Multilayer End Coupled Band Pass Filter using Low-temperature Co-fired Ceramic Technology for Broadband Fixed Wireless

Received Jan 5, 2018 Revised Mar 2, 2018 Accepted Mar 18, 2018 This paper presents design approach for realizing multilayer End Coupled Bandpass Filter (ECBPF) using low temperature co-fired ceramic (LTCC) technology at TMRND's LTCC Lab. Design method for the ECBPF is based on the coupled-resonator filter which was realized in LTCC multilayer substrate and operating at the center frequency of 42GHz. Three samples of EC BPF have been designed, simulated, fabricated and investigated in terms of performance and structure size. This multilayer ECBPF were fabricated in the 8 layers LTCC Ferro A6S materials with dielectric constant of 5.8, loss tangent of 0.002 and metallization of gold. The measured insertion loss for ECBPF was 2.43dB and return loss was 22.81dB at the center frequency at 42GHz. The overall size of the fabricated filter was 6.0 mm x 2.5 mm x 0.77 mm.


INTRODUCTION
The rapid development in microwave and millimeter wave communication technology is demand for high quality, miniaturization, and low-cost fabrication of passive components such as the microstrip bandpass filters (BPF) and antennas. The next generation of wireless communication networks envisages operation at millimeter-wave frequencies (>30GHz) to achieve the high data speed where larger allocable bandwidth is available for gigabit/s transmissions.Several applications were developed in microwave and millimeter wave (mm-wave) band to achieve the high-speed data transmission including for mm-wave Radio over Fiber (RoF) application as reported in [1][2][3]. In general, the bandpass filter is one of an important passive component in microwave and millimeter wave communication system because of its function for permitting signal in the desired range of frequencies and rejecting all other. There are many filter topologies at microwave and mm-wave frequencies as reported such as the parallel-coupled-resonator filters [4] and the hairpin resonator filters [5] which have a problem with a large size, the high loss due to the increased resonator capacitance, low stopband rejection, and spurious response at the filter's harmonics.We need to use different feed topologies and multilayer structure to reduce the size and improve insertion loss such as parallel-coupled-feed structure [6], tapped-line [7], and the end-coupled structures [8].

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One of the important packaging technologies to integrate passives elements such as filters and antenna is the Low-Temperature Co-fired Ceramic (LTCC) technology. This LTCC technology allows for the realization of compact devices which makes use of ceramic tapes with low loss material systems such as Ferro A6S material which has a dielectric constant of 5.8 and loss tangents of 0.002 [9]. The main drawback implementation of the planar bandpass filter at higher microwave and millimeter wave frequencies are the dimensional limitations in fabrication technologies such as printed circuit board (PCB) and Low-Temperature Co-Fired Ceramic (LTCC) technology. For example, conventional planar end-coupled BPF is impossible to be implemented because of the tight coupling between the resonator, the band gap is narrow and the LTCC print screen process with the minimum gap between line resolution is less than 100um [10]. In this paper, we proposed design from planar to multilayer end coupled bandpass filter (ECBPF) to overcome the problem of the dimensional limitation of fabrication technologies. The multilayer ECBPF structure with vertically stacked 2 stage resonators, 6 grounded layers, a fractional bandwidth of 7.14% and size reduction of 30% compared with an equivalent LTCC end coupled microstrip filter [10] was designed and developed at a center frequency of 42GHz using LTCC technology. The fabricated multilayer ECBPF size was 6.0 mm x 2.5 mm x 0.77mm.

