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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Medium wave direct gain receiver. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception Broadcast receivers are currently built mainly according to the superheterodyne scheme. There are many reasons for this - these are high sensitivity and selectivity, which change little during tuning and when changing ranges, high stability and repeatability of parameters in mass production. For short wave reception, it is difficult for a superheterodyne receiver to find an adequate replacement. But for the medium wave range, much simpler direct amplification receivers are also suitable. Their main disadvantage is their low selectivity. But they, as a rule, give better reception quality, make less noise, do not create interference whistles and do not have side reception channels. The quality factor of the circuits in the MW range can reach 200 or more, while the bandwidth of the circuit is even less than necessary for normal reception of AM signals. Therefore, the circuits can be combined into bandpass filters, forming a more or less rectangular frequency response of the radio path. But this is difficult to do, since the contours must be tuned in range, and a multi-circuit receiver turns out to be difficult to manufacture and tune. There is another way to increase the selectivity of the direct amplification receiver, which is rarely used. It consists in applying the so-called pseudo-synchronous reception method, in which the signal level at the carrier frequency of the desired station is raised in the radio path by a narrow-band circuit. The amplitude detector of the receiver tends to suppress weak signals of interfering stations in the presence of a strong useful signal, and the magnitude of this suppression is proportional to the square of the ratio of the amplitudes of the interfering and useful signals (see; Chistyakov N. I., Sidorov V. M. Radio receivers .- M .: Communication, 1974, §13.3). By amplifying the carrier several times, very significant interference suppression can be obtained. Raising the carrier also reduces distortion when detecting a useful signal. But a narrow-band circuit that raises the carrier will inevitably attenuate the edges of the sidebands of the received signal, corresponding to the upper frequencies of the audio (spectrum. This drawback can be easily eliminated by providing an appropriate rise in the upper frequencies in the CNY after the detector. It was this way of increasing the selectivity that was chosen when developing the described receiver . This receiver is designed to receive local and powerful distant stations in the CB range of 530 ... 1600 kHz. In terms of sensitivity, it is not much inferior to class III-IV superheterodynes, but provides a noticeably better reception quality. Its selectivity, measured by the usual single-signal method, is rather low (10 ... 20 dB at a detuning of 9 kHz), however, the interfering signal in the adjacent channel, equal in amplitude to the useful one, is suppressed by 26 ... 46 dB due to the effect described above. The output power of the built-in ULF does not exceed 0,5 W, which is enough to listen to radio broadcasts through headphones or a loudspeaker in an ordinary living room (the main attention during development was paid not to volume, but to sound reproduction quality). The receiver is powered by any source with a voltage of 9 ... 12 V, the current consumption in silent mode does not exceed 10 mA. We will analyze the operation of the receiver in more detail, referring to its circuit diagram shown in Fig. 1.
