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 Filter, Distributed Filter, Active Filter, Digital Filter, Switched 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 "FilterSolutionsPRO™" 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 AddressLocked or "DongleLocked singleuser 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
Switched Capacitor Filters 
FilterSolutions


FilterQuick

15 Available FIR Selections 
Yes




Cascaded Parallel and BiQuads 
Yes


Yes

IIR Digital Selections of Bilinear, Matched Z, Impulse Invariant, Step Invariant, Modified Impulse Invariant, and Modified Step Invariant 
Yes


Yes

Band Pass topology selection with Band Pass or Low Pass, High Pass stages 
Yes




Switched Capacitor circuit synthesis for Cascade and Parallel Biquads 
Yes


Yes

Userselected minimum Capacitance 
Yes


Yes

Large order synthesis: up to 20 Poles 
Yes


Yes

Automatic minimum Capacitance spread selection 
Yes




Gain changeable Biquads. 
Yes


Yes

OffAxis Quadruplet Zero placement for delay equalization 
Yes


Yes

Locked or Floating License 
Yes


Yes

Available Analyses: 
FilterSolutions


FilterQuick

Exportable text netlists 
Yes


Yes

Easytoread exportable graphical circuit displays 
Yes


Yes

User modified Time, Frequency, Impedances, Reflection Coefficients and Transfer Functions 
Yes


Yes

Random element value updates for Monte Carlo analyses 
Yes


Yes

Allows Separate Analysis for each stage 
Yes


Yes

Rearrange and reanalyze Cascaded Biquads. 
Yes


Yes

Swap Wn's from Biquad to Biquad. 
Yes


Yes

Detailed component sensitivity analyses, Including sensitivity tables and plots 
Yes




