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

1. The stability of an oscillator is enhanced with the use of:

a) high Q tuning network

b) passive elements

c) appropriate feedback methods

d) none of the mentioned

View Answer

Explanation: The stability of an oscillator is enhanced with the use of high Q tuning circuits. At microwave frequencies, lumped elements cannot provide high Q factor. Waveguide tuning circuits cannot be integrated with small microwave circuits easily. Hence dielectric resonators are used to provide high Q factor.

2. In oscillator tuning circuits, dielectric resonators are preferred over waveguide resonators because:

a) they have high Q factor

b) compact size

c) they are easily integrated with microwave integrated circuits

d) all of the mentioned

View Answer

Explanation: Dielectric resonators have all the above properties mentioned. Also, they are made of ceramic materials which also increases the temperature stability of resonator which further increases the fields of application.

3. A dielectric resonator coupled with an oscillator operates in:

a) TE_{10δ}

b) TE_{01δ}

c) TM_{10δ}

d) TM_{01δ}

View Answer

Explanation: A dielectric resonator is coupled to an oscillator by positioning it in close proximity to a microstrip line. The resonator operates in TE

_{10δ}mode, and couples to the fringing field magnetic field of the microstrip line.

4. A dielectric resonator is modeled as __________ when it is used as a tuning circuit with a oscillator.

a) series RLC circuit

b) parallel RLC circuit

c) LC circuit

d) tank circuit

View Answer

Explanation: The strength of coupling between a microstrip line and the resonator is determined by the spacing between them. The resonator appears as a series load on the microstrip line. The resonator is modeled a s a parallel RLC circuit and the coupling to the feed line is modeled by the turns ratio of the transformer.

5. The coupling factor between the resonator and the microstrip line is the ratio of external Q to the unloaded Q.

a) true

b) false

View Answer

Explanation: The coupling factor the resonator and the microstrip line is the ratio of unloaded Q to external Q. The simplified expression for coupling factor is N

^{2}R/2Z

_{0}.

6. If the reflection coefficient seen on the terminated microstrip line looking towards the resonator is 0.5, then the coupling coefficient is:

a) 0.5

b) 0.25

c) 0.234

d) 1

View Answer

Explanation: Coupling coefficient for a line in terms of reflection coefficient is Г/ (1-Г). Substituting the given reflection coefficient in this expression, the coupling coefficient is 1.

7. A dielectric resonator can be incorporated into a circuit to provide _________ using either parallel or series arrangement.

a) frequency stability

b) oscillations

c) high gain

d) optimized reflection coefficient

View Answer

Explanation: Oscillators can be obtained in various transistor configurations, their instability can be enhanced by using series or shunt elements. A dielectric resonator can be incorporated into a circuit to provide frequency stability using either parallel or series arrangement.

8. It is desired to design a frequency oscillator at 2.4 GHz and the reflection coefficient desired is 0.6, then the coupling coefficient between the feed line and the dielectric resonator is:

a) 1.5

b) 1

c) 0.5

d) none of the mentioned

View Answer

Explanation: Coupling coefficient for a line in terms of reflection coefficient is Г/ (1-Г). Substituting the given reflection coefficient in this expression, the coupling coefficient is 1.5.

9. If the reflection coefficient between the feed line and the resonator is -0.6, then the equivalent impedance of the resonator at resonance given that the characteristic impedance of the microstrip line is:

a) 50 Ω

b) 12.5 Ω

c) 25 Ω

d) none of the mentioned

View Answer

Explanation: The equivalent impedance of the resonator at resonance is Z

_{0}(1 + Г)/(1- Г). Substituting the given values in this expression, equivalent impedance of the resonator is 12.5 Ω.

10. If the equivalent impedance of the resonator at resonance is 12.5 Ω and the characteristic impedance of the feed line is 50 Ω, then the coupling coefficient is:

a) 0.25

b) 0.5

c) 0.75

d) 1

View Answer

Explanation: Coupling coefficient is defined as the ratio of the equivalent impedance of the resonator to the characteristic impedance of the feed line. Substituting accordingly, coupling coefficient is 0.25.

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