Towards Reliable Fully Integrated Radios for DR

1
Towards Reliable Fully Integrated Radios for DR

• Michael Mark
• Jesse A. Richmond
• Nathan M. Pletcher
• DR Seminar
• April 30, 2007

2
History of ULP Radios for DR in Our Group
FBAR-based low power oscillator
2-Channel Prototype Transceiver
Super-Regenerative Transceiver
VLSI 2004 (Otis et al)
ISSCC 2005 (Otis /Chee)
ESSCIRC 2002 (Otis)
ISPLED 2003 (Roundy et al)
2004 (Pletcher / Otis)
Passive Weak-up Receiver
Transmit Beacon
3
History of ULP Radios for DR in Our Group
Future Work Topic of This Talk
Active Antenna Transmitter
PicoCube
VLSI 2006 (Chee)
2006 (Burghardt et al)
JSSCC 2006 (Chee)
2006 (Pletcher / Gambini)
Injection Locked Transmitter
Fully Integrated WuRx
4
Film Bulk Acoustic Resonator (FBAR)
Avago 1.9 GHz FBAR (BAW)
100µm
5
Film Bulk Acoustic Resonators (FBARs)

• Why FBAR?
• provide highly accurate high Q resonances at RF
  
• can be used for frequency references and RF
  filter
• low power and low insertion loss due to high Q
• small form factor
• possible candidate to replace quartz (for
  frequency references) and bulky SAW for filter
  applications
• FBAR Limitations
• limited frequency tuning range (tuning range
  1/Q)
• worse temperature coefficient than quartz
• economical integration of high quality FBARs and
  CMOS still an issue

100µm
6
Limited Frequency Tuning Range

• Problems Interference and Fading
• Interference
• Receiver might be jammed by a strong signal at a
  frequency at - or close to - the receive
  frequency and has no way of escaping that
  scenario
• Fading
• Fading results from superposition of transmitted
  signals that have experienced differences in
  attenuation, delay or phase shift, or might be
  simply due to the attenuation of a single signal
  

100µm
7
Example Multipath Fading
r
d
8
Temperature Dependency of Resonances

• Problems
• temperature coefficient of FBAR approx. -25
  ppm/ºC (crystal 5 ppm/ºC)
• Frequency variation with temperature might cause
  TX frequency to fall out of receive band
• Clock-frequency drift requires more pessimistic
  logic timing

100µm
(courtesy B. Otis)
9
FBAR - CMOS Integration

• Many different ways of connecting CMOS with FBAR
• chip-on-board mainly for proof of concept
  circuits
• chip-on-chip mainly for proof of concept
  circuits
• flip-chip yield issues no hermetic sealing
• above IC integration ideal way of doing it
  however expensive and up to now Q of FBAR fully
  integrated on CMOS is lower that Q of standalone
  FBAR

100µm
(pictures courtesy by B. Otis)
10
How to Deal with These Limitations?
100µm
11
How to Achieve More Tuning Range?
100µm
12
Interpolative Oscillator

• Allows to generate a frequency of oscillation by
  interpolation between two or more resonator
  frequencies
• Maximum spacing of the resonance frequencies of
  single resonators ?1 and ?2 is given by

100µm
13
Frequency Division and Mixing

• Principle high
  frequency reference generated by FBAR-based
  oscillator, Fractional-N divider generates
  variable lower frequencies and single sideband
  up-conversion mixer produces a signal with
  frequency fRef fDiv
• Issues circuit and
  power overhead, spectral impurity

100µm
14
High Reference Frequency PLL

• Idea is to build a fractional-N PLL with a FBAR
  reference oscillator (at e.g. 500 MHz) and a wide
  PLL bandwidth, relaxing the close in phase noise
  requirements for the VCO, which might lead to a
  low overall power consumption.
• Similar to Divide-and-Mix approach but promises
  higher spectral purity

100µm
Ref W. Rhee et al. JSSC08
15
Temperature Compensated Oscillator

• Key Idea Cancellation of temperature dependence
  by adding two different frequencies with equal
  absolute frequency shift per degree
• Analogous to the idea of a bandgap voltage
  reference
• Enabled by the fact that FBARS with different
  resonance frequencies can be made to have
  different relative temperature coefficients as
  well.
• E.g. Resonator 1 f0 2.1 GHz Tc -25 ppm
  Resonator 2 f0 1.8 GHz Tc
  -29.2 ppm ? absolute frequency
  shift 52.5 kHz/ºC

100µm
16
High Reference Frequency PLL

• Circuit Implementation
• Difference oscillator uses two oscillators and
  a mixer to subtract one frequency from the other
• Leads to a temperature stable output frequency of
  300 MHz for our previous example

100µm
17
FBAR CMOS Integration

• FMOS a novel approach for integrating FBARs
  with CMOS
• Based on Avagos Microcap technology, CMOS and
  FBAR will be combined into one hermetic sealed
  package
• In close collaboration with Avago Technologies
  and University of Washington (Prof. B. Otis)

100µm
courtesy R. Ruby, Avago Technologies
18
FBAR CMOS Integration Microcap

• New Generation packaging for further integration
  and miniaturization used for Avagos FBAR filter
• A blank wafer is bonded on top of FBAR wafer to
  for a hermetic sealing for FBAR
• The packaged FBARs can then be wire-bond or
  flip-chip assembled

19
FBAR CMOS Integration FMOS

• Principle Idea Replacing the cap wafer with an
  actual CMOS wafer with circuit on it
• Requires matched wafer sizes for FBAR and CMOS
  wafer (6 inch), as well as wafer level technology
  access
• Therefore an Avago In-house 0.4 um CMOS process
  without any special RF features is used
• Process is characterized for low-frequencies but
  untested at RF
• Shrinking of transistor
  gate length down to
  
  0.28 um possible

20
FMOS Timeline

• First tape out in October 2006
  mainly device test structures for RF
  characterization and some oscillators and
  dividers
• Measurements and characterizations showed
  sufficient accuracy of CMOS models for
  applications around 2 GHz
• Next tape out planned in June 2007. Focus on
  interpolative and temperature compensated
  Oscillators

21
Summary

• FBAR based radios have proven to be excellent
  candidates for ultra low power radios for DR-like
  applications, however they have some limitations
• Ongoing research on how to overcome these
  limitations was presented
• We are on our way to overcome at least some of
  the obstacles towards more reliable ultra-low
  power radios for DR like applications

100µm