Tunable bandpass filter

Abstract

This invention provides a tunable bandpass filter capable of improving distortion characteristics for high-frequency signals. [Solution] The tunable bandpass filter comprises a first input / output terminal IO1, a second input / output terminal IO2, a plurality of resonators RE having a first resonator REa and a second resonator REe, a plurality of variable capacitance circuits CI, a voltage supply terminal TP, a plurality of first inductors L1, and a first capacitor C1. Each resonator RE is formed by a microstrip line. Each variable capacitance circuit CI has a first variable capacitance diode D1, a second variable capacitance diode D2, a first electrical resistor R1, a second capacitor C2, a third variable capacitance diode D3, a second electrical resistor R2, and a third electrical resistor R3.

Description

Technical Field

[0001] Embodiments of the present invention relate to a tunable band-pass filter.

Background Art

[0002] In a radar system, a band-pass filter (BPF) is used to remove spurious emissions other than a desired wave at a stage before a frequency converter that performs frequency conversion (down-conversion) of a high-frequency signal (RF signal) received by an antenna. Also, a BPF is used to remove spurious emissions other than a desired wave generated by a frequency converter or the like at a stage after a frequency converter that performs frequency conversion (up-conversion) to a high-frequency signal. Thereby, when transmitting a frequency-converted signal from the antenna, spurious components that can be emitted from the antenna can be removed. In particular, since transmitted waves are subject to regulations under the Radio Law, removal of spurious emissions is an important issue.

[0003] In recent years, the frequency range used in radar systems has been expanding. When trying to remove spurious emissions with a BPF having a wide frequency range as a passband, spurious emissions in a frequency range close to the desired wave cannot be removed. For this reason, there is a demand for a tunable BPF that can remove many spurious emissions other than the desired wave even when expanding the range in which the frequency of the passband of a BPF having a relatively narrow passband can be varied.

[0004] For example, a received wave, which is a weak signal, is amplified by a low-noise amplifier (LNA) or the like at a stage before the BPF. In order to ensure the quality of the received signal, the BPF requires linearity with respect to the signal amplified by a low-noise amplifier or the like, and requires not only low pass loss but also high distortion characteristics.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] U.S. Patent No. 8922305 [Patent Document 2] Japanese Patent Application Publication No. 6-343002 [Patent Document 3] Japanese Patent Publication No. 2000-349509 [Patent Document 4] Japanese Patent Publication No. 2004-297277 [Overview of the project] [Problems that the invention aims to solve]

[0006] This embodiment provides a tunable bandpass filter capable of improving distortion characteristics for high-frequency signals. [Means for solving the problem]

[0007] One embodiment of a tunable bandpass filter is: First input / output terminal and, Second input / output terminal, A plurality of resonators having a first resonator and a second resonator, Multiple variable capacitance circuits connected one-to-one to the aforementioned multiple resonators, Voltage supply terminals and A plurality of first inductors, each of which has one end connected to the voltage supply terminal and the other end connected to one of the plurality of variable capacitance circuits, The system comprises a first capacitor having one end connected to a first node between one end of the plurality of first inductors and the voltage supply terminal, and the other end being grounded. Each of the resonators is formed by a microstrip line including a ground conductor and a strip conductor connected to the ground conductor. The strip conductor of the first resonator is connected to the first input / output terminal, The strip conductor of the second resonator is connected to the second input / output terminal, Each of the aforementioned variable capacitance circuits is, First variable capacitance diode, A second variable capacitance diode having an anode connected to a ground wire and a cathode connected to the cathode of the first variable capacitance diode, A first electrical resistor connected between the second node between the cathode of the first variable capacitance diode and the cathode of the second variable capacitance diode, and the other end of the first inductor, A second capacitor having one end connected to the strip conductor and the other end, A third variable capacitance diode having an anode connected to the anode of the first variable capacitance diode and a cathode connected to the other end of the second capacitor, A second electrical resistor is connected between the third node between the other end of the second capacitor and the cathode of the third variable capacitance diode, and the other end of the first inductor. A third electrical resistor has one end connected to a fourth node between the anode of the first variable capacitance diode and the anode of the third variable capacitance diode, and the other end is grounded. The signal input to one of the first input / output terminals and the second input / output terminal has its passband frequency adjusted by the variable capacitance circuit passed through and output from the other of the first input / output terminals and the second input / output terminals.

