Voltage Controlled Oscillator - Linear IC Questions and Answers

This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “Voltage Controlled Oscillator”.

1. Which device is used for diagnostic purposes and for recording?
a) Low pass filter
b) Monolithic PLL
c) Voltage Controlled Oscillator
d) None of the mentioned
View Answer

Answer: c
Explanation: A Voltage Controlled Oscillator (VCO) is used for converting low frequency signals such as EEGs, EKG into an audio frequency range. These audio signals can be transmitted over two way radio communication systems for diagnostic purposes or can be recorded on a magnetic tape for further reference.

2. If the output of the Schmitt trigger is given below. Estimate the output at the pin 3 of VCO.

a)

b)

c)

d)

View Answer

Answer: a
Explanation: In VCO, the output of Schmitt trigger is fed to the input of inverter. Therefore, the output at pin3 would be an inverted output. As the input is a square wave, the output obtained will be an inverted square wave.

3. Write the equation for time period of VCO?
a) (2×V_cc×C_T)/i
b) (V_cc C_T)/(2×i)
c) (V_cc×C_T×i)/2
d) (2×V_cc)/(i×C_T)
View Answer

Answer: b
Explanation: The time period of VCO is given as T=2×△t =(2×0.25×V_cc ×C_T)/i =(0.5 V×_cc×C_T)/i = (V_cc×C_T)/(2×i).

4. Determine the value of current flow in VCO, when the NE566 VCO external timing resistor R_T =250Ω and the modulating input voltage V_c=3.25V.(Assume V_cc=+5v).
a) 3mA
b) 12mA
c) 7mA
d) 10mA
View Answer

Answer: c
Explanation: Current flowing in VCO, i =(V_cc– V_c)/ R_T = (5V-3.25V)/250 = 1.75/250
=>i =7mA.

5. From the circuit given, find the value of output frequency?

a) 178.484 Hz
b) 104.84 Hz
c) 145.84 Hz
d) 110.88 Hz
View Answer

Answer: b
Explanation: Output frequency, f_o =[2×(V_cc– V_c) ]/(C_T×R_T×V_cc )= [2x(8-1.5)]/(0.47µFx33kΩx8v) =13/0.124
=> f_o=104.84 Hz.

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6. The output frequency of the VCO can be changed by changing
a) External tuning resistor
b) External tuning capacitor
c) Modulating input voltage
d) All of the mentioned
View Answer

Answer: d
Explanation: The output frequency of VCO, f_o = [2×(V_cc– V_c)]/(C_T×R_T×V_cc).
From the equation, it is clear that the f_o is inversely proportional to C_T & R_T and directly proportional to V_c.Therefore, the output frequency can be changed by changing either voltage control, C_T or R_T.

7. Calculate the value of external timing capacitor, if no modulating input signal is applied to VCO. Consider f_o=25 kHz and R_T=5 kΩ.
a) 6nF
b) 100µF
c) 2nF
d) 10nF
View Answer

Answer: c
Explanation: When modulating input signal is not applied to VCO, the output frequency becomes f_o=1/(4×R_T×C_T)
=> C_T =1/(4×R_T×f_o) =1/(4×5kΩ×25kHz) = 2×10^-9 =2nF.

8. What is the advantage of using filter?
a) High noise immunity
b) Reduce the bandwidth of PLL
c) Provides dynamic range of frequencies
d) None of the mentioned
View Answer

Answer: a
Explanation: The charge on the filter capacitor gives a short time memory to the PLL. So, even if the signal becomes less than the noise for a few cycles, the dc voltage on the capacitor continues to shift the frequency of VCO, till it picks up the signal again. This produces high noise immunity.

9. Which filter is used in VCO?
a)

b)

c)

d) All of the mentioned
View Answer

Answer: d
Explanation: The loop filter used in the VCO can be one of the three types of filter shown above.

10. Choose the VCO for attaining higher output frequency.
a) NE566
b) SE566
c) MC4024
d) All of the mentioned
View Answer

Answer: c
Explanation: MC4024 is used for attaining high output frequency, because the maximum output frequency of NE566 and SE566 is 500kHz.

11. Voltage to frequency conversion factor for VCO is
a) K_v = △V_c/ △f_o
b) K_v = △f_o/△V_c
c) K_v = △f_o × △V_c
d) K_v = 1/(△f_o×△V_c)
View Answer

Answer: b
Explanation: The voltage to frequency conversion factor is defined as the change in frequency to the change in modulating input voltage.
=> K_v=△f_o/△V_c.

12. Calculate the voltage to frequency conversion factor, where f_o=155Hz and V_cc=10V.
a) 130
b) 124
c) 134
d) 116
View Answer

Answer: b
Explanation: The voltage to frequency conversion factor, K_v = △f_o/△V_cc= 8×f_o/V_cc = (8×155)/10=124.

13. Find the equation for change in frequency of VCO?
a) △f_o = (2×△V_c)/(R_T×C_T×V_cc)
b) △f_o = △V_c/(4×R_T×C_T×V_cc)
c) △f_o = △V_c/(2×R_T×C_T×V_cc)
d) △f_o = (4×△V_c)/(R_T×C_T×V_cc)
View Answer

Answer: a
Explanation: The original output frequency, f_o =2×[V_cc-V_c+△V_c]/[R_T×C_T×V_cc].
The new frequency f₁ =2×[V_cc-V_c]/[R_T×C_T×V_cc].
Change in frequency, △f_o= f_o– f₁ = 2×[V_cc-V_c+△V_c]/[R_T×C_T×V_cc]-{2×[V_cc-V_c]/[ R_T×C_T×V_cc] => △f_o = (2×△V_c)/(R_T×C_T×V_cc).

14. Using the given specifications, determine the voltage to frequency conversion factor.

a) 8.32
b) 8.90
c) 8.51
d) 8.75
View Answer

Answer: c
Explanation: △f_o = 2×△V_c/(R_T×C_T×V_cc)
=>△V_c= (△f_o×R_T×C_T×V_cc)/2 = (4.7µFx10kΩx5x112)/2 = 13.16V.
K_v= △f_o/ △V_c = 112Hz/13.16V
=>K_v=8.51.

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