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Distributed Filter Module


Feature Comparison Summary

FilterSolutions® and Filter Quick™ are Windows® based software programs for the synthesis and analysis of electronic filter circuits. The modules available in these programs comprise: Passive FilterDistributed FilterActive FilterDigital FilterSwitched Capacitor, and Zmatch™ (used for creating impedance matching circuits). FilterQuick, which is included in FilterSolutions. offers a simplified interface for initiating designs which are then usable with either the FilterQuick intreface or FilterSolutions' advanced feature sets.

Licenses for FilterSolutions can be purchased as "FilterSolutions-PRO™" which contains all the modules. Alternately, the modules can each be licensed individually.

The "PRO" licenses include the Zmatch Impedance Matching Circuit module. The Distributed Element License will include the Distributed Element version of Zmatch, and the Passive Module will include the Lumped Element version of Zmatch.

FilterSolutions and its individual modules are available in Network license configuration,or  MAC Address-Locked or "Dongle-Locked single-user licenses.

 FilterFree is a "freeware" version of FilterSolutions with minimal functionality (DOWNLOAD LINK). FilterFree analysis is limited to 3rd Order Analog designs or to 10 tap FIR designs. All analyses in FilterFree are limited to ideal transfer functions


Distributed Filter Synthesis: FilterSolutions   FilterQuick
Distributed designs with Inductor translations only Yes   -
Composite designs including lumped elements and distributed stubs Yes   Yes
Design with Single Stub or Dual Stub Resonators Yes   Yes
Easily change Single Stub Resonators to Dual Stub and vice-versa Yes   -
Single Terminated, Unequally Terminated, and Equally Terminated Filters Yes   -
User modified Time, Frequency, Impedances, Reflection Coefficients and Transfer Functions Yes   -
Large order synthesis of up to 20 poles Yes   Yes
Perform "em" optimized synthesis with Sonnet internal Co-calibrated Ports Yes   -
Create Impedance matching networks with built-in Zmatch program** Yes   -
Diplexers and Multiplexers Yes   -
Microstrip and Stripline Geometries Yes   Yes
Parallel Edge and Shunt Stub coupled Resonator Bandpass designs: Stripline and Microstrip Yes   -
Radial and Delta Microstrip and Stripline Stubs Yes   -
Composite Filters comprising Passive Elements and Distributed Filter Stubs Yes   -
Suspended Substrates and Covers Yes   Yes
Tapped or Untapped Combline, Hairpin or Interdigital Microstrip or Stripline filter designs Yes   -
Multi-Band Filter Synthesis* Yes   -
Locked or Floating License Yes   Yes
Third Party Tool Connections: FilterSolutions   FilterQuick
Export DXF files for Stripline, Microstrip and Suspended Substrate layers to AutoCAD Yes   Yes
Direct export to Sonnet Software "em" analysis tools Yes  


Direct export to NI - AWRCorp software tools (where available) Yes   Yes
Supports full Modelithics Select models in NI-AWR analyses Yes   Yes

Direct export to CST Microwave's STUDIO SUITE™


Yes   Yes
Available Analyses and Output: FilterSolutions   FilterQuick
Exportable Text Netlists and Transfer Functions Yes   Yes
Easy-to-Read Exportable graphical circuit displays Yes   Yes
Frequency, Impedance and Time analyses Yes   Yes
User selected dimensional systems Yes   Yes
Parasitic Resistance and Conductance Analyses Yes   -
Random element value updates for Monte Carlo analyses Yes   -
DCR and ESR Parasitic Analysis for lumped elements in Composite Filters Yes   -
Easily change single stub to dual stub or dual stub to single stub designs Yes   -
Smith Chart and Jones Chart outputs Yes   Yes

* Multi-Band Filter networks are defined as a cascade of an arbitrary number of passbands and stopbands in a single lumped LC, active or digital IIR filter

** The Zmatch program allows for matching filter circuits to any arbitrary real or imaginary, input or output impedance values

FilterSolutions and FilterQuick support a wide variety of distributed filter designs. Filters may be singly terminated, or doubly terminated with equal or unequal load and source impedance. Diplexers are supported for all filter types. Finite resistance and conductance analyses are supported for all distributed filters.

All-pass sections are supported in order to create Delay Equalizers. All synthesized filters are checked for proper frequency response prior to being displayed. All filters are displayed in graphic, easy to read formats. The programs provide netlist selections that display the filter netlist in a text format, ready for AC or transient analysis. FilterSolutions, (but not FilterQuick), creates both balanced and unbalanced filters.

