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PLL
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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
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