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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Receiver in a soap dish. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception I often go fishing. And so I want to listen to music, other radio programs in nature. One day I was looking through the binder of the Radio magazine, and I came across a diagram of the simplest direct amplification receiver with 3 transistors. I tried to make it, and I succeeded," Sasha Stepankov from the Belarusian city of Mogilev shares his joy. The circuit of this radio receiver was developed specifically for beginner radio amateurs by the famous Moscow master of radio reception Vladimir Timofeevich Polyakov. It is not only simple and economical, but also uncritical to the voltage of the power source, and requires virtually no adjustment. Here is what Sasha tells about how he reproduced this amazing scheme: “I assembled the receiver from the simplest materials. I took a soap dish for the case. I drew the location of the details on a sheet from a notebook in a cage. in the places where the radio elements were installed. I inserted the parts into the holes, and then, in accordance with the diagram, connected them with a thin tinned wire. From a broken radio receiver, I used a variable capacitor and a piece of a ferrite rod. I wound two layers of thick paper on the rod, glued it, and then wound two windings. And the receiver is ready. Now I'm not bored when I'm out of town." The receiver is loaded on a miniature telephone capsule or headphones (headphones) and has only one adjustable circuit (Fig. 1). There is no power switch, the receiver is turned off by simply pulling the headphone plug out of the jack. The inductor L1 is wound on a magnetic (or loop) antenna WA1. The circuit is tuned to the frequency of the received radio station by a variable capacitor (KPI) C1. The radio frequency (RF) signal from the circuit L1C1 through the coupling coil L2 is supplied to three stages of RF amplification, made according to a scheme with direct connection between the stages on transistors VT1 - VT3.
The amplified RF signal is detected by the diode VD1, and its high-frequency component is filtered out by the capacitor C2, and the low-frequency component through the coupling coil L2 (which is a very low resistance for audio frequency currents) is again fed to the base of the transistor VT1 for further amplification. This is usually done in reflex receivers, where the same stages amplify both RF and AF currents. But our receiver can be considered reflex rather conditionally. The fact is that through the detector diode VD1 the circuit of one hundred percent OOS (negative feedback) is closed, which operates only at direct current and audio frequencies, but does not affect RF in any way. As a result, the mode of all three transistors is rigidly stabilized, and in the absence of a signal, the voltage on the collector of transistor VT3 is equal to the sum of the opening voltage of transistor VT1 (about 0,5 V) and diode VD1 (also about 0,5 V). In this case, the bias current of the base of the transistor VT1 will be such that the diode will start to work in the section of the current-voltage characteristic with maximum curvature. And this is exactly what is required for good detection. In the presence of an RF signal, the diode VD1 detects its positive half-waves, opening the transistor VT1 more strongly. Following it, transistors VT2 and VT3 also open more. Thus, the average (over the period of the RF signal) voltage at the VT3 collector drops, and the current consumed by all three transistors increases. This explains the rare efficiency of this receiver. After all, it consumes a large current only when there is a useful signal, and the consumption is proportional to the signal. When the latter is absent, the consumption is minimally necessary. If we connect an oscilloscope to the collector and emitter of the transistor VT3, we will get the picture shown in Figure 2. It can be seen from it that, as a result of detection, the positive half-waves of the modulated RF signal are rigidly "tied" to the voltage level of +1 V, while the negative half-waves modulated by an AF signal with twice the usual amplitude.