In order to generate singly terminated filters, a source impedance of "0" is entered in the source resistance entry field. To generate an equally terminated filter, the same number is entered for both source and load impedance.  If differing source and load resistances are entered, the programs will create both singly and doubly terminated filters fitting the filter characteristics selected in the Control Panel.

Sonnet, CST, and DXF Exports

Sonnet Software, Inc (www.sonnetsoftware.com) is totally dedicated to the development of commercial Planar High-Frequency EM (electromagnetic) software.  FilterSolutions and FilterQuick's Distributed Filter designs in Stripline, Microstrip, or Suspended Substrate, may be exported as Sonnet format files, or directly into the Sonnet program, for detailed Sonnet electromagnetic analysis.

In conjunction with CST Microwave (www.cst.com) Nuhertz provides the ability to integrate filter synthesis with a full-wave 3-dimensional electromagnetic analysis tool.  Working with CST STUDIO SUITE® a new capability in analytic tool integration is now available.

FilterSolutions also support the export of Stripline, Microstrip, and Suspended Substrate design layouts to AutoCAD™ in .DXF format.  Synthesized filter design files may be imported into any planar microwave analysis tool that supports DXF imports.

Distributed Components

Transmission line filters use distributed element theory to translate Lumped Element LC filters into distributed stubs. Inductors and Parallel LC resonators may be replaced with shorted distributed stubs, Capacitors and series LC Resonators may be replaced with open distributed stubs. Replacing LC Resonators with single stubs is more efficient, but results in significantly more aliasing. 

Inductors may be replaced with 90 degree distributed segments.  Series resonators may be replaced by 180 degree distributed segments.  Checking the "Combine Stubs" box will cause filters to be synthesized with single stub resonators.  Checking the "Use Segments" box will cause filters to be synthesized using distributed segments instead of cascaded stubs.

Inductor and capacitor components and resonators, with their distributed replacement element equivalents, are shown below:

Distributed Displays

All FilterSolutions graphical filter displays are user interactive. It is possible to change the values of any stub or segment, add new components, or delete stubs. At that point a frequency, time reflection or impedance analysis of the modified filter can be generated.   The stub or segment can be highlighted, and then changed, by passing the cursor over the element. When highlighted, a left-click of the mouse changes the stub or segment, while a right-click adds or deletes a stub (shown below).  Selectable parameters include impedance, length, resistance, conductance, or propagation speed.

After a stub or segment is changed, it appears in blue for visual reference. If a frequency value changes as a result of the new element values, the new value is also displayed in blue.

Changing, Adding, or Deleting a Distributed Filter Component

Adding stubs is a useful technique when modeling parasitics or evaluating the effect of other attached circuits. Resonant stub pairs have the optional property of automatically updating the other stub in the pair, so that the resonant frequency is maintained. All-pass sections may be added by right clicking a stub adjacent to the load or source Resistor, or by adding All-pass poles and zeros to the pole/zero plot.

Changing distributed values is a useful tool for evaluating the effect of real distributed elements in the filter

Lumped Elements in Distributed Filters

FilterSolutions offer the capability of creating filters with a mixture of lumped and distributed stubs. This is a flexible method of substituting the low Q lumped elements with distributed stubs, and using lumped elements where high Q parts are available.

A coupled resonator bandpass filter is shown with open stubs replaced by lumped capacitors. The shorted stubs remain as substitutions for inductors.  The impedance of the shorted stubs has been manually set to 100 Ohms using the change control panel.

Lumped Element Substitution

Frequency, Reflection, Impedance, and Time Analysis

Frequency, reflection, impedance, or time analysis of filters generated by Filter Solutions and FilterQuick are performed by selecting the appropriate control on the circuit window.  Frequency, reflection, and impedance analysis include magnitude, phase, and group delay. Time analysis includes step, ramp, and impulse response.  By clicking the left mouse key at any location, a cursor trace with the frequency and trace information is activated. These analyses include all user modifications made to the filter.

When a filter has been modified by changing, adding, or deleting a stub, or, if any elements or segments containing nonzero resistance or conductance, then the "Ideal" analysis trace appears in dark blue for comparison.

Attenuation due to the source Resistor may be included or removed as desired using the "Inc Gen Bias" box in the Lumped Element control panel. Examples are shown below:

Frequency Analysis

Time Analysis

Finite Resistance and Conductance

FilterSolutions allow the simulation of nonzero resistance and conductance in Distributed Filters. The specific filter frequency, reflection, impedance, and time analyses will use the resistance and conductance provided in the analysis. A dark blue background trace shows the ideal filter (with zero resistance and conductance) for quick comparison. For singly terminated and unequally terminated filters, degradation effects based on non-zero resistance and conductance may be manually compensated by moving the pole locations in the pole/zero plot. (The compensation is done in real time if the RTC box on top of the pole/zero plot is checked).

