Passband filter having an asymmetrical filter response

Details Passband filter having an asymmetrical filter response Passband filter having an asymmetrical filter response

2001-02-14

2002-10-08

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S187000

Reexamination Certificate

active

06462631

ABSTRACT:

TECHNICAL FIELD
The invention relates generally to acoustic resonators and more particularly to tailoring the filter response for a passband filter having film bulk acoustic resonators.
BACKGROUND ART
In different communications systems, the same signal path functions as both an input to a receiver and an output from a transmitter. For example, in a cellular or cordless telephone, an antenna may be coupled to the receiver and to the transmitter. In such an arrangement, a duplexer is often used to couple the common signal path to the input and to 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 a “full duplexer.” A full duplexer operates properly only if the transmit signal is carried at a frequency that is different than the frequency of the receive signal. The full duplexer utilizes passband 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 includes an antenna port
20
which is connected to an antenna
22
.
The duplexer
14
employs a transmit passband filter
24
, a receive passband 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 from the transmit port
12
and the receive signal to which the receiver is tuned.
The requirements of the passband filters
24
and
26
of the duplexer
14
are stringent. The passband filters must isolate low intensity receive signals generated by the antenna for input to the low noise amplifier
18
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. 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 that may be mixed with the receive signal from overloading the low noise amplifier
18
.
One standard for use in PCS devices for a mobile telephone is the code division multiple access (CDMA) standard. A CDMA 1900 MHz mobile phone has a transmit filter
24
with a passband of 1850 MHz to 1910 MHz and has a receive filter
26
with a passband of 1930 MHz to 1990 MHz. A filter response
30
for the transmit filter is shown in FIG.
2
. The filter response is defined by poles and zeros (i.e., nulls) of acoustic resonators. The poles and zeros are equidistantly spaced from a center frequency
32
. During ideal conditions, the attenuation within the range of frequencies from 1850 MHz to 1910 MHz is relatively small. That is, the filter response
30
exhibits a relatively small insertion loss. On the other hand, the attenuation beyond the target passband is substantial. As shown in
FIG. 2
, there is a steep roll-off at both the high frequency end and the low frequency end of the filter response. The steep roll-off at the high frequency end ensures isolation from the passband of the receive filter
26
, which is only 20 MHz above the passband of the transmit filter.
There are a number of available approaches to fabricating a duplexer. The conventional approach is to use ceramic technology. That is, ceramic-based half-wave and quarter-wave resonators are fabricated and connected to provide the poles and zeros which define the desired filter response. A significant built-in advantage of ceramic filters is that the temperature coefficient of such a filter is close to zero. Thus, the filter response does not materially change in shape or location as a result of temperature variations.
One concern with the use of ceramic duplexers is that there is a relationship between the quality factor “Q” of the filter and the size of the filter. For a ceramic filter, Q decreases with the decreasing size of the filter. In applications such as the CDMA market, the guard band between the transmit passband and the receive passband is very narrow (20 MHz). Since Q affects the steepness of the roll-off of the filter response, the Q must remain within a set range if the roll-off of the filter response is to meet the specifications set forth by the requirements of the system. Therefore, the duplexer that is fabricated using ceramic technology has a certain minimum volume that is relatively large. In fact, of the components of a CDMA 1900 MHz telephone, only the battery is larger than a ceramic-based duplexer.
Alternative approaches to using ceramic-based duplexers include fabricating surface acoustic wave (SAW) duplexers or film bulk acoustic resonator (FBAR) duplexers. Both of these types of duplexers occupy much smaller volumes than the ceramic duplexers, since the limiting factors for the Q are governed by the properties of sound waves, rather than electrical resistance. A typical SAW or FBAR die size (e.g., silicon chip size) is on the order of 0.25 mm. The height is governed by the die package requirements, but can be made under 2 mm. A drawback for both SAW and FBAR duplexers is that both technologies suffer from frequency shifts as a result of temperature variations. As the duplexer increases in temperature, the stiffness of the resonating materials decreases. The decrease in material stiffness results in a shift in the sound wave velocity, since the sound velocity is dependent upon the square root of the mass density divided by the stiffness. It follows that the filter response shifts downwardly in frequency as the temperature rises. SAW duplexers also have problems with power handling capabilities and achieving a relatively high Q. It has not yet been shown that SAW duplexers can meet the performance requirements for use in CDMA 1900 MHz telephones.
FBAR technology has three advantages over SAW technology. First, FBAR duplexers have been shown to have excellent power handling abilities. Second, FBAR resonators demonstrate Qs that are significantly higher than those identified in publications regarding SAW resonators. Using FBAR resonators, it is possible to achieve a 10.5 MHz roll-off (from 3.3 dB to 47.5 dB) for the transmitter portion of a CDMA PCS duplexer. In comparison, ceramic duplexers have approximately a 20 MHz roll-off. The third advantage of FBAR duplexers over SAW duplexers is that they tend to have a lower temperature coefficient. SAW resonators made from lithium niobate have a frequency shift of approximately 90 ppm/° C., and SAW resonators made with lithium tantalate have a frequency shift of approximately 34 ppm/° C. In comparison, FBAR duplexers have been measured to have a frequency shift between 20 and 30 ppm/° C.
As previously noted, within the CDMA PCS specification, there is a 20 MHz guard band between the transmitter and receiver passbands. The goal of a duplexer is to allow as much energy through each passband, while rejecting nearly all energies outside of the passband. If a realistic FBAR duplexer has a 50 dB roll-off in 10 MHz, this leaves 10 MHz for process variation and temperature shift. In percentage terms, this is slightly greater than 0.5 percent (i.e., 10 MHz/1920 MHz). If it is assumed that an FBAR filter has a temperature-dependent frequency shift of 30 ppm/° C., and it is assumed that system requirements must meet specifications over a temperature range of −20° C. to 60° C., the total temperature shift may be as great as 4.8 MHz. Additionally, heating of the FBAR filter as a result of absorption

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