Envelope Detector

Envelope detector circuit uses a diode, a capacitor and a resistor and it is just like a half wave rectifier followed by a low-pass filter. It is a linear detector which takes high frequency RF signal as input and gives an output which is the envelope of the input signal. A diode detector is a type of envelope detector and is used for the detection of AM signal.

Circuit Operation

Here the input signal is rectified by the series diode D. The combination of capacitor C and resistor R behaves like a low-pass filter. The input signal contains both the original message and the carrier wave where the capacitor helps in filtering out the RF carrier waves. The capacitor gets charged during the rising edge and discharges through the resistor R in falling edge. Thus the capacitor helps in giving an envelope of the input as output.

Advantage

• This technique is highly used in AM receivers as it is very simple and cheap. 
• The circuit acts as a linear detector as the output varies as per the voltage of input and the distortion here is very less.

Disadvantage

• The circuit does not have an amplification capability to amplify the rectified signal. Amplification should be done by separate devices.
• Before giving input to the circuit the signal must be bandpass filtered otherwise the circuit will demodulate unnecessary signals.

Compared to advantages the disadvantages are very negligible and hence these circuits are extensively used for AM detection.

Analog vs Digital Communication

Analog

The signals which are continuous are called analog signals. For example a sound coming from a car is an analog signal.
Advantage

• Analog signals are less complicated and less expensive.
• Mathematical implementation of analog signal is easier.
• In presence of heavy noise, analog signals never completely degrades.
• Unlike digital signals analog signals does not have any quantization loss.
• It also does not require highly complicated ICs for processing. If some analog components will be present then it can be processed easily.

Disadvantage

• Analog signals are sensitive to noise.
• If analog signal undergoes degradation, it is difficult to verify where and when the degradation occurred.
• It is difficult to enhance the degraded analog signals as it may lead to amplification of noise as well.

Digital

The signals which are represented by binary values 0 and 1 is called digital signals. 0 and 1 corresponds to low voltage and high voltage respectively.

Advantage

• It is easier to store in memory as the entire signal comprises of a series of 0 and 1.
• Digital signal comparatively more immune to noise.
• There are a lot of channel coding and error correction codes are available for an effective and error free transmission of digital signals.
• Degraded digital signals can be easily reconstructed by using advanced algorithms and digital signal processors.

Disadvantage

• Digital signals may be completely lost at the presence of excessive noise.
• Digital signal processing is highly complicated and require a lot of mathematical calculation hence the digital processors are bit expensive.
• Digital signals undergoes quantization loss.
• Digital signals require a bunch of heavily complicated  ICs for its processing.