RF Electronics Chapter 7: RF Filters Page 249
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
resonator
fp
= 0.5559*70 MHz = 38.9 MHz. The coupling gap is re-tuned to obtain a
difference of 38.9 MHz between the peaks. This corresponds to a coupling gap of 2.5 mm.
This procedure is then repeated for all the remaining coupling gaps for the filter using the
respective coupling coefficients from tables or equations 4 and 5.
Figure 7.62. Circuit for the interdigital filter using calculated loading and coupling values.
Simulating this circuit of figure 7.62 gives a filter with a response close to the desired
value, as shown in figure 7.63. To obtain the precise desired specification, the filter needs
to be optimised. For a lossless Butterworth filter, the coupling gaps are symmetrical.
However, for lossy filters or different filter types, asymmetrical coupling gaps result. For
a practical Butterworth Bandpass filter, the design procedure can assume a lossless filter.
Subsequent optimisation for the desired filter performance, accommodates the losses by;
allowing changes in resonator lengths, coupling points and coupling gaps. Note that after
optimisation, the resonators in this design are not of equal length and the filter is no longer
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