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Review Questions:
1. Define analog transmission.
It is a transmission method of conveying information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that information. It could be the transfer of an analog source signal, using an analog modulation method such as frequency modulation (FM) or amplitude modulation (AM), or no modulation at all.
2. Define carrier signal and its role in analog transmission.
In analog transmission, the sending device produces a high-frequency signal that acts as a base for the information signal. This base signal is called the carrier signal or carrier frequency. A carrier is a single-frequency signal that has one of its characteristics (amplitude, frequency, or phase) changed to represent the baseband signal.
3. Define digital-to-analog conversion.
The process of changing one of the characteristics of an analog signal based on the information in digital data is called digital-to-analog conversion. It is also called modulation of a digital signal. The baseband digital signal representing the digital data modulates the carrier to create a broadband analog signal.
4. Which characteristics of an analog signal are changed to represent the digital signal in
each of the following digital-to-analog conversion? a. ASK b. FSK c. PSK d. QAM
a) ASK changes the amplitude of the carrier. b) FSK changes the frequency of the carrier. c) PSK changes the phase of the carrier. d) QAM changes both the amplitude and the phase of the carrier.
5. Which of the four digital-to-analog conversion techniques (ASK, FSK, PSK or QAM)
is the most susceptible to noise? Defend your answer.
We can say that the most susceptible technique is ASK because the amplitude is more affected by noise than the phase or frequency.
6. Define constellation diagram and its role in analog transmission.
A constellation diagram can help us define the amplitude and phase of a signal element, particularly when we are using two carriers. The diagram is useful when we are dealing with multilevel ASK, PSK, or QAM. In a constellation diagram, a signal element type is represented as a dot. The bit or combination of bits it can carry is often written next to it. The diagram has two axes. The horizontal X axis is related to the in-phase carrier; the vertical Y axis is related to the quadrature carrier.
Exercises:
10. Calculate the baud rate for the given bit rate and type of modulation.
a. 2000 bps, FSK b. 4000 bps, ASK
c. 6000 bps, QPSK d. 36,000 bps, 64-QAM
a) r = log 2 2 = 1 โ S = (1/1) ร (2000 bps) = 2000 baud b) r = log 2 2 = 1 โ S = (1/1) ร (4000 bps) = 4000 baud c) r = log 2 4 = 2 โS = (1/2) ร (6000 bps) = 3000 baud d) r = log 2 64= 6 โ S = (1/6) ร (36,000 bps) = 6000 baud
11. Calculate the bit rate for the given baud rate and type of modulation.
a. 1000 baud, FSK b. 1000 baud, ASK
c. 1000 baud, BPSK d. 1000 baud, 16-QAM
a) r = log 2 2 = 1 โ N = (1) ร (1000 bps) = 1000 bps b) r = log 2 2 = 1 โ N = (1) ร (1000 bps) = 1000 bps c) r = log 2 2 = 1 โ N = (1) ร (1000 bps) = 1000 bps d) r = log 2 16 = 4 โ N = (4) ร (1000 bps) = 4000 bps
12. What is the number of bits per baud for the following techniques?
a. ASK with four different amplitudes b. FSK with 8 different frequencies
c. PSK with four different phases d. QAM with a constellation of 128 points.
We use the formula r = log 2 L to calculate the value of r for each case. a) Log 2 4 = 2 b) Log 2 8 = 3 c) Log 2 4 = 2 d) Log 2 128 = 7
Thank you
Preceded by:
Dr. Eng. Samy Elmokadem
