Communication is necessary whether it is two people, or a huge group of people. The word communication refers to the exchange of information from one place to another. Communication is necessary, in order to keep everyone update about what’s going on in today’s world. This gives a brief idea on ‘what’, ‘when’, and ‘how’ things goes on, resulting in creating a better world tomorrow.
In order to make communications simpler and convenient, communication systems were developed by the telecommunication engineers. A communications system is a group of components that works together to transfer information from one place to another. Any communication system includes at least three basic components; which includes a transmitter, medium, and a receiver.
A transmitter could be anything which transmits something. But in engineering world this differs a bit, even though the main concept is still the same. In engineering, a transmitter is a set of components which modulates the message signal and transmits this signal as electromagnetic waves (EM). And the medium could be any think which the message propagates. Today the most commonly used mediums are copper wires, air (using radio waves) and fiber optics (using light waves). The final component, which is the receiver, receives the message signal and passes it to the destination. When the receiver receives a signal it demodulates and provides the destination with the original message.
This study focus on the simulation of frequency modulated transmitter, simulation of the circuit. In addition it also focuses on the problems faced and the possible solutions that have been discovered so far.
There are two common types of radio transmission. Those are amplitude modulated (AM) transmissions, and frequency modulated (FM) transmissions. Even though AM circuits are cheaper and the signals can be propagated to a longer distance today FM has taken over AM transmissions. This is because in AM the attenuation ration is higher compared to FM. In addition, in AM the bandwidth is limited, which in FM it is unlimited.
In frequency modulation, the frequency of the carrier signal is varied with respect to the original signal. Figure below shows the process of FM modulation:
The information signal is the original source signal coming from the source. This signal is taken into the FM transmitter through a transducer: example microphone. In order to modulate the information, a higher frequency compared to information signal is in the modulator. When the information signal and the carrier signal are multiplexed inside the FM modulator, it gives out the output which is known as the FM modulated wave.
Figure #1: image shows example of a carrier wave, information signal, AM modulated wave & FM modulated wave.
The figure #1 shows that, the frequency of the carrier wave is changed according to the original signal without changing the amplitude of the carrier. In FM, the frequency is highest when the amplitude of the signal is highest. And the frequency is the lowest when the amplitude of the information signals.
The main objective of this assignment is to construct a simple FM transmitter circuit and to test it in the laboratory.
Basically an FM transmitter is a combination of components working together, which accepts audio input and converts it into FM radio waves. An FM transmitter consists of a modulator, and transmitter. It takes audio as the inputs and modulates the signal into FM radio wave. In frequency modulation (FM), the frequency of the electromagnetic (EM) wave is varied without changing the amplitude of the signal. Then the modulated signal is released on to the medium through an antenna.
Modulation index is the measure of the relative quantity of information to carrier amplitude in the modulated signal. It is always used to distinguish the spectral power distribution of the modulated wave. The increase in modulation increase the number of sideband also increases. Thus a higher bandwidth is required to capture most of the sidebands. Below shows the formula to find the modulation index of an FM signal.
Deviation ratio can be defined as the maximum value of modulation index that is permitted. A frequency of 75 kHz is the maximum carrier deviation for commercial. Below shows the formula used to find Deviation Ration:
Even though human’s ears can pick up frequencies between 20 Hz to 20 kHz, sometimes the frequencies above 15 kHz are ignored. Therefore the 15 kHz of frequency is chosen as the maximum modulating frequency.
Carrier swing is the total deviation of a frequency modulated wave from the lowest to the highest frequency. The carrier swing of a wave is calculated by using the formula below:
The percentage modulation is the ratio of actual frequency to the maximum frequency deviation expressed in percentage. Below shows the formula used to find percentage modulation:
FM transmitter is a device which is used to convert audio signals in to frequency modulated signal and propagate the modulated wave to the destination through a medium. FM transmitter consists of a microphone as the input. This microphone is a transducer which is convert input audio signal into electromagnetic signal. The EM wave from the microphone is amplified by and transistor and is passed. This amplified EM wave goes through demodulation process as gets demodulated before it is been transmitted.
