JP5682888B2 - Tunable high frequency bandpass filter - Google Patents

Description

  The present invention relates to a band pass filter that passes a high-frequency signal in a specific frequency band among frequency components of a high-frequency signal and attenuates other high-frequency signals. Specifically, the band-pass characteristics are tunable, and in particular, in frequency domain communications where the frequency used in mobile communications is overcrowded, it can be applied to cognitive radio communications and software defined radio communications that perform communications by arbitrarily selecting the operating frequency. A tunable high-frequency bandpass filter.

  In recent years, mobile communication systems in the gigahertz band (1 to 5 GHz) have been developed, and there has been a demand for microwave integrated circuits and monolithic microwave integrated circuits for high-frequency circuits that can handle the band. It is difficult to make an inductor element that can deal with a frequency range of GHz and generally has a large Q value (a dimensionless number that mainly represents the state of vibration). Therefore, the band-pass filter in the GHz band requires a different configuration from that of the LC resonator. For example, a λ resonator, a resonator electrode for operating as a dual mode bandpass filter, or the like is used (Patent Documents 1 to 4).

In order to effectively use a frequency that is a finite resource, a filter having a steep skirt characteristic is required. A band pass filter in a wireless system is an indispensable part for selecting the frequency of received radio waves and the frequency of transmitted radio waves.
However, the bandpass filter has a difficulty in increasing the insertion loss when the passband is narrowed.

Conventionally, a filter using a dielectric resonator has been used as a band pass filter. According to the dielectric resonator filter, a filter characteristic having a narrow pass band, a small insertion loss, and a high load Q value can be obtained.
However, if the dielectric constant is increased to reduce the size of the device, the temperature characteristics of the dielectric resonator become sensitive and the resonance frequency varies with temperature. In addition, since the resonance frequency fluctuates depending on the pressure, the adjustment has to rely on experience and intuition, which causes manufacturing difficulty and cost increase.

  Patent Documents 1 to 4 also disclose a single-wavelength resonator, a resonator electrode for operating as a dual-mode bandpass filter, and the like, but the conventional band-pass filter achieves sufficient miniaturization. In addition, there were problems in terms of performance reproducibility and manufacturing costs.

JP 2007-68123 A JP 2004-134894 A JP 2004-135102 A JP 2004-349960 A

  Therefore, the present invention is based on performing a perturbation analysis on the hairpin resonator tap feed filter and examining the attenuation pole and the matching pole, and is easy to manufacture and inexpensive, but with low loss and narrow bandwidth. It is an object of the present invention to provide a tunable high-frequency bandpass filter that is small in size and whose pass center frequency can be arbitrarily changed and can be applied to cognitive radio and software radio.

  In order to solve the above problems, a tunable high-frequency bandpass filter of the present invention has the following configuration. That is, a high-frequency circuit board having a ground layer formed on the lower surface of the dielectric, a resonator having a microstrip line provided on the upper surface of the high-frequency circuit board, and a high-frequency circuit that is electrically connected to the microstrip line. In a high-frequency bandpass filter having an input / output line for output, two hairpin-shaped microstrip lines having approximately a half wavelength of the pass center frequency are used, and the microstrip line is connected from an input line for inputting a high frequency. Complementing the relationship between the two lengths to both open ends of the wire and the two lengths from the connection position with the output line that outputs high frequency to both open ends of the microstrip line, the connection to the high frequency input line The lengths of the two microstrip lines from the position to the connection position with the high-frequency output line are made equal, By allowing the free ends to oppose each other through a predetermined gap, the open ends are electrically coupled to generate electric field coupling at the resonance frequency, and two matching frequencies are generated between the two attenuation frequencies to obtain band pass characteristics. Thus, a capacitive load is provided at each of the connection position with the input line and the connection position with the output line, and the value of the passing center frequency is controlled by the capacitance value.

  Here, a chip capacitor having a variable capacitance may be used for the capacitive load provided at the connection position with the input / output line.

  A varactor diode having a variable capacitance may be used for the capacitive load provided at the connection position with the input / output line.

  An input / output line for inputting / outputting a high frequency may be tapped to an arbitrary position of the microstrip line, and the attenuation frequency may be adjusted depending on the connection position.

  The attenuation frequency may be adjusted by the gap between the open ends of the two microstrip lines.

  The attenuation frequency may be adjusted by the length of the side surfaces of the two open ends facing each other of the two microstrip lines.

  The attenuation frequency may be adjusted according to the line width of the two microstrip lines.

  The passing center frequency may be determined by the following equation.

