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Westbay Compufilt v2

Line Interference Analysis. EMI Suppression Filter Modelling

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    Westbay Compufilt, first introduced in 1994, has been fully updated for V2.0, with the introduction of many new features. The principle of Compufilt is to provide ready built models of Feedthrough, Mains (Line), and PCB filters, to allow insertion loss versus frequency characteristics to be plotted. A range of pulse shapes can also be applied to filter circuits, allowing behaviour in data circuits and to transient events to be modelled. A fully-featured Search Wizard is also available to provide a filter circuit and component values to match particular performance requirements.

   Feedthrough Filter Modelling

        Six feedthrough style filter models are provided: capacitor, L-C filter, C-L filter, Ferrite Bead, Pi filter and T filter. There is a choice of eight Source Circuits and six Load Circuits. Inductors can have self-resistance and self-capacitance. Inductors can have a Core Characteristic assigned, allowing inductance and resistance to be frequency-variable. A Saturation Characteristic can also be defined, allowing inductance to be varied with current.
        Capacitors can have a temperature coefficient of capacitance characteristic (TCC) assigned, such as Z5U, to allow variation of performance with temperature to be modelled.


        A Spot Analysis can be made of insertion loss, attenuation, input and output impedances at a given frequency, along with phase information.
        A graph of performance versus frequency can be plotted between 10Hz and 10GHz, with up to five models plotted at once. Frequency and insertion loss data can also be exported to a file suitable for opening in a spreadsheet. Parameter Sweeps can be performed on a range of items, such as Source Resistance, allowing the performance variations of a filter to be displayed over a range of values.
        A range of waveforms can be applied to a filter using the Waveform Plots facility.


Mains Filter Modelling

        Seven Mains (or Line) filter models are provided, ranging from the basic inlet filter with a sngle Cx capacitor, bigfilar inductor and pair of Cy capacitors, through to a two stage filter with two sets of inductors, two pairs of Cy capacitors and three Cx capacitors. Both differential mode and common mode insertion loss is calculated. The differential mode circuit can have the full range of Source and Load circuits, whilst the common mode circuit can have resistive or capacitive coupling and loading, reflecting real conditions.
        A LISN (Line Impedance Stabilising Network) can be placed between the filter and the line voltage. The noise source can be external, where the filter is intended to improve the immunity of the equipment, or internal, where the filter is required to reduce emissions from the equipment.
        As with the Feedthrough Filters, Spot Analysis and Parameter Sweeps are provided.


PCB Filter Modelling

        Six Printed Circuit Board filter models are provided, being C, L-C, C-L, inductor, Pi filter and T filter. They differ from the Feedthrough models in that the capacitors have self-inductance, which dominates the performance at high frequencies. All the inductor and capacitor characterisations provided by the Feedthrough models are also available.

Custom Circuit Modelling

        Whilst the Feedthrough, PCB and Mains configurations will cover many filter circuit needs, the Custom Circuit Modeller offers the opportunity to define many other styles. It consists of seven series and six shunt elements, interspersed with one another. Each element can be a choice of R, L, C, C-R, L-R, L-C or L-R-C. The standard range of source and load circuits is again available.
        Spot analysis and swept frequency graphs can be obtained, and the circuit linked to the Waveform Plots facility.

Source and Load Circuits

        EMI Suppression Filters are normally characterised in a 50 ohm source and load system, although insertion loss levels vary considerably with other source and load conditions. Compufilt not only allows any value of source and load resistance to be specified, but allows other circuits to be used.
        The Source circuit can be R, L, C, series L//C, series L//R, series L-R, series R-C, or series R with shunt C to ground. The Load circuit can be R, C, L, L-R, C-R or L-C-R.

Parameter Sweeps

        The Parameter Sweeps facility offers an excellent means of investigating filter performance variations under varying operating conditions. Factors such as Source and Load impedance will often not be   precisely known, and it is useful to look at worse case performance by varying these factors over a range.
        According to circuit model, some or all of the following parameters can be swept over a range of values of the user's choice:
Source and load resistance, filter capacitance, inductance, some component parasticis, current and temperature.
        The parameter step sequences can either be linear, with equal sized steps, or logarithmic. The latter is useful when stepping capacitance for example, providing equally spaced graph curves on the logarithmically arranged frequency scales.
        A Sweep facility is provided for Feedthrough, Mains and PCB Filter Styles.


Waveform Plots

        EMI Suppression Filters are often used on data lines, carrying digital signals for example. Higher data rates can include frequencies close to that of the interference band, and in these circumstances it is necessary to know the effect of the filter on the signal. Capacitive filters tend to slow the rising edge of a pulse, and if the capacitance is large enough, to attenuate the signal. The switching threshold can also be delayed.
        Filters are also often the 'first' component in an equipment, at the interface between the circuitry and external cabling. As such, they may be subjected to electrical transients, which can take the form of a fast rising voltage edge followed by an exponential decay for example. A number of things are of interest here; can the filter attenuate the transient to any extent; wil inductive components in the filter cause resonance and voltage magnification.
        The Waveform Plots facility takes the current filter model (Feedthrough, PCB, Mains or Custom Circuit), and applies one of five waveforms: Sinewave; Trapezoid; Ringing Trapezoid; Damped Sinusoid; Decaying Exponential. An amplitude versus time plot is produced; alternatively, a Fourier Harmonics graph can also be produced.


The Search Wizard

        It is often required to match a filter to a set of insertion loss requirements, or an increase in a margin between observed currents and specification limits perhaps. The Filter Search Wizard leads the user through a series of stages to make the process of finding a suitable filter more straightforward, collecting data as it progresses.

The URL Database and Web Browser

        Users of Westbay Compufilt will often want to reference filter manufacturers' data, and to assist that process, Compufilt incorporates a database of manufacturers' web addresses. This can be viewed and edited within the program, and a simple in-built browser enables rapid connection to the company's site. (Please note that Microsoft Internet Explorer 4 or later needs to be installed on your system for the in-built Browser to operate).

The Tools Facility

        A 'Tools' facility gathers together a range of inductance and capacitance calculations, dB conversions and other useful information, for the convenience of the user.

Westbay Compufilt Lite

    This is a free of charge program that can be copied and installed as required. It provides unlimited modelling of Feedthrough Capacitors, and unlimited access to the URL Database and Web Browser. All other circuit models can also be accessed, with restrictions on source and load circuits and component values. Compufilt Lite can be downloaded.

System Requirements

Windows 95 or later. 20Mb of Hard Disk space.

Westbay Compufilt is a trademark of Westbay Technology Ltd.
Microsoft and Windows are trademarks of Microsoft Corporation
Copyright Westbay Technology 1995 - 2011.


Westbay Technology Ltd, Main Street, Baycliff, Ulverston, Cumbria LA12 9RN, England

Tel: 01229 869 798    (from the United Kingdom)    Tel: +44 1229 869 798    (International)                                 Contact by e-mail