Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-03-07
2002-10-29
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R, C310S31300R
Reexamination Certificate
active
06472959
ABSTRACT:
BACKGROUND OF THE INVENTION
It is well known to use SAW resonators for bandpass filtering in communications equipment, especially cellular radio equipment, to provide desirable characteristics such as low insertion loss (attenuation within the filter pass band), low ripple in the pass band, high stop band rejection (attenuation outside the pass band), small size, and low cost. Known SAW resonator filters include one-pole resonators, ladder circuit structure (LS) filters comprising a plurality of one-pole resonators, waveguide coupled double mode resonator (WCR) filters, and longitudinally coupled double mode resonator (LCR) filters. It is also known in such filters to provide a capacitance arrangement in order to provide the filter with sharper band edges, i.e. steeper transitions between the pass and stop bands of the filter.
For example, Hikita et al. U.S. Pat. No. 4,803,449 issued Feb. 7, 1989 and entitled “Filter Combining Surface Acoustic Wave Resonators” discloses a filter comprising a plurality of one-pole SAW resonators coupled together via capacitance elements. Mineyoshi U.S. Pat. No. 5,694,095 issued Dec. 2, 1997 and entitled “Surface Acoustic Wave Resonance Device Adapted To Simple And Precise Adjustment Of Resonant Frequency” describes frequency adjustment of a single mode SAW resonator by providing an additional capacitance which may be in parallel with the resonator. Examples of LS filters are described in a paper by Ikata et al. entitled “Development Of Low-Loss Band-Pass Filters Using SAW Resonators For Portable Telephones”, Proceedings of the IEEE Ultrasonics Symposium, 1992, pages 111 to 115.
Double mode SAW resonators include WCR (also referred to as a transversely coupled resonator) and LCR (also referred to as an in-line coupled resonator) arrangements. For example, Xu et al. U.S. Pat. No. 5,821,834 issued Oct. 13, 1998 and entitled “Surface Wave Device With Capacitance” discloses a filter having two WCRs connected in cascade with one or more shunt capacitances to narrow the filter pass band. Other WCR filters with capacitance arrangements are disclosed for example by Gopani et al. U.S. Pat. No. 5,077,545 issued Dec. 31, 1991 and entitled “Surface Acoustic Wave Waveguide-Coupled Resonator Notch Filter”, and by Japanese Documents No. 4-230108 dated Aug. 19, 1992, No. 4-249907 dated Sep. 4, 1992, and No. 5-55855 dated Mar. 5, 1993, each in the name of Morii and each entitled “Surface Acoustic Wave Filter”.
In addition, Japanese Document No. 3-222512 dated Oct. 1, 1991, in the name of Komazaki et al. and entitled “Polarized Type SAW Resonator Filter”, discloses a plurality of one-pole resonators arranged transversely, or side by side, with a capacitance in parallel with the plural resonators to provide a steepened cut-off characteristic of the filter. WCR filters have a small fractional bandwidth (ratio of pass band width to center frequency of the pass band of the filter) suitable for example for IF (intermediate frequency) filtering in cellular communications systems. For filtering RF (radio frequency) signals in a cellular communications system, a much greater fractional bandwidth is typically required, for which it is known to use either LS filters as discussed above or LCR filters.
As is well known in the art, an LCR comprises two or more IDTs (interdigital transducers) arranged longitudinally, i.e. in line in the direction of SAW propagation, typically between two reflection gratings. LCR filters with capacitance arrangements are also known. For example, Nagaoka Japanese Document No. 2-130010 dated Nov. 10, 1988 and entitled “Surface Acoustic Wave Device” discloses an LCR filter in which a capacitance, which may be constituted by parts of the IDTs, is arranged in parallel with an LCR to provide attenuation poles above and below the pass band of the filter, thereby providing steeper transitions between the pass and stop bands.
