FM transmitter Project Using only transistors

What is this FM transmitter Circuit About?

There are so many FM related questions that always catch headlines, among these questions are; How does an FM transmitter circuit works?, What are FM transmitters used for?, What is FM transmitter and Reciever?, Why do we use transmitter?.
If you are also asking similar questions, then you are not alone. Sir Boateng at Electrotek has your back because this post is all about DIY FM transmitter (Do-It-Yourself FM transmitter). There will also be a detailed explanation of what the FM transmitter is. If you are interested in microcontrollers, you can start with Arduino Programming for Beginners.

What is Modulation

An audio signal (Voice and musical instruments) is within a frequency of 20hz to 20khz. This frequency can only travel a few meters from the source. But we need to transmit this audio signal to our audience in a long distance away from us. This is only possible using high frequency, which is the backbone of an FM transmitter.

This issue of distance transmission gave birth to modulation. The phenomenon behind modulation is to combine an audio signal and high a frequency signal (called the carrier) together. This combination transmitted over the air as shown below.

Block diagram of Frequency Modulation System

Modulation is therefore the process of combining information (audio or data signals) and high frequency (carrier) for transmission.

wikipedia.org

A device at the receiving end has a specialized circuit to separate the audio signal from the high frequency.

There are many types of modulation in all aspects of telecommunication but the common category of modulation in radio waves are the Frequency Modulation and the Amplitude Modulation.

What is FM?

FM means Frequency Modulation: It is the type of modulation used for an FM transmitter. With this type of modulation, the audio frequency modulates the carrier frequency.

There are different types of FM transmitters, some are very powerful to the extent of transmitting to a very long distance, others are stable with advance PLL control device. Apart from their stability and power, some FM transmitters use RDS (Radio Data System) to attach text information (such as the name of the station, and their frequency).

Recently, technology has made it possible to use Bluetooth FM transmitter modules as a method of transmitting FM signals. Using a few tiny semiconductor chips, one can transmit high-quality audio and data signals with this Bluetooth FM transmitter.

Building a Simple FM transmitter Using Only Transistors

The project focus on designing a simple FM transmitter circuit that can transmit radio signals within a range of 1 to 2 kilometers if a good antenna is used. Unlike, Bluetooth FM transmitter, this circuit uses only a few transistors to transmit the audio signal from one place to another using the frequency modulation spectrum. The first two transistors Q1 and Q2 form a normal audio pre-amplifier. This increase the audio signal before feeding it to the output stage. The amplified audio signal to be propagated is then passed to the base of transistor Q3 through the 0.47uf capacitor (C5).

Circuit diagram.

A simple FM transmitter circuit diagram.

All the resistors are used for biasing. the Output transistor (Q3) modulates the oscillating frequency with the audio signal and also as the final power amplifier. The passive oscillation is performed by capacitor C9 and inductor L1 which form the tank circuit. Inductor L2 uses mutual induction to induce the radio frequency from the tank circuit to the antenna. This phenomenon is what we called electromagnetic induction.

Things To Consider in this FM transmitter project.

  1. As in all radio circuit, you have to follow PCB design rules. That is to choose a good and quality Printed Circuit Board (PCB).
  2. Your circuit track must be free from stray capacitance as u keep all components pins short as possible.
  3. You can supply this circuit with a range of DC voltages from as little as 6v to a maximum of 12V.
  4. L3 is 220uH. You can make one with number 28 SWG 30 turns on 6mm diameter former with 5mm long. This is not critical as you can use any inductor available
  5. L1 and L2 are on the same core close to each other. L1 is 3 turns of 1mm enameled copper wire on a 10mm diameter plastic core. L2 is 2 turns of 1 mm enameled copper wire.
  6. You can vary the frequency from 88 to 108 by adjusting the trim-pot capacitor (C9).
  7. The audio gain is controlled by the variable resistor (R9.
  8. You can adjust the trim-pot capacitor (C8) for RF gain and optimum performance.
  9. For this type of FM transmitter, using a stabilized DC source such as a battery for powering the circuit will reduce noise to a great margin.
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