“DESIGN OF FM RADIO TRANSMITTER”

Designers:

Fermante, Joseph Greg

Gonzales, Lalaine Joy

Leonor, Erwin Gabriel

Instructor:

Engr: Vincent E. Malapo

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ABSTRACT

An FM transmitter functions as an electronic device specifically designed to transform an

audio signal, such as music emanating from a mobile phone, into a radio wave. This radio wave

adheres to the designated FM frequency range, typically encompassing a spectrum of 88 MHz to

108 MHz. The transmitter implements a modulation technique, meticulously manipulating the

frequency of the radio wave in accordance with the variations in amplitude of the audio signal.

This process ensures faithful representation of the original audio content within the transmitted

radio wave.

This document presents a comprehensive outline for the design and subsequent

fabrication of a low-power FM radio transmitter. The primary objective is the realization of a

functional transmitter capable of broadcasting audio signals over a restricted range. To achieve

this, an audio modulator circuit will be incorporated, tasked with modulating the frequency of a

carrier wave in direct correspondence with the characteristics of the input audio signal, typically

derived from a music source. The fabrication process will entail the meticulous selection of

suitable electronic components, followed by their assembly on a designated circuit board in strict

adherence to a predefined schematic diagram. To guarantee optimal performance, the document

will encompass detailed calibration procedures, ensuring the transmitter operates at the

designated frequency while minimizing any potential interference with established radio

broadcasts.

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PURPOSE OF THE DESIGN

The design and application of FM radio transmitters encompass a multifaceted

engineering discipline dedicated to the development of electronic devices capable of wirelessly

transmitting audio information across designated radio frequency bands. This technology

underpins a wide range of communication applications, fostering the dissemination of audio

content such as music, speech, and data signals over extended distances without the constraints

of physical cables. FM radio transmission offers distinct advantages, including resistance to

interference from electrical noise and the ability to broadcast high-fidelity audio signals over a

broad coverage area.

The successful design of an FM transmitter hinges on the execution of several critical

processes. Firstly, the generation of a high-frequency carrier wave forms the foundation for the

transmission process. Subsequently, this carrier wave undergoes modulation, a technique that

meticulously varies its characteristics based on the information to be transmitted, typically in the

form of an audio signal. Finally, the resulting encoded signal is efficiently transmitted through an

antenna, enabling its wireless propagation over designated radio frequencies.

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LIST OF COMPONENTS AND TOOLS

TABLE 1.

COMPONENTS FUNCTION

1. Transistors ➢ Act as amplifiers, boosting weak

● 2 − 2𝑁3904 electrical signals from the audio source to

a stronger level for further processing in

the circuit..

2. Resistors ➢ Resistors help to set the voltage levels at

● 1 − 100𝑘Ω different points in the circuit.

● 1 − 100Ω

● 1 − 1𝑀Ω

● 1 − 1𝑘Ω

● 3 − 10𝑘Ω

3. Inductor ➢ Stores energy in a magnetic field and

● 0. 1μ𝐻 inductor (Air coil) helps tune the circuit to the desired FM

radio frequency.

4. Capacitors ➢ Capacitors are used for tuning, filtering

● 2 − 0. 1μ𝐹 unwanted frequencies, and blocking DC

● 1 − 40𝑝𝐹 trimmer current while allowing AC current to

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 ● 1 − 4. 7𝑝𝐹 pass.

● 1 − 10𝑝𝐹 ➢ Trimmer is a special type of capacitor that

allows for fine-tuning of the circuit's

frequency.

5. Antenna ➢ Radiates the FM radio waves carrying the

audio information out into the air,

allowing them to be received by FM

radios tuned to the transmitter's

frequency.

6. 9V Battery ➢ Provides power to the entire circuit.

7. Battery Clip ➢ A battery clip creates a connection to the

battery in a circuit.

8. PCB ➢ PCB holds all the electronic components

together and provides the connections

between them.

9. 3.5mm Female Jack ➢ A 3.5mm female jack, the jack serves as

the audio input for the signal to be

broadcast.

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 Table 1 provides a comprehensive list of all the components used in the FM transmitter

circuit. It also details the specific function of each component, explaining how it contributes to

the overall operation of the transmitter.

BLOCK DIAGRAM

An FM transmitter is an electronic device that creates radio waves by converting

electrical signals. These radio waves carry audio information which can be received by FM

radios.

The Input, which represents the electrical signal that carries the audio information you

want to transmit. This signal may come from a microphone, music player, or any other device

that generates an electrical audio signal.

