Avionics Multiple Choice Questions on “Tuned Circuits”.
1. What is reactance?
a) Resistance offered to ac current by resistor only
b) Resistance offered to dc current by resistor
c) Resistance offered to ac current by coil and capacitance
d) Resistance offered to dc current by coil and capacitance
Answer: c
Clarification: Both coils and capacitors offer an opposition to alternating current flow known as reactance, which is expressed in ohms. Like resistance, reactance is an opposition that directly affects the amount of current in a circuit.
2. What happens to the current when it passes through a capacitor?
a) Phase of voltage leads the current
b) Phase of current leads the voltage
c) Phase of current and voltage cancel out each other
d) Phase of current and voltage gets added
Answer: b
Clarification: In addition to resistance of alternating current flow, a capacitor alters the phase difference between the current and voltage. The capacitor lowers the phase of the voltage and hence the current leads the voltage.
3. What happens to the voltage when it passes through an inductor/coil?
a) Phase of voltage leads the current
b) Phase of current leads the voltage
c) Phase of current and voltage cancel out each other
d) Phase of current and voltage gets added
Answer: a
Clarification: In addition to a resistance of alternating current flow, an inductor/coil alters the phase difference between the current and voltage. The inductor/coil lowers the phase of the current and hence the voltage leads the current.
4. A capacitor used in an ac circuit continually charges and discharges.
a) True
b) False
Answer: a
Clarification: In an ac power supply the voltage increases and decreases with a particular frequency. During the first half cycle, the capacitor gets charged and in the other half, it gets discharged. The same process repeats throughout the wave.
5. What happens to capacitative reactance when operating frequency is increased?
a) Increases
b) Decreases
c) Remains constant
d) goes to infinite
Answer: b
Clarification: Capacitative reactance is inversely proportional to the operating frequency and is given by the formula Xc= 1/2πfC where C is the capacitance value in Farad.
6. Calculate the reactance of a 100-pF capacitor at an operating frequency of 2 MHz?
a) 200Ω
b) 700.16Ω
c) 796.2Ω
d) 2Ω
Answer:b
Clarification: Capacitative reactance =Xc= 1/2πfC = 1/(2 x 3.14 x 2 x 106 x 100 x 10-12) = 796.2Ω.
7. Which one of the following is false with respect to stray capacitance?
a) Reduces with decrease in size of lead wires
b) Reduces when chip capacitors are used
c) Increases when lead wires a are lengthy
d) Increases with less capacitance value
Answer: d
Clarification: The lead wires of capacitors behave as a complex RLC circuit and generate stray capacitance. This can be reduced by shortening the length of the lead wires or using chip capacitors. However, decreasing or increasing the value of capacitance does not affect stray capacitance.
8. What happens to inductive resistance when operating frequency is increased?
a) Increases
b) Decreases
c) Remains constant
d) Goes to infinite
Answer: a
Clarification: Inductive reactance is directly proportional to the operating frequency and is given by the formula Xf= 2πfL where L is the inductance value in Henry.
9. What is the inductive reactance of a 40-μH coil at 18 MHz?
a) 4000Ω
b) 4522Ω
c) 4522H
d) 0
Answer: b
Clarification: Inductive reactance = XL=2πfL = 2 x 3.14 x 18 x 106 x 40 x 10-6 = 4522Ω.
10. What is the Q factor for a 3-μH inductor with a total resistance of 45Ω at 90 MHz?
a) 37.68
b) 37.68Ω
c) 29.22Ω
d) 32.75Ω
Answer: a
Clarification: Quality factor = Q = 2πfL/R = (2 x 3.14 x 90 x 106 x 3 x 10-6)/45 = 1695.6 / 45 = 37.68.
11. What is the resonant frequency of a 2.7-pF capacitor and a 33-nH inductor?
a) 512MHz
b) 720GHz
c) 250MHz
d) 533MHz
12. What is the bandwidth of a resonant circuit with a frequency of 28 MHz and a Q of 80?
a) 350KHz
b) 30KHz
c) 42.7KHz
d) 57.05MHz
Answer: a
Clarification: Bandwidth = resonant frequency / Quality factor = 28 x 106 /80 = 350KHz.