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Gigahertz Integrated Circuits Group |
Recent and on-going research projects Frequency Converters  A
major theme of our
research work is in the subject of RF mixers. These circuits are
categorized as frequency converters because
they transfer spectral energy from one frequency band to
another. In communications links, for example, mixers are
used in the modulation and demodulation process and they are also used
to upconvert and downconvert information-bearing signals for
long-distance transmission. In addition, mixers are key
components in radar and microwave imaging systems. Recently we demonstrated a wideband active mixer with a minimum double-sideband noise figure (NF) of 2.4 dB. This is the lowest noise figure for an active mixer reported to date, to the best of our knowledge. In another recent achievement, we demonstrated the first dual-band self-oscillating mixer (SOM) for C-band and X-band applications. Dual-band operation was achieved by using both the fundamental and harmonic signals generated by the oscillator core of the SOM. Listed below are some of our most recent mixer papers.
Gallium Nitride Circuits  While silicon-based
integrated circuits are commonly used in cases where low-power
radio-frequency (RF) circuits are needed, there are
applications
that require large amounts of RF signal power and for which silicon is
not suitable. One such category of circuits are the power
amplifiers used in transmitters for long-range wireless communications
links. Since those power amplifiers must often deliver tens
or hundreds of watts of RF power, specialized transistors using
semiconductor materials such as Gallium Nitride (GaN) are needed to
meet the specifications. The design of GaN-based circuits is
a subject of major interest for industry and governments worldwide
because there is a strong demand for power amplifiers and related
integrated circuits (IC's) for the telecommunications and defence
markets. Aerospace is also an important market for
GaN because it is a wide bandgap semiconductor, which enables
it to withstand high levels of ionizing radiation in satellite systems.
We are developing power amplifier circuits with broadband
performance and high linearity in a 0.5 um GaN process.Ultra-Wideband Operational Transconductance Amplifiers (OTA's)  OTA's can
have much higher cutoff frequencies than
operational
amplifiers (op amps) and for this reason OTA's are more commonly used
at RF and microwave frequencies. We
have fabricated and measured an advanced fully-differential operational
transconducatance amplifier (OTA) in CMOS with the largest
frequency
bandwidth
known to date, exceeding
10 GHz. In contrast
to OTA's that use a feedback
topology, our OTA employs a feedforward-regulated cascode configuration
which significantly improves the operating speed of the circuit. Given
that an OTA has a voltage input and a current output and that microwave
circuits are predominantly characterized using s-parameter network
analyzers we therefore designed several special test circuits in order
to determine the microwave performance of our OTA. There are several
important microwave application areas that benefit from using a very
high-speed OTA like the one developed in our lab. Such applications
include active filters, circulators, phase shifters, and
oscillators.
Advanced OTA Applications  Using the
ultrawideband OTA's we have demonstrated a 5.4
GHz reconfigurable quadrature
amplitude modulator (QAM)
that can generate 4-QAM, 16-QAM or higher
constellation levels as needed. In a different application,
we used OTA's to build an active
CMOS circulator. Circulators are very useful
microwave
components that can be used to isolate networks from each other and
they are also found in reflection phase shifters and reflection
amplifiers.
It is well-known that
most
microwave
receivers operate at a fixed frequency band because the filters used
for band selection are not tunable. Therefore, using
a
tunable RF filter allows for a single
receiver to be used over multiple bands and hence reducing the system
costs. Using OTA's we have demonstrated the smallest
active filter known to date
operating in the 2.0 GHz band
and having 0.5 GHz of center frequency tuning
range.
Intermodulation Distortion Cancellation Circuits  When
an
RF
signal passes through an amplifier it experiences a certain amount of
distortion, meaning that the output waveform will contain not only the
amplified signal but also unwanted spectral lines. The unwanted spectra
appear due to non-linear phenomena inside the amplifier, and the
problem gets worse as the RF signal power increases. We have developed
a new method to reduce the amount of non-linear distortion in
amplifiers. The approach consists of generating an inverted replica of
the distortion spectra and then adding it to the output of the
amplifier so that the distortion cancels out and only the desired
signal remains.
Phase Shifting and Phase Control Circuits  A
number of techniques
exist for phase shifting involving vector summation, signal reflection,
distributed time-delay networks and more. Our most recent
phase shifter design can produce
360º of
continuous, variable, phase
shift in the 2 to 3 GHz. That phase
shifter relies on the vector sum method and, to our knowledge,
it is the smallest
phase shifter chip relative
to operating wavelength. We also demostrated another highly compact
variable
phase shifter based on the principle of signal reflection.
That circuit uses a quasi-circulator circuit and an OTA-based
active inductor. While it is usually more desirable to have
a
circuit that can produce a variable phase shift, there are
circumstances where it is essential to produce just one precise phase
shift (e.g., 90º),
over a range of frequencies. By
employing feedback techniques, we have shown how a specific
phase
angle can be produced over multiple gigahertz on-chip.