The poles may be "stretched" along the real axis, and flattened slightly along the imaginary axis. (This works reasonably well for singly terminated filters and largely unequal terminated filters). For filters whose source-to-load ratio is greater than 0.2 and less than 5.0, non-zero resistance and conductance compensation is marginal.

Equally terminated filters cannot use compensation in this manner.

Since the effect of non-zero resistance and conductance is unique to each distributed filter, the effects of non-zero resistance and conductance are only included in the specific filter being analyzed. The control panel analysis functions assume an ideal filter.

To display the non-zero resistance and conductance in Ohms and Mhos per unit length of the distributed stub, one checks the "Other Info" check box in the distributed filter display. The check box also displays stub inductance and capacitance in Henries, and Farads per unit length.

Nonzero Resistance and Conductance

Coupled Resonator Filters

Coupled resonator bandpass Filters are narrow band approximations of bandpass filters. The advantages of using coupled resonators rather than classic bandpass filters are the attainment of more desirable element values at high frequencies, and flexible element value selections. (See the Lumped Element Filter section on coupled resonator band pass filters for more information).

FilterSolutions supports Coupled Resonator bandpass filters with distributed Stubs. Stub values may be selected based upon equivalent capacitance or inductance of one of the resonator legs. Lumped elements may be substituted for distributed stubs where practical. Resonators may be constructed with separate L and C equivalent stubs or one LC resonator stub

Coupled Resonator Example:

Below is an example of 1GHz Capacitor Coupled bandpass filter. Only the first stage resonator and coupling elements are shown.

Lumped Element Coupled Resonator Band Bass Filter

Here is the distributed equivalent of the same Coupled Resonator bandpass filter. The first two stubs replace the LC resonator. The third element replaces the coupling capacitor. This filter has an alias frequency of 5 GHz.

Distributed Coupled Series Resonator Filter

The following schematic shows the same Coupled Resonator Filter with a single stub Resonator. Using one stub instead of two moves the alias frequency down from 5 to 2 GHz

Filter with Single Stub Resonator

Here is the same filter with only the inductors replaced by distributed stubs. This substitution results in minimal aliasing, and fewer stubs.

Filter with Inductors replaced by Stubs.

Parallel Coupled and Shunt Stub Filters

Parallel Edge Coupled and Shunt Stub bandpass Filters can be synthesized in FilterSolutions.  Both offer superior geometries over classic designs or Lumped Element Coupled Resonator equivalent designs.  Parallel Coupled Filter models support both open and shorted end stubs, as shown below.  The outer sections may be replaced with narrow band single line approximations to remove narrow gaps.

Shunt-Stub Bandpass Filters may be produced with all stubs being identical when designing Chebyshev I and Butterworth filters, as shown below.

Multi-Conductor Filters

Interdigital, Hairpin, and Combline Filters can be synthesized by the programs.  Interdigital and Hairpin filters may be calculated with or without Tapped and Hairpin Resonator end-couplings, and with or without ground end couplers.  Combline Filters are available with or without Tapped end-couplings.  The grounded end couplers generally produce more practical geometry but tend to be more costly.  Tapped and Hairpin coupled ends frequently eliminate impractically small outer gap geometries.

Following are examples of the various multi-conductor filter options.

Miniaturized Hairpins and Ring Resonators

Standard hairpin filters are inherently inefficient due to wasted space in the hairpins and the inability to narrow the band with the use of transmission zeros.  Miniaturized hairpins solve this problem by folding the hairpin legs back inside the hairpin, and providing a geometry suitable for synthesizing transmission zeros.  Ring resonators improve space utilization in the vertical direction and also providing a geometry suitable for synthesizing transmission zeros.

Below are the four topologies shown to scale. It is easily seen that the cross coupled miniaturized hairpin is by far the most space efficient topology.

Cross Coupled Miniaturized Hairpins with the Most Efficient Space Utilization  (670 Square mm)

EM Optimization

EM Optimization may be performed in NI -AWR with Sonnet Port Tuning, and EM extractions with Axiem, Sonnet, or other third party EM solvers. 

Net Lists

When "Netlist" is selected in the Distributed Element circuit control bar, the filter’s netlist is shown in a text window. The netlist is set up for AC and transient analyses on the load resistance, and is ready to plug into any application that uses netlists. When finite Q is selected, the parasitic resistances are included in the netlist. Resistance values used are based upon the resonant frequency of LC pairs, and the cutoff or center frequency for all other components. The pulse source is commented out to prevent conflicts with the AC source. Selecting "Netlist" again removes the net list window.

