Page 304 - Analog and Digital Filter Design

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Filter Desigr! Software

The program then asks for source and load resistance values. These are simply
for the netlist that could be used by a circuit analysis program; but note that if
the source resistance is significant, compared with the resistors used to define
the filter response, the resultant frequency response can be in error.

The program displays a Component list. with a description of each capacitor’s
purpose.

If a highpass design is required, enter a ‘2’’ at the beginning of the prograni
(when asked if a highpass or lowpass is required). Follow all the other steps as
given above, entering a value for all the series capacitors between the input ana
the op-amp’s noninverting input. The result is a component list that describes
resistors used in the final design.

In both cases a file “active.ckt” will be produced and will contain a “Spice-like”
circuit analysis program netlist of the filter design.

Filter2

FilterLexe is a passive filter design program. This can design highpass, lowpass.
bandpass, and bandstop filters, with Butterworth, Chebyshev. and Bessel
responses. Filter orders from three to nine are possible, although Chebyshev
designs are limited to odd order only because equal terminations are used.

When the program is run, it first asks whether a lowpass, highpass, bandpass,
or bandstop filter is required. Entering a number “1” at this point produces a
highpass design; entering a “2” causes a lowpass design to be produced; enter-
ing a “3” or a “4” produces bandpass or bandstop designs, respectively. Enter-
ing a zero allows the user to quit the program. This quit facility is present at all
program entry points.

The cutoff frequency is required. This can be entered as an exponent (i.e.> Ie6)
or as a value and coefficient (i.e., 1 M). If a bandpass or bandstop filter is being
designed the program will ask for two frequencies: the lower cutoff point and
the upper cutoff point. In the case of a bandpass filter these are the two pass-
band edges, and between these frequencies the filter has little insertion loss. In
the case of a bandstop filter there is very little insertion loss below the lower
cutoff frequency or above the upper cutoff frequency. Between the lower and
upper frequencies the filter has a high insertion loss.

The program then asks whether the filter type is Butterworth, Bessei, or
Chebyshev. There are passband ripple options of 0.1 dB, 0.25dB, 0.5dB, and 1 dB.
The program requires a number between one and six to be entered at this point,
or zero to quit.