[0008] Furthermore, a tunable bandpass filter according to one embodiment is First input / output terminal and, Second input / output terminal, A plurality of resonators having a first resonator and a second resonator, Multiple variable capacitance circuits connected one-to-one to the aforementioned multiple resonators, Voltage supply terminals and A first capacitor having one end connected to the first node between the plurality of variable capacitance circuits and the voltage supply terminal, and the other end being grounded, The first low-pass filter, It includes a second low-pass filter, Each of the resonators is formed by a microstrip line including a ground conductor and a strip conductor connected to the ground conductor. The strip conductor of the first resonator is connected to the first input / output terminal via the first low-pass filter. The strip conductor of the second resonator is connected to the second input / output terminal via the second low-pass filter. The first low-pass filter is, A first inductor connected between the first input / output terminal and the strip conductor of the first resonator, A fourth capacitor having one end connected to the sixth node between the first input / output terminal and the first inductor, and the other end being grounded, The second low-pass filter is, A second inductor connected between the second input / output terminal and the strip conductor of the second resonator, A fifth capacitor having one end connected to the seventh node between the second input / output terminal and the second inductor, and the other end being grounded, Each of the aforementioned variable capacitance circuits is, First variable capacitance diode, A second variable capacitance diode having an anode connected to a ground wire and a cathode connected to the cathode of the first variable capacitance diode, A first electrical resistor connected between the second node between the cathode of the first variable capacitance diode and the cathode of the second variable capacitance diode, and the voltage supply terminal, A second capacitor having one end connected to the strip conductor and the other end, A third variable capacitance diode having an anode connected to the anode of the first variable capacitance diode and a cathode connected to the other end of the second capacitor, A second electrical resistor connected between the third node between the other end of the second capacitor and the cathode of the third variable capacitance diode, and the voltage supply terminal, A third electrical resistor having one end connected to a fourth node between the anode of the first variable capacitance diode and the anode of the third variable capacitance diode and the other end grounded. Pass the frequency of the passband adjusted by the variable capacitance circuit among the signals input to one of the first input / output terminal and the second input / output terminal, and output the signals from the other of the first input / output terminal and the second input / output terminal.

Brief Description of Drawings

[0009] [Figure 1] FIG. 1 is a circuit diagram showing a tunable bandpass filter according to a comparative example. [Figure 2] FIG. 2 is a cross-sectional view showing one resonator among a plurality of resonators according to the above comparative example. [Figure 3] FIG. 3 is a circuit diagram showing a tunable bandpass filter according to a first embodiment. [Figure 4] FIG. 4 is a circuit diagram showing a tunable bandpass filter according to Modification 1 of the first embodiment. [Figure 5] FIG. 5 is a circuit diagram showing a tunable bandpass filter according to a second embodiment. [Figure 6] FIG. 6 is a circuit diagram showing a tunable bandpass filter according to Modification 1 of the second embodiment. [Figure 7] FIG. 7 is a circuit diagram showing a tunable bandpass filter according to Modification 2 of the first embodiment. [Figure 8] FIG. 8 is a circuit diagram showing a tunable bandpass filter according to Modification 3 of the first embodiment.

Embodiments for Carrying Out the Invention

[0010] Hereinafter, the tunable bandpass filter according to the comparative example and each embodiment will be described in detail with reference to the drawings.

[0011] (Comparative Example) First, we will explain the configuration of the tunable bandpass filter (tunable BPF) in the comparative example. Figure 1 is a circuit diagram showing the tunable BPF in the comparative example.

[0012] As shown in Figure 1, the tunable BPF comprises a first input / output terminal IO1, a second input / output terminal IO2, multiple resonators RE, multiple variable capacitance circuits CI, a voltage supply terminal TP, multiple first inductors L1, and a first capacitor C1. The multiple resonators RE and the multiple variable capacitance circuits CI operate as a filter circuit.

