Amplitude modulation (AM) is a fundamental concept in analog communication systems. This experiment demonstrates how the amplitude of a high-frequency carrier signal can be varied by a lower-frequency modulating signal, such as an audio input. By following this guide, you will understand how AM works and observe it in action using simple electronic components.
Follow these step-by-step instructions to assemble the AM circuit for this experiment:
1. Use an oscillator circuit, such as a crystal oscillator or an astable multivibrator, to generate a stable carrier signal. Select a carrier frequency within the desired range (e.g., 500 kHz to 1 MHz for AM radio).
2. Connect the output of the oscillator to the base of a transistor, which will serve as the modulation stage.
1. Use an audio signal generator or a microphone circuit to produce the modulating signal. Ensure the signal frequency is much lower than the carrier frequency.
2. Couple the modulating signal to the base of the transistor through a coupling capacitor. This ensures that only the AC component of the modulating signal is applied, preventing DC interference.
1. Connect a resistor in series with the transistor's collector terminal to limit the current. This acts as the load for the circuit.
2. Use a decoupling capacitor at the output to block DC components and allow only the modulated AC signal to pass to the load.
1. Connect the emitter of the transistor to the ground of the power supply.
2. Provide a DC power supply (e.g., 9V) to the circuit, ensuring that the voltage and current ratings are within the limits of the components used.
1. Double-check all connections to ensure the components are correctly placed on the breadboard or PCB.
2. Verify polarity-sensitive components like electrolytic capacitors and ensure proper orientation.
1. Connect an oscilloscope to the circuit output. Observe the waveform on the oscilloscope screen.
2. Adjust the amplitude of the modulating signal and verify that the carrier signal’s amplitude changes accordingly, demonstrating amplitude modulation.
Assemble the oscillator circuit to generate a stable carrier frequency. Use an adjustable resistor (potentiometer) to fine-tune the carrier frequency if needed.
Connect the modulating signal to the modulation input of the carrier signal circuit. Ensure that the modulating signal's amplitude is within the circuit's acceptable range.
Use an oscilloscope to observe the output signal. You should see the amplitude of the carrier signal varying according to the modulating signal.
The amplitude of the carrier signal will vary in accordance with the modulating signal. For example, if you use an audio signal as the modulating input, the carrier signal's amplitude will mimic the waveform of the audio signal.
The observed output should show a high-frequency carrier wave with its peaks following the shape of the modulating signal.