Surface acoustic wave filter and saw filter package

Abstract

A polar SAW (Surface Acoustic Wave) filter includes a band-pass ladder type SAW filter having a SAW resonator. A two-terminal pair circuit is serially connected to the band-pass ladder type SAW filter and includes a plurality of inductors. The polar SAW filter has attenuation poles in each of the higher-frequency and lower-frequency attenuation ranges of a pass band.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a SAW (Surface Acoustic Wave) filter with an attenuation pole advantageously applicable to, e.g., a transmitter filter or a receiver filter included in a mobile communication terminal such as a cellular phone terminal.[0003]2. Description of the Background Art[0004]SAW filters with an attenuation pole for the above-mentioned application are disclosed in, e.g., Japanese patent laid-open publication Nos. 93382 / 1998 and 163808 / 1998, hereinafter referred to as Documents 1 and 2, respectively. Another SAW filter with an attenuation pole is proposed in Sato et al., “Small Loss, Band-Pass Filter Using SAW Resonators”, Papers A, the Institute of Electronics, Information and Communication Engineers of Japan, Vol. J76-A, No. 2, pp. 245-252, 1993 (Document 3 hereinafter).[0005]Small size, high performance parts are essential with cellular phones and other handy mobile communication terminals that a...

AI Technical Summary

Benefits of technology

The present invention provides a polar SAW filter with attenuation poles at both lower and higher frequencies in a pass band, which solves the problems of previous filters. The filter has a steep characteristic in both attenuation ranges. This is achieved by using a band-pass ladder type SAW filter with a SAW resonator and a two-terminal pair circuit including inductors.

Problems solved by technology

However, if the attenuation value in the higher-frequency attenuation range of the transmission band is small, then it is likely that a radio wave radiated from a mobile communication terminal turns round into the receiver band of the same terminal, lowering the reception quality.

Such an attenuation value in the higher frequency range is not always enough.

However, even the filter characteristic shown in FIG. 5 cannot effect sufficient attenuation in the lower-frequency attenuation range.

It follows that the filter characteristic available with the SAW filter taught in Document 1 is not sufficient when applied to the CDMA system in the United States as a receiver filter and is therefore likely to degrade the reception quality.

More specifically, if the SAW filter taught in Document 1 and lacking a good filter characteristic is applied to the CDMA system in the United State as a transmitter filter, then, the receiver filter fails to reduce the influence of the turn-round of a radio wave from the transmitter side of the same mobile communication terminal to a satisfactory degree.

Further, the receiver filter cannot sufficiently reduce, e.g., the influence of an interference wave that may arrive at the mobile communication terminal from another radio communication apparatus.

It is therefore quite probable that the SAW filter will fail to satisfy a required standard in the lower-frequency attenuation range, although satisfying a required standard in the higher-frequency attenuation range.

In practice, however, such a great inductance L is difficult to realize.

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