FM TRANSMITTER Avelino, Anne Loraine L., Galang, Vincent N., Nañoz, Allona Jane M., Punzalan, Justine Roy A. College of Engineering School of Technology First Asia Institute of Technology and Humanities Abstract— This paper aims to design a low-power FM transmitter that is powered by a 9V battery. Electret microphone and audio jack were used as audio input. The transmitter operating frequencies are from 89-90 MHz and 103-104.1 MHz. The range of the said frequencies are both 2 meters.
II. CIRCUIT DESIGN FM uses VHF radio frequencies usually 87.5 – 108.0 MHz to transmit and receive the FM signals. For the circuit shown in Fig. 2.1, the process of transmission is done by using pre- amplifier and modulator circuit.
I. INTRODUCTION Frequency modulation (FM) is widely used for a variety of forms in radio communications. It is an important form of modulation and being used nowadays. FM has developed immensely since it was created by Edwin Howard Armstrong last 1931. The idea of having an FM transmitter is done by the process of audio amplification, modulation and then transmission. Audio input from the electret microphone or any other device like audio jack is first amplified using the common emitter configuration. 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 variable capacitor. The output signal from the emitter of the transistor is coupled to the input of the power amplifier transistor using the coupling capacitor. As this signal is amplified, the variable capacitor in the power amplifier section tends to maintain an output matching with that of the oscillator. The amplified RF (radio frequency) signal is then transmitted using antenna. The frequency is set at anywhere between the FM frequency range from 87.5 MHz to 108 MHz. There are many applications including its use for room monitoring, baby listening, nature research, low-power broadcasting to a very limited audience in near vicinity and others. Also, it can be used at any place to transmit audio signals using FM transmission, particularly at institutions and organizations. This allows portable audio devices to make use of the better sound quality of a home audio system or car stereo without requiring a wired connection. In addition, they are often used to broadcast a stationary audio source like computer or television. This project is a hand-sized transmitter. The transmitter operating frequencies are 89-90 MHz and 103-104.1 MHz. The transmitter is powered by a 9V battery. It is a low-power transmitter composed of capacitors, resistors, a screw adjustable trimmer capacitor and an inductor. Being low-powered, it has a range of up to one-fourth of a mile radius depending on the quality of the receiver, obstructions and elevation.
Figure 2.1 FM Transmitter Circuit
A Common Emitter Amplifier is used as the pre- amplifier of the circuit. CE configuration is one of the most commonly used pre- amplifier due to its stability and medium level gain. There are two input signal option for the project. These are through condenser/electret microphone and audio jack. A condenser microphone is used to accept the sound signals. Inside the mic, a capacitive sensor diaphragm is present. It vibrates according to the air pressure changes and generates AC signals. These signals serve as the input for transmission. In addition, audio jack is used to feed directly audio signal to the circuit. That is, an audio file from a mobile device or laptop is played in which the audio jack is connected. Use of audio jack provides clearer intelligence signal for transmission. These signals are fed to the base of the configured 2N3904 pre- amplifier. Output signal of the amplifier would be the input for the modulator circuit. In FM Transmission, a tank circuit is needed. For this project, it is implemented with an inductor paralleled to a variable capacitor. The 4pf capacitor serves as the negative feedback to the oscillating tank circuit. As long as the current exists across the inductor coil and the variable capacitor, tank circuit will oscillate at the resonant carrier frequency for FM modulation. The oscillator circuit produces the needed RF carrier waves for FM transmission. LC configuration serve as storage of energy needed for oscillation. An FM Transmitter with this configuration is very susceptible to noise. Placement of components is very crucial. Board Layout is shown on Figure 2.2 and is made as small as possible. As much as possible the group did not use wires of any kind for jumpers. Wires are only used when there is a need of connection/ interface for casing (switches and power supply). Making the circuit as
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compact as possible would somehow decrease noise level and provide stability during operation.
After several unsuccessful attempts, the group decided to fabricate it, taking the chance that it might have a progress when the components are fixed in their position and more compact. Figure 3.2 shows the first fabrication done and the electret microphone is first used as the input. With the auto-search capability of the cellular phone used, a transmission was detected at 99.2 MHz.
Figure 2.2 FM Transmitter PCB Layout
Figure 2.3 demonstrates the location of each component used for this project.
Figure 3.2
Figure 2.3 Parts Layout
III. RESULTS AND DISCUSSION Converting the schematic of the transmitter in actual is one of the critical points in the project. A compact circuit is to be maintained in breadboard and in the fabricated ones to be able to have a proper transmission.