Due to the action of the FOS, the detection is linear. If the signal level is too high and the negative half-waves reach zero, the oscillation envelope (with amplitude modulation) will inevitably become limited and the receiver will begin to work with distortions. You can eliminate them by attenuating the RF signal by detuning the input circuit or by simply rotating the antenna. If this does not seem very convenient, then it is possible to solder a constant resistor with a resistance of 1 ... 20 Ohms into the emitter circuit of the transistor VT100. Of course, the sensitivity of the receiver will decrease in this case. To make it "omnivorous", working with equal success both in the city and outside it, instead of a constant resistor, you will have to install a variable; by selecting the resistance (up to 200 ... 220 ohms), it will be possible to set the optimal sensitivity. Since all three transistors are “current” amplifiers for AF oscillations, and their collector currents are added in a common power wire, the BF1 phone is also included in it, and the receiver starts working immediately as soon as the phone plug is inserted into the XT 1 socket. Capacitor C3 prevents RF currents from entering the circuit of the telephone BF1 and the battery GB1. About the details of the receiver. Transistors VT1 and VT3 can be KT315 or KT312 with any letter indexes. The same applies to the transistor VT2 (KT361). You can try to use transistors KT3102 (VT1, VT3) and KT3107 (VT2). The selection of transistors according to the current transfer coefficient is still not required. It's just that transistors with a high coefficient will provide a slightly higher sensitivity of the receiver, and their mode of operation will still be rigidly stabilized. How to deal with excessive sensitivity, we already know. Diode VD1 - any low-power high-frequency, but certainly silicon. The functions of a tuning capacitor can be performed by any suitable KPI with an air (better!) or solid dielectric. Suitable, for example, KP-180, produced in sets for children's technical creativity, or one section of the KPE block from any transistor receiver. The maximum capacitance of the KPI must be at least 180 pF. Coils L1 and L2 can be wound on a round or rectangular magnetic antenna rod made of ferrite grade 400...1000НН. The length of the rod is not less than 50 mm. To receive MW-band stations, the L1 coil must contain 55 ... 70, and the communication coil L2 - 5 ... 7 turns of wire with a diameter of 0,25 ... 0,35 mm in insulation (PEL or PEV brands). The winding is carried out in one spy, turn to turn, the distance between the coils is 5 ... 7 mm. To increase the quality factor of the L1 coil, and, consequently, the selectivity (selectivity) of the receiver, it is better to use a "litz wire", ready-made or home-made - from three to five PEL 0,07 ... 0,15 wires, folded together and slightly twisted. V. Polyakov also tested a loop antenna with a square section of 55x55 mm, made of 60 turns of PEL 0,25 wire wound in bulk. The frame served as the body of the receiver. A communication coil of 5 turns of the same wire is wound over the contour. As you know, the loop antenna does not require any magnetic circuit, you only need to install the receiver housing on the priest. However, this is convenient if you keep the receiver in your breast pocket. The sensitivity of the device with such an antenna turned out to be quite sufficient for receiving central radio stations in the conditions of Moscow and the region. For reception in the LW range, the number of turns of both coils should be approximately tripled. The BF1 phone can be a miniature capsule TM-2A or any other phone with a coil resistance of about 50 ohms. With such a telephone, the receiver can operate at a supply voltage of 1,2 V and higher. The current consumed by it is 1,2 mA when powered by a single disk battery and 1,8 mA when powered by a single 316 V A1,5 cell. 6...180 V (two disc batteries or two A2,4 cells). The current consumption will then increase to 3,0...316 mA. Excellent results were obtained when using TDS-1 stereo headphones, the phones of which are connected in parallel. Loud and high-quality sound is then provided with a supply voltage of 3 V and a current consumption of 5 mA. High-resistance phones can also be used with the receiver, for example, TA-4 with a DC resistance of 4,4 kOhm, but the supply voltage will have to be increased to 4,5 ... 9,0 V (one or two 3336L batteries or one Krona battery , "Korund" or "Oreol-1"). The current consumption will be 1...2 mA. In general, the receiver allows wide variations in load resistance and supply voltage without deteriorating the quality of work and, most importantly, without selecting elements. Since not only audio frequency currents pass through the phone, but also the constant current component of transistors, when connecting a phone, especially with a weak magnet, it is advisable to observe the polarity. So, the telephone terminal marked with a "+" sign must be connected to the battery. If there is no marking on the phone case, the desired polarity is determined experimentally by the best sound. The design of the receiver can be very diverse. Most likely it will be a small box with a tuning knob and a connector for connecting a phone, placed in a breast pocket. The second knob, as already mentioned, can be the knob for selecting the optimal sensitivity. Mounting parts on the board - both printed and hinged. True, with an unsuccessful arrangement of parts, self-excitation is possible, which manifests itself in the form of extraneous noise. The reason is most likely that the collector circuit of the transistor VT3 is located too close to the loop coil L1 or to the capacitor C1. Then a parasitic capacitive coupling occurs between them. Self-excitation is eliminated by spacing the specified elements in space. If this does not help, it is recommended to surround (shield) the collector circuit of the VT3 transistor with an insulated strip of copper foil, which is "grounded", that is, electrically connected to a common wire (conditional case). Sometimes it is useful and easy to swap the leads of the loop coil or coupling coil. If there are not many radio stations in your area, or if you want to tune the receiver to one favorite (say, 1-hour music) program, the receiver is made with a fixed tuning. In this case, the variable capacitor C50 is replaced by a constant one, the capacitance of which is in the range of 200 ... 1 pF, and the number of turns of the coil LXNUMX is selected so as to ensure fine tuning to your favorite radio station. Then you will hardly need the second sensitivity knob, and the controls will disappear on the receiver. Author: V.Bannikov
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