Similarly, Japanese Document No. 9-172342 dated Jun. 30, 1997, in the name of Honmo et al. and entitled “Dual Mode Surface Acoustic Wave Resonator Filter”, discloses a filter having cascaded LCRs each with a parallel capacitance, connected between the input and output of the LCR, providing attenuation poles just above and just below the filter pass band, thereby steepening the transitions between the filter pass and stop bands. As shown clearly in FIG. 4 of Honmo et al., as the magnitude of the capacitance is increased, the attenuation poles are moved more closely to the center frequency of the filter pass band, the transitions between the filter pass and stop bands become more steep, and attenuation in the stop bands becomes substantially more degraded.
In addition, Sakamoto et al. U.S. Pat. No. 4,931,755 issued Jun. 5, 1990 and entitled “Surface-Acoustic-Wave Device With A Capacitance Coupled Between Input And Output” discloses a SAW filter with multiple (e.g. five) IDTs coupled in line with one another, serving alternately as input and output IDTs, and a parallel capacitance connected between the input and output. The capacitance shifts an antiresonant frequency of the SAW device, on a high frequency side of the pass band, to a lower frequency closer to the center frequency of the pass band, thereby increasing close-in attenuation on the high frequency side of the pass band of the device. As shown in
FIG. 3
of this reference, attenuation in the lower stop band, and far-out attenuation in the upper stop band, are substantially degraded.
Also, Shindo et al. Japanese Document No. 9-172350 dated Jun. 30, 1997 and entitled “Multiple-Mode Surface Acoustic Wave Filter” discloses a filter comprising two LCRs connected in cascade, with a capacitance divider at an output of the second LCR having its tapping point connected to a junction between the cascaded LCRs, to feed back part of the output signal and thereby provide a steeper attenuation characteristic of the filter. This is an IF filter for which only a small fractional bandwidth is required.
It is well known, for example from Sakamoto et al., Mineyoshi, and Xu et al. referred to above, that capacitances can be conveniently provided in a SAW device by capacitance between conductors on the substrate of the SAW device, for example in the form of an interdigital structure (IDS) on the substrate.
For filtering RF signals in current cellular communications systems, a fractional bandwidth of the order of 2 to 4% is typically required. The LCR filters of the prior art discussed above use the first (symmetric) and second (asymmetric) modes as the two modes of the LCR, and use piezoelectric substrates and orientations (crystal cuts and acoustic wave propagation directions) which do not provide filters with such a large fractional bandwidth. For example, the Honmo et al. reference discloses achieving a fractional bandwidth of about 0.3 to 0.5% using a substrate of 45° X-Z (i.e. 45° rotated X-cut Z-propagation) lithium tetraborate (LBO).
In contrast, for filtering RF signals with a large fractional bandwidth, it is desirable to use an LCR filter having a piezoelectric substrate and orientation that provides a high electromechanical coupling coefficient and a high SAW propagation velocity.
In order to address these desires, an article by Morita et al. entitled “Wideband Low Loss Double Mode SAW Filters”, Proceedings of the IEEE Ultrasonics Symposium, 1992, pages 95 to 104 discloses using LCR filters using the first (symmetric) and third (symmetric) modes with a substrate of 36° Y-X lithium tantalate (LiTaO
3
) or 64° Y-X lithium niobate (LiNbO
3
) providing high electromechanical coupling. This is a leaky SAW arrangement in which the acoustic waves are shallow bulk acoustic waves (SBAWs), specifically surface skimming bulk waves (SSBWs), and not Rayleigh wave SAWs. More particularly, there is no significant propagation of Rayleigh waves with these substrates. Conversely, in the other prior art discussed above the substrates are selected to minimize the propagation of bulk acoustic waves, including SBAWs, because these would detract from the performance of the filters using Rayleigh wave SAWs
Beaudin Steve A.
Cameron Thomas P.
Damphousse Simon
Nortel Networks Limited
Pascal Robert
Summons Barbara
LandOfFree
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