JPH10247833A - Tunable filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small sized filter with a small insertion loss and a high attenuation by shifting a pass band. SOLUTION: This filter is provided with two resonators 11, 11' which are connected in a cascade by a coupling inductor 13 and connected to an input (output) terminal 14 via an input (output) inductor 15 respectively. The resonators 11, 11' are connected to ground via an additional inductor 18 and a switch circuit respectively, a control terminal 23 is connected between both the inductors 18 via a control circuit, and the center frequency of the filter is shifted towards the low frequency by applying a control voltage to the control terminal 23.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile communication device having a center frequency of several hundred MHz to several GHz.
Particularly, the present invention relates to a tunable filter used in a plurality of different pass bands.

[0002]

2. Description of the Related Art FIG. 13 and FIG.
This will be described with reference to FIG. FIG. 13 is a diagram illustrating a configuration of a filter according to the related art, and FIG. 14 is a diagram illustrating characteristics of the filter according to the related art.

For example, as shown in FIG.
A first mobile communication system requiring a first filter comprising a pass band A and a first attenuation band C, and a second filter comprising a second pass band B and a second attenuation band D. In a second mobile communication system, a system that can share the first and second mobile communication systems is considered. According to the related art, the first mobile communication system is configured as shown in FIG.
The input / output terminals of the first filter and the second filter are switched by the switch 100, or a third filter that simultaneously satisfies the characteristics of the first filter and the second filter as shown in FIG. Had to be designed.

[0004]

In recent years, the technical progress in the mobile communication field has been remarkable, and since various radio communication systems use different frequencies and other systems in each country, various systems are required to receive various services. I had no choice but to have individual terminals. Recently, taking advantage of the features of each system,
There is a movement to receive a plurality of services by one terminal, and a dual band in which two services are received by one terminal is generally considered. As described above, a terminal is required to be small and lightweight because of its portability when one terminal receives different services.

However, in the case of the filter described in the prior art, there is a problem that it is difficult to reduce the size because the switch 100 (see FIG. 13) is connected to the two filters. In addition, there is a problem in that the deterioration of the insertion loss caused by the switch 100 leads to a reduction in transmission time during transmission and a deterioration in sensitivity during reception.

Even if a filter is designed to satisfy two different filter characteristics at the same time, a filter with a wide band and a high attenuation is required. Becomes smaller, and
If an attempt is made to increase the attenuation, the band becomes narrower and the insertion loss increases, making it difficult to satisfy the characteristics originally required of the filter. The third described in the prior art
Not only does not satisfy the insertion loss of the pass band, but the first filter and the second filter are composed of two resonators, whereas the third filter is composed of four resonators. Therefore, the mounting area is about twice as large, and miniaturization is difficult.

An object of the present invention is to provide a filter having a small size, a small insertion loss and a high attenuation by shifting a pass band.

[0008]

According to the present invention, one or more resonators connected to each input / output terminal by an input / output inductor or capacitor and cascaded to each other by a coupling inductor or capacitor are provided. Prepared,
An additional inductor is connected to the one or more resonators via a switch circuit, and a control terminal is connected between the one or more additional inductors and the switch circuit via a control circuit, A tunable filter is obtained in which a control voltage is applied from the control terminal to shift the center frequency of the filter to a lower frequency side.

According to the present invention, each of the input / output terminals is connected by an input / output inductor or a capacitor, and includes one or more resonators cascaded to each other by a coupling inductor or a capacitor. An additional inductor is grounded via a pin diode to each of the at least one resonator, and a control terminal is connected between the additional inductor and the pin diode via a control circuit. A tunable filter is obtained in which the filter is applied from the control terminal to shift the center frequency of the filter to a lower frequency side.

Further, according to the present invention, each of the input / output terminals is connected by an input / output capacitor or an inductor, and is provided with one or more resonators cascaded to each other by a coupling capacitor or an inductor. An additional capacitor is grounded to each of the one or more resonators via a switch circuit, and a control terminal is connected between the one or more additional capacitors and the switch circuit via a control circuit. Is applied from the control terminal to shift the center frequency of the filter to the lower frequency side, thereby obtaining a tunable filter.

According to the present invention, each of the input / output terminals is connected by an input / output capacitor or an inductor, and is provided with one or more resonators cascaded to each other by a coupling capacitor or an inductor. In each of the one or more resonators, an additional capacitor is grounded via a pin diode, and a control terminal is connected between the additional capacitor and the pin diode via a control circuit. A tunable filter is obtained in which the filter is applied from the control terminal to shift the center frequency of the filter to a lower frequency side.

