Motivation: Reactive Radio

1
Motivation Reactive Radio
Main Radio
High data rate out
Wakeup signal
Power modeling shows that reactive radio must
consume lt100µW to save power over duty cycled
approach Design goal ? 50µW
Carrier Sense Ckt

• Reactive radio detects broadcast wakeup signal
  and activates main radio for data transmission
• Conserves power by only turning on main
  transceiver when it is needed
• High cost for false positive wakeups

2
Digital Oscillator Calibration

• Calibrate LC oscillator with high accuracy
  reference (FBAR)
• Convert control voltage to digital signal and
  control oscillator frequency digitally
• Turn off FBAR oscillator and control loop after
  calibration to save power. Control word from ADC
  is latched while loop is off.

Low-accuracy LC oscillator (lt100µW)
To calibrate over 200MHz span better than 500kHz
accuracy, 400 steps (9 bits) required
9
FBAR osc
ADC
PD
LPF
.
.
.
High-accuracy (500ppm) FBAR oscillator (300µW)
3
Low Power Oscillator Strategies

• Subthreshold biasing
• greater transconductance (gm) for a given bias
  current
• device ft lower in subthreshold, but CMOS
  scaling helps

• Low supply voltage
• CMOS supply is always shrinking
• previous oscillator work shows operation without
  performance degradation down to 300mV supply

4
Oscillator Core
Vdd

• Vdd 500mV (or lower)
• Ibias 200µA
• f0 1.8 - 2.0GHz
• L 3nH (bondwire)
• Device W/L300/.13 (deep subthreshold regime)
• Fully differential output with 50O buffers
  (500mVp-p)
• Independent control of capacitor banks on each
  output

off-chip
L
L
Nominal power consumption 100µW
Digitally controlled capacitor bank (0-511fF)
5
LSB Capacitor Design

• ?C1fF, Cunit20fF, N19
• Finger caps for good matching and Q
• Switch parasitics can be absorbed into array of N
  caps
• Form binary arrays of this basic unit for larger
  caps

Cunit
Vctrl1
Cunit
Cunit
Cunit

N
2
1
6
MSB Capacitor Design

• ?C32fF, Cunit36fF
• Finger caps for good matching and Q
• Switch parasitics can NOT be absorbed into array
  of caps
• Form binary arrays of this basic unit for larger
  caps

(5/.13)
1
Cunit
2
Cunit

(.15/1)
(.15/1)
N
Vctrl1
7
Capacitor Bank Segmentation
Simulation results Bit 1? ?f 410kHz Bit 2? ?f
830kHz
N-ratio scheme
Cunswitched 600fF
(extraction shows about 800fF with routing)
Single switched capacitance
Cunswitched 150fF
8
Effects of Segmentation Mismatch
Design for overlapping bands to maintain
frequency resolution
Cmsb lt 31 Clsb

• Overlapping bands, no missing codes
• Reduced tuning range

Alternative
Cmsb gt 31 Clsb

• Increased tuning range
• Missing codes

9
Whats Next
Future Work

• Digitally tuned oscillator test chip just back
  from fab and ready for test
• Both bondwire inductor and integrated inductor
  versions on the die
• Digital serial controller (SPI) on-chip to
  expedite frequency programming
• Verify power and tuning resolution performance
• Investigate open-loop frequency drift over time
• Design calibration loop to periodically tune
  oscillator to FBAR reference oscillator

Utilize this low power LO in reactive radio
receiver