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Introduction
The basic Frequency Modulation communications system has:
1) Transmitter, which is the sub-system that takes the information signal and processes it prior to transmission. The transmitter modulates the information onto a carrier signal and broadcasts it over the channel
2) Channel: The medium which transports the modulated signal to the receiver. In this project, LED will be connected to FM transmitter end. Signals then will be picked up by the Photo Detector diode right before the FM Receiver
3) Receiver that is the sub-system that takes in the transmitted signal from the channel and processes it to retrieve the information signal. The receiver must be able to discriminate the signal from other signals which may use the same channel medium, amplify the signal for processing and demodulate (remove the carrier) to retrieve the information.
Parts List
To design the FM transmitter and receiver from the beginning we estimate that 25 man-hours would be needed. The majority of those man-hours would be devoted to discovering the frequency characteristics of the transmitter’s VCO (VCO voltage input vs. VCO frequency output) by changing the resistor and capacitor components that determine it. However, we estimate that only 5 hours would be needed to produce the printed circuit boards and install all the components of the transmitter and receiver.
FM Transmitter
Description
The FM Transmitter uses FM waves (frequency modulated waves) to send sound. Frequency modulation transmits data (in our case an audio signal) over a carrier wave by changing the frequency of the carrier wave, where the frequency of the carrier wave corresponds to the voltage level of the audio signal. In order to use electromagnetic transmission, the audio signal must first be converted into an electric signal. The conversion is accomplished by a transducer, in our case the microphone. After conversion, the audio signal is used to modulate a carrier signal.
The process of modulation means to systematically use the information signal (what you want to transmit) to vary some parameter of the carrier signal. The carrier signal can be a sinusoidal, as shown above, but in our case the carrier signal will be a square-wave that is generated by a voltage-controlled oscillator (VCO).
Design
Amplifier
In our circuit we used an amplifier in order to amplify the audio signal and increase the amplitude of the VCO input. We chose an LM741 operational amplifier that was readily available in our lab, and configured it for use as a non-inverting amplifier. Our amplifier was designed to have gain of 20, therefore if the input signal is 100mV the output of amplifier will be 2V.
Voltage Controlled Oscillator (VCO)
A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied voltage, while in our case signals will be fed into the VCO to cause frequency modulation (FM).
A voltage-controlled oscillator (VCO) provides a periodic signal where the frequency of the signal is related to the level of an input voltage control signal supplied to the VCO.
We collected data to plot the voltage-frequency characteristics of the VCO of the MC14046B.
Due to limitation of the frequency response of the LED and photodiode, we limited our frequency range to a maximum of approximately 1MHz. Therefore we chose the characteristics of the pink line, which reaches 1MHz at approximately 5.9V.
For the choice of the center frequency, we considered the capture frequency range of the receiver. We believed that the PLL would safely be able to lock onto a frequency range of 300kHz. This meant a frequency range of the VCO between approximately 700kHz and 1MHz. We chose a center frequency of 860kHz which corresponds to a VCO input voltage of 4.9V.
Pspice Design: Amplifier and VCO
• Input audio signal is reproduced by V4 voltage source.
• Output signal from pin 4 of U2 (MC14046B PLL IC not found in PSpice, therefore CD4046 IC used. Both IC’s have same pin configurations) will be passed to LED circuit.