JPH052009B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to the configuration of a bandpass filter comprising a resonant line and an input/output line.

[Background of the invention]

A typical configuration example of an unnecessary wave removal bandpass filter is described in Japanese Patent Application Laid-Open No. 17900/1983. This bandpass filter consists of an input/output line and a resonant line terminated at one end with a capacitor, and tunes at the signal frequency and removes the image signal. A bandpass filter needs to have low insertion loss and sufficiently suppress unnecessary waves. Generally, in a bandpass filter consisting of an input/output line and a resonant line, in order to suppress the transmission of unnecessary waves due to direct coupling between the input/output line, the coupling between the input/output line and the resonant line is made loose, or the resonant line is connected in multiple stages. The configuration is such that it can be used for However, such a configuration has the disadvantage that insertion loss becomes large.
Furthermore, a configuration in which multiple resonant lines are used has the disadvantage that the circuit becomes large.

[Purpose of the invention]

An object of the present invention is to provide a fixed-frequency or variable-frequency bandpass filter that is small, has low loss, and can sufficiently suppress unnecessary waves.

[Summary of the invention]

In order to achieve the above object, in the bandpass filter of the present invention, the input circuit is configured with a trap circuit in which a capacitor is connected in series to an input coupling inductance, and a parallel resonant circuit in which a capacitor is connected in parallel to the trap circuit. . Select the center frequency of the trap circuit to be lower than the passband center frequency,
By making the resonant frequency of the parallel resonant circuit equal to the pass band center frequency, a band pass filter that can sufficiently suppress unnecessary waves with low insertion loss is obtained.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail according to embodiments shown in the drawings. FIG. 1 shows an embodiment of the present invention. The circuit of the present invention consists of an input circuit, a resonant circuit, and an output circuit, and transmits signals through inductive coupling. The input circuit includes a capacitor 13 and a variable capacitance diode 9 connected in series to the input terminal 1, and an inductance 3 with one end grounded.
and a variable capacitance diode 10 whose input end of the variable capacitance diode 9 is grounded. The resonant circuit consists of inductances 4 and 5 and variable capacitance diodes 7 and 8, both ends of which are grounded. Also, terminal 1
4, a DC voltage is applied via high resistances 11 and 12 to change the resonance frequency. Furthermore, the output circuit consists of an inductance 6, one end of which is grounded, and a signal is taken out from the output terminal 2. Generally, in a band pass filter, an input circuit is constructed by arranging an input coupling inductance close to a resonant circuit as shown in FIG. In the bandpass filter of the present invention, a trap circuit consisting of an inductance 3 of the input coupling line and a variable capacitance diode 9 connected in series with the inductance 3, and a variable capacitance diode 10 connected in parallel to the trap circuit are used to reduce the inductance of the trap circuit. min and variable capacitance diode 10
The input circuit consists of a parallel resonant circuit consisting of At this time, inductance 3 and variable capacitance diode 9
The trap frequency of the trap circuit is set to be lower than the tuning frequency of the bandpass filter, and the inductance 3 is set so that the resonant frequency of the parallel resonant circuit of the inductance of the trap circuit and the variable capacitance diode 10 is equal to the tuning frequency of the bandpass filter. and select variable capacitance diodes 9 and 10. Thereby, it is possible to obtain a bandpass filter with a small insertion loss and a large unnecessary wave removal effect without increasing the insertion loss.

Figure 3 shows a characteristic comparison diagram with a conventional circuit. 16
is the characteristic of a conventional bandpass filter, and 17 is the characteristic of the circuit of the present invention. It is clear that the insertion loss is smaller than in the conventional circuit, and unnecessary waves are suppressed.

FIG. 4 shows the characteristics of the circuit of the present invention shown in FIG. 1. This is a bandpass filter for the 1 to 2 GHz band by appropriately selecting the inductances 4 and 5 of the resonant circuit and the inductance 3 of the input circuit according to the conditions described above. Here, 18 is a center frequency of 950MHz (voltage applied to terminal 14 of 0.5V), 1
9 shows the characteristics of center frequency 1350MHz (applied voltage 4V), 20 shows the characteristics of center frequency 1750MHz (applied voltage 11V), insertion loss 3dB or less, 3dB bandwidth
It has good characteristics of 150MHz. When using this band-pass filter, for example, in a BS tuner of a satellite broadcast receiver, the image signal is
1750MHz, 2150MHz for 19, 2550MHz for 20
Therefore, high attenuation of 45 dB or more can be obtained in both cases. The insertion loss is 2 to 3 dB lower than that of conventional filters, and the image signal compression is improved by about 20 dB.

As described above, in the present invention, the input circuit of the bandpass filter is configured with a low-frequency trap circuit using an input coupling line and a parallel resonant circuit with a resonant frequency equal to the passband center frequency. A bandpass filter with low loss and sufficient suppression of unnecessary waves can be obtained.

I have described the circuit shown in Figure 1 above, but
The present invention is not limited to this. FIG. 5 shows a second embodiment. Basically, the configuration is the same as that shown in FIG. 1, but a low-frequency trap circuit is constructed by an inductance 3 and a fixed capacitor 21 of the input circuit. Further, in the embodiment shown in FIG. 6, the resonant circuit is composed of an inductance 15 and variable capacitance diodes 7 and 8. It is clear that the same effect as the circuit shown in FIG. 1 can be obtained in either case.

The variable bandpass filter using variable capacitance diodes has been described above, but the present invention is the seventh
It is clear that a bandpass filter using fixed capacitance in the resonant circuit and input circuit as shown in the figure has similar effects.

〔Effect of the invention〕

According to the present invention, the input circuit of a bandpass filter consisting of an input/output coupling line and a resonant circuit placed between them is formed by a low-frequency trap circuit using the input-coupling line and a parallel resonant circuit using the low-frequency trap circuit. By configuring this, attenuation can be increased in a band away from the center frequency without increasing the loss at the center frequency of the passband, which is effective in reducing the loss of the bandpass filter and suppressing unnecessary waves.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a bandpass filter according to the present invention, FIG. 2 is a circuit diagram of a conventional circuit, FIG. 3 is a characteristic comparison diagram of the circuit of the present invention and a conventional circuit, and FIG. 4 is a diagram of the circuit of the present invention. Characteristic diagrams showing the characteristics, Fig. 5, Fig. 6,
FIG. 7 is a circuit diagram of another circuit diagram embodiment of the present invention. 3, 4, 5, 6, 15...inductance,
7, 8, 9, 10...variable capacitance diode, 1
3, 21, 22, 23, 24... capacitor.

Claims (1)

[Claims] 1. One end of a circuit in which a fixed or variable capacitor is connected in series to a first inductance is grounded, the other end is used as a signal input terminal, and the signal input terminal is grounded through the fixed or variable capacitor. a first circuit section; a second circuit section in which a second inductance with one end grounded and the other end serving as a signal output terminal is arranged parallel to the first inductance; and a second circuit section arranged parallel to the first inductance; A circuit section in which one end of the inductance of No. 3 is grounded as it is and the other end is grounded via a fixed or variable capacitor, or both ends are grounded via a fixed or variable capacitor is called the first circuit section. and a second circuit section. 2. In the bandpass filter according to claim 1, a circuit in which a fourth inductance arranged in parallel with the first inductance and a fifth inductance arranged in parallel with the second inductance are connected in parallel. A circuit section formed by having one end grounded as it is and the other end grounded via a fixed or variable capacitor, or both ends grounded via a fixed or variable capacitor, is
A bandpass filter, characterized in that it is disposed closely between a circuit section and a second circuit section.