# Microwave Engineering Questions and Answers – RF Oscillators

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This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “RF Oscillators”.

1. _________ is a non linear circuit that converts DC power to an AC waveform of desired frequency based on the oscillator design.
a) Attenuator
b) Amplifier
c) Oscillator
d) None of the mentioned

Explanation: Oscillator is a non linear circuit that converts DC power to an AC waveform. Most RC oscillators provide sinusoidal outputs, which minimizes undesired harmonics and noise sidebands.

2. The transfer function of an RF oscillator is given by:
a) A/ (1-AH (ω))
b) A/ (1+AH (ω))
c) A/ (-1+AH (ω))
d) 1/ (1-AH (ω))

Explanation: Transfer function of an RF oscillator is given by A/ (1-AH (ω)). Here, A is the gain of the transistor multiplier used. H(ω) is the function representing the feedback network. In an oscillator, positive feedback is used.

3. The criterion on which oscillations are produced in the oscillator circuit is called:
a) Shannon’s criteria
b) Barkhausen criteria
c) Colpitts criteria
d) None of the mentioned

Explanation: When the condition 1-AH (ω) =0 occurs, it is possible to achieve non zero output voltage for zero input voltage, thus forming an oscillator. This is called Barkhausen criteria.
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4. The necessary condition for oscillation in a Colpitts oscillator is:
a) C2/C1=gm/Gi
b) C1/C2=gm/Gi
c) C2/C1= gm*Gi
d) None of the mentioned

Explanation: The condition for sustained oscillation in a Colpitts oscillator is C2/C1 = gm/Gi. Here C1 and C2 are the capacitance in the feedback network, gm is the transconductance of the transistor and Gi is the input admittance.

5. Colpitts oscillator operating at 50 MHz has an inductor in the feedback section of value 0.10µH. then the values of the capacitors in the feedback section is:
a) 100 pF, 100 pF
b) 100 pF, 50 pF
c) 70 pF, 130 pF
d) 80 pF, 60 pF

Explanation: The equivalent value of series combination of the capacitors is given by 1/ω2L. This gives the equivalent capacitance value of 200 pF. C1C2/ (C1+C2) =200 pF. C1 and C2 values can be chosen in several ways. One of the way is C1=C2=100 pF.

6. The necessary condition for oscillation in a Hartley oscillator is:
a) L1/L2 = gm/Gi
b) L1/L2 =Gi /gm
c) L2L/L1 = gm/Gi
d) None of the mentioned

Explanation: Necessary condition for oscillation in a Hartley oscillator is L1/L2 = gm/Gi. Here, L1 and L2 are the inductances in the feedback network and gm is the transconductance of the transistor and Gi is the input admittance.

7. An inductor is operating at frequency of 50 MHz. Its inductance is 0.1 µH, and then the series resistance associated with the inductor is: (Qo=100)
a) 0.31 Ω
b) 1.32 Ω
c) 1 Ω
d) 1.561 Ω

Explanation: Series resistance associated with an inductor is given by ωL/Qₒ. Substituting in this equation, the series of an inductor is given by 0.31.

8. Hartley oscillator has inductance values of 12 mH and 4 mH in the feedback section and a capacitor of 4 nF. Then the resonant frequency of the circuit is:
a) 19.89 kHz
b) 25 kHz
c) 45 kHz
d) 12 kHz

Explanation: Resonant frequency of Hartley oscillator is given by 1/ 2π√(C1 (L1 + L2)). Substituting the given values in the above equation, cut-off frequency is 19.89 kHz.

9. Colpitts oscillator in the feedback section has an inductance of 4 mH and capacitors of 12 nH and 4 nH. Then the resonant frequency of Colpitts oscillator is:
a) 50.4 kHz
b) 35.1 kHz
c) 45.9 kHz
d) None of the mentioned

Explanation: Resonant frequency of Colpitts oscillator is given by 1/2π√LCₒ, where C0 is the equivalent capacitance given by C1C2/ (C1+C2). Substituting and solving the equation, resonant frequency is 45.9 kHz.

10. For Colpitts oscillator, the capacitors C1 and C2 in the feedback network are 1 µF and 25 µF respectively. Then the β value of the transistor is:
a) 35
b) 000.76
c) 25
d) 0.0025

Explanation: β for a transistor is defined as the ratio of transconductance of the transistor to the input admittance, which is equal to the ratio of C2/C1. Substituting the given values, β of the transistor is 25.

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