Assertion (A) : In applications such as FM and FSK, VCO plays an important role. Reason (R) : The frequency control is easily possible by varying d.c. voltage.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both are true, and the reason states precisely the property that makes the assertion true — option 1.
What FM and FSK both require. Frequency modulation demands an oscillator whose frequency follows the message continuously:
\(f(t)=f_{c}+k_{f}\,m(t)\)
and frequency-shift keying demands one that jumps between two frequencies as the data bit changes — the same requirement, restricted to two values. In both cases what is needed is an oscillator controlled by a voltage, which is what VCO stands for.
How the control is achieved. The commonest method uses a varactor diode, whose junction capacitance depends on reverse bias:
\(C_{j}=\dfrac{C_{0}}{\left(1+\dfrac{V_{R}}{V_{bi}}\right)^{n}}\)
Placed in the tank circuit of an LC oscillator, it makes the resonant frequency
\(f=\dfrac{1}{2\pi\sqrt{LC_{j}}}\)
a function of the applied DC voltage. Widening the reverse bias widens the depletion layer, which is the capacitor's dielectric gap, so capacitance falls and frequency rises. Since the junction is reverse biased it draws no current, so the modulating source needs to supply no power — only a voltage.
Why direct voltage control is such an advantage. The modulating signal can be applied straight to the tuning input with no transducer, no moving part and no mechanical delay, so modulation to megahertz rates is straightforward. In an integrated VCO the same job is done by a current-starved ring oscillator or an emitter-coupled multivibrator, where the control voltage sets a charging current and hence the period — different circuit, identical principle.
The wider importance is that this one block appears far beyond modulation: a VCO is the element that a phase-locked loop steers, which makes it the heart of frequency synthesisers, clock recovery circuits, FM demodulators and FSK decoders alike. The same voltage-to-frequency relation that lets a VCO generate FM lets a PLL recover it, since the loop's control voltage must then be a copy of the original modulation.
Hence, both (A) and (R) are true and (R) is the correct explanation of (A).
Similar Questions
The phase locked loop (PLL) is one of the interesting applications of the lock-in amplifier. Apart from FM stereo decoders, tracking filters, motor speed control, FM demodulators, etc. it has found wide applications in generation of local oscillator frequencies in house-hold TV and FM tuners as automatic frequency control (AFC). Indeed, PLL has emerged as one of the fundamental building blocks in electronics and it is commercially available as a single package. Basically, a PLL is a lock-in amplifier in which the reference signal is provided by its own output, converted to frequency by a voltage controlled oscillator (VCO). When locked to the input frequency the dc output is small but sufficient to drive the VCO to produce a frequency which is equal to that of the signal. In this tracking situation, the input signal and the VCO output are almost in phase quadrature and the lock-in amplifier produces a small dc voltage which is often referred to as error voltage. The moment input signal is fed, the VCO frequency starts changing and the PLL is said to be in the capture mode. The VCO continues to change its frequency until it equals that of the input and stays there ; the PLL is then in the phase-locked state. In this state, if there is any change in the input frequency, the loop automatically tracks it through its repetitive action.
Consider the following statements regarding an RC phase shift oscillator :
i. amplifier gain is positive.
ii. amplifier gain is negative.
iii. phase shift introduced by the feedback network is 180°.
iv. phase shift introduced by the feedback network is 360°.Which is correct ?
Which of the following oscillations makes use of both positive and negative feedback ?
For a FET based phase shift oscillator, what should be the value of capacitor (C) for oscillator operation at 1 kHz. The resistor (R) in the feedback network is 20 kΩ.
The current amplification factor in radian square of Colpitts oscillator is :
The voltage controlled oscillator is used for :
The PLL is in the free-running state when :
Assertion (A) : A monostable multivibrator can be used to alter the pulse width of a repetitive pulse train.
Reason (R) : Monostable multivibrator has a single stable state.
Select your answer using the codes given below :
In an RC phase shift oscillator the frequency of oscillation is given by
Important Questions from Types of Oscillators
Electronic ohmmeter uses OP-AMP as a/an:
Which of the following statements about the Wien Bridge Oscillator is CORRECT?
Hartley Oscillator is a:
Which of the following is the fixed frequency oscillator?
If R = 51 kΩ and C = 0.001 μF, the resonant frequency of a Wien Bridge oscillator is:
