Design Of High Frequency Balanced Filter Module Based On LTCC Technology

In modern wireless communication, the requirement of higher performance RF component leads to miniaturization, high frequency and integration of RF component. As an important component for RF module, the design and development of balanced filter also has got great improvement. The balanced filter based on LTCC technology can realize the requirement of small size, high frequency and low cost. Using LTCC technology, the balanced filter is analyzed, which is widely used for Bluetooth and wireless LAN.In this paper, the circuit simulation and magnetic simulation are used to analyze and design the LTCC balanced filter with 2.45GHz frequency. The main contents are as follows:1. Bandpass filter design. The design in the passband is derived from low-pass prototype filter, making use of J inverter enables us to convert the low-pass prototype filter to two kinds of bandpass rudiment, one is bandpass filter with capacitive coupling, and the other is bandpass filter with inductive coupling. In this paper, the circuit configuration utilizes the bandpass filter with inductive coupling and utilizing the shorted coupled line with inductive coupling composes of a second-order combine bandpass filter. It introduces the analysis of network which explains the circuit configuration in detail. Transmission zeros are introduced in the stopband, and the theory of transmission zeros is described. A modified resonator introduces a transmission zeros in the low stopband or high stopband and the coupling between the resonator also can introduce transmission zeros in the low stopband or high stopband. The results of circuit and 3-D magnetic simulation accord with the design requirement.2. Balun design. The 4-port network parameters are reduced to 3-port network parameters, which indicate the relation between balun network parameters and input impedance. The main configuration is composed of three coupled line with capacitance load. The kuroda rule explains that the design realize balun function. The design contains two types of the bandpass filter configuration, which is combline filter and interdigital filter. Thus,traditional coupled-resonator filter theory can be used to design and analyze such the device in order to obtain excellent bandpass transmission characteristic. The results of circuit and 3-D magnetic simulation accord with the design requirement.3. The design of balanced filter with transmission zeros. A parallel resonator is used at input port to produce a transmission zero at the lower stopband. The proposed filter was configured by using modified resonators, which can be used to provide a transmission zero located at upper stopband frequency. A 2.45GHz single-to-balanced bandpass filter with a size of 2.0mm×1.25mm×0.9mm has been implemented in a multilayered low-temperature co-fired ceramic (LTCC) substrate, The simulated results are that the maximum of 4.6 dB insert loss, the maximum of 0.3 dB amplitude imbalance and maximum 4.1o°f phase imbalance are achieved in the passband, respectively.