JP3136867B2 - Tunable bandpass filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image filter in a satellite broadcast receiver.

[0002]

2. Description of the Related Art In a satellite broadcast receiver, an input 1st-IF signal in a 1 GHz band is tuned to a 400M signal.
It is converted into a 2nd-IF signal in the Hz band. As the frequency at this time, the upper local is generally adopted,
The frequency is the 1st-IF frequency plus twice the 2nd-IF frequency (Reference: Yukihiko Miyamoto, "Design and Implementation of High-Frequency Circuits": Published by Japan Broadcasting Publishing Association in 1987). The signal or noise that depends on the image frequency is
As in the case of the first-IF signal, the signal is converted into the second-IF signal, and thus becomes an interference signal. Therefore, an image filter for suppressing the image frequency is required.

[0003] In Japanese satellite broadcasting, the frequency band of the 1st-IF signal is 1049.48 MHz to 1318 MHz. Since the second intermediate frequency is 402.78 MHz,
The image frequency band is 1855.04 MHz to 212.
3.56 MHz. Therefore, an image filter for attenuating the image frequency band of 1855.04 MHz to 2123.56 MHz is required. Also 1st-I
When the F signal band is wide and the number of channels is large, a third-order distortion is likely to be hindered in an amplifier or the like at the subsequent stage. Therefore, a tunable bandpass filter has been employed. As already proposed, Japanese Patent Laid-Open No.
For example, there is JP-A-135111. Hereinafter, the tunable bandpass filter described in Japanese Patent Application Laid-Open No. 3-135111 will be described with reference to the drawings.

FIG. 5 is a circuit diagram of a conventional tunable band-pass filter (Japanese Patent Laid-Open No. 3-135111).
In FIG. 5, reference numeral 18 denotes an input terminal. 19, 23,
24, 25, 26 and 27 are microstrip lines. 20 and 29 are variable capacitance diodes and 21, 31
Is a capacitor, 22 and 30 are resistors, 32 is a control terminal,
28 is an output terminal.

FIG. 6 shows an example of frequency characteristics when the control voltage of the conventional tunable bandpass filter is changed.
FIG. 7 shows a simulation of a conventional tunable bandpass filter.
Shows the performance characteristics.

The operation of the conventional tunable bandpass filter configured as described above will be described below.

A first resonant circuit comprising a first variable capacitance diode 20, a second microstrip line 23, and a coupled line constituted by microstrip lines 24 and 25, and a second variable capacitance diode. 2
9. A second resonance circuit composed of a coupled line constituted by the third microstrip line 26 and the microstrip lines 24 and 25 is coupled by the coupled line, and the connection to the input / output terminal is made by the first and fourth terminals, respectively. This is realized by a microstrip line. When the voltage of the control terminal 32 is equal to the first and second resistors 22 and 3
0, the voltage is applied to the variable capacitance diodes 20 and 29.
The capacitance values of 0 and 29 change. As a result, the tuning frequency of each resonance circuit changes, so that a tunable bandpass filter can be realized.

The first and fourth microstrip lines 19 and 27 are for matching the input / output impedance to 50Ω. The second and third microstrip lines 23 and 26 are for forming traps in order to improve the out-of-band high-frequency characteristics of the bandpass filter. However, from FIG. 6 and FIG. 7, the out-of-band characteristics fluctuate depending on the distance between the first resistor 22 and the second resistor 30 and the variable capacitance diodes 20 and 29, and the first resistor 22 and the second
Is connected to the variable capacitance diodes 20 and 29 via a line having a width of about 0.2 mm and a length of about 15 mm, for example, a chip connected between the variable capacitance diodes 20 and 29 and the ground. The capacitors 21 and 31 have an impedance in which the L component is dominant within the band. Therefore, a width of 0.2 m where the first and second resistors 22 and 30 and the variable capacitance diodes 20 and 29 are connected.
With a line of m and a length of about 15 mm, a high frequency band is raised, and in order to secure an image interference suppression ratio in a high band, it is necessary to attenuate considerably by a low-pass filter in a subsequent stage. .

[0009]

However, in the above configuration, the first resistor 22 at the control terminal 32 is not provided.
The out-of-band characteristics vary depending on the distance between the second resistor 30 and the variable capacitance diodes 20 and 29, and the first resistor 22 and the second resistor 30 are, for example, 0.2 mm wide and 15 mm long.
When connected to the variable capacitance diodes 20 and 29 through the following lines, the attenuation of the image frequency band fluctuates,
As shown in FIGS. 6 and 7, there were cases where the amount of attenuation could not be secured in the high frequency range.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a tunable bandpass filter capable of securing a certain amount of sufficient attenuation in a high frequency range.

