The Latest Development of Tunable Microwave Filters

Abstract

There have been many types of filters for various communication implications such as surface acoustic wave (SAW) filters, bulk wave (BAW) filters, transmission line filters, cavity filters, substrate integrated waveguide (SIW) filters, etc. As the radio frequency (RF) front end continues to develop, bandpass filters (BPFs) with excellent frequency response and tunability are becoming more and more competitive. In this chapter, three different types are reported, including On-chip-based TBPF, microstrip-based TBPF, and SIW-based TBPF, and some of them are theoretically analyzed carefully. Filters with frequency tuning, bandwidth tuning, and transmission zeros tuning are shown comprehensively. In addition, the limitations and future trends of tunable microwave filters are objectively expounded.

1. Introduction

In recent years, to adapt to the technical requirements of wireless communication, the RF front-end is developing towards multi-band, tunable, miniaturized, low-cost etc. As a key frequency-selective component in the RF front-end, tunable filters increase the range of frequency and reduce the complexity in multi-band communication systems. The performance of a bandpass filter has attracted more attention. Electronically tuned bandpass filters are the most promising frequency selection scheme for tunable RF front end. Filters with different structure are introduced in the order of frequency tuning, frequency and bandwidth tuning, transmission zero tuning, and band pass filter with special function. This chapter introduces chip TBPF, micro-strip line TBPF, and substrate-integrated waveguide TBPF based on the form of tunable bandpass filters.

2. On-Chip-based TBPF

For microwave frequencies, a varactor-tuned LC filter is a popular choice in tunable BPF circuit designs, though its limited capacitance tuning range is still a problem.

In 2016, a 5/60 GHz CMOS receiver with a transformer-coupled Q-enhanced tunable channel-select active filter was proposed in [1]. The partial diagram of the circuit for the 5 GHz receiver path is shown in Figure 1. Two varactors, C_t1 and C_t2 (V_T control), tune frequencies from 5.15 to 5.65 GHz. The Q factor is compensated by tuning the tail current (V_q1 control) to get a good frequency response. By adjusting the frequency and the Q value at the same time, the filter maintains a good frequency response in different operating frequencies. In 2017, a similar receiver architecture using a negative resistance to enhance Q achieved a wider tuning range [2].

Figure 1.

Partial diagram of the circuit for the 5GHz mode in [1].

In [3], a passive LC filter is demonstrated, which consists of a 3rd-order pi-section structure to meet the out-of-band attenuation requirement. The filter provides a 300–882 MHz operating frequency range using metal-insulator-metal (MIM) capacitors and NMOS switches. In [4], a 0.8/2.4 GHz tunable active-RC BPF is reported. An elliptic filter structure is adopted to realize a large out-of-band rejection. An operational amplifier (OPAMP) offers enough gain to maintain a high Q factor. The tunability of this BPF is achieved by resistor and capacitor banks.

Nicolo Testi et al. have fabricated a 4th-order tunable Q-enhanced BPF [5]. Figure 2 shows the complete structure of the 4th-order BPF and each stage. As shown in Figure 2(b), an array of differential pairs on the left is used to provide an overall gain in the 72 dB range. The switched capacitor array on the right is used to tune the center frequency. In addition, the negative resistance network (a cross-coupled pair M6, M7) compensates for the loss of the LC tank during frequency tuning. The filter covers a frequency response from 2.35 to 2.48GHz and the frequency step is 0.4 MHz.

Figure 2.

Schematic of (a) the 4th-order LC TBPF and (b) each of the TBPF stages of the BPF in [5].

An N-path structure is equivalent to a series LC coupling network is proposed in [6]. As is shown in Figure 3, quarter-wave T-lines are added on either side to transform a shunt LC into a series LC. This structure completes the N-path equivalence theory. Moreover, T-lines also eliminate the influence between each stage of an N-path filter. Figure 4 shows the diagram of the proposed 6th-order filter. The quarter-wave T-lines are realized by lumped capacitor-inductor-capacitor T-type circuits. The designed filter covers the 600-850 MHz tuning range. The BPF has excellent filter shape and OOB rejection but increases in-band loss.

Figure 3.

Conversion for parallel LC and series LC to N paths.

Figure 4.

Diagram of the designed 6th-order N-path filter in [6].

3. Microstrip-based TBPF

On-chip filter has the advantage of a small area and easy integration with the entire RF front-end, but the design flexibility is greatly reduced and limited by process. In addition, its performance is poor, and cost is higher. Planar BPFs, such as microstrip filters and substrate-integrated waveguide (SIW) filters, are a more popular choice due to their combination of performance, design flexibility, and ease of integration with other active elements.

In [7], a method that uses a transmission line, stub, and capacitor to form a basic resonator was proposed. An N^th-order filter can be designed by cascading multiple resonators. And it controls all varactors by a single bias, which simplifies the control circuit. In [8], a 4th-order band-pass filter was designed using two loop hairpin resonators and two combined resonators. This structure has a more symmetrical in-band response. In [9], the best impedance- to length ratio of the stepped-impedance resonator (SIR) was demonstrated. Fabricated SIR-loaded BPF achieved the widest bandwidth tuning. In [10], a novel filter using comb line and split-ring resonators achieved a tuning range of 1.1–1.88 GHz.

