University of the East College of Engineering
ECE Department
Amplitude Shift Keying (ASK)
Experiment No. 1 ECN 422 – 1ECT 7:30 – 10:30AM / ECELAB2
Name: FAJARDO, Shiela Monique A. Student Number: 20101115224
Date Performed: November 19, 2013 Grade
Date Submitted: November 26, 2013
Introduction
Amplitude shift keying (ASK) in the context of digital communications is a modulation process which imparts to a sinusoid two or more discrete amplitude levels (also called on-off keying or OOK). It is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave.
For a binary message sequence there are two levels, one of which is typically zero. Thus the modulated waveform consists of bursts of a sinusoid. Figure 1 illustrates a binary ASK signal (lower), together with the binary sequence which initiated it (upper). Neither signal has been band-limited.
Figure 1.1 An ASK signal (below) and the message signal (above)
Any digital modulation scheme uses a finite number of distinct signals to represent digital data. ASK uses a finite number of amplitudes, each assigned a unique pattern of binary digits. Usually, each amplitude encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular amplitude. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the amplitude of the received signal and maps it back to the symbol it represents, thus recovering the original data. Frequency and phase of the carrier are kept constant.
Like AM, ASK is also linear and sensitive to atmospheric noise, distortions, propagation conditions on different routes in PSTN, etc. Both ASK modulation and demodulation processes are relatively inexpensive. The ASK technique is also commonly used to transmit digital data over optical fiber. For LED transmitters, binary 1 is represented by a short pulse of light and binary 0 by the absence of light. Laser transmitters normally have a fixed "bias" current that causes the device to emit a low light level. This low level represents binary 0, while a higher-amplitude light wave represents binary 1.
The simplest and most common form of ASK operates as a switch, using the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero. This type of modulation is called on-off keying, and is used at radio frequencies to transmit Morse code (referred to as continuous wave operation).
Answers to Questions
Question 1: What is the relationship between the digital signal and the presence of the carrier in the ASK signal?
ASK signal is time-coincident with the digital signal. When the digital signal is logic 0, the ASK signal has a signal voltage of 0V.
Question 2: What is the ASK signal’s voltage when the digital signal is logic 0? The ASK signal’s voltage is 0V when the digital signal is logic 0.
Question 3: What feature of the ASK signal suggests that it is an AM signal?
The presence of the ASK signal’s upper and lower limits (or envelopes) suggests that it is an AM signal.
Question 4: Why is the recovered digital signal not a perfect copy of the original signal? The internal noise affects the recovery of the original digital signal.
Question 5: What can be used to “clean-up” the recovered digital signal? A comparator circuit can be used to clean up the recovered digital signal.
Question 6: How does the comparator turn the slow rising voltages of the recovered digital signal into sharp transition?
Experimental Discussion
Part A deals with generating an ASK signal. This was done with the use of the dual analog switch module.
Figure 1.2 Digital signal (red) and the generated ASK signal (yellow)
After generating the ASK signal, the digital signal was then overlaid to the ASK signal’s envelope.
Figure 1.3 Digital signal overlaid with ASK signal’s envelope
Part B deals with demodulating an ASK signal by Envelope Detector method. An Envelope Detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the envelope of the original signal.
Figure 1.4 Demodulated signal using Envelope Detector
Part C deals with restoring the digital signal using a comparator. A comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals V+ and V- and one binary digital output VO. The output is ideally
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The extremely large open-loop gain of this circuit makes it an extremely sensitive device for comparing its input with zero.
Figure 1.5 Recovered signal using Comparator
Another part of the experiment is about noise. Noise is a random, undesirable electric energy that enters the communication system via the communicating medium and interferes with the transmitted message.
Figure 1.6 Noise (20dB input) added to the ASK signal
Figure 1.7 Noise (6dB input) added to the ASK signal
Figure 1.8 The Original signal (red) and the Recovered signal (yellow)
In the end, the original signal could not be recovered because of the noise present on the channel.
Conclusion
Amplitude Shift Keying is a modulation scheme used for multiplexing digital data. This is also called On-Off Keying (OOK). From the term “On-Off”, this means that the ASK signal’s voltage is zero (0) when the digital signal’s logic level is zero (0). This was seen in Figure 2.
Amplitude Shift Keying is a kind of Amplitude Modulation (AM), only difference is that the message signal is in digital form and not in analog. This could be concluded because of the presence of the upper and lower limits (envelopes) of the ASK signal are present. The ASK signal’s upper and lower envelopes are of the same shape as the data stream.
A comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. The extremely large open-loop gain of this circuit makes it an extremely sensitive device for comparing its input with zero. That is why the comparator can turn slow rising voltages of the recovered digital signal into sharp transition.
Like AM, ASK is also linear and sensitive to atmospheric noise and distortions. Noise affects the recovery of the original signal which results to the recovered signal not being a perfect copy of the original signal. When noise is extremely high, it may be impossible to recover the data. This was seen in Figure 8.
Glossary
Amplitude Shift Keying (ASK) - in the context of digital communications is a modulation process, which imparts to a sinusoid two or more discrete amplitude levels.
Binary – represents numeric values using two symbols: typically 0 and 1.
Comparator – a device that compares two voltages or currents and outputs a digital signal indicating which is larger.
Envelope Detector – an electronic circuit that takes a high-frequency signal as input and provides an output which is the envelope of the original signal.
Noise – a random, undesirable electric energy that enters the communication system via the communicating medium and interferes with the transmitted message.
References
Envelope Detector. Retrieved November 25, 2013 from http://en.wikipedia.org/wiki/Envelope_detector
Comparator. Retrieved November 25, 2013 from
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar8.html
