Demodulation

Demodulation is the reverse process of modulation. Modulation is defined as the process to convert the incoming signal into the suitable form of transmission. It improves the efficiency of the incoming signal and sends it to the receiver. Demodulation converts the received signal back to its original form.

During the modulation, the message signal is superimposed on the carrier signal for efficient transmission. At the receiving end, the message signal is recovered from the carrier wave (message signal + carrier signal), which is termed as demodulation. Thus, demodulation is defined as the process to recover the original signal from the carrier wave.

The original signal is often known as the baseband signal. The modulation and demodulation process comprises of two signals, message signal and the carrier signal. Message signal contains the information that needs to be transmitted from the transmitter to the receiver. Carrier signal is a signal with similar characteristics as the message signal. It is a high frequency signal which is transmitted along with the message signal on the same communication channel.

Various demodulation techniques such as synchronous detection, carrier recovery, error detection, and error correction are incorporated by demodulators. Demodulators can perform none or any of the above functions depending on the requirements. Some demodulators are created specifically for different purpose.

Transmission process

The transmission is carried through the communication channel. The frequency of carrier signal is always high than the message signal because it helps in the increasing the frequency of the message signal.

The three terms involved in a communication process are termed as the transmitter, communication channel, and receiver, which are the components of the communication system. Transmitter transmits the signal, sends to the receiver through the communication channel. The signal is transmitted through various communication mediums, such as optical fiber, telephone lines, twisted pair cables, and coaxial cable.

Types of Analog modulation

Amplitude Modulation
Pulse Modulation
Angle Modulation

Amplitude Modulation

In AM, the amplitude of the carrier varies with the amplitude of the message signal. Amplitude Modulation techniques are used to improve the bandwidth utilization and total transmission power of the transmission medium.

Demodulation in AM

There are two methods that perform the demodulation process in AM, coherent detection method, envelope detector, and the product detector.

Coherent Detection

It is used in the suppressed carrier demodulation process, such as DSBSC (Double Sideband Suppress Carrier) and SSBSC (Single Sideband Suppress Carrier). It requires a coherent detector, which generates a carrier signal similar to the carrier used during modulation. The suppressed carrier modulates transmits only the message signal by suppressing the carrier for better efficiency. Hence, the carrier is generated at the receiving end for successful recovery of the original message signal. The filters are further used to separate the message signal from the carrier wave.

Envelope Detector

It is one of the simple demodulation techniques because it does not require any coherent detector. It is commonly used in the demodulation process with carrier, such as DSBC (Double Sideband with Carrier) and SSBC (Single Sideband with Carrier). Here, the circuit of demodulator comprises of a RC network connected in parallel with the diode. The capacitor stores the charge, which is utilized when the diode is in OFF state. The output wave generated by the circuit is a sawtooth wave.

Product Detector

Product refers to the multiplication. A product detector multiplies the incoming signal with the carrier frequency generated by the local oscillator. It is equal to the carrier signal used for various amplitude modulation techniques. The filters are further used to recover the original message signal.

Types of AM

AM is further categorized as:

Single Sideband Modulation (SSB)
Double Sideband Modulation (DSB)
Vestigial Sideband Modulation (VSB)
Quadrature Amplitude Modulation (QAM)

Single Sideband Modulation (SSB)

SSB is the amplitude modulation method to produce sidebands on either side of the carrier wave. Sidebands are the frequency bands on either or both sides of the carrier wave, which are produced during the modulation process.

Double Sideband Modulation (DSB)

DSB is the amplitude modulation method to produce sidebands on both sides of the carrier wave, which are symmetrical about the carrier frequency.

Vestigial Sideband Modulation (VSB)

Vestige means a section of the signal. Thus, a section of the signal is modulated by adding a vestige to the produced sidebands.

Quadrature Amplitude Modulation (QAM)

It sends two message signals over the same spectrum, while other modulation techniques send only one message signal over the same spectrum.

Pulse Modulation

In Pulse Modulation, the pulse carries the information. The amplitude, width, and position of the pulse vary with the message signal. It is used in various applications, such as Ethernet and digital transmission.

Demodulation

The demodulation of pulse modulation techniques (PWM, PPM, and PAM) is quite easy compared to other modulation types. In PWM, a ramp signal is taken starting at the positive edge. It is stopped when the negative edge comes. The width of the ramp signal is different and thus follows different heights depending on the amplitude and width of the pulse. When it passes through the LPF (Low Pass Filter), it forms an envelope and appears as the demodulated signal at the output.

RC (Resistor and Capacitor) combinations and transistors can be used to generate the ramp signal.

Types of Pulse Modulation

Pulse Modulation is further categorized as:

Pulse Amplitude Modulation
Pulse Width Modulation
Pulse Position Modulation

Pulse Amplitude Modulation

PWM is the modulation based on the regulation of the amplitude of the pulse. The amplitude of the pulse varies with the sample value of message signal.

Pulse Width Modulation

PWM is the modulation based on the regulation of the width of the pulse. The width of the pulse varies with the sample value of message signal.

Pulse Position Modulation

PPM is the modulation based on the regulation of the position of the pulse. The position of the pulse varies with the sample value of message signal.

Angle Modulation

Angle Modulation is the combination of frequency and phase modulation. Here, the frequency of phase of the carrier varies with the instantaneous value of the message signal.

Demodulation

The demodulation methods for angle modulation are selective carrier method, discriminator method, and PLL (Phase Locked Loops).

Selective Carrier Method

It consists of a frequency selective network and the envelope detector. The output wave generated by the frequency selective method is both frequency modulated and amplitude modulated. The frequency should be proportional to the instantaneous frequency of the received signal.

Discriminator method

It is an IC (Integrated Circuits) based demodulation technique. The circuit comprises of the AM demodulator and the electronic filter that helps in recovering the original message signal.

PLL

It works on the concept of the two phase signals and minimizes the differences between them. It tracks the components of the FM and PM (frequency and phase) of the carrier from the signal received from the transmitter. It is also used in AMSC (Amplitude Modulation Suppressed Carrier) for synchronous detection.