RF Electronics Chapter 7: RF Filters Page 231
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
slightly change the frequency response and the group delay. To achieve the best input and
output match and a flat group delay, the component values linked to variables, calculated
using the equations in the Global Definitions folder on the Project palette are replaced
with the fixed values, which can be optimised. The filter is optimised to provide a flat
group delay and meet the bandwidth specification. To achieve the lowest insertion loss at
the centre frequency, S
11
and S
22
are optimised to be better than -30 dB at 18.48 MHz.
The group delay is optimised to be as close to 325 nS as possible. Having a constant group
delay results in a Bessel type filter and the filter bandwidth is determined by the group
delay. The 325 nS group delay results in a 2 MHz bandwidth. The component values after
optimisation are shown in figure 7.36. Figure 7.37 shows the corresponding frequency
response. The resulting filter has a very flat group delay.
Figure 7.36. Optimised coupled resonator LC filter.
Figure 7.37. Frequency response of the filter of figure 7.36.
HF Filters
At higher frequencies, tuneable inductors become difficult to make and the inductors can
be replaced by Microstrip inductors as shown in figure 7.38. This is a capacitive coupled
LC resonator filter with a 160 MHz centre frequency and a 27 MHz bandwidth. The
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