Reflex radio receiver for local reception. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Reflex radio receiver for local reception. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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To receive several local radio stations operating in the MW band, you can assemble a simple radio receiver with just three transistors (Fig. 1). Sufficient sensitivity and sound volume are provided due to the implementation of the receiver according to the reflex circuit, when the same transistor stages amplify both RF and AF signals.

Reflex radio receiver for local reception

Reception of radio stations is carried out on a magnetic antenna WA1. The oscillatory circuit is made up of an inductor L1 and a variable capacitor C1 - they are tuned to the desired radio station.

The RF signal isolated by the circuit through the coupling coil L2 and the capacitor C2 enters the first RF amplification stage, made on the transistor VT1. From its load - resistor R1 - the signal enters the second stage, in which the transistor VT2 operates. The role of the loading of the cascade for RF signals is performed by the inductance of the voice coil of the high-impedance dynamic head BA1. Both stages are covered by DC feedback (through resistors R2 and R3), which stabilizes their mode of operation.

The amplified radio signal is fed from the collector of the transistor VT2 through the capacitor C4 of relatively small capacity (which prevents the passage of AF signals) to the detector, assembled on the transistor VT3. The radio frequency component of the signal is "cut off" by capacitor C6, and the component of the AF signal is fed through the chain R5, C3 to the same two-stage amplifier. Now the dynamic head, which is the load of the amplifier, will perform its direct role - to convert the AF vibrations into sound.

All resistors - MLT-0,125, capacitors - KP-180 (C1), K53-1 (C3) and KLS - the rest. Instead of those indicated in the diagram, it is permissible to use KT315 transistors with the same letter indices. The polarity of connecting the power supply and capacitor C5 will have to be reversed. Dynamic head - 0,25GDSH-7 or other small-sized with a voice coil with a resistance of 50 ohms or more. For the manufacture of a magnetic antenna, you will need a piece of a 400NN ferrite rod with a diameter of 8 and a length of 50 mm. A turn to turn is wound on it with a PELSHO 0,15 wire of coils L1 (120 turns) and L2 (15 ... 20 turns), placing them side by side on the rod. Power source - any small-sized battery with a voltage of 9 V ("Korund", "Krona", 6PLF22, etc.).

Establishing the receiver begins with the selection of the resistor R2 of such a resistance at which the collector current of the transistor VT2 will be set within the limits indicated in the diagram. Similarly, resistor R4 is selected to obtain the desired collector current of transistor VT3. Sometimes, due to the high inductance of the dynamic head, self-excitation of the receiver occurs, which can be easily eliminated by shunting the dynamic head with a resistor of about 1 kOhm.

You may want to listen to broadcasts on a miniature phone such as the TM series. In this case, the supply voltage can be reduced to 3 V, but you will have to use five resistors of a different value: R1 - 2,7 kOhm, R2 - 30 kOhm, R3 - 150 Ohm, R4 - 180 kOhm and R6 - 3,9 kOhm. The current consumed by the cascade on the transistor VT2 will be halved.

Continuing the theme of listening to radio stations on the phone, we can offer to assemble the receiver in the form of a key fob. This is achieved by replacing the magnetic antenna with miniature high-frequency chokes of the D-0,1 type (Fig. 2): L1 should be an inductance of 400-500 μH, L2 - about 100 μH. The "antenna" in this version will be ... the listener himself - the sensor E7 is connected to the oscillatory circuit through the capacitor C1 in the form of a metal ring worn on the finger. In addition, you will have to limit yourself to one radio station and tune the circuit to it by selecting capacitor C1.

Reflex radio receiver for local reception

And another option is to embed the micro-earphone into the key fob body. Then you will be able to use the microreceiver without disturbing others, simply by pressing it with your palm to your ear. Of course, in the "keychain" version, it is more expedient to use a battery of three disk batteries, for example, D0,06, connected in series.

If self-excitation occurs in the receiver, which cannot be eliminated by the proposed method, try the following: swap the coil L2 and capacitor C2, and connect the left terminal of capacitor C3 according to the diagram to the point of their connection.

Author: Yu.Prokoptsev, Moscow

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