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Mobile Phone Patent Abstract
A filter arrangement for use in a multiband dual-mode mobile phone.
In particular, the mobile phone is operable in a W-CDMA mode and
a band-reject filter comprising at least a series and a shunt acoustic-wave
resonators is used to create a deep notch at the receive frequency
band. The band-reject filter further includes a plurality of inductive
elements for matching static capacitance of the resonators at the
passband frequency. The mobile phone is also operable in 1800 and
1900 GSM bands. The band-reject filter is operatively connected
to a harmonic trap in order to eliminate the third harmonic of these
GSM bands. The mobile phone is also operable in the lower GSM band,
for which a harmonic filter can share an antenna path with the band-reject
filter.
Mobile Phone Patent Claims
What is claimed is:
1. A method of filtering transceiver signals in a mobile terminal,
the mobile terminal operable in at least one transmit frequency
band and one receive frequency band separated from the transmit
frequency band, wherein the mobile terminal has a signal source
for providing transmit signal in the transmit frequency band, said
method comprising the steps of: providing a signal path between
the signal source and an antenna; and providing a band-reject filter
in the signal path so as to attenuate signal in the receive frequency
band, wherein the band-reject filter comprises: at least one series
resonator operatively connected between the signal source and the
antenna, the series resonator having a first end and an opposing
second end, with the first end connected to the signal source; at
least one shunt resonator connecting between the second end of the
series resonator and a grounding point, the shunt resonator and
the series resonator forming an L-section; and at least one inductor
connected in parallel with the L-section.
2. The method of claim 1, wherein the transmit frequency band substantially
occupies a frequency range of 1920 to 1980 MHz, and the receive
frequency band substantially occupies a frequency range of 2110
to 2170 MHz.
3. The method of claim 2, wherein the mobile terminal operates
in a code-division multiple access (CDMA) mode.
4. The method claim 3, wherein the further transmit frequency band
substantially occupies a frequency range of 1710 to 1770 MHz.
5. The method of claim 3, wherein the further transmit frequency
band substantially occupies a frequency range of 1710 to 1785 MHz.
6. The method of claim 3, wherein the further transmit frequency
band substantially occupies a frequency range of 1850 to 1910 MHz.
7. The method of claim 3, wherein the mobile terminal is operable
in a code division multiple access (CDMA) mode in said at least
one transmit frequency band and one receive frequency band, and
also operable in a GSM mode in said further transmit frequency band.
8. The method of claim 7, further comprising the step of providing
a harmonic trap, operatively connected to the band-reject filter,
for attenuating a harmonic of the GSM mode.
9. The method of claim 1, wherein the mobile terminal is also operable
in a further transmit frequency band, the mobile terminal further
having a further signal source for providing a further transmit
signal in the further transmit frequency band, said method further
comprising the step of operatively connecting the further signal
source to the signal path at the first end of the series resonator.
10. The method of claim 1, wherein the series resonator comprises
an acoustic wave resonator and the shunt resonator comprises another
acoustic wave resonator.
11. The method of claim 1, wherein the series resonator comprises
a bulk acoustic wave resonator and the shunt resonator comprises
another bulk acoustic wave resonator.
12. A band-reject filter for use in a mobile terminal, the mobile
terminal operable in at least one transmit frequency band and one
receive frequency band separated from the transmit frequency band,
wherein the mobile terminal has a signal source for providing transmit
signal in the transmit frequency band, and a signal path between
the signal source and an antenna, said filter comprising: at least
one series resonator operatively connected to the signal path between
the signal source and the antenna, the series resonator having a
first end and an opposing second end, with the first end connected
to the signal source; at least one shunt resonator connecting between
the second end of the series resonator and a grounding point, the
shunt resonator and the series resonator forming an L-section; and
at least one inductor connected in parallel with the L-section.
13. The filter of claim 12, wherein said at least one inductor
is connected between the first end of the series resonator and the
grounding point, said band-reject filter further comprising a further
inductor connected between the second end of the series resonator
and the grounding point.
