Amplitude Shift Keying

In digital communication, different modulation techniques are used to transmit data or message to receiver over a communication channel . One such technique is Amplitude Shift Keying (ASK). It is a modulation technique that alters the amplitude of a carrier signal to transmit the information over channel. It is a modulation scheme having wide range of application in real world which includes radio, television, and digital data transmission.

Amplitude Shift Keying

Amplitude Shift Keying (ASK) is a digital modulation technique . It transmits the digital information by varying the amplitude of a carrier signal. In ASK, a high-amplitude carrier signal is used to represent a binary ‘1,’ and a low-amplitude carrier signal represents a binary ‘0.’

It involves the superimposition of a carrier signal and a digital message signal. The carrier signal is often a high-frequency sinusoidal waveform, which serves as the carrier for the digital information. The binary message signal, consisting of ‘1’s and ‘0’s, is used to control the amplitude of the carrier signal. The resultant signal formed after the superimposition of message and carrier is transmitted over the communication channel.

Circuit diagram of ASK

Working

• Carrier signal is chosen to be of very high frequency as compared to the message signal. It is usually a sinusoidal waveform.
• Message signal consists of a binary stream of data. In ASK the high amplitude is represented by ‘1’ and low amplitude is represented by ‘0’.
• Modulated signal is generated as the resultant of the product of message signal and carrier signal(Modulation). If the message is represented by m(t), carrier as c(t) the the modulated waveform f(t)=m(t)*c(t) and this is transmitted over the communication channel.
• At receiver end, the modulated signal is demodulated to get the message binary stream(Demodulation). At receiver various mechanisms are used to recover the original message.
  
  

  Amplitude Shift Keying

ASK Modulation

In ASK modulation, a carrier wave (usually a sinusoidal wave) is modified in accordance with the digital message signal. The amplitude of the carrier signal is changed between two predefined values i.e. 0 and 1 to represent binary data.

The carrier wave is represented by continuous signal. The carrier’s amplitude changes between two levels, particularly ‘0’ and ‘1’. When the input data is ‘0’, the amplitude might be lower, and when the input data is ‘1’, the amplitude could be higher. The changes in amplitude effects the carrier signal in accordance with the digital signal. This modulation technique is simple but can be susceptible to noise and interference, which can affect the accuracy of the received data.

Amplitude Shift Keying MODULATION

ASK Demodulation

Demodulation in Amplitude Shift Keying (ASK) involves extracting the original digital message signal from the modulated carrier wave. The process reverses the modulation applied to the carrier signal. The demodulation of ASK can be performed through various methods, such as envelope detection or coherent detection.

Types Of ASK Demodulation

There are mainly two types of Demodulation which is performed namely envelope detection and coherent detection technique:

• Envelope detection
• Coherent detection

Envelope detection

Envelop detection is a relatively simpler method as compared to coherent detection which is used for demodulating Amplitude Shift Keying (ASK) signals. It primarily focuses on extracting the envelope of the modulated signal to recover the original digital data.

The steps involved in this technique is as follows

• The modulated ASK signal, which contains the carrier signal and the digital data, is received by the demodulator. This is called receiving of signal.
• The received signal is then passed through a diode. The diode rectifies the signal, allowing only the positive portion of the waveform (the envelope) to pass through it. This process is called Diode Rectification.
• Following the rectification process, a low-pass filter is applied to the rectified signal. This filter helps remove or discard the high-frequency carrier signal, leaving the varying amplitude that represents the original digital data. This process is called Low-Pass Filtering of the modulated signal.
• Once the signal has been filtered by passing it through the LPF, a threshold detection mechanism is used to interpret the amplitude changes. Comparing the amplitudes to a predefined threshold helps to determine the transmitted digital data. If the amplitude is above the threshold, it might be interpreted as one bit ‘1’, and if it’s below the threshold, it might be interpreted as the other bit ‘0’. This is called Threshold Detection.
• The message signal is recovered from the modulated signal.

