A Low-Noise Frequency Synthesizer for Infrastructure Applications

Shayan Farahvash, William Roberts, Jake Easter, Rachel Wei, Dave Stegmeir, Li Jin RFMD, USA

Outline

• Motivation

• Design Challenges

• VCO

• Capacitor Array

• Charge Pump

• Calibration

• Conclusion

Motivation

Why an integrated infrastructure-grade synthesizer?

• Semi-integrated solution with an external VCO is expensive• Recent wireless standards (e.g., WCDMA) have a lax

requirement on the phase noise compared to legacy ones (e.g., GSM)

• Power consumption requirements for infrastructure products are not as tight as those for handsets

Design Challenges

• Phase Noise

• Tuning Bandwidth

• Kv Variation

• Design Stability• Manufacturing variation• Process variation• Temperature

• Layout Constraints

• Current Consumption

Phase Noise

Stability

Kv

Bandwidth

Current

VCO

• Colpitts active core

• No tail current

• Linearization of the

tuning curve using C3

and C4

• Tuning sensitivity

adjustment using C1

and C2

• On-chip low-noise

regulator

Simulated VCO Performance

Similar to other Colpitts variations, there is an optimum n

87

7

65

5

CCC

CCCn

Simulated VCO Performance

Optimum n coincides with voltage-limited operation

87

7

65

5

CCC

CCCn

Temperature Stability

Variation in the VCO frequency as a function temperature is roughly 20MHz

Kv Variation

The operating voltage of the VCO is selected such that Kv variation isminimized

Switch-Capacitor Array

• Segmented design to keep the area small while simultaneously providing monotonicity

• Frequency tuning resolution determined by the smallest MIM capacitor allowed

B0

C

B1

2C 4C

B3

8C

B4

16C

B5

32C

B6

32C

B7

32C

Thermometer Coded Elements

Binary Weighted Elements

B2

Unit Element of the Switch-Capacitor ArrayDrain of a switch transistor experiences a negative voltage swing

Unit Element of the Switch-Capacitor Array

Charge Pump

• The charge pump was required to work up to 6V• The potential of each isolated P-well ( where M3 and M4 residing) was

adjusted by a tracking circuit

Tracking Circuit Performance Bias voltage of each isolated P-well is a function of the output voltage of the charge pump.

VVSS VVDD

Out of Loop Dividers

VCO frequency can be scaled down using a collection of “out-of-loop” frequency dividers

Calibration

During calibration, number of VCO cycles are counted in one PFD cycle

Measured Synthesizer Performance

Calibration algorithm is virtually instantaneous

Measured Synthesizer Performance

• VCO Phase Noise : -134dBc/Hz @ 600kHz• Better phase noise using “out-of-loop” dividers

Performance of the Calibration Algorithm

Calibration algorithm can maintain the Vtune within 100mV from what is set by the VC word

Die Micrograph

Performance Summary

Conclusion

• Fully integrated synthesizer fabricated in 0.18um SiGe BiCMOS process using a switch-capacitor array for wideband tuning

• VCO phase noise of -134dBc/Hz at 600kHz

• CP is capable of having an output voltage in excess of 6V albeit the breakdown voltage of MOS devices is only 4V

• Six “out-of-loop” dividers provide the means for frequency scaling

• Calibration algorithm with no delay overhead and less than 100mV error in setting the VCO tune voltage

Auxiliary: Tank Voltage and Current

Voltage-limited operation with swing twice as large as supply

Auxiliary: Tank Voltage and Current

Startup is guaranteed even without a tail current source

Auxiliary: VCO FOM

fLPkT

ff

d

2

0log10FOM2 fLPkT

fBW

d

2

log10FOM1

fLf

fBW

20log10FOM3

This Work [3] [1] [2] [4]

FOM1 (dB) -9.6 2.0 -15.7 -4.5 0.7FOM2 (dB) 6.0 8.2 4.5 16.7 10.9FOM3 (dB) 197.1 193.4 193.0 190.1 189.8

• FOM1: Bandwidth, Power Dissipation and Phase Noise• FOM2: Frequency, Power Dissipation and Phase Noise• FOM3: Bandwidth, Frequency, Phase Noise

Auxiliary: Out-of-loop Dividers

• CML logic based• Design emphasis on low noise floor

• Large signal amplitudes and faster slew rates are necessary• Dominant noise source are in clock buffering and output paths• Critical to reduce the current reference bias noise