Wireless communication systems require larger frequency bandwidth because multimedia transmissions have become commonplace. Through frequency aggregation, wider bandwidth can be accommodated; thus, high-speed data can be transmitted via the combined wider bandwidth. Recently, for spectrum sharing, the TV white space (TVWS) band has been open for wireless communication applications. Therefore, this band must be aggregated with a conventional mobile frequency band. Such mobile communication systems require bandpass filters that cover more than two frequency bands in order to aggregate two separate frequency bands. These multiband bandpass filters have been studied by many researchers [1–5]. Most design methods focus on two or more narrow communication bands; however, in this new design, a TVWS band with a large bandwidth is combined with conventional narrow communication bands. In this paper, a design method for a dual bandpass filter (BPF), in which the two passbands include a wide TVWS band and a narrow long-term evolution (LTE) band, is proposed. To acquire sufficient coupling for the wide bandwidths, the method proposed in [6] is used. The proposed filter has two bands whose lower passband covers 470–698 MHz bandwidth, while the higher passband occupies 824–894 MHz.

A dual-band BPF is designed based on the procedures for standard Chebyshev BPFs [7]. The filter consists of two BPFs at the center frequencies of 584 MHz and 859 MHz, respectively. The two BPFs are designed, and then two filters are combined. The conventional BPF design for the TVWS band is shown in Fig. 1, where air coils (0806SQ; Coilcraft Inc., Cary, IL, USA) are used to reduce the insertion loss. Since the two passbands are located very close to each other, in order to obtain sufficient isolation between the two passbands, transmission zeros are introduced in the stopband. In the proposed design, cross couplings are included as shown in Fig. 2(a) [8,9]. The influence of the cross-coupling capacitor is shown in Fig. 2(b). The larger the coupling capacitances, the nearer they become to the passband, as shown in Fig. 2(b). Too large cross-coupling can cause deterioration of passband characteristics.

The designed BPF for the TVWS band is shown in Fig. 3. The frequency responses of the designed BPF together with the conventional BPF (Fig. 1) [10] are compared in Fig. 4, where the isolation by transmission zeros is pronounced. In a similar manner, the BPF for the LTE band is designed. In this band, the bandwidth is narrow, and dielectric resonators (DRs) are used to reduce the insertion loss. The conventional three-pole BPF is shown in Fig. 5(a), where resonators made of lumped elements are replaced with DRs [11,12].

To increase the isolation characteristics, transmission zeros are also introduced in this design as shown in a modified circuit in Fig. 5(b). As the passband is narrow, the series coupling inductors are replaced with parallel LC resonant circuits; the transmission zeros are located at the stopband between the two passbands. A comparison of the simulated performances is shown in Fig. 6, where the solid line represents the responses with transmission zeros.

Next, the two designs are combined in two ways. The resulting filter network has a common input port and a common output port, as shown in Fig. 7, while in the other design a common input port together with two output ports, the TVWS band output and the LTE band output, are separated as a diplexer (Fig. 8).

Figs. 7 and 8 show two different types of filter networks. Since the two designed BPFs have very good isolation characteristics, optimization using ADS software was performed without much difficulty combining the two filters. The element values of the two networks are presented in Tables 1 and 2.

The proposed dual-band BPF was fabricated on a 0.8-mm-thick FR-4 (relative dielectric constant of 4) substrate. Fig. 9 shows the simulated and measured S-parameters of the two-port design. The insertion loss (S₂₁) of the TVWS and LTE bands was measured as 1.9 dB and 2.1 dB, respectively. The return loss characteristics (S₁₁) were measured at lower than –10 dB within both passbands. The isolation levels between two bands were obtained below –30 dB. Fig. 10 shows the S-parameter of the three-port design. The insertion loss was measured at less than 1.3 dB within both passbands, and the return loss was lower than –10 dB together with about 30 dB of isolation characteristics. Table 3 summarizes the measured return loss, insertion loss, and isolation characteristics. In Fig.11, photographs of the proposed ual-band BPF and the diplexer re presented.

In this paper, a dual-bandd BPF that can cover the whole TVWS band as well as the LTE band 5, is proposed. To isolate the two closely located bands, a transmission zero is inserted using a cross-coupling scheme as well as a modified inverter scheme. The proposed dual-band filter has 40% and 8% fractional bandwidth for each bandwidth, which is unusual in a dual-band filter design. Therefore, the proposed dual-band BPF can be applied to cognitive radio applications in the TVWS band.