3.5 GHz filter, LTE, band 42

Here, we would like to report on the development of a 3.5 GHz filter operating in band 42 of the LTE frequency band[1]. The filter consists of 8 resonators, each with an unloaded Quality (Q) factor of over 2,800 and has internal dimensions of 40 x 25 x 25 mm³. Figure 1 depicts the internal structure of the resonator. From this figure, the resonator is slightly loaded with a matrix of 5 x 5 resonant elements, necessary to a) reduce the frequency of operation and b) keep the dimensions within the prescribed form factor and c) reduce post-fabrication filter tuning requirements. The specifications of the filter include passband loss of less than 1.25 dB at band edges and at least -15 dB attenuation at 10 MHz offset from the band edges. In addition, the return loss is to be kept at at least -18 dB.

Figure 1: Modelled resonator with a resonant matrix of 5x5 (left) and fabricated resonator (right) — Figure 1: Modelled resonator with a resonant matrix of 5×5 (left) and fabricated resonator (right)

The designed filter is shown in Figure 2. The total internal dimensions of the filter are 110 x 41 x 52 mm³. The filter has 3 cross couplers necessary to introduce 4 transmission zeroes – 2 on the lower skirt and the remaining two on the upper skirt of the response, Figure 3.

Figure 2: Designed 8-pole 3.5 GHz filter (left) and fabricated filter, top view (right)

Figure 3: Simulated (blue) and measured (red) reflection coefficient shown on the left; and simulated (blue) and measured (red) transmission coefficient of filter of Figure 2

Highlights:

The correspondence between the simulated and measured filter responses is excellent.
Both simulated and fabricated filters meet all specifications of band 42, LTE standard.
As an example, the return loss is no less than 18.2 dB and insertion losses are kept well below 1.25 dB at the band edges.
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