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Design Notes

18 GHz MICROSTRIP LOWPASS FILTER

This project shows a Printed Microstrip Filter designed on a 15 mil Alumina substrate. Simulations show a comparison between a pure EM simulation with AXIEM and a more efficient hybrid approach that combines EM analysis (using X-Models) with conventional circuit simulations. Both approaches match the measured data. The "LPF_Basic" schematic shows potential problems when using a schematic tool in that the final layout is not taken into account in the schematic - it is assumed all stub fingers are isolated with the associated errors in results.

The use of X-Models shows the vast speed improvements using a linear schematic over that taken by a true electromagnetic approach.

Data Courtesy of Hughes Aircraft Company

(The warnings, shown as yellow inverted triangles, are the first indication that the linear schematic elements are being pushed passed their recommended limits.  Whenever this occurs, an EM simulation is recommended to confirm the design accuracy)

Overview

Within this project are three designs.  The "LPF_AXIEM"  EM Structure shows the filter in the AXIEM 3D planar electromagnetic simulator, which may also be viewed in three dimensions.

    This compares very closely to measured data.  The "LPF_Advanced" schematic makes use of advanced X-Models (EM-based models), coupled lines and other linear schematic elements, including a substrate definition.

    This too gives excellent correlation.  The "LPF_Basic" indicates potential errors in schematic entry with the associated errors in results.

 

The agreement can clearly be seen in the graphs.  The added benefit is that the linear schematics takes seconds to simulate and the full EM design takes minutes to simulate. 

Simulation Plots

The traces on both graphs show good agreement between the EM tool and the schematic linear engine against measured data, IF the schematic design is entered correctly.

   

 

LPF_AXIEM

When setting up the enclosure definition, we recommend a minimum of three mesh cells across the narrowest line or gap within a design, in order that the tool may approach edge current calculations.  Setting the mesh size any finer will increase simulation time without necessarily adding any more accuracy to the results. This structure is set up to use the Advanced Frequency Sweep  feature. 

LPF_Advanced

The LPF_Advanced schematic makes use of advanced X-Models for the MTEE junctions and also coupled line models.  The X-Models can be found in the various technology element folders (Microstrip, Stripline and Coplanar) often under the junction’s folder in each technology.  These discontinuities are often the cause of simulation errors which can be vastly reduced by using these advanced EM-based X-Models with no increase in simulation time compared to standard equation based junction models.  These X-Models are reporting a small warning of geometry errors.  Even with this warning, the results with the X-models are nearly identical to the EM and measured results. 

LPF_Basic

This schematic makes use of no X-Models and no coupled sections.  This if often how designers make mistakes with a linear tool.

Modeling and Simulation

The X-Models rely on pre-generated equations based on a given substrate definition.  Many of these have been simulated by AWR and can be found under the <Root AWR installation>\EM_Models folder.  You can generate new X-Models for your specific substrate definition if they do not already exist is simple.  Please select File > Open Example  and then type “autofill” to find these preconfigured examples.