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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A radio receiver powered by... a multimeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur [an error occurred while processing this directive] A radio receiver can now be bought on almost every corner. The question is, why then do it yourself? You can answer immediately and to the point: this is how radio amateurs learn, gain knowledge and experience, improve their skills and abilities, and master radio engineering. In addition, they get incomparable pleasure from listening to the air with the help of a home-made design, every detail of which is familiar, selected and installed by their own hands, and these actions are accompanied by thoughts about further improvement of the receiver. For those radio amateurs who are content with practical purposes, you can see that modern factory receivers suffer from many shortcomings, and almost the most important of them is low efficiency. Indeed, to loudly sound a room, a few milliwatts of power is enough, for the operation of telephones - microwatts. At the same time, even miniature modern receivers with telephones consume tens and hundreds of milliwatts from batteries. Such extravagance is not justified by anything other than the desire of manufacturers of receivers and batteries to get more profit. These considerations, as well as idle experiments with a dozen popular radio components, led to the creation of a very simple receiver that works on headphones and is powered by a multimeter (or avometer) connected by an ohmmeter, i.e. set to resistance measurement mode. Why a universal measuring device, and not a separate battery? Just because it is the most necessary device in the laboratory of a beginner radio amateur, even if the entire "laboratory" consists of this one device, which is small in size and always at hand. I had the cheapest YX-1000A pointer multimeter from Sunwa, in which one "316" or "AA" battery with an EMF of 1,5 V is installed. full scale (1 ohm). When the receiver is operating, the arrow deviates by 0,3/0...1/10 of the scale, which corresponds to the current consumption of 1...3 μA. A very useful property of such a solution was also found - the multimeter serves both as an indicator of the received signal level and as an indicator of fine tuning - when a radio station signal arrives, the consumed current increases and the instrument arrow deviates to the right, although not to the full scale. The receiver operates in the MW range on the built-in magnetic antenna WA1 (Fig. 1). To receive weak and distant stations, an external antenna (XS1 socket) and grounding (XS2 socket) are connected. The input (and only) oscillatory circuit of the receiver is formed by a coil of a magnetic antenna and KPI C2. The small capacitance of the coupling capacitor C1 with an external antenna reduces its influence on the tuning frequency.
The main part of the receiver, which detects and amplifies the signal, is assembled on two common transistors VT1 and VT2 according to the scheme of a composite emitter follower. Transistors operate in microcurrent mode. The RF signal from the input circuit is fed to the base of the transistor VT1 through the capacitor C3. Required to bring the transistors to the initial section of their characteristics, the base bias current sets the resistor R1. High-impedance telephones are connected to the sockets of the XS3 connector, and the multimeter is connected to the sockets of the XS4 connector. Capacitor C4 closes the radio frequency currents remaining after detection, and capacitor C5 closes the audio frequency currents so that their power is not wasted on the internal resistance of the multimeter. Now it is necessary to explain why the composite emitter follower was chosen. It does not amplify the signal voltage, having a transmission coefficient of about 0,9. But it amplifies the current: to find the current gain, it is necessary to multiply the current transfer coefficients of transistors VT1 and VT2. It will turn out about several thousand, even despite the fact that in the microcurrent mode, the transfer coefficient of transistors decreases. The input impedance of the cascade increases by the same factor in comparison with the load resistance. The resistance of high-resistance phones to direct current is 3,2-4,4 kOhm, and to alternating current of audio frequencies - 10 ... 20 kOhm. As a result, the input impedance of the cascade reaches tens of megaohms and practically does not shunt the input circuit, making it possible to fully realize its resonant properties. In a tuned circuit, the signal voltage increases by a factor of Q (Q is the quality factor) compared to the EMF induced by the signal in the antenna. When using a good magnetic antenna coil, Q reaches 250 ... 280 in the CB range. This is the voltage amplification. The high quality factor of the circuit also provides selectivity - the suppression of signals from neighboring stations in frequency. Of course, with one circuit it is small. The signal is detected as follows: since the operating point is chosen at the initial section of the characteristic, the positive half-waves of the signal cause a significant increase, while the negative half-waves cause only a slight decrease in the already small initial current through the transistors. As a result, a constant current component appears, which increases with increasing signal amplitude. The details of the receiver can be very different. Magnetic antenna - any, from a broadcasting receiver. Of the coils, only medium-wave should be left on the rod, although if desired, you can install a switch and make the receiver dual-band - LW and MW. Preferably long rods, for example, 200 long and 10 mm in diameter, made of 400NN ferrite. Coils wound with a litz wire (a wire twisted from many insulated veins) have a high quality factor. KPI - also from broadcasting receivers, with a solid or air dielectric. In a two-section KPI, it is better to connect the sections in parallel to increase the tuning range. It is permissible to use any low-power high-frequency transistors, for example, the KT315, KT361, KT3102, KT3107 series. When using p-n-p transistors, the polarity of the multimeter connection is reversed. Capacitors - ceramic, the only resistor - any type. To power the receiver, any pointer multimeter with a resistance measurement mode is suitable. Its typical scheme is shown in Fig. 2. It is better to put the variable zero-setting resistor R1 in the position of minimum resistance (maximum deviation of the instrument needle). Keep in mind that the positive probe of the multimeter (XP2) will have a negative voltage in ohmmeter mode! It is also desirable to find out what current the device gives at the selected limit of resistance measurements with closed terminals. This can be done with another multimeter. The current must be within 0,05 ... 0,5 mA.
In the absence of high-resistance phones, you can also use low-resistance ones, from the player, for example, by connecting them through a matching transformer T1 (Fig. 3). An output transformer from old transistor receivers is suitable (the middle output of its primary winding remains free), a transformer of a broadcasting radio point, and any other, low-frequency, small-sized, with a transformation ratio from 30:1 to 10:1. Of course, the step-down winding is connected to telephones.
Setting up the receiver is not difficult: first, by connecting telephones and a multimeter, they achieve a deviation of the instrument’s arrow by about 1/10 of the scale. Then they try to receive radio stations and, if necessary, correct the range of the receiver by moving the coil along the magnetic antenna rod or unwinding its turns. In the middle part of Russia, it is convenient to navigate using the Mayak radio station at a frequency of 549 kHz. The receiver should be tuned to it when the KPI capacity is close to the maximum. In the case of self-excitation of the receiver in the high-frequency part of the range, which is manifested by a strong whistle of a changing tone when tuning to the frequencies of radio stations, the following measures are useful: installing a blocking capacitor with a capacity of 1000 ... 4700 kΩ between the base of the transistor VT1 and the elements R10C22. Turn the receiver on and off by connecting or disconnecting telephones or a multimeter, or switching it to another measurement mode, such as voltage. The test results of the receiver turned out to be quite good. Although not loud, but in Moscow he received all the central radio stations on a magnetic antenna. Late at night, at least a dozen and a half NE radio stations were received on a wire several meters long, including stations in European capitals (Bucharest, Warsaw, Stockholm, etc.) and several Middle Eastern stations. Author: V.Polyakov, Moscow
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