Push-pull transmitter for remote control. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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This transmitter is suitable for operation as part of telecontrol systems or in radio security devices that do not have strict requirements for the stability of the frequency of the transmitted signal (for example, if the receiver is a super-regenerator). Amplitude manipulation.
The transmitter at a 50-ohm load when powered from a +12V source develops a power of 0,2 W. Operates at frequencies in the 27 MHz range. Maximum current consumption 80 mA.
The diagram is shown in the figure. It resembles the circuit of a push-pull high-frequency erasure and magnetization generator from a tape recorder (GSP). In principle, just such a scheme was taken as a basis, only its parameters were changed in accordance with the fact that the transmitter must operate at a much higher frequency (27 MHz versus 100 kHz for GPS). The high-frequency generator itself is made of two transistors VT1 and VT2, connected according to a symmetrical multivibrator circuit with an inductive load. Capacitors C2 and C3 are feedback elements, and the two halves of the coil L1 serve as loads for the multivibrator arms. Resistors R3 and R4 create the necessary bias at the bases of these transistors. The supply voltage is supplied to the tap of the coil L1, as well as to the base circuits through R3 and R4.
The generation frequency is determined by the circuit, which includes L1 and C4, as well as capacitances C2 and C3 and the capacitance of transistors VT1 and VT2. The output signal is taken from the coupling coil L2, the number of turns of which depends on the equivalent resistance of the antenna.
The circuit is characterized by a large RF voltage swing at the L1 pins, which can reach 50 V or more.
The manipulator is made on transistor VT3, this is a transistor switch, which, when a high logical level voltage is applied to its input (base) (via R5), opens and closes the power circuit of the transmitter, putting it into action. The maximum equivalent frequency of input pulses is 3000 Hz; at a higher frequency, the RF generator may start unsteadily.
If it is necessary that the manipulation is not 100%, or if you need to arrange amplitude modulation, you can apply some bias voltage to the VT3 base so that it is open or slightly open (depending on what modulation mode is required) in the absence of an input modulating signal.
Coils L1 and L2 are wound on a ceramic frame with a diameter of 8 mm, there is no core. L1 contains 20 turns with a tap from the middle, 12 - 5 turns located on the surface of L1, in its center. Wire PEV-0,56 (0,5...0,7 is possible).
Transistors KT608 can be replaced with KT603, KT630.
By changing the capacitance of capacitor C4 within 6...500 pF, you can configure the generator to any frequency in the range of 10...35 MHz, and changing the values of any other circuit elements is not required. If C4 is replaced with an air variable capacitor of the KPE-2V type (12..495 pF), you can get a tunable high-frequency generator with a relatively powerful output, which can be very useful in a radio amateur workshop when setting up receiving or transmitting equipment.
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