[0013] Multiple resonators RE include a first resonator REa, a second resonator REe, and so on. Multiple variable capacitance circuits CI are connected to multiple resonators RE on a one-to-one basis. Here, the last digits of the symbols of the resonators RE and variable capacitance circuits CI that are connected on a one-to-one basis are matched. For example, a variable capacitance circuit CIa is connected to a resonator REa. Furthermore, the last digits of the symbols of the internal components of the resonators RE are matched to the last digits of the resonators RE, and the last digits of the symbols of the internal elements of the variable capacitance circuits CI are matched to the last digits of the variable capacitance circuits CI.

[0014] Each first inductor L1 has one end connected to a voltage supply terminal TP and the other end connected to one of the multiple variable capacitance circuits CI. The first capacitor C1 has one end connected to a first node n1 between one end of a plurality of first inductors L1 and a voltage supply terminal TP, and the other end is grounded.

[0015] Figure 2 is a cross-sectional view showing one of the multiple resonators RE related to this comparative example. In Figure 2, resonator REa is shown as representative of the multiple resonators RE. As shown in Figures 1 and 2, the tunable BPF comprises a substrate BA, an insulating layer IL, a ground conductor GC, and a plurality of strip conductors SC. The substrate BA is formed of, for example, gallium arsenide (GaAs). The insulating layer IL is formed on the substrate BA. The ground conductor GC is formed on the side of the substrate BA opposite to the side where the insulating layer IL is provided and is grounded. Each strip conductor SC is formed on the insulating layer IL and is connected to the ground conductor GC at one end through via holes formed in the substrate BA and the insulating layer IL, and is grounded. The other end of the strip conductor SC is connected to CI.

[0016] Each resonator RE is formed by a microstrip line SL that includes a ground conductor GC and a strip conductor SC connected to the ground conductor GC. The ground conductor GC and the strip conductor SC are each formed of a metal layer. Multiple microstrip lines SL share the ground conductor GC. The strip conductor SCa of resonator REa is connected to the first input / output terminal IO1. The strip conductor SCe of resonator REe is connected to the second input / output terminal IO2. Of the multiple strip conductors SC, strip conductor SCa and strip conductor SCe are located at the ends, respectively.

[0017] As shown in Figure 1, each variable capacitance circuit CI includes a first variable capacitance diode D1, a second variable capacitance diode D2, and a first resistor (first electrical resistance) R1. Focusing on the variable capacitance circuit CIa, the second variable capacitance diode D2a has an anode connected to the ground wire and a cathode connected to the cathode of the first variable capacitance diode D1a. The first resistor R1a is connected between the second node n2a between the cathode of the first variable capacitance diode D1a and the cathode of the second variable capacitance diode D2a, and the other end of the first inductor L1a.

[0018] A tunable BPF can pass through the frequency of the passband adjusted by the variable capacitance circuit CI from the signal (RF signal) input to one of the first input / output terminal IO1 and the second input / output terminal IO2, and output it from the other of the first input / output terminal IO1 and the second input / output terminal IO2.

[0019] A DC power supply and a DAC (Digital Analog Converter) are connected to the voltage supply terminal TP. The positive bias voltage Vtune applied to the voltage supply terminal TP is applied to the second node n2a via the first resistor R1a, and a reverse bias voltage is applied to the first variable capacitance diode D1a and the second variable capacitance diode D2a. The first variable capacitance diode D1a and the second variable capacitance diode D2a function as variable capacitances. By adjusting the bias voltage Vtune, the center frequency of the passband of the tunable BPF can be varied. The tunable BPF can remove spurious emissions other than the desired wave.

[0020] To suppress leakage of RF signals (high-frequency signals) between the resonators RE, an LC filter (in this case, a low-pass filter) consisting of a first inductor L1a and a first capacitor C1 is provided. Even if an RF signal leaks from the strip conductor SCa to the variable capacitance circuit CIa and passes through the first inductor L1a, the RF signal can be discharged to the ground wire via the first capacitor C1. This suppresses leakage of RF signals to the voltage supply terminal TP, DC power supply, and DAC, as well as leakage of RF signals to another variable capacitance circuit CI (e.g., the first inductor L1b).

[0021] If the first variable capacitance diode D1a and the second variable capacitance diode D2a are elements with high breakdown voltage when a reverse voltage is applied between the anode and cathode, the sensitivity of the capacitance adjustment amount to the reverse bias voltage becomes low, which is effective for reducing distortion characteristics.

