A PLL consists of a phase detector, filter, voltage controlled oscillator (VCO), and optional divider. The phase detector compares the phase of the input signal to the VCO output signal and generates an error voltage. The filter smooths the error voltage which is fed to the VCO. The VCO then adjusts its output frequency according to the error voltage to minimize the phase difference between its output and the input signal. An optional divider may be included to scale the VCO output frequency before feeding it back to the phase detector for comparison to the input signal. In this way, the PLL is able to lock its output phase to the input phase or some multiple of the input phase.

1. Introduction
 What is a PLL?
 Parts of a PLL.
 Working of PLL.

2. What is a PLL?
 Digital frequency control system
 Generate high speed oscillations
 Acquire and track signals
• Radio Frequency Demodulation
• DX-ing
• RF communications

3. Control system representation

4. Modeling a PLL

5. PLL Control System

6. Parts of a PLL
 Phase Detector
 Filter
 Voltage Controlled Oscillator
 Programmable Counter

7. Parts of a PLL
 Phase Detector
 Acts as comparator
 Produces a voltage proportional to the phase difference
between input and output signal
 Voltage becomes a control signal
 A phase detector or phase comparator is a frequency mixer,
analog multiplier or logic circuit
 It generates a voltage signal which represents the difference
in phase between two signal inputs.

8. Parts of a PLL
 Filter
 Determines dynamic characteristics of PLL
 Specify Capture Range (bandwidth)
 Specify Tracking Range
 Receives signal from Phase Detector and filters
accordingly

9. Parts of a PLL
 Filter (Cont.)
 It provides a smoother form of a signal, removing
the short-term fluctuations, and leaving the
longer-term trend.
 PLL loop filter (usually a low pass filter) generally
has two distinct functions.
 The primary function is to determine loop
dynamics, also called stability. This is how the
loop responds to disturbances, such as changes in
the reference frequency.

10. Parts of a PLL
 Filter (Cont.)
 The most common and
easily understood active
filter is the Active Low Pass
Filter.
 It uses an op-amp for
amplification and gain
control.
 The simplest form of a low
pass active filter is to
connect an inverting or non-inverting
amplifier.
 The advantage of this
configuration is that the op-amps
high input impedance
prevents excessive loading
on the filters output .

11. Parts of a PLL
 Voltage Controlled Oscillator
 Changes a DC input voltage into a pulsed waveform.
 The output frequency is proportional to the
magnitude of input control voltage.
 Its output can be of any waveshape.

12. Parts of a PLL
 Voltage Controlled Oscillator (Cont.)
 The ouput frequency of a VCO is given by:
fout = fmin + KVCOVin
where fmin = output frequency when Vin=0
KVCO = VCO conversion gain, Hz/V
 In PLL, the VCO output frequency (fo) is fed to one
input of the phase detector.
 Phase detector compares it with the frequency of
input signal (fi) which is at its other input.

13. Parts of a PLL
 Divider
 Divides the VCO output by the degree of the open
loop gain
 Feedback loop allows phase comparison

14. Parts of a PLL
 Divider (Cont.)
 A phase-locked loop or phase lock
loop (PLL) is a control system that
generates an output signal whose
phase is related to the phase of an
input signal.
 The oscillator generates a periodic
signal.
 The phase detector compares the
phase of that signal with the phase of
the input periodic signal and adjusts
the oscillator to keep the phases
matched.
 It then brings the output signal back
toward the input signal for
comparison called a feedback loop
since the output is 'fed back' toward
the input forming a loop.