Encyclopedia of radio electronics and electrical engineering / radio reception

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Often summer residents carry with them a superheterodyne portable or small-sized ("pocket") radio receiver. One of the disadvantages of such a receiver is that transmissions are often accompanied by various noises and whistles. Under such conditions, a direct amplification receiver responds much better, but it is, fortuitously, less sensitive than a superheterodyne receiver. The author of the proposed article has developed a direct amplification receiver, which has both a sufficiently high sensitivity and good sound quality. Long-term operation of this receiver has shown that it may well be recommended for use in the country.

The receiver is designed to operate only in the MW range (525 ... 1605 kHz), has a sensitivity when receiving on a magnetic antenna no worse than 1,5 mV / m (one of the modifications of the Speedol receiver has 0,5 mV / m) and good selectivity. It is powered by a 9 ... 12 V source, but it also works when the voltage drops to 6 V.

The receiver circuit is shown in fig. 1. It contains a dual circuit input circuit, a radio frequency (RF) amplifier, a cascade detector, and an audio frequency (3CH) amplifier. The RF signal received by the magnetic antenna through a bandpass filter (PF), consisting of inductors L1, L2 and capacitors C1 - C5, is fed to the input of a two-stage RF amplifier. The filter increases the selectivity of the receiver in the adjacent channel, it is tuned over the range by a variable capacitor (KPI) C2. The first stage of the amplifier is made on a field-effect transistor VT1 according to a common-source circuit, which makes it possible to maintain a sufficiently high input resistance and connect the PF oscillatory circuit directly to the amplifier [5]. At the same time, such a stage provides more gain compared to using a transistor in source follower mode.

(click to enlarge)

The load of the first stage is the resistor R2. From it, the signal enters through the capacitor C9 to the second stage - a conventional aperiodic voltage amplifier assembled on a transistor VT2 according to a common emitter circuit. From the output of the cascade (load resistor R6), the amplified RF signal is fed through the capacitor C11 to the cascade detector, assembled on diodes VD2, VD3, VD5, VD6 and capacitors C12-C14. Such a detector significantly increases the amplitude of the detected signal compared to a conventional detector based on one or two diodes, and also improves selectivity and reduces the likelihood of high-frequency signal components penetrating into the 3-frequency amplifier, which, as is known, is one of the causes of self-excitation [1].

Connecting a diode VD1 to a cascade detector leads to compression of the dynamic range of the signal before its detection and is used instead of an automatic gain control system [3]. The compression effect is enhanced by connecting the diode VD4. If desired, you can include switches in the cathode circuit of these diodes and put the diodes into operation at your discretion. The main and additional diodes of the detector should be only germanium [5].

From the detector load (resistor R8), a 3H signal is fed through resistor R9 to the volume control - a variable resistor R10, and from it to the input of a two-stage 3H amplifier assembled on bipolar transistors according to a well-known transformerless circuit [4]. Capacitor C16 prevents self-excitation of the receiver at maximum volume (the variable resistor engine is in the extreme position according to the circuit) and additionally filters the oscillations P4 after the detector.

From the output of the amplifier, the signal enters through the capacitor C18 to the dynamic head BA1. Power is supplied to the receiver by switch SA1.

In addition to those indicated in the diagram, you can use transistors KPZ0ZG, KPZ0ZD (VT1), KT312B, KT312V (VT2), KT315E, KT315Zh (VT3), any of the MP37, MP38 (VT4, VT7) series, any of the MP39-MP42 (VT5, VT6). It is desirable to select the transistor VT1 with the greatest steepness of the characteristic, VT2 - with a base current transfer coefficient of 100 ... 110, VT3 - 120 ... 130, VT4-VT7 - 60 ... 70. Diodes VD1-VD6 - any of the D9 series. Fixed resistors - MLT-0,125, VS-0,125, variable - SP-Ill or similar of the same rating. When using a variable resistor combined with a switch, a separate power switch is not needed. Fixed capacitors - any type, oxide C7, C9, C10, C15, C17, C18 - K50-6 or others for a rated voltage of 16-25 V, tuning C1, C3 - KPK-1, variable capacitor - two-section, with air dielectric and capacitance change from 12 to 495 pF (in extreme cases, you can use KPI with a maximum capacitance of 365 pF). Capacitor C4 is made in the form of two pieces of wire with a diameter of 2 and a length of 10 mm, located at a distance of 10 mm from each other [2].

Coil L1 is wound on a rod with a diameter of 10 and a length of 200 mm from ferrite 400NN turn to turn and contains 49 turns of LESHO 7x0,07 wire (this is how a litz wire is denoted - a wire containing seven strands with a diameter of 0,07 mm). The coil is placed at a distance of 8...10 mm from one of the ends of the rod. Since the coil may have to be moved along the rod during the adjustment of the receiver, it is desirable to make a paper ring for it and place the turns of the coil on it.