[0022] However, because a wide voltage dynamic range is required for the DC power supply and DAC that provide the reverse bias voltage, commonly available low-voltage output type DACs cannot be used, resulting in the disadvantage of increased system costs.

[0023] Furthermore, the variable capacitance of the first variable capacitance diode D1a and the second variable capacitance diode D2a are changed by the reverse bias voltage applied between the anode and cathode. Diode elements with low reverse breakdown voltage are applied to the first variable capacitance diode D1a and the second variable capacitance diode D2a of the variable capacitance circuit CIa, and the case in which a large RF signal is input to the first input / output terminal IO1, and the RF signal leaks to the second variable capacitance diode D2a via the variable capacitance of the first variable capacitance diode D1a connected to the strip conductor SCa.

[0024] The RF signal causes slight fluctuations in the voltages applied across the first variable capacitance diode D1a and the second variable capacitance diode D2a, resulting in a slight shift in the center frequency of the passband of the tunable BPF. This then causes variations in the passband loss due to differences in the flatness of the passband loss within the passband and the amount of attenuation outside the passband, leading to a deterioration of the distortion characteristics for the RF signal. In particular, when the propagating frequency is high, that is, when the variable capacitance of the variable capacitance circuit CIa is small, the fluctuation of the center frequency of the passband in response to capacitance changes becomes relatively large, further worsening the distortion characteristics for the RF signal.

[0025] For the reasons stated above, there is a need for a tunable bandpass filter that can remove spurious emissions other than the desired wave, has a variable passband center frequency, and improves the distortion characteristics for RF signals.

[0026] (First embodiment) Next, the configuration of the tunable BPF according to the first embodiment will be described. Figure 3 is a circuit diagram showing the tunable BPF according to this embodiment. The tunable BPF is configured in the same way as the tunable BPF of the comparative example described above, except for the configuration described in this first embodiment.

[0027] As shown in Figure 3, the tunable BPF is composed of an MMIC (Monolithic Microwave Integrated Circuit) and is a BPF whose passband center frequency can be varied by the bias voltage Vtune. In this first embodiment, a technique is provided that can improve the distortion characteristics for RF signals (input signals) with large amplitude without increasing the bias voltage Vtune.

[0028] The variable capacitance circuit CI of this first embodiment includes a first variable capacitance diode D1, a second variable capacitance diode D2, and a first resistor R1. Furthermore, the variable capacitance circuit CI includes a second capacitor C2, a third variable capacitance diode D3, a second resistor (second electrical resistance) R2, and a third resistor (third electrical resistance) R3.

[0029] The main difference from the comparative example is that a second capacitor C2 with a fixed capacitance for DC blocking and a third variable capacitance diode D3 were added, thereby applying diode elements with low reverse breakdown voltage (first variable capacitance diode D1, second variable capacitance diode D2, and third variable capacitance diode D3) to the variable capacitance circuit CI.

[0030] Focusing on the variable capacitance circuit CIa, the anode of the second variable capacitance diode D2a is directly connected to the ground wire. The second capacitor C2a has one end connected to the strip conductor SCa and the other end. The third variable capacitance diode D3a has an anode connected to the anode of the first variable capacitance diode D1a and a cathode connected to the other end of the second capacitor C2a. The second resistor R2a is connected between the third node n3a, which is between the other end of the second capacitor C2a and the cathode of the third variable capacitance diode D3a, and the other end of the first inductor L1a. The third resistor R3a has one end connected to the fourth node n4a, which is between the anode of the first variable capacitance diode D1a and the anode of the third variable capacitance diode D3a, and the other end is grounded.

[0031] According to the tunable BPF of the first embodiment configured as described above, the tunable BPF includes a first input / output terminal IO1, a second input / output terminal IO2, a plurality of resonators RE, a plurality of variable capacitance circuits CI, a voltage supply terminal TP, a plurality of first inductors L1, and a first capacitor C1. Each variable capacitance circuit CI has a first variable capacitance diode D1, a second variable capacitance diode D2, a first resistor R1, a second capacitor C2, a third variable capacitance diode D3, a second resistor R2, and a third resistor R3.