14. The filter of claim 12, wherein the series resonator comprises
an acoustic wave resonator and the shunt resonator comprises another
acoustic wave resonator.
15. The filter of claim 12, wherein the series resonator comprises
a bulk acoustic wave resonator and the shunt resonator comprises
another bulk acoustic wave resonator.
16. The filter of claim 12, wherein the transmit frequency band
substantially occupies a frequency range of 1920 to 1980 MHz, and
the receive frequency band substantially occupies a frequency range
of 2110 to 2170 MHz.
17. A mobile terminal operable in at least one transmit frequency
band and one receive frequency band separated from the transmit
frequency band, said mobile terminal comprising: a signal source
for providing transmit signal in the transmit frequency band; a
signal path between the signal source and an antenna; and a band-reject
filter operatively connected to the signal path so as to attenuate
signal in the receive frequency band, wherein the band-reject filter
comprises: at least one series resonator operatively connected between
the signal source and the antenna, the series resonator having a
first end and an opposing second end, with the first end connected
to the transmission source; at least one shunt resonator connecting
between the second end of the series resonator and a reference point,
the shunt resonator and the series resonator forming an L-section;
and at least one inductance connected in parallel with the L-section.
18. The mobile terminal of claim 17, wherein the transmit frequency
band substantially occupies a frequency range of 1920 to 1980 MHz,
and the receive frequency band substantially occupies a frequency
range of 2110 to 2170 MHz.
19. The mobile terminal of claim 18, wherein said transmit signal
comprises a code division multiple access (CDMA) signal.
20. The mobile terminal of claim 18, further operable in a GSM900
transmit band, said mobile terminal further comprising a further
signal source, operatively connecting to the antenna, for providing
a further transmit signal in the GSM900 transmit band.
21. The mobile terminal of claim 17, also operable in a further
transmit frequency band, said mobile terminal further comprising:
a further signal source, operatively connecting to the signal path
at the first end of the series resonator, for providing a further
transmit signal in the further transmit frequency band.
22. The mobile terminal of claim 21, wherein the further transmit
frequency band substantially occupies a frequency range of 1710
to 1770 MHz.
23. The mobile terminal of claim 21, wherein the further transmit
frequency band substantially occupies a frequency range of 1710
to 1785 MHz.
24. The mobile terminal of claim 21, wherein the further transmit
frequency band substantially occupies a frequency range of 1850
to 1910 MHz.
25. The mobile terminal of claim 21, wherein the transmit signal
comprises a code division multiple access (CDMA) signal, and the
further transmit signal comprises a GSM signal.
26. The mobile terminal of claim 25, further comprising a harmonic
trap, operatively connected to the band-reject filter, for attenuating
a harmonic of the GSM signal.
27. The mobile terminal of claim 26, wherein the harmonic trap
is connected to the band-reject filter through a diode which is
unbiased when the mobile terminal is in the CDMA mode for transmitting
the CDMA signal.
28. The mobile terminal of claim 17, wherein the series resonator
comprises an acoustic wave resonator and the shunt resonator comprises
another acoustic wave resonator.
29. The mobile terminal of claim 17, wherein the series resonator
comprises a bulk acoustic wave resonator and the shunt resonator
comprises another bulk acoustic wave resonator.
Mobile Phone Patent Description
FIELD OF THE INVENTION
The present invention relates generally to thin-film bulk acoustic
wave filters and, more particularly, to bulk acoustic wave filters
in mobile phone front-end.
BACKGROUND OF THE INVENTION
It is known that a bulk acoustic-wave (BAW) device is, in general,
comprised of a piezoelectric layer sandwiched between two electronically
conductive layers that serve as electrodes. When a radio frequency
(RF) signal is applied across the device, it produces a mechanical
wave in the piezoelectric layer. The fundamental resonance occurs
when the wavelength of the mechanical wave is about twice the thickness
of the piezoelectric layer. Although the resonant frequency of a
BAW device also depends on other factors, the thickness of the piezoelectric
layer is the predominant factor in determining the resonant frequency.