According to studies, when the antenna used in transmission is aligned with the antenna of the receiving medium, a feedback will take place, mixing a feedback sound in the transmitting sound (voice or music). This is what happened in the first fabrication attempt, which proved that the transmitter made transmits at 99.2MHz. But then a problem was encountered since the input voice being transmitted is not heard in the receiver. This is due to the reason that there is already a high-power radio station transmitting at 99.1 MHz, and a 0.1 increment is included in its bandwidth, so this shows that the input of the group’s transmitter will not be recognized and only a feedback sound will be produced.
Following the usual routine when adapting the simulation to actual, the transmitter is first performed in breadboard. A cellular phone is used as the receiver, for it is one of the best medium to find frequencies transmitting either in a close distance or not. However, no matter how compact the circuit is, as shown in Figure 3.1, transmitting is not that easy and this also goes to finding what frequency is being transmitted. Figure 3.3
A second fabrication attempt is done, and this time, it is more compact compared to the previous fabrication and is evident in Figure 3.3. Also, a trimmer capacitor is used in the tank circuit instead of a ceramic capacitor used in the first fabrication, and the coiling of the inductor is changed. The expected result should be a result better than the previous one, but instead, it is the opposite. During the radio scanning, only the frequencies of the high-power radio stations are detected. So the trimmer capacitor is adjusted, that should have the same effect to its frequency. But still, no other frequencies are detected. Figure 3.1
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Figure 3.4
The third fabrication attempt is done and from a gauge of 22 with 7 turns, the inductor is changed to a gauge of 18 with 4 turns and a total of 12mm length as shown in Figure 3.4. Without adjusting the 4pF – 40pF trimmer capacitor, a transmission is detected at 102.2 MHz. With a maximum distance approximately 11 ft., the voice input is clearly heard with some noise included. Changing the input medium to an audio jack, the transmission in the said frequency was gone. The antenna was moved, but still, the transmitting frequency is not detected. Carefully adjusting the antenna to its original position, the clear transmission is changed to 102.1 MHz, but there’s still no sign of the transmission when the input is audio jack. Setting aside the circuit for a few hours, and trying it again, the transmitting frequency jumped to 101.4 MHz. Despite being in the center of two high-power operating radio stations, 101.1 and 101.9 MHz, the voice input is still transmitting clearly.
Figure 3.5
The group then carefully included the switches for the main circuit and the control for which input will be used as shown in Figure 3.5. But upon simulating the revised circuit, the previous transmission was gone. The trimmer capacitor is adjusted so that the operating frequency of the circuit will also be adjusted but no matter how much adjusting is done in the capacitor, no transmission is being detected. Another fabrication is done which follows the same specifications of inductor (turns, gauge, and length), and an unturned trimmer capacitor. This time, the switches are already included in the circuit before performing a trial to avoid the same thing that happened in the previous one. The output product is shown in Figure 3.6, which can be considered a replica of the third fabrication.
Figure 3.6
The circuit is then simulated and transmission is detected in 89.1 MHz for it is the station which provides a feedback sound in the receiver. Slightly calibrating the trimmer capacitor, the frequency jump to 89.5 MHz. While the trial is being done, bad reception happens and the frequency is being shifted by a 0.1 increment. As observed and tested, the transmission using electret microphone and the audio jack has a range of 89.0 – 90.0 MHz. The speaker or the input voice shouldn’t be too close to the electret microphone, for it transmits greater when the voice is far but of enough distance. The operating frequency can be considered critical during simulation because three high-power radio stations are included in the bandwidth which are 89.1, 89.5, and 89.9 MHz. This affects the maximum distance the receiver can receive the signal being transmitted by the circuit. Despite of having a very short distance, it transmits clearly and minimum noise is being tolerated. Once the receiver exceeds the given distance, the high-power transmission will be detected and the transmission of the circuit will be gone. The group fixed the circuit in its case to avoid the sensitive components from being moved and retain its operating frequency. Shown in Figure 3.7 is the final output for the FM transmitter that operates at a range of 89.0 – 90.0 MHz.
Figure 3.7
Since the fourth fabrication is an exact replica of the third in terms of the values and placements of the components, the group decided to fix and change the inductor of the third fabrication then, set the wires of the switches to a shorter one. When
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simulated, the operating frequency was detected at the range of 103.7 – 104.2 MHz. Unlike the transmitter operating at 89.0 – 90.0 MHz that clearly transmits the input signal, this revised fabrication clearly transmits the input using audio jack with a maximum length of 22 ft. In terms of the input using electret microphone, the operating signal jump to 103.8 MHz but the reception is very noisy. The input signal is audible though not understandable.
