Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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Most radio receivers of the middle class have a mechanical tuning system, in which a relatively large diameter pulley is fixed on the rotor axis of the variable capacitor (KPI), rotated by a cable or gear mechanism. The angle of rotation of the KPI rotor is usually 180°. With this design, the position of the pulley is rigid (if there is no play) associated with the tuning frequency of the input and heterodyne oscillatory circuits. A similar relationship exists for radio receivers with varicap tuning, tuned by a variable resistor. If the variable resistor is not multi-turn, then the angle of rotation of its engine does not exceed, as a rule, 270 ... 300 °.
By tracking the position of the pulley with a position sensor, it is possible at the right time, that is, in a certain part of the frequency tuning range, to perform some necessary switching. For example, in a VHF receiver with an extended range (65,8 ... 108 MHz), switch stereo decoders: with polar modulation - in the frequency section 65,8 ... 73 MHz (VHF-1) and with a pilot tone - in the frequency section 88 ... 108 MHz (VHF-2). Thus, it is possible to exclude the use of rather complex systems for recognizing the method of encoding stereo transmissions.
In dual-band VHF receivers based on K174XA34, KXA058, K174XA42 microcircuits, etc., the pulley position sensor can replace the manual range switch. In this case, the sensor contact is used as a commutator of circuit elements or varicap setting voltages. With appropriate tuning parameters, it is possible to achieve conjugation of the "densely populated" frequencies of the VHF-1 and VHF-2 bands without the characteristic empty section between them, which is characteristic of receivers with a continuous extended range. Such switching of ranges is also possible in MW - LW receivers of direct amplification.
An optocoupler, a pair of permanent magnet - magnetoresistor or a permanent magnet - reed switch can work as a position sensor. The latter option is preferable, as it is simple, reliable and does not require the use of power supplies, which is important when using such a device in portable equipment.
Structurally, the reed switch is a magnet 2, rigidly fixed on the setting pulley 3, and a reed switch 1, located on the printed circuit board 4 under the pulley in the zone of the magnet movement arc (see figure).
In the proposed simple design, a 25x10x5 mm element from standard furniture magnetic latches is used as a permanent magnet. Due to its relatively large length, this magnet is able to keep the reed switch closed for a sufficiently large segment of the pulley travel, which is required for proper switching. Reed switch - any type with a miniature design.
Making the sensor is easy. The magnet is removed from the magnetic latch and glued into the KPE pulley window. Then, empirically, such a position of the reed switch on the board is found, at which its contact switches and is held at the required segment of the pulley stroke. In this position, the reed switch is glued to the board with Moment glue. The vertical gap between the magnet and the reed switch must be within 1...3 mm. The conclusions of the reed switch are soldered according to the scheme of a specific device.
Author: A.Pakhomov, Zernograd, Rostov region
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