The netlist supports distributed segments between stubs, ideal distributed stubs, and real LCRG distributed stubs. Ideal distributed stubs are set to the quarter wavelength of the distributed stubs and segments. Dummy Resistors are inserted to allow the net list to execute in SPICE programs.

The netlist may be printed, copied to the Windows clipboard, or saved to a text file. Individual stub values may be selected and copied to the Windows clipboard for ease in retrieving component values for other applications.   If nothing is selected when using the “Copy” function, then everything in the net list will be copied.

Shown below is the generated netlist for the filter shown on the right.

Geometry Layouts

Microstrip and Stripline filters are most easily visualized with accurate geometry layouts. FilterSolutions and FilterQuick offer a layout view to augment the schematic view. Schematic views include distance scale, overall width and length information.


As can be experienced in the design of Digital Filters, RLGC Distributed Element Filters produce aliasing in the frequency axis. The aliasing in turn, causes warping in the frequency range of interest.  The warping causes a reduction in the bandwidth of Bandpass and Band-stop Filters. As in the case of Digital Bilinear Filters, the bandwidth of these filters may be preserved by prewarping the filter prior to synthesis.  When possible, the programs automatically apply the prewarping to Distributed Element bandpass and band-stop filters so that the bandwidths are properly maintained.

Below is an illustration of the effect of pre-warping on the pass band of a Distributed Element Bandpass filter.

No Prewarping

With Prewarping

Effect of Prewarping on a Distributed BandpPass Filter

The Pass Band is designed for 6GHz to 9 GHz.

The Null Frequency is set to 20GHz.

Microstrip and Stripline

FilterSolutions quickly calculates conductor widths of Microstrip and Stripline for implementation in Distributed Element Filters, by selecting the option, and entering the dielectric parameters.  For Microstrip, the program will calculate new line lengths compensated for the effective dielectric constant, due to the absence of dielectric material above the transmission line.

All-pass functions and balanced filters cannot be implemented as Microstrip and Stripline designs. These features are disabled when Microstrip or Stripline configurations are selected.

Microstrip designs are supported in grounded substrate, suspended substrate, and covered substrate topologies.

Coupled Lines

FilterSolutions supports coupled Stripline and Microstrip high pass and bandpass filters, including coupled line resonator filters.  The graphic syntax is shown in Filter 1, below, along with the uncoupled line models.  Stripline and Microstrip widths and gaps are calculated and displayed, provided the width and gap calculations are of reasonable dimensions. 

High pass filters are implemented as wide band coupled line bandpass filters; such that the upper band edge notch is very narrow.

Conductor and Dielectric Losses

Transmission line losses are modeled for accurate filter simulations.  For RGLC lines, standard modeling techniques, to compute complex impedance Beta, are used with the calculated L and C values, and user entered R and G values.  For Stripline and Microstrip designs, attenuation is calculated with the calculated impedance and conductor dimensions, and the user-entered conductor resistivity, dielectric dimensions and dielectric loss tangent. The schematic displays attenuation in dB/length at the center frequency.  For coupled lines, both the even and odd mode attenuation are calculated, modeled, and displayed at the center frequency.

Smith Charts

Smith Charts, Jones Charts, and polar plots are provided for frequency and reflection responses for easy to read graphical feedback.  Left and right mouse keys provide read cursor data from the impedance grid.

Smith Chart Display

Radial and Delta Stubs

Open-end Microstrip and Stripline stubs have the disadvantages of occupying large amounts of space, or performing poorly with higher order modes.  FilterSolutions offers Radial and Delta stubs to help correct these shortcomings.  To illustrate the geometry advantages, the Low Pass filter shown in "Geometry Layouts, above, " is re-designed below using Radial and Delta stubs.  The total width is reduced from 16.1 mm to 10.9 mm using Radial stubs, and 9.3 mm using Delta stubs.

Radial and Delta Stubs

Suspended Substrate

FilterSolutions supports Suspended Substrate Microstrip for single, coupled, and multiple coupled lines, both with and without covers.  Suspended Substrate Microstrip has an advantage over standard Microstrip: it results in less dispersion of the propagation constant, greater structural accuracies, more precise electrical properties, and permits higher characteristic impedances. [1]

Impedance Matching Networks

Devices with differing impedances occasionally have to be matched to other devices or designs to minimize reflections.  FilterSolutions and FilterQuick accomplish the matching function by designing LC matching networks.  In these networks, the impedances are matched such that one device or design sees a conjugate impedance to itself when looking into the matching network attached to the other device. The technique eliminates or reduces reflections.  Multiple, discrete frequency or broadband frequency matching are both supported.

[1] A. Lehtovuori and L. Costa, Model for Shielded Suspended Substrate Microstrip Line, ISBN 951-22-4202-8, Page, 2