JPH0611640Y2 - SAW filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Field of the Invention The present invention provides a SAW filter, and in particular, a multi-mode filter formed by coupling a plurality of SAW resonators on the same piezoelectric substrate at intervals of at least two. And a SAW in which each multimode filter is electrically connected in cascade.
Regarding filters.

2. Description of the Related Art The SAW filter described above has excellent filter characteristics such as a narrow band compared to other filters and a large amount of attenuation outside the band. Therefore, it is used as a high frequency (RF) filter such as a pager. ing. Here, it is desirable that the RF filter has a large amount of attenuation at the center frequency ± 2 × (IF frequency) in consideration of image interference to the IF stage. Since the IF frequency is usually 455 KHz,
Specifically, it is expected that the amount of attenuation at a distance of 910 KHz above and below the center frequency is large.

By the way, the present inventor manufactured the following filter as an example of the SAW filter. That is, two SAW resonators are acoustically coupled by bringing them close to each other in the cross width direction,
Along with constructing a multiple mode filter, a pair of these filters were arranged on the quartz substrate at intervals in the cross width direction, and both were electrically cascaded to manufacture a SAW filter with a center frequency of 280 MHz and four poles. .

However, according to experiments, in the above filter, some spurious is generated at a position higher than the center frequency by 910 KHz. Therefore, when the image disturbance to the IF stage is taken into consideration, the above SAW is generated. R for filter, such as pager
It turns out that there is a problem in using it as an F filter.

Considering the cause of the spurious emission at a frequency 910 KHz higher than the center frequency, it was found that it was due to acoustic leakage between two cascaded pole double mode filters.

As a measure to prevent this acoustic leakage, it is conceivable to provide a sound absorbing material between the two multi-mode filters, but this method requires a space for providing the sound absorbing material, so the chip size becomes large. In addition, there is a disadvantage that a step of providing a sound absorbing material is required and the number of manufacturing steps is increased.

At this point, the present invention is devised so that acoustic leakage is effectively scattered and the two multimode filters are not acoustically coupled, and further, the chip size can be reduced and the number of manufacturing steps can be reduced. It is intended to provide an excellent SAW filter.

Means for Solving the Problems The above object is achieved by forming a lead electrode for external connection on a piezoelectric substrate between the multimode filters.

Action A wire bonding wire or the like is connected to the extraction electrode for external connection. When the extraction electrode connected with such a line is formed on the piezoelectric substrate between the two multimode filters,
As a result of acoustic leakage existing between the two multi-mode filters being scattered by the wire bonding wire or the like, it becomes difficult to couple the two multi-mode filters due to the acoustic leakage. Therefore, it is possible to improve the filter characteristics as compared with the conventional case.

Further, since the extraction electrode is an electrode that is naturally necessary for the SAW filter, the number of manufacturing steps does not increase, but rather the number is reduced compared to the configuration in which the sound absorbing material is provided, and instead of providing the sound absorbing material, the SAW filter is provided at that location As a matter of course, since the extraction electrode necessary for the equipment is provided, the overall size is reduced.

Embodiment FIG. 1 shows a center frequency of 280 as an embodiment of the present invention.
In the schematic plan view showing the SAW filter of MHZ, 1 is a substrate made of, for example, 38 ° rotation Y-cut crystal (ST-cut crystal), and 2 to 5 are SAW resonators. As shown in FIGS. 2 and 3, each SAW resonator has a comb-shaped electrode (hereinafter, I
It is called DT. ) 2a to 5a and grating-shaped reflectors 2b, 2c to 5b, 5c arranged on both sides of the electrode in the SAW propagation direction P. SAW above
Two resonators (2, 3, 4 and 5) are acoustically coupled by arranging them two by two (3, 4 and 5) close to each other in the cross width direction Q to form a two-pole multimode filter 6, 7.

An example of the optimized design of the multimode filters 6 and 7 is as follows.

Number of IDT electrodes: 150 vs. number of reflector strip lines: 300 IDT and reflector materials: aluminum IDT and reflector film thickness: 0.02λ (where λ is the wavelength at the center frequency of 280 MHz λ ≈ 11μ
m) Pitch ratio between IDT and reflector P1 / P2 = 0.994 Distance between IDT and reflector l = 1.15λ SAW resonator aperture length W≈10λ Distance between two SAW resonators G≈1λ Optimization design as above Multimode filters 6 and 7
ST cut quartz substrate 1 with 400 μm (≈36λ) apart
A SAW filter is formed by arranging it on the upper side and providing extraction electrodes 8a, 8b for external connection on the upper surface of the substrate between the multimode filters 6 and 7.

The lead-out electrodes 8a, 8b ... Are used as bonding pads or soldering pads, for example.
The conductive patterns 9a, 9b, ... Formed on the piezoelectric substrate 1 connect the a, 8b ... And the IDTs of the SAW resonators 2-5. In this case, the ground side can also partially use the strip line of the reflector. FIG. 1 shows the state. Also, some reflectors may be used for the connection on the hot side. The size of the extraction electrodes 8a, 8b ... Is set to a size necessary to scatter acoustic leakage between the two multimode filters 6 and 7, and the number of electrodes is not limited.

With this configuration, a coil was inserted in series with the input and output of the filter to perform tuning and measurement (measurement was performed in a 50Ω system). With the same configuration, no extraction electrode was formed between the dual mode filters. Compared to the SAW filter, spurious near the center frequency (280 MHz) and 910 KHz higher was significantly improved. FIG. 4 is a frequency characteristic curve showing this state. The broken line is the SAW filter of the present invention, and the solid line is the SAW without the extraction electrode between the dual mode filters.
The frequency characteristic of the filter is shown.

The multimode filter has two SAW resonators 12 and 13 as shown in FIG.
Can be also configured by arranging in the SAW propagation direction, and it goes without saying that the present invention can be applied to a SAW filter using a multimode filter having such a configuration.

Effect of the Invention Since the present invention is configured as described above, it is possible to effectively scatter acoustic leakage between two multi-mode filters and prevent spurious emission due to acoustic leakage. In addition to having an excellent effect, it also has a manufacturing and structural effect that the overall size can be reduced without increasing the number of manufacturing steps.

[Brief description of drawings]

FIG. 1 is a schematic plan view of a SAW filter as an embodiment of the present invention, FIG. 2 is a detailed plan view of a SAW resonator, FIG. 3 is a side sectional view showing a part of the SAW filter, and FIG. FIG. 5 is a diagram showing filter characteristics, and FIG. 5 is a diagram showing another example of usable multimode filters. 1 ... Piezoelectric substrate, 2, 3, 4, 5 ... SAW resonator 6, 7 ... Multimode filter 8a, 8b, 8c, 8d ... Extraction electrode

Claims (1)

[Scope of utility model registration request] 1. A multi-mode filter comprising a plurality of SAW resonators coupled to each other on the same piezoelectric substrate.
A SAW filter in which at least one of them is arranged at intervals and each of the multi-mode filters is electrically connected in cascade. A lead-out for external connection is provided on the piezoelectric substrate between the multi-mode filters. A SAW filter having electrodes formed.