The modulation is the technique by which data is converted into electrical or digital signals for transferring that signal over a medium. Signal modulation is done to carry the signal over large distances. After the signal transmission, the modulated signal must be converted back into the original signal to retrieve the original information.

This back-conversion is done with the help of various demodulation techniques. Hence, it means the separation of the signal from the modulated carrier. In this article, we will study these techniques in detail, their applications, advantages and disadvantages, etc.

What is Demodulation?

The Demodulation is the process of extracting the original information from a modulated carrier wave. It is an important function used in the communication systems which allows the recovery of transmitted data at the receiver end. There are different types of the demodulators with different output signals which will be discussed further.

Demodulation Process

To understand the concept of Demodulation, we need to study the Modulation first. Modulation is the process where properties of the carrier signal like amplitude, frequency, or phase change according to the baseband or message signal. It is the reverse process of the modulation to recover the signal from modulated signal and hence obtain the original signal, so the need of the demodulation is to get the original information or signal from the modulated one.

Demodulation

Demodulation is the technique to recover the original signal from the modulated signal. The demodulation is done with the help of a demodulator. A demodulator will convert the carrier variation of amplitude, frequency, or phase back to the message signal. There are three different types of demodulators for converting the AM (amplitude modulation), FM (frequency modulation), and PM (phase modulation) modulation schemes.

Need for Demodulation

The Demodulation is important as the it ensure that the transmitted audio signals can be correctly received and processed. Demodulation is the process of recovering the original signal from the modulated signal. It allows the separation of the audio frequency from the higher frequency carrier which makes the signal audible and usable to the human listener and compatible with standard audio playback devices. Without using the demodulation the information carried by the high-frequency carrier waves would remain inaccessible

Demodulation Techniques

There are various demodulation techniques depending on the type of modulation. Some of the technique are listed below:

• Diode rectifier envelope detector
• Product detector
• Synchronous detection

Diode Rectifier Envelope Detector

It is used to demodulated the amplitude modulated signal. The simple diode detector circuit is given below. It is the simplest form of detector because it comprises of a single diode and a capacitor. Since it does not meet the standard of the other techniques related to it but this is the most basic type of the detector and detects the envelope of the AM signal.

Simple Diode Detector Circuit

The working the circuit is as follows:

• The signals from the previous stages is applied to the diode. The capacitor helps in removing the unwanted radio frequency (RF) elements.
• When the radio signal is applied to the diode, only half the waveform passes through it and half of the waveform is removed. This is due to the property of diode i.e., current can flow only in one direction through the diode.
• The resulting signal is applied to the audio output which is seen in the above diagram. This resulting signal is the demodulated signal.

Diode rectifier envelope detector is a very simple circuit. Due to its simplicity, it is very low cost. But it also has some disadvantages like it is not linear due to the diode characteristic which leads to the distortion in the signal. It is less sensitive and its performance is affected by the selective fading.

Product Detector

The product detector circuit is used to demodulate the SSB (single side band) signal. In SSB amplitude modulation, the first carrier signal is suppressed, and then either the upper-side band or lower-side band is suppressed. The circuit diagram of the product detector is given below.

Product Detector

The working the circuit is as follows:

• The SSB signal is passed from the IF transformer. It uses a crystal oscillator which produces the local oscillator signal. The generated local oscillator signal has a frequency similar to the carrier signal.
• The SSB signal is then multiplied by the local oscillator signal. This leads to the creation of different frequencies i.e., sum and difference of the frequencies.
• The resulting signal is then passed to the low-pass filter. It will pass only the low frequency signal which is the baseband signal. Hence, in this way, we extract the original signal.

This technique is used in the various communication application during demodulation. Also, this circuit can be used to intercept Morse Code signal.

Synchronous Detection

It is also known as coherent detection and is used to demodulate the amplitude-modulated signal. It also comprises the oscillator which generates the local oscillator signal. It is then multiplied with the incoming modulated signal. The resulting signal is then passed through the low-pass filter to remove the high-frequency components. The block diagram for the Synchronous Detector is given below.

Synchronous Detection

The filtered signal contains the original baseband signal. The received signal is then decoded to extract the original information. It helps in providing better performance. It is used in various applications of broadcast receivers like radio communication equipment or walkie-talkies because it is easy to incorporate.

Demodulation Techniques in AM, FM, and PM Radio

Given Below is the Demodulation techniques for the Radio system

AM Radio

The AM radio System follow these Demodulation steps

• Rectification: The Received Signal is first send for rectification which converts the varying amplitude into corresponding voltage variations.
• Filtering: The rectified signal is them passed to the low pass filter which removes the high frequency components leaving the baseband audio signal.
• Audio Amplification: Then this filter audio is amplified to a suitable level for using in a speaker or headphones.

FM Radio

The FM radio System follow these Demodulation steps

• Frequency Discriminator or Phase-Locked Loop (PLL):These circuits are used for the demodulation techniques in the FM signal, the frequency discriminator is used to detect changes in the frequency of the received signal and converts them into proportional voltage changes, PLL is used to lock into the frequency of the received signal and compares it with a stable reference frequency.
• Voltage-Controlled Oscillator (VCO):The Output of the Discriminator are used control VCO which can be used for generating audio signal proportional to the frequency change of the received signal.
• Audio Amplification: Then this audio is amplified to a suitable level for using in a speaker or headphones.

PM (Phase Modulation) Radio

The PM radio System follow these Demodulation steps

• Conversion to AM Signal: The Signal is first converted back into a amplitude-modulated signal using demodulators like homodyne detector.
• Amplitude Demodulation: Then the AM Signal is demodulated using amplitude demodulation techniques like envelope detection.
• Extraction of Original Signal: The demodulated signal contains the original modulating with any phase information encoded in the PM signal. This phase information is processed to recover the original signal.

Challenges for Demodulation

Some challenges for modulation is discussed below:

Challenges

There are two main challenges while performing demodulation. The challenges are:

• Synchronization: It is very typical to maintain the synchronization between the incoming modulated signal and the demodulator. To overcome this challenge carrier synchronization is used.
• Noise and interference: While recovering the original signal, it might be distorted due to the noise and interference in the channel. To overcome this problem, error correction methods are used.

Difference between Modulation and Demodulation