# Linear Integrated Circuit Questions and Answers – Basic Principles of Phase-Locked Loops

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This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “Basic Principles of Phase-Locked Loops “.

1. Free running multivibrator is also called as
a) Stable multivibrator
b) Voltage control oscillator
c) Square wave oscillator
d) Pulse stretcher

Explanation: Free running multivibrator operates at a frequency which is determined by an external tuning capacitor and a resistor. On applying a dc control voltage the frequency can be shifted on either sides. This frequency deviation is directly proportional to the dc control voltage and hence it is called as ‘voltage controlled oscillator’.

2. The output voltage of phase detector is
a) Phase voltage
b) Free running voltage
c) Error voltage
d) None of the mentioned

Explanation: The phase detector compares the input frequency with the feedback frequency and produces output dc voltage called as error voltage.

3. At which state the phase-locked loop tracks any change in input frequency?
a) Free running state
b) Capture state
c) Phase locked state
d) All of the mentioned

Explanation: In the phase-locked, the output frequency is exactly same as the input signal frequency. So the circuit tracks any change in the input frequency through its repetitive action.
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4. Match the list I with list II which represents the three stages of phase locked loop.(PLL)

 List I List II 1.Before input frequency applied i. PLL-Phase locked state 2.When the input frequency applied ii.PLL=Free running state 3.After input frequency applied iii. PLL-Capture mode

a) 1-ii, 2-iii, 3-i
b) 1-iii, 2-ii, 3-i
c) 1-i, 2-ii, 3-iii
d) 1-ii, 2-i, 3-iii

Explanation: Before the input is applied, the PLL is in a free running state. Once the input frequency is applied, the VCO frequency start to change & PLL is said to be in capture mode. When the VCO frequency continues to change until it is equal to the input frequency, the PLL is said to be in phase locked state.

5. What is the function of low pass filter in phase-locked loop?
a) Improves low frequency noise
b) Removes high frequency noise
c) Tracks the voltage changes
d) Changes the input frequency

Explanation: The output voltage of a phase detector is a dc voltage and is often referred to as error voltage. This output is applied to the low pass filter which removes the high frequency noise and produces a dc level.

6. What is the need to generate corrective control voltage?
a) To maintain the lock
b) To track the frequency change
c) To shift the VCO frequency
d) All of the mentioned

Explanation: The output frequency(fo) of VCO is identical to input frequency(fs) except for a finite phase difference(φ), which generates a corrective control voltage to shift VCO frequency from fo to fs, thereby maintains the lock once locked and PLL tracks the frequency changes of the input signal.

7. At what range the PLL can maintain the lock in the circuit?
a) Lock in range
b) Input range
c) Feedback loop range
d) None of the mentioned

Explanation: The change in frequency of the incoming signal can be tracked when the PLL is locked. So, the range of frequencies over which PLL maintains the lock with the incoming signal is called as the lock in range.

8. The pull-in time depends on
a) Initial phase and frequency difference between two sign
b) Overall loop gain
c) Loop filter characteristics
d) All of the mentioned

Explanation: The pull-in time depends on the above mentioned characteristics to establish lock in the PLL circuit.