Miniaturized 38 GHz Circular Substrate Integrated Waveguide Band Pass Filter using Low Temperature Co-Fired Ceramic Technology

This study presents design approach for realizing miniaturized Substrate Integrated Waveguide (SIW) Band Pass Filter (BPF) using Low Temperature Co-fired Ceramic (LTCC) technology at TMRND’s LTCC Lab. Design method for the SIW BPF is based on the circular cavity structure with four pole Chebyshev and operating at center frequency of 38 GHz. This SIW BPF is an important part of the Remote Antenna Unit (RAU) transceiver for the Radio over Fiber (RoF) system. Two types of circular SIW BPF have been designed and investigated in term of performance and structure size which are planar SIW BPF and compact SIW BPF. Both SIW BPF were developed using LTCC Ferro A6M materials with dielectric constant of 5.8, loss tangent of 0.002 and metallization of gold. The insertion loss for planar SIW BPF and compact SIW BPF were measured at 6.2 dB and 6.3 dB, respectively. The passband return losses for both types of the SIW BPF were measured at more than 10 dB. The size of the compact SIW BPF is 6.94×6.94 mm meanwhile size for planar SIW BPF is 12.15×4.145 mm . The size of the compact SIW BPF is reduced to nearly 10% compared to a planar SIW BPF structure.


INTRODUCTION
A band pass filter is a key component for passive element in Remote Antenna Unit (RAU) transceiver for Radio over Fiber (RoF) systems. Generally, RoF systems have some advantages, such as low-loss and high speed wireless and wireline data transmission over fiber links which support high speed multimedia services and high defination video. The millimeter wave (mm-wave) frequency is started from 30 to 300 GHz have received more attracted because of possible miniaturization of the analog components such as filter and antennas. Although it has been known for many decades, this technology has attracted significant interest from researcher, academia and industry when advances in silicon process technology and low cost integration solution in Hizan et al. (2014). Huge demand of the higher data-speeds and high bandwidth applications is increasing recently, such as point to point wireless communication was reported (Van Heijningen and Gauthier, 2004), wireless indoor communication network (Dang et al., 2007), radio over fiber (Yaakob et al., 2014).
One of the main issue to realize the mm-wave radio system is related with cost effective packaging and the size of the mm-wave modules structure like filter modules and amplifier modules. The waveguide filters are widely used because of their high Q value, high power capability and outstanding selective. However, they are bulky, heavy and not suited for high density integration. The present microwave and mm-wave filters are bulky and heavily that give a significant limitation when it comes to fabrication. Several microwave filter have been reported with new innovative design on size reduction. There are many types of filter topologies and structures to realize miniaturized microwave and mm-wave filter such as Substrate Integrated Waveguide (SIW), Stub Loaded Resonator (SLR) and Stepped Impedance Resonator (SIR) as reported by Vidhya and Jayanthy (2013). Wu et al. (2003) introduced the substrate integrated circuit in 2003 which is the new concept for high-frequency electronics and optoelectronics. Losses of SIW components are lower than the corresponding microstrip devices. In terms of size, SIW is more compact and easy to integrate with other microwave and millimeter-wave circuits in the same substrate compared to the conventional waveguide. SIW circuits are fabricated by application of either a standard Printed Circuit Board (PCB) or a Low Temperature Co-fired Ceramic (LTCC) process. Generally, the conventional structure of SIW filters are predominantly based on rectangular and circular cavities as reported by Xu et al. (2013) andDe Carlo andTringàli (2010).
In this study, in regard tominiaturized which combine performance of circular Substrate Integrated Waveguide (SIW) Band Pass Filter (BPF) using Low Temperature Co-fired Ceramic (LTCC) technology was proposed. Design method for the SIW BPF is based on the circular cavity structure with four pole Chebyshev and operating at center frequency of 38 GHz. Two types of experimental SIW BPF structures at the same central frequency of 38 GHz are fabricated using Low Temperature Co-fired Ceramic (LTCC) technology. Design details are decribed and both simulated and experimental results are presented and discussed.

SIW BAND PASS FILTER DESIGN
This study focuses on miniaturizing the circular Substrate Integrated Waveguide (SIW) band pass filter which will be employed as important part of the Remote Antenna Unit (RAU) for RoF applications as shown in Fig. 1. SIW band pass filter provides a lowprofile, low-cost, possible integration and low-weight scheme while maintaining high performance, which is satisfied with the needs perfectly.
A basic structure of an SIW band pass with 1-pole Chebyshev characteristic was reported by Hizan et al. (2015) as shown in Fig. 2. The measured results of an insertion loss of -1.954 dB and the passband return loss of greater than 10 dB with 640 MHz bandwidth operating at 38 GHz.
In this study, a forth order Chebyshev SIW BPF with 4 pole Chebyshev characteristic is designed and developed using in-house TMRND's LTCC technology. Two types of experimental SIW band pass filter structures have been designed and developed which are planar SIW BPF as shown in Fig. 3 and compact SIW BPF as shown in Fig. 4. Table 1 shows the geometric dimension for both SIW BPFs.
The similar design technique as reported by Hizan et al. (2015) was applied. The filter's specification was shown in Table 2. However, in this design the number of order was changed from first degree Chebyshev to four orders for both filter types. The operating mode for the forth order SIW filters are TM010. From the filter design theory in Hunter (2001) and Matthaei et al. (1980), the admittance inverter and capacitance values of the low pass prototype can be determined using the following formulas: where, by N is the degree of the network and ƞ is defined as: sinh (3) And ɛ related to the insertion loss ripple and hence the passband return loss: The lowpass prototype is transformed into bandpass filter at 38 GHz centre frequency with -10 dB passband return loss bandwidth of 100 MHz. Under the transformation, the inverter values are invariant. Then, the lowpass ptototype equivalent circuit can transformed into a bandpass equivalent circuit using the formulas (5-7):

TS AND DISCU
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