistor VT1, a diode mixer (VD1, VD2), a local oscillator (VT2) and a three-stage audio frequency amplifier (3H) with a high transmission coefficient on transistors VT3-VT5, loaded with BF1 headphones.

The desired range is selected by switch SA1. under. connecting to the input of the RF amplifier one of the bandpass filters, and to the mixer - the corresponding local oscillator circuit. The latter is tuned in frequency by a variable capacitor C27 and generates oscillations, the frequency of which is half the frequency of the received RF signals. To reduce the dependence of the local oscillator frequency on the supply voltage, a simple stabilizer on a VD3 zener diode was used.

(click to enlarge)

The amateur radio signal amplified by the transistor VT1 is fed to the mixer (VD1, VD2) through the broadband RF transformer T1. The 3H oscillations resulting from direct conversion are fed through the L16C32 low-pass filter to the volume control - a variable resistor R7. and from its engine - to the input of the 3H amplifier.

To prevent self-excitation of the receiver due to parasitic connections of its cascades through a common power supply, decoupling filters R6C18, R9C34, R15C35 and capacitor C38 are used. For the same purpose, the wires leading to the moving contacts of the SAI switch sections are shielded.

The receiver parts are mounted on a printed circuit board with dimensions of 170x100 mm (in fig. 2 shows part of it, which contains elements of RF and 3H amplifiers, as well as a local oscillator). All coils are wound on polystyrene frames with a diameter of 7 mm with carbonyl iron trimmers (frames of IF filters of television receivers were used). The distance between the axes of the coils of the input bandpass filters is approximately 16 mm. The winding data of the coils are given in Table 1 (ordinary winding, turn to turn).

Table 1

Choke inductance L16 (the design can be any) -100 mH. Broadband transformer T1 is wound on a ferrite (100НН) ring with an outer diameter of 10 mm. Each of its windings contains seven turns of enameled wire with a diameter of 0,3 mm (winding is performed simultaneously with three wires).

The terminals of the capacitors that are part of the band-pass filters and local oscillator circuits, and the wires from the range switch are soldered to contacts pressed into the plastic bases of the frames, which serve as coil terminals. Brass screens are installed on the board between the coils of the band-pass filters and the local oscillator, as well as between them and the rest of the receiver parts.

Editorial note. Instead of those indicated on the diagram in the receiver, you can use domestic transistors of the KT325, KT355 series. KT368 (VT1, VT2) and KT373 (VT3-VT5), diodes KD503A (VD1, VD2), Zener diode KC175A (VD3). Headphones BF1 - electromagnetic resistance 3 ... 5 kOhm. Choke L16 can be wound on a ferrite (3000NM-1) ring size K20X12X6. The winding must contain 220...240 turns of PELSHO 0,1 wire.

Author: Janeczek A.; Publication: N. Bolshakov, rf.atnn.ru

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