  Where Z is the characteristic impedance of the microstrip line, Z0 is the external impedance, Cg is the capacitance value between the open ends of the microstrip line, Ct is the capacitance value of the capacitive load that controls the matching frequency, and ΔCt is the capacitance of the capacitive load The amount of change in value, ω 0 is the angular frequency when there is no capacitive load, and f is the pass center frequency.

  The conductor may be thickened until the two matching poles are integrated to reduce the passage loss.

  PTFE, glass epoxy, or alumina may be used for the high-frequency circuit board.

  A copper foil may be used for the conductor.

  The gap between the open ends of the two microstrip lines may be 1 to 20 mm.

  According to the present invention, since the microstrip line is used, it is easy to manufacture and is small and inexpensive, but the performance is stable, and electrical coupling is generated between the open ends of the two microstrip lines. It is possible to provide a filter having a low loss and a narrow bandpass characteristic generated during Then, the value of the pass center frequency is controlled by the capacitance value of the capacitive load provided at the connection position with the input / output line, thereby realizing a tunable band pass filter.

Explanatory drawing which shows the tunable bandpass filter of the Example of this invention The graph which shows the characteristic of the tunable high frequency band pass filter of the Example of this invention Circuit diagram of an embodiment of a bandpass filter according to the invention Same as above, graph showing frequency characteristics Similarly, a graph showing changes in frequency characteristics when the value of the tuning capacitor Ct is changed Same as above, a graph showing the change of the passing center frequency when the value of the tuning capacitor Ct is changed

  Hereinafter, embodiments of the present invention will be described based on examples shown in the drawings. It should be noted that the design of the embodiment can be appropriately changed by using a conventionally known technique.

FIG. 1 is an explanatory diagram showing a tunable high-frequency bandpass filter according to an embodiment of the present invention.
The bandpass filter of the present embodiment is formed on the upper surface of the dielectric of the high frequency circuit board in which the ground layer is formed on the lower surface of the dielectric. Since the high-frequency circuit board is well known, it is not shown here. In this band pass filter, a plurality of pairs of λ / 2 microstrip line resonators formed by coupling two λ microstrip line resonators with a microstrip line having a length of λ / 2 are arranged. One λ / 2 microstrip line resonator in the pair of λ / 2 microstrip line resonators and one λ / 2 microstrip line resonator in the other pair of λ / 2 microstrip line resonators Edge joined. By making these open ends face each other through a predetermined gap, the open ends are electrically coupled to generate electric field coupling at the resonance frequency, and two matching frequencies are generated between the two attenuation frequencies to pass the band. The characteristics can be obtained.
Thus, since the bandpass filter of this embodiment is a parallel coupled stripline filter, a plurality of λ / 2 microstripline resonators can be arranged in an oblique direction, so that the filter shape is linear. It is advantageous for miniaturization in that it can be prevented from being stretched.

Capacitive loads are provided at the connection position with the input line and the connection position with the output line, respectively, and the value of the passing center frequency is controlled by the capacitance value.
As the capacitive load provided at the connection position with the input / output line, a variable-capacitance chip capacitor or varactor diode can be used.

FIG. 2 is a graph showing characteristics of the tunable high-frequency bandpass filter according to the embodiment of the present invention.
This shows the transfer characteristic assuming 2 GHz as the pass center frequency, and S21 shows the pass characteristic.
It is a graph when the capacitive load Ct provided at the connection position with the input / output line is changed to 0.1 to 3.1 pF. It can be seen that the passing center frequency decreases as the capacitive load increases.

  The passing center frequency is determined by the following equation.

  Where Z is the characteristic impedance of the microstrip line, Z0 is the external impedance, Cg is the capacitance value between the open ends of the microstrip line, Ct is the capacitance value of the capacitive load that controls the matching frequency, and ΔCt is the capacitance of the capacitive load The amount of change in value, ω 0 is the angular frequency when there is no capacitive load, and f is the pass center frequency.

  In order to adjust the attenuation frequency, an input / output line for inputting and outputting a high frequency is configured to be tapped at an arbitrary position of the microstrip line, and the attenuation frequency can be adjusted depending on the connection position.

  Similarly, the gap between the open ends of the two microstrip lines, the length of the side surfaces of the two open ends facing each other of the two microstrip lines, and the length of the side surfaces of the two open ends of the two microstrip lines facing each other The attenuation frequency can also be adjusted by the line widths of the two microstrip lines.

  In order to reduce the passage loss, a configuration in which the conductor is thickened until the two matching electrodes are integrated can be used.

  The gap between the open ends of the two microstrip lines is 1 to 20 mm, PTFE, glass epoxy, alumina as a high-frequency circuit board, and copper foil as a conductor can be suitably used.