According to the present invention, each of the input / output terminals is connected by an input / output capacitor or an inductor, and is provided with one or more resonators cascaded to each other by a coupling capacitor or an inductor. A varicap is grounded to each of the one or more resonators, and a control terminal is connected to the one or more varicaps via a control circuit. A control voltage is applied from the control terminal to the filter. Is shifted to the lower frequency side or the higher frequency side.

Further, according to the present invention, there is provided a tunable filter wherein the switch circuit is a pin diode.

Further, according to the present invention, there is provided a tunable filter wherein the switch circuit is a GaAs switch.

Further, according to the present invention, there is provided a tunable filter, wherein the switch circuit is a mechanical switch such as a relay.

[0016]

The band elimination filter using two resonators as shown in the equivalent circuit of FIG. 9 will be described as an example. The two resonators 11, 11 'are
Are connected to each other by a coupling inductor 13, and are connected to the input / output terminal 14 by an input / output inductor 15. The center frequency f0 of the filter is substantially determined by the resonance frequencies of the two resonators 11, and they are in a proportional relationship. Since the coupling inductor 13 and the input / output inductor 15 are substantially proportional to the center frequency, the values of the two differ before and after the shift (the values after the shift are smaller). Regardless
There is no problem even if the coupling inductor 13 and the input / output inductor 15 are set to constant values. Therefore, if the resonance frequency of the resonator 11 can be shifted, the pass band and the attenuation band of the filter can be shifted.

The resonance frequency fr is represented by the following equation (1).

[0018]

(Equation 1) Therefore, if an inductor L 'is added in parallel to the resonator,
The resonance frequency f'0 is represented by the following equation (2).

[0019]

(Equation 2) And the resonance frequency can be lowered. If the inductor 18 to be added is connected via the switch 16, two different resonance frequencies can be realized by on / off control of the switch 16, and the center frequency of the filter can be shifted (see FIG. 10).

Next, a band-pass filter using two resonators as shown in the equivalent circuit of FIG. 11 will be described as an example. The two resonators 31 and 31 ′ are connected to each other by a coupling capacitor 33, and are connected to the input / output terminal 12 by an input / output capacitor 34. The center frequency f0 of the filter is substantially determined by the resonance frequencies of the two resonators 31, 31 ', and they are in a proportional relationship. Since the coupling capacitor 33 and the input / output capacitor 34 are in an inversely proportional relationship with the center frequency, when the center frequency is largely shifted, the characteristic value is degraded due to deviation from the theoretical value. Even if the coupling capacitor 33 and the input / output capacitor 34 have constant values regardless of before and after the frequency shift, the characteristics do not deteriorate. Therefore, if the resonance frequency of the resonators 31 and 31 'can be shifted, the center frequency of the filter can be shifted.

The resonance frequency f0 is represented by the following equation (3).

[0022]

(Equation 3) Therefore, if a capacitance C is added to the resonator in parallel, the resonance frequency f'0 can be expressed by the following equation (4).

[0023]

(Equation 4) And the resonance frequency can be lowered. If the capacitor C to be added is connected via a switch, two different resonance frequencies can be realized by ON / OFF control of the switch 40, and the center frequency of the filter can be shifted. (See FIG. 12A).

If an element such as a varicap 38 whose capacitance can be varied by an external control current is used in place of the additional capacitance C and the switch, the center frequency can be continuously shifted within the range of capacitance change according to the current. (See FIG. 12B).

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As a first embodiment according to the present invention, a band elimination filter using two resonators as shown in FIG. 1 will be described. Resonator 1
1, 11 'are connected to each other by a coupling inductor 13, and an input / output terminal 14 is connected to the input / output inductor 1.
5 are connected. The additional inductor 18 is PI
It is grounded via an N diode 20. Further, a choke coil 19 and a bypass capacitor 2 are provided between the additional inductor 18 and the PIN diode 20 as a control circuit.
2 and a control terminal 23 are connected via a current control resistor 21.

Since the PIN diode 20 is open when no forward current is flowing, the additional inductor 18 is not grounded, and the center frequency of the filter is substantially the same as that when the filter is constituted only by the resonator represented by the above equation (1). Same but P
When a forward current flows through the IN diode 20, the additional inductor 18 is grounded, so that the center frequency of the filter shifts to a lower frequency in accordance with the additional inductor 18 shown in the above equation (2).

