Page 306 - Analog and Digital Filter Design

Filter Design Software 303

The angle is given by the expression 6 = sin-'(l/wJ, where os is the normalized
frequency where the stopband begins.

The program describes filters in terms of their shorthand notation, such as GO#
20 50 degrees. The CO indicates a Cauer type filter. All designs have a 20% pass-
band reflection coefficient, which equates to a VSWR (voltage standing wave
ratio) of 1.5, and this is indicated by the 20 in the notation. The final number
is the angle (in degrees) that was described earlier.

The cutoff frequency must be entered before the design process can take place.
In lowpass or highpass filters this is the passband edge, or -3dB point. This
should be entered as a number and exponent (Le., le6). If a bandpass or band-
stop lilter is being designed the program will ask for two frequencies: the lower
cutoff point and the upper cutoff point. The upper cutoff point must have a
higher frequency than the lower cutoff point.

In a bandpass filter design the upper and lower cutoff frequencies are at the two
passband edges. Between these frequencies the filter has little insertion loss. The
filter has a high insertion loss below the lower cutoff point and above the upper
cutoff point. In a bandstop filter design 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 now has enough specifications to carry out the design process. The
output is a display of the component values, with some explanation given of the
component placement in the ladder network. The netlist is saved in a file called
"filter.ckt," which will be overwritten if a new design is undertaken.

Diplexer

Diplexer.exe designs passive diplexer filters. Two complementary odd-order
ladder networks are designed, which are used to separate signals into high and
low bands or passband and stopband. The ladder networks can be based on
either a Butterworth or a 0.1 dB Chebyshev response.

When the program is run, the user is asked whether the design is a lowpasdhigh-
pass design or a bandpasslbandstop design. A number 1 or 2 should be entered,
corresponding to the two options. At this point the program can be exited by
typing a zero; this also applies to most of the other data entry points.

The user is then asked whether the design should be based on a Butterworth or
a Chebyshev response. For a Butterworth design enter number 1, or number 2
for Chebyshev. The Chebyshev design will have a small amount (0.1 dB) of rippie