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1983-10-04
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57 lines
b. BANDPASS FILTER
A common structure in microwave design is the edge-coup-
led bandpass filter, either in microstrip or stripline. We
can easily model such circuits with the CPL element. Here, we
shall construct only half of the filter, then utilize the IMA
(image) transformation to obtain the other half, joining the
two to form a complete filter. This is a good technique for
decreasing execution time. For our example, we will consider
a 3-pole filter having a bandwidth of 200 MHz and a center
frequency of 2 GHz.
ELEMENT PARAMETER
TYPE NUM #1 #2 #3
1 CPL 1 77.67 38.04 2
2 CPL 2 56.17 45.06 2
3 CAS 3 1 2
4 IMA 4 3
5 CAS 5 3 4
6 END 5
The bandpass performance of this filter, from 1.8 GHz to
2.2 GHz in .05 GHz steps, is found to be:
FREQ S11 S21 S12 S22
in GHz in dB in dB in dB in dB
1.8 -0.07 -18.16 -18.16 -0.07
1.85 -0.36 -10.95 -10.95 -0.36
1.9 -2.97 -3.05 -3.05 -2.97
1.95 -17.63 -0.08 -0.08 -17.63
2 -153.3 0 0 -153.3
2.05 -17.63 -0.08 -0.08 -17.63
2.1 -2.97 -3.05 -3.05 -2.97
2.15 -0.36 -10.95 -10.95 -0.36
2.2 -0.07 -18.16 -18.16 -0.067
Note that this is only one way of describing the filter.
We could have chosen to use the EQU command as in the previous
example, and simply make the third pair of coupled lines equal
to the second, and the fourth equal to the first. This, how-
ever, would have required three cascades per frequency, where-
as we have obtained the same results here using only two.
When execution time is important, little things like this can
make a difference, particularly during optimization.
C. DEVICE MODELLING
Every once in a while, a device is encountered which is
not characterized in any two-port parameters at all. A good
example of this is the J309 J-FET, which we will be using in
later examples. Before we use this device in any circuits,
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