mixed-signal socs with in situ self-healing circuitry
TRANSCRIPT
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Mixed-Signal SoCs With
In Situ Self-Healing CircuitryChristopher Maxey
Booz Allen Hamilton
Sanjay Raman
Defense Advanced Research Projects Agency
Kari Groves, Tony Quach, Len Orlando,
and Aji Mattamana
Air Force Research Laboratory
Gregory Creech
Ohio State University
Jay Rockway
SPAWAR Systems Center Pacific
h ADVANCES IN INTEGRATED circuit technologieshave increasingly enabled the single-chip integra-
tion of multiple analog and digital functions, result-
ing in the development of complex mixed-signal
systems-on-a-chip (SoCs). In particular, scaling
channel lengths in CMOS processes to 32 nm and
below has pushed the fmax and ft of silicon CMOS
transistors in excess of 230 and 440 GHz, respec-
tively [1] and the fmax and ft of SiGe BiCMOS transis-
tors in excess of 350 and 300 GHz, respectively [2].
In this regime, it is possible to realize state-of-the-art
RF performance on the same platform used to fa-
bricate digital processors and dense memory cores.
On the other hand, pattern variations partly due to
the complex masks needed for subwavelength litho-
graphy and random variables such as dopant loca-
tion effects become statistically significant at these
dimensions. Therefore, a
major consequence of the
drive towards ever smaller
transistor gate lengths is a
dramatic increase in intra-
wafer and intradie process
variability (as measured by
standard deviation) in crit-
ical device p aramet ers
such as threshold voltage,
effective channel length, etc. Figure 1 plots intra-wafer variability for several process nodes based on
lithographic limitations published by the ITRS [3]
along with projections of yield reduction in these
nodes from Monte Carlo simulations corroborated
by measurements of representative circuits [4]. In
aggregate, variability in several parameters simulta-
neously can greatly exacerbate circuit performance
degradation. Such variation can have significant
impact on digital circuits, but the effect is magni-
fied for analog and mixed-signal circuits due to the
heightened sensitivity of such designs to device
mismatch and the integration of numerous individ-
ual subblocks that can vary greatly in noise charac-
teristics, operating frequency, etc. Consequently,
mixed-signal circuit performance yield degrades
more rapidly with technology scaling than with
digital circuits.
To deal with the impact of variability in advanced
technologies, designers often must conservatively
accommodate worst-case corner case simulations
and relax target performance specifications to
Editors notes:
This article discusses the goals and recent achievements of the HEALICs
program. The program_s aim is to enhance wireless systems with sensors,
actuators, and mixed-signal control loops in order to improve their
performance yield.
VHaralampos-G. Stratigopoulos, TIMA Laboratory, and
Alberto Valdes-Garcia, IBM T. J. Watson Research Center
0740-7475/12/$31.00 B 2012 IEEENovember/December 2012 Copublished by the IEEE CEDA, IEEE CASS, IEEE SSCS, and TTTC 27
Digital Object Identifier 10.1109/MDT.2012.2226014
Date of publication: 23 October 2012; date of current version:
17 January 2013.
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guarantee a sufficient postfabrication parametric
yield. These traditional corner-based design techni-
ques require coverage of an extensive and expo-
nentially growing parameter space that becomes
intractable for large designs. Furthermore, efforts tocontrol variability through aggressive fabrication
process management or extensive Design-for-Yield
(DFY) procedures are costly and ultimately limited
in effectiveness [5]. An alternative approach advo-
cated in the Self-HEALing Mixed Signal Integrated
Circuits (HEALICs) program initiated by the
authors is to recover lost performance by employ-
in g on -ch ip sen sors ( meters) , act uat ors
(knobs) and analog/digital control loops that
measure the effects of device variability in situ and
consequently adjust tunable circuit parameters todrive the chip towards a more optimal performance
point. This control circuitry can be applied at both
the sub-block and system level allowing the designer
to focus on performance goals and not on yield
related issues. As with any control system, chal-
lenges related to stability, response times, system
bandwidth, etc. cannot be overlooked when im-
plementing self-healing, and overcoming these
challenges while maintaining performance has
been an important aspect of the program. Further-
more, HEALICs design teams must also address the
possibility that process variability adversely im-
pacts the self-healing circuitry itself, and compen-
sation for this effect has been a particular point of
emphasis.Calibration techniques for mixed-signal circuits
such as analog-to-digital converters (ADCs) have
been studied extensively; however, they typically
focus on fully digital techniques that rely on
known-good states stored in on-chip memory [6].
