Repetition of different designs of wireless microphones showed their shortcomings. So, without a quartz oscillator frequency stabilization observed «withdrawal» transmitter frequency, and in schemes with a crystal — a small frequency deviation, that is, NFM. The generator has been described with two connected in parallel quartz resonators — «Super VXO».
Indeed, in this arrangement, the deviation of oscillation frequency with change of capacitance in series with a quartz resonator, is much greater. In addition, «Super VXO» choke is used, and if it excluded the overlap in frequency is reduced, but remains quite sufficient for obtaining a frequency deviation of ± 10 kHz after three frequency multiplier crystal oscillator that provides a good quality of the emitted radio microphone signal.
Schematic diagram of the FM radio microphone shown in Fig. The scheme used quartz crystals to 25.0 MHz, and thus multiplying the frequency of the signal into three wireless microphone misses the bandwidth of standard broadcast band (100-108 MHz and 65,8-74). Many imported radios cover the frequency range 64-108 MG n (for example, the receiver WAXIBAXB-222 FM-TV) and well receive the signal of the wireless microphone.
The master oscillator can be applied widespread quartz resonators 26.6 MHz; while after tripling the output frequency will be 79.8 MHz.
On-chip DA1 assembled microphone amplifier, transistors VT1, VT2 — generator under the scheme «Super VXO» (easy, no throttle), the transistor VT3 — frequency multiplier of three.
The coil L1 is wound on a paper frame 05.4 mm (with a brass core of television PTK) and contains 7 turns of wire of SEW-2 0.5 mm diameter outlet — from the 2nd round of counting from the upper terminal. Coil L2 — frameless with an inner diameter of 5 mm and comprises 13 turns of wire of SEW-2 0.5 mm in diameter.
Configuring the wireless microphone is reduced to a tuning circuit L1-C14 at 75 MHz and the output P-circuit L2-C20-C21-C22 for maximum return power to the antenna. Antenna — telescopic 1 m long.
Wireless Microphone transmits data over a distance of about 700 meters in an open area.