A Wideband LC VCO in CMOS

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A Wideband LC VCO in CMOS
BWRC Winter 2003 Retreat

• Axel Berny
• Professor R. G. Meyer
• Professor A. M. Niknejad

Monday, January 13rd, 2003
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Introduction

• During the past decade, VCO research has largely
  focused on optimizing phase noise, not tuning
  range.
• Several applications need wideband VCOs with low
  phase noise
• Most common example modern cable systems
• ? growing demand for bandwidth suggests that
  wideband low-phase-noise VCOs will play an
  increasingly important role in future
  transceivers.
• Goal design VCOs that concurrently achieve low
  phase noise and a wide tuning range (gt30).

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Example Cable Tuner System

• Modern cable tuners are based on the
  dual-conversion architecture shown above.
• VCO requirements are extremely demanding
• Very low phase noise -85dBc/Hz _at_ 10kHz offset !
• Very wideband 1.2-2.1GHz (55) !!

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Wideband VCOs

• Several VCO topologies can achieve a wide tuning
  range
• Ring oscillators
• Relaxation oscillators
• DDS
• ? all these have poor phase noise performance.
• Cable systems have relied on multiple LC VCOs
  (each covering a small band) and/or external
  varactors (with 30V tuning range) ? expensive
  and inefficient!
• Question can LC VCOs be designed to achieve a
  wide tuning range and maintain low phase noise?

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Wideband LC VCOs

• Answer YES!
• CMOS technology offers standard varactors AND
  excellent switches.
• ? Combine continuous and coarse tuning

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Predicted Tuning Characteristic
Tuning Characteristic (N3, L5.8nH,
Cv,min1.2pF, C260fF)

• Wide frequency range gets divided into smaller
  sub-bands where tuning char. is linear KVCO
  is low.
• Non-linear varactors can be made a rel. small
  of total tank capacitance.
• Trade-off between inductance and tuning range.

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VCO Implementation

• All pmos to reduce 1/f noise
• Only 2 gain devices to minimize cap loading.
• Large area tail device since main 1/f contributor
• P/Nwell varactors
• Very compact integrated varactor bias chokes. LB
  100nH at only 100um/side!!

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Die Photo

• Dimensions
• 1600 x 1500 ?m2
• Technology IBM 0.25?m RF CMOS process (5 Al
  metal layers).

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Measured Tuning Range

• Measured tuning range
• 1.06-1.41 GHz or 28.3.
• Simulated tuning range 1.06-1.46 GHz or 31.7.
• ? good agreement!

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Phase Noise Measured vs Simulated
fo 1.446 GHz (B2B1B0000)
fo 1.228 GHz (B2B1B0100)
VDD 2 V, Vtune 2 V, Icore 3.7 mA
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Performance Summary

• Need to address amplitude variations.
• Tuning range can be improved
• Use mos cap
• Increase size of cap array (lower L)

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Amplitude Variations

• Amplitude varies considerably across wide tuning
  range

Parasitic series resistance of the coil

• Consequences
• Start-up reliability needs careful consideration
  since
• Large amplitude variations can affect the
  performance of the mixer or the prescaler, and
  can cause significant variations in the phase
  noise.

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Traditional Amplitude Control

• Pros
• FB provides tight control of tank amplitude
• FB forces VCO into start-up
• Cons
• Injected noise into VCO core ? very difficult to
  remove!
• Loop stability
• Area/power overhead

Automatic Amplitude Control (AAC) loop
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Proposed Amplitude Control Scheme

• Calibration-like ? loops is open once desired
  amplitude has been reached. ? No noise added!!
• VCO Amplitude is fully programmable
• VCO amplitude can be made constant over the
  entire tuning range
• Can be programmed to set amplitude only as high
  as needed to meet phase noise spec, saving
  considerable power.
• Start-up guaranteed by starting calibration
  routine at high current.
• Calibration circuitry consumes very little
  area/power.

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Using MOS cap as Varactor

• Achieves a larger Cmax/Cmin than pn junctions
• Cmax/Cmin is independent of VDD!
• Tank swing is no longer limited!

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Improved VCO Implementation

• Tail current is digitally controlled (for
  amplitude calibration)
• Cap array size increased (L decreased) to improve
  tuning range
• Inversion-mode MOS varactor

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Simulation Results tuning range
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Simulation Results Phase Noise

• fo 1.6 GHz
• Icore 5 mA
• Vtune 2.5 V

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Simulation Results Calibration
Vosc (V)
3.0
2.0
1.0

• Calibration to 2.75 V
• fo 1.3 GHz
• fclk 2 MHz

0.0
-1.0
-2.0
-3.0
-4.0
time (?s)
1.0
2.0
3.0
4.0
5.0
6.0
0.0
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Conclusion Future Work

• Demonstrated a LC VCO with good phase noise
  performance and fairly wide tuning range.
• Designed a wideband VCO that achieves low phase
  noise and a 1.2-2.1 GHz tuning range.
• Designed an amplitude control scheme that does
  not degrade phase noise and contributes
  negligible power/area overhead.
• Future work
• Tape out!
• Measure validate simulated performance