As the thickness of the piezoelectric layer is reduced, the resonance
frequency is increased. BAW devices have traditionally been fabricated
on sheets of quartz crystals. In general, it is difficult to achieve
a device of high resonance frequency using this fabrication method.
When fabricating BAW devices by depositing thin-film layers on passive
substrate materials, one can extend the resonance frequency to the
0.5-10 GHz range. These types of BAW devices are commonly referred
to as thin-film bulk acoustic resonators or FBARs. There are primarily
two types of FBARs, namely, BAW resonators and stacked crystal filters
(SCFs). An SCF usually has two or more piezoelectric layers and
three or more electrodes, with some electrodes being grounded. The
difference between these two types of devices lies mainly in their
structure. FBARs are usually used in combination to produce passband
or stopband filters. The combination of one series FBAR and one
parallel, or shunt, FBAR makes up one section of the so-called ladder
filter. The description of ladder filters can be found, for example,
in Ella (U.S. Pat. No. 6,081,171). As disclosed in Ella, an FBAR-based
device may have one or more protective layers commonly referred
to as the passivation layers. A typical FBAR-based device is shown
in FIGS. 1a to 1d. As shown in FIGS. 1a to 1d, the FBAR device comprises
a substrate 501, a bottom electrode 507, a piezoelectric layer 509,
and a top electrode 511. The electrodes and the piezoelectric layer
form an acoustic resonator. The FBAR device may additionally include
a membrane layer 505. As shown in FIG. 1a, an etched hole 503 is
made on the substrate 501 to provide an air interface, separating
the resonator from the substrate 501. Alternatively, an etched pit
502 is provided on the substrate 501, as shown in FIG. 1b. It is
also possible to provide a sacrificial layer 506 separating the
resonator and the substrate, as shown in FIG. 1c. It is also possible
to form an acoustic mirror 521 between the bottom electrode 507
and the substrate 501 for reflecting the acoustic wave back to the
resonator. The substrate can be made from silicon (Si), silicon
dioxide (SiO2), Gallium Arsenide (GaAs), glass or ceramic materials.
The bottom electrode and top electrode can be made from gold (Au),
molybdenum (Mo), tungsten (W), copper (Cu), nickel (Ni), titanium
(Ti), Niobium (Nb), silver (Ag), tantalum (Ta), cobalt (Co), aluminum
(Al) or a combination of these metals, such as tungsten and aluminum.
The piezoelectric layer 130 can be made from zinc oxide (ZnO), zinc
sulfide (ZnS), aluminum nitride (AlN), lithium tantalate (LiTaO3)
or other members of the so-called lead lanthanum zirconate titanate
family. Additionally, a passivation layer typically made from a
dielectric material, such as SiO2, Si3N4, or polyimide, is used
to serve as an electrical insulator and to protect the piezoelectric
layer. It should be noted that the sacrificial layer 506 in a bridge-type
BAW device, as shown in FIG. 1c, is, in general, etched away in
the final fabrication stages to create an air interface beneath
the device. In a mirror-type BAW device, as shown in FIG. 1d, the
acoustic mirror 521 consists of several layer pairs of high and
low acoustic impedance materials, usually a quarter-wave thick.
The bridge-type and the mirror-type BAW devices are known in the
art.
The disadvantages of the membrane type approach are that it is
difficult to produce the layers 507, 509, 511 on top of the membrane
505 so that they have sufficiently small mechanical stress, which
would break or bend the membrane 505. Moreover, the membrane structure
is not very rugged mechanically, which complicates the handling
and dicing of the wafers. The mirror structure is clearly more rugged
because the whole structure is solidly mounted on the substrate
501. As such, the mirror structure provides better thermal relief
to the substrate in high power applications.
