JP2004023655A - Antenna branching filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an antenna branching filter whose insertion loss is small and attenuation characteristics are good. <P>SOLUTION: A 1st ladder type SAW filter has a 1st pass band and is connected to an antenna via a phase circuit, a 2nd ladder type SAW filter has a 2nd pass band for lower frequency than the 1st pass band and is commonly connected to the antenna via a phase circuit, a SAW resonator has an antiresonating point in the pass band of the 1st ladder type SAW filter installed between the 2nd ladder type SAW filter and the phase circuit, and an inductor for circuit matching. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an antenna duplexer having low loss and high attenuation characteristics using a surface acoustic wave filter.
[0002]
[Prior art]
2. Description of the Related Art In recent years, small communication devices such as mobile phones have been frequently used. In such devices, simultaneous communication in which transmission and reception are performed simultaneously is performed.
Therefore, as shown in FIG. 7, the transmission signal Fh and the reception signal Fl having different frequencies are separated from each other. For example, the ladder-type SAW filters 1 and 2 are used to share the antenna 3.
The ladder-type SAW filters 1 and 2 and the antenna 3 are phase-matched by interposing phase circuits 4 and 5 respectively.
[0003]
In a typical resonance type SAW filter, it is desirable to pass a signal within a predetermined pass band with low loss and to give a large amount of attenuation to a signal outside the band.
However, in the case of an antenna duplexer, the resistance given by the S11 characteristic in the reflection region must be small in addition to the characteristics desired in such a normal filter.
[0004]
FIG. 8 is a characteristic diagram showing a transmission characteristic (curve A) and a reflection characteristic (curve B) of a general ladder-type SAW filter. The horizontal axis represents frequency, and the vertical axis represents response level.
FIG. 9 is a Smith chart of the S11 characteristic of the SAW filter.
As is clear from the Smith chart, on the high frequency side (illustrated C), the propagation loss due to the bulk wave significantly increases in addition to the general conductor loss and dielectric loss, so that the frequency characteristic of the Smith chart is inward. The circuit wraps around and has a resistance component in the circuit, resulting in an increase in circuit loss, that is, deterioration in return loss (D in the figure).
[0005]
On the other hand, in order to function as an antenna duplexer, the impedance of the high-pass filter and the low-pass filter viewed from the antenna port must be on the open side (right end) on the Smith chart.
In order to satisfy such a condition, a phase circuit is required for the output of each duplexer.
[0006]
However, when a phase circuit is used, the above-described S11 characteristic is further deteriorated due to the loss, and the return loss characteristic in the reflection region of the filter is significantly deteriorated. FIG. 10 shows a Smith chart when a phase circuit is added to the characteristics of FIG. For example, assuming that the resistance components of the reflection band and the pass band of the high-pass filter viewed from the branch portion of the antenna are Rh and Rl, the amount of power of the pass band of the high-pass filter input to the antenna flowing into the low-pass filter. That is, the influence of the S11 characteristic of the low-pass filter on the loss IL of the high-pass filter at the time of antenna demultiplexing is given by the following equation.
[0007]
IL = 10 * log (Rh / (R1 + Rh))
For example, assuming that the values read from the chart of FIG. 9 are Rh = 374Ω and Rl = 50Ω as calculation examples, the influence of the low-pass filter on the high-pass filter is calculated as IL = −0.545 dB, and the high-pass filter is calculated as IL = −0.545 dB. The insertion loss of the filter increases by 0.545 dB, which has a very undesirable effect.
[0008]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an antenna duplexer in which deterioration of insertion loss can be minimized and attenuation characteristics are improved.
[0009]
[Means for Solving the Problems]
Claim 1 of the present invention is a first ladder-type SAW filter having a first pass band and connected to an antenna via a phase circuit, and a second pass band having a lower frequency than the first pass band. And a second ladder-type SAW filter commonly connected to the antenna via a phase circuit, and a first ladder-type SAW filter provided between the second ladder-type SAW filter and the phase circuit. An antenna duplexer comprising: a SAW resonator having an anti-resonance point disposed in a band; and a circuit matching inductor.
[0010]
According to a second aspect of the present invention, in the first aspect, the SAW resonator and the inductor for circuit matching are connected in series and inserted between the second ladder-type SAW filter and the phase circuit. An antenna duplexer according to claim 3, wherein the SAW resonator and the inductor for circuit matching are connected in parallel to each other and the second ladder-type SAW filter is provided. An antenna duplexer interposed between a connection point of a phase circuit and a ground potential.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the block diagram shown in FIG.
Reference numeral 11 denotes a first ladder type SAW filter having a pass band Fh.
Reference numeral 12 denotes a second ladder-type SAW filter having a pass band Fl having a lower frequency than the pass band of the first ladder-type SAW filter 11.
