Small size and ruggedness are two important factors in the selection of bandpass filters
for military and OEM applications. Monolithic ceramic block combline bandpass filters
not only offer a size advantage in UHF through L-band frequencies; they also have other
characteristics that make them extremely attractive when compared to other technologies.
The filters are characteristically lower in cost and have relatively good insertion loss
due to their high Q material (Q>10,000). This paper describes the design technique
used for ceramic bandpass filters.
The procedure for designing ceramic bandpass filters is straightforward and relies
on standard filter theory. It is only in the construction stage of the
realization that the structure becomes unique and the commercial attractiveness becomes
apparent. A design example is provided here together with an equivalent circuit for a
Chebyshev equal-ripple filter constructed with a material possessing a dielectric constant
of 37. Ceramic materials with low loss tangents (67 x 10-6) and high dielectric
constants (37 and 78) provide a means to create small coaxial structures which could
be coupled to form combline bandpass filters. The sketch in Figure 1 shows the basic
foreshortened quarter-wavelength coaxial resonator structure. The resulting filters are
compact, rugged devices with low insertion loss in bandwidths of 0.5 to 6 percent. It
is also possible to realize transmission zeros in these devices and structures.
The design procedure for these combline filters is based on papers by Matthaei (1)
and Cristal (2) which include descriptions of the physical structures required for their
realization. It is necessary to determine the order of a filter based on a given bandwidth,
rejection, loss, etc. Using Reference 1, a low pass to bandpass transformation is performed.
For a Chebyshev response, n is obtained from:
Since n cannot be a fraction, it will be rounded up to the next highest integer. Once
n is calculated, the low pass prototype element values (or g values) are obtained (1).
Using the above information, coupling coefficients are given by (1):
To excite the TEM mode, resonators are located in close proximity to one another.
In doing that they become electromagnetically coupled via their associated electric and
magnetic fields. While designing such devices, the desired degree of coupling is usually
known, and it is required in order to determine the spacing necessary to achieve this
coupling. By using coupling coefficients (from equation 4), Reference 2, and transmission
line theory, coupling coefficients are adjusted.