The production of a crystal oscillator frequency-controlled broadband FM transmitter

Numerous experiments have shown that the modulation frequency deviation of the FM transmitter is the key to the true reproduction of the sound at the receiving end. Although the ordinary LC oscillation modulation circuit can obtain a larger frequency deviation, the negative impact it brings - frequency drift is unbearable. Therefore, people use the high Q value and low access coefficient characteristics of quartz crystals as frequency stabilization devices and apply them to FM transmitters. This measure does solve the frequency drift phenomenon, but the fatal weaknesses such as low modulation sensitivity and narrow frequency deviation that follow seriously damage the clarity and frequency response of the receiver demodulation signal. This circuit uses the TEK622 wide-band deviation crystal oscillator FM transmitter module as the core component, and uses its unique frequency stabilization measures and frequency deviation expansion technology to make the whole machine not only have a very stable transmission frequency, but also show an extraordinary high-definition wireless transmission effect. According to the actual measurement of HK-1000 digital frequency counter (gate time 1S), the frequency drift of the evaluation board is less than 0.007MHz after 24 hours of uninterrupted operation. Moreover, the frequency is not sensitive to voltage fluctuations. On the other hand, the modulation signal-to-noise ratio of this machine is better than 70dB, which greatly improves the ripple interference phenomenon of AC power supply that is common in typical transmitting circuits. As long as the power supply quality at the power supply end is guaranteed, the receiver sound background is very quiet, and basically no AC interference sound can be heard. The circuit is shown in the figure below. It is recommended to use double-sided fiberglass board for the printed board. The design should comply with the wiring principles of high-frequency lines. All resonant inductors are wound with 0.5mm enameled wire with an inner diameter of 4mm. The component parameters in the figure design the transmission frequency to 100HMz, which can be received by ordinary household FM radio.

When properly debugged, the working current is 180mA, the RF output power is 0.5W (under 50 ohm pure resistive load), and the transmission distance in open area can reach 1000-1500 meters. In addition, the frequency range of TEK622R is 20-180HMz, and the frequency can be adjusted by changing the parameters of JT and each LC circuit.