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Radio microphones with frequency modulation (FM) is usually quite complex. So, in FM wireless microphone [1] the electrodynamic signal from the microphone is amplified in an operational amplifier, and then fed to the base of transistor high frequency generator. thereby effecting a mixed amplitude-frequency modulation.


Figure 1

To simplify the design of the FM wireless microphones possible when using small condenser microphones included directly in the high-frequency oscillating circuit of the generator. Possible schemes of such inclusion are shown in figures 1-3.


Figure 2

As you know, a condenser microphone is made in the form of a flattened capacitor with two flat fixed electrode, which is fixed parallel to the membrane (thin foil, metallized dielectric film, etc.), electrically insulated from the fixed electrodes. Speaking element of the oscillator circuit, it is, thus, performs frequency modulation.


Figure 3

Power FM-wireless microphones is a fraction of a few mW for the circuit in figure 1, the units, the tens of mW for the circuit in figure 2, and dozens, hundreds (with radiators) mW for the circuit in figure 3. Range, respectively, varies from tens of meters to several kilometers when using the FM radio receivers with a sensitivity of at least 10 mV/m.

The parameters of the inductors is similar to that given in [1].

Literature

1. Rickos V. FM wireless microphone. - Ham. -1991, No. 4, pp. 22-23.

M. SHUSTOV, Tomsk

(RL 9/91)