Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2003-02-28
2004-12-28
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S196000, C310S31300R
Reexamination Certificate
active
06836197
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a surface acoustic wave (SAW) reflector filter and, more particularly, to a SAW reflector filter employing dual tracks each including an input transducer, an output transducer and at least one reflector, where the reflectors have reflection functions that are equal in magnitude and opposite in phase so that the reflected waves combine at the output transducers.
2. Discussion of the Related Art
Surface acoustic wave (SAW) filters for use in mobile phone communications systems are designed to be small in size, exhibit good out-of-bandwidth rejection, and provide narrow bandwidths with steep transition edges. Conventional SAW filters include an input transducer and an output transducer formed on a piezoelectric substrate. The input transducer is electrically excited with the electrical input signal that is to be filtered. The input transducer converts the electrical input signal to surface acoustic waves, such as Rayleigh waves, lamb waves, etc., that propagate along the substrate to the output transducer. The output transducer converts the acoustic waves to a filtered electrical signal.
The input and output transducers typically include interdigital electrodes formed on the top surface of the substrate. The shape and spacing of the electrodes determine the center frequency and the band shape of the acoustic waves produced by the input transducer. Generally, the smaller the width of the electrodes, or the number of electrodes per wavelength, the higher the operating frequency. The amplitude of the surface acoustic waves at a particular frequency is determined by the constructive interference of the acoustic waves generated by the transducers.
The combined length of the transducers determines the length of the overall filter. To design a conventional SAW filter with ideal filter characteristics, the filter's impulse response needs to be very long. Because the length of the impulse response is directly proportional to the length of the transducer, the overall length of a conventional SAW filter having ideal characteristics would be too long to be useful in mobile phone communication systems.
Reflective SAW filters have been developed to satisfy this problem. Reflective SAW filters generally have at least one input transducer, one output transducer and one reflector formed on a piezoelectric substrate. The reflector is typically a reflective grating including spaced apart grid lines defining gaps therebetween. The acoustic waves received by the reflector from the input transducer are reflected by the grid lines within the grating so that the reflected waves constructively and destructively interfere with each other and the wave path is folded. The constructively interfered waves are reflected back to the output transducer having a particular phase. Because of the folding, the length of the transducer is no longer dependent on the duration of the impulse response. Reflective SAW filters are, therefore, smaller in size and have high frequency selectivity, and thus are desirable for mobile phone communications systems.
The frequency response of a reflective SAW filter is further improved by weighting the individual reflectors to achieve a desired net reflectivity. The frequency response sets the phase and magnitude of the reflected acoustic waves. Existing weighting methods include position-weighting, omission-weighting and strip-width weighting. Other methods of weighting reflectors include changing the lengths of open-circuited reflective strips within an open-short reflector structure. Weighting the reflector helps to reduce the physical size of the filter and to improve the filter's frequency response.
FIG. 1
is a top plan view of a known dual track SAW reflector filter
10
including a first track
12
and a second track
14
. The first track
12
includes a bi-directional input interdigital transducer
16
, a bi-directional output interdigital transducer
18
, a first reflector
20
positioned on one side of the input transducer
16
and a second reflector
22
positioned on an opposite side of the output transducer
18
, all formed on a piezoelectric substrate
24
, as shown. Likewise, the second track
14
includes a bi-directional input interdigital transducer
28
, a bi-directional output interdigital transducer
30
, a first reflector
32
positioned on one side of the input transducer
28
and a second reflector
34
positioned on an opposite side of the output transducer
30
, all formed on the piezoelectric substrate
24
, as shown. The reflectors
20
,
22
,
32
and
34
can be any one of a number of suitable reflector devices, such as a reflective grating including a series of grid lines. The interdigital transducers
16
,
18
,
28
and
30
include a plurality of uniformly spaced interdigital electrode fingers
38
attached at opposite ends by bus bars
40
.
An electrical input signal to be filtered is applied to the input transducers
16
and
28
on an input line
42
. The input transducers
16
and
28
convert the electric signal into surface acoustic waves that propagate outward from the input transducers
16
and
28
along a top surface of the substrate
24
. Some of the acoustic waves from the input transducer
16
are directed towards the reflector
20
and some of the acoustic waves from the input transducer
16
are directed towards the output transducer
18
and the reflector
22
. Likewise, some of the acoustic waves from the input transducer
28
are directed towards the reflector
32
and some of the acoustic waves from the input transducer
28
are directed towards the output transducer
30
and the reflector
34
.
The reflectors
20
,
22
,
32
and
34
are tuned to the wavelength &lgr; at the center frequency of the frequency band of interest that is to be filtered, and have the same length L
1
. The reflected waves from the reflectors
20
and
22
are directed back to the output transducer
18
and the reflected waves from the reflectors
32
and
34
are directed back to the output transducer
30
where they are converted to a filtered electrical signal on a common output line
36
.
The input transducer
16
and the output transducer
18
are spaced the same distance apart (L
3
) as the input transducer
28
and the output transducer
30
. Also, the output transducers
18
and
30
have opposite polarities. Therefore, the surface acoustic waves directly received by the output transducer
18
from the input transducer
16
are 180° out of phase with the surface acoustic waves directly received by the output transducer
30
from the input transducer
28
. Hence, these waves cancel on the output line
36
and will not be converted into electrical signal at the output transducers
18
and
30
. These waves pass through the output transducers
18
and
30
with little attenuation and are reflected by the reflector
22
in the track
12
and the reflector
34
in the track
14
, respectively.
It is necessary to prevent cancellation of the reflected acoustic waves from the reflectors
20
,
22
,
32
and
34
on the output line
36
. The reflectors
20
and
32
and the reflectors
22
and
34
are thus offset relative to each other by &lgr;/4. Particularly, the distance between the input transducer
16
and the reflector
20
and the distance between the output transducer
18
and the reflector
22
is L
2
. However, the distance between the input transducer
28
and the reflector
32
and the distance between the output transducer
30
and the reflector
34
is L
2
+&lgr;/4. Thus, the acoustic waves reflected by the reflectors
20
and
32
travel a different distance to the transducers
18
and
30
, respectively, by &lgr;/2, and are thus 180° out of phase with each other when they reach the output transducers
18
and
30
. In other words, the acoustic waves in the second track
14
are delayed relative to the acoustic waves in the first track
12
. Therefore, the output signals add on the output line
36
. Likewise, the acoustic waves reflect
Garber Edward M.
Kong Alvin M.
Yip Davis S.
Miller John A.
Northrop Grumman Corporation
Summons Barbara
Warn, Hoffmann, Miller & LaLone, P.C.
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