Description of an FM Transmitter Circuit

An FM transmitter is an electronic circuit that broadcasts speech and music over radio waves. FM, or frequency modulation, is the manner in which the circuit transmits audio signals. The transmitter circuit takes in an audio signal and uses it to shift the frequency of a radio signal, and then sends it over the airwaves. The transmitter consists of a few components, including an audio amplifier, radio-frequency oscillator, a radio amplifier and an antenna.

Input Amplifier
The FM transmitter circuit has an input that accepts audio signals. These signals can come from CD players, tape players, microphones or other equipment. The first stage in the transmitter is an amplifier that boosts the level of the audio signal. The amplifier has a gain control you adjust to obtain an optimum signal strength.

High-Frequency Oscillator
The heart of the FM transmitter is an oscillator that produces a signal in the range of FM broadcasts: 88.1 to 108.1 MHz. The oscillator generates an FM carrier wave. The incoming audio modulates, or shifts, the oscillator’s frequency up and down. An FM receiver, tuned to the oscillator’s main frequency, detects the frequency shifts and recovers the original audio signal.

Power Amplifier
The frequency-modulated signal from the oscillator goes into a radio-frequency power amplifier. The power of FM transmitters varies from about 100 milliwatts to dozens of kilowatts. A handheld, battery-powered FM transmitter, for example, has under a watt of radio frequency power. Commercial FM radio stations have power amplifiers ranging from a few watts to over 20 kW.

Antenna
The transmitter’s power amplifier sends its signal to the antenna, which converts the signal to radio waves. The size of the antenna, as well as its above-ground height, depends on the application. A low-power FM antenna can be a few inches in length. Some commercial FM broadcasting antennas are among the tallest structures in the world, reaching over 1,700 feet in height.

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