Figure #2: FM transmitter circuit
The FM transmitter circuit consists of two transmitters. The first transistor (Q1) is used for the amplification and the second transistor (Q1) is used for demodulation processor. The voice / audio signal is picked by the microphone and is converted into electrical signal. Then this signal is passed through capacitors to smoothen the signal into the transistor Q1. The transistor Q1 amplifies the EM waves passing through it. After the amplification process, the signal undergoes modulation process in the transistor Q2. In addition to the modulation process, Q2 also amplifies the demodulated signal further more. Amplification is important in order to propagate the signal to a longer distance without any damages. An Inductor and a variable capacitor are connected in the collector of the Q2. Variable capacitor and the inductor works in a group to give the function of a tuner. The frequency of the output signal is set by varying the capacitance of the variable capacitor. And finally the modulated signal is passed onto the antenna for transmission. Below shows a simple block diagram to explain how the circuit works.
Figure #4: image shows the FM transmitter circuit done on a breadboard.
The FM transmitter is used to vary commonly in communication systems today. It gained the interest of almost all the engineers because FM transmitter uses frequency modulation. People use FM over M due to the following reasons.
1. 1. FM modulated waves can carry more information compared to AM.
2. 2. FM modulated wave can travel to a longer distance.
3. 3. High frequency signals have less change of getting damaged by noise.
4. 4. High frequency signals will not get distorted easily.
In an FM transmitter there are two main basic block: the amplifier and the modulator. The amplifier amplifies the EM signal. The modulator varies the frequency of the carrier signal with respect to the message signal. As the system deals with frequency modulation, the circuit is embedded with a FM modulator.
In frequency modulation, when the amplitude of the message signal is at highest peak, the frequency of the carrier signal is at the highest. And when the amplitude of the message signal is at the lowest, the frequency of the carrier signal is at its least.
In the FM transmitter circuit used in this assignment a microphone (transducer) is used to convert audio signal into electromagnetic waves. This EM signal is then passed through the transistor Q1 for amplification. Amplification is necessary because the input signal will be weak thus to get a better waveform. After the amplification process the EM wave is passed through another transistor Q2 for modulation process. In addition to modulation, Q1 also does the amplification of the modulated signal so that it can be propagated to a longer distance without getting distorted.
A variable capacitor and an inductor are used as a tuner circuit in order to set an output frequency of the circuit. In order to find the output frequency the following formula is used:
The preferred frequency modulated wave is propagated through the antenna.
In the circuit that was used in this study has a very weak tank circuit. So it is advised to use a powerful circuit tank circuit so that it can detect very little differences in the frequency.
The hardware of the FM transmitter circuit was done on a breadboard. The FM transmission was detected on my FM radio when the circuit is switched ON. From this it’s concluded that the output signal of the circuit is a frequency modulated circuit. FM radio can only detect FM modulated waves.
Andrews. (2000). AM waves. Available:
http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/RadCom/part9/page4.html.
Last accessed 20th Sep 2011.
Xing. (2004). Modes and modulation. Available: http://www.dxing.com/modesand.htm. Last accessed 21st Sep 2011.
Ratana. (2006). The advantages of AM and FM modulation. Available: http://www.edaboard.com/thread86386.html. Last accessed 21st sep
Burkholder. What is an FM transmitter?. Available: http://www.wisegeek.com/what-is-an-fm-transmitter.htm. Last accessed 23th Sep 2011.
Wardsworth. Frequency modulation. Available: http://wadsworthsales.com/fm.aspx. Last accessed 23rd Sep 2011.
Pike, J. . (2005). INTRODUCTION TO TACTICAL RADIO COMMUNICATIONS THEORY. Available: http://www.globalsecurity.org/military/library/policy/army/accp/ss0002/le1.htm. Last accessed 22nd Sep 2011.
Frequency modulation (FM) equation. Available: http://ecelab.com/FM-equations.htm. Last accessed 23rd Sep 2011.
