Telecom Mask
What is the 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. 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 FM transmitter is. Keep your finger cross.
What is Modulation
Audio signal (Voice and musical insruments) is within a frequency of 20hz to 20khz. This frequency can only travel for few meters from the source. But we need to trasmit this audio signal to our audience in long distance away from us. This is only posible using high frequency.This issue of distance transmission gave birth to modulation. The phenomenom behind modulation is to combine an audio signal and high a frequency signal (called the carrier) together. This combination is then transmitted over the air as shown below.
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| Frequency Modulation block diagram |
Modulation is therefore the process of combining information (audio or data signals) and high frequency (carrier) for transmission.
A device at the receiving end has a specialized circuit to seperate 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 in which 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).
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. 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.
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| A simple FM transmitter circuit diagram. Click to the image to enlarge |
Things To Consider.
- As in all radio circuit, you have to follow PCB design rules. That is to choose a good and quality Printed Circuit Board (PCB).
- Your circuit track must be free from stray capacitance as u keep all components pins short as possible.
- You can supply this circuit with a range of DC voltages from as little as 6v to a maximum of 12V.
- 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
- 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.
- You can vary the frequency from 88 to 108 by adjusting the trim-pot capacitor (C9).
- The audio gain is controlled by the variable resistor (R9.
- You can adjust the trimpot capacitor (C8) for RF gain and optimum performance.
- Using stabilized DC source such as a battery for powering the circuit will reduce noise.