Design of LTCC Second-Order Inductively Coupled Bandpass Filter

0 Introduction

Modern mobile communication systems, from GSM to GPRS to CDMA, have frequencies from hundreds of Hz to 900 MHz, 1.8 GHz, 2.4 GHz, 5.8 GHz, and even higher. At the same time, the requirements for miniaturization and high performance of devices are constantly increasing. In the microwave band, passive devices of multilayer ceramic dielectrics, such as filters, are increasingly valued due to their advantages of miniaturization, easy integration, and flexible design. In order to reduce the loss of devices while miniaturizing them and obtain higher quality factors, it is necessary to seek new materials and technologies. Among the many microwave dielectric plates, LTCC has more advantages than HTCC (high temperature cofired ceramic). It combines the advantages of co-firing technology and thick film technology, reduces the expensive and repeated sintering process, and all circuits are laminated and hot-pressed and sintered at one time, saving time, reducing costs, and reducing the size of the circuit; for the RF microwave field, more importantly, it has the advantages of high quality factor, high stability, and high integration. Therefore, LTCC has become an ideal material for civilian and military electronic systems. At present, microwave devices based on LTCC technology have begun to be used in various mobile communication devices such as mobile phones, PHS, cordless phones, etc., and are also very useful in modules such as Bluetooth, wireless LAN cards, antenna switches, etc.

Low-temperature ceramic co-firing (LTCC) technology uses thick film materials. According to the pre-designed layout pattern and stacking order, the metal electrode material and the ceramic material are co-fired at one time to obtain the required passive devices and module components. The metal strip stacking technology can easily realize the coupling of capacitance and inductance between layers. The cross-capacitor coupling method can be used to obtain the transmission zero point in the stop band that can improve the transmission characteristics. In addition, LTCC uses high-conductivity metals such as gold and silver as conductive media, which will not oxidize during the sintering process, so no electroplating protection is required; the composition of the LTCC ceramic substrate is variable, and dielectric materials with different electrical properties can be generated according to different ingredients. Each parameter can be adjusted within a certain range, thereby increasing the flexibility of the design.

l Multilayer filter structure and principle

The classical filter design theory is relatively mature. The multilayer dielectric filter uses a stacked circuit structure to realize the function of the filter circuit. This technology not only makes the filter small in size, but also has good high-frequency performance, but the distribution of the electromagnetic field inside the device is not easy to determine, and tends to be complex as the number of layers increases. Figure 1 shows the general structure of a multilayer dielectric filter. In Figure 1, the microstrip circuit (black part) is printed on the LTCC substrate (gray part), the upper and lower layers are shielding layers, and the middle is a circuit structure that plays a filtering role (usually called a circuit layer). The specific style and number of layers of the pattern layer depend on the designed filter parameters (such as center frequency, insertion loss in the passband, stopband attenuation, etc.). In the same way, microwave devices such as antennas, balanced or unbalanced converters (baluns) can be obtained.

In the design process of LTCC, it is more common to design a capacitive coupled bandpass filter in a lumped component manner. Considering the convenience of LTCC process, the filter is generally not more than 3 orders. The bandpass filter circuit composed of lumped components is composed of resonators connected in series and parallel. Here, a bandpass filter theory that generates 3 additional transmission zeros is used to combine LTCC technology with bandpass filters. The main technical indicators of the filter are given here. By studying the equivalent circuit of the LC bandpass filter, the filter is simulated and optimized using the three-dimensional electromagnetic simulation software HFSS. A second-order coupled resonant bandpass filter is used as a prototype, which is an inductively coupled π-type structure. On the basis of this core circuit, matching capacitor CI, ground inductor LG, and parallel capacitor Cp are added. This circuit can generate 3 transmission zeros, and the equivalent circuit structure is shown in Figure 2. The design of transmission zeros is because there are many wireless system applications at present, and the frequency bands used by each system are very close, which can easily cause interference between each other. Therefore, the interference between systems can be reduced by designing transmission zeros. This circuit can synthesize large capacitors and small inductors. Cs is about PF level, and Ls is about 0.1 nH level, so it is more suitable for low-temperature co-fired ceramic substrates.

2 LTCC multilayer filter process

The dielectric layer material of the filter is ULF140 microwave dielectric ceramic, with relative dielectric constant εr=13.4, quality factor Q>2 100, frequency temperature coefficient τF≈0, and silver electrode for internal and external electrodes. The multilayer structure design of the device uses microstrip lines to form a two-stage resonator, and the coupling capacitor layer C12 input/output capacitor is on the same layer as the coupling capacitor. For every 2.5% change in the dielectric constant of the material, the center frequency will move 32~42 MHz. Since the coupling capacitance and load capacitance between layers increase with the increase of the dielectric constant, the center frequency of the device will decrease with the increase of the dielectric constant, and the center frequency will move to a low frequency. Therefore, when designing the filter, a margin must be left in the performance. This paper uses HFSS to simulate the filter structure. Figure 3 shows a stripline structure filter widely used in design, which consists of 3 pattern layers. At the same time, it can be seen from the figure that this is a two-stage resonant filter, and the structure of the two resonant units is the same, and they are connected by electromagnetic coupling. Since the multilayer ceramic microwave filter uses non-ferromagnetic media, the inter-stage coupling is mainly achieved by capacitive coupling. Therefore, when discussing the coupling situation, only capacitive coupling is considered. After experiments and analysis, the inductance L of the resonant unit of the filter is provided by the self-inductance LN of the conductor N, and the resonant capacitance of the resonant unit is provided by the self-capacitance CN of the conductor N and the coupling capacitances CR and CS between the conductor N and the conductor R and between the conductor N and the conductor S. The coupling capacitance between the resonant units is composed of the sum of the respective coupling capacitances between the corresponding NN, RR, and SS in the two resonant units. In this way, by solving the capacitance and inductance matrix formed by all conductors, the specific values ​​of each parameter are obtained, and then the response of the filter is obtained by circuit analysis of this equivalent circuit. The processing and production of LTCC chip filters must go through the process of casting, punching, through-hole filling, printing electrodes, lamination and isostatic pressing, slicing, and co-firing. Whether the process accuracy can be controlled well is the guarantee of producing qualified devices. The actual produced LTCC chip filter has a size of 3.75 mm×1.38 mmX 0.97 mm. The simulated electrical performance parameters and the actual electrical performance parameters of the samples produced are shown in Table 1. The instrument used for the sample test is Agilent E8363B vector network analyzer.

As can be seen from Table 1, the simulation value is close to the actual value, but there are certain differences. There are many factors that lead to poor device performance, such as inconsistent thickness of the cast dielectric substrate. Misalignment caused by printing lamination and hot pressing, deviation during slicing and device deformation, and uneven shrinkage during co-firing. In addition to improving the process level, good early design is also one of the solutions to these problems. For example, try to avoid too small coupling spacing and too many layers in the design. At the same time, more simple circuit structures should be used to reduce unnecessary process processes.

3 Conclusion

A LTCC second-order inductively coupled bandpass filter with a center frequency of 2.45 GHz and three transmission zeros is designed. A set of analytical methods for synthesizing the filter is used to give the values ​​of the circuit components, and a filter with good performance is synthesized using circuit and electromagnetic simulation software. Compared with traditional discrete devices, the multilayer filter based on LTCC technology has many advantages such as small size, light weight, and good performance. The filter given in this paper has good performance. As long as the two reflection zeros and two matching quality factors can be set in advance, the values ​​of each device can be effectively synthesized. The design has a certain flexibility and different parameters can be set according to different filter specifications. It has good practical value in wireless systems.