The mirror operates basically as a .lambda./4 transformer in that
it consists of multiple pairs of alternating layers with high and
low acoustic impedance, each of which is acoustically about one
quarter wavelength thick. Thus, the whole stack transforms the impedance
of the substrate to a very low impedance at the mirror/bottom electrode
interface, creating an acoustically reflective interface similar
to the air-interface in membrane type structures. The optimal operation
of the mirror requires that the difference in the high and low impedance
is as large as possible. In a mirror type BAW, the air-interface
and the acoustic mirror form a cavity therebetween for confining
the acoustic energy in an acoustic resonance, as shown in FIG. 2.
The equivalent circuit of anyone of the BAW resonators mentioned-above
is shown in FIG. 3. The equivalent circuit includes an equivalent
resistance (R), an equivalent inductor (Lm), an equivalent capacitance
(Cm), and a parallel parasitic capacitance (Co). As such, BAW resonators
can be used as building blocks in impedance elements filters, such
as ladder and lattice filters. Both ladder and lattice filters are
known in the art. For example, ladder filters are disclosed in Ella.
The basic unit in a ladder filter is an L-section 600, as shown
in FIG. 4a. The L-section 600 includes two BAW resonators, a series
resonator 500a and a shunt resonator 500b. When the L-section is
used as a bandpass filter, the resonance frequency of the shunt
resonator is designed to be slightly lower than that of the series
resonator. The idea is to have the parallel resonance (=ideally
infinite impedance) of the shunt resonator and the series resonance
of the series resonator (=ideally zero impedance) at or close to
the center frequency of the passband. The equivalent circuit for
the L-section 600 is shown in FIG. 4b. The frequency response of
a typical bandpass filter consisting of several (in this case 3)
L-sections 600 is shown in FIG. 5. As shown in FIG. 5, the frequency
response has a passband section centered about 940 MHz. In the response
as shown in FIG. 5, the notch below the passband is due to the series
resonance of the shunt resonator (signal being effectively grounded),
and the notch above the passband is caused by the parallel resonance
of the parallel resonator (signal seeing an infinite series resistance).
The passband characteristics of BAW resonator combination can be
improved by using two or more L-sections. The number of L-sections
mostly influences the amount of attenuation outside the passband.
It should be noted that ladder filters are usually referred to as
having complete stages. Those ladder filters include equal number
of series and shunt resonators. However, a ladder filter can have,
for example, 3 shunt resonators and two series resonators (referred
to as 2.5 stages).
In addition to using L-sections as passband filters, it is known
in the art to make band-reject filters by switching the roles of
series and shunt resonators. In this case, the series-connected
resonators (500a, 500c) have their parallel resonance and the shunt
resonators (500b, 500d) their series resonance at the intended stop-band,
so as to provide a deep and steep notch at this frequency. The topology
of such a filter with 2 L-sections is shown in FIG. 6, and the typical
response of such filter is shown in FIG. 7. The response is, in
principle, a mirror image of the passband ladder filter (see FIG.
5), in that the minimum insertion loss occurs at the "notch"
frequencies below and above the stop band. The filter in such a
format is, however, not very useful because the insertion loss outside
the designated reject frequencies is still very large, typically
from 3 to 6 dB, depending on the design. Accordingly, such a filter
is not very useful as a band-reject filter in a multiband, double-mode
mobile phone engine front-end.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to provide in a
transmitter module a filter having a good attenuation at a certain
frequency and, more particularly, at the W-CDMA receive frequency
and a small attenuation at the W-CDMA transmit frequency. This object
can be achieved by disposing an acoustic-wave filter comprising
one or more L-sections of a ladder filter and frequency matching
components in the transmitter module. The L-section comprises a
series resonator and a shunt resonator such that a deep and steep
notch for attenuating the W-CDMA receive frequency is achieved by
the resonator characteristics, and that the resonators appear at
the transmit frequency as passive coupling elements.
Thus, according to the first aspect of the present invention, there
is provided a method of filtering transceiver signals in a mobile
terminal, the mobile terminal operating in at least one transmit
frequency band and one receive frequency band separated from the
transmit frequency band, wherein the mobile terminal has a signal
source for providing transmit signal in the transmit frequency band.