Furthermore, analog calibration techniques have
been investigated for individual circuit blocks [7],
[8], but they have not been studied extensively for
full SoC applications. HEALICs technology com-
bines analog and digital techniques for low-power
overhead, embedded comprehensive healing forSoCs that compensates not only for process
variation, but also for the extreme environments
and long operational lifetimes experienced by the
Department of Defense (DoD) electronic systems,
as well as other systems: aviation, automotive,
industrial automation, etc. For this reason, we
anticipate that the technology developed under
this program will greatly enhance long-term
reliability in addition to improving performance
yield.
Figure 1. Variability in critical transistor parameters for submicron process nodes
plotted along with the associated impact on performance yield for digital and
mixed signal circuits [3], [4].
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Program objectives and planTo measure the efficacy and power efficiency of
self-healing, the program design teams have targeted
the yield-improvement and power consumptionoverhead goals described in Table 1.
Performance yield is defined as the percentage of
die per wafer that meets or exceeds a set of prede-
fined circuit performance metrics. The performance
yield for the baseline (or nonself-healing) die is
calculated using the following formula:
Baseline Performance Yield DBaseline
NBaseline(1)
where DBaseline is the number of baseline die that
meet or exceed the same set of performance metricsand NBaseline is the maximum number of baseline die
that can be fabricated on a single wafer. Without self-
healing, baseline die are expected to yield poorly if
held to the same performance standards as the self-
healing chips. In most cases, the defined perfor-
mance criteria exceed state-of-the-art for each
design. In Phase I, the program targeted mixed-signal
cores or sub-blocks, and in Phase II the program
targets larger system-on-a-chip designs.
Posthealing performance yield can be calculated
using:
Posthealing Performance Yield DHEALICs
NBaseline(2)
where DHEALICs is the number of self-healing die per
wafer that meet or exceed all metrics. By dividing
DHEALICs by NBaseline, the performance yield is subject
to an area overhead correction to ensure that the
portions of the chip that are exclusively for self-
healing do not consume an excessive fraction of the
die area. Combining (1) and (2) yields the following
formula used for calculating posthealing performance
yield and accounting for self-healing area overhead:
Posthealing Performance Yield
%PosthealingYieldNHEALICs
NBaseline(3)
where NHEALICs is the number of self-healing die that
can be fabricated on a single wafer and %Postheal-
ingYield is the percentage of self-healing die that
meet all defined metrics. In some cases, self-healing
can enable a smaller chip to meet the performance
metrics compared to a baseline design, i.e.,
NHEALICs > NBaseline. In this case, (3) allows for the
performance yield to exceed 100%, which can be
interpreted as reducing the fabrication costs for agiven part. While not an explicit program require-
ment, we also expect that self-healing will result in a
reduction of performance variance as measured over
the full set of die, indicating that techniques are truly
compensating for process variability and not for poor
initial design or inaccurate device models. This effect
will be readily apparent in the yield measurements
reported in the following section of this paper.
Self-healing designs are also required to incur
minimal power consumption overhead relative to
the baseline design. Power overhead may be mea-
sured at an appropriate healing duty cycle if self-
healing is not expected to be continuously operating.
While the extreme scaling of silicon transistors is
primarily responsible for the variability described as
the motivating factor for self-healing technology, it
simultaneously makes it possible to incorporate
substantial processing capabilities on-chip for self-
healing at low area and power overhead.
To date, the HEALICs program has demonstrated
advancements in self-healing yield and performance
Table 1 HEALICs program goals for self-healing.
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in RF transceivers and subblocks, including ADCs,
phase-locked loops (PLLs), and power amplifiers
(PAs). These designs have been fabricated in
processes ranging from 180 nm SiGe BiCMOS to
32 nm silicon-on-insulator (SOI) CMOS, demonstrat-
ing the efficacy of self-healing across a range oftechnologies and variability effects. Table 2 sum-
marizes the individual program design teams and
the mixed-signal SoCs they have targeted for
demonstrating their self-healing methods.
