| Spectrum resources used in modern wireless communication are continuously extended to higher frequencies, it is necessary to design a dual communication system which can be compatible with existing spectrum resources and higher frequency spectrum resources. Filter is an important device to filter signal for communication system. The design of dual-band bandpass filter is the key of dual-band communication system, it has great practical and commercial value. Miniaturized, convenient, high performance, highly integrated devices are pursued in the field of modern communication. With low-temperature co-fired ceramic(LTCC) technology, a large number of passive components can be integrated into the 3D multi-layer substrates, and active devices also can be embedded into the substrates as functional modules. LTCC technology is very suitable for dual-band bandpass filer design due to the characteristics such as high performance, low cost, miniaturization, etc.This paper introduces the research progresses and the main design methods of dual-band bandpass filter in both domestic and foreign field, describes the basic theory about microwave filter, uses a design of a 2.4GHz single-band bandpass filter as a example to introduces the general method of designing filters based on LTCC technology, based on the theory that both the combination of single-band bandpass filters and a secondary frequency conversion can form the characteristics of a dual-band bandpass filter, two kinds of 2.4GHz/5.2GHz dual-band bandpass filters based on LTCC process and a 1.8GHz/2.4GHz dual-band bandpass filter based on microstrip process are designed.The shortcoming of the bandpass filter of capacitive coupling structure is that the stop-band attenuation characteristic is not steep enough in the vicinity of the upper cutoff frequency. The stop-band attenuation characteristic can be well optimized by introducing a transmission zero.To avoid the impedance mismatch problem which will occur when assemble two single-band bandpass filters in parallel directly, two independent single-band bandpass filters with the minimum interference are designed. A 2.4GHz/5.2GHz dual-band bandpass filter based on LTCC process is designed, its size is only4.5mm×2.3mm×1.2mm.Simulation results show that the return losses at both the center frequencies of 2.4GHz and 5.2GHz are more than 30 d B, passband insertion losses are about 2d B. Due to the two transmission zeros introduced artificially, the stop-band attenuation at 2GHz and 2.9GHz reach to 35 d B, and more than 20 d B at 5.8GHz.A 1.8GHz/2.4GHz dual-band bandpass filter based on the principle of secondary frequency conversion is realized by microstrip process after simplify its circuit structure with admittance converter. Its size is cm7cm7 ?,center frequencies are 2.4GHz and 5.2GHz. In the passband, return loss is more than 25 d B, insertion loss is less than 1d B. The intermediate stop-band has obvious attenuation, insertion loss at 2.09 GHz reaches to 76 d B.A 2.4GHz/5.2GHz dual-band bandpass filter based on the principle of secondary frequency conversion is realized by LTCC process whose size is only ??3.16.12.3mmmmmm. The return loss in the two passband is about 20 d B and 30 d B respectively, and insertion loss in passband is less than 1d B. The attenuation of the stop-band between the two passband is about 20 d B. By introducing an adjustment capacitor connected to the filter network in parallel, the frequency response characteristics at high frequency is improved, so that the insertion loss at 7.7GHz reaches to 57 d B. Comparing with the filter realized by the microstrip process, it can be seen that the filter based on the LTCC process can be greatly volume reduced, LTCC technology has its advantages in realize miniaturized devices. |
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