Abstract:
Magnetically coupled passive transformers are increasingly integrated on-chip for
various analog and radio frequency (RF) applications including direct current
(DC) isolation, impedance transformation/matching, and conversion between
single-ended and differential signals. A primary motivation for the on-chip
integration of transformers is the overall size reduction and reduced cost.
However, the performance of on-chip transformers is adversely affected by
increased losses in the conductors and silicon substrate as well as limited values
of achievable inductance, coupling coefficient and quality factor. Consequently,
accurate, broadband transformer models compatible with transient circuit
simulation are needed for analog and RF integrated circuit design.
This thesis presents a new two-step methodology for automated broadband
model generation for monolithic transformers integrated on silicon substrates.
First, an equivalent circuit model (ECM) in single or multi-PI topology is
extracted from available four-port scattering parameters obtained by
electromagnetic full-wave simulation or measurement. To further enhance the
broadband characteristics of the extracted ECM, an automated augmentation
method based on linear least squares fitting is adapted. The new augmentation
procedure adds physically motivated circuit elements to the ECM resulting in a
guaranteed stable and passive circuit model with improved accuracy over a broad
frequency range. The modeling algorithm has been implemented in a MATLAB
code. A typical SPECTRE model extraction over the frequency range of 0.1-10
GHz takes less than 4 minutes on an Intel-based 2.13 GHz Xeon computer. The
new extraction approach is validated for a variety of transformer configurations
built on standard BiCMOS and CMOS processes with stacked and interleaved
topologies, different shapes, and various turns ratios, sizes and process
parameters. The extracted models show good agreement over a broad frequency
range with the corresponding measurement data obtained by 3-D full wave
electromagnetic simulation.