The method comprises the steps of:
providing a signal path between the signal source and an antenna;
and
providing a band-reject filter in the signal path so as to attenuate
signal in the receive frequency band, wherein the band-reject filter
comprises:
at least one series resonator operatively connected between the
signal source and the antenna, the series resonator having a first
end and an opposing second end, with the first end connected to
the signal source;
at least one shunt resonator connecting between the second end
of the series resonator and a grounding point, the shunt resonator
and the series resonator forming an L-section; and
at least one inductor connected in parallel with the L-section.
The mobile terminal is operable in a code-division multiple access
(CDMA) mode, and the transmit frequency band substantially occupies
a frequency range of 1920 to 1980 MHz, and the receive frequency
band substantially occupies a frequency range of 2110 to 2170 MHz.
The mobile terminal is also operable in a GSM mode, and also has
a further signal source for providing a further transmit signal
in the further transmit frequency band. The method further comprises
the step of operatively connecting the further signal source to
the signal path at the first end of the series resonator.
The further transmit frequency band includes a frequency range
substantially of 1710 to 1785 MHz and a frequency range substantially
of 1850 to 1910 MHz. The transmit frequency band can also substantially
occupy a frequency range of 1710 to 1770 MHz for the forthcoming
W-CDMA.
The band-reject filter is operatively connected to a harmonic trap,
for attenuating a harmonic of the GSM mode.
Preferably, the series resonator comprises at least an acoustic
wave resonator and the shunt resonator comprises another acoustic
wave resonator. These acoustic wave resonators can be bulk acoustic
wave or surface acoustic wave resonators.
According to the second aspect of the present invention, there
is provided a band-reject filter for use in a mobile terminal, the
mobile terminal operating in at least one transmit frequency band
and one receive frequency band separated from the transmit frequency
band, wherein the mobile terminal has a signal source for providing
transmit signal in the transmit frequency band, and a signal path
between the signal source and an antenna. The filter comprises:
at least one series resonator operatively connected to the signal
path between the signal source and the antenna, the series resonator
having a first end and an opposing second end, with the first end
connected to the signal source;
at least one shunt resonator connecting between the second end
of the series resonator and a grounding point, the shunt resonator
and the series resonator forming an L-section; and
at least one inductor connected in parallel with the L-section.
Said at least one inductor is connected between the first end of
the series resonator and the grounding point, and the band-reject
filter further comprises a further inductor connected between the
second end of the series resonator and the grounding point.
The series resonator comprises a bulk or surface acoustic wave
resonator and the shunt resonator comprises another bulk or surface
acoustic wave resonator.
The transmit frequency band substantially occupies a frequency
range of 1920 to 1980 MHz, and the receive frequency band substantially
occupies a frequency range of 2110 to 2170 MHz in W-CDMA mode.
According to the third aspect of the present invention, there is
provided a mobile terminal operating in at least one transmit frequency
band and one receive frequency band separated from the transmit
frequency band. The mobile terminal comprises:
a signal source for providing transmit signal in the transmit frequency
band;
a signal path between the signal source and an antenna; and
a band-reject filter operatively connected to the signal path so
as to attenuate signal in the receive frequency band, wherein the
band-reject filter comprises:
at least one series resonator operatively connected between the
signal source and the antenna, the series resonator having a first
end and an opposing second end, with the first end connected to
the transmission source;
at least one shunt resonator connecting between the second end
of the series resonator and a reference point, the shunt resonator
and the series resonator forming an L-section; and
at least one inductance connected in parallel with the L-section.
The transmit frequency band substantially occupies a frequency
range of 1920 to 1980 MHz, and the receive frequency band substantially
occupies a frequency range of 2110 to 2170 MHz in W-CDMA mode.
The mobile terminal is also operable in a GSM mode in further transmit
frequency bands. The mobile terminal further comprises a further
signal source, operatively connecting to the signal path at the
first end of the series resonator, for providing a further transmit
signal in the further transmit frequency band. The further transmit
frequency band substantially occupies a frequency range of 1710
to 1785 MHz and a frequency range of 1850 to 1910 MHz.