[0032] The capacitance of the variable capacitance circuit CIa is the combined capacitance of the first variable capacitance diode D1a, the second variable capacitance diode D2a, the third variable capacitance diode D3a, and the fixed capacitance of the second capacitor C2a. The center frequency of the passband of the tunable BPF is determined by this combined capacitance.

[0033] The fluctuations in variable capacitance caused by the fluctuations in the voltages applied across the first variable capacitance diode D1a, the second variable capacitance diode D2a, and the third variable capacitance diode D3a, due to the amplitude of the RF signal, are also determined by the combined capacitance.

[0034] By adding the fixed capacitance of the second capacitor C2a, the proportion of the fixed capacitance in the combined capacitance increases. Therefore, even if an RF signal leaks into the variable capacitance circuit CIa, the fluctuations in the combined capacitance of the variable capacitance of the first variable capacitance diode D1a, the fluctuations in the combined capacitance of the variable capacitance of the second variable capacitance diode D2a, and the fluctuations in the combined capacitance of the variable capacitance of the third variable capacitance diode D3a can be reduced.

[0035] Compared to the comparative example, the variation in the passband center frequency of the tunable BPF is smaller, which reduces the deterioration of distortion characteristics for RF signals.

[0036] As described above, a tunable bandpass filter (BPF) can be obtained that can remove spurious emissions other than the desired wave, has a variable passband center frequency, and improves the distortion characteristics for RF signals.

[0037] By adjusting the size of the second capacitor C2a, which has a fixed capacitance, and the sizes of the first variable capacitance diode D1a, the second variable capacitance diode D2a, and the third variable capacitance diode D3a, it becomes possible to easily adjust the voltage sensitivity of the variable capacitance, which is determined by the bias voltage Vtune and the first variable capacitance diode D1a, the second variable capacitance diode D2a, and the third variable capacitance diode D3a, without changing the design of the resonator RE.

[0038] Since the second capacitor C2a also functions as a DC filter, it continues to function as a fixed-capacity capacitor even when the voltage between the resonator REa and the second capacitor C2a becomes higher than the bias voltage Vtune due to the input of a DC voltage or RF signal to the first input / output terminal IO1 or the second input / output terminal IO2.

[0039] Furthermore, as described above, in this first embodiment, diode elements with low reverse breakdown voltage can be applied to the first variable capacitance diode D1, the second variable capacitance diode D2, and the third variable capacitance diode D3 of the variable capacitance circuit CI, and commonly available low-voltage output type DACs and DC power supplies can be used, and these DACs and DC power supplies can be connected to the voltage supply terminal TP.

[0040] The anode of the second variable capacitance diode D2a is directly connected to the ground wire. The anode of the second variable capacitance diode D2a is not indirectly connected to the ground wire via a capacitor. Therefore, it is possible to avoid a situation where the range in which the center frequency of the passband of the tunable BPF is varied becomes narrow. Note that the range in which the above center frequency can be varied is determined by the combined capacitance of the variable capacitance circuit CIa, and the larger the fixed capacitance, the narrower the range in which the above center frequency can be varied.

[0041] The number, shape, and spacing of the strip conductors SC should be determined considering the passband and cutoff band (restricted band) of the tunable BPF. Furthermore, the characteristics of the first inductors L1a to L1e, the first variable capacitance diodes D1a to D1e, the second variable capacitance diodes D2a to D2e, the third variable capacitance diodes D3a to D3e, the first resistors R1a to R1e, the second resistors R2a to R2e, and the third resistors R3a to R3e are all the same, but may differ from each other. However, it should be noted that optimization of the characteristics is necessary, for example, when the characteristics of the first inductors L1a to L1e are different.

[0042] (Modification 1 of the first embodiment) Next, the configuration of the tunable BPF according to Modification 1 of the first embodiment will be described. Figure 4 is a circuit diagram showing the tunable BPF according to Modification 1. The tunable BPF is configured in the same way as the tunable BPF of the first embodiment, except for the configuration described in Modification 1.