[0012]
The first and second ladder-type SAW filters 11 and 12 are connected to an antenna 15 via phase circuits 13 and 14, respectively.
A SAW resonator 16 and an inductor 17 having an anti-resonance point in the pass band of the first ladder-type SAW filter 11 are provided between the second ladder-type SAW filter 12 and the phase circuit 14.
[0013]
The SAW resonator 16 and the inductor 17 may be connected in series as shown in FIG. 1 and inserted between the second ladder-type SAW filter 12 and the phase circuit 14, As shown in FIG. 2, a SAW resonator 16 and an inductor 17 are connected in parallel between a connection point between the second ladder-type SAW filter 12 and the phase circuit 14 and a ground potential as a circuit. You may do so.
[0014]
Here, the frequency characteristic of the SAW resonator 16 whose anti-resonance point is Fh is shown in FIG.
In FIG. 3, the horizontal axis represents the frequency and the vertical axis represents the response level, and shows the reflection characteristic (curve E) and the transmission characteristic (curve F).
In the figure, G is a resonance point, and H is an anti-resonance point.
[0015]
When the inductor 17 is connected in series to the SAW resonator 16 having the frequency characteristic of the anti-resonance point of Fh, the frequency characteristic changes as shown in FIG. 4 using the same reference numerals as in FIG.
That is, the anti-resonance point H hardly changes, but the resonance point G shifts largely toward the low frequency side according to the value of the inductance 17.
Therefore, it is possible to set the anti-resonance point in the first pass band Fh with the second pass band Fl by using the appropriately designed inductance 17 using the SAW resonator 16 which is appropriately designed. it can.
[0016]
Here, as shown in FIG. 1, when the SAW resonator 16 and the inductor 17 are connected in series to the second ladder type SAW filter 12, the frequency characteristics are as shown in FIG. 5 using the same reference numerals as in FIG. .
FIG. 6 shows a Smith chart of the reflection characteristics.
That is, as is clear from FIG. 5, the attenuation characteristic of the SAW resonator 16 is added to the attenuation characteristic of the ordinary ladder-type SAW filter, so that the attenuation characteristic is improved as a whole.
[0017]
Further, as is apparent from the Smith chart of FIG. 6, the S11 characteristic is significantly improved near the first pass band Fh, the resistance is reduced, and the insertion loss can be reduced.
The phase circuit is added from FIG. 6, and the resistance in the reflection band and the pass band of the low-pass filter viewed from the antenna branch point is 1015Ω and 50Ω, and the power in the pass band of the high-pass filter entering the antenna is low. The amount flowing into the band filter, that is, the loss IL is -0.209 dB.
[0018]
Therefore, in the above embodiment, the loss can be improved by about 0.3 dB to 0.4 dB as compared with the conventional example shown in FIG.
As described above, by improving the S11 characteristic of the second ladder-type SAW filter, the pass loss in the band of the first ladder-type SAW filter when the antenna is shared can be significantly improved.
[0019]
【The invention's effect】
As described in detail above, according to the present invention, it is possible to provide an antenna duplexer having a small insertion loss and good attenuation characteristics.
[Brief description of the drawings]
FIG. 1 is a block diagram of one embodiment of the present invention.
FIG. 2 is a block diagram of another embodiment of the present invention.
FIG. 3 is a diagram illustrating an example of a frequency characteristic of a SAW resonator.
FIG. 4 is a diagram illustrating frequency characteristics of a series circuit of a SAW resonator and an inductor.
FIG. 5 is a diagram showing frequency characteristics of the circuit of the present invention shown in FIG. 1;
FIG. 6 is a Smith chart of the S11 characteristic of the circuit of the present invention shown in FIG. 1;
FIG. 7 is a block diagram illustrating an example of a conventional antenna duplexer.
FIG. 8 is a diagram illustrating a frequency characteristic of the circuit illustrated in FIG. 7;
FIG. 9 is a Smith chart of the S11 characteristic of the circuit shown in FIG. 7;
FIG. 10 is a Smith chart of S11 characteristics when a phase circuit is added to the circuit shown in FIG. 9;
[Explanation of symbols]
11 First ladder-type SAW filter 12 Second ladder-type SAW filter 13, 14 Phase circuit 15 Antenna 16 SAW resonator 17 Inductor

Claims (3)

A first ladder-type SAW filter having a first pass band and connected to the antenna via a phase circuit;
A second ladder-type SAW filter having a second pass band at a lower frequency than the first pass band and commonly connected to the antenna via a phase circuit;
A SAW resonator having an anti-resonance point disposed in a pass band of the first ladder-type SAW filter provided between the second ladder-type SAW filter and the phase circuit, and a circuit matching inductor;
An antenna duplexer comprising: 2. The antenna demultiplexer according to claim 1, wherein a SAW resonator and a circuit matching inductor are connected in series and inserted between a second ladder-type SAW filter and a phase circuit. vessel. 2. The device according to claim 1, wherein the SAW resonator and the inductor for circuit matching are connected in parallel and inserted between the connection point of the second ladder-type SAW filter and the phase circuit and the ground potential. Characteristic antenna duplexer.

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