Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-11-27
2004-03-23
Budd, Mark (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S364000
Reexamination Certificate
active
06710508
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to acoustic resonators and more particularly to adjusting and stabilizing the resonant frequency of a film bulk acoustic resonator (FBAR).
BACKGROUND ART
In many different communications applications, a common signal path is coupled to both an input of a receiver and an output of a transmitter. For example, in a cellular or cordless telephone, an antenna may be coupled to the receiver and the transmitter. In such an arrangement, a duplexer is often used to couple the common signal path to the input and the output. The function of the duplexer is to provide the necessary coupling to and from the common signal path, while preventing the signals generated by the transmitter from being coupled to the input of the receiver.
One type of duplexer is referred to as the half duplexer. A half duplexer uses a switch to connect the common signal path to the receiver or the transmitter on a time division basis. The half duplexer has the desired attenuation properties, but is unacceptable in many telephony applications, since it does not allow parties of a call to speak and be heard simultaneously.
A type of duplexer that is more acceptable for telephony applications is the full duplexer. A full duplexer operates only if the transmit signal has a frequency that is different than the frequency of the receive signal. The full duplexer incorporates band-pass filters that isolate the transmit signal from the receive signal according to the frequencies.
FIG. 1
illustrates a conventional circuit used in cellular telephones, personal communication system (PCS) devices and other transmit/receive devices. A power amplifier
10
of a transmitter is connected to a transmit port
12
of a full duplexer
14
. The duplexer also includes a receive port
16
that is connected to a low noise amplifier (LNA)
18
of a receiver. In addition to the transmit port and the receive port, the duplexer
14
includes an antenna port
20
, which is connected to an antenna
22
.
The duplexer
14
is a three-port device having the transmit port
12
, the receive port
16
and the antenna port
20
. Internally, the duplexer includes a transmit band-pass filter
24
, a receive band-pass filter
26
and a phase shifter
28
. The passbands of the two filters
24
and
26
are respectively centered on the frequency range of the transmit signal that is input via the power amplifier
10
and the receive signal to which the receiver is tuned.
The requirements for the band-pass filters
24
and
26
of the duplexer
14
are stringent. The band-pass filters must isolate low intensity receive signals generated by the antenna
22
from the strong transmit signals generated by the power amplifier
10
. In a typical embodiment, the sensitivity of the low noise amplifier may be in the order of −100 dBm, while the power amplifier may provide transmit signals having an intensity of approximately 28 dBm. Thus, the duplexer
14
must attenuate the transmit signal by approximately 50 dB between the antenna port
20
and the receive port
16
to prevent any residual transmit signal mixed with the receive signal at the receive port from overloading the low noise amplifier.
One type of PCS that is used in a mobile telephone employs code division multiple access (CDMA). The CDMA PCS wireless bands are approximately 1900 MHz and have an especially stringent regulatory requirement for duplexer performance. Some concerns will be identified with reference to
FIG. 2. A
passband
30
is defined by at least two of poles and at least two of zeros. The poles and zeros are equidistantly spaced from a center frequency
32
. For the transmitter passband
30
, the transmitter-to-antenna insertion loss
34
is preferably less than −3 dB over most of the band. The isolation from the transmitter to receiver ports exceeds 50 dB across most of the transmitter band and 46 dB in the receiver band. The crossover between the transmitter and receiver bands occurs around 1900 MHz, which is 20 MHz below the CDMA specification. As will be explained more fully below, the lower-frequency pole is determined by the characteristics of shunt resonators, while the higher-frequency pole is determined by the characteristics of series resonators.
Another challenge for the duplexer is achieving power handling requirements. The power amplifier
10
in the transmitter of
FIG. 1
can deliver 1 Watt of power to the transmit port
12
of the duplexer
14
. The band-pass filter
24
must be capable of handling such power without being destroyed and without its performance being degraded.
The duplexer
14
of
FIG. 1
will be described in greater detail with reference to FIG.
3
. The duplexer includes a transmit film bulk acoustic resonator (FBAR) array
40
and a receive FBAR array
42
. The transmit FBAR array
40
is a 2½-stage ladder circuit having three series FBARs
44
,
46
and
48
and two shunt FBARs
50
and
52
. The series FBARs are connected in series between the transmit port
12
and the antenna port
20
, while the shunt FBARs are connected between electrical ground and nodes between the series FBARs. Each full stage of an FBAR array is composed of one series FBAR and one shunt FBAR. A half stage is limited to either one series FBAR or one shunt FBAR. In the exemplary array
40
, the half stage is the series FBAR
48
.
The receive FBAR array
42
is a 3½-stage ladder circuit. The FBAR array includes three series FBARs
54
,
56
and
58
and four shunt FBARs
60
,
62
,
64
and
66
. The series FBARs are connected in series between the ninety degree phase shifter
28
and the receive port
16
. The shunt FBARs are connected between electrical ground and nodes between the series FBARs.
Circuits suitable for use as the ninety degree phase shifter
28
are known in the art. As examples, the phase shifter may be composed of inductors and capacitors or may be a &lgr;/4 transmission line.
Within the transmit FBAR array
40
, each series FBAR
44
,
46
and
48
may have the same resonant frequency (f
r
). Similarly, the shunt FBARs
50
and
52
may have the same resonant frequency, but the resonant frequency of the series FBARs is approximately 3.0 percent greater than that of the shunt FBARs. As a result, the two poles that were described with reference to
FIG. 2
are provided.
The receive FBAR array
42
of the receive band-pass filter
26
may also be composed of series FBARs
54
,
56
and
58
having the same f
r
and shunt FBARs
60
,
62
,
64
and
66
having the same f
r
that is 3.0 percent different than the resonant frequency of the series FBARs. However, in the duplexer
14
of a CDMA PCS device, the receive band-pass filter
26
is required to attenuate the transmit signal to such a low level that the residual transmit signal mixed with the receive signal at the receive port
16
does not overload the low noise amplifier
18
of FIG.
1
. Consequently, the transmit signal rejection requirements of the receive band-pass filter are considerably more stringent than the receive signal rejection requirements of the transmit band-pass filter
24
. This is most easily achieved if the shunt FBARs
60
,
62
,
64
and
66
of the receive FBAR array have more than one resonant frequency.
In comparing the transmit FBAR array
40
to the receive FBAR array
42
, the resonant frequency of the series FBARs
54
,
56
and
58
of the receive FBAR array may be approximately 80 MHz higher than the resonant frequency of the series FBARs
44
,
46
and
48
of the transmit FBAR array. As a result, the center frequencies of the two arrays will be offset.
In view of the duplexer
14
, it is important that each FBAR in the transmit FBAR array
40
and the receive FBAR array
42
be tuned to its target frequency, so that the transmit band-pass filter
24
and the receive band-pass filter
26
can properly isolate the transmit signals from the receive signals without any objectionable crossover noise. Furthermore, optimal performance of the duplexer
14
can only be achieved if the band-pass filters
24
Ruby Richard C.
Wen Joe Qingzhe
Agilent Technologie,s Inc.
Budd Mark
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