[0043] As shown in Figure 4, the variable capacitance circuit CI further includes a third capacitor C3 and a fourth resistor (fourth electrical resistance) R4. Focusing on the variable capacitance circuit CIa, the third capacitor C3a has one end connected to the anode of the second variable capacitance diode D2a and the other end connected to the ground wire. The fourth resistor R4 has one end connected to a fifth node between the anode of the second variable capacitance diode D2a and one end of the third capacitor C3a, and the other end is grounded.

[0044] The variable capacitance of the variable capacitance diode D changes as the voltage across the diode D changes. If one end of the second variable capacitance diode D2a is directly grounded, the variable capacitance will change directly if the potential of the opposite terminal of the second variable capacitance diode D2a changes. However, in this modified example 1, the impedance of the anode side of the second variable capacitance diode D2a is increased by the third capacitor C3a, which reduces the change in the variable capacitance of the second variable capacitance diode D2a, etc. Therefore, it is more effective in improving the distortion characteristics with respect to RF signals if the variable capacitance circuit CI also has a third capacitor C3.

[0045] (Second embodiment) Next, the configuration of the tunable BPF according to the second embodiment will be described. Figure 5 is a circuit diagram showing the tunable BPF according to this second embodiment. The tunable BPF is configured in the same way as the tunable BPF of the first embodiment, except for the configuration described in this second embodiment.

[0046] As shown in Figure 5, no inductor is connected between the variable capacitance circuit CI and the voltage supply terminal TP. Instead, the tunable BPF further comprises a first low-pass filter LP1 and a second low-pass filter LP2. The strip conductor SCa is connected to the first input / output terminal IO1 via the first low-pass filter LP1. The strip conductor SCe is connected to the second input / output terminal IO2 via the second low-pass filter LP2.

[0047] The first low-pass filter LP1 includes a first inductor L1 and a fourth capacitor C4. The first inductor L1 is connected between the first input / output terminal IO1 and the strip conductor SCa. The fourth capacitor C4 has one end connected to the sixth node n6 between the first input / output terminal IO1 and the first inductor L1, and the other end grounded.

[0048] The second low-pass filter LP2 includes a second inductor L2 and a fifth capacitor C5. The second inductor L2 is connected between the second input / output terminal IO2 and the strip conductor SCe. The fifth capacitor C5 has one end connected to the seventh node n7 between the second input / output terminal IO2 and the second inductor L2, and the other end grounded. Focusing on the variable capacitance circuit CIa, the anode of the second variable capacitance diode D2a is directly connected to the ground wire.

[0049] In the tunable BPF according to the second embodiment configured as described above, the same effects as in the first embodiment can be obtained. The tunable BPF is equipped with two inductors, a first inductor L1 and a second inductor L2. Regardless of the number of stages of the resonator REa to REe, the tunable BPF only needs to be equipped with two inductors, which simplifies the layout of the elements and contributes to miniaturization.

[0050] For example, a tunable BPF is a BPF that passes frequencies in the range between +500MHz and -500MHz, centered around 5GHz. The first low-pass filter LP1 and the second low-pass filter LP2 can help remove (suppress) harmonics (e.g., the second harmonic at 10GHz) which are distortion components, when the center frequency of the passband is 5GHz.

[0051] (Modification 1 of the second embodiment) Next, the configuration of the tunable BPF according to Modification 1 of the second embodiment will be described. Figure 6 is a circuit diagram showing the tunable BPF according to Modification 1. The tunable BPF is configured in the same way as the tunable BPF of the second embodiment, except for the configuration described in Modification 1.

[0052] As shown in Figure 6, the variable capacitance circuit CI further includes a third capacitor C3 and a fourth resistor R4. Focusing on the variable capacitance circuit CIa, the third capacitor C3a has one end connected to the anode of the second variable capacitance diode D2a and the other end connected to the ground wire. The fourth resistor R4 has one end connected to a fifth node between the anode of the second variable capacitance diode D2a and one end of the third capacitor C3a, and the other end is grounded.

[0053] The third capacitor C3a increases the anode impedance of the second variable capacitance diode D2a, thereby reducing changes in the variable capacitance of the second variable capacitance diode D2a and improving the distortion characteristics for RF signals.

