Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1999-06-02
2001-07-17
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S189000, C333S191000, C310S322000, C310S366000
Reexamination Certificate
active
06262637
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to electronic components and, in particular, to electronic components for use in high-power radio-frequency circuits.
BACKGROUND OF THE INVENTION
In many different communications applications, a common signal path is coupled both to the input of a receiver and to the output of a transmitter. For example, in a transceiver, such as a cellular or cordless telephone, an antenna may be coupled to the input of the receiver and to the output of the transmitter. In such an arrangement, a duplexer is used to couple the common signal path to the input of the receiver and to the output of the transmitter. The duplexer provides the necessary coupling while preventing the modulated transmit signal generated by the transmitter from being coupled from the antenna back to the input of the receiver and overloading the receiver.
One long-established form of duplexer is the half duplexer. Half duplexers are described by C. K. Campbell in S
URFACE
A
COUSTIC
W
AVE
D
EVICES FOR
M
OBILE AND
W
IRELESS
C
OMMUNICATION
., pp. 253-272, Academic Press, New York, (1998). A half duplexer uses a switch to connect the antenna to the receiver or the transmitter on a time division basis. The half duplexer has good coupling and attenuation properties, but is nevertheless an unacceptable solution for telephony applications because it does not allow both parties to speak (and be heard) simultaneously.
A more acceptable form of duplexer for telephony applications is the full duplexer, also described by Campbell. To enable a full duplexer to be used, the transmit signal must be at a different frequency from the receive signal. The full duplexer lacks a switch and incorporates band-pass filters that isolate the transmit signal from the receive signal according to the frequencies of the signals.
FIG. 1
shows a conventional front-end circuit
10
such as that used in a cellular telephone, personal communication system (PCS) device or other transmit/receive apparatus. In this, the output of the power amplifier
12
of the transmitter
14
and the input of the low-noise amplifier
16
of the receiver
18
are connected to the duplexer
20
, which is a full duplexer. Also connected to the duplexer is the antenna
22
.
The duplexer
20
is a three-port device having a transmit port
24
, a receive port
26
and an antenna port
28
. The antenna port is connected to the transmit port through the band-pass filter
30
and to the receive port through the series arrangement of the 90° phase shifter
34
and band-pass filter
32
. The pass bands of the band-pass filters
30
and
32
are respectively centered on the frequency range of the transmit signal generated by the transmitter
14
and that of the receive signals to which the receiver
18
can be tuned. In the example shown, band-pass filters are configured such that the high-frequency stop band of the band-pass filter
30
overlaps the pass-band of the band-pass filter
32
and the low-frequency stop band of the band-pass filter
32
overlaps the pass-band of the band-pass filter
30
.
The requirements for the band-pass filters
30
and
32
constituting the duplexer
20
are quite stringent. The band-pass filters isolate the very weak receive signal generated by the antenna
22
and fed to the input of the low-noise amplifier
16
from the strong transmit signal generated by the power amplifier
12
. In a typical embodiment, the sensitivity of the low noise amplifier
16
is of the order of −100 dBm, and the power amplifier
12
can feed power levels of about 28 dBm into the duplexer. In such an example, the duplexer must attenuate the transmit signal by about 50 dB between the antenna port
28
and the receive port
26
to prevent the residual transmit signal mixed with the receive signal at the receive port from overloading the low-noise amplifier.
One type of mobile telephone that is becoming increasingly popular is the personal communication system (PCS) that uses Code Division Multiple Access (CDMA). CDMA PCS is described in T. S. Rapport, ed., C
ELLULAR
R
ADIO
& P
ERSONAL
C
OMMUNICATIONS, VOL
. 2, pp.501-509, IEEE Press, Piscataway, N.J., (1996). CDMA PCS devices operate in frequency bands at about 1,900 MHz and impose especially stringent requirements on the duplexer performance. The guard band between the portions of the spectrum assigned to the transmit signal and the receive signal is only about 1% of the carrier frequency, i.e., 20 MHz. The bandwidth of the portions of the spectrum assigned to the transmit signal and the receive signal are about 3% of the carrier frequency, i.e., 60 MHz. This means that the band-pass filters
30
and
32
are required to have an extremely sharp roll-off.
FIG. 2
shows the basic arrangement of the transmit and receive bands. The required characteristics of the band-pass filters
30
and
32
are shown at
36
and
38
, respectively.
Cellular telephones and PCS devices are constantly being made smaller and lower in cost. Several stacked printed circuit boards are typically used to accommodate the circuitry of the PCS device in the overall package size. Not only must the components mounted on the printed circuit boards be miniaturized, they must meet stringent height requirements. Components taller than the height limit require the printed circuit boards to be spaced further apart, which reduces the packing density that can be achieved. Alternatively, over-height components require that holes be cut in at least one adjacent printed circuit board to accommodate them, which both reduces the packing density and increases assembly costs.
Another challenge for the duplexer
20
is its power handling capability. The power amplifier
12
in the transmitter
14
can deliver up to 1 Watt of power to the transmit port
24
of the duplexer
20
. Miniaturized as just described, the band-pass filters
30
and
32
must be capable of transmitting such power without being destroyed, or without its characteristics degrading with use.
Current-generation PCS devices use a ceramic filter as the duplexer
20
. However, such ceramic filters are bulky, measuring some 28×8×5 mm, are over-height components and are expensive. Samples of such filters show evidence of having been individually tuned, which accounts for some of the cost of such devices.
Surface acoustic wave (SAW) filters have also been used as duplexers in cellular telephones and PCS devices, see, for example, O. Ikata, N. Nishihara, Y. Satoh, H. Fukushima and N. Hirisawa,
A Design of Antenna Duplexer Using Ladder Type SAW Filters, P
ROC
. 1998 IEEE I
NTERNATIONAL
U
LTRASONICS
S
YMPOSIUM
, S
ENDAI
, J
APAN
, paper O-1 (Oct. 1998). The roll-off of a SAW filter with sufficient power handling capability is insufficiently steep for the CDMA application just described. Instead, two SAW filters and an electronic switch have to be used. One of the filters covers the upper half of the transmit and receive bands, the other covers the lower half of the transmit and receive bands. The electronic switch selects the appropriate filter depending on the portions of the transmit and receive bands in which the PCS device is operating. Thus, a duplexer based on SAW filters is also unacceptable bulky, complex, expensive and may be subject to failure in the event of a surge in the transmitter output power.
What is needed, then, is a duplexer that has sufficiently steep filter characteristics to enable it to be used in applications, such as CDMA PCS devices, in which the separation between the transmit and receive bands is only about 1% of the operating frequency and in which power levels exceeding one Watt do not impair the reliability of the duplexer or the long-term stability of the filter characteristics. The duplexer should be substantially smaller than current duplexers based on ceramic filters or SAW filters, and should not require individual tuning so that the cost of manufacture can be kept low.
SUMMARY OF THE INVENTION
The invention provides an FBAR-based duplexer that comprises a first port, a second port, a third port, a
Bradley Paul D.
Larson, III John D.
Ruby Richard C.
Agilent Technologie,s Inc.
Hardcastle Ian
Pascal Robert
Summons Barbara
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