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

1. In microwave oscillators, negative resistance transistors and diodes are used in order to generate oscillations in the circuit.

a) True

b) False

View Answer

Explanation: In microwave oscillator, for a current to flow in the circuit the negative impedance of the device must be matched with positive impedance. This results in current being non-zero and generates oscillation.

2. Any device with negative impedance as its characteristic property can be called:

a) Energy source

b) Energy sink

c) Oscillator

d) None of the mentioned

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Explanation: A positive resistance implies energy dissipation while a negative resistance implies an energy source. The negative resistance device used in the microwave oscillator, thus acts as a source. The condition X

_{in}+ X

_{L}=0 controls the frequency of oscillation. X

_{in}is the impedance of the negative resistance device.

3. In a microwave oscillator, a load of 50+50j is connected across a negative resistance device of impedance -50-50j. Steady state oscillation is not achieved in the oscillator.

a) True

b) False

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Explanation: The condition for steady state oscillation in a microwave oscillator is Z

_{in}=-Z

_{L}. Since this condition is satisfied in the above case, steady state oscillation is achieved.

4. For achieving steady state oscillation, the condition to be satisfied in terms of reflection coefficients is:

a) Г_{in}=Г_{L}

b) Г_{in}=-Г_{L}

c) Г_{in}=1/Г_{L}

d) None of the mentioned

View Answer

Explanation: The condition for steady state oscillation to be achieved in terms of reflection coefficient is Г

_{in}=1/Г

_{L}. Here Г

_{in}is the reflection coefficient towards the reflection coefficient device and Г

_{L}is the reflection coefficient towards the load.

5. A one port oscillator uses a negative resistance diode having Г_{in}=0.9575+j0.8034 (Z0=50Ω) at its desired frequency point. Then the input impedance of the diode is:

a) -44+j123

b) 50+j100

c) -44+j145

d) None of the mentioned

View Answer

Explanation: The input impedance of the diode given reflection coefficient and characteristic impedance is Z0 (1+Г

_{in})/ (1-Г

_{in}). Substituting in the given equation, the input impedance is -44 +j123 Ω.

6. If the input impedance of a diode used in the microwave oscillator is 45-j23 Ω, then the load impedance is to achieve stable oscillation is:

a) 45-j23 Ω

b) -45+j23 Ω

c) 50 Ω

d) 23-j45 Ω

View Answer

Explanation: The condition for stabilized oscillation is Z

_{in}=-Z

_{L}. According to this equation, the load impedance required for stabilized oscillation is – (45-j23) Ω. The load impedance is thus -45+j23 Ω.

7. To achieve stable oscillation, Z_{in} + Z_{L}=0 is the only necessary and sufficient condition to be satisfied by the microwave oscillator.

a) True

b) False

View Answer

Explanation: The condition Z

_{in}+ Z

_{L}=0 is only a necessary condition for stable oscillation and not sufficient. Stability requires that any perturbation in current or frequency is damped out, allowing the oscillator to return to its original state.

8. In transistor oscillators, the requirement of a negative resistance device is satisfied using a varactor diode.

a) True

b) False

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Explanation: In a transistor oscillator, a negative resistance one port network is created by terminating a potentially unstable transistor with impedance designed to drive the device in an unstable region.

9. In transistor oscillators, FET and BJT are used. Instability is achieved by:

a) Giving a negative feedback

b) Giving a positive feedback

c) Using a tank circuit

d) None of the mentioned

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Explanation: Oscillators require a device that has high instability. To achieve this condition, transistors are used with a positive feedback to increase instability.

10. In a transistor amplifier, if the input impedance is -84-j1.9 Ω, then the terminating impedance required to create enough instability is:

a) -84-j1.9 Ω

b) 28+j1.9 Ω

c) – (28+j1.9) Ω

d) None of the mentioned

View Answer

Explanation: Relation between terminating impedance and input impedance is Z

_{s}=-R

_{in}/3. Z

_{s}is the terminating impedance. Substituting in the given equation, the terminated impedance is 28+j1.9 Ω.

**Sanfoundry Global Education & Learning Series – Microwave Engineering.**

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