纵向耦合谐振滤波器旁瓣抑制的改进

Spurious Rejection Improvement of Longitudinally Coupled Resonator Filter

介绍了一种在石英基片上两端对纵向耦合谐振滤波器阻带抑制的方法.一般纵向耦合谐振器的频率响应有旁瓣较高的缺点,影响其实际应用.分析这些旁瓣形成的原因后,引入了hiccup结构.它改变换能器指条结构分布,并且对指条进行了不同方式的加权.这种结构有利于多方面调节滤波器的结构参数,对其频响进行细微的控制和调整.运用耦合模理论对纵向谐振模式进行了分析,在计算机上这种结构的滤波器特性作了模拟,理论计算表明器件响应中部分旁瓣得到了有效的抑制.实验结果证明了理论分析的准确性与实用性.

Progress in the communication industry has led to an increasing need for RF filters with large bandwidth and low insertion loss. SAW longitudinally coupled resonator filters are such kind of devices that meet the requirement of working with high frequency and small-size equipment. To increase the performance of traditional filters, we analyze the cause of spurious peaks and put forward the improved methods.As we know, it is so complicated to make out the cause of all the spurious peaks that we cannot use one simple way to restrain them efficiently. Side gratings can be used to depress the spurious peaks far from passband, moreover, the hiccup structure is employed to control the peak around the center frequency, which is due to the coupled resonance modes. The input and output transducers are divided into four parts and a center grating is inserted among transducers. Coupling-of-modes(COM) theory is used to calculate the insertion loss and phase frequency. Every building block is described by its transmission matrix which gives the relation between the waves at their two ends. The Y-parameters and S-parameters of the whole SAW filter can be calculated by cascading the individual transmission matrixes. But because the IDTs of SAW resonator filters are not uniformly weighed, their transmission matrixes can not be derived from the COM models directly. To solve this problem, we divide IDTs into several different building blocks, uniformly distributing IDTs parts and acoustic transmission lines. From these building blocks' respective COM models their transmission matrixes can be derived. Then the total transmission matrixes of IDTs can be calculated out. Several filters are designed, simulated, fabricated and measured. Phases of two couples of transducers are found nearly opposite at the low frequency spurious peaks. Not only are proper weighting functions taken on transducers, but also parameters of center grating and transducers are adjusted to decrease the side peak. The simulation results agree well with the experiment on ST Quartz. The spurious peak of low frequency decreases from -23 dB to -52 dB in calculation results and from -32 dB to -43 dB in the experiment. Although this peak almost disappears, the high frequency only gets depressed 4 dB lower than before. It will be studied in the future.