Phase Locked Loop (PLL)

What is a PLL?

• A PLL is a negative feedback system where an oscillator-generated signal is phase and frequency locked to a reference signal.

• Analogous to a car’s “cruise control”

2

PLLs are widely used in a variety of applications include:

•line synchronisation and colour sub-carrier recovery in TV receivers

•synthesised local oscillators and FM demodulators in radio receivers

•frequency synthesisers in transceivers and signal generators

Introduction

Introduction

Parts of a PLL:

• Phase Detector

• Filter

• Voltage Controlled Oscillator

• Programmable Counter

PLL Control System

Detecting the input / reference signal’s phase

Most of the time, RC low pass filter is used

Voltage controlled oscillator / Voltage controlled

multivibrator

Or sometimes also refer to as frequency divider

Phase Detector

• Acts as comparator

• Produces a voltage proportional to the phase difference between input and output signal

• Voltage becomes a control signal

1) Phase Only Sensitive Detector

Diode ring mixer phase detector:

• This is the simplest form of phase detector and it can be synthesized from a diode ring mixer.

1) Phase Only Sensitive Detector

XOR circuit

• provide a very useful simple phase detector for some applications.

2) Phase-Frequency Detector

• Edge-triggered - Input duty-cycle doesn’t matter• Pulse-widths proportional to phase error

GoFasterD

CK

Q

DFF

DLY

GoSlower

Ref

Vdd

Vdd

FB

D

CK

Q

DFF

Q

CK

D

R

R

2) Phase-Frequency Detector

States:

GoFaster GoSlower Effect:0 0 No Change0 1 Slow Down1 0 Speed Up1 1 Avoid Dead-Zone

2) Phase-Frequency Detector

Example:

Ref

FB

GoFaster

GoSlower

Vctl

What is Dead-zone?

• “Dead-zone” occurs when the loop doesn’t respond to small phase errors - e.g. 10 ps phase error at PFD inputs.

• To overcome this one solution is to add a delay in the phase detector reset path, i.e. on the output of the NAND gate.

Filter

• Determines dynamic characteristics of PLL– Specify Capture Range (bandwidth)– Specify Tracking Range

• Receives signal from Phase Detector and filters accordingly

Filter

• The PLL filter is needed to remove any unwanted high frequency components which might pass out of the phase detector and appear in the VCO tune line.

• The filter also affects the ability of the loop to change frequencies quickly. If the filter has a very low cut-off frequency then the changes in tune voltage will only take place slowly, and the VCO will not be able to change its frequency as fast.

Voltage Controlled Oscillator (VCO)

• Set tuning range

• Set noise margin

• Creates low noise clock oscillation

VCO Requirement

VCO tuning range:

• It is obvious that the voltage controlled oscillator must be able to tune over the range that the loop is expected to operate over.

• This requirement is not always easy to meet and may require the VCO or resonant circuit to be switched in some extreme circumstances.

VCO Requirement

VCO tuning gain:

• The gain of the voltage controlled oscillator is important.

• It is measured in terms of volts per Hz (or V/MHz, etc).

• The voltage controlled oscillator will gain affects some of the overall loop design considerations and calculations.

VCO Requirement

VCO V/f slope:

• It is a key requirement the voltage to frequency curve is monotonic, i.e. it always changes in the same sense

• If it changes, this can cause the loop to become unstable.

VCO Requirement

Phase noise performance:

• particular importance in some PLL applications - particularly where they are used in frequency synthesizers.

VCO Feedback

• Like any oscillator, a VCO may be considered as an amplifier and a feedback loop.

• The gain of the amplifier may be denoted as A and the feedback as B.

VCO Feedback

• For the oscillator to oscillate on a given frequency, the system includes a resonant circuit to ensure that the oscillation occurs on a given frequency.

• The resonant circuit can be one of a number of configurations from an LC resonant circuit in either series or parallel resonance dependent upon the circuit, or a quartz crystal, etc.

• The most common types of oscillator used in Colpitts and Clapp oscillator.

VCO Tuning

• To make a VCO, the oscillator needs to be tuned by a voltage.

• This can be achieved by making the variable capacitor from varactor diodes.

• The tune voltage for the VCO can then be applied to the varactors.

VCO Tuning

Voltage controlled oscillator circuit

Programmable Counter

Divider

• Divides the VCO output by the degree of the open loop gain

• Feedback loop allows phase comparison