As a characteristic example, when a control voltage is applied from the control terminal 23 to a filter having a frequency characteristic as shown in FIG. 2A, the pass band and the attenuation band become approximately as shown in FIG. 2B. The frequency shifted to the low frequency side of 35 MHz.

Next, a second embodiment of the present invention will be described with reference to FIG. In the present embodiment, as shown in FIG. 3, the choke coil 19 can be omitted by connecting a capacitor 24 having a capacitance represented by the following mathematical expression 5 to the additional inductance 18. This is because the impedance at the node 25 is large for the signal having the frequency of f0.

[0029]

(Equation 5)

By shifting the center frequency of one filter as described above, it is possible to correspond to two bands, so that it is smaller and lighter than the conventional case where two filters are used, and two filters are used. Since a switch circuit for switching between is not required, a filter without insertion loss deterioration can be realized. In addition, since the band is narrower than a filter that satisfies the two filter characteristics at the same time, a high-attenuation filter can be easily designed.
Since the number of resonators is small, miniaturization is easy.

Next, a third embodiment of the present invention will be described with reference to FIG. In the present embodiment, as shown in FIG. 4, the shift can be performed in two stages by separately providing the resonators 11 and 11 'for the additional inductor 18, the PIN diode 20, and the control circuit. Of course, it is needless to say that the shift can be made multi-stage by adding a large number.

In the first to third embodiments described above, the case where a PIN diode is used as a switch has been described. However, a GaAs MMIC switch, a mechanical switch, or the like may be used.

Next, a fourth embodiment according to the present invention will be described with reference to FIG. This embodiment is a bandpass filter using two resonators as shown in FIG. 5, and is configured using a capacitor instead of the inductor in the above-described embodiment. The resonators 31 and 31 ′ are connected to each other by a coupling capacitor 33, and are connected to the input / output terminal 14 by an input / output capacitor 34. Additional capacity 32 is PIN
It is grounded via a diode 20. Further, a control terminal 23 is connected between the additional capacitance 32 and the PIN diode 20 via a choke coil 19, a bypass capacitor 22, and a current control resistor 21 as a control circuit. The additional capacity 32 also has a DC cut function.

Since the PIN diode 20 is open when no forward current is flowing, the additional capacitance 32 is not grounded, and the center frequency of the filter is substantially the same as that when the filter is constituted by the resonator expressed by the above equation (3). However, when a forward current flows through the PIN diode 20, the additional capacitance 32 is grounded, so that the center frequency of the filter shifts to the lower frequency side in accordance with the additional capacitance C shown in the equation (4).

As a specific example, the resonance frequency f0 of the resonators 31 and 31 '= 960 MHz, the coupling capacitor 33 is 0.9 pF, the input / output capacitor 14 is 1.8 pF, the additional capacitance 32 is 1.8 pF, and the bypass capacitor 23 is used. 1
FIG. 6A shows the attenuation characteristics of the filter when the current control resistor 21 is 300Ω and 000 pF. When a voltage of 3 V was applied thereto from the control terminal 20, the center frequency shifted to a lower frequency side of about 35 MHz as shown in FIG. 6B.

As described above, by shifting the center frequency of one filter, it is possible to correspond to two bands, so that it is smaller and lighter than in the case where two filters are used as in the conventional case, and the conventional structure is used. As described above, since a switch circuit for switching between two filters is not required, a filter without deterioration of insertion loss can be realized. Also, since the band is narrower than a filter that satisfies the two filter characteristics at the same time, a filter with a high attenuation can be easily designed.

Next, a fifth embodiment according to the present invention will be described with reference to FIG. In the present embodiment, FIG.
As shown in (2), by adding an additional capacitor, a PIN diode, and a control circuit, the shift can be made in two stages. Of course, it is needless to say that the shift can be made multi-stage by adding a large number.

As described above, the PIN diode is used as an example of the switch circuit. However, a GaAs switch or a mechanical switch such as a relay may be used.

Next, a sixth embodiment of the present invention will be described with reference to FIG. In this embodiment, as shown in FIG. 8, a varicap 38 is connected in parallel with the resonator. Additional capacitor 32 and PIN diode 20
Is the same as that described in the first embodiment except that is replaced by the varicap 38.

The varicap 38 changes the inter-terminal capacitance of the varicap 38 according to the amount of reverse current. By changing the applied voltage from the control terminal 23,
The center frequency of the filter can be shifted continuously.

