How does an envelope detector extract the original modulating signal from the FM carrier?
1 Answer
An envelope detector, also known as an envelope demodulator or detector, is a simple and widely used technique for extracting the original modulating signal from a frequency-modulated (FM) carrier wave. In FM, the amplitude of the carrier wave is kept constant, but its frequency varies according to the modulating signal. The goal of the envelope detector is to recover the original modulating signal from these frequency variations.
The process of extracting the original modulating signal using an envelope detector involves the following steps:
Reception of the FM signal:
The FM signal, which consists of the carrier wave modulated by the audio or baseband signal, is received by an antenna or other receiving device. The FM signal is typically in the radio frequency (RF) range.
Conversion to baseband:
The first step in envelope detection is to convert the received RF signal back to its original baseband signal, which contains the modulating audio signal. This can be done using a frequency demodulator, such as a quadrature detector or a phase-locked loop (PLL). The output of this stage is the baseband signal, which is a frequency-shifted version of the original audio signal.
Rectification:
In the next step, the baseband signal is passed through a rectifier. A rectifier is a circuit that converts an AC signal to a pulsating DC signal, allowing only the positive or negative portions of the signal to pass through. For envelope detection, a diode is commonly used as a rectifier. When the baseband signal is positive, the diode allows the signal to pass through unaltered, but when the baseband signal is negative, the diode blocks the signal.
Smoothing:
The rectified signal contains amplitude variations that correspond to the original modulating audio signal. However, it still has some ripples due to the rectification process. To remove these ripples and obtain a smoother representation of the envelope, a low-pass filter is used. This filter allows only the slow variations of the signal to pass through, effectively smoothing out the signal.
Resultant envelope signal:
After smoothing, the output signal of the envelope detector is a pulsating DC signal that represents the envelope of the modulating audio signal. This envelope signal closely follows the variations in the original audio signal that caused the frequency modulation of the carrier wave.
By following these steps, the envelope detector successfully extracts the original modulating signal from the FM carrier, allowing the audio or data signal to be recovered and reproduced at the receiver. Keep in mind that while envelope detection is a simple and commonly used technique, it may not be as effective as more complex demodulation methods in situations with high noise or interference. In such cases, more sophisticated demodulation techniques like phase-locked loops (PLLs) or digital signal processing methods may be employed.
The process of extracting the original modulating signal using an envelope detector involves the following steps:
Reception of the FM signal:
The FM signal, which consists of the carrier wave modulated by the audio or baseband signal, is received by an antenna or other receiving device. The FM signal is typically in the radio frequency (RF) range.
Conversion to baseband:
The first step in envelope detection is to convert the received RF signal back to its original baseband signal, which contains the modulating audio signal. This can be done using a frequency demodulator, such as a quadrature detector or a phase-locked loop (PLL). The output of this stage is the baseband signal, which is a frequency-shifted version of the original audio signal.
Rectification:
In the next step, the baseband signal is passed through a rectifier. A rectifier is a circuit that converts an AC signal to a pulsating DC signal, allowing only the positive or negative portions of the signal to pass through. For envelope detection, a diode is commonly used as a rectifier. When the baseband signal is positive, the diode allows the signal to pass through unaltered, but when the baseband signal is negative, the diode blocks the signal.
Smoothing:
The rectified signal contains amplitude variations that correspond to the original modulating audio signal. However, it still has some ripples due to the rectification process. To remove these ripples and obtain a smoother representation of the envelope, a low-pass filter is used. This filter allows only the slow variations of the signal to pass through, effectively smoothing out the signal.
Resultant envelope signal:
After smoothing, the output signal of the envelope detector is a pulsating DC signal that represents the envelope of the modulating audio signal. This envelope signal closely follows the variations in the original audio signal that caused the frequency modulation of the carrier wave.
By following these steps, the envelope detector successfully extracts the original modulating signal from the FM carrier, allowing the audio or data signal to be recovered and reproduced at the receiver. Keep in mind that while envelope detection is a simple and commonly used technique, it may not be as effective as more complex demodulation methods in situations with high noise or interference. In such cases, more sophisticated demodulation techniques like phase-locked loops (PLLs) or digital signal processing methods may be employed.