One way to implement a BPF with tunable bandpass and frequency is to cascade two filter units with different frequency responses. In [11], a BPF with an adjustable pass-band edge was designed by cascading a high-pass filter (HPF) and a low-pass filter (LPF) directly. Figure 5 shows the layout of proposed filter. HPF is on the left and LPF is on the right. The passband edges are the cutoff frequency of LPF and HPF controlled by loaded varactors. Its tuning range is determined by capacitance of varactor and the electric length of the coupled line. The fabricated filter has a sharp roll-off and good in-band shape.

Figure 5.

Layout and fabricated filter.

Another way to vary bandwidth is to adjust coupling, including interstage coupling and input-output coupling. A tunable filter based on cross-shaped multi-mode resonators are shown in Figure 6 [12]. The input/output coupling circuits (J/K inverter cascaded with a transformer) control the resonator to operate at different mode frequencies. The odd mode is controlled by varactor C₁ while the even mode and transmission zeros are controlled by C₂ and C₃. The center frequency of the fabricated filter can be tuned from 600 to 1450 MHz. The return loss is more than 15 dB and insertion loss is less than 4 dB over tuning range. The Bandwidth can vary from 120 to 950 MHz when center frequency is 1 GHz.

Figure 6.

Configuration of the proposed filter.

In ultra-wideband (UWB) system, many narrowband interference signals affect the information reception. Thus, a notch filter is needed to protect the system from unwanted interference. Notch bandpass filter that combines pre-selection and interference suppression is a multi-functional filter. Existing notch design schemes mainly include using additional notch resonators [13], embedded open stubs [14], and asymmetric coupled fed lines [15]. To suppress in-band interference dynamically, notch tunable filters are required [16, 17, 18]. Recently, reconfigurable notch BPF with tunable bandwidth has also been reported [19, 20, 21, 22].

Based on a pair of parallel-coupled lines (PCLs) and a terminated cross-shaped resonator (TCSR), a series of wideband reconfigurable BPFs with notch control are designed in [19], including tunable notch frequency, tunable notch bandwidth, and tunable broadband bandwidth. Each part can be controlled independently, which greatly improves the tuning freedom of notch bandpass filters. Figure 7 illustrates filter A. The notch bandwidth is controlled by S1 and SF independently, and the notch frequency is affected by l_l-H and l_l-V. Figure 8 presents the filter B. Varactors are inserted to vary the parameters of TCSR. The BPF bandwidth can be tunable by the variable parameters of TCSR. Measured results present that notch frequency and bandwidth keep fixed when the BPF operate in different bandwidth. The bandwidth tuning range is 1.22–2.46 GHz.

Figure 7.

Schematic of filter A.

Figure 8.

Schematic of filter B.

4. SIW-based TBPF

In [23], a tunable dual-band filter is reported using half-mode substrate-integrated waveguide (SIW) structure. Two pairs of stubs are loaded at the open ends of the HMSIW structure to form the two passbands. Frequency tuning is achieved by loading variable capacitors at the end of stubs. Changes in one pair of load lines have little effect on the other passband.

In [24], two 4-pole SIW filters were designed with different dual-mode resonator configurations. The basic SIW cavity is excited by 50 Ω microstrip line, which loaded coupling slots at either side of transmission lines to impedance match. An H-shaped slot is introduced in the proposed rectangular cavity to generate second resonant mode. Figure 9 shows two higher-order structures based on basic resonator. Two resonators are placed vertically or horizontally and coupled by a pair of U-shaped slots. A varactor was placed to tune frequency. The frequency tuning range of Filter A is 2.7–3.4 GHz, and filter B is 3.1–3.9 GHz. The IL of both of them is less than 1.5 dB.

Figure 9.

Proposed filter in (a) vertical configuration (b) horizontal configuration.

An eighth-mode SIW tunable filter was presented in [25]. As shown in Figure 10, varactor C_v1 is loaded at stubs to achieve frequency tuning. Varactor C_v2 is loaded between two resonant cavities to vary the electrical coupling strength, which enables bandwidth tuning. The frequency tuning range is 2.17–2.72 GHz and the BW tuning range is 140-435 MHz. In addition, the edge electrical coupling between two resonators offers a transmission zero out-of-band, which enables the filter to obtain better out-of-band rejection.

Figure 10.

Diagram of proposed SIW TBPF.

In [26], a half-mode SIW filter with a superposed two layers PCB was demonstrated. As shown in Figure 11, PCB2 is extended outside of PCB1 to load feed lines and varactors. The metal semicircular not only helps to improve the accuracy of frequency tuning but also reduces the filter size. Varactor C_v1 is loaded between the center via hole to tune frequency. Varactor C_v2 is loaded between the holes of resonator to tune the bandwidth by affecting the electric coupling and magnetic coupling. Varactor C_v3 could provide a suitable Q factor when the frequency and bandwidth is tunable. The frequency turning range is 1.11–1.5 GHz and bandwidth is from 46 to 130 MHz.

Figure 11.

Top view and the side view of proposed filter.

5. Conclusion

There is no doubt that tunable microwave filters are pushing towards more compact, more functions but less control components. In this process of development, on-chip-based TBPF, microstrip-based TBPF, and SIW-based filter are seen as three potential ways. On-chip BPF provide the possibility of full integration of system-on-chips. Microstrip line TBPF demonstrates flexible filtering performances. SIW-based filter has less loss in the band. However, the low Q value of filters in CMOS and GaAs process, the dispersion of microstrip-based filter in high frequency, and the relatively large area of SIW-based filters are still unresolved issues. Moreover, a more universal accurate design process and a simpler control method are very meaningful to research.