RMM radio microphone. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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The RMM radio microphone provides a range of at least 500 m, a radiated power of 5 mW and a current consumption of not more than 23 mA from a 9 V power supply. The schematic diagram of the radio microphone is shown in Fig. 1.
Fig.1 (click to enlarge)
The printed circuit board of the radio microphone, together with all the elements of the circuit, is shown in the figure.
Rice. 2 (click to enlarge)
The radio microphone is assembled on only three transistors KT3102, KT315, KT503E. The main input signal is taken from a highly sensitive electret microphone MKE-3 and fed through the L1C2 correction circuit to a microphone amplifier assembled on a VT1 transistor. Resistors R2 and R3 provide DC transistor mode. Capacitors C4 and C5 limit the bandwidth from above to reduce the noise level. Nevertheless, the bandwidth of transmitted audio frequencies is quite wide: from 50 to 15000 Hz.
The XS1 plug connector is designed to use the auxiliary input to connect the signal from other audio sources: tape recorder, receiver, etc. At the same time, the signal from the output of the microphone amplifier is turned off. This is followed by the modulation depth regulator R5, from where, through the pre-distortion chain R6, C9, emphasizing the upper audio frequencies, the signal is fed to modulate the carrier to the output high-frequency generator assembled on the transistor VT2.
The generator is assembled according to the capacitive three-point scheme and operates at a frequency in the range of 66-73 MHz. The oscillatory circuit contains an inductor L3 with capacitors C13, C14 connected in parallel and a feedback capacitor C15 with the capacitance of the emitter junction of the transistor. This capacitance, under the influence of the modulating voltage supplied to the base of the transistor, changes, due to which the frequency modulation of the high-frequency signal is carried out, which is fed from the circuit L4, C9, inductively coupled to the coil L3, to the transmitting antenna. The radio channel bandwidth is 180 kHz.
Power is supplied to the radio microphone from a 1 V battery GB9 through a voltage stabilizer on a VT3 transistor. Elements L2, C7 and Rl, C1 are decoupling filters along the power circuit. Signal reception is possible with any VHF-FM radio receiver.
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