Digital Communication Tanmay Gurav TE-EXTC -B 20104B0043
ASK Amplitude Shift Keying
ASK - Amplitude Shift Keying Amplitude Shift Keying ASK - is a type of Amplitude Modulation which represents the binary data in the form of variations in the amplitude of a signal.
Generation
Detection ASK demodulation can be done in two ways.
High end explaination : If amplitude is the only parameter of the carrier wave to be altered by the information signal, the modulating method is called amplitude-shift keying (ASK). ASK can be considered a digital version of analog amplitude modulation. In its simplest form, a burst of radio frequency is transmitted only when a binary 1 appears and is stopped when a 0 appears. In another variation, the 0 and 1 are represented in the modulated signal by a shift between two preselected amplitudes.
ASK Applications Modulation has an important role in communications. Amplitude shift keying applications are mentioned below. They are: Low-frequency RF applications Home automation devices Industrial networks devices Wireless base stations Tire pressuring monitoring systems Thus, Ask (amplitude shift keying) is a digital modulation technique to increase the amplitude characteristics of the input binary signal. But its drawbacks make it so limited and these drawbacks can be overcome by the other modulation technique which is FSK.
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NOTES The binary ASK system was one of the earliest form of digital modulation used in wireless telegraphy. In a binary ASK system binary symbol 1 is represented by transmitting a sinusoidal carrier wave of fixed amplitude Ac and fixed frequency fc for the bit duration Tb whereas binary symbol 0 is represented by switching of the carrier for Tb seconds. This signal can be generated simply by turning the carrier of a sinusoidal oscillator ON and OFF for the prescribed periods indicated by the modulating pulse train. For this reason the scheme is also known as on-off shift testing.
The simplest method of modulating a carrier with a data stream is to change the amplitude of the carrier wave every time the data changes. This modulation technique is known as “AMPLITUDE SHIFT KEYING”. The simplest way of achieving amplitude shift keying is by switching on the carrier whenever the data bit is 1 & switching off whenever the data bit is ‘0’ This technique is known as ONOFF KEYING.
Note: ASK wave from is generated by balanced modulator circuit which is also known as a linear multiplier. In order to generate ASK wave from we apply the digital data stream and modulation input as a input to the linear multiplier. The method of demodulate the ASK wave from is to rectify it pass it through the filter & square up the resulting wave from, the output is the original data stream. Amplitude shift keying is less efficient because the noise inherent in the transmission channel can deteriorate the signal so much that the amplitude changes in the modulated carrier wave due to noise addition. This may lead to the incorrect decoding at the receiver. Hence THIS TECHNIQUE IS NOT WIDELY USED IN PRACTICAL application & it is however used in diverse areas and old emergency radio transmissions and fiber-optic communication.
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Definition: Amplitude Shift Keying (ASK) is a form of digital modulation technique in which the amplitude of the analog sinusoidal carrier signal is varied to represent the input digital data.
Both frequency and phase of the carrier signal remains constant while its amplitude changes in accordance with the levels of digital data.
vASK(t)=
Vc cos(2₶fct) 0
for binary 1 for binary 0
where Vc is the maximum amplitude of analog carrier signal in volts, and fc is the carrier frequency in Hz. • Thus, the modulated ASK signal is either the carrier signal (for binary 1) or no carrier signal (for binary 0). • That simply means that the carrier signal is either ON or OFF, depending on whether the digital information data is either binary 1 or 0, respectively. • This is the reason that sometimes amplitude-shift keying is also referred to as On-Off Keying (OOK).
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for every change in the input binary data stream, either from logic 1 to logic 0 or vice-versa, there is corresponding one change in the ASK signal waveform. or the entire duration of the input binary data logic 1 (high), the output ASK signal is a constant-amplitude, constant-frequency sinusoidal carrier signal. For the entire duration of the input binary data logic 0 (low), the output ASK signal is zero (no carrier signal).
In ASK, baud rate = bit rate In general, the transmission bandwidth for ASK signal is given by
Where r (typically 0 < r < 1) is related to the modulation process by which the signal is filtered to establish a bandwidth for transmission; and fb is the bit rate.
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Power spectral density (PSD) for ASK signal The ASK signal is basically the product of the binary digital data sequence and the sinusoidal carrier signal.
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Lets have an experiment for understanding ASK Generation and Detection
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Aim: To study the generation and detection of ASK Requirements:
Trainer/ Simulink/Matlab / Hardware
Amplitude Shift Keying (ASK) is a type of Amplitude Modulation which represents the binary data in the form of variations in the amplitude of a signal. Any modulated signal has a high frequency carrier. The binary signal when ASK modulated, gives a zero value for Low input, while it gives the carrier output for High input. The following figure represents ASK modulated waveform along with its input. ASK Modulator The ASK modulator block diagram comprises of the carrier signal generator, the binary sequence from the message signal and the band-limited filter. Following is the block diagram of the ASK Modulator.
Observations:
Draw Waveforms X= Y=
Conclusion:
1.If the bit rate of an ASK signal is 1200bps,what is the baud rate? 2.Is ASK highly susceptible? 3.What are the characteristics of transmission medium which effect speed of transmission in ASK? 4.Find the minimum band width for an ASK signal transmitting at 2000bps.The transmission made is half duplex? 5.If B.W is 5000Hz for an ASK signal, what are the baud rate? 6.What isthe advantage of ON-OFF keying ( ) in ASK? 7.Given the bandwidth of 10KHz( 1Hz to 1KHz), Find the band width for upper side & lower side band of carrier in full duplex ASK? 8.For the above problem, what are the carrier frequencies in upper & lower side bands?