[0054] Next, modifications of the variable capacitance circuit CI of the first and second embodiments will be described. The following describes modifications of the variable capacitance circuit CI of the first embodiment, but the descriptions below are also applicable to the variable capacitance circuit CI of the second embodiment.

[0055] (Modification 2 of the first embodiment) Next, the configuration of the tunable BPF according to Modification 2 of the first embodiment will be described. Figure 7 is a circuit diagram showing the tunable BPF according to Modification 2. The tunable BPF is configured in the same way as the tunable BPF of the first embodiment, except for the configuration described in Modification 2.

[0056] As shown in Figure 7, each variable capacitance circuit CI may further include a fourth variable capacitance diode D4 and a fifth resistor (fifth electrical resistance) R5. Focusing on the variable capacitance circuit CIa, the fourth variable capacitance diode D4a has a cathode connected to the cathode of the third variable capacitance diode D3a and an anode connected to the other end of the second capacitor C2a. The fifth resistor R5 has one end connected to the eighth node n8a between the other end of the second capacitor C2a and the anode of the fourth variable capacitance diode D4a, and the other end is grounded.

[0057] As described above, the number of stages of the variable capacitance diode D in the variable capacitance circuit CI may be four. The same effects as in the first embodiment can be obtained in this modified example 2 as well.

[0058] (Modification 3 of the first embodiment) Next, the configuration of the tunable BPF according to Modification 3 of the first embodiment will be described. Figure 8 is a circuit diagram showing the tunable BPF according to Modification 3. The tunable BPF is configured in the same way as the tunable BPF of Modification 2 described above, except for the configuration described in Modification 3.

[0059] As shown in Figure 8, each variable capacitance circuit CI may further include a fifth variable capacitance diode D5 and a sixth resistor (sixth electrical resistance) R6. Focusing on the variable capacitance circuit CIa, the fifth variable capacitance diode D5a has an anode connected to the anode of the fourth variable capacitance diode D4a and a cathode connected to the other end of the second capacitor C2a. The sixth resistor R6 is connected between the ninth node n9a, which is between the other end of the second capacitor C2a and the cathode of the fifth variable capacitance diode D5a, and the other end of the first inductor L1a.

[0060] As described above, the number of stages of the variable capacitance diode D in the variable capacitance circuit CI may be five or more. The same effects as in the first embodiment can be obtained in this modified example 3 as well.

[0061] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. [Explanation of Symbols]

[0062] IO1…First input / output terminal, IO2…Second input / output terminal, TP…Voltage supply terminal, LP…Low-pass filter, C1…First capacitor, L1,L2…Inductors, CI…Variable capacitance circuit, C2~C5…Capacitors, D1~D5…Variable capacitance diodes, R1~R6…Resistors, RE…Resonator, SL…Microstrip line, SC…Strip conductor, GC…Ground conductor, n…Node, Vtune…Bias voltage.

Claims

1. First input / output terminal and, Second input / output terminal and, A plurality of resonators having a first resonator and a second resonator, Multiple variable capacitance circuits connected one-to-one to the aforementioned multiple resonators, Voltage supply terminals and A plurality of first inductors, each of which has one end connected to the voltage supply terminal and the other end connected to one of the plurality of variable capacitance circuits, The system comprises a first capacitor having one end connected to a first node between one end of the plurality of first inductors and the voltage supply terminal, and the other end being grounded. Each of the resonators is formed by a microstrip line including a ground conductor and a strip conductor connected to the ground conductor. The strip conductor of the first resonator is connected to the first input / output terminal, The strip conductor of the second resonator is connected to the second input / output terminal, Each of the aforementioned variable capacitance circuits is, First variable capacitance diode, A second variable capacitance diode having an anode connected to a ground wire and a cathode connected to the cathode of the first variable capacitance diode, A first electrical resistor connected between the second node between the cathode of the first variable capacitance diode and the cathode of the second variable capacitance diode, and the other end of the first inductor, A second capacitor having one end connected to the strip conductor and the other end, A third variable capacitance diode having an anode connected to the anode of the first variable capacitance diode and a cathode connected to the other end of the second capacitor, A second electrical resistor is connected between the third node between the other end of the second capacitor and the cathode of the third variable capacitance diode, and the other end of the first inductor. A third electrical resistor has one end connected to a fourth node between the anode of the first variable capacitance diode and the anode of the third variable capacitance diode, and the other end is grounded. The signal input to one of the first input / output terminals and the second input / output terminal has a passband frequency adjusted by the variable capacitance circuit, which is then passed through and output from the other of the first input / output terminals and the second input / output terminals. Tunable bandpass filter.

