In recent years, advances in wireless communication systems have continued at a high pace with increasing use of the electromagnetic spectrum. For example, fifth generation (5G) wireless systems will use new frequencies in addition to the legacy ones of the fourth generation (4G) and Long Term Evolution (LTE) systems. Concurrently, wireless devices offer more and more applications that rely on wireless communications thereby increasing data rates and spectrum usage. As these technologies continue to evolve, the need for covering multiple frequency bands in a single device is bound to continue to increase. Already, current wireless devices support wireless local area networks (WLAN) at multiple frequencies, multiple 4G/LTE frequencies, legacy global system mobile communications (GSM) frequencies, worldwide interoperability for microwave access (WiMax) bands, and other industrial, scientific and medical (ISM) bands for near field communication (NFC) and wireless charging. To efficiently accommodate such a wide range of frequencies, high performance dual-band and multi-band microwave devices are needed.

The term dual-band refers to microwave devices that are able to operate simultaneously at two separate frequencies. This approach leads to: (i) increased circuit integration, (ii) size reduction, (iii) cost minimization and (iv) reduced power consumption for wireless systems. As filters are critical microwave components for wireless devices, Dual-band Bandpass filters (D-BPFs) are key enablers for dual band wireless communication systems. D-BPFs allow two different modulated signals to be extracted simultaneously from other undesired signals and interferers by attenuating out-of-band signals thereby improving the system’s signal to noise ratio (SNR). Currently, D-BPFs are the subject of intense and continued research where issues such as improving selectivity, decreasing size, accommodating wide/narrow fractional bandwidths and operating frequency selection are being investigated.

Depending on the target application and the desired performance, D-BPFs can be realized using different topologies and technologies including waveguide, microstrip and other planar structures and lumped-elements. The choice of a given filter fabrication technology is dictated by the desired filtering characteristics as well as size and cost constraints. The key filtering characteristics are center frequency, bandwidth, Q-factor, in-band insertion loss and out-ofband rejection. Conventional waveguide D-BPFs have the lowest insertion loss and offer better power handling. However due to their bulkiness and poor integration capability with other circuit blocks, they are generally not suited for most portable systems particularly those operating at frequencies less than 6 GHz. Compared to waveguide D-BPFs, planar microstrip structures have the advantage of smaller size and easy integration, although they show higher insertion loss and lower power handling capability. Despite being less bulky than waveguides, distributed microstrip filters can still be quite large, particularly at lower frequencies, and may be impractical for many portable mobile devices.

Lumped-elements, which are passive components having all their dimensions smaller than operating wavelength, offer yet another option for filter realization with the possibility of designing very compact size D-BPFs for a wide range of microwave applications. In terms of insertion loss and power handling, lumped elements filters offer similar performance to distributed microstrip ones. It should be noted however that for higher frequencies, the realization lumped-elements becomes more and more difficult because of the required fabrication tolerances and techniques as well as the limitation imposed by the element’s self-resonance frequency (SRF). Lumped element D-BPFs can be realized on-chip of off-chip. On-chip realization uses lumped-elements built on the semiconductor substrate and offer the smallest filter size that can be achieved. However, these substrates tend to have relatively high loss which leads to low Q-factor for the elements and, hence limiting the achievable filter performance. Additionally, on-chip lumped element inductors tend to occupy large chip areas, which increases cost. Off-chip lumped element realization can overcome the low Q limitation of the on-chip technology but with typically larger size components. To achieve both high Q and small size in off-chip lumped element implementations, the use of 3D circuit fabrication technologies, such as Low Temperature Co-fired Ceramic (LTCC), is considered as one of the most viable options.

TCC is a suitable technology for developing miniaturized advanced filters for applications where low loss, small size, easy integration, temperature stability and high relative permittivity are required. A typical LTCC module consist of several dielectric tapes, or layers, connected by vias and on which good or resistive conductors are printed as well as very high dielectric constant pastes. Resistors, inductors and capacitors can therefore be fabricated using this technology by combining buried and/or surface structures.

Narrowband standards such as the global positioning system (GPS)Liu et al. (2010), personal communication services (PCS)Kim et al. (1999) and wireless communication services (WCS)Chetouah et al. (2016) are commonly referred with respect to the industrial applications f the single narrow bandpass filters (SNBPFs). These filters would allow narrow-bandwidth modulated signals to be extracted from multiple interfering signals and improve system signal to noise ratio (SNR). Over the past few years, several classes of narrow bandpass filters have been developed with various features. Typical reported strategies for the SNBPFs realizations include the following: substrate integrated waveguides (SIW)He et al. (2014), cavity resonatorsLorente et al. (2013), parallel coupled lines, split ring resonators and hightemperature superconductors (HTS) using planar microstrip technology Shao & Lin (2015); Liu et al. (2016); Li et al. (2006), active SNBPFs implemented in CMOSSu & Tzuang (2012) and realized lumped-element structures in LTCC Brzezina et al. (2009); Xu et al. (2016b). The properties of existing reported strategies have been summarized,  Utilization of active SNBPFs is limited due to the power consumption. SIWs and filters using microstrip technology have a relatively bulkier size at low frequencies, which limits their integration with other blocks. Lumped-element topologies are highly recommended for filters operating at less than 5GHz in order to reach a very compact size.

Among various options for realizing lumped-elements, monolithic microwave integrated circuits (MMIC), surface mount devices (SMD) and LTCC, the last is more attractive. The 3-D layout realization in LTCC provides a very compact size, apart from other features such as low loss at high frequencies, integration capabilities in 3-D environment and low cost.

In the literature, several design methods of LTCC bandpass filters (BPFs) have been reported with various degrees of performance and FWB. In Choi et al. (2003), using half-wavelength coupled resonators leads to a bulky size for a designed circuit at microwave frequencies. In Deng et al. (2005) and Yang et al. (2010), the limited number of TZs degrade due to band rejection. In Xu et al. (2016b), the passband frequency responses are obtained based on a direct synthesis method, but the very narrow FBW is not supported by the proposed circuit. In Zhang et al. (2014) and Dai et al. (2014), the desired frequency responses cannot be precisely specified, due to the proposed optimization design methodologies.

In this paper, a new topology of a lumped-element LTCC bandpass filter for applications with small FBW is introduced. Existing three coupled inductors in the input of proposed circuit model lead to an increase in the loaded Q-factor of the circuit at the given resonant frequency which concludes narrowband frequency responses. Through an even/odd modes analysis, the proposed SNBPF is supported by a straightforward design procedure that uses closed-form synthesis equations to control even/odd mode poles. This analytical solution provides flexibility, allowing the application of the new topology in different frequencies of spectrum and, therefore, creates a narrow bandpass response of the network. To reject out-of-band spurious signals, two TZs are appointed in both sides of the passband.

Further, it is demonstrated that the single bandpass is generalized to dual-band frequency responses through manipulation of the proposed SNBPF circuit model. In the light of increasing demand for multiband wireless communication systems, some researches have been conducted on LTCC filters with dual-band frequency responses, (see Lin et al. (2014); Xu et al. (2013); Oshima et al. (2010); Chen et al. (2009); Chen (2005); Pourzadi et al. (2019)). Similar to the SNBPF, a direct synthesis method with closed-form equations is provided to synthesize a dual-narrow bandpass filter (DNBPF). In the new DNBPF, existing independent poles are asset, for they have independent and controllable external quality factors at both bands. The center frequencies can be adjusted, whereas wide or tight frequency separation between passbands is possible.