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Modern element base allows you to collect the radio microphone all on a single chip. Commercially available chip CRT contains the necessary set of transistors. Installation of wireless microphones compact, but low power transmitter and rational layout reduce the possibility of self-excitation and parasitic coupling to a minimum.

 

Scheme of wireless microphones is presented in figure. The microphone amplifier on the elements DA1.1 and DA1.2 amplifies the voice signal from the electret microphone WM to a level that ensures the desired frequency deviation. The transistors connected in the circuit with common emitter and feedback voltage.

The capacitance of the capacitors C1 and Sz are taken less than usual, which emphasizes high frequency audio signal.

With engine tuning resistor R6 is amplified and processed signal is fed to the modulator. As the varicap element DA1.3. Frequency modulation is achieved by changing the capacitance in the circuit of quartz: Quartz resonator is excited in the master oscillator at the fundamental frequency of 13.5 MHz.

With the emitter of the transistor DA1.4 frequency-modulated signal supplied by the multiplier DA1.5. The circuit L2, C9 tuned to the frequency of 94.5 MHz, highlighting the seventh harmonic oscillator. Through the condenser With 10 RF oscillation circuit is received in the antenna.

Such a construction of the transmitter has its advantages. In the applied circuit of the master oscillator are stable even inactive quartz. Big difference frequency of the master oscillator and multiplier reduces the impact of radiation output circuit for the operation of the generator.

DA1 chip — CRT with any letter index. Quartz — 13.5 MHz in a metal case.

The coil L2 is wound wire sew 0.6 mm mandrel diameter of 5 mm and contains 12 turns with a branch of the middle. Inductors LI, L3 is wound wire sew 0.4 mm mandrel with a diameter of 2.5 mm and contain 25 turns. Capacitor C4 — C50-40, the remaining capacitors — ceramic, any type. Trimmer — SDR-196, fixed resistors — MLT-0,125. The microphone MKE-3 can be replaced by FEM-84.

Establishing wireless microphones start with the microphone amplifier. Current consumption at specified values of resistors R2, R4 is 1 mA. High impedance phones controlling the passage of sound into the negative electrode C4.

RF probe or oscilloscope to check the work of the master oscillator to the junction point of the capacitors C6, C7, C8. The current consumption of generator — 2...3 mA.

The circuit L2, C9 tune into resonance, shifting and pushing the coils of the coil L2. The selection of resistor R12 sets the current consumed by the multiplier, order b... 8 mA. Complete the building, installing a resistor R6 frequency deviation of 75 kHz.

Literature
1. The A. Allik FM transceiver for 144 MHz. Radio, 1987, No. 3, Pp. 19-21.
2. Polyakov V. T. Hams about the technique of direct conversion. — M.: Patriot, 1990.

A. KURGUZOV, 129282, Moscow, Studeny proezd, 10 — 49.

(RL 3/98)