The mobile terminal further comprises a harmonic trap, operatively
connected to the band-reject filter, for attenuating a harmonic
of the GSM mode.
The mobile terminal is further operable in a GSM900 transmit/receive
mode. The mobile terminal further comprises
a further signal source, operatively connecting to the antenna,
for providing a further transmit signal in the GSM900 transmit band.
The present invention will become apparent upon reading the description
taken in conjunction with FIGS. 8 to 13.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a cross-sectional view illustrating a typical bulk acoustic
wave device having a resonator and a membrane formed on a substrate,
wherein the substrate has a through hole for providing an air interface
for the membrane.
FIG. 1b is a cross-sectional view illustrating a typical bulk acoustic
wave device having a resonator and a membrane formed on a substrate,
wherein the substrate has an etched section for providing an air
interface for the membrane.
FIG. 1c is a cross-sectional view illustrating a typical bulk acoustic
wave device having a resonator and a membrane formed on a substrate,
wherein a sacrificial layer is formed between the membrane and the
substrate.
FIG. 1d is a cross-sectional view illustrating a typical bulk acoustic
wave device having a resonator and a membrane formed on a substrate,
wherein an acoustic mirror is formed between the substrate and the
bottom electrode of the resonator.
FIG. 2 is a schematic representation illustrating how a resonance
is formed in a typical mirror-type BAW device.
FIG. 3 shows an equivalent circuit of a typical BAW resonator.
FIG. 4a shows an L-section of a ladder filter formed by a series
resonator and a shunt resonator.
FIG. 4b shows an equivalent circuit of an L-section of a ladder
filter of FIG. 4a.
FIG. 5 is a plot showing a typical frequency response of a ladder
filter with 3 L-sections.
FIG. 6 is a schematic representation illustrating the topology
of a prior art 2-stage band-reject filter.
FIG. 7 is a plot showing a typical frequency response of a 2-stage
band-reject filter of FIG. 6.
FIG. 8 is a schematic representation showing a band-reject filter
for use in a transmitter module in multiband, double-mode mobile
phone engine front-end, according to the present invention.
FIG. 9a is a plot showing a typical frequency response of the band-reject
filter of the present invention.
FIG. 9b is a plot showing a typical frequency response of the band-rejection
filter cooperatively engaged with a harmonic trap.
FIG. 10a is a circuit diagram showing a typical matching component
which can be used to match and filter two transmit signals.
FIG. 10b is a circuit diagram showing another version of the matching
component.
FIG. 11 is a schematic representation showing the linkage between
the transmitter module of the present invention to a GSM transceiver
module operating at 900 MHz.
FIG. 12 is a schematic representation showing a band-reject filter
having two L-sections, according to the present invention.
FIG. 13 is a schematic representation illustrating a mobile terminal
having a band-rejection filter, according to the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
In a multiband mobile terminal operating both in W-CDMA mode and
in GSM mode, currently the front end is arranged to have two separate
antennas, one for the W-CDMA frequency band and one for the GSM
frequency bands. In the GSM mode, the transmitter and the receiver
are separated by switching. In the W-CDMA mode, a duplexer or a
diplexer is used. The present invention combines bandpass and band-reject
filters using acoustic-wave resonators. The combined filter can
be made to have a very deep notch at the W-CDMA receive frequency,
while retaining wide passband (at least 30 MHz) with very low loss.
The combined filter is shown as a band-reject filter 40 in FIGS.
8, 11 and 12. In particular, the combined filter is used in a front-end
of a multiband, dual-mode mobile terminal (GSM+W-CDMA), as shown
in FIGS. 8 and 11. The front-end is used herein to refer to the
RF (radio frequency) parts that are disposed between a typical RF-asic(s)
and an antenna. The front-end has a transmitter module for the 1800
& 1900 MHz GSM mode and a European or U.S. W-CDMA mode, and
transceiver module for the lower GSM bands (see FIG. 11).