The narrow-band contour, emphasizing the carrier of the received signal, is the circuit of the magnetic antenna L1C1C2 with a quality factor of at least 200 ... 250. Its 0,7-level bandwidth is, when tuned in range, from 2,5 to 6 kHz. The received signal selected by the circuit is fed to the URF, made according to the cascode circuit on field-effect transistors VT1 and VT2. The cascade amplifier has a high input impedance and practically does not shunt the magnetic circuit, i.e., does not reduce its quality factor. The first transistor VT1 is selected with a low cutoff voltage (0,5 ... 3 V), and the second VT2 - with a much larger one (8 V). This made it possible to connect the gate of the second transistor to a common wire and get by with a minimum of parts in the amplifier. The total drain current of the amplifier is equal to the initial drain current I s.nach of the first transistor (0,5 ... 2,5 mA), and its drain voltage is equal to the bias voltage of the second transistor (2 ... 4 V). The load of the cascode amplifier is the second tunable resonant circuit L3C6C7, connected to the output of the amplifier through the coupling coil L2. This circuit has a much lower quality factor (no more than 100 ... 120) and passes the spectrum of the AM signal with only a slight attenuation at the edges of the sidebands. The introduction of another circuit into the receiver is necessary because, as practice has shown, the selectivity of one circuit of a magnetic antenna is not enough for complete detuning from the signals of powerful local stations, even those far removed in frequency from the receiver tuning frequency. In addition, the second circuit sharply limits the bandwidth, and hence the power of the noise coming from the RF to the detector. Structurally, it is easy to introduce a second circuit, since the vast majority of KPIs are produced in the form of dual blocks. The second, aperiodic, URF cascade is assembled on a VT3 field-effect transistor. It is loaded on the diode detector VD1, VD2, made according to the voltage doubling scheme. The AGC signal of negative polarity from the detector load - resistor R7 through the filter circuit R4C4 is fed to the gate of the first RF transistor. VT1 and locks the egr when receiving powerful stations. This reduces the total current of the cascode amplifier and its gain. The capacitance of the blocking capacitor C/0, which shunts the load of the detector, is chosen to be small. This is very significant, since interference suppression in the detector occurs only under the condition that the difference frequency of beats between the carriers of the useful and interfering stations is allocated at the detector load. The detected sound signal through the corrective chain R8R9C11 is fed to the gate of the source follower VT4. By moving the slider of the resistor R8, you can change the amount of rise in the upper frequencies of the sound spectrum, weakened by the magnetic antenna circuit. This variable resistor successfully serves as a tone control. The source follower VT4 matches the output of the detector with the low-pass filter L4C14C15C16. The LPF has a bandwidth of about 7 kHz and a pole (i.e. maximum) attenuation at a frequency of 9 kHz, corresponding to the beat frequency between the carriers of stations operating in adjacent frequency channels. The LPF filters this and other beat frequencies of the noisy useful signal and thereby further improves the two-signal selectivity of the receiver. At the output of the low-pass filter, a volume control R12 is connected through a terminating resistor R13. Resistor R12 is needed only so that the low-pass filter output is not closed by the regulator at the very low volume levels. Any ULF or input of a tape recorder recording amplifier can be connected to the receiver output. In this case, the volume control R13 is not needed, the output signal is taken from the low-pass filter capacitor C15, and the resistor R12 is transferred to the low-pass filter input and connected in series with the decoupling capacitor C12. The receiver's own ULF is made according to a simple scheme shown in fig. 2.
Transistor VT7 amplifies the input signal voltage. The output stage - a power amplifier - is a push-pull signal repeater assembled on composite transistors of various types of conductivity. Diode VD1, included in the collector circuit of the pre-amplifier VT7, creates on the bases of the transistors of the output casing, and a small initial offset, which is necessary to reduce distortion of the "step" type, so that the output transistors open more fully with positive half-cycles of the signal, when the current of the transistor VT1 decreases, a voltage boost was used - positive feedback through the load resistor of the pre-amplifier R1, connected to the power wire through the dynamic head, to which the output voltage of the amplifier is applied. The voltage boost makes both half-waves of voltage at the output of the amplifier symmetrical, thus reducing non-linear distortions. Distortion is also reduced by the OOS chain. through resistor R2, which simultaneously stabilizes the DC amplifier mode. At low volumes, the OOS increases due to a somewhat unusual scheme for turning on the volume control (R13 in Fig. 1), further reducing distortion. Indeed, the depth of the