Coil L2 can be wound on a K16x8x4 ferrite ring with a magnetic permeability of 100 - it contains 64 turns of LESHO 7x0,07 wire. Coil inductance - 200 uH. If capacitor C2 is used with a maximum capacitance of 365 pF, the inductance of the coil should be 270 μH, which means that the number of turns will have to be increased to 75. The number of turns of coil L1 is increased to 57.

Dynamic head VA1 - 0,5GDSH-2 with a voice coil with a resistance of 8 ohms. You can also use a 0,5GD-37 head or a head from a subscriber loudspeaker with a 4 Ohm voice coil.

Most of the receiver parts are mounted on a printed circuit board (Fig. 2) made of one-sided foil fiberglass, the jumpers between the conductive tracks are made with a single-core mounting wire in insulation. Trimmer capacitors C1 and C3 are mounted on a fiberglass bar. The board with its foil pad is attached with screws to the body of the KPE block. The output of the KPI rotor is soldered to the common wire of the receiver.

(click to enlarge)

The case for the receiver was used ready - from the loudspeaker "Ob-305", but any other of the appropriate dimensions will do. The location of the board and parts of the receiver in the housing is shown in fig. 3. Of course, the KPI, volume control and power switch can be placed on the front of the case.

Setting up the receiver begins with checking and setting the operating modes of the transistors. You will need an avometer with a relative input resistance of at least 20 kOhm / V. First, by selecting the resistor R12, the voltage on the collectors of the output transistors is set equal to half the supply voltage (the modes are indicated for a voltage of 9 V). Next, a milliammeter is turned on in parallel with the open contacts of the SA1 switch and, by selecting the VD7 diode, the quiescent current is set to about 9,5 mA. The voltage at the drain and source of the transistor VT1 is set by selecting the resistor R1, at the terminals of the transistor VT2 - by selecting the resistor R4.

To adjust the PF, unsolder the capacitor C4 and the output of the L1 coil, which is right according to the diagram, and connect an external antenna to the transistor gate through a capacitor with a capacity of 10 ... 15 pF - a wire about two meters long. Having moved the KPE rotor to a position of almost maximum capacity, tune in to the Mayak radio station, operating at a frequency of 549 kHz. By selecting the number of turns of the L2 coil, achieve the highest sound volume.

After that, connect the L1 coil and the C4 capacitor, and turn off the temporary antenna. By moving the L1 coil along the rod, achieve the highest volume of the same radio station. The pairing of the filter circuits at the low-frequency end of the range can be considered completed.

Proceed to a similar operation at the high-frequency end of the range, for which again unsolder coil L1 and capacitor C4, connect an external antenna and try to tune in to some radio station in the position of almost the minimum capacitance of the KPI. Trimmer capacitor C3 achieve maximum sound volume. It remains to solder the coil L1 and the capacitor C4, turn off the external antenna, set the highest volume with the tuning capacitor C1 - and the pairing at the high-frequency end of the range is completed.

The operation of pairing the settings of the MF contours at both ends of the range should be repeated several times in order to achieve the best results.

With the method of conjugating the contours given by the author, the capacitance of the external antenna upsets the PF, especially at the high-frequency end of the range. The best results in PF tuning can be achieved in this way. Set trimmer capacitors C1 and C3 to the middle position. Disconnecting capacitors C2.2, C3 and coil L2 and replacing C4 with a jumper, select the position of the coil L1 on the antenna rod so that tuning to the mentioned radio station "Mayak" takes place in the position of almost maximum capacitance C2. Leaving C2 in this position and restoring the PF circuit completely, select the number of turns of the L2 coil to obtain the maximum reception volume. Turn off C2.2, C3, L2 again and tune the receiver to any station in a position of almost minimum capacity. Without changing the position of the rotor C2, restore the PF circuit and use trimmer capacitors C3 and C1 to achieve maximum reception volume.

Literature

1. Prokoptsev Yu. Cascade detector. - Radio, 1994, No. 4, p. 41.
2. Polyakov V. Double-circuit preselector of a direct amplification receiver. - Radio, 1993, No. 12, p. 12-14.
3. Veryutin V. Modernized receiver "Yunost-105". - Radio, 1987, No. 12, p. 33.
4. Polyakov V. Medium-wave direct amplification receiver: Sat: "To help the radio amateur", vol. 95, p. 41-51. - M.: DOSAAF, 1986
5. Shulga G. Direct amplification receiver with fixed tuning for three programs. - Radio, 1982, No. 6, p. 52, 53.

Author: R. Plyushkin, Ekaterinburg

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