Simple FM transmitter

FM transmitter is the abbreviation of FM radio transmitter, which is mainly used to transmit voice and music programs of FM radio stations wirelessly. As a simple communication tool, the FM transmitter can carry out effective mobile communication without the support of a relay station, so it is very popular among people. At present, it is widely used in small-scale mobile communication projects in the fields of production, security, and field engineering.

Usually, the transmitter includes three parts: high frequency part, low frequency part and power supply part. The high frequency part generally includes the main oscillator, buffer amplifier, frequency multiplier, intermediate amplifier, power amplifier drive stage and final stage power amplifier. The role of the main oscillator is to generate a carrier with stable frequency. In order to improve frequency stability, the main oscillator stage often uses a quartz crystal oscillator, and a buffer stage is added after it to weaken the influence of the latter stage on the main oscillator. The low-frequency part includes microphone, low-frequency voltage amplifier stage, low-frequency power amplifier stage and final low-frequency power amplifier stage. The low frequency signal is gradually amplified to obtain the required power level at the final stage power amplifier to modulate the high frequency final stage power amplifier. Therefore, the final low-frequency power amplifier stage is also called a modulator. Modulation is the process of loading the information to be transmitted onto a certain high-frequency oscillation (carrier frequency) signal. So the final high frequency power amplifier stage becomes the modulated amplifier.

The first stage of the circuit is a pre-stage based on transistor Q1. This is the collector of a basic bias amplifier stage with collector current and R1 provides the necessary collector-to-base bias resistor R2 to set the collector. C1 is the base input audio signal of the input DC decoupling capacitor and Q1. C8 is the power supply bypass capacitor. The next stage is around the transistor Q2 oscillator and modulation stage. The output electrolytic capacitor C2 couples the first stage to the second stage. The bias resistors R3 and R4 of Q2. R5 is the emitter resistance of Q2. Inductor L1 and trimmer capacitor C5 form a resonant circuit, which is necessary to create oscillation. The modulated FM signal is at the collector of Q2, which uses capacitor C9 to couple the antenna.

Precautions

1. The circuit can be powered from any 6 to 12V DC.

2. Use battery-powered circuits to improve performance and reduce noise.

3. Fine-tuning C5 can be used to adjust the transmission frequency.

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