Page 307 - Analog and Digital Filter Design

304 Analog and Digital Filter Design

within the passband, but it will have a steeper rate of attenuation outside the
passband.

The filter order is required next and this can be an odd number, between three
and nine. Filter sections with a high order will have a greater degree of rejec-
tion to signals in the other section’s passband.

The load impedance (for both filter sections) is required. Any integer value from
“1” to ‘’10,000” can be entered. As described in Chapter 8, each filter section is
designed for zero source impedance, but when the two sections are combined
the input impedance remains constant and equal to the load. The source is there-
fore required to have the same impedance as both loads, which are also equal.

The cutof‘f frequency must be entered next, as a number and exponent (e.g.,
1.7e5 representing 170kHz). This is the frequency where one filter section’s
passband stops and the other filter section’s passband begins. If a passband
stopband option was chosen, a second cutoff frequency is requested, which must
be higher than the one entered previously. The passband of one filter section is
defined by these frequency limits. The other filter section passes all signal fre-
quencies outside these limits.

The output from this program is a display of the circuit’s component values.
Included is a description of their connection in the two ladder networks. A
circuit netlist is produced and saved in a file, “diplexer.ckt,” that will be over-
written if the user decides to design a new diplexer.

Match2A

The Match2A program allows users to match a source to a load. When the
program is run, the user is asked to enter a value for R1 (the source). A numer-
ical value (e.g., 50) should be entered. Next, the user is asked for a value for R2
(the load). Again, a numerical value (e.g., 100) should be entered.

Once the source and load are entered, the program needs to know the frequency
range over which matching is required. The start frequency (in Hertz) needs to
be entered first, say le6 (for 1MHz). The stop frequency needs to be entered
next, say 2e6 (for 2MHz). The VSWR limit, which is the maximum VSWR
within the frequency range specified, should now be entered (e.g., 2.0).
The load may be complex and have a series or shunt reactance included. To
allow for this the program asks for the load type (L or C or none). Enter “none”
if the load is a pure resistance. If the load has series or parallel inductance, enter
the letter L. If the load has series or parallel capacitance, enter the letter C.
Suppose you enter the letter C. The program will then ask whether the reactance