OOS is determined by the ratio of the resistance between the engine and the upper output of the volume control according to the circuit to the resistance of the resistor R2 (see Fig. 2). When moving the slider down, the first of the mentioned resistances increases, increasing the depth of the FOS. In the receiver, it is desirable to use transistors of exactly those types that are indicated in the schematic diagram of Fig. 1. In extreme cases, instead of KP303A, you can use KP303B, V, I, Zh. Instead of KP303E, you can try to use KP303G, D. Diodes VD1, VD2 - any high-frequency germanium. The dual KPI unit can be taken from any broadcasting receiver. Very convenient units with a built-in vernier, which facilitates tuning to radio stations. Resistors and capacitors can be of any type, trimmer capacitors C1 and C6 are of the KPK-M type. For a magnetic antenna, a ferrite rod with a magnetic permeability of 400 ... 1000 is suitable. Its length can be within 140..180mm, diameter 8...10 mm. To obtain the highest possible quality factor, the coil of the magnetic antenna L1 should be wound with LESHO 21X0,07 litz wire or, in extreme cases, LESHO 7x0,07. If you cannot find a litz wire, you should twist together 15 ... 20 conductors of the PEL 0,1 type and wind the coil with the resulting bundle. When stripping and soldering the litz wire, care should be taken to ensure that there are no broken or unsoldered veins left. The coil is wound on a cardboard frame with a wall thickness of 0,5 ... 1 mm. The frame should move along the ferrite rod with little friction. The winding is carried out turn to turn, the number of turns is 45 ... 55 (a smaller number corresponds to larger dimensions and higher magnetic permeability of the core). To protect against moisture, the frame with the coil can be impregnated with molten paraffin. For coils L2 and L3, standard fittings are suitable - an armored core with a shield from the IF circuits of portable receivers, such as the Sokol receiver. The communication coil L2 contains 30, and the loop coil L3 - 90 turns of PEL wire 0,1. The location of the coils on a common frame does not really matter. The L4 low-pass filter coil with an inductance of 0,1 H is wound on a ring with an outer diameter of 16 mm and a height of 5 mm (K16X8X5) made of 2000NM ferrite. It contains 260 turns of any insulated wire with a diameter of 0,1.. 0,25 mm. You can also pick up a ready-made coil, for example, one of the windings of a transition or output transformer from ULF portable receivers. By connecting a capacitor with a capacity of 5000 pF and an oscilloscope in parallel with the coil, the sound generator signal is fed to the resulting circuit through a resistor with a resistance of 100 kOhm ... 1 MΩ. Determining the resonant frequency of the circuit by the maximum voltage on the name, it is necessary to select such a coil (or its number of turns) so that the resonance is observed at a frequency of 6,5 ... 7 kHz. This frequency will be the cutoff frequency of the low-pass filter. In the absence of a suitable coil, it can be replaced (with worse results, of course) with a 2,2 kΩ resistor. Capacitor C16 in this case can be assembled from a ULF receiver circuit on a variety of transistors. As VT1, KT315, KT301, KT201 with any letter index or any other silicon low-power npn transistor is suitable. It is desirable that its transfer coefficient be at least 100. Any germanium low-frequency low-power transistors of the corresponding type of conductivity are suitable for the output stage, for example, MP10, MP11, MP37, MP14-16, MP39 -42. To reduce distortion, it is useful to choose approximately equal current transfer coefficients for pairs of transistors VT2 and VT3, as well as VT4 and VT5. Diode VD1 - any low-power germanium. The rest of the parts can be of any type. Dynamic head B1 - any type with a resistance of 4 ... 16 ohms. However, in order to realize good reception quality, it is better to use a sufficiently powerful broadband head in a large case or a ready-made industrial speaker system. The receiver (without ULF) is mounted on a printed circuit board, a sketch of which is shown in fig. 3.
There are no actual conductive tracks on the board - the foil serving as a common wire occupies its entire surface (the board is shown from the side of the foil). The conclusions of the parts are passed, as usual, into the holes of the board. Those conclusions that, according to the scheme, should be connected to a common wire, are soldered to the foil. The solder points are shown in the sketch with black circles. Other conclusions are connected, in accordance with the diagram, with a single-core wire in insulating tubes, laid directly on the surface of the foil. In order to avoid short circuits, the holes for these conclusions must be countersunk - they are shown in the sketch with light circles. Such printed-on-board mounting is easy to perform; in addition, due to the large area of the "grounded" foil, parasitic couplings between individual stages are reduced, and, consequently, the risk of self-excitation of the receiver. The ULF receiver is mounted on a separate board (Fig. 4) using the most common printed wiring. The trace pattern is simple, and the board is easy to manufacture with a sharp knife without the need for chemical etching.
