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Bandpass filter Based on Ring Resonator at RF Frequency above 20 GHz

Received Sep 24, 2017 Revised Dec 27, 2017 Accepted Jan 16, 2018 This paper presents two dual-mode rectangular ring resonators, designed at RF frequency above 20 GHz for bandpass filter applications. The first resonator is designed at 20 GHz using single layer microstrip technology, on Rogers Duroid TMM10 substrate with the following characteristics; relative dielectric constant (εr) = 9.2, substrate thickness (h) = 1.270 mm, dielectric loss tangent (tan δ) = 0. The second resonator is built using multilayer CMOS technology at 75 GHz. The resonator is simulated using fluorinated silicon glass (FSG) and silicone rich oxide (SRO) with relative dielectric constant (εr) equals to 3.7 and 4.2 respectively. Both filter designs are built using fullwave electromagnetic simulation tool. For filter design using microstrip technology, the return loss is found at 9.999 dB and the insertion loss is at 3.108 dB while for filter design using CMOS technology, the return loss is found at 11.299 dB and the insertion loss at 0.335 dB. Both results had shown good passband performance with high rejection level at the out-of band.


INTRODUCTION
Bandpass filter is well known as one of the essential component at the front-end system in wireless communication and RF system. Amongst the popular resonators used in RF microwave application are coaxial resonators, dielectric resonators; surface acoustic wave (SAW), yttrium iron garnet (YIG) crystal resonators and ceramic resonators. Understanding each characteristic of different resonators is a must in order to design good filters. Advancing through the digital world, there are a lot of obstacles need to be considered as the trend nowadays are towards the 5 G technology. Hence, the existing techniques and topologies may be further explored to fit into the 5 G network system [1][2][3]. In terms of topologies, ring resonator can be exploited due to their low loss, compact size, high frequency selectivity and high quality (Q) factor performance which are vital in the evolution of future wireless communication system [3].
An original research proposed on dual-mode ring resonator was familiarized first by Wolff and Knoppik for microwave substrate measurement [4]. More research on dual-mode bandpass filter have been presented since then such as reported in [5][6][7][8] that allowed for various parameters to be investigated and derived, including the transmission poles, transmission zero and the resonance frequencies. In the case of coupling constant, the even/odd resonance frequency locations are utilized using lumped capacitors [9][10]. Besides this, there are several approach proposed to design a compact dual-mode bandpass filter using high- permittivity materials and variation of the rectangular ring layout [11][12][13]. The adaptation of coupling capacitance and frequencies filtering can also deliver varied applications in microwave circuits over alterations in electrical length by means of typical dual-mode double ring filter design whereby the filter is developed from single ring configurations to double ring structures [10], [14].
Having said these, it may be useful to also explore in the field of IC circuits, whereby cost of filter and the occupied volume can be reduced further. Complementary metal oxide semiconductor (CMOS) has a great integration proficiency towards incorporating both millimeter wave as well as digital circuits into one single chip in order to achieve low cost for the mass manufacture as well as production [14][15][16]. In terms of bandpass filter design, it enhanced the production of a millimeter wave which is extensively used in wireless communication systems. For instance, a millimeter-wave CMOS on-chip passive filter was discussed in [15] and millimeter-wave CMOS-based bandpass filters by means of small insertion-loss were proposed in [5], [17]. Even though designing on microstrip is much easier as the material are easily obtain while, designing on CMOS technology might be a bit hustle as few institutions has the capability on handling the material itself. However, on the bright side, CMOS can be produced in mass production at a lower cost.
The concern here is that there has yet existence of a filter design, explored and implemented both on microstrip and CMOS technology. In this paper, we present the design and analysis of a dual mode rectangular ring resonator that used the topology in [18]. In this work, the topology is explored for microstrip and CMOS technology for much higher frequency that is applicable for 5 G network system. The resonance frequencies of these filters are chosen at 20 GHz and 75 GHz. At 20 GHz, the filter is designed on single layer microstrip technology while at 75 GHz, the filter is proposed on multi layer CMOS technology. Both designs are simulated using full wave simulator and results had shown good performance in terms of return loss and insertion loss with high frequency selectivity.