Another parameter to be considered is the antenna. Similar to the inductor, the antenna should be fixed in terms of its position because based on the simulations done in this project, when the antenna is moved, even slightly, the transmission will be gone and the operating frequency will be shifted. This is due to the absence the coupler that makes any antenna resistive. Since the group didn’t use a coupler in the design, the antenna is not resistive and therefore, should be fixed. In terms of resistor, small wattage is recommended, for it occupies minimum spaces in the fabricated circuit and contributes less in the noise transmitted. Also, wires used in connecting explicit components of the circuit should be of minimum length for it also affects the transmission of the circuit. Lastly, the circuit should very compact to have a better transmission. Transmitter circuits with very large spacing among components is less likely to perform the desired operation. IV. CONCLUSION
Figure 3.8
Also fixing this circuit to its corresponding case, the output hardware is shown in Figure 3.8 and even though the transmission using electret microphone is not clear enough to be understand, the functionality of this circuit in terms of the audio jack input is beyond expectations enough to be fixed and presented. The final FM transmitters produced are shown in Figure 3.9, carefully labelled of its operating frequency.
To conclude, the FM transmission is clearly demonstrated by the circuit used in this project. The tank of the circuit which dictates the frequency of operation is one of the most vital part. The computation through the use of formula discussed in the Results and Analysis is observed and used. However, there is a big difference in the expected frequency and the actual frequency. This is because of the limitation in making the inductor, to produce a desired inductance. The coiling, the diameter, the length of the inductor, and the gauge of the wire used is found to very significant in making the inductor. To get the inductance needed, the said parameters should all be considered in approximating its value. The feedback between the FM transmitter and the receiver is the indicator that the transmission of signals is happening in that frequency. The noise which aids in the transmission affects the clarity of the signal. There are cases that the noise is more evident than the modulating signal. That is why, the placement of the components in the circuit board must also be considered together with length of wires used if any, and the condition of the atmosphere. The condition of the atmosphere is observed to be stable in dawn or in early morning.
Figure 3.9
Summarizing all the results obtained in all the simulations done to achieve the desired results, there are important parameters that needs to be considered. First is the tank circuit which includes the trimmer capacitor and the inductor. Based on the formula,𝑓𝑜 = 1 , the capacitance and the inductance contributes to the 2𝜋√𝐿𝐶 operating frequency and when one of it is changed, so is the frequency. That’s why when the trimmer capacitor is adjusted, the operating frequency will jump to another band of frequency. In terms of inductor, the length, spacing, number of turns, and gauge of the wire is important and should be monitored from time-to-time. If possible, the inductor should be fixed during simulation.
Lastly, the bandwidth or the range of transmission is wide. Also, the device is very sensitive, small movements of the antenna affects the point of frequency in the FM range. The first good transmission is happened at 102.1 MHz to 102.2. The second attempt happened at a range of frequency of 101.4 MHz to 101.6 MHz, and the last try is at 100.5 MHz but more noise than the signal. Thus, the device, specially its antenna is very sensitive. The conclusion in this case is that the length of the antenna also affects the frequency of operation of the circuit. Since there are many high-power FM transmitters established and built, it is therefore concluded that those high power transmitters can easily transmit signals and interfere those lowpower ones. The audio signals coming from the high-power
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transmitter can easily be heard in an FM receiver than that of signals came from low-power. However, if the receiver is near to a low power transmitter, the signals from the low-power transmitter will be received by the said receiver.
Anne Loraine L. Avelino
Vincent N. Galang
Allona Jane M. Nañoz
Justine Roy A. Punzalan REFERENCES [1]
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W. Tomasi, Electronic Communications Systems (Fundamentals through Advanced), 5th ed., Upper Saddle River, New Jersey: Pearson Education, 1987. Frequency Modulation. [Online]. Available at: https://en.wikipedia.org/wiki/Frequency_modulation Frequency Modulation. [Online]. Available at: http://www.radio-electronics.com/info/rf-technology-design/fmfrequency-modulation/what-is-fm-tutorial.php LC Circuit. [Online]. Available at: https://en.wikipedia.org/wiki/LC_circuit List of Radio Stations in Manila. [Online]. Available at: https://en.wikipedia.org/wiki/List_of_radio_stations_in_Metro_Manila FM Transmitter Bug [Online]. Available at: http://hackaweek.com/hacks/?p=283 Ultimate FM Transmitter Bug [Online]. Available at: http://www.instructables.com/id/The-Ultimate-FM-Transmitter/ Mini FM Radio Broadcast Transmitter [Online]. Available at: http://electronics-diy.com/mini-fm-radio-broadcast-transmitter.php
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