The audio pre- amplifier, amplifies the weak electrical signal from the microphone.

Transistors can amplify weak signals by using a small current at the base to control a much larger

current flowing between the collector and emitter. In this circuit, the microphone converts sound

waves into a small electrical signal.

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 Next, the signal moves to the Modulator. The modulator merges the audio signal with the

RF carrier wave. This process creates a new radio wave that has the characteristics of both the

original audio signal and the carrier wave. The FM radio picks up on these variations in the

carrier wave, which it translates back into the original audio information.

The signal then proceeds to the Oscillator. The oscillator generates a constant and precise

radio frequency (RF) signal. This RF signal acts as a carrier for the audio information.

The audio amplifier stage, further amplifies the audio signal from the pre-amplifier. The

amplified signal is then output from the circuit through headphones connected between the

collector of transistor Q2 (2N3904) and ground (GND).

The final stop is the Antenna. The antenna radiates the modulated radio wave out into

the open air. This radio wave can now be picked up by FM radio receivers within range of the

transmitter.

SCHEMATIC DIAGRAM

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Input Stage:

The circuit starts with a microphone (MK1) that captures sound waves and converts them

into electrical signals. This microphone serves as the input for the circuit. These weak electrical

signals from the microphone travel through resistor R5 (100kΩ) and might be coupled to the

base of transistor Q1 (2N3904) through capacitor C5 (4.7pF). Resistors R5 and R6 and capacitor

C5 (4.7pF) might form a simple high-pass filter circuit, allowing higher frequency audio signals

to pass through while blocking lower frequencies. Transistor Q1 (2N3904), resistors R1 (10kΩ)

and R2 (1MΩ), and capacitor C2 (10pF) could potentially function as a voltage amplifier circuit.

This circuit would amplify the weak electrical signals from the microphone.

Voltage-Controlled Oscillator (VCO):

The amplified microphone signal is likely coupled to the base of transistor Q2 (2N3904) through

capacitor C2 (10pF). Transistor Q2 and its surrounding components (L1, C4, L2, C3, R3, R4,

C1) form the voltage-controlled oscillator (VCO) circuit. The VCO circuit generates a periodic

electrical signal at a specific frequency. The frequency of this oscillation is determined by the

values of the capacitors (C3, C4) and inductors (L1, L2) in the circuit. The voltage on the base of

transistor Q2 can also affect the frequency. When the voltage on the base increases, the

frequency of the oscillation tends to increase as well. Conversely, when the voltage on the base

decreases, the frequency of the oscillation tends to decrease. This is how the circuit achieves

voltage control of the oscillation.

Output:

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The antenna is connected to the circuit through inductor L2. The VCO's oscillating electrical

signal is coupled to the antenna through L2, radiating the signal wirelessly.

AUDIO INPUT

The microphone converts sound waves into electrical signals. These electrical signals are

then coupled to the base of transistor Q1 (2N3904) through resistor R5 (100kΩ). Resistor R5

limits the current flowing into the base of the transistor, protecting it from damage.

The audio input in this circuit is a microphone that converts sound waves into electrical

signals. These signals are then amplified by transistors Q1 and Q2 before being output to

headphones.

AUDIO PRE-AMPLIFIER

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 The audio pre-amplifier in the circuit is transistor Q1 (2N3904). The transistor is used to

amplify the weak electrical signal from the microphone (MK1) before it is further amplified by

transistor Q2.

Transistors can amplify weak signals by using a small current at the base to control a

much larger current flowing between the collector and emitter. The transistor amplifies this weak

signal according to the gain of the transistor, which is set by the resistors surrounding the

transistor. Transistor Q1 is the pre-amplifier in the circuit because it amplifies the weak signal

from the microphone before it is sent to the next amplification stage.

MODULATOR

A modulator is an electronic circuit that takes a low-frequency information signal (like an audio

signal) and combines it with a high-frequency carrier signal to create a modulated signal. This

modulated signal is then transmitted over long distances through mediums like air or cable.

The reason for modulation is that low-frequency signals are inefficient for transmitting over long

distances. High-frequency signals, on the other hand, can travel long distances with less signal

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degradation. By combining the information signal with a carrier signal, the modulator creates a

composite signal that can be efficiently transmitted.

OSCILLATOR

The oscillator in an FM transmitter is the engine that drives the whole process. It's responsible

for creating the basic radio wave frequency that carries the signal. This frequency is what your

radio tunes into to pick up a particular station. So, when you're dialing through stations, you're

essentially adjusting your radio to pick up different frequencies generated by the oscillator. It's

like the master clock that keeps everything in sync, ensuring that your radio can catch the signal

and deliver your favorite tunes or talk shows without missing a beat.