DESIGN OF THE MULTILAYER END COUPLED BANDPASS FILTER
In this paper, the basic structure of the Multilayer ECBPF is based on the coupled-resonator filter which was realized in LTCC multilayer substrate and operating at the center frequency of 42GHz. This multilayer ECBPF is important parts of the Remote Antenna Unit (RAU) Transmitter for mm-wave RoF applications. Figure 1 shows the proposed block diagram of RAU Transmitter for Radio over Fiber system where operating at Broadband Fixed Wireless Access frequency of 40GHz. The photodetector (PD) converts optical input signal within 1100-1650nm wavelength to the radio frequency signal within the range of 37-43.5GHz. The radio frequency signal is amplified by the Low Noise Amplifier (LNA) module and then passed through the band pass filter. The filtered signal is then amplified again to the desired power level by the Power Amplifier (PA) module before being transmitted to the antenna. In this work, multilayer ECBPF was designed fabricated and measured. The design methodology for implementing the structure of the multilayer ECBPF begins from finalizing the specification, design the schematic circuit by computer-aided techniques and then generate the physical layout realization through electromagnetic (EM) simulation tools. The general design procedure to design and develop the multilayer ECBPF can be summarized as: a. define and finalize the specification b. derive a schematic circuit c. circuit optimization d. generate a physical layout based on the circuit e. tune the layout to meet required specifications f. fabricate LTCC prototypes g. testing to verify the performance The design specification for multilayer ECBPF using LTCC technology is shown in Table 1. It was designed using 8 layers LTCC Ferro A6S tapes systems with relative dielectric constant and a loss tangent of 5.8 and 0.002 respectively. The thickness of single layers was 96m. It consists of the two layers were required for end coupled resonator BPF and 6 layers used as Grounded planes as illustrated in Figure 2(a). Figure 2(b) shows the 3D view design layout which generated through CST electromagnetic (EM) simulation tools. The total dimension of proposed bandpass filter was 6.0mm x 2.5mm x 0.77 mm. Insertion Loss (S21) < 3 dB 5 Return Loss (S11) >10 dB 6 Impedance 50  Three samples of multilayer ECBPF as marking as sample A, B and C (see Figure 7) were designed, fabricated, measured and analyzed in order to check the filter performances in term of insertion loss and structure size. The performances of the 3 samples of the multilayer ECBPF with Ground Signal Ground (GSG) ports were investigated. Expected result for the end coupled BPF at the center frequency of 42GHz with insertion loss, S21 less than 3dB and return loss more than 10dB.
In theory, according to J.Hong and M.J Lancaster [11], the general configuration of an end coupled microstrip bandpass filter is illustrated in Figure 3 where each open end microstrip resonator is approximately a half guided wavelength long at the center frequency f o of the bandpass filter. The coupling from one resonator to the other is through the gap between the two adjacent open ends and hence is capacitive. The gap can be represented by the inverters, which are of the form in Figure 4. These J-inverters tend to reflect high impedance levels to the ends of each of the half wavelength resonators and it can be shown that this causes the resonators to exhibit a shunt type resonance.
, ; J=1 to n-1 Where by Jj,j+1: Characteristic admittances of J-inverters Yo: Characteristic admittances of the MSL g 0,g 1….,g n: Element of a ladder type low pass prototype with a normalized cutoff frequency FBW: a fractional bandwidth of the band pass filter = (f high -f low /f center) The value of characteristic impedance, J-inverter can be obtained with equation (1) to (3) if the type order and ripple (in Chebyscheff type) of the mm-wave bandpass filter are determined. For the wideband filter, we need tight coupling between the resonator and the band gap must be very narrow. From the calculation, the filter has FBW of 7.14%, hence this filter can be categorized as a narrowband filter because of fractional bandwidth (FBW) less than 20%. BPFs can be classified as a narrow bandpass filter and wide bandpass filter on the basis of their FBW. With the FBW of 20% or less is classified as narrow bandpass filter whereas the wideband pass filters are those who are FBW much higher than the 20%. A good bandpass filter requires low passband insertion loss and large suppression in the rejection area including the image signal and in-band signal harmonics. When the frequency is low, e.g., for most wireless communication applications, filters are usually implemented by LC element type. In our LTCC design rules, the resolution of print screen process, the minimum gap between lines is 100um. Assuming the capacitive gaps act as perfect, series capacitance discontinuities of susceptance B j,j+1 as shown in Figure 5. Where the Bj,j+1, and are evaluated at fo Coupling gaps, Sj,j+1 of the microstrip end coupled resonator filter can be determined as obtained the series capacitances that satisfy: Whereby ω 2πf is the angular frequency of the mid-band. The physical lengths of resonators are given by Effective length From the above mathematical equations, design parameter of the band gap and physical length for multilayer ECBPF can be obtained as shown in Table 2.

LTCC FABRICATION AND MEASUREMENT OF MULTILAYER END COUPLED BANDPASS FILER
The multilayer ECBPF was fabricated using state-of-the-art in-house TMRND's LTCC process was described in [12]. Figure 7 shows fabricated multilayer end coupled BPF with 3 samples mark as sample A, B, and C.The detail physical dimensions of the LTCC BPF are listed in Table 3. All the designs complied with in-house TMRND's LTCC design guidelines with minimum spacing at 100m between lines.    Figure 9 shows the multilayer ECBPF has centered at 42GHz with insertion loss of 2.28dB and return loss of 11.66dB. The simulated and measured responses of the design were summarized in Table 4 for three (3) samples A, B, and C. All the measurements were done by using the R&S ZVA50 network analyzer and Cascade Microtech 450m probe tips. From the results, we found that the sample C gave the best result compared to others, with the insertion loss and return loss of 2.43dB and 22.81dB, respectively. However, the measured insertion loss at center frequency was 0.15dB larger than simulated results. A different might due to shrinkages of the conductor layer which results in an increase of spacing and gap of each filter section. Additionally, the increased insertion loss might be caused roughness of the fired circuits. Furthermore, the measured result has good agreement compared to simulated result.

CONCLUSION
This paper proposed the multilayer ECBPF with vertically stacked 2 stage resonators and 6 grounded layers, a fractional bandwidth of 7.14% and size reduction of 30% comparing with an equivalent LTCC end coupled microstrip filter. The advantage of the proposed structure is to overcome the problem with the planar structure of end coupled BPF due to dimensional limitations in fabrication technologies. From the results, we found that sample C gave the best result compared to others, with the insertion loss and return loss of 2.43dB and 22.81dB, respectively at the center frequency of 42GHz which good agreement with simulated results. As summarized, the variations between the measured and designed dimensions were defined and measured relative to the guided wavelength at the design center frequency. Therefore, we need to optimize the design in the next stage for further improvement.