Selectivity-tuned bandpass filter Z. Brito-Brito, I. Llamas-Garro and L. Pradell A bandpass filter able to electrically fine tune selectivity is presented. The reconfigurable filter topology has four poles and a quasi-elliptic bandpass filter response. The device is tuned by pin diodes which select tiny stubs that reconfigure filter selectivity without degrading filter return loss throughout the passband. Simulations and measure- ments are in good agreement. Introduction: Filters having transmission zeros at finite frequencies have improved selectivity compared to Chebyshev filters, and can be used to separate narrow adjacent channels in communication systems. It is well known that temperature variations of microwave substrates [1, 2], or fab- rication tolerances [3], can slightly mistune a filter out of an optimum response. Fine tuning techniques can adjust a filter response to overcome temperature variations or fabrication tolerances. Previous work involving reconfigurable transmission zeros, focuses on changing the position of a single transmission zero from one side of the passband to the other [4– 6]. In this Letter we focus on selectivity fine tuning where a pair of trans- mission zeros are moved closer or farther to the filter centre frequency. Four selectivity states are obtained using two switchable tiny stubs, which modify the coupling between a pair of resonators. K14 variable Q e1 Q e2 K 12 K 34 K 23 Fig. 1 Filter general coupling diagram Table 1: Fixed coupling coefficients K 12 ¼ K 34 0.0581 K 23 0.0543 Q e1 ¼ Q e2 9.5356 Table 2: Variable coupling coefficient State K 14 1 0.0097 2 0.0123 3 0.0173 4 0.0198 RF input feed line resonators pin diode RF choke limiting resistor DC port Fig. 2 Photograph of fabricated filter Filter topology: The proposed selectivity reconfigurable device is based on the filter presented in [7], and consists of a four-pole quasi-elliptic bandpass filter using microstrip open-loop resonators. The filter is com- posed by four inter-resonator couplings K 12 , K 23 , K 34 , K 14 , and input/ output couplings to the circuit Q e1 and Q e2 as shown in Fig. 1. In this filter topology, coupling K 14 in Fig. 1 is variable, and can take four different values resulting in a reconfigurable selectivity. The filter was designed to have a 10% fractional bandwidth with a 1.7 GHz centre fre- quency. The theoretical fixed couplings for the device are shown in Table 1, and the theoretical values of the reconfigurable coupling K 14 are shown in Table 2, where filter selectivity increases as K 14 increases. The fabricated filter is shown in Fig. 2, and consists of four open-loop resonators with a tapped input and output coupling to resonators 1 and 4. The variable electric coupling K 14 is tuned using two tiny stubs of different length connected by pin diodes. The proposed filter is able to switch between four different selectivity states discretely, defined by selecting a combination of pin diodes in the off or on state. The coupling coefficients are found using the method described in [7] by carrying out full-wave electromagnetic simulations, including lumped element models for the pin diodes and choke inductors [8]. The filter was fabricated on a 1.524 mm-thick Rogers substrate having a 35 mm copper metallisation. The diodes were HPND-4028 Avago Technologies beam lead pin diodes. The bias network consisted of a choke inductor and a 1 kV resistor to limit the current on each diode to 10 mA in the forward bias state; a voltage of 210 V was supplied in the reverse bias state. Results: Figs. 3 and 4 show the simulated [8] and measured response for the four discrete states, respectively. Measurements were taken using an HP 8510C network analyser. The comparison between simulated and measured results shows very good agreement in terms of selectivity tuning and passband return loss. Selectivity increased from the off-off state to the on-on state as expected. When both diodes are reverse- biased, none of the stubs are selected, producing the off-off state. When the left diode is reverse-biased and the right diode is forward- biased the shorter stub is selected, increasing the selectivity and produ- cing the off-on state. The longer stub is selected by polarising the left diode in a forward-biased state while keeping the right one reverse- biased, defining the on-off state with a higher selectivity than the pre- vious state. Finally, when both diodes are forward-biased the highest selectivity is achieved corresponding to the on-on state. 1.5 –60 –30 –20 –10 0 off-off state off-on state on-off state on-on state –40 S 11 , dB S 21 , dB –20 0 1.6 1.7 frequency, GHz 1.8 Fig. 3 Simulated filter response 1.5 –60 –30 –20 –10 0 –40 S 11 , dB S 21 , dB –20 0 1.6 1.7 frequency, GHz 1.8 off-off state off-on state on-off state on-on state Fig. 4 Measured filter response ELECTRONICS LETTERS 10th September 2009 Vol. 45 No. 19