250+ TOP MCQs on Phase Detector / Comparator & Answers

Linear Integrated Circuit Multiple Choice Questions on ” Phase Detector / Comparator “.

1. Analog phase detector is often referred as
A. Full wave detector
B. Half wave detector
C. Rectifier wave detector
D. None of the mentioned
Answer: B
Clarification: Analog phase detector is called as half wave detector because, the phase information for only one-half of the input waveform is detected and averaged.

2. What happens when VCO output is 90^o out of phase with respect to input signal?
A. Perfect lock
B. Attenuation
C. Shift in phase of comparator
D. Error signal is removed
Answer: A
Clarification: The error voltage is zero when the phase shift between the two inputs is 90^o. So, for the perfect lock, the VCO output should be 90^o out of phase with respect to the input signal.

3. Find the error voltage of phase comparator whose input signal is V_s= V_ssin(2πf_st) and the output signal V_o= V_osin(2πf_ot+φ).
A. V_e=[k×(V_s/2)]×[cos(-φ)-cos(2πf_ot+φ)].
B. V_e=[k×V_s×(V_o/2)]×[cos(-φ)-cos(2πf_ot+φ)].
C. V_e=[k×V_s×(V_o/2)]×[cos(-φ)+cos(2πf_ot+φ)].
D. V_e=[k×V_s×V_o]×[cos(-φ)-cos(2πf_ot+φ)].
Answer: B
Clarification: A phase comparator is basically a multiplier which multiplies the input signal by the VCO signal. Thus, the phase comparator output = V_s×V_o = V_s×V_osin(2πf_st)× sin(2πf_ot+φ) =k×V_s×V_o ×sin(2πf_st)×sin(2πf_ot+φ)
Where k – phase comparator gain.= k×V_s×V_o/2[cos(-φ)-cos(2πf_ot+φ)]
When at lock, f_s =f_o
=> V_e = V_e={k×V_s× V_o/2}x[cos(-φ)-cos(2πf_ot+φ)].

4. How to overcome the problem associated with switch type phase detective?
A. Increase loop gain depending on input signal
B. Phase shift is made linear
C. Limit the amplifier of input signal
D. All of the mentioned
Answer: C
Clarification: The problems can be eliminated by limiting the amplifier of the input signal that is converting the input to a constant amplified square wave. This can be achieved by using a balanced modulator used as full wave switching phase detector.

5. If the average error voltage & the phase shift are given as 6.2v & π/4.Determine the phase angle to voltage transfer coefficient of full wave switching phase detector.
A. -0.19
B. -0.09
C. -0.03
D. -0.13
Answer: D
Clarification: The phase angle to voltage transfer coefficient
=>kφ = (φ-π/2)/ V_{e (avg)} =[π/4-π/2]/6.2 =[π-2π/4]/6.2 = -(π/4)/6.2 = -π/24.8
=>kφ = -0.13.

6. When does a digital phase detector can be used, where f_o->output frequency, f_s->input frequency.
A. Both f_o & f_s signals should be square wave
B. f_o should be square wave & f_s can be any non-sinusoidal wave
C. f_s should be square wave & f_o can be any non-sinusoidal wave
D. Both f_o & f_s can be any non-sinusoidal wave
Answer: A
Clarification: The XOR gate produces high output when only one of the input signal f_o or f_s is high. So, to detect high or low value of waveform, square waves are used.

7. The maximum dc output voltage in digital phase detector occurs
A. When the phase difference is π/2
B. When the phase difference is π
C. When the phase difference is 3π/4
D. When the phase difference is 2π
Answer: B
Clarification: The maximum dc output voltage occurs when the phase difference is π, because the output of the XOR gate phase detector remains high throughout.

8. Given the DC output voltage versus phase difference φ curve. Find the conversion gain values.

A. 15.7V/rad
B. 1.26V/rad
C. 1.59V/rad
D. 0.8V/rad
Answer: C
Clarification: The slope gives the conversion gain. Therefore, conversion gain=V_cc/φ =5V/π =1.59V/rad.

9. Which among the following has better capture tracking & locking characteristics?
A. XOR phase detector
B. Edge triggered phase detector
C. Analog phase detector
D. All of the mentioned
Answer: B
Clarification: Edge triggered phase detector has better capture tracking and locking characteristics as the dc output voltage is linear upto 360^o compared to other detectors.