Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2002-03-26
2004-05-18
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000, C333S195000
Reexamination Certificate
active
06737936
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to mobile communication equipment and, more particularly, to a surface-acoustic-wave duplexer for use in the radio-frequency section of, for example, a mobile telephone.
2. Description of the Related Art
Two types of surface-acoustic-wave (SAW) duplexers are known. In the first type, a transmitting SAW filter and a receiving SAW filter are formed on two separate piezoelectric substrates, which are mounted in separate cavities in a single duplexer package. In the second or monolithic type, a single piezoelectric substrate, on which both a transmitting SAW filter and a receiving SAW filter are formed, is mounted in the duplexer package. In the monolithic type of SAW duplexer, the shunt-arm SAW resonator of the transmitting SAW filter and the series-arm SAW resonator of the receiving SAW filter are conventionally disposed side by side.
FIG. 1
schematically illustrates the layout of a SAW duplexer of the monolithic type, wherein a transmitting SAW filter and a receiving SAW filter are formed on a single piezoelectric substrate or chip
10
. These two filters include four SAW resonators, which are laid out in a row in the following order: a transmitting series-arm SAW resonator
11
, a transmitting shunt-arm SAW resonator
12
, a receiving series-arm SAW resonator
13
, and a receiving shunt-arm SAW resonator
14
. In this and subsequent drawings, Tx is used as an abbreviation for transmitting, Rx for receiving, S for series-arm, and P for shunt-arm.
When a monolithic SAW duplexer of this type is employed, problems of crosstalk from the transmitting circuits to the receiving circuits are observed. The transmit-to-receive isolation characteristic is influenced by many factors, such as the composite frequency characteristic of the signal path from the transmitting terminal to the antenna terminal and the signal path from the antenna terminal to the receiving terminal, the impedance characteristic of the signal path from the transmitting filter to the fifty-ohm (50-&OHgr;) antenna termination to the receiving filter, and interference (coupling) due to the monolithic layout of the SAW resonators of the two filters.
In some mobile communication systems, such as the J-cdma (Japanese code division multiple access) system, the transmitting band and the receiving band are both divided in two. In this type of system, a duplexer with a full-band configuration, i.e., a duplexer that covers all parts of each band, requires comparatively wide passbands. For J-cdma, the passband width must be thirty-eight megahertz (38 MHz). This forces the transmitting band and the receiving band to be closely adjacent, and when the SAW resonators are all disposed on one chip, interference between them becomes inevitable.
The basic principle of a SAW filter is the propagation of surface waves on a piezoelectric substrate (e.g., LiTaO
3
) that is patterned to function as a bandpass filter by modal resonance. For a J-cdma mobile phone, the isolation characteristic of interest is that in the transmitting frequency band (887 MHz to 925 MHz). The receiving frequency characteristic in this band is dominated by the series-arm SAW resonator of the receiving filter. Since a two-chip duplexer is free of isolation problems and a monolithic duplexer is not, it is evident that the series-arm SAW resonator of the receiving filter is affected by interference from the adjacent shunt-arm SAW resonator of the transmitting filter.
By increasing the number of SAW resonators in the series arm of the receiving filter or the shunt arm of the transmitting filter, it is possible to increase the attenuation of one filter in the passband of the other filter. However, as long as the shunt-arm SAW resonators in the transmitting filter and the series-arm SAW resonators in the receiving filter are disposed side by side, some surface acoustic waves will propagate from the shunt-arm SAW resonators of the transmitting filter to the series-arm SAW resonators of the receiving filter on the surface of the piezoelectric substrate disposed between the shunt arm of the transmitting filter and the series arm of the receiving filter, degrading the isolation characteristic between the two filters by reducing the attenuation of each filter in the passband of the other filter.
A more detailed explanation of this problem will be given with reference to a simulated example.
FIGS. 2
,
3
,
4
, and
5
show characteristics, as determined by simulation, of SAW resonators of the general type employed in the present invention.