ASK Demodulator

Coherent detection

This is another method which is used for demodulation of ASK signal. It involves phase synchronization with the carrier wave.

The steps involved in this technique is as follows

• The received signal is mixed with a local oscillator, aligning its phase and frequency with the original carrier signal. This process is called Reception.
• Filtering isolates the part of the mixed signal that corresponds to the data signal.
• The demodulator sets an amplitude threshold to distinguish between the ‘1’ and ‘0’ states.
• The amplitude of the received signal is compared against the threshold to determine the transmitted data.

In this way the original message signal is recovered.

ASK Coherent detection demodulation

Advantages

There are various advantages of using ASK as a modulation technique.

• It is one of the simplest modulation techniques which makes it easy to implement in both analog and digital systems.
• It is a cost-effective modulation technique, which is one reason for its widespread use in various applications.
• It is compatible with various transmission media, including radio waves, optical fibers, and wired communication systems.
• It is efficient in terms of bandwidth utilization as it only requires two levels of amplitude i.e. 0 and 1.

Disadvantages

• Susceptibility to Noise: ASK is more sensitive to noise and interference compared to other modulation techniques like Frequency Shift Keying (FSK) or Phase Shift Keying (PSK). Fluctuations in the amplitude caused by noise can lead to errors in data recovery at the receiver end.
• Bandwidth Inefficiency: ASK can be less bandwidth efficient compared to other modulation techniques. It requires more bandwidth because it uses changes in amplitude to encode data, which can limit the number of bits transmitted in a given bandwidth.
• Power Inefficiency: Transmitting signals at varying amplitudes requires more power compared to constant amplitude transmission. This can be a disadvantage in scenarios where power efficiency is critical.
• Limited Data Rate: ASK is generally limited in terms of achievable data rates compared to other modulation techniques. This limitation arises from the constraints on how fast the amplitude can be changed without distortion.
• Signal-to-Noise Ratio Sensitivity: ASK’s performance is highly dependent on the signal-to-noise ratio (SNR). As the ratio between the signal power and noise power decreases, the accuracy and reliability of data transmission can be significantly affected.
• Lack of Phase or Frequency Information: ASK only utilizes changes in amplitude to represent data, unlike other modulation techniques that use phase or frequency changes. This limitation might make it more vulnerable to certain types of interference and distortions.

Applications

Here comes a wide range of application of ASK

• ASK is commonly used in wireless communication systems, such as keyless entry systems, remote controls, and radio frequency identification (RFID) tags.
• In optical fiber communication, ASK is used to transmit digital data over long distances.
• ASK is often used in digital broadcasting, including television and radio transmissions, for transmitting audio and video signals.
• ASK can be used for low to medium data rate communication, such as in binary data transmission over short distances.
• ASK is used in medical telemetry systems for monitoring and transmitting patient data.

Conclusion

In conclusion, Amplitude Shift Keying (ASK) is a basic and straightforward modulation technique that encodes digital data by altering the amplitude of a carrier wave. While it has its advantages, including simplicity and ease of implementation, it also comes with several limitations such as susceptibility to noise, bandwidth inefficiency, power inefficiency, limited data rate, sensitive to SNR etc.

While ASK might not be the most efficient or modulation technique, it finds application in scenarios where simplicity and cost-effectiveness are more crucial than achieving high data rates or robustness against noise. It’s commonly found in applications such as remote controls, RFID systems, and some low-cost wireless communication systems.

FAQs on Amplitude Shift Keying

1. Where is ASK used?

ASK is commonly used in applications like RFID systems, remote controls, and some low-cost wireless communication systems.

2. How does ASK work?

ASK works by changing the amplitude of the carrier wave to encode binary data. It uses different amplitudes to represent different symbols or bits.

6. How is ASK demodulated?

ASK can be demodulated using techniques such as envelope detection or coherent detection. Envelope detection involves rectification, low-pass filtering, and threshold detection to extract the digital data.