This paper will describe results obtained by the
IBM, Raytheon, and UCLA teams, however, several
other important self-healing research activities in this
program are not covered in detail because of space
constraints. Examples include self-healing image
reject mixers [9] and LNAs [10] developed at
Georgia Tech (a member of the NGES team), novelI-gate MOSFETs for self-healing circuits [11] devel-
oped at University of Texas at Dallas (a member of
the IBM team), and built-in self-test synthesis tech-
niques for self-healing [12] developed at Auburn
University (a member of the BAE Systems team).
Recent accomplishmentsThis section will cover the measurement and
yield improvement results for several of the fabri-
cated and measured Phase I designs.
Self-healing PA and PLL designs
A research team led by IBM has demonstrated a
self-healing 28 GHz PA in 45 nm SOI and a self-
healing 2127 GHz PLL in 32 nm SOI. Together, these
components were designed for a Ka-band radio
transceiver.The high-level schematic of the self-healing PA
designed by CalTech is shown in Figure 2 [13]. The on-
chip integrated sensors include RF power sensors at
the input and output to estimate gain, junction
temperature sensors to assist with the estimation of
power added efficiency (PAE), and dc current sensors
to determinepower consumption. Thepower sensor is
calibrated by measuring the output of a few represen-
tative die and calculating a ratio between sensed
power and actual power. These data are included in
the healing algorithm for each of the chips. On-chipcontrol knobs include bias actuation mechanisms at
each of the PA stages and tunable transmission lines at
the PA output used to adjust matching impedances.
An embedded fully custom microprocessor is used to
determine the knob settings necessary to minimize
power consumption for a desired output power (thus
maximizing PAE). The control algorithm affords a
level of flexibility in operation and the target output
power can be designated at run time. Through this
control loop, the PAE is increased from 5.6% in the
Table 2 Summary of target self-healing demonstration circuits.
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baseline case to 7.3% in the healed state. PAE is
defined as:
PAE100RF Output PowerRF Input Power
dc power consumption:(4)
Furthermore, the 1 decibel compression point
and the gain are increased from 11.3 to 13.8 dBm
and from 20.3 to 23.7 dB, respectively.
This PA chip was also an excellent platform for
investigating the potential efficacy of self-healing for
Figure 2. Self-healing PA: (a) high-level schematic and (b) die photograph (1.8 mm 0 1.7 mm).
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improving RF/mixed-signal circuit reliability. To this
end, CalTech performed an extreme test of reliability
healing by intentionally and sequentially inducing
damage in one of the PA output stages through theuse of laser ablation and initiating the healing
algorithm after each stage of damage. Notably,
self-healing was capable of recovering significant
amounts of lost output power by autonomously ad-
justing the output matching to compensate for the
change in impedance loading. A set of curves show-
ing output power data for the undamaged and for
three successive ablations is shown in Figure 3 along
with before and after photos of the PA. Ultimately,
the self-healing algorithm is shown to recover up to
4.8 dBm of output power compared to the baselinesettings.
Concurrent with the design of the PA, the IBM
team designed and tested a 25-GHz self-healing
PLL fabricated in 32 nm SOI technology. The PLL
architecture was a hybrid analog/digital design
with proportional and integral paths following the
phase/frequency detector. A peak detector at the
output of the oscillator acts as the primary sensing
path for the self-healing algorithm, which, in this
case, is primarily responsible for reducing phase
noise at a 10-MHz offset from the carrier to
G124 dBc/Hz. The bias values and VCC are
measured along with the frequency to complete
the in situ virtual phase noise model of the PLLperformance [14]. During testing, a quadratic
relationship is derived that relates the output
amplitude, output frequency, bias voltage, bias
current, tuning band, and VCC value to the mea-
sured phase noise. Once this equation is deter-
mined, the coefficients for each term are stored in
on-chip memory. After self-healing is initiated, the
knob settings are stepped through incrementally. An
on-chip processor then evaluates the stored equa-
tion for the sampled sensor values and returns an
estimate of the phase noise. If phase noise is at aminimum, the algorithm stops and the knob settings
are held constant. The algorithm is implemented
completely on-chip in a 16-bit arithmetic logic unit
which uses a 32-word data memory and a 256-word
instruction memory for low healing overhead. Using
this algorithm, the performance yield of the PLL is
improved from $20% in the baseline case to 100%
posthealing for 55 measured die. Healing power
overhead was $10% and healing area overhead
was 12.7%.
