What is an envelope detector in AM receivers?
1 Answer
An envelope detector is an essential component in amplitude modulation (AM) receivers. Its purpose is to extract the modulating audio signal from the incoming AM signal, which is typically a high-frequency carrier wave modulated by a lower-frequency audio signal. The AM signal's amplitude varies with the audio signal, and the envelope detector is designed to recover this varying amplitude (envelope) of the AM signal.
The process of envelope detection involves the following steps:
Reception of the AM signal: The antenna of the AM receiver captures the modulated AM signal, which contains both the carrier wave and the modulating audio signal.
Demodulation: The AM signal is passed through an envelope detector circuit. The envelope detector's main function is to convert the amplitude variations of the AM signal (resulting from modulation) into a varying direct current (DC) voltage that represents the audio signal.
Rectification: The incoming AM signal is usually a bipolar signal that swings both above and below the zero voltage line. The envelope detector rectifies this signal, converting it into a unipolar signal that only varies in the positive direction.
Low-pass filtering: The rectified signal, also known as the "envelope," still contains the carrier frequency and high-frequency noise components. To remove these high-frequency components and retain only the audio frequency variations, a low-pass filter is used. This filter allows only the slower audio variations to pass through while attenuating the carrier frequency and higher frequencies.
Smoothing: The output of the low-pass filter is typically a pulsating DC signal that follows the envelope of the AM signal. This pulsating signal is then smoothed using a capacitor, which charges and discharges to reduce the pulsations and create a more continuous representation of the audio signal.
Audio amplification: After the smoothing process, the resulting audio signal is usually very weak. Therefore, an audio amplifier is used to boost the signal to a suitable level, which can then be further processed and fed into a speaker to reproduce the original audio content.
By performing envelope detection, an AM receiver can successfully demodulate and extract the original audio signal from the incoming AM signal, allowing us to listen to the broadcasted audio content, such as music, speech, or any other transmitted information.
The process of envelope detection involves the following steps:
Reception of the AM signal: The antenna of the AM receiver captures the modulated AM signal, which contains both the carrier wave and the modulating audio signal.
Demodulation: The AM signal is passed through an envelope detector circuit. The envelope detector's main function is to convert the amplitude variations of the AM signal (resulting from modulation) into a varying direct current (DC) voltage that represents the audio signal.
Rectification: The incoming AM signal is usually a bipolar signal that swings both above and below the zero voltage line. The envelope detector rectifies this signal, converting it into a unipolar signal that only varies in the positive direction.
Low-pass filtering: The rectified signal, also known as the "envelope," still contains the carrier frequency and high-frequency noise components. To remove these high-frequency components and retain only the audio frequency variations, a low-pass filter is used. This filter allows only the slower audio variations to pass through while attenuating the carrier frequency and higher frequencies.
Smoothing: The output of the low-pass filter is typically a pulsating DC signal that follows the envelope of the AM signal. This pulsating signal is then smoothed using a capacitor, which charges and discharges to reduce the pulsations and create a more continuous representation of the audio signal.
Audio amplification: After the smoothing process, the resulting audio signal is usually very weak. Therefore, an audio amplifier is used to boost the signal to a suitable level, which can then be further processed and fed into a speaker to reproduce the original audio content.
By performing envelope detection, an AM receiver can successfully demodulate and extract the original audio signal from the incoming AM signal, allowing us to listen to the broadcasted audio content, such as music, speech, or any other transmitted information.