# Microwave Engineering Questions and Answers – Crystal Oscillators

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

1. One condition to be satisfied in an oscillator circuit so that stable oscillations are produced is:
a) positive feedback is to be achieved
b) negative feedback is to be achieved
c) 1800 phase shift is required between the transistor input and output.
d) none of the mentioned

Explanation: In an oscillator a total of 3600 of phase shift is to be achieved in the entire circuit to produce oscillations. The transistor used in the oscillator circuit must produce a phase shift of 1800 to achieve stable oscillations. Hence this condition has to be satisfied by the oscillator.

2. In an oscillator, the resonant feedback circuit must have must have a low Q in order to achieve stable oscillation.
a) true
b) false

Explanation: If the resonant feedback circuit has a high Q, so that there is random phase shift with frequency, the oscillator will have good frequency stability.

3. Quartz crystals are more efficient as a feedback network because:
a) less circuit complexity
b) cost effective
c) crystals operate at high voltage levels
d) LC circuits have unloaded Q of a few hundreds

Explanation: At frequencies below a few hundred MHz, where LC resonators seldom have unloaded Qs greater than a few hundred. Quartz crystals have unloaded Q of about 10000 and have a temperature drift of 0.001%/C0.

4. Quartz crystal and tourmaline used in oscillators work on the principle of:
a) photo electric effect
b) piezo electric effect
c) Raman effect

Explanation: Quartz crystals work on the principle of piezo electric effect. When electrical energy is applied to these crystals, they vibrate in a direction perpendicular to the application of energy producing oscillations.
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5. A quartz crystals equivalent circuit is a series LCR circuit and has a series resonant frequency.
a) true
b) false

Explanation: A quartz crystal has an equivalent circuit such that a series LCR network is in parallel with a capacitor. A quartz crystal thus has both series and parallel resonant frequencies.

6. Quartz crystal is used in the _______region, where the operating point of the crystal is fixed.
a) resistive
b) inductive reactance
c) capacitive reactance
d) none of the mentioned

Explanation: Quartz crystal is always operated in the inductive reactance region so that the crystal is used in place of an inductor in a Colpitts or pierce oscillator.

7. In the plot of reactance v/s frequency of a crystal oscillator, the reactance between series resonant frequency and parallel resonant frequency is:
a) capacitive
b) inductive
c) both capacitive and inductive
d) none of the mentioned

Explanation: In the plot of reactance v/s frequency of a crystal oscillator, the reactance between series resonant frequency and parallel resonant frequency is inductive. In this region between the series and parallel and series resonant frequencies, the operating point of the crystal is fixed and hence can be used as part of other circuits.

8. In the equivalent circuit of a quartz crystal, LCR arm has an inductance of 4 mH and capacitor has a value of 4nF, then the series resonant frequency of the oscillator is:
a) 0.25 MHz
b) 2.5 MHz
c) 25 MHz
d) 5 MHz

Explanation: The series resonant frequency of a crystal oscillator is given by 1/√LC. Substituting the given values of L and C in the expression, the series resonant frequency is 0.25 MHz.

9. Parallel resonant frequency of quartz crystal is given by:
a) 1/ √(LCₒC/(Cₒ+C))
b) 1/√LC
c) 1/√LCₒ
d) 1/ √(L(Cₒ+C) )

Explanation: Parallel resonant frequency of an oscillator is given by√(LCₒC/(Cₒ+C)). Here L and C are the inductance and capacitance in the LCR arm of the equivalent circuit of the crystal. Co is the capacitance existing in parallel to this LCR arm.

10. The equivalent circuit of a quartz crystal has LCR arm capacitance of 12nF and inductance of 3mH and parallel arm capacitance of 4nF. Parallel resonant frequency for the circuit is:
a) 3 MHz
b) 0.3 MHz
c) 6 MHz
d) 9 MHz

Explanation: The parallel resonant frequency of a crystal oscillator is given by 1/√(LCₒC/(Cₒ+C)). Substituting the given values in the equation, the parallel resonant frequency is found to be 3 MHz.

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