RF Electronics Chapter 7: RF Filters Page 248
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
of the resonator, as shown in the graph of figure 7.60. After tuning the tapping point for
the correct bandwidth, the correct q
1
and q
n
for the filter has been determined.
The tapping point in the circuit of figure 7.60 of 3.125 mm (+5 mm for TL7) results in
the required 113.2 MHz bandwidth. The total resonator length of Ltot = 46.46 mm places
the maximum response of the loaded resonator at 1 GHz.
Figure 7.60. Left: circuit for input tap determination, right: frequency response.
Figure 7.61. Left: circuit for coupling determination, right: frequency response.
The coupling gap is tuned to obtain the correct coupling between resonators. To reduce
the effect of loading, the tapping length Lct is made as small as possible, while keeping
the total resonator length the same as before, resulting in the test circuit shown in
figure 7.61. When the second resonator is tuned and brought close to the first resonator,
a double peak response will result as shown in figure 7.61. The frequency difference
between the peaks is given by equation 9.4.3 in Zverev [1] as:
dB fp
BW k
3 12
Eqn. 7.30
For this filter k
12
= 1, so that
fp
=70 MHz for the peaks in frequency response of the test
circuit, coupling between the first and second resonator as shown in figure 7.61. This
corresponds to a coupling gap of 1.54 mm. The same test circuit is used to determine the
coupling gaps for the other resonators. For the coupling between the second and the third
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