Finite-difference Transmission Line Matrix Research Articles

FD-TLM electromagnetic field simulation of high-speed Josephson junction digital logic gates

The finite-difference transmission line matrix (FD-TLM) method is extended to modeling low-T/sub c/ Josephson junction (JJ) digital logic integrated circuits (IC's), providing comprehensive simultaneous time-domain, three-dimensional (3-D) full-wave electromagnetic field and JJ device analysis. Techniques for FD-TLM modeling of a Josephson Atto-Webber switch (JAWS), a two-junction superconducting quantum interference device (SQUID), and modified variable threshold logic (MVTL) logic gates are discussed and simulation results are presented. Interconnection lengths are intentionally short so that the full-wave FD-TLM simulation results can be validated with results of conventional quasistatic-based circuit simulations. Good agreement between the simulation techniques validates the FD-TLM JJ logic circuit modeling approach. In the FD-TLM method the electromagnetic behavior of the circuit is modeled from the material properties and dimensions of the circuit, avoiding separate extractions of parasitic capacitance and inductance as needed in conventional circuit simulations.

Study and characterization of optoelectronic photoconductor-based probes

Photoconductor sampling probes are important for optoelectronic device and circuit measurement applications. We use the finite-difference transmission line matrix (FD-TLM) method to characterize the ultrafast photoconductor sampling probes' fundamental performance limits imposed by electromagnetic constraints. Both on-wafer and free-standing photoconductor probes are investigated as applied to microstrip lines found in millimeter-wave integrated circuits and as applied to coplanar strip lines found in ultrafast device characterization systems. Localized invasiveness, distributed loading, and measurement accuracy are investigated for various probing configurations and orientations. Localized invasiveness is small for all our simulations. Distributed loading is negligible for cases where the guided-mode field confinement is tighter than the spatial separation between the probe support structure and the transmission line. Measurement accuracy is acceptable for simulated structures, but amplitude calibration precision may be compromised. We also study photoconductor probes for high-bandwidth signal generation on transmission line structures. The generated signals follow the photoconductor switch photocurrent signal for cases where both the probe and the transmission line form well-matched guiding-wave structures but are attenuated and distorted for other cases.

Coplanar transmission lines on thin substrates for high-speed low-loss propagation

We investigate the attenuation and phase velocity characteristics of coplanar strip (CPS) transmission lines designed for high-speed, low-loss propagation at sub-THz frequencies. Photoconductor switches driven by femtosecond optical pulses were used to generate propagating picosecond electrical transients. External electrooptic sampling was used to measure the time-domain impulse-response characteristics with subpicosecond time resolution. The finite-difference transmission-line-matrix (FD-TLM) numerical method was used to model picosecond pulse propagation on identical transmission lines. The experiment and the numeric simulations have clarified nonquasistatic high-frequency effects and were shown to agree over a 500 GHz frequency range. Additionally, analytic quasi-static velocity and characteristic impedance formulas have been verified and their frequency range of validity established for the investigated CPS geometries. Radiation into the substrate is the dominant loss mechanism at frequencies above /spl sim/100 GHz for the CPS lines on thick substrates. CPS transmission line fabrication on thin substrates has been proposed as a method for reducing high-frequency loss and increasing the microwave propagation velocity. CPS transmission lines fabricated on 8-/spl mu/m-thick Si membranes have been studied and demonstrated to possess the desired high-speed, low-loss properties.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A finite-difference transmission line matrix method incorporating a nonlinear device model

A variable-mesh combination of the expanded-node transmission line matrix (TLM) and finite-difference-time-domain (FD-TD) methods for solving time-domain electromagnetic problems is described. It retains the physical process of wave propagation and the numerical stability of the former and it has the computational efficiency of the latter. This full-wave finite-difference transmission line matrix (FD-TLM) method utilizes transmission lines of differing impedances to implement a three-dimensional variable mesh, which makes practical the simulation of structures having fine details, such as digital integrated circuits (ICs). Circuit models for lumped resistors, capacitors, diodes, and MESFETs have been developed and included for use in simulating digital and microwave ICs. The validity of the variable mesh implementation is verified by comparing an FD-TLM simulation of a picosecond pulse generator structure with electrooptical measurements, and the validity of the device model implementation is verified by comparing an FD-TLM simulation of a MESFET logic inverter with a SPICE simulation. >