As shown in FIG. 8, the transmitter module 10 comprises a signal
path connecting the antenna 12 to two transmitter branches: one
branch comprises a power amplifier 20 and an impedance matching
element 22 for conveying transmit signals in the 1800 & 1900
GSM bands, and the other branch comprises a power amplifier 30 and
an impedance matching element 32 for conveying transmit signals
in the European or U.S. W-CDMA transmit frequency band. The branches
are connected to a node 34. In order to attenuate the W-CDMA receive
frequency, a band-reject filter 40 is connected between the antenna
12 and the node 34. The band-reject filter 40 has at least one L-section
of a ladder filter. As shown, the L-section has two acoustic-wave
resonators: a series resonator 42 connected between the antenna
12 and the node 34, and a shunt resonator 44 connected between one
end of the series resonator 42 and a circuit ground. The band-reject
filter 40 further comprises two small coils 46, 48. The coils 46,
48 are used to match the static capacitance of the resonators at
the passband frequency. Typically, the coils are in the range of
0.8 nH to 3.5 nH. But these values depend strongly upon the filter
design as well as the frequency and also upon the general impedance
level. As such, the band-filter 40 creates a very deep notch at
the W-CDMA receive frequency, while retaining wide passband (at
least 300 MHz) with very low loss for the transmit frequencies.
For example, if the series resonance of the series resonator 42
is chosen to be substantially equal to 2.06 GHz, which is slightly
above the passband of W-CDMA transmit frequency band of 1920-1980
MHz, and the parallel resonance of the series resonator 42 is substantially
equal to 2.114 GHz and the corresponding frequencies of the shunt
resonator 44 are Fs=2.17 GHz and Fp=2.226 GHz, a deep notch is formed
in the stopband beyond the transmit frequency band. A typical response
of the band-reject filter 40 is shown in FIG. 9a. Such a filter
arrangement can be used as a separate Tx module, as part of an antenna
switch module. Furthermore, a harmonic trap 26, together with a
diode 24 can be added to the transmitter module 10 in order to create
a third harmonic notch for the 1800 and 1900 MHz bands. The response
is shown in FIG. 9b. The harmonic trap 26 can be a coil or a simple
resonant circuit consisting of a coil and a capacitor, for example.
The diode 24 is used to minimize loss when the mobile terminal is
operated in the W-CDMA mode and such a third harmonic notch is not
necessary. Thus, when the mobile terminal is operated in the W-CDMA
mode, the diode 24 is unbiased.
In the transmitter module 10, two Tx power amplifiers 20, 30 are
used: one for the upper GSM bands and one for W-CDMA. It should
be noted that, in a dual-mode mobile terminal, only one power amplifier
is "on" at a time. Thus, it is possible to use switches
to connect one power amplifier at a time to the filter 40. These
switches can be part of a simple lowpass/highpass diplexer so that
the switches can be arranged in parallel (instead of being in series)
to improve losses. However, the power amplifiers 20, 30 can be connected
in parallel, as shown in FIG. 8, through separate matching elements
22, 32 to match the output impedance of the respective power amplifiers
in the "off"-stage. With proper matching, the power amplifier
in the "off"-stage appears as an open circuit at the node
34. In the optimal case, the output impedance of an "off"-stage
power amplifier can be defined, and only passive matching is required.
An exemplary matching element is shown in FIG. 10a. As shown in
FIG. 10a, only passive elements such as coils and capacitors are
used. Alternatively, diodes are added as switches, as shown in FIG.
10b. The diode connected to the power amplifier in "off"-stage
will be biased on, to present an open circuit at the input node.
The switches can be PIN diodes, CMOS or any suitable solid state
switches, for example.
