1(a) In your own words and with the aid of a diagram, describe the concept of magnetic flux density. [2 marks]
1(b) Two bar magnets of 50mm height and 50mm width at the pole faces are fixed in position, with their respective N and S poles in close proximity. If the total flux is 0.1 webers, calculate the flux density between the two pole faces. Assume that fringing effects are negligible; that is, all of the flux passes between the pole faces. [2 marks]
1(c) Why is more economical to distribute electricity over long distances using AC rather than DC form? Please examine: http://en.wikipedia.org/wiki/War_of_Currents [3 marks]
1(d) Using a figure, describe the magnetic field effects that are created by a current flowing through a long straight conductor. In your own words and with the aid of a figure, describe the right-hand rule. Why is it important that data cables are not run alongside power cables?
One of the most common computer Ethernet cables is Cat5e UTP (i.e., unshielded twisted pair). The lack of shielding reduces the cost and allows the cable to be more flexible. However, research on what twisting the cable means and describe why it is performed.
[3 marks]
2(a) Several different diode configurations are illustrated in Figure 2.1, where the measured voltage is displayed at the anode and cathode terminals of the diode. Describe whether the configurations (i)–(vi) are forward or reverse biased, and in each case determine if the current i can flow in the direction that is indicated. The diode is a silicon diode that has a forward-biased junction potential of 0.7V.
2(b) Figure 2.2 illustrates two diode circuits. In each case, calculate the current flowing through the 10kΩ resistor and calculate the voltage drop across the germanium diode. Please assume that a germanium diode has a forward-biased junction potential of 0.3V.
2(c) Using the description of the operation of a n-p-n transistor on page 3-39 and page 340 of your notes, describe the operation of a p-n-p transistor in your own words.
3(a) If we take a two-input NAND gate and tie the inputs permanently together then the gate will behave like a different logic gate. Which gate will it behave like? Using only the same NAND gate, examine the truth table and determine how we could get a similar behaviour using a different wiring configuration. [3 marks]
3(b) Demonstrate using truth-tables that a two input AND gate with an inverter on the output is equivalent to a 2-input OR gate with an inverter on each input. Write the algebraic symbols to represent this relationship. [3 marks]
3(c) Using only AND, OR and NOT logic gates, determine a circuit that behaves in the same way as a 2-input XOR gate. Validate that it works correctly using a truth table.
[4 marks]
4(a) Show, using a truth table only, that C BA ABC
. Illustrate the two equivalent
circuits. What is this relationship called?
[7 marks]
4(b) Prove that B AB AA using only the remaining rules of Boolean algebra and the three additional laws, stating each step by referring to the rule number (i.e. this is Rule 11 on page 3-50 of the notes – use the other rules to prove rule 12). [3 marks]
Question 5 [15 marks]
5(a) An industrial robot has three sensors (A, B, C) that determine if it is working correctly. A digital system needs to be designed that uses a green light emitting diode (LED) to signal that the system is working correctly and an alarm that sounds if the LED is not lit. The industrial robot is functioning correctly when any of the following conditions arise:
• When sensor A is not active and sensor B is active, regardless of C. • When sensor A and sensor B are active at the same time, regardless of C. • When sensor A is not active and sensor B is not active, regardless of C.
Present a truth table for this system.
[3 marks]
5(b) Derive, using a Boolean algebra, a minimal sum-of-products function for this system. [4 marks]
5(c) Implement the circuit using logic gates.
[2 marks]
5(d) Using De-Morgan’s Theorem, convert the circuit output expression to a NAND gate only format and illustrate it (assume that only two-input NAND gates are available).
[6 marks]
6(a) In your own words, distinguish between the following types of memory: ROM, PROM, EEPROM and RAM. Give examples of where EEPROMs and RAM might be used in a home PC and other home devices/appliances. [6 marks]
6(b) In your own words, briefly explain the differences between: (i) Simplex, half-duplex and full-duplex channels (ii) Serial and parallel transmission
[4 marks]
7(a) In your own words, describe the concept behind and advantages/disadvantages of each of amplitude, frequency and phase modulation. Use a graph of a signal over time to help describe the differences. [6 marks]
7(b) A communication system employs a phase shift signalling technique that has four possible phase shifts. It has a signalling rate of 1200 baud. What is the information rate? If we upgrade the signalling technique to allow for eight possible phase shifts, what will the information rate be then? [4 marks]
8(a) Two mobile phone are 7.5 miles apart are linked by a 24-gauge twisted-pair cable. Using the graph on page 5-4 of your notes (note: in miles), what is the highest frequency signal that can be sent between them if the maximum acceptable attenuation between the two stations is 75dB? [2 marks]
8(b) If a 2.6/9.5 coaxial-pair tube (page 5-7 of your notes) is installed instead of the twisted-pair cable in 8(a), what will be the attenuation of a signal with the form s = 50sin(2×106 t) between each exchange? [5 marks]
8(c) In your own words, describe the skin-effect that occurs at high frequencies in twisted pair cables. If at 1MHz the skin effect causes the effective radius of the cable to be reduced to half its value, what effect would this have on the resistance of the cable, compared to its DC resistance value? [3 marks]