Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-06-15
2003-01-21
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S191000
Reexamination Certificate
active
06509814
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to filters, and in particular to filters constructed using bulk acoustic wave resonators. Such filters may be used in communications equipment as band pass filters which enable selection of a frequency band in which transmission channels are located, and with rejection of frequencies outside the band of interest. The invention also relates to communications equipment (for example, a radio frequency receiver and/or transmitter) comprising such filters.
High-performance radio-frequency (RF) filters typically use high dielectric constant ceramic resonators or surface acoustic wave resonators. The former devices are rather bulky, whereas the latter are smaller but have higher insertion loss (generally>3 dB) and generally rather poor stop-bands. As a result, neither provides an ideal solution for channel band selection in small communications devices such as mobile phones. Filters for such applications need deep stop-bands to reject unwanted signals, as well as low pass-band insertion loss (typically<2 dB) to achieve adequate signal-to-noise ratio. There is therefore a requirement for very small resonators with high Q-factor (typically>500). To achieve this aim, with potential for integration on silicon, thin-film bulk-acoustic-wave (BAW) resonators have been proposed. These are applicable to the frequency range 0.5 to 10 GHz, and are therefore appropriate for the third generation mobile telephony standard, as well as for already established wireless standards, such as GSM, W-CDMA, Bluetooth, HomeRF, DECT and GPS.
The need for low insertion loss and high stop-band attenuation can not be achieved with a single resonator. Filters are therefore typically made up of a number of resonators, and a conventional thin-film BAW filter configuration is a ladder construction, shown in simplified schematic form in FIG.
1
. This has alternating series sections
2
and shunt sections
4
, each of which can be a single resonator, or one or more resonators on the same frequency connected in series or parallel (which is electrically equivalent). The anti-resonant frequency of the shunt element is chosen to be the resonant frequency of the series elements to provide minimum insertion loss at that frequency.
The individual resonators are typically arranged as so-called solidly-mounted resonators (SMRs), an example of which is illustrated in FIG.
2
. The required conversion between electrical and mechanical energy is achieved by a layer of piezoelectric material
10
(for example zinc oxide, aluminium nitride, PZT, PLZT) between two metal layers
12
,
14
in which electrodes are formed. The piezoelectric material
10
is provided over one or more acoustically mismatched layers
16
, which are mounted on an insulating substrate
18
, for example glass. The acoustically mismatched layers act to reflect the acoustic wave which results from resonance of the piezoelectric layer
10
at the resonant frequency.
In
FIG. 2
, a number of high impedance layers
16
a
and low impedance layers
16
b
are shown. Porous silicon oxide (aerogel) may be used for the low-impedance
16
b
layers, and a single layer may in fact be adequate to achieve sufficiently high Q, due to the very low acoustic impedance of this material. The high impedance layers
16
a
may comprise tungsten.
In
FIG. 2
, the upper metal layer
12
defines both terminals
12
a
,
12
b
of the resonator, and the lower metal layer
14
effectively acts as an intermediate electrode between two series-connected resonators. This avoids the need to make electrical contact to the lower metal layer
14
through the piezoelectric layer
10
. A single pair of series-connected resonators then acts as the basic building block of the filter and may be considered as the basic resonator element.
FIG. 2
also shows a plan view, with contact pads
20
providing the input and output of the device.
Ladder filter arrangements such as shown in
FIG. 1
have demonstrated good performance, for example less than 2 dB insertion loss and very low-level of spurious response. However, there are also some disadvantages, which can be understood from an approximate electrical equivalent circuit of the resonator, shown in FIG.
3
.
C
o
is an (unwanted) static capacitance of the resonator, whereas C
m
, L
m
and R
m
characterise the mechanical resonance. These are, respectively, the motional capacitance, motional inductance and motional resistance of the resonator. The resonator appears as a pure capacitor C
o
at frequencies removed from the resonance (except at other significant mechanical resonances such as harmonics, which are not accounted for in this simple model). In designs reported to date, the shunt and series resonators have similar areas, and therefore similar static capacitances. This gives only about 6 dB attenuation, in the frequency bands to be rejected by the filter (the “stop-band”), per combination of series and shunt sections. This is the result of the static capacitance of each resonator. A T-section, comprising two series-connected resonators and an intermediate shunt resonator may can be considered as the basic building block of a ladder filter. A single resonator element
2
i
,
2
o
(
FIG. 1
) is then at the input
6
and output
8
of the filter, and the intermediate series resonators elements
2
b
each comprise two series-connected resonator elements.
To achieve the desired low pass-band insertion loss and high stop-band insertion loss, each individual building block should meet these two requirements. Although increasing the number of sections adds to the stopband loss (as required), this also increases pass-band loss (and also the overall filter size). The pass-band and stop-band requirements therefore conflict with each other. Typically, several such building blocks are required for even moderate stop-band rejection. Consequently, both the area occupied and the insertion loss in the pass-band are increased without improving filter selectivity.
It has been recognised, for example in U.S. Pat. No. 5,471,178, that the stop band performance for a ladder filter is determined in part by the static capacitance ratio between the series and shunt resonators, as the resonators act as a capacitive voltage divider at frequencies removed from the resonant frequencies.
SUMMARY OF THE INVENTION
According to the invention, there is provided a ladder filter comprising a plurality of bulk acoustic wave resonators, the resonators comprising a plurality of series resonators in series between an input port and an output port of the filter, and one or more shunt resonators each connected between a junction between two series resonators and a common terminal, the series resonators comprising an input series resonator connected to the input port and an output series resonator connected to the output port, and wherein the or each shunt resonator has a static capacitance which is more than four times the static capacitance of the input or output series resonators.
The ladder filter of the invention provides increased shunt resonator capacitance (compared to conventional designs in which the series and shunt resonators have substantially the same area). This reduces the effective coupling across the section thereby enabling a smaller number of series-shunt filter sections to be used to achieve good stop-band rejection, while still providing good performance in the pass-band. The filter of the invention can be impedance matched to both input and output impedances of the filter, so as to minimise the pass-band insertion loss. The increased shunt capacitance does, however, reduce the filter bandwidth, and the invention is based on the recognition that filter bandwidth can be traded for improved out-of-band rejection.
The series resonators may further comprise one or more intermediate series resonators having a static capacitance which is approximately half the static capacitance of the input or output series resonators. In this way, the ladder filter can be made up of identical T-section building blocks. For equal
Biren Steve R.
Lee Benny
Summons Barbara
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