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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Direct amplification radio receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur In the Center for Technical Creativity of Students (TSTU) of the Ministry of Education of the Russian Federation, a radio laboratory has been operating for several years, led by an experienced radio amateur, candidate of technical sciences Vasily Ivanovich Veryutin, a long-term author of the Radio magazine. This laboratory has become a kind of branch of the editorial laboratory and intends to develop designs for repetition by novice radio amateurs. Today we publish a description of its first development. More than a decade of experience of the author of these lines with circle members in pioneer camps and out-of-school institutions confirms the expediency of using a simple radio receiver as the basic design, which has a minimum of parts, high consumer qualities and repeatability during assembly. A diagram of one of the variants of such a receiver is shown in Fig. 1. It is built according to a reflex circuit on two transistors: silicon VT1 and germanium VT2. Transistor VT2 is taken germanium because two silicon, switched on by direct current as one composite transistor, require a power supply with a voltage of 2 ... 3 V for normal operation, which complicates the design and increases its dimensions.
It is known that reflex receivers are critical to changes in the amplitude of the received radio carrier. After all, the transistors in them amplify both the radio frequency (RF) signal and the audio frequency (AF) signal at the same time. In other words, the amplitudes of the RF and AF signals are added and, if the total amplitude goes beyond the linear section of the amplifying properties of the transistor, signal distortion occurs, manifested in the form of squeaks and squeaks of various tones. These limitations do not allow making a reflex radio receiver of high sensitivity and make it necessary to calculate the signal amplification path, focusing on powerful nearby stations. Such radio receivers cannot receive weak signals due to the low gain of the RF path. A way out of this situation can be found in the case of using a device for compressing the dynamic range of signals (see the article "Modernized receiver" Yunost 105 "in" Radio ", No. 12, 1987). Then, when the amplitude of the input RF signals changes by more than 100 once the signal at the output of the detector changes by a maximum of XNUMX. This property of the compression device allows the circuit to be calculated for the maximum gain of the cascades without fear of strong signal distortions. Since we are talking about a radio receiver designed for wide repetition by novice radio amateurs, we should dwell on the range of received frequencies. The choice was made in favor of the long-wave (LW) range, since transmissions on medium waves (MW) and even more so on ultrashort waves (VHF) are listened to near large cities at relatively short distances, while in the LW range it is possible to receive two or three radio stations within a radius of up to 200 km. Let's consider in more detail the operation of the receiver. The RF signal isolated by the oscillatory circuit L1C1 of the magnetic antenna WA1 is fed through the coupling coil L2 to the base of the transistor VT1. Cascades on transistors VT1, VT2 amplify the RF signal by more than 100 times. This signal is also facilitated by the L4L3 radio frequency transformer with a transformation ratio of about five. The coil L4 of the transformer is connected to the collector circuit of the transistor VT2, and the coil L3 is shunted by back-to-back diodes VD1, VD2. They are selected germanium because they start working at an RF signal amplitude of 10 mV, while silicon diodes need a signal with an amplitude of 500 mV. Diode VD3 is used to detect the RF signal emitted by the diodes VD1 and VD2. The AF signal is removed from the capacitor C3, which is filtered by the R2C4 circuit and fed to the same transistors VT1 and VT2 for subsequent amplification. Now the AF signal is allocated to the load, which can be used as a headphone or a telephone capsule with a resistance of 25-250 ohms, plugged into the X1 socket. Miniature stereo phones are also suitable, for example, from a player, the capsules of which must be connected in series. It is easy to see that the G1 battery is connected to the receiver through headphones, which made it possible to abandon the power switch. But in this option, you have to take into account the load resistance: the smaller it is, the louder the sound of the receiver and the greater the current consumption from the source, and vice versa. On average, the current consumed is about 10 mA, i.e. a galvanic cell or battery with a capacity of 0,5xAh will work continuously for almost 50 hours. This radio does not have a volume control. This was done deliberately - the sound volume in it is small, but sufficient for listening to programs on the street and in a not very noisy room. Due to the relatively high sensitivity of the receiver, it is recommended that certain requirements be observed when it is repeated. First of all, one should not strive for over-miniaturization. In a matchbox, for example, a good receiver is unlikely to work out, since the RF transformer radiates electromagnetic waves into space, which are immediately picked up by a closely spaced antenna, i.e., the probability of self-excitation increases. It is also necessary, when finalizing the receiver, to provide for such a spatial orientation of the transformer, in which there will be no squeaks and squeaks accompanying the reception of stations. The KT3107K transistor can be replaced with KT3107I, KT361B, and GT308V - with P416B or a similar high-frequency germanium. Instead of GD507V diodes, it is permissible to use D18 or D20, diodes of the D9 series work somewhat worse. The magnetic antenna is made on a ferrite rod 400NN or 600NN with a diameter of 8 mm and a length of 60...80 mm. A paper sleeve is put on the rod, and coils are already wound on it: L1 should contain 240 turns of PEV-2 0,1 wire, and L2 - 20 turns of the same wire wound over L1. The RF transformer is wound on a K10x6x3 ring made of 2000NN ferrite; L3 contains 150 turns of PELSHO 0,1 wire; L4 - 30 turns of PEV-2 0,1. The turns of the coils are evenly distributed over the entire ring, which reduces the radiation of electromagnetic waves into space. It is desirable to mount the details of the receiver on a printed circuit board made of foil fiberglass. The version of the board is not given. The simplicity of the circuit design allows radio amateurs to independently draw up a print drawing, depending on the parts used. The variable capacitor is KP-180, but it is not necessary to install it. One option is to replace this capacitor with a constant one, choosing a capacitance (within 100 ... 200 pF) depending on the frequency of the received radio station (if the receiver is designed for one fixed setting). For more precise tuning, the ferrite rod of the magnetic antenna is moved inside the frame. The option of fixed tuning of the receiver to several radio stations is possible. Then a small-sized P2K switch is installed on the case, and circuit capacitors are placed on its terminals. It is also necessary to place a small-sized connector on the case - the counterpart of stereo phones. The recommended dimensions of the receiver case are at least 80x60x20 mm, the battery is placed between the magnetic antenna (with a gap of at least 10 mm) and other parts to form a kind of magnetic and electric screen. The appearance of such a radio receiver and the installation view are shown in fig. 2 and 3.
Developed in the laboratory of CTTU. Author: V.Veryutin, Moscow
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