RESEARCH METHOD
Explaining research chronological, including research design, research procedure (in the form of algorithms, Pseudocode or other), how to test and data acquisition [1], [3]. The description of the course of research should be supported references, so the explanation can be accepted scientifically [2], [4]. Various research and studies had been conducted to gain necessary insight and information specifically on the technologies used, transmission line, resonance frequency and the techniques implementation. Typically, the common parameters involved when designing bandpass filter are insertion loss, return loss, transmission zero, transmission poles, fractional bandwidth and coupling. In this work, two types of bandpass filters with different resonance frequency are proposed here.
The first design is proposed on Rogers Duroid TMM10 using microstrip technology with relative dielectric constant (εr) = 9.2, substrate thickness (h) = 1.270 mm, dielectric loss tangent (tan δ) = 0.0023 and operating frequency at 20 GHz. The second design is performed on CMOS technology. The materials used are fluorinated silicone glass (FSG) with relative dielectric constant (εr) = 3.7 and silicone rich oxide (SRO) with relative dielectric constant (εr) = 4.2. These materials was obtained from the CMOS18 FSG standard process that offers a single poly with three layers metal (3LM), four layer metal (4LM), five layer metal (5LM) or six layer metal (6LM). Both filters are design to meet specifications as shown in Table I.   Figure 1(a) shows the topology of the 2 nd order bandpass filters using two rings resonator. Figure  1(b) indicates the first layer of the two rectangular rings with CMOS technology. The ground panel is formed by connecting M1 and M2 together while the top metal, M6 is where the designed structure is placed.  The ideal topology of the ring circuit is transformed into distributed element and final layout is displayed in Figure 2(b). The layout clearly shows two rectangular rings with 50Ω transmission feed line and also coupling line. This filter design is proposed on microstrip technology, using Rogers Duroid TMM10 substrate specifically built to be used in RF applications systems with the following characteristics; relative dielectric constant (ε r ) = 9.2, substrate thickness (h) = 1.270 mm, dielectric loss tangent (tan δ) = 0.0023 and operating frequency at 20 GHz. Based on the dimensions in Table 2, length of coupling lines, top rings as well as the bottom rings need to be the same in order for the ring to be in a symmetrical shape. L1a indicate length of the top rings, L2a indicate the bottom rings and LC indicate the coupling length which all measure as 6440um with the gap coupling, S2 of 350um. Parameters L1 and L2 both show the length of the ring side which is 4466um respectively. The lengths of the side also need to take into account as to maintain the rectangular shape. Widths of the rings are all designated by w. The second design of bandpass filter is proposed at 75 GHz. Figure 3(a) shows the simulated response on dual mode bandpass filter using CMOS technology at 75 GHz on a wider range in which the next figure will show the enhanced response on more specific frequency range. Two resonance frequencies are found at 71 GHz and 79 GHz with 38.397 dB and 27.191 dB respectively. The attenuation level of the insertion loss at 75 GHz is found at 0.335 dB. The transmission zero frequencies are obtained at 56 GHz and 100 GHz respectively. The markers at m4 and m5 signify the levels of both transmission zeroes of aforementioned frequencies at 45.906 dB and 8.471 dB respectively. The fractional bandwidth of this filter is around 24% slightly higher than that of the first filter. The layouts along with the dimensions of every element of the bandpass filter are displayed on Figure 3(b) and Table 3 respectively. The lengths, widths and gap of the second filter follow the same labels with the first filter as in Figure 2(b).    Table 4 summarized the simulated responses and performances of the ring resonators using microstrip and CMOS technologies respectively. Based on the results obtained, both resonators had shown acceptable performance for bandpass filter applications. The resonators achieved good selectivity with transmission zeroes at the stopband and two transmission poles in the passband region. The rejection levels for the resonators exceeding 20 dB which indicate good rejection level. In terms of bandwidth, the resonators are suitable to be used for narrowband application since the fractional bandwidths are less than 30%.

CONCLUSION
In a nutshell, two bandpass filters were presented using dual-more rectangular resonator as a base cell. The first filter was designed at 20 GHz using single layer microstrip technology while the second filter was designed at 75 GHz using multilayer CMOS technology. Both filters were successfully simulated with acceptable responses in terms of insertion loss and return loss. In view of the experimental results, this arrangement scheme realized a filter not only with high selectivity and sharp skirt but also compact size, good performance and simple design and therefore is possible to be applied at high frequency above 20 GHz.