The design of the receiver can be very different, for example, in the case of a subscriber broadcast loudspeaker, using the dynamic head available in it. It is also possible to perform the receiver as a separate structure connected to a loudspeaker or acoustic system. The recommended location of boards, magnetic antenna and controls is shown in fig. 5 (top view, from the side of the details). The design of the receiver scale can also be any, in accordance with the tastes and capabilities of the radio amateur. To mount the magnetic antenna, it is preferable to use plastic fittings so as not to introduce additional losses that reduce the quality factor of the input circuit. If a network unit is used to power the receiver, it should be located to the left of the ULF board (see Fig. 5), away from the magnetic antenna. If the mains transformer creates a large stray field, it is possible to induce an alternating current background on the low-pass filter coil of the L4 receiver. They can be weakened by choosing the mutual orientation of the coil and the transformer, increasing the distance between them and, finally, shielding the coil with a magnetic shield. Induction from the mains transformer decreases sharply if it is rewound, increasing the number of turns of all windings by 15 ... 20%. The receiver is set up with ULF. By applying a supply voltage of 9 ... 12 V, the resistance of the resistor R2 is selected so that the voltage on the collectors of transistors VT4 and VT5 is equal to half the supply voltage. By including a milliammeter in the break of the power wire, select the type and instance of the diode (VD1 in Fig. 2) until a quiescent current of no more than 4 ... 5 mA is obtained. If the quiescent current is excessively large and it cannot be reduced in this way, you can connect several Diodes in parallel or shunt the diode with a resistor with a resistance of 150 ... 300 Ohms. You should not solder the diode when the ULF is on, since the current consumption increases sharply and the terminal transistors may fail. Having connected the receiver, they check the voltage at the istom of the transistor VT4 (2 ... 4 V) (see Fig. 1), the drain of the transistor VT3 (3 ... 5 V) and the connection point of the drain of the transistor VT1 with the source of the transistor VT2 (1,5 ...3 V). If the voltages are within the specified limits, the receiver is operational and you can try to receive the signals of the stations. The lower limit of the range (530 kHz) is set by moving the L1 coil along the magnetic antenna rod. The best way to do this is to receive a powerful radio station of the second All-Union Mayak program at a frequency of 549 kHz - it should be heard with the KPE rotor plates almost completely inserted. At the frequency of this station, the settings of the receiver circuits are matched, adjusting the inductance of the L3 coil with a tuning core according to the maximum reception volume. Then, having received some station in the short-wave portion of the range (the rotor plates - KPI are removed), the pairing operation is repeated by adjusting the capacitance of the tuning capacitors C1 and C6. To fine-tune the contours, repeat the pairing operation 2-3 times alternately at the low-frequency and high-frequency edges of the range. With self-excitation of the URF, which manifests itself in the form of whistling and distortion when receiving stations, it is necessary to reduce the resistance of the resistor R2 and try to rationally arrange the conductors leading to the KPI stator plates (they should be as short as possible, located away from each other and closer to the "grounded" surface fees). In extreme cases, these wires will have to be shielded. For more precise tuning to the frequency of the radio station, the receiver can be equipped with a tuning indicator - a pointer instrument included in the break in the power wire of the cascode URC in series with resistor R3. Any device with a deviation current of not more than 1 ... 2 mA will do. The device must be shunted with a resistor, the resistance of which is selected so that the arrow deviates to the full scale in the absence of a received signal. When a radio station signal is received, the AGC system locks the URCH and the arrow deviation decreases, indicating the signal strength. Tests of the receiver in Moscow gave quite good results. In the daytime, almost all local stations listened to on any superheterodyne-type transistor receiver were received. In the evening and at night, when long-range passage opens on the NE, many stations were received several thousand kilometers away. Due to the low single-signal selectivity, several stations can be heard at the same time, but when finely tuned to a sufficiently strong signal, the effect of "suppression" is noticeable, and the program is heard cleanly or with only slight interference.
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