Lowpass Filter Design

Three models of filter will be designed and analyzed using WIPL-D powerful built in feature Filter Designer. Filter Designer is user-friendly, wizard-like GUI with straightforward two-stage filter. It is intended for automated design of lowpass, highpass, bandpass and bandstop filters of Chebyshev and Butterworth type.

Here, the filter type is lowpass and approximation is considered to be Chebyshev. The first model is implemented as LC ladder, the second one as transmission line while the third is microstrip filter. The third model is also simulated in WIPL-D 3D EM solver by using full electromagnetic analysis.

Operating band of interest is between 0.014 GHz and 2.8 GHz. Due to powerful built-in interpolation, it is sufficient to run the model in 11 frequency points (EM simulation) at any standard desktop or laptop. Simulation time for circuit models is negligible, while for full-wave EM model is a couple of seconds per frequency point.

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Metallic Cover over Wilkinson Power Divider

The Wilkinson power divider is a rather simple microwave circuit that allows equal splitting the input power to two of the identical output ports. The EM simulation carried out in the WIPL-D Pro 3D EM solver is itself simple, regardless the high operating frequency of 25 GHz and extremely thin substrate (0.005 mm with Er=3).

The application note demonstrates the most efficient way to model microstrip ports using two trapezoidal plates with a short/thin wire in between. Both wire ends are connected to metallic quads via triple junctions, which instructs the kernel to consider all three nodes to be in electrical connection. Such feeding mechanism inherently enables very low reflection loss. The lumped resistor is realized via concentrated loading.

A metallic box is added to the model of the divider to decrease EM coupling with neighboring devices. The performance of the divider with and without the box is compared. The results are as expected: very low return loss for microstrip ports, equal power division (-3 dB) between the input and the outputs, very good isolation between the output ports and rather low influence of adding the metallic cover.

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Design Flow for Microstrip Bandpass Filter

The accurate and efficient design of a microstrip bandpass filter can be easily accomplished by using WIPL-D design suite. It integrates microwave circuit, EM, radiation and near-field analysis within a single design environment.

The built-in tool Filter Designer allows direct synthesis of a microstrip filter circuit based on the user specification through user-friendly, wizard-like interface. The filter comprising analytic microstrip elements can be optimized to meet the specifications and then smoothly transferred to the EM model. A metallic box can be added to enclose the filter and study the effects of the shielding to the filter performance. The effects occurring in the box can alter the performance related to fundamental and especially to parasitic bandwidths. The effectiveness of techniques to suppress the box modes can be tested by full wave EM simulator immediately at simulation stage without expensive and time consuming rework once filter samples have been made.

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Microstrip Combline Bandpass Filter

A successful simulation of microstrip combline bandpass filter is demonstrated in this application note. The software tool used for the simulation is WIPL-D Pro software, a full wave 3D EM Method-of-Moments (MoM) based solver.

Simulation of one of the classical printed structure, the microstrip combline bandpass filter, was performed in a reasonable time on cost-effective, moderate computer platform. The fabrication of the microstrip filter prototype can follow immediately after the optimization.  The optimal dimensions of the filter resonators, input and output lines and the gaps between the lines are all realizable in any standard microstrip technology. In addition, dielectric parameters have been chosen targeting a popular, commercial substrate.

The combline filter was simulated between 4.0 GHz and 5.5 GHz. The efficient Fitter, which is part of the WIPL-Graph window, enabled the wideband response of this microstrip filter to be accurately interpolated from model simulation at 37 discrete frequency points.

Described bandpass filter is very simple, but it still has very good properties as calculated S-parameters confirm the existence of a transmission zero above the passband which can be used to increase the selectivity at high passband edge.

According to all of the analysis details presented, it can be concluded that WIPL-D software is very suitable for the simulation of various microstrip filter structures for both, commercial and academic purposes.

In addition, the practical and educative value of this document should be recognized. This document with published dimensions of the structure can be used as a starting point for microstrip combline bandpass filter design and further research.

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Microwave Circuits Design in WIPL-D Microwave

Simulation of complex microwave circuits presents a challenge for modern computational software. Using circuit level analysis is recommended option for the starting calculations in order to get results quickly. In circuit solvers, all circuit elements are modeled by predefined library components whose calculations are performed faster than calculations in full-wave EM analysis, even over a wide frequency range, practically on-the-fly.

WIPL-D Microwave is fast, accurate and easy-to-use software tool, what is proofed by several simulation examples. WIPL-D Microwave is compatible with many other software tools because of supported Touchstone file import. It is integrated with WIPL-D EM solver, WIPL-D Optimizer and WIPL-D Time Domain Solver.

The examples of circuit designs include single-stub tuner with rectangular waveguides (matched at 10 GHz), Diplexer operating at 2 GHz and 2.2 GHz, and Chebyshev impedance transformer in coaxial technology from 2 GHz up to 8.5 GHz. Computer required for these simulations is any standard desktop or laptop PC.

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Microstrip Bandpass Filters

This application note presents S-parameters obtained with effective usage of WIPL-D Fitter, number of unknowns, computer memory required and simulation time per frequency obtained after simulation of two passive, microstrip, band pass filters. The first simulated filter is interdigital, while the second one is filter with coupled resonators.

The software tool used for simulations is WIPL-D Pro, a full wave 3D EM Method-of-Moments based solver. Interdigital BPF was simulated from 1900 MHz to 2600 MHz, while coupled resonator filter was simulated from 2000 MHz to 2700 MHz. Both simulations were performed in 9 frequency points. The S11-parameter curve is very smooth, owing WIPL-D Fitter efficient interpolation of the frequency response.

Simulations show extremely low number of unknowns and can be run at any desktop or laptop PC, with simulation times measured in seconds.

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Six-Port as Wireless Communication Receiver

The analysis and simulation of a six-port receiver for 24 GHz QPSK signal usign WIPL-D Microwave design environment has been demonstrated. The set of simulation tools available within the program allows not only for the accurate and reliable design of the individual components comprising a receiver system, but for detailed analysis of the system itself. Example of studying an impact of imperfections of branch-line hybrids to receiver performance has been presented.

Additional simulations of the receiver system are possible. The simulations should address impact of phase imbalance larger then considered 1° to receiver performance. Voltages at ports 1 and 2, both been set to 1 V for the demonstration, can be set to more realistic values. Measured voltages can be multiplied by a coefficient to take into account real conversion characteristic of the detector. Knowing sensitivity threshold of DSPU, minimum received signal level can then be determined. Simulation blocks of branch line and Wilkinson combiner circuits can be replaced with the blocks of S parameters measured on fabricated samples to explore signal constellations with real-world circuits. All these simulations can be carried out using WIPL-D Microwave.

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