Hybrid leaky surface acoustic wave resonator filter

Details Hybrid leaky surface acoustic wave resonator filter Hybrid leaky surface acoustic wave resonator filter

2002-04-25

2004-09-28

Nguyen, Minh (Department: 2816)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S133000, C310S31300R

Reexamination Certificate

active

06798318

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a hybrid bandpass filter using a combination of a longitudinally coupled leaky surface acoustic wave (LSAW) resonator and a network of LSAW impedance-element filters, and more particularly, to a hybrid bandpass filter with improved performance which is manufacturable using standard fabrication techniques resulting from compatible metalization thicknesses of the coupled resonator and impedance-element components.
BACKGROUND OF THE INVENTION
As the telecommunications industry and society continue to push for mobile communications devices which are smaller, lighter, less expensive, and more energy efficient, the requirements for bandpass filters within these devices become increasingly stringent. Where once transversely coupled surface acoustic wave (SAW) resonator filters were widely used, high-performance transversal SAW filters or longitudinally coupled SAW or LSAW filters have begun to take their place. Transversal SAW filters have the advantages of high flexibility, wide bandwidth, and flat group delay time. However, with newer digital mobile communications protocols requiring smaller size and even less insertion loss, transversal filters simply cannot meet the requirements.
Longitudinally coupled LSAW resonator filters have shown much promise in meeting many of the application requirements because of their wide achievable bandwidth and low insertion loss. Conventional longitudinally coupled LSAW resonator filters typically consist of a plurality of LSAW resonator filter tracks connected in series. Each track generally includes a pair of reflective gratings, between which are disposed a plurality of interdigital transducers (IDTs). In each track, one or more non-adjacent IDTs act together to form a signal input for the track, and the remainder of the IDTs form an output. Adjacent tracks are connected together in series such that the output of the first track is connected to the input of the second, whose output is connected to the input of the third, etc. The input of the first track and the output of the last track comprise the electrical input and output of the bandpass filter. The most common configurations employ only two tracks with two, three, or five IDTs in each track. By way of example,
FIG. 2
a
illustrates a schematic representation of a two-track longitudinally coupled LSAW resonator filter of the prior art with three IDTs per track, and
FIG. 2
b
illustrates one with five IDTs per track. In both cases, the filters include two separate tracks, each with input IDTS
20
, output IDTs
21
, and gratings
22
.
Good bandpass characteristics can be achieved with longitudinally coupled LSAW resonator filters by introducing resonant cavities between adjacent IDTs and between the gratings and the IDTs adjacent to them. As described in U.S. Pat. No. 5,485,052, the resonant cavities are introduced by inserting spacers between each IDT and its neighboring IDT or grating. The length of these spacers can be either positive (i.e. moving the IDTs/gratings further apart) or negative (i.e. moving the IDTs/gratings closer together). Spacers between adjacent IDTs are typically on the order of ±&lgr;/4, where &lgr; is the acoustic wavelength, and the spacers between the gratings and the adjacent IDTs is usually much smaller (e.g. ±&lgr;/40).
Although longitudinally coupled LSAW resonator filters exhibit good passband characteristics and strong rejection of frequencies substantially removed from the passband, they are typically plagued by inadequate rejection of frequencies close to the passband, especially those frequencies just above the passband. This renders them unusable in many applications requiring strong near-in rejection.
FIG. 3
demonstrates this phenomenon.
Another technology used prolifically for mobile communications applications includes use of a “ladder” filter, the name of which comes from its architecture of repeated series-connected and shunt-connected impedance-element filters. Impedance-element filters are simple one-port resonators, as illustrated with reference to FIG.
4
. They consist of a simple IDT
23
disposed between two reflective gratings
24
. At the resonant frequency of one of these devices, the impedance is extremely low; at the anti-resonant frequency, on the other hand, the impedance is very high. By utilizing these devices as impedance elements in an electrical circuit, various filter characteristics can be achieved. The series connection of such a device acts as a crude low-pass filter with a deep notch corresponding to the anti-resonant frequency where the device's impedance substantially inhibits the signal from getting through. This Is shown as the thin line (Series) plotted in FIG.
5
. The shunt connection of such a device, on the other hand, acts as a high-pass filter, with a deep notch corresponding to the resonant frequency, where the impedance is so low as to short much of the signal to ground. The frequency response in this configuration is plotted with the thick line (shunt) in FIG.
5
. Hence, by repeated series-shunt combinations, or “ladders”, as illustrated in
FIG. 6
a
and shown schematically in
FIG. 6
b
, bandpass filters can be realized with low insertion loss and excellent rejection of frequencies close to the pass band. However, ladder filters suffer from poor rejection of frequencies substantially removed from the passband, as demonstrated in the transfer function plot of FIG.
7
.
The idea of combining longitudinally coupled LSAW resonator filters and LSAW impedance-element filters in order to achieve good near- and far-frequencyrejection is not novel. An allusion to this combination can be found in U.S. Pat. No. 5,610,566. However, U.S. Pat. No. '566 fails to recognize that LSAW impedance-element filters require thick metalization in order to reap the benefits of high reflectivity and high piezoelectric coupling, thereby achieving a high Q. As will be herein described, this requirement renders LSAW impedance-element filters incompatible with conventional LSAW coupled resonator filters at the same relative metal thickness.
FIG. 8
a
and
FIG. 8
b
demonstrate the variation of the piezoelectric coupling coefficient, K
2
, and the reflectivity, &kgr;, as a function of aluminum metalization thickness on 42° Y-rotated lithium tantalate (LiTaO
3
). On that substrate, impedance element filters require aluminum metalization thickness of at least 9% of the acoustic wavelength.
Conventional longitudinally coupled LSAW resonator filters, on the other hand, require a thinner metalization. This is because the velocity of the LSAW mode is in very close proximity to the slow shear bulk acoustic wave (BAW) mode. Whenever a discontinuity is encountered by the propagating LSAW, energy is reflected backwards and, due to the close proximity of the BAW, a significant portion of that energy can be converted into BAW energy and lost into the bulk of the substrate. This energy loss is commonly referred to as “radiation” or “scattering” loss. As reflectivity goes up, BAW radiation losses at the discontinuities go up as well. The spacer-type resonant cavities constitute significant phase discontinuities. Thus, conventional longitudinally coupled LSAW resonator filters are usually limited to metalization thicknesses of 8.5% or less. As thickness is increased above that value, losses due to BAW radiation outgrow the gains from increasing piezoelectric coupling and reflectivity.
Thus, in order to achieve a low-loss filter, the combination of the two aforementioned technologies would require different metalization thicknesses for the coupled resonators and the impedance-element filters, despite the suggestion otherwise by U.S. Pat. No. 5,610,566. This would require complicated fabrication steps, thus rendering the device unmanufacturable by reasonable standards. The present invention overcomes this problem by utilizing a novel longitudinally coupled LSAW resonator filter with chirp-type resonant cavities, rather than spacers. This structure exhibits significantly les

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