Differential Filter Design and Optimization

This note explains how to analyze and optimize a general differential microwave circuit using WIPL-D design environment. Weather it is the circuit or EM component the differential/common mode analysis can be carried out within WIPL-D Microwave Pro by adequate connection of several transformer elements to convert single-mode S parameters to differential/common mode S Parameters.

The innovative differential filter (taken from the literature) has been used to illustrate the analysis and optimization procedures. The filter has been analyzed as ideal transmission line circuit, as microstrip schematic and as 3D EM component. Converting ideal transmission line circuit into the realistic microstrip layout, causes several parasitic effects to occur, not included in the ideal model. The optimization process was applied to EM model since it includes all of the parasitic effects.

The modeling method demonstrated here can be applied to directly obtain differential/common mode S parameters of other differential components, such as amplifiers, couplers, antenna matching networks, etc.

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Rectangular Waveguide
Iris-Coupled Filter

In this application note, simulation of a band-pass, iris-coupled waveguide filter is described. The irises will be located on the both sides of the standard WR-90 waveguide. The iris-coupled filter was created from the scratch in WIPL-D Pro CAD. Two symmetry planes (Symmetry and Anti-Symmetry) have been applied to reduce the complexity of the simulations. Two Waveguide Ports were used as feeders.

The model of iris-coupled filter was simulated from 9 GHz to 12 GHz in 16 frequency points. To ensure high accuracy, the convergence of the results has been checked. After the convergence has been confirmed, dimensions of the filter were tuned to set values of S11 in the pass-band (10.19 GHz to 10.82 GHz) below -20 dB.

Computer used for these simulations is regular desktop or laptop, running the problem in couple of seconds per frequency point. Very low number of unknowns was used, resulting in usage of inexpensive hardware platform.

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Effective Antenna Design by
EM-Circuit Co-Simulation

The goal of a circuit-EM co-simulation is to alleviate the computational complexity decomposing the complete system into parts that need to be simulated using full-wave approach, and parts that would be modeled by predefined library components. The major benefit is that any antenna or antenna component of interest can be included and electromagnetically simulated, on-the-fly, at the circuit simulation runtime.

WIPL-D 3D EM solver and WIPL-D MW circuit solver present dynamically connected software. They are fast and accurate in co-simulation. In co-simulation, number of unknowns is dramatically reduced because of separating one big EM computational problem into many small problems.

Here, circuit is implemented in microstrip technology. It consists of two microstrip patch antennas and feeding network: microstrip lines, microstrip T junction and bends. System is analyzed in frequency band 9 GHz up to 11 GHz in 9 uniformly distributed points. Central operating frequency is 10 GHz. Simulation time is measured in seconds on any standard desktop or laptop.

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Rectangular Waveguide Interdigital Filter

This application note describes simulation of a band-pass, interdigital filter in rectangular waveguide technology. The filter was modeled using WIPL-D Pro CAD by using native editor and built-in primitives.

Symmetry plane has been utilized to reduce the complexity of the structure and two Waveguide Ports were used as feeders. The model was simulated from 5 GHz to 14 GHz in 26 frequency points. To ensure high accuracy, the convergence of the results has been checked. After the convergence study, dimensions of the filter were tuned to set values of s11 in the pass-band (8.30 GHz to 10.86 GHz) below -20 dB.

Computer used can be any desktop or laptop, desirably with higher number of CPU cores, with simulation time per frequency measured in
seconds.

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Bond Wires as Interconnect Technology

Accurate modeling of the bonding wires is essential to successfully utilize semiconductor bare dies and achieve a large scale of integration of modern microwave front ends. WIPL-D Microwave provides a complete design environment where electromagnetic and circuit simulation can be efficiently applied to examine the influence of interconnects to the performance of an integrated system.

The limitations of the single wire bonds regarding the relatively narrow return loss bandwidth have been illustrated through the analysis of performance degradation of an amplifier. The improvement of the characteristics when double wire bonds are utilized are presented and the mechanism of the improvement explained.

The selected examples concentrate on the applications in the frequency range around 24 GHz. As the operating frequency goes higher, the effects of interconnects become more severe and some means of compensation of bond wire connections described in the literature must be utilized to keep the return loss at reasonable values or a different interconnect technology, such as flip chip of micro ball grid array (µBGA), has to be considered.

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Dielectric Resonator Filter [Verified by Measurements]

The application note deals with complex dielectric resonator filters with numerous tuning elements. Careful comparison to measured results is carried out in each step. The story unfolds from a simple single cavity with the loop made of coax cable. The cubic dielectric puck is inserted at the middle to modify the resonance. In the next step, the puck is hollow, which changes the resonance again. WIPL-D sweeper is used to demonstrate how the resonance shifts by merely changing puck length. Such models are simple and the simulation is carried out in seconds in the entire frequency band. Single cavity with solid puck is the most important model. It is tuned to achieve a wide and deep resonance, which coincide with the measured results. A single cavity model is very fast and lasts only a couple of seconds per frequency point, where powerful built-in interpolation allows using only 11 frequency points.

In the last step, a single cavity is mirrored and connected to the second cavity with goal post. The simulation is still carried out in seconds per frequency point at regular desktop or laptop PC.

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Matching Network Design for Power Amplifiers

The adequate modeling of matching networks is crucial for the success of power amplifier design. Due to a set of simulation tools included, WIPL-D Microwave provides a complete environment for accurate and efficient modeling and design of power amplifiers.

Example of a commercially available power transistor has been presented to explain the matching network topologies preferred for the power amplifier application and to demonstrate the outline of the complete design flow. The necessity to introduce electromagnetic analysis to accurately model typical power amplifier matching networks or otherwise face the significant inaccuracy due to the effects of coupled discontinuities has also been explained.

The design has been carried out for the microstrip substrate recommended by a manufacturer. If necessary, it can be easily adapted to any other substrate preferred providing that the substrate thickness conforms with the height of the transistor lead and the first transmission line element in the networks remains wider than the width of the transistor lead.

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3-Port Wilkinson at High Microwave Frequencies

The accurate modeling of the effects occurring at high microwave frequencies is the key to successful design of a Wilkinson power combiner/divider. WIPL-D Microwave provides a complete environment required for a design of these circuits including circuit and electromagnetic co-simulation.

Example of a design cycle has been provided. The cycle starts with the analysis of an ideal, by-the-book circuit schematic, continues to a more detailed schematic with models for microstrip discontinuities, and is then further expanded with the introduction of an EM component to model a complete microstrip circuit with high accuracy. Finally, the impact of a real-world resistor is demonstrated. The impact of each modeling step to degradation of circuit performance comparing to an ideal circuit is illustrated and explained in details. A designer is therefore provided with a clear understanding of what to expect and how to mitigate the potential problems at early stages of the design – it will be a good practice to utilize the smallest resistor size available, and, if possible, pick a substrate so that line-to-resistor-pad length ratio is maximized.

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Linear Transistor Modeling Using Equivalent Circuits

The accurate transistor characterization is a starting point to carry out an amplifier design. Lumped circuit equivalent transistor models are compact and versatile means of providing reliable S parameter data. WIPL-D Microwave provides a complete environment to implement these models.

The modeling of a commercially available low power packaged GaAs HEMT has been illustrated. The partitioning of the transistor equivalent circuit to intrinsic and extrinsic elements has been presented and physical grounds behind each of the elements explained in brief. It has been shown that S parameters calculated using the model are highly accurate and can be utilized to overcome the limitation imposed with sparsely tabulated transistor data, e.g. to find the optimal biasing point for a desired application.

WIPL-D Microwave design environment provides required flexibility to implement a schematic of any commonly used linear transistor model.

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