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.
The voltage controlled oscillator is used for :
Voltage to frequency conversion
The name states the function: a voltage-controlled oscillator produces an output whose frequency is set by an input voltage. It is therefore a voltage-to-frequency converter — option 1.
\(f_{out}=f_{0}+K_{v}V_{c}\)
where \(f_{0}\) is the free-running frequency with no control voltage and \(K_{v}\) is the conversion gain in hertz per volt. Linearity of that relation over the working range is the VCO's principal specification.
How the conversion is achieved. Two families dominate:
| Type | Mechanism | Typical use |
|---|---|---|
| LC with varactor | Control voltage changes the varactor's junction capacitance, hence \(f=\dfrac{1}{2\pi\sqrt{LC}}\) | RF, radio tuners |
| Relaxation / current-starved | Control voltage sets the current charging a timing capacitor, hence the ramp rate | Integrated PLLs, function generators |
Its role in the loop, as the passage describes. The phase detector compares the input signal with the VCO output and produces an error voltage; the loop filter smooths it; and the VCO converts that voltage back into a frequency. The loop therefore closes only because the VCO performs precisely this conversion — it is the element that turns the correction signal back into the quantity being controlled.
The complementary function belongs elsewhere. Frequency-to-voltage conversion — option 2 — is what the whole PLL performs when used as an FM demodulator: since the VCO's control voltage must track the input frequency, that control voltage is the demodulated output. Notice the neat inversion: a frequency-to-voltage converter is built by putting a voltage-to-frequency converter inside a feedback loop.
Where VCOs appear beyond the PLL: as the tuning element of a frequency synthesiser, as the modulator in a direct FM transmitter (where the audio signal is the control voltage), as the sweep source in a spectrum analyser, and as the clock generator in every digital system that must adjust its frequency.
Hence, the VCO performs voltage to frequency conversion.
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 ?
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.
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 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:
