Master of Science / Department of Electrical and Computer Engineering / W. B. Kuhn / An analysis of frequency pulling in a varactor-tuned LC VCO under coupling from
an on-chip PA is presented. The large-signal behavior of the VCO's inversion-mode
MOS varactors is outlined, and the susceptibility of the VCO to frequency pulling
from PA aggressor signals with various modulation schemes is discussed. We show
that if the aggressor signal is aperiodic, band-limited, or amplitude-modulated, the
varactor-tuned LC VCO will experience frequency pulling due to time-modulation
of the varactor capacitance. However, if the aggressor signal has constant-envelope
phase modulation, VCO pulling can be eliminated, even in the presence of coupling,
through careful choice of VCO frequency and divider ratio. Additional mitigation
strategies, including new inductor topologies and system-level architectural choices,
are also examined.
The analysis is then applied to improve a fully-integrated half-duplex UHF micro-
transceiver in which signal coupling between the LO and PA caused frequency pulling
that prevented the use of QPSK signaling at certain data rates. We determine that a
VCO operating at 4x transmit frequency will be naturally insensitive to pulling from
QPSK signals. To validate the proposed solution, a prototype IC containing a pair of
QPSK transmitters with integrated 100mW Class-C PAs was designed and fabricated
in 0.18um SOI. The transmitters--one utilizing a 2x VCO, one utilizing a 4x VCO--
were designed to closely match the performance of the original microtransceiver when
transmitting QPSK data. The transmitter with the 2x VCO experienced frequency
pulling from the PA while transmitting QPSK data, but the transmitter with the 4x
VCO did not, thereby confirming the analysis in this work.
A revision of the microtransceiver was designed in 0.5 [mu]m SOS utilizing an off-
chip PA inductor to reduce signal coupling with the VCO. A second revision of the
microtransceiver with two prototype transmitters was designed in 0.25 [mu]m SOS uti-
lizing 4x VCOs and figure-8 VCO inductors for maximum insensitivity to pulling
from QPSK and band-limited modulation, as well as other design improvements that
leverage the higher f[subscript]t of the smaller process. Both revisions also include a hardware
FSK modulator, a new charge pump, and a redesigned fractional-N synthesizer to
attenuate a divided-reference spur in the IF output. These revisions of the radio will
enable future researchers to focus on system-level applications where highly-integrated
medium-power transceivers with fully-functioning IQ modulation are needed.
| Identifer | oai:union.ndltd.org:KSU/oai:krex.k-state.edu:2097/19755 |
| Date | January 1900 |
| Creators | Sobering, Ian David |
| Publisher | Kansas State University |
| Source Sets | K-State Research Exchange |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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