3.6 GHz filter mixed ceramic conductor

FILTER SPECIFICATIONS I

The preliminary specifications as supplied by the vendor are given on the right-hand side.
The filter is expected to operate at a centre frequency of 3.6 GHz with a maximum insertion loss of 1.3 dB at pass-band edges.
The filter needs to be free of spurious emissions up to 12.75 GHz. The spurios response can be taken care of by the addition of a low-pass filter, however, that will come at the expense of additional insertion loss.
Passband specs can be meet both with standard ceramics supplied by the vendor and with our standard distributed resonator solutions, however, out-of-band specs are more difficult to achieve. Here, standard TEM solutions have the first spurious response at 3 times the fundamental frequency, standard waveguide ceramics at less that 2 times, with our standard distributed resonator approach being in the similar range.
Here we present approach that combines the vendor’s strength in ceramics combined with a resonator design to provide a wide range spurious response.

The passband filter specifications can be met with a resonator of an unloaded Q-factor of about 1,300. The entire filter needs to fit into an envelope of 92.5 x 20.5 x 6 mm3 including connectors. If space allocated to the connectors is excluded, the total space available to the filter is 82.5 x 20.5 x 6 mm3.
The topology of the filter has to be chosen in accordance to meeting the filter specifications, but also bearing in mind the manufacturing ease.
For example, the passband specs can be achieved using an-inline coupled filter of the 9th order (9 resonators with easy fabrication) or with cross-coupled 8th order (8 resonators). The required cross-coupling for the 8th pole filter makes fabrication difficult.
The passband response of the 9th order inline filter is shown on the right hand side.

The ceramic (Kyocera εr = 20 with tan(δ) = 2e-5) loaded mini coaxial resonator internally developed is unable to reach the desired unloaded Q-factor of over 1,400 in an internal volume of 17.3 x 9 x 4.9 mm3 at 3.6 GHz.
The resonator with these dimensions is shown on bottom of the slide, while the Eigenmode analysis for the first 10 modes is shown on the table opposite.
The standard resonator has the first spurious response at 15 GHz which is approximately 4.1 times greater than the fundamental frequency.

Due to heavy loading (percentage bandwidth of 11.1 %), the ceramic ring is removed. This results in no or only a very mild influence on performance
The connector is SMP-MAX.
The proximity to the resonator determines coupling bandwidth and filter performance

Discrete ports used for tuning in a circuit simulator
Silver conductivity in full-wave simulations reduced by 20 % to take into account possible manufacturing imperfections

Meets specs
Silver conductivity in full-wave simulations reduced by 20 % to take into account possible manufacturing imperfections

Meets and exceeds specs
Silver conductivity in full-wave simulations reduced by 20 % to take into account possible manufacturing imperfections

Main contributor to performance degradation of the filter as function of temperature is thermal expansion of resonators, in particular, their length.
The nominal length of resonators is:

Under the assumption of the resonators made out of aluminium (CTE = 22.2X 10^6 ), the percentage change in length for a change from 20 C to 80 C is + 0.125 %, while the change in length for temperature change from 20 to -40 is 0.15 %.
Under the assumption of the resonators made out of invat (CTE = 1.5X 10^6 ), the percentage change in length for a change from 20 C to 80 C is + 0.00975 %, while the change in length for temperature change from 20 to -40 is 0.009 %.
The small change in the length using invar is not expected to influence the performance of the filter for changes in the temperature