RF AMPLIFIER

The RF amplifier is like the muscle that boosts the power of the signal before it goes out

over the air. It takes the signal, which starts off pretty weak, and amps it up to a stronger level.

This boost is necessary because the original signal might not have enough power to travel far or

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get through obstacles. The RF amplifier works specifically within the radio frequency range used

for FM transmission, making sure the signal stays in the right lane. By beefing up the signal, the

RF amplifier helps the transmission reach more people and cover a wider area. This is super

important for FM radio stations, as it helps them broadcast to as many listeners as possible. Plus,

a good RF amplifier helps keep the signal clear and stable, so listeners get the best experience.

In order to achieve the target operating frequency of the FM transmitter, the following

equation was employed:

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𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 =
2π 𝐿𝐶

Where:

● Frequency is the radio frequency in Hertz (Hz)

● L is the inductance in Henrys (H)

● C is the capacitance in Farads (F)

● π (pi) is a mathematical constant approximately equal to 3.14159

Given:

𝐿1 = 0. 1μ𝐻

𝐶4 = 30 𝑝𝐹

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𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 = −12 −6
2π (30𝑥10 )(0.1𝑥10 )

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𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 = 91. 888149237𝑥10

𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 = 91. 88𝑀𝐻𝑧

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 1
The equation 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 = was employed to determine the target operating frequency of
2π 𝐿𝐶

the FM transmitter. After substituting the specified values of L1 = 0.1 μH and C4 = 30 pF, the

resultant frequency was approximately 91.88 MHz.

Available FM Radio Signals in CSPC-Nabua

FM Radio Frequency FM Radio Station Name

87.5 Citizens’ Choice Radio

94.3 FMR University FM Nabua

96.7 Good Vibes Radio

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CIRCUIT SIMULATION

The FM transmitter circuit embarks on a captivating mission: to propel audio signals on a

wireless voyage through the airwaves of the FM radio spectrum. The circuit begins with the

microphone, a dedicated device that serves as the bridge between the physical world of sound

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and the electrical realm. Sound waves, carrying the essence of music or speech, are meticulously

transformed into electrical signals by the microphone. However, these initial electrical signals

are often faint and require amplification. This is where the audio amplifier steps in, acting as a

stalwart champion, bolstering the strength of the electrical signals.

The amplified audio signal then embarks on the next leg of its journey, entering the

domain of the frequency modulator circuit. Here, a fascinating process unfolds, known as

frequency modulation (FM). The modulator meticulously varies the frequency of a separate,

high-frequency radio wave, generated by the oscillator circuit. This radio wave, designated as the

carrier wave, serves as the foundation for our wireless transmission. The variations in the carrier

wave's frequency are meticulously tailored to correspond to the fluctuations in amplitude of the

original audio signal. In essence, the audio information becomes embedded within the

characteristics of the carrier wave.

The newly minted modulated signal now carries the essence of the original audio

information. However, this signal still requires additional strength for effective transmission.

Thus, it proceeds to the final amplification stage. Here, the signal is significantly amplified,

granting it the power necessary to travel a substantial distance.

The amplified and information-rich signal is finally presented to the antenna. The

antenna, acting as a tireless broadcaster, radiates the electromagnetic waves, carrying the

encoded audio information, into the surrounding space. The design and placement of the antenna

are paramount for ensuring efficient transmission of the signal.

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 The FM transmitter circuit, fueled by a dedicated power supply, meticulously executes

each stage of this process. This power supply, typically batteries or a DC source, provides the

essential energy that drives the entire operation.

In conclusion, the FM transmitter circuit serves as a maestro, orchestrating a remarkable

transformation. It takes a fragile audio signal, modulates it onto a carrier wave, amplifies it for

strength, and finally broadcasts it wirelessly through the antenna. It's crucial to adhere to

regulations governing frequency use and transmission power to ensure harmonious coexistence

with other radio stations and devices. The utilization of various electronic components, such as

ceramic capacitors for stability and filtering, inductors for storing energy, resistors for controlling

current flow, and transistors for signal amplification, all play vital roles in the captivating saga of

the FM transmitter.

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DOCUMENTATION

Functional Testing

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Transferring The Layout With A Marker

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Etching Process

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Drilling

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Board Assembly

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BIBLIOGRAPHY:

[1]J. Y. Channel, “How to Make FM Transmitter,” Instructables.

https://www.instructables.com/How-to-Make-FM-Transmitter/

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