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1983-10-04
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57 lines
it would be wise to initiate the design using a Smith chart,
matching the input and output at opposite ends of the band de-
sired. Then, use these values as a starting point, specifying
the overall desired performance as objectives for the optimiza-
tion process.
C. EXAMPLES
a. LOWPASS FILTER
Let us assume, for the moment, that we desire a slightly
better stopband performance from the lowpass filter used as an
example on page 18. In this exercise, let us seek to achieve
20 dB or better of attenuation or better from 1.4 GHz and on,
while at the same time improving the passband return loss to
10 dB or better for 0.9 GHz and below. We could, of course,
achieve this performance by simply designing a lowpass filter
with a larger number of poles, but for the purposes of our ex-
ample we will assume that we have limitations (physical space,
parts count, cost, etc.) that are preventing us from doing so.
We will also have to accept a less than minimum passband rip-
ple, and initially we shall specify a 0.2 dB ripple.
By accepting a marginal passband return loss, we have
made a compromise in the performance that should enable us to
achieve our objective. Speaking of which, we need to write a
set of objectives for our optimization. We should specify a
few points in both the passband and stopband. For the pass-
band, frequencies of 0.3, 0.6, and 0.9 GHz should suffice.
As for the stopband, 1.4 and 1.7 GHz can be used. We need not
specify any more for the stopband, as the performance for the
higher frequencies can be assumed to be better than that at
1.4 GHz, due to the nature of the filter. Using the objectives
editor (see the following chapter for a description), we write
the following:
FREQ OBJECTIVES
in GHz S11 WGT S22 WGT S21 WGT
0.3 -10 10 -10 10 -0.1 10
0.6 -10 10 -10 10 -0.1 10
0.9 -10 10 -10 10 -0.1 10
1.4 0 0 0 0 -20 -10
1.7 0 0 0 0 -20 -10
The purpose in specifying the passband S21 as -0.1 dB
rather than -0.2 dB is that we achieve equal weighting either
side of this value, thereby giving us -0.2 dB overall. The
stopband return loss is of no concern to us, therefore is has
been eliminated from the error function by setting the weight-
ing to zero. Setting the weighting to a negative value for
the stopband S21 tells the program that anything less than
-20 dB is perfectly acceptable. The weightings applied to
our objectives are purely arbitrary at this point, a value of
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