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 PLL is in the free-running state when :
No input is applied.
"Free-running" means exactly what it says: the loop has nothing to lock to, so the VCO runs at its own natural frequency. With no input signal there is no phase error, the phase detector produces no correction, and the VCO oscillates at \(f_{0}\) — the frequency set by its own R and C or L and C. That is option 4.
The three states of a PLL follow one another as a signal appears and is acquired:
| State | Condition | VCO frequency |
|---|---|---|
| Free running | No input | \(f_{0}\), its natural value |
| Capture | Input present, not yet matched | Sliding towards fin |
| Locked (tracking) | Input matched | \(f_{VCO}=f_{in}\), held there |
The passage describes the transition between the last two: "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". Free running is the state before that sequence begins.
Two ranges characterise the loop, and the distinction matters:
• The capture range is the band of input frequencies, centred on \(f_{0}\), over which an unlocked loop can acquire lock. It is limited by the loop filter, since the beat note between input and VCO must pass through the filter to steer the VCO at all.
• The lock range is the band over which an already-locked loop can follow the input. It is wider, being limited only by the VCO's tuning range and the phase detector's output swing.
\(\text{Capture range}\le\text{Lock range}\)
always — it is easier to hold lock than to acquire it, which is why a loop that has slipped may not immediately recapture.
Why the other options are not states at all. They compare input and output voltages, but a PLL locks on frequency and phase; the amplitude relationship between input and VCO output is irrelevant to whether the loop is locked, since the input is typically limited or squared up before reaching the phase detector.
Hence, the PLL free-runs when no input is applied.
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 voltage controlled oscillator is used for :
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:
