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.
A phase locked loop (PLL) consists of :
A low pass filter
A PLL has three blocks, and the filter between the other two is a low-pass filter — option 3.
| Block | Function |
|---|---|
| Phase detector | Compares input and VCO phases, outputs a voltage proportional to the difference |
| Low-pass loop filter | Removes the sum-frequency and noise, leaving the DC error |
| VCO | Converts the error voltage back into a frequency |
Why it must be low pass. A phase detector is essentially a multiplier, and multiplying two signals at \(f_{in}\) and \(f_{VCO}\) produces components at both the difference and the sum frequencies:
\(\cos A\cos B=\tfrac{1}{2}\cos(A-B)+\tfrac{1}{2}\cos(A+B)\)
Only the difference term carries the phase information the loop needs; when locked it becomes a DC level. The sum term, at roughly twice the carrier frequency, is useless and would frequency-modulate the VCO if allowed through. A low-pass filter passes the wanted DC and rejects the unwanted high-frequency product — the passage's "small dc voltage… referred to as error voltage".
The filter does far more than clean up the signal, though — it sets the loop's dynamics. Its bandwidth determines:
• the capture range, since the beat note must fall inside the filter's passband for the loop to pull in;
• the lock-in time, since a narrow filter responds slowly;
• the noise bandwidth, since a narrow filter rejects more input noise;
• the stability, since the filter contributes phase shift to a feedback loop that already has a pole from the VCO's integration.
The design compromise is therefore unavoidable: a wide filter captures quickly and tracks fast-changing inputs but admits noise and jitter; a narrow one is quiet and highly selective but slow to acquire and prone to losing lock. Most practical loops use a lag-lead network rather than a simple RC, so that the low-frequency gain needed for accuracy can be combined with the phase margin needed for stability.
Hence, a PLL contains a low pass filter.
Similar Questions
The active filter works well
What will be the 'change in roll off' when the order of a filter changes from 1st to 2nd ?
Which of the following sentences are correct ?
A. Gain of active filter can be more than 1.
B. Gain of passive filter can be more than 1.
C. Butterworth filter changes the roll off of output as its order changes.
D. Phase of output at cutoff frequency in butterworth filter does not change with its order.
E. Feedback is not needed in any filter circuit
Choose the correct answer from the options given below :
A band pass filter response has :
(a) Two critical frequencies
(b) Quality factor depends on the center frequency and the bandwidth
(c) Quality factor depends on the critical frequencies
(d) A voltage gainOptions :
Important Questions from Filters
In choke input filter circuit, the first element is _______.
In the frequency response graph of an amplifier the 3 dB point refers to :
Which of the following statements is NOT correct for a Chebyshev Filter?
Which of the following statements is NOT correct about Butterworth filter?
The characteristic equation for the output voltage of an All‐pass filter is given by:
