Download Low-Noise Amplifier

Phase Locked Loops Continued
VCO
Ref
1/M
PFD
Loop
Filter
1/N
LO fLO=fref*N/M
PhaseLocked
Loop
• Basic blocks
– Phase frequency detector (PFD)
– Loop filter (including charge pump)
– Voltage controlled oscillator
– Frequency divider
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Phase Locked Loops Continued
• Key specs
– hold range: the frequency range over which
phase tracking can be statically maintained
– pull-in range: the frequency range over which
PLL can become locked
– pull-out range: dynamic limit of frequency
range for stable operation
– lock range: frequency range within which a
PLL locks within one single-beat note
between reference frequency and output
frequency
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Illustration of Static Ranges
Very slowly vary input frequency
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Phase Frequency Detector
• Generates phase difference between the input
signal and VCO output signal
• Distinguish if VCO is faster or slower
• Different types
– Analog vs digital
– Linear vs nonlinear
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Analog phase detector: multiplier
• Linear multiplier
• Functions the same way as a mixer
• But converting to DC (same frequencies)
• Same mixer circuits can be used
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Simple BJT 2-Quadrant Multiplier
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Gilbert cell
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Waveforms
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CMOS versions
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• 2V, High-Frequency CMOS Multiplier
– K-K Kan, D. Ma, K-C Mak and H.C. Luong, “Design Theory and Performance of
a 1-GHz CMOS Downconversion and Upconversion Mixers,” Analog Integrated
Circuit and Signal Processing, Vol. 24, No. 2, pp. 101-111, July 2000
.
Based on the Gilbert cell
•Can operate at a lower supply voltage because the mixer does not use stacking
• Source followers give better linearity
• Has a smaller mixer gain because sharing the bias currents with the followers
reduces gm
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• A Quarter-Square CMOS Multiplier
– J.S. Pen˜a-Finol and J.A. Connelly, “A MOS Four-Quadrant Analog
Multiplier Using the Quarter-Square Technique,” J. of Solid-State
Circuits, vol. SC-22, No. 6, pp. 1064-1073, Dec. 1987.
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• CMOS Four-Quadrant Multiplier
– Babanezhad and Temes - JSSC, Dec. 1985.
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Digital phase-frequency detector
• Compares edges of reference and divided
clocks.
• If reference clock leads the divided clock, the UP
signal is asserted.
• If the divided clock leads the reference clock ,
the DWN signal is asserted.
• In an ideal PFD no pulses are present at the
output in the locked state.
• Duty cycle of inputs is not relevant to the circuit
operation.
• The width of the UP/DWN pulses is proportional
to the phase difference between the clock
inputs.
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Digital Xor
phase detector
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Digital phase-frequency detector
• Conceptual diagram
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Conventional Digital PFD
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Delay in the Conventional PFD
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Output of PFD for locked state
• In locked state, narrow pulses are
generated in both UP/DWN outputs.
• The width of these pulses determines the
amount of noise introduced to the VCO
output by the charge-pump.
• Timing mismatch between the UP/DWN
pulses is a source of spurious tones.
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• The Charge-Pump converts the phase error
information provided by the PFD into a voltage
that controls the VCO frequency.
• If UP is high, top switch is closed and charge is
injected into capacitor, increasing voltage Vout
• If DWN is high, bottom switch is closed and
charge is extracted from capacitor,decreasing
voltage Vout
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State diagram
Up=0;
Dn=0;
Up=0;
Dn=1;
Up=1;
Dn=0;
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Non-idealities
• In practical PFD the delay of the gates
creates non-idealities in the phase
input/output characteristic.
• The PFD can no longer resolve very small
phase errors, and a dead zone is created.
• To solve this problem, extra delay is
introduced in the feedback path of reset
signal.
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Dead zone problem
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Non-ideal effects
of charge pumps
• Current mismatch
– Mismatch between source and sink currents in the charge pump
introduces a finite phase error.
• Current leakage
– When the source/sink currents are off, leakage currents can flow and
modify the VCO control voltage of the VCO by charging/discharging the
loop filter. Spurs are introduced.
• Charge sharing
– Parasitic capacitances from the switches share charge with the loop
filter when the nodes they are connected to have a large change in their
voltage.
• Charge injection
– Occurs when switches are turned off and the charge in their channels is
injected/extracted to the loop filter. Spurs are introduced
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Precharge PFD
– S. Kim, et. al., “A 960-Mb/s/pin Interface for Skew-Tolerant Bus Using
Low Jitter PLL, IEEE J. of Solid-State Circuits, Vol. 32, No. 5, may 1997,
pp. 691-700.
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Modified Precharge PFD
– H. O. Johansson, “A Simple Precharged CMOS Phase Fequency Detector,”
IEEE J. of Solid-State Circuits, Vol. 33, No. 2, Feb. 1998, pp. 295-299.
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