The transmitter module 10, according to the present invention,
can be integrated with a transceiver module 60 of the lower 900
GSM bands. As shown in FIG. 11, the antenna 12 in the front-end
5 is operatively connected to a band-reject filter 40 of the present
invention and a Tx harmonic filter 80 for the lower GSM bands. In
the transceiver module 60, a 900 MHz Tx bandpass filter 62, a balun
64, a power amplifier 66 are operatively connected to the antenna
12 via a diode 68 for signal transmission. For reception, a 900
MHz Rx bandpass filter 70, a balun 72, and a matching filter 76
are operatively connected to the antenna 12. The matching filter
76 can be a .lambda./4 transmission line. The diodes 68, 74 are
both unbiased in the 900 MHz Rx mode so that the 900 MHz power amplifier
66 appears to have a very high impedance. The diodes 68, 74 are
both biased in the 900 MHz Tx mode so that the 900 MHz power amplifier
66 sees a low insertion loss path to the antenna 12, while the matching
filter 76 transforms the low impedance of the diode 74 to appear
as open circuit at the antenna port. As such, only one of the 900
MHz branches is electrically "visible" at the antenna
port when the mobile terminal is in the 900 MHz Tx or Rx mode.
It should be noted that the band-reject filter 40 as shown in FIG.
8 consists of only one L-section of the ladder filter. It is possible
to use two or more L-sections in such a band-reject filter. A band-reject
filter 40' with two L-sections is shown in FIG. 12. As shown, two
series resonators 42, 52, two shunt resonators 44, 54 and three
coils 46, 48, 56 are used.
When the front-end 5 is used in a mobile terminal 1, as shown in
FIG. 13, the size and performance (insertion loss and attenuation
on the Rx frequency) of the Tx filtering can be significantly improved,
especially in the mobile terminal that includes both GSM and CDMA.
The transmitter module 10 is also useful in a mobile terminal operating
only in GSM or CDMA. The major advantage of the transmitter module
10 is that only one acoustic-wave filter can be used to filter all
the upper bands (1800, 1900 and W-CDMA).
In sum, the band-reject filter for use in the front-end of a multiband
mobile phone, according to the present invention, provides a reasonably
wide passband below the rejection frequency region. If the rejection
region is chosen to be the W-CDMA receive frequency (in Europe 2110-2170
MHz), the bandpass region can cover the W-CDMA transmit frequency
as well as the 1800 and 1900 GSM transmit frequencies: 1710-1785,
1850-1910 and 1920-1980 MHz. With additional resonators, it is also
possible to create a notch for the blue-tooth frequency.
The present invention has been disclosed in conjunction with bulk
acoustic wave (BAW) resonators and filters. However, the same principle
can be applied to surface acoustic wave (SAW) counterparts. Thus,
the resonators 42, 44, 52, 54 as depicted in FIGS. 8 and 12 can
be BAW or SAW resonators. Furthermore, the present invention has
been disclosed in conjunction with European GSM and W-CDMA bands
to demonstrate the benefits of the invention. It should be appreciated
that the same invention can be applied to certain other bands and
band combination as well. For example, the upper bands could be
one or more of the European GSM 1800 band (Tx frequency band of
1710-1785 MHz, Rx frequency range of 1805-1880 MHz), the U.S. GSM
1900 band (Tx frequency range of 1850-1910 MHz, Rx frequency range
of 1930-1990 MHz), and the forthcoming U.S. W-CDMA (Tx frequency
range of 1710-1770 MHz, Rx frequency range of 2110-2170 MHz). It
is also possible to replace the GSM 1900 by the US 1900CDMA, which
is located substantially at the same frequency as the GSM 1900.
Here the GSM branch would probably need some additional filtering.
Moreover, separate power amplifiers 20 and 30 are used to amplify
the transmit signals in the GSM branch and the transmit signals
in the W-CDMA branch. However, if the linearity and efficiency of
a certain power amplifier can meet the requirements of transmit
signals in both branches, it is possible to use a signal amplifier
for carrying out the amplification. In that case, the matching of
such single amplifier would be different from the matching of two
separate amplifiers as shown in FIG. 8. Nevertheless, the band-reject
filter of the present invention is still applicable.
Thus, although the invention has been described with respect to
a preferred embodiment thereof, it will be understood by those skilled
in the art that the foregoing and various other changes, omissions
and deviations in the form and detail thereof may be made without
departing from the scope of this invention.
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