  FIG. 3 is a circuit diagram of an embodiment of the band-pass filter according to the present invention, FIG. 4 is a graph showing the frequency characteristics, and FIG. 5 is a change in frequency characteristics when the value of the tuning capacitor Ct is changed. FIG. 6 is a graph showing changes in the pass center frequency when the value of the tuning capacitor Ct is changed.

  The design of the passing center frequency of the circuit of the example is 1.99979 GHz according to the following formula, which is almost the same as the circuit simulation value of 2.002 GHz, and a steep skirt characteristic was obtained as shown in FIG. .

  As shown in FIGS. 5 and 6, when the tuning capacitor Ct was changed, only the passing center frequency was changed while the attenuation pole frequency was not changed. The capacitance of the capacitor Ct and the passing center frequency have a linear relationship.

  According to the high-frequency bandpass filter of the present invention, it is easy to manufacture, is small and inexpensive, has stable performance, has low loss and narrow bandpass characteristics, and near the boundary between the signal passband and the stopband. It can also support UWB standards where it is desired that the signal pass rate change is steep, mobile radio communication systems with many frequency allocations, mobile base stations, terrestrial digital broadcast base stations, terrestrial digital broadcast repeaters, radio communication equipment, It can be used in various situations such as communication systems such as mobile phone terminals, various measuring devices suitable for distance measurement, and wireless devices for traffic systems such as aircraft where interference with adjacent channels is a problem.

Claims (12)

A high-frequency circuit board having a ground layer formed on the lower surface of the dielectric, a resonator having a microstrip line provided on the upper surface of the high-frequency circuit board, and a high-frequency circuit that is electrically connected to the microstrip line In a high-frequency bandpass filter comprising an input / output line,
Using two hairpin-shaped microstrip lines that are approximately half the wavelength of the center frequency of the pass, two lengths from the connection position to the input line that inputs high frequency to both open ends of the microstrip line, and output that outputs high frequency Two microstrips from the connection position to the high-frequency input line to the connection position to the high-frequency output line are complemented by the relationship between the two lengths from the connection position to the line to both open ends of the microstrip line. By equalizing the lengths of the lines and making their open ends face each other with a predetermined gap, the open ends are electrically coupled to generate electric field coupling at the resonance frequency, and the two matching frequencies are attenuated by two. Generate between the frequencies to get the bandpass characteristics,
The connection position of the connecting position of the input line and an output line, each provided with a capacitive load, by varying the capacitance value, wherein the attenuation frequency Ru changeable der the passband center frequency without changing Tunable high frequency band pass filter. The tunable high-frequency bandpass filter according to claim 1, wherein the capacitive load provided at the connection position with the input / output line is a chip capacitor having a variable capacitance. The tunable high-frequency bandpass filter according to claim 1, wherein the capacitive load provided at the connection position with the input / output line is a varactor diode having a variable capacitance. The tunable high-frequency bandpass filter according to any one of claims 1 to 3, wherein an input / output line for inputting / outputting a high frequency is tapped at an arbitrary position of the microstrip line, and the attenuation frequency is adjusted by the connection position. The tunable high-frequency bandpass filter according to any one of claims 1 to 4, wherein an attenuation frequency is adjusted by a gap between open ends of two microstrip lines. The tunable high-frequency bandpass filter according to any one of claims 1 to 5, wherein the attenuation frequency is adjusted by the lengths of the side surfaces of two opposed open ends of two microstrip lines. The tunable high-frequency bandpass filter according to any one of claims 1 to 6, wherein an attenuation frequency is adjusted by a line width of two microstrip lines. Passing center frequency is

(Where Z is the characteristic impedance of the microstrip line, Z0 is the external impedance, Cg is the capacitance value between the open ends of the microstrip line, Ct is the capacitance value of the capacitive load that controls the matching frequency, and ΔCt is the capacitive load. (The amount of change in capacitance value, ω0 is the angular frequency when there is no capacitive load, and f is the pass center frequency.)
The tunable high-frequency bandpass filter according to any one of claims 1 to 7, defined by: The tunable high-frequency bandpass filter according to any one of claims 1 to 8, wherein the conductor is thickened until the two matching poles are integrated to reduce the passage loss. The tunable high-frequency bandpass filter according to any one of claims 1 to 9, wherein the high-frequency circuit board is PTFE, glass epoxy, or alumina. The tunable high-frequency bandpass filter according to any one of claims 1 to 10, wherein the conductor is a copper foil. The tunable high-frequency bandpass filter according to any one of claims 1 to 11, wherein a gap between open ends of two microstrip lines is 1 to 20 mm.

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