About Switched Capacitor Filters
Switched Capacitor Filters are normally used in an Integrated Circuit product environment as Resistors are more expensive and less easily controlled. Switched Capacitors may be used to simulate Resistors to an acceptable degree of accuracy. Capacitors and MOSFETS are generally less costly than Resistors. The switching function of the MOSFET produces a discrete response rather than a continuous response from the filter. Therefore, Z Transforms are employed rather than STransforms. Just as in digital filters, aliasing effects occur. Any ZTransform approximation to a continuous function may be used to design a Switched Capacitor Filter. However, it is generally desirable to use the ZTransform that results in the smallest capacitor value spread.
FilterSolutions and FilterQuick offer the choice of six IIR ZTransforms, giving the option of selecting the design with the smallest capacitor spread. Care must be taken to insure that the ZTransform is acceptably accurate, given the design requirements. The design with the best capacitor spread is not necessarily the best design, as the accuracies may be unacceptable.
Switched Capacitors
Switched Capacitors may simulate positive or negative Resistors. Positive Resistors may float: they are treated as constants in the ZTransform. Negative Resistors may not float and are treated as a constant with a single frame delay in the ZTransform.
Note that the negative Switched Capacitors do not float. It is also critical that the clock DOES NOT OVERLAP with its invert. Switch polarities are user optional, but the polarity definition must be followed everywhere in the filter.
Biquad Architectures
FilterSolutions and FilterQuick support both Cascade and Parallel Biquads. The cascade architecture produces higher quality stop band zeros, while the Parallel form may minimize errors to opamp parasitics. Following are examples of a 1 MHz, thirdorder Elliptic Filter:
High/Low Q Biquads
Second order Biquad stages may be arranged in either a high Q or low Q configuration. In general, the low Q configuration produces a more desirable capacitor spread for Q below 1.0. The high Q configuration performs better for Q values above 1.0. As this “rule of thumb” does not always apply; the program calculates both stages; then uses the stage with the lowest Q.
The user has the capability of selecting the other stage configuration by leftclicking on an opamp in the schematic. The following example is of a fourth order Butterworth filter composed of a low Q stage and a high Q stage.
IIR ZTransforms
The programs offer a choice of six IIR ZTransforms to approximate continuous filters.
In summary, they are:
Bilinear
The Bilinear Transform uses trapezoidal integration to implement the digital ZTransform. It is usually the most accurate digital implementation for filtering a continuous variable. However, significant warping of the frequency spectrum occurs. Prewarping is also an available option.
MatchedZ
The Matched ZTransform employs a basic method of translating Analog poles and zeros to Digital poles and zeros on the cascaded continuous transfer function.
Impulse Invariant
The Impulse Invariant Transformation retains the exact impulse response of the analog system. Its use is desirable in cases where retention of the impulse time response of the filter is important. The transformation is properly performed on the Parallel transfer function.
Step Invariant
Like the Impulse Invariant Transform, the Step Invariant Transformation also retains the exact impulse response of the analog system. Its use is desirable in cases where retention of the impulse time response of the filter is important Again, the transformation is properly performed on the Parallel transfer function.
Modified Impulse Invariant
The Modified Impulse Invariant Transformation performs an Impulse Invariant computation on the Cascaded rather than the Parallel transfer function. This creates some error in the transformation, but may useful in simplifying switched capacitor designs
Modified Step Invariant
The Modified Step Invariant Transformation performs a Step Invariant computation on the Cascaded rather than the parallel transfer function. This creates some minor error in the transformation, but may be useful in simplifying Switched Capacitor designs.
FIR ZTransforms
Switched Capacitor Filters may generally be designed to meet any desired FIR ZTransform. All FIR ZTransforms supported for Digital Filters are also available for Switched Capacitor Filter design. Finite Impulse Response Filters have the advantage, as compared to IIR filters, of allowing group delay to be held constant. FIR filters have the disadvantage of creating a passband that is less controlled. Further, the filter may be excessively large to be of practical use in Switched Capacitor applications.
Band Pass Architectures
Just as for Active Filter designs, the programs allow the user to design Bandpass Filters from bandpass stages or from lowpass and highpass stages. Generally, bandpass stages are more suited for narrow band filters, while lowpass high pass stage designs are more suited for wider band filters. Selection of the incorrect stage type may result in excessively high stage gains.
Net Lists
The programs allow the creation of SPICEexecutable net lists for all Switched Capacitor Filter designs, including those with user modifications. This feature allows for the easy validation of the software and its output, as well as the ability to perform analyses in other tools, for modifications to the filter beyond the capabilities of FilterSolutions. Switched Capacitor Filters are only supported by transient analyses in the program.
User Modification Analysis
Users have the capability to alter Capacitor values and reanalyze the filter. By left clicking on any capacitor, as shown, one can enter the new desired value in the “popup” window. Clicking on any analysis button on the schematic allows the regeneration of frequency, time, input impedance, or Z Transform analyses.
Modify a Capacitor Value
Monte Carlo Analysis
A Monte Carlo statistical analysis may easily be performed visually with the programs. After creating and displaying a Switched Capacitor filter and its frequency, impedance, or time responses, one can left click a capacitor requiring further study. The study can be of one capacitor, or all capacitors at once. In the Change Control Panel, selecting "Random", allows the entry of the maximum tolerance or standard deviation, in per cent.
Monte Carlo analyses may be launched manually by repetitive clicks of "Apply". Alternately, the analyses may be run automatically by entering the desired number of iterations.
Graph traces may be overwritten or retained as desired. Both Uniform and Gaussian distributions are provided for inserting element value error.
The following graph shows an example of the effect of random error from 5% capacitors on the magnitude of a third order Elliptic Filter:
Random Error Due to 5% Capacitor Tolerance
Real and Quadruplet Zero Delay Equalization
Phase angle and group delay may be altered by the presence of dual and quadruplet off axis zeros. Unlike AllPass stages, the mere addition of dual and quadruplet offaxis zeros also affects the passband magnitude response. Therefore additional calculations are needed to adjust the pole locations needed to restore the pass band. Delay equalization with real and quadruplet zeros result in a flatter Chebyshev passband and steeper attenuation near the cutoff frequency than a comparable size filter equalized with traditional AllPass stages. This technique may provide a more efficient filter, depending on the specific design requirements.
Filter Solutions, (but not FilterQuick), offers a fast and easy approach to real and quadruplet delay equalization for low pass, high pass, and bandpass Switched Capacitor Filters. Poles and group delay are updated in real time in response to manipulation of zeros to flatten the pass band into an equiripple (Chebyshev I) or maximally flat (Butterworth) shape. Switched Capacitor Filters are calculated instantly with the positioned zeros.
Quadruplet Zero Equalized Low Pass Chebyshev Passive Filter, Frequency Response and Pole/Zero Plane
Filter Solutions offers efficient Switched Capacitor designs for this filter.
Quadruplet Zero Equalized Low Pass Chebyshev Passive Filter Schematic