Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-01-10
2004-06-08
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R
Reexamination Certificate
active
06747530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a surface-active wave filter (SAW) and, specifically, to a SAW filter of the reactance filter type with improved stop band suppression as well as to a method for the optimization of the stop band suppression.
2. Description of the Related Art
Reactance filters are known from classical filter technology. When SAW resonators are employed for the individual resonators instead of discrete elements (inductors and capacitors), then this is called a SAW filter according to the reactance filter type.
Given SAW filters of the reactance filter type, SAW resonators are employed as impedance elements.
FIG. 1
shows the schematic structure of a known resonator. It comprises metallic structures on the surface of a piezo-electric substrate and has a terminal pair
1
-
1
and
1
-
2
to which an interdigital transducer
1
-
4
is connected for the transformation of electrical energy into acoustic energy. A reflector
1
-
3
and
1
-
5
is respectively arranged at both sides of the interdigital transducer
1
-
4
along the acoustic axis in order to prevent the acoustic energy from escaping.
FIG. 2
, on the left, shows the equivalent circuit diagram for a SAW resonator R and shows the symbol employed for the resonator at the right. A series resonant circuit composed of dynamic inductance L
1
, dynamic capacitor C
1
and dynamic resistor R
1
(when taking losses into consideration) is located in the first branch of the parallel circuit, and the static capacitor C
0
of the interdigital transducer is located in the second branch. The series resonant circuit reflects the behavior of the resonator in the resonance case, i.e., in the range of the resonant frequency f
r
. The static capacity reflects the behavior in the frequency ranges f<<f
r
and f>>f
r
. The dynamic capacitor C
1
is proportional to the static capacitor C
0
of the interdigital transducer:
C
1
-C
0
(1.1)
A resonator has a resonant frequency f
r
and an anti-resonant frequency f
a
. The following applies to the resonant frequency f
r
:
f
r
=
1
2
⁢
π
⁢
L
1
*
C
1
(
1.2
)
The following applies for the anti-resonant frequency f
a
of a resonator:
f
a
=
f
r
*
1
+
C
1
C
0
(
1.3
)
The basic unit of a SAW reactance filter is a “basic element” as shown in FIG.
3
. It is composed of a first resonator R
1
with resonant frequency f
rp
and appertaining anti-resonant frequency f
ap
in the parallel branch and of a second resonator R
2
with resonant frequency f
rs
and appertaining anti-resonant frequency f
as
in the serial branch. The frequency curve of the admittance Y
p
of the resonator R
1
in the parallel branch and the frequency curve of the impedance Z
s
of the resonator R
2
in the serial branch are shown in FIG.
4
. For producing a band-pass filter with the middle frequency f
0
, the resonant frequencies of the two resonators have the following relationship:
f
ap
≈f
rs
≈f
0
(1.4)
Each basic element is fundamentally viewed as a two-port element with the terminals
3
-
1
or
3
-
2
of port
1
and the terminal
3
-
3
or
3
-
4
of port
2
(see FIG.
3
). At the same time, the terminal
3
-
1
is the input and the terminal
3
-
3
is the output of the series resonator. The input of the parallel resonator is connected to the terminal
3
-
1
. The terminals
3
-
2
and
3
-
4
represent the reference ground for an asymmetrical operation. The output
3
-
4
of the parallel resonator that faces toward the reference ground is referred below as the output side or ground side of the parallel resonator. The inductance L
ser
that lies between the output side of the parallel resonator and the reference ground reflects the connection to the housing ground in the real structure.
The selection level of the SAW filter according to the reactance filter type is defined, first, by the relationship C
0p
/C
0s
of static capacitor C
0p
in the parallel branch and static capacitor C
0s
in the series branch and is defined, second, by the number of basic elements connected following one another (in a cascaded manner, i.e., in series).
The basic elements when they are connected in series are usually circuit-adapted, i.e., they are respectively mirrored. FIG.
5
and
FIG. 6
show two examples of a reactance filter in which respectively two basic elements are cascaded. The output impedance
5
-
1
(Z
out
) or
6
-
1
(Z
in
) of the first basic element is equal to the input impedance
5
-
2
or
6
-
2
of the second basic element, resulting in a minimization of losses due to mismatching. Many structures are possible or known for reactance filters with respect to the number and arrangement of the basic elements.
Resonators of the same type (series resonator or parallel resonator) lying immediately behind one another can also be respectively combined to form one in which the overall capacity remains the same. The interconnection of a filter according to
FIG. 7
corresponds in effect to that of a filter according to FIG.
8
.
FIGS. 9 and 10
show the typical, actual structure of a SAW filter on a piezoelectric substrate
9
-
1
in a ceramic housing
9
-
0
and the typical connecting technique with bond wires
9
-
8
through
9
-
12
or
10
-
9
.
At the output side
9
-
15
through
9
-
17
, the parallel resonators R
1
, R
3
and R
5
are connected to the housing ground pads
9
-
4
,
9
-
5
and
9
-
7
via bond wires
9
-
9
,
9
-
10
and
9
-
12
.
As a result of the typical structuring technique (see FIG.
9
and FIG.
10
), series inductances between, for example, the output side
9
-
17
of the parallel resonator R
5
on the substrate (chip)
9
-
1
and the ground
10
-
5
next to the outer housing pin
9
-
4
are obtained given the connection of the parallel branches to ground. These essentially include the inductive part of the stripline on the chip, the inductance of the bond connection
9
-
9
and that of the housing lead-through
10
-
3
.
These series inductances influence the behavior of the filter both in the passband as well as in the stop band. f<<f
0
applies for the pass band. The resonant frequency and, thus, the bandwidth of a resonator can, as known, be modified by an external wiring belonging to the resonator. An inductance serially with the resonator increases the effective dynamic inductance, resulting in a drop in the resonant frequency f
r
. Since the anti-resonant frequency f
a
is shifted to only a very slight extent, the bandwidth &Dgr;=f
a
-f
r
of a resonator is increased with the serial inductance. The bandwidth of the SAW filter is also increased in the case of a parallel resonator.
f<<f
0
and f>>f
0
applies for the stop band. Here, the equivalent circuit diagram of a resonator is reduced to its static capacitance C
0
since the series resonant circuit is extremely high-impedance beyond f
0
and corresponds to a no-load condition. An inductance L
ser
connected serially with the resonator produces the series resonant circuit shown in
FIG. 11
b
having a resonant frequency.
f
pol
=
1
/
2
⁢
π
⁢
L
ser
*
C
0
(
1.5
)
In the case of an inductance connected serially with a parallel resonator, this means that the energy of the filter can flow off directly to ground given the frequency f
pol
; a “pole point” thus forms in the filter curve, i.e., an increased suppression in the stop band. A plurality of pole points in the stop band corresponds to the plurality of parallel branches with series inductance. Pole points f
pol1
and f
pol2
that can be distinguished from one another in terms of frequency derive only given different products II
1
=L
ser1
*C
01
and II
2
=L
ser2
*C
02
. When the products are identical, then the pole points lie at the same frequency; a double pole point f
pol
=f
pol1
=f
pol2
is obtained with a higher suppression than that of a simple single pole point.
FIG. 11
b
shows the attenuation behavior of a resonator in the parallel branch to which an inductance L
ser
is serially connected to the output si
Epcos AG
Schiff & Hardin LLP
Summons Barbara
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