FM Radio Transmitter

Circuit PrincipleOur task is transmit sound signal in 200m range without no wire using frequency modulation. FM transmission is done by the process of audio pre amplification, Mixing audio signal with high frequency carrier waves (modulation) and then transmission through antenna .

Block diagram

Circuit Schematic

Circuit PrincipleAudio signal from the microphone is first amplified using the common emitter configuration of BJT. This amplified signal is then given to the oscillator circuit through the coupling capacitor. The oscillator circuit generates a signal with a frequency determined by the value of the trimmer (variable) capacitor. The output signal from oscillator circuit is a mixture of the input signal and the oscillating signal, producing a frequency modulated signal. Then signal transmitted using antenna in radio frequency range (87-108 MHz).

FM modulationAudio signal have low frequency (Low power) so we can’t transmit directly, so we need to modulate audio signal with high frequency carrier wave in radio frequency range.

Radio frequency

MicrophoneMicrophone convert audio signal into electrical signal but signal will be in µV so we need to amplify these signal.

Voltage signal from microphone

Audio Pre-amplifier

VOLTAGE-DIVIDER BIAS CONFIGURATION

Here c1 and c2 are coupling capacitor.Coupling capacitor c1 and c2 for limiting the frequency Large value indicates low frequency ,whereas lesser value increases higher frequency.

Purpose of R1 is to limit current through microphone

For high amplification

βR5 > 10R3

Input Output

Amplifier input and output

Input audio signal

Amplified output

Tank circuit

Oscillator CircuitFrequency of oscillations

Here we require a frequency between 88 MHz to 108 MHzLet us select L1 inductor 0.1uH. This gives value of C4 to be around 20pF. Here we select a variable capacitor in the range 4 to 40pF.

Capacitor C5 acts as a negative feedback to the oscillating tank circuit and keep the tank circuit to vibrate without damping.

Selection of Antenna

we can prepare an antenna using a stick antenna or a wire approximately which would be about 1/4th of the transmitting wavelength.

Uses… This circuit can be used at any place to transmit audio signal using

FM transmission.

Wide array of FM transmitters used in many different environments. Some of the more popular uses are radio stations ,sporting events, homes, cars, and correctional facilities.

Compatible with your laptop, smart phone, FM transmitters will broadcast your audio source to the cars radio without an auxiliary input in low range.