RF Electronics Chapter 7: RF Filters Page 245
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Where h (figure 7.57b) is the spacing between the walls and d is the rod diameter.
Knowing K from the filter tables and d and h from the design specifications, allows the
spacing C between the resonators to be calculated.
In addition, the impedance of the resonator is given by:
) tanh(
4
138
0
h
E
d
h
Log Z
Eqn. 7.27
The singly loaded Q for the input (Q
1
) and output (Q
n
) resonators are:
)
2
(
1
4
1
2
0
1
L
Sin
Z
R
Q
Eqn. 7.28a
)
2
(
1
4
2
0
L
Sin
Z
R
Q
n
n
Eqn. 7.28b
Since Q
1
and Q
n
can be obtained by de-normalising q
1
and q
n
from filter tables, R is the
desired termination impedance, (usually 50 Ω) and Z
0
is the characteristic impedance of
the resonator as obtained from equation 7.27, that the tapping points l
1
and l
n
can be
determined using equation 7.28.
Several companies produce commercial interdigital filters, K&L Microwave [12] is one
of them. Interdigital filters have two forms:
Round Rod Interdigital Filters
Figure 7.58. Interdigital filter for operation at 940 MHz, with top plate removed (Kikkert).
RF Electronics: Design and Simulation
245 www.cadence.com/go/awr
