Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Finding and receiving long-distance radio stations on HF is a hobby of many radio amateurs around the world. They study the passage of radio waves, geography, foreign languages, they are always up to date with the latest world events - and all this without interrupting radio engineering. From the available parts, you can assemble a simple, economical and sensitive KB receiver, quite suitable for listening to distant radio stations around the world. Despite the obvious shortcomings (lack of an accurate digital scale, the complexity of tuning with simultaneous frequency control and feedback), it will allow you to spend unforgettable hours traveling "on the waves of the ether".

The proposed regenerative receiver is assembled according to the 2-V-2 direct amplification scheme (two cascades of the URF, a detector and two cascades of the ultrasonic frequency converter) on four bipolar transistors and two semiconductor diodes. Having experimented with various types of regenerators, the author came to the conclusion that the receiver assembled according to the proposed scheme provides the highest quality, stable and noiseless detection of AM signals. The circuit diagram of the receiver is shown in fig. 1.

The UHF receiver is two-stage, it is assembled on transistors VT1 and VT2. The signal from the antenna enters the input (and only) circuit through a small capacitor C1. The connection of the input circuit with the first UHF stage is carried out through a capacitive divider C4C5. Regeneration serves to increase the gain and sharpen the resonant characteristic of the input circuit. It is provided by high frequency positive feedback using capacitor C3. By selecting this capacitor, a rough setting of the regeneration level is carried out. This level can be smoothly adjusted using a variable resistor R4, which changes the mode and, consequently, the gain of the first UHF stage.

The audio signal detected by the diodes VD1, VD2 through the low-pass filter C11R9C10 is fed to the ultrasonic frequency converter assembled on transistors VT3 and VT4. Its gain at a frequency of 1000 Hz with a supply voltage of 1,2 V is about 150. When an audio signal with an amplitude of 0,5 mV and a frequency of 1000 Hz is applied to the ULF input, the sound in telephones is well audible. The constant component of the detected signal through the integrating circuit R5C7R1 controls the operating point of the transistor VT1, automatically adjusting the level of regeneration. The modes of all stages of the receiver are stabilized using bias circuits that provide negative feedback. Diode VD3 prevents battery discharge through the solar panel VD4-VD7.

The receiver is powered by one disk nickel-cadmium battery G1 with a voltage of 1,2 V. The current consumption is 1,5 ... 2 mA.

The accumulator is recharged from the solar battery in the presence of sufficient illumination. The load of the receiver is low-resistance phones or even one telephone capsule with a resistance of 50 ... 200 Ohms. High-resistance phones will also work with a slightly lower volume, while the current consumed by the receiver will drop to 1 mA.

The sensitivity of the receiver from the antenna input with an output power of 0,1 mW, a telephone impedance of 100 ohms and a modulation depth of 30% is 30 μV, the maximum sensitivity when listening in a quiet room and a modulation depth of 100% reaches 2 μV. The sensitivity was measured with the feedback controller set close to the excitation threshold.

All applied semiconductor devices are silicon, non-polar capacitors are ceramic, polar ones are oxide. Resistors - brand MLT-0,125. The variable resistor R4 is used of the SP-1-A-1 W type, but it is better to use a similar imported one, for example, from TESLA, which provides smoother rotation of the engine. The VD4-VD7 solar battery can be taken from a used calculator or assembled from four silicon photocells.

Coil L1 for the 12 MHz band (25 meters) should have an inductance of 1,45 uH. In my design, a frame with a diameter of 9 mm was used, on which 12 turns of wire in enamel insulation with a diameter of 0,45 mm were wound round to round. The inductance of such a coil without a trimmer is about 1,3 μH. When a trimmer made of HF ferrite with a length of 10 and a diameter of 6 mm is screwed into the coil, its inductance increases to 1,5 μH. If the frame and wire are of a different diameter, the inductance of a single-layer coil in microhenry can be calculated by the formula

L=Dn²/(1000nh/D+440),

de D - coil diameter, mm; h - winding pitch, mm; n is the number of turns.

Since the total capacitance of the circuit is about 120 pF, using the Thomson formula, it is not difficult to calculate the frequency of the circuit. A variable capacitor C2 can be made independently from one movable and one fixed plate, or you can use a standard KPE5 ... 180 pF by connecting a 27 pF capacitor in series with it. You can also use a trimmer capacitor with an air dielectric ti KPV or a varicap suitable for capacitance changes, for example, KB 109V, but to power it you will have to make a micropower converter that provides an adjustable output voltage of 1 ... 10 V.

For more stable operation of the receiver, in particular, to eliminate the influence of "hands" on the setting, the front panel with control knobs placed on it must be made of metal or foil material. Such a panel, among other things, (shields parasitic capacitive pickups.

A sketch of the printed circuit board from the side of the printed conductors is shown in fig. 2.

For the manufacture of a printed circuit board, one-sided foil getinax or textolite is used. In the foil, grooves are etched or cut with a cutter in accordance with Fig. 2. In the places marked with dots, holes with a diameter of about 1 mm are drilled. After mounting and soldering, it is necessary to carefully check whether there are short circuits between the printed conductors and, if necessary, remove excess solder and scratch the grooves. However, it should be noted that a device made by hinged, volumetric mounting on a common "grounded" plate and mounting strips with petals turns out to be more compact, and with a single "production" it is also made much faster.

A properly assembled receiver almost does not need to be adjusted. Nevertheless, it is useful to check the voltage on the collectors of transistors VT2 and VT3 with a high-resistance voltmeter. They should be approximately 0,8 ... 1 V. If necessary, resistors R6 and R10 are selected. The desired range and frequency tuning limits are determined respectively by the number of turns of the coil L1 and the maximum capacitance of the capacitor C2. They can be corrected directly when receiving radio stations.

The last operation is the selection of the positive feedback capacitor C3. Its capacitance should be such that generation occurs approximately in the middle position of the resistor R4 slider. In the absence of ready-made capacitors of small capacity, it is permissible to replace them with two insulated mounting conductors twisted over a length of 1 ... 2 cm.

Author: S. Kovalenko, Kstovo, Nizhny Novgorod Region

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