Figure 3. Self-healing result for intentionally damaged PA: (a) no damage; (b) 1/8 stage removed;
(c) 1/4 stage removed; and (d) 1/2 of stage removed. Micrographs of PA: (e) before and
(f) after damage.
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Self-healing pipelined ADC
R esearchers f rom UC LA
have pursued the design of a
self-healing ultralow-power pi-
pelined 1 Gsps ADC with a flash
output stage in 65 nm CMOS
[15]. Therefore, canonical self-healing involved adding control
loops and processors to the
baseline chip with the con-
straint of minimizing the over-
head created by these auxiliary
blocks. For this ADC design,
however, the application of
self-healing was found to signif-
icantly decrease the size of the
yielding part relative to a non-
yielding baseline part by en-abling the use of minimum
sized devices in critical analog
parts of the design.
Flash ADCs are well known
for realizing extremely high
sample rate data converters at
the expense of high comparator
cou nt f or h igh n umbers of
digital output bits. Further-
more, these comparators ex-
hibit significant capacitanceand gain mismatch in deeply
scaled CMOS that would other-
wise be an attractive fabrication
process for such ADCs. One
method to combat the mis-
match (the baseline approach)
is to use devices with larger
than minimum dimensions that
exhibit lower variability stan-
dard deviation; but this ob-
viates the speed, the size, andthe power advantages of using
65 nm or better technology.
Alternatively, a self-healing al-
gorithm designed to mitigate
the effect of comparator mis-
match would enable the use of
minimum size devices. Specifically, in this imple-
mentation, capacitance and gain mismatch healing
is employed in the 4-bit input stage and capaci-
tance mismatch healing is employed in the follow-
ing four stages. The final three stages have sufficient
performance margin to not require additional
healing. Figure 4(a) shows the general ADC
architecture schematic.
Figure 4. (a) High-level ADC schematic showing the self-healing required
for each stage. Inset: op-amp comparator schematic and (b) comparison
of self-healing design to recently published ADCs in various submicron
CMOS technologies. (All data points except for self-healing points
compiled from [16]).
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Through this approach, the ADC demonstrated
56.7 dB of signal-to-noise-and-distortion ratio
(SNDR) equivalent to 8.38 effective bits with a
491 MHz input signal sampled at 1 Gsps. More
importantly, the power consumption was merely
33 mW, resulting in an ADC figure-of-merit (FOM)
of$99 fJ/conversion step. The ADC FOM is calcu-lated as follows:
FOM Power
2ENOB Sampling Frequency: (5)
Compared to a baseline design utilizing larger
comparator transistors, this design was 41 smaller
and $100 less power consumptive. A conservative
clock tree design consumed approximately a third of
the power and could have been further reduced
with aggressive design. Furthermore, simulation re-
sults indicate approximately a 16% reduction in theFOM if the design was ported to 45 nm technology. A
plot comparing this result to others in recent liter-
ature is shown in Figure 4(b).
Self-healing radar receivers
In addition to subblocks dedicated for use in
communications systems, self-healing has been
shown to dramatically increase performance yield
for DoD-relevant radar and EW receivers as well. For
example, a team led by Raytheon developed a
618 GHz self-healing radar receiver with several on-chip sensors and actuators designed to reduce I/Q
phase mismatch, PLL output spur level, and gain
fluctuations across the spectral field of regard [17].
Figure 5a shows the design schematic.