2. The anode of the second variable capacitance diode is directly connected to the ground wire. The tunable bandpass filter according to claim 1.

3. Each of the aforementioned variable capacitance circuits is, A third capacitor having one end connected to the anode of the second variable capacitance diode and the other end connected to the ground wire, The fourth electrical resistor further comprises one end connected to a fifth node between the anode of the second variable capacitance diode and the one end of the third capacitor, and the other end being grounded. The tunable bandpass filter according to claim 1.

4. Each of the aforementioned variable capacitance circuits is, A fourth variable capacitance diode having a cathode connected to the cathode of the third variable capacitance diode and an anode connected to the other end of the second capacitor, The fifth electrical resistor further comprises one end connected to the eighth node between the other end of the second capacitor and the anode of the fourth variable capacitance diode, and the other end being grounded. The tunable bandpass filter according to claim 1.

5. First input / output terminal and, Second input / output terminal and, A plurality of resonators having a first resonator and a second resonator, Multiple variable capacitance circuits connected one-to-one to the aforementioned multiple resonators, Voltage supply terminals and A first capacitor having one end connected to the first node between the plurality of variable capacitance circuits and the voltage supply terminal, and the other end being grounded, The first low-pass filter, It includes a second low-pass filter, Each of the resonators is formed by a microstrip line including a ground conductor and a strip conductor connected to the ground conductor. The strip conductor of the first resonator is connected to the first input / output terminal via the first low-pass filter, The strip conductor of the second resonator is connected to the second input / output terminal via the second low-pass filter. The first low-pass filter is, A first inductor connected between the first input / output terminal and the strip conductor of the first resonator, A fourth capacitor having one end connected to the sixth node between the first input / output terminal and the first inductor, and the other end being grounded, The second low-pass filter is, A second inductor connected between the second input / output terminal and the strip conductor of the second resonator, A fifth capacitor having one end connected to the seventh node between the second input / output terminal and the second inductor, and the other end being grounded, Each of the aforementioned variable capacitance circuits is, First variable capacitance diode, A second variable capacitance diode having an anode connected to a ground wire and a cathode connected to the cathode of the first variable capacitance diode, A first electrical resistor connected between the second node between the cathode of the first variable capacitance diode and the cathode of the second variable capacitance diode, and the voltage supply terminal, A second capacitor having one end connected to the strip conductor and the other end, A third variable capacitance diode having an anode connected to the anode of the first variable capacitance diode and a cathode connected to the other end of the second capacitor, A second electrical resistor is connected between the third node between the other end of the second capacitor and the cathode of the third variable capacitance diode, and the voltage supply terminal. A third electrical resistor has one end connected to a fourth node between the anode of the first variable capacitance diode and the anode of the third variable capacitance diode, and the other end is grounded. The signal input to one of the first input / output terminals and the second input / output terminal has a passband frequency adjusted by the variable capacitance circuit, which is then passed through and output from the other of the first input / output terminals and the second input / output terminals. Tunable bandpass filter.

6. The anode of the second variable capacitance diode is directly connected to the ground wire. The tunable bandpass filter according to claim 5.

7. Each of the aforementioned variable capacitance circuits is, A third capacitor having one end connected to the anode of the second variable capacitance diode and the other end connected to the ground wire, A fourth electrical resistor having one end connected to a fifth node between the anode of the second variable capacitance diode and one end of the third capacitor, and the other end being grounded, It further possesses, The tunable bandpass filter according to claim 5.

8. Each of the aforementioned variable capacitance circuits is, A fourth variable capacitance diode having a cathode connected to the cathode of the third variable capacitance diode and an anode connected to the other end of the second capacitor, The fifth electrical resistor further comprises one end connected to the eighth node between the other end of the second capacitor and the anode of the fourth variable capacitance diode, and the other end being grounded. The tunable bandpass filter according to claim 5.

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