Frequency Modulated Oscillator (for FM Transmitter)
An oscillator can be built using LC (inductor-capacitor) tank circuit. When we vary the capacity of the capacitor in LC tank circuit then the frequency of the oscillator will vary according to capacitance change. The idea of this modulator is by employing a varactor (voltage-dependent variable capacitor) in the LC tank circuitry. As the capacitance is controlled by a voltage level, then it can be controlled using audio signal from microphone. A signal from direct microphone output will be too small, so we need a pre-amp before feeding the signal to this circuit. This circuit assume a standard line level voltage for the input, so you can feed it with audio mixer, pre-amp, equalizer, or other various audio equipment. We can use this circuit for FM wireless audio, microphone, other applications where a stable frequency modulated oscillator is needed. Just remember that this circuit is not a transmitter, so you’ll need an RF power amplifier to amplify the output in order to transmit via antenna. We can vary the LT to cover 75 to 150 MHz, as needed.