Unique to this team, an on-board ARM process-
ing core was synthesized and fabricated to run a
modified Nelder-Mead optimization algorithm used
to efficiently minimize mismatch, suppress spurs
and flatten gain response. The ARM core utilizes
130.4 KB of memory, but only 36 KB are specific to
the healing circuitry. In this case, HEALICs techni-
ques are shown to take advantage of baseline pro-
cessing and memory capabilities to reduce healing
area and power overhead. To reduce phase mis-
match, for example, an auxiliary mixer is used to
produce a dc value proportional to the amount of
phase offset between the Iand Q paths. Feedback to
variable amplifiers in each path are used to fine tune
the phase offset, and the Nelder-Mead algorithm is
used to establish values for each amplifier that mini-
mizes mismatch. The search routine is applicable to
bound-constrained and discretized optimization
problems and has shown efficacy for the types of
self-healing required for the radar receiver designed
in this effort. Furthermore, Nelder-Mead is particu-
larly robust in the presence of noisy measurement
data, which is expected from the low-power sensors
typically incorporated in self-healing designs. Toheal PLL spur levels, a similar algorithm is used with
slightly different knobs and sensors. A power de-
tector on the output is fed to the algorithm, which
independently sets two knobs: charge injection on
the VCO control line and a trigger setting to establish
the precise timing of the charge injection. Finally, to
heal gain flatness, the output power sensor is mo-
nitored and the tunable amplifiers are further ad-
justed while maintaining optimum phase matching.
Figure 5b shows histograms for healed mismatch
and gain flatness. The area shaded in green repre-sents the performer-defined metric that sets the stan-
dard for yield in the case of this design. For both of
these metrics, yield was improved from 0% in the
baseline case to 100% in the healed case.
The Raytheon team is also investigating the
effects of ageing on circuits, which is of particular
interest given that some defense, as well as com-
mercial, electronic systems are fielded for decades.
The main cause of aging in PMOS is negative bias
temperature instability (NBTI) which shifts the
threshold voltage over time and is especially delete-rious for gate lengths shorter than 100 nm. Hot-
carrier injection (HCI) is an important in ageing
mechanism for NMOS and has an amplified impact
on mixed signal circuits compared to pure digital
circuits. NCSU, a member of the Raytheon team, is
developing techniques for the purposes of designing
self-healing knobs and sensors specifically for
ageing and has published initial results in [18].
Self-healing 60 GHz radio transceiver
Another research group at UCLA has successfully
incorporated self-healing sensors, actuators and
control loops to substantially improve the perfor-
mance yield of a 60 GHz, 4 Gbps communications
transceiver fabricated in 65 nm CMOS [19]. The
schematic for this design, identifying additions spe-
cific to self-healing, and the photograph of the fabri-
cated die are shown in Figure 6.
Circuitry dedicated solely to self-healing repre-
sented 8% of the total area. Likewise, the power
consumption of these blocks amounted to only 3.7%
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of the chips total. On-chip sensors include an envel-
ope sensor for tracking the output spectrum; a tem-
perature sensor for estimating the kT noise level; apower sensor for estimating the path gain; and an
auxiliary ADC for quantizing the sensor outputs. Self-
healing control knobs include phase offset and dc
offset for the individual I/Q paths; PA and mixer
current biases; variable gain LNAs; and an auxiliary
DAC for generating control signals. The transceiver
utilizes a loop-back healing approach, where the Tx
is used to drive the Rx for the purposes of measuring
the radios performance. This minimizes the need
for off-chip calibration of the on-chip sensors. The
central component for mediating the self-healing
algorithms is the parameter estimator (PE) block
shown in the block diagram in Figure 6a. The (PE)comprises a 128-point FFT processor synthesized in
digital logic along with spectrum magnitude estima-
tion for both the Iand Q channels. The PE is respon-
sible for estimating LO leakage power, image tone
power, among other parameters and consumes only
0.56 mm2 of die area and 4.8 mW. The DAC control-
ler, meanwhile, provides test tones for measuring
circuit response to 1-tone and 2-tone tests.
Critical performance parameters for this design
include receiver noise figure; transmitter linearity;
Figure 5. (a) Radar receiver schematic showing location and connections for self-healing sensors and
knobs; histograms showing improvement in performance yield for phase error (b) and gain flatness (c).
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Figure 6. 60 GHz, 4 Gbps self-healing transceiver: (a) high-level schematic; (b) die
photograph (4 mm4 mm); and performance yield enhancement for (c) image tone
rejection and (d) receiver NF.