Further, it goes without saying that any element capable of changing the capacitance by an external control current other than the varicap may be used as the means for changing the capacitance.

[0042]

As is apparent from the above description, according to the present invention, by changing the resonance frequency of the resonator, one filter can cope with two bands, so that the size of the filter is smaller than that of the prior art. A lightweight filter can be realized. Further, since a SW circuit is unnecessary, a filter with little deterioration of insertion loss can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of a tunable filter according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating an attenuation characteristic of a tunable filter according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating a configuration of a tunable filter according to a second embodiment of the present invention.

FIG. 4 is a diagram illustrating a configuration of a tunable filter according to a third embodiment of the present invention.

FIG. 5 is a diagram illustrating a configuration of a tunable filter according to a fourth embodiment of the present invention.

FIG. 6 is a diagram illustrating an attenuation characteristic of a tunable filter according to a fourth embodiment of the present invention.

FIG. 7 is a diagram illustrating a configuration of a tunable filter according to a fifth embodiment of the present invention.

FIG. 8 is a diagram illustrating a configuration of a tunable filter according to a sixth embodiment of the present invention.

FIG. 9 is an equivalent circuit diagram showing a basic configuration of a filter when the inductor of the present invention is used as a coupling element.

FIG. 10 is an equivalent circuit diagram in which an additional inductor and a switch circuit are provided in a resonator.

FIG. 11 is an equivalent circuit diagram showing a basic configuration of a filter when the capacitor of the present invention is used as a coupling element.

FIG. 12A is an equivalent circuit diagram in which an additional capacitor and a switch circuit are provided in a resonator, and FIG. 12B is an equivalent circuit diagram in which a varicap is provided in the resonator.

FIG. 13 is a diagram showing a configuration of a conventional filter.

FIG. 14 is a diagram showing characteristics of a conventional filter.

[Explanation of symbols]

 11, 11 'resonator 12 capacitor 13 coupling inductor 14 input / output terminal 15 input / output inductor 16, 40 switch 18 additional inductor 19 choke coil 20 PIN diode 21 control resistor 22 bypass capacitor 23 control terminal 24 capacitor 25 node 31, 31 'resonator 32 additional capacitor 33 coupling capacitor 34 input / output capacitor

Claims (8)

[Claims] An input / output terminal includes one or more resonators connected to each other by an input / output inductor or a capacitor and cascade-connected to each other by a coupling inductor or a capacitor. Each has an additional inductor grounded via a switch circuit, a control terminal is connected between the one or more additional inductors and the switch circuit via a control circuit, and a control voltage is supplied from the control terminal A tunable filter, which is applied to shift the center frequency of the filter to a lower frequency side. 2. One or more resonators connected to each input / output terminal by an input / output inductor or capacitor and cascaded to each other by a coupling inductor or capacitor. Respectively, the additional inductors are grounded via pin diodes,
A control terminal is connected between the additional inductor and the pin diode via a control circuit, respectively.
A tunable filter, wherein a control voltage is applied from the control terminal to shift a center frequency of the filter to a lower frequency side. 3. One or more resonators connected to each input / output terminal by an input / output capacitor or inductor and cascaded to each other by a coupling capacitor or inductor. Each of the additional capacitors is grounded via a switch circuit, a control terminal is connected between the one or more additional capacitors and the switch circuit via a control circuit, and a control voltage is supplied from the control terminal. A tunable filter, which is applied to shift the center frequency of the filter to a lower frequency side. 4. One or more resonators connected to each input / output terminal by an input / output capacitor or inductor and cascaded to each other by a coupling capacitor or inductor. Respectively, additional capacitors are grounded via pin diodes,
A control terminal is connected between the additional capacitor and the pin diode via a control circuit, respectively.
A tunable filter, wherein a control voltage is applied from the control terminal to shift a center frequency of the filter to a lower frequency side. 5. One or more resonators connected to each input / output terminal by an input / output capacitor or inductor and cascaded to each other by a coupling capacitor or inductor. Each has a varicap grounded, and a control terminal is connected to the one or more varicaps via a control circuit.
A tunable filter, wherein a control voltage is applied from the control terminal to shift a center frequency of the filter to a low frequency side or a high frequency side. 6. The tunable filter according to claim 1, wherein the switch circuit is a pin diode. 7. The tunable filter according to claim 1, wherein the switch circuit is a GaAs switch. 8. The tunable filter according to claim 1, wherein the switch circuit is a mechanical switch such as a relay.