Frequency Multipliers  A multiplier
is a frequency converter where the objective is to
multiply an input fundamental frequency by an integer number to obtain
a higher-frequency harmonic signal. This is a very useful operation in
communications transceiver systems where it is necessary to generate
high-frequency signals from low phase-noise dielectric resonator
oscillators (DRO's). By taking advantage of the complementary nature of
the current-voltage characteristics of NMOS and PMOS devices in CMOS,
we have invented several new multipliers with very low conversion loss
and that have excellent suppression of the fundamental signal at the
output, which eliminates the need for off-chip filtering structures.
Photo right: 6 GHz tripler.
Integrated circuits for biomedical applications: Deep-Brain stimulation  The
body tremors
experienced by people with Parkinson's disease are caused when bundles
of neurons in the subthalamic nucleus region of the brain start firing
in a continuous fashion. Essentially, the neurons are operating in a
type of positive feedback loop. When probes are inserted into
the brain and they release electrical pulses, the feedback loop is
disrupted, the abnormal neuron activity stops, and the tremors cease.
The frequency of the pulses needed to alleviate the tremors varies from
person to person, but it is typically in the range of 130 Hz to 185 Hz
and above. Electrical stimulation of other cells besides neurons has
also been extensively investigated. In vitro
cardiac tissue cultures, for instance, can be electrically stimulated
during and after growth to better mimic their natural environment. Of
course, the stimulation of in vivo
cardiac tissue through the use of pacemakers is a well established
technology. The signal frequencies used to stimulate cardiac
tissue ranges from about 0.1 Hz to 10 Hz or more. In our
research group we have employed advanced circuit design techniques to
create chips that can generate very low
frequencies
in the range of 0.03 Hz to 185 Hz for the biomedical applications
described above. Another one of our chip designs
won
second place
at the 2010
student paper competition organized by Professional Engineers
Ontario (PEO),
Kingston Chapter.
Quadrature Oscillator Circuits  Differential
oscillators generate two output signals of the same frequency but 180
degrees out of phase. By designing two differential oscillators at
identical frequencies and properly injection-locking them it is
possible to synchronize them so that their phases are in quadrature. As
a result the circuit will have four output signals with the same
frequency but 90 degrees out of phase with one another, which is very
important in the design of modulators and demodulators using QPSK
modulation or its derivatives. In our group we have demonstrated a 3
GHz quadrature oscillator (photo) that uses a super-harmonic coupling
mechanism between two differential oscillators. The circuit has low
phase noise by using cross coupled NMOS and PMOS transistors in the
constituent oscillators.
Phase Shift Keying Modulators  Various
forms of
phase shift keying modulation are used in many important communications
and wireless applications. For example, Binary Phase Shift Keying
(BPSK) is used in the Global Positioning System (GPS) and Radio
Frequency Identification (RFID). Quadrature Phase Shift Keying (QPSK)
is used for WCDMA, Bluetooth, WLANs, and a host of other systems. We
first demonstrated a BPSK modulator using a balun and two transistors
operating in a complementary switching configuration. The circuit had a
throughput of 200 Mbps at a carrier frequency of 2.4 GHz. Subsequently
we designed and tested a QPSK modulator using two BPSK unit cells and
we used a novel vector summation method to combine the outputs of the
two BPSK modules. Right: photograph of the BPSK unit cell.
Microwave Variable Attenuator Circuits  Voltage-Variable
attenuator (VVA) circuits are found in a variety of communications
applications such as transmitters and receivers in order to
counterbalance the fluctuations in gain of the amplifier stages over
temperature. Other applications include microwave signal source power
control, power backoff in RF transmitters, vector modulators, and beam
forming networks. In our research group we have designed a variety of
attenuators using both bipolar and FET technologies. The IC to the
right depicts a microwave CMOS attenuator with over 25 dB attenuation
range.
Frequency Divider Circuits  A
frequency
divider
circuit is one that takes periodic input signal at a particular
frequency and delivers another periodic signal whose frequency is a
fraction of the original input signal. Frequency dividers are key
components in phase shift keying demodulators, phase-locked loops, and
signal generators, to name but a few prominent applications. In our
laboratory we have developed a novel divide-by-four frequency divider
capable of operating at an input frequency of 2.2 GHz. The
microphotograph on the right shows the fabricated integrated circuit
frequency divider using CMOSP18 technology.
Hybrid Microwave Circuits  A
hybrid microwave circuit is one in which a packaged device is used in a
printed circuit board which contains passive microwave components such
as power splitters, power combiners, filters, matching networks, and
others. Once the passive circuitry has been designed and fabricated on
a microwave substrate, the packaged active components such as the
transistors or diodes are soldered or epoxied on the substrate. At
Queen's University we have the capability of generating high-resolution
printed circuit boards for microwave applications. To the right is an
example of a three-port quasi-circulator circuit using slow-wave
couplers and packaged HFET devices.
Passive Microwave Circuits  Our
work in passive circuits is primarily in the area of filters. We are
interested, for example, in multi-band filtering structures. In the
photo is a four-band multiplexer, which means that the filter can
separate an incoming signal into four separate channels. Conversely
since it is a fully passive structure it also works in reverse: it can
be used to combine signals in four separate bands into a single
composite signal.
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