RF Electronics Chapter 2: Computer Simulation Page 12
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
The easiest way to produce the diplexer is to produce a separate high-pass filter and then
use that and the low-pass filter as sub-circuits to produce the diplexer. The low–pass filter
can be the Butterworth low-pass filter of figure 2.7. The high-pass filter shown in figure
2.8, uses the component values from table 2.1. The full diplexer is shown in figure 2.9,
and the resulting S parameter frequency response is shown in figure 2.10.
Practical Note: Since the high-pass filter and low-pass filter are in parallel, the low-pass
filter must have an open-circuit input impedance at high frequencies and the high-pass
filter must have an open circuit input impedance at low frequencies. This is achieved by
using filters that have the first element as a series element for the input.
Figure 2.9. Diplexer using low-pass and high-pass sub-circuits.
In this example, one could optimise each of the 8 individual components of the high-pass
and low-pass filters. To facilitate the optimisation, it is desirable to keep the number of
parameters to be optimised as small as possible, and optimise the 4 values Zn1 to Zn4,
shown in figure 2.9. These are located in the Global Definitions and are used to calculate
the element values for both the high-pass and low-pass filters, using the equations in
figures 2.7 and 2.9. The corner frequency Fc and Impedance R, are also located in the
Global Definitions to allow those to be changed without affecting the optimisation.
Figure 2.10. Frequency response of the diplexer.
RF Electronics: Design and Simulation
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