FIG. 2
shows the frequency characteristic of a series-arm SAW resonator. The attenuation loss is one-half decibel (−0.5 dB) at a frequency of 860 MHz, −2 dB at 870 MHz, −4 dB at 880 MHz, −18 dB at 890 MHz, −3 dB at 900 MHz, =2 dB at 910 MHz, and −2 dB at 920 MHz. The return loss is −23 dB at 860 MHz, −11 dB at 870 MHz, −3 dB at 880 MHz, −2 dB at 890 MHz, −3 dB at 900 MHz, −6 dB at 910 MHz, and −8 dB at 920 MHz.
FIG. 3
shows the reflection coefficient characteristic of a reflector that can be used in a series-arm SAW resonator. The reflection coefficient is 0.65 at a frequency of 860 MHz, 0.95 at 880 MHz, 0.95 at 900 MHz, and 0.95 at 920 MHz.
FIG. 4
shows the frequency characteristic of a shunt-arm SAW resonator. The attenuation loss is −13 dB at a frequency of 860 MHz, −24 dB at 870 MHz, −4 dB at 880 MHz, −2 dB at 890 MHz, −2 dB at 900 MHz, −2 dB at 910 MHz, and −2 dB at 920 MHz. The return loss is −2 dB at 860 MHz, −2 dB at 870 MHz, −5 dB at 880 MHz, −17 dB at 890 MHz, −10 dB at 900 MHz, −7 dB at 910 MHz, and −6 dB at 920 MHz.
FIG. 5
shows the reflection coefficient characteristic of a shunt-arm SAW resonator. The reflection coefficient is 0.55 at a frequency of 860 MHz, 0.95 at 880 MHz, 0.95 at 900 MHz, and 0.95 at 920 MHz. In the frequency band between 860 MHz and 840 MHz, however, the reflection coefficient is significantly reduced, becoming about 0.20 to 0.40, only about 30% of the reflection coefficient in the band between 860 MHz and 920 MHz.
The frequency characteristics and reflector reflection coefficient characteristics shown in
FIGS. 2
,
3
,
4
and
5
were obtained by simulating the operation of, for example, a series-arm SAW resonator having one interdigital transducer (IDT) with an aperture of one hundred micrometers (100 &mgr;m) and one hundred pairs of electrode fingers. The attenuation loss characteristics in
FIGS. 2 and 4
are the characteristics of a resonator provided with a reflector.
FIG. 6
illustrates the frequency characteristic
15
of the transmitting filter and the frequency characteristic
16
the receiving filter in a conventional duplexer, indicating the resonance frequency f
RxP
of the shunt-arm SAW resonator in the receiving filter, the resonance frequency f
RxS
of the series-arm SAW resonator in the receiving filter, the resonance frequency f
TxP
of the shunt-arm SAW resonator in the transmitting filter, and the resonance frequency f
TxS
of the series-arm SAW resonator in the transmitting filter.
FIG. 7
schematically illustrates the isolation characteristic of a SAW duplexer. In region A (shaded), which is the region in which interference occurs, the isolation characteristic is determined predominantly by the frequency characteristic of the series-arm SAW resonator transducer of the receiving filter.
In the series-arm SAW resonator shown in
FIG. 2
, the attenuation loss in the series resonance band centered at the lower resonance frequency of 860 MHz is only about 0.5 dB, indicating that little leakage of surface acoustic waves occurs at this frequency. As can be seen in
FIG. 3
, the reflection coefficient characteristic is about 60% or more, so the small amount of leakage is further reduced and causes little problem.
On the other hand, in the shunt-arm SAW resonator shown in
FIG. 4
, the attenuation loss in the series resonance band cen
Oki Electric Industry Co. Ltd.
Rabin & Berdo P.C.
Summons Barbara
LandOfFree
Surface-acoustic-wave duplexer with improved isolation does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Surface-acoustic-wave duplexer with improved isolation, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surface-acoustic-wave duplexer with improved isolation will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3211205