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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Receiver band switching indication. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / radio reception The use of modern element base, including specialized microcircuits, in the design of broadcasting receivers leads to the unification of circuit solutions. For amateur creativity, this opens up new possibilities in creating a wide range of service devices. In this regard, the author of the published article shared a very interesting improvement in the method of indicating the switching of ranges. Modern fixed and portable radios are usually designed to operate in the LW, MW, HF and VHF bands. Recently, designs have appeared with only a few MW, HF and VHF bands. Indication of the included range in most cases is carried out by indicating the risks on the receiver case when the switch slider is moved. However, this does not provide sufficient visibility, especially when using modern small-sized switches, in which the step of movement during switching is very small and does not exceed 3–5 mm. And it is absolutely impossible to determine the position of the included range late in the evening or at night. Obviously, in this case, it is necessary to use some kind of light-emitting elements, such as LEDs. Their low forward currents (0,5...1,0 mA) provide reliable indication and low power consumption. In some radio receivers, especially of the previous years of production, the indication of the included range was carried out by igniting the corresponding incandescent lamps (much less often than LEDs). For this, additional groups of contacts of the range switch were used. In a modern industrial design, it is not possible to implement such a method, since the number of groups of switching contacts in the switches used is limited and all of them are involved. Another common way to indicate the position of the switch was to use a non-switchable incandescent lamp that illuminates the window of a mechanically rotating drum with alphanumeric indices (range numbers or its conventional abbreviated name). There is another method for indicating the included range using CMOS logic chips - in this case, additional contacts in the switch or special mechanical devices are not required to turn on the indicator LEDs [1]. We will consider the principle of operation of such a device using the example of a circuit (Fig. 1), which is the simplest input device of a radio receiver in the form of an oscillatory circuit with switching to two ranges. It should be noted that for the considered method of indication, both the input and heterodyne circuits can be used. Schematic changes made to indicate range switching are shown as thick lines.
In the first position of the range switch, the tuning frequency of the circuit, i.e., the received frequency range, is determined by the inductance L1 and the capacitance of the capacitors C1 and C3 connected in series. When the switch is moved to another position, instead of capacitor C1, C2 is connected to the circuit circuit. In the position of the range switch "1" from the power source of the receiver to the voltage divider R1, R2, a constant voltage is supplied through the resistor R5. This voltage is separated from the inductance L1 of the circuit by the capacitor C1, so its shunting effect is excluded. From the middle point of the divider, the voltage level is log. 1 is fed to the input of the inverter DD1, which causes a zero voltage level to appear at the output of the element. This leads to the flow of current through the LED HL1, the glow of which indicates the inclusion of the first range of the radio. The current through HL1 is set and limited by resistor R6. When the switch is moved to the second position of the voltage level, the log. 1 from the divider R3, R4 enters the input of the element DD2, and is removed from the input DD1. In accordance with this, the HL2 LED turns on, and HL1 turns off. In order for the display device not to affect the operation of the receiver, in particular, not to cause a decrease in the quality factor of the circuit, it is necessary to use resistors R1, R2, R3, R4 and R5 with large resistance values. If CMOS digital microcircuits are used in the device, then the resistance of these resistors can be in the range from hundreds of kilo-ohms to several mega-ohms. In particular, the resistances of the resistors R2 and R4 are determined by the value of the input currents of the inverter elements. Resistors R1 and R3 are introduced to eliminate the influence of the input capacitance of the inverter microcircuit on the resonant frequency of the receiver circuit, and R5 eliminates the shunting of the circuit by the power source and protects it from short circuits in case of failure of capacitors C1 - C3. At the same time, in particular, for the first position of the switch, it is necessary that the ratio of the total resistance (R1 + R5) and R2 provide a log level voltage. 1 at the input DD1 is not lower than 0,7 of the power supply voltage. A similar condition must be satisfied for the second position. A practical diagram of the display device introduced into the five-band radio receiver "Meridian RP-248" (earlier name "Meridian RP-348") is shown in fig. 2. The binding of the elements of the indication circuit and the receiver is made in accordance with the diagram given in the "Operating Guide" [2].
The inverters are made on a 564LN2 chip, HL1 and HL2 LEDs are AL307A. A filter is introduced into the display device: diode VD1 (KD522B) and capacitor C1, which excludes the influence of a change in the voltage of the power source on the operation of the inverters of the display device. In the VHF range, where there are no switching elements of the oscillatory circuits, to turn on the indicator LED (HL5), the supply or removal of the supply voltage of the VHF unit is used. Structurally, the device is made on a printed circuit board, on which a microcircuit, resistors, a diode and a capacitor are placed. The LEDs are located on the front panel of the receiver above the tuning scale in such a way that each of them is located above the section of the scale that corresponds to the included range. The use of the 564 series microcircuit is preferable, since their analogues of the K561 series have large dimensions and are less convenient for installation in the limited volumes of the industrial design of the receiver. The five-band (without VHF band) Neiva RP-205 radio receiver was also redesigned in a similar way. In conclusion, it should be noted that the considered principle of band switching indication can be used not only in radio receivers, but also in other devices (in transmitters, measuring instruments, etc.). In order for the inverters of the DD1 microcircuit to receive as much input voltage as possible (in this case, the current consumption in the microcircuit power circuit is minimal), the terminals of resistors R2 and R4 (Fig. 1), which are upper in the circuit, should be connected to the terminals of resistors R1 and RЗ, which are upper in the circuit. Similarly, the top terminals of resistors R6-R9 (Fig. 2) should be connected to the left terminals of resistors R2-R5. Literature
Author: B.Sergeev, Yekaterinburg
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