Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-04-07
2002-07-09
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R
Reexamination Certificate
active
06417746
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to surface acoustic wave (SAW) filters, and, more specifically, to enhancing the performance of SAW filters.
2. Background
A conventional longitudinally-coupled SAW filter is illustrated in FIG.
1
. The filter itself is identified with numeral
10
. The filter is shown as it would typically be configured in operation, i.e., terminated with source and load impedances and coupled to a signal source. The signal source is identified with numeral
1
; the source impedance, with numeral
2
, and the load impedance, with numeral
7
. The filter itself comprises two outer reflectors, identified respectively with numerals
3
and
6
, and two interdigital transducers (IDTs), identified respectively with numerals
4
and
5
, situated within the cavity
9
defined by the two outer reflectors
3
and
6
. IDT
4
, being coupled to the signal source
1
through the source impedance
2
, functions as a source IDT, and IDT
5
, being coupled to the load impedance
7
, functions as a load IDT. In the conventional filter, the gap
8
between the source and load IDTs
4
and
5
is typically the spacing between IDT fingers plus roughly 0.7 Bragg lengths where a Bragg length is approximately defined as ½&lgr; (&lgr; being the wavelength of the acoustic wave generated within the SAW filter). The Bragg length is a fixed value whereas the wavelength varies with frequency. The number of poles (natural frequencies) in the transfer function of the filter of
FIG. 1
is limited to 3.
A second embodiment of the conventional SAW filter is illustrated in FIG.
2
. As in
FIG. 1
, the filter, which is identified by numeral
19
, is shown as it would typically be configured in operation, i.e., terminated with source and load impedances and coupled to a signal source. The signal source in this embodiment is identified with numeral
11
; the source impedance, with numeral
12
; and the load impedance, with numeral
18
. The filter itself comprises two outer reflectors, identified respectively with numerals
13
and
17
, with three IDTs, identified respectively with numerals
14
,
15
, and
16
, in the cavity defined by the two outer reflectors
13
and
17
. IDT
15
, being coupled to signal source
11
through source impedance
12
, functions a source IDT, while IDTs
14
and
16
, being coupled to the load impedance
18
, function as load IDTs. In this embodiment of the conventional SAW filter, the gaps
19
and
20
between the source and load IDTs are each typically the same as the first embodiment. Again, the number of poles in the transfer function of the filter of
FIG. 2
is limited to 3.
As is known, each of the filters of
FIGS. 1 and 2
are mounted on a piezo-electric substrate made of a material such as lithium tantalate (LiTaO
3
), lithium niobate (LiNbO
3
), or the like.
FIGS. 1 and 2
each represent top views of the filters as they might appear on the surface of the substrate. In operation, when the A/C signal from the signal source is applied to the filter, an alternating electric field is created between the fingers of the IDTs making up the filter. This electric field causes the substrate to expand and contract at the frequency of the A/C signal applied by the signal source. The result is that a longitudinally-propagated acoustic wave is generated within the cavity defined by the outer two reflectors.
The frequency selectivity of a filter is defined by the number of poles in the transfer function of the filter. Generally speaking, the greater the number of poles, the greater the frequency selectivity of the filter. The situation is illustrated in
FIG. 3
, in which the frequency response of a filter with a small number of poles is identified with numeral
21
, and the frequency response of a filter with a greater number of poles is identified with numeral
20
. As can be seen, the frequency selectivity of the frequency response identified with numeral
20
(determined by the steepness of the slope of the sides) is greater than that of the frequency response identified with numeral
21
.
As stated previously, the number of poles that is available in the filters of
FIGS. 1 and 2
is three. If greater frequency selectivity is desired beyond that available through a three-pole filter, the conventional approach is to cascade one or more of the filters of
FIGS. 1 and 2
in order to achieve a greater number of poles, and hence, greater frequency selectivity. The problem is that this results in excessive space being consumed by the filter, and also excessive insertion loss.
Consequently, there is a need for a SAW filter which is able to provide increased frequency selectivity while avoiding the excessive space consumed and insertion loss exhibited in the conventional approach.
SUMMARY OF THE INVENTION
In accordance with the purpose of the invention as broadly described herein, there is provided a surface acoustic wave filter comprising first and second reflectors defining a cavity, first and second interdigital transducers (IDTs) within the cavity and spaced by a gap, and at least one reflector within the gap. In one embodiment, the number of reflectors in the gap is equal to the desired number of filter poles−3. The spacing between the reflectors in the gap, and between the IDTs and the reflectors in the gap; the number of fingers in the IDTs, and in the reflectors in the gap; the pitch of the IDTs and of the reflectors in the gap; and the aperture of the IDTs and of the reflectors in the gap provide the necessary degrees of freedom to achieve a desired frequency response shape, which is determined in part by the placement of the poles and zeroes of the filter transfer function. In one implementation, one or more of these parameters are selected to achieve an optimal filter in terms of one or more of the following criteria: 1) minimum ripple in the passband; 2) maximum attenuation in the stopband; 3) minimum insertion loss; 4) minimum sensitivity to parameter changes; and 5) maximum ease of buildability.
In a second embodiment, first, second, and third IDTs are placed within the cavity. The spacing between the first and second IDTs defines a first gap, whereas the spacing between the second and third IDTs defines a second gap. In this embodiment, at least one reflector is placed in the first gap, and at least one reflector is placed in the second gap. In this implementation, the filter exhibits mirror-image symmetry, such that the number of reflectors in the first gap is equal to the number of reflectors in the second gap. In this implementation example, the number of reflectors placed in each gap is equal to the number of desired poles−3.
A surface acoustic wave filter system is also provided. In this system, an embodiment of a filter configured in accordance with the subject invention is terminated with a parallel load impedance and a series source impedance. The series source impedance is placed in series with an A/C signal source.
In one application, the filter system of the subject invention forms the RF filter of the front-end of a super-heterodyne transceiver.
REFERENCES:
patent: 4731595 (1988-03-01), Wright
patent: 5485052 (1996-01-01), Seki et al.
patent: 5717367 (1998-02-01), Murai
patent: 5874869 (1999-02-01), Ueda et al.
patent: 5936488 (1999-08-01), Taguchi et al.
patent: 6163236 (2000-12-01), Thomas
patent: 11-17494 (1999-01-01), None
patent: 11-298287 (1999-10-01), None
Rockwell brochure onCollins Torsional Mechanical Filters, Rockwell Semiconductor Systems—Filter Products; 2990 Airway Avenue, Costa Mesa, CA 92626-6018; Feb. 1996.
Conexant Systems Inc.
Howrey Simon Arnold & White , LLP
Pascal Robert
Takaoka Dean
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