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and image tone power. The target noise figure was
6 dB, and self-healing was able to correct the
baseline value of 9.4 dB to 4.9 dB for 100% of the
chips. Likewise, the target Tx third-order output
intermodulation product level (OIM3) and the target
image tone rejection level were both 40 dBc, and
the on-chip algorithm was able to correct baselinevalues of32.4 dBc and 32.8 dBc to 42.6 dBc
and 41.8 dBc, respectively, on 100% of the die.
To heal the image tone power, the envelope
sensor captures the spectrum of the transmitter out-
put and the PE estimates the magnitude of the power
in each of the FFT frequency bins. After the estima-
tion is performed, the phase offset, relative gains and
dc offsets between the I/Q channels are adjusted to
minimize the power at the image frequency. A
coarse/fine search algorithm is implemented in cus-
tom logic to close the control loop and minimize the
measured image power. Likewise, to heal the noise
figure, the temperature sensors are sampled by the
auxiliary ADC to estimate the current kT level. Given
the relationship between stage gain and overall
noise figure, the NF can be optimized by adjusting
the gain in each of the three stages. Figure 6c and d
show histograms for each of these performance pa-rameters comparing the healed state to the baseline
state. Since the area of the baseline design is con-
strained by the number of bond pads and since the
self-healing circuitry fit in vacant parts of the die,
there is no area overhead correction necessary for
this part.
IN SITU SELF-HEALING has been shown to dramat-
ically improve the performance yield of RF/mixed-
signal designs at deep-submicron nodes. A wide
range of cores and subblocks from PAs to entire
Table 3 Summary of yield improvement through self-healing for the circuits described in this paper.
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transceivers have been co-integrated with sensors,
actuators, and control loops, resulting in the resto-
ration of lost performance at low power and die area
overhead. A summary of some of the performance
yield improvement capabilities highlighted in this
paper is included in Table 3.
Activities that promise to extend these capabil-ities to the system-on-a-chip level are already
underway, and a repository of self-healing IP has
been established at the Air Force Research Labo-
ratory to facilitate a more widespread adoption of
these techniques throughout the DoD design
community.
AcknowledgmentsThe authors would like to thank all of the per-
formers on the HEALICs program for their effortstowards achieving the program goals; in particular,
those who contributed to the work outlined in this
paper: principal investigators Dr. Jose Tierno and
Mr. Daniel Friedman from IBM, Prof. Behzad Razavi
from UCLA, Mr. Gerry Sollner from Raytheon, Prof.
Frank Chang also from UCLA, and their teams. The
HEALICs program was developed at and funded by
the Defense Advanced Research Projects Agency.
The work presented in this paper was supported
through Air Force Contracts FA8650-09-C-7924,
FA8650-09-C-7925, and FA8650-09-C-7926; Navygrants N66001-09-1-2029 and N66001-09-1-2030; and
Navy contract N66001-09-C-2023. The views, opi-
nions, and/or findings contained in this article are
those of the authors and should not be interpreted
as representing the official views or policies, either
expressed or implied, of the Defense Advanced
Research Projects Agency or the Department of
Defense. h
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Christopher Maxey is an Associate with Booz
Allen Hamilton. His research interests include mixed-
signal and RF circuits. He has a Masters degree in
electrical engineering from Virginia PolytechnicInstitute and State University (Virginia Tech) in
2004. He is a member of the IEEE.
Sanjay Raman is a Program Manager at DARPA,
currently on assignment from Virginia Polytechnic
Institute and State University (Virginia Tech), where
he is a Professor of Electrical and Computer
Engineering. His research interests include silicon-
based RF/microwave millimeter-wave circuits; self-
correcting RF/mixed-signal circuits; heterogeneous
integration; and sensor microsystems. He has a PhD
in electrical engineering from the University of
Michigan in 1998. He is a senior member of the IEEE.
Kari Groves is an RF Engineer with the Air Force
Research Laboratory.
Tony Quach is an RF Engineer with the Air Force
Research Laboratory.
Len Orlando is an RF Engineer with the Air Force
Research Laboratory.
Aji Mattamana is an RF Engineer with the Air
Force Research Laboratory.
Gregory Creech is recently retired from the AirForce Research Laboratory. He is now with Ohio
State University.
Jay Rockway is with SPAWAR Systems Center
Pacific.
h Direct questions and comments about this article
to Chri stopher M axey , B ooz A ll en Hami lton;
November/December 2012 39