Filter device

Abstract

There is provided a low-loss and wide-band filter device capable of obtaining a sufficient attenuation amount. In a communication system including a first band pass filter having a relatively low pass band or a second band pass filter having a relatively high pass band, the filter device (1) is used as the first band pass filter. In the filter device (1), series arm oscillators (S21 to S23) are inserted into a series arm connecting an input terminal (2) and an output terminal (3). Parallel arm oscillators (P21, P22) are connected to a parallel arm connecting the series arm and the reference potential. At least one of the parallel oscillators (P21, P22) is connected in series to inductances (L1, L2). The resonance frequency of sub-resonance generated by insertion of the inductances (L1, L2) is positioned in the pass band, or in its vicinity, of the band filter of the reception side or the transmission side of the partner filter with respect to the ladder type filter.

Description

technical field [0001] The present invention relates to a filter arrangement having a plurality of resonators connected to provide a ladder circuit configuration, such as a transmitter bandpass filter and a receiver bandpass filter used in a communication system. Background technique [0002] In general, a ladder filter having a plurality of connected surface acoustic wave resonators is widely used as a receiver bandpass filter or a transmitter bandpass filter of a surface acoustic wave device. For example, Patent Document 1 discloses a ladder filter having a plurality of single-terminal pairs of surface acoustic wave resonators arranged alternately in a plurality of parallel branches and a series branch from the input side to the output side. on the road. In Patent Document 1, as Figure 24 As shown, a parallel branch resonator P1 is inserted in the parallel branch, while a series branch resonator S1 is inserted in the series branch. Although Figure 24 shows a one-stage ...

AI Technical Summary

Problems solved by technology

Although the circuit structure described in Patent Document 2 allows large attenuation in the high frequency passband, it is difficult to provide a wide passband bandwidth

Therefore, it is difficult to provide a filter with sufficient attenuation and capable of operating in a wide frequency range, such as a transmitter bandpass filter for a WCDMA branch filter in the 2GHz band

[0009] On the other hand, in the ladder filter described in Patent Document 1, the inductor L connected in series with the parallel branch resonator P1 provides a wide bandwidth and a large attenuation

However, the optimal inductance value of the inductor L is not specifically disclosed

Furthermore, Patent Document 1 does not disclose a specific structure for improving attenuation in the high-frequency passband.

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