[0011]

According to the present invention, there is provided a tunable bandpass filter according to the present invention, in which one terminal of a first microstrip line is used as an input terminal and the other terminal is provided with a first variable capacitance. An anode terminal of the diode is connected, the cathode terminal is grounded with a first capacitance, and a fifth microstrip line is connected to a connection point between the cathode terminal and the first capacitance. The other terminal is connected to a control terminal via a first resistor, the other terminal of the first microstrip line is connected to a second microstrip line, and the other terminal is connected to the other terminal. Connected to one terminal of a coupled line grounded to one terminal of each of the microstrip lines, and the third microstrip line is connected to the other terminal of the coupled line. One terminal of the trip line is connected, the other terminal is connected to the anode terminal of a second variable capacitance diode, and the cathode terminal is grounded by a second capacitance, and the cathode terminal is connected to the other terminal. A sixth microstrip line is connected to a connection point between the third microstrip line and a control terminal via a second resistor, and the other terminal of the third microstrip line is connected to the other terminal of the third microstrip line. Is connected to one terminal of a fourth microstrip line, and the other terminal is used as an output terminal.

[0012]

According to the present invention, a first resonance circuit comprising a first variable capacitance diode, a second microstrip line, and a coupled line constituted by a microstrip line, and a second variable capacitance are provided. A second resonant circuit comprising a diode, a third microstrip line, and a coupled line constituted by the microstrip line is coupled by the coupled line, and the connection to the input / output terminal is made by the first and fourth microstrip lines, respectively. It is realized by the line.

Since the voltage of the control terminal is applied to the variable capacitance diode via the first and second resistors, the capacitance value of the variable capacitance diode changes according to the terminal voltage. As a result, the tuning frequency of each resonance circuit changes, so that a tunable bandpass filter can be realized. First
And the fourth microstrip line is for matching the input / output impedance to 50Ω. The second and third microstrip lines are for forming traps in order to improve the out-of-band high-frequency characteristics of the bandpass filter. The frequency is selected to match the image frequency of the 1st-IF signal, and if a microstrip line of an appropriate length is inserted between the first and second resistors and the control terminal, two frequencies are obtained. Despite the tunable bandpass filter using the resonance circuit, 4
A large image suppression of about 0 dB can be realized.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of a tunable bandpass filter according to an embodiment of the present invention. In FIG. 1, 1 is an input terminal, 2, 3, 4, 5, 11, 1
2, 8, 15 are microstrip lines, 6, 14 are variable capacitance diodes, 7, 16 are capacitors, 9, 1
7 is a resistor, 10 is a control terminal, and 13 is an output terminal.

FIG. 2 shows an example of frequency characteristics when the control voltage of the tunable bandpass filter of the present invention is changed. FIG. 3 shows an example of the simulation characteristics of the tunable bandpass filter of the present invention. FIG. 4 shows an example of frequency characteristics when the control voltage of the tunable bandpass filter of the present invention is varied. The operation of the tunable bandpass filter of the present invention configured as described above will be described.

The microstrip lines 4 and 5 are electrically coupled to form a so-called coupled line. The width is 1.2mm each, and the interval is
0.3 mm and length were each selected to be 7 mm. The degree of coupling is determined by the width, length and spacing of each microstrip line and the microstrip lines 8 and 15. Reference numerals 6 and 14 denote variable capacitance diodes having a capacitance of approximately 4.5 pf (when Vt = 1 V) and 0.9 pf (Vt = 1).
5v). Reference numerals 7 and 16 denote capacitors whose values are set to 1 in consideration of tracking characteristics with the voltage controlled oscillator.
I chose 5pf. Reference numerals 9 and 17 denote resistors for applying the control voltage Vt.

1 will be described in more detail. One terminal of the first microstrip line 2 is an input terminal 1 and the other terminal is a first variable capacitance diode.
The anode terminal of node 6 is connected, and its cathode terminal is connected to the first terminal.
And a fifth microstrip line 8 at the connection point between the cathode terminal of the first variable capacitance diode 6 and the first capacitance 7.
The other terminal is connected to the control terminal 6 via the first resistor 9, the other terminal of the first microstrip line 2 is connected to the second microstrip line 3, and the other terminal is connected to the other terminal. One terminal of each of the two microstrip lines 4 and 5 coupled to the terminal is connected to one terminal of a coupled line 40 grounded, and the other terminal of the coupled line 40 is connected to a third microstrip line 11. Is connected to the other terminal, the anode terminal of the second variable capacitance diode 14 is connected, the cathode terminal is grounded by the second capacitance 16, and the cathode terminal is connected to the other terminal. A sixth microstrip line 15 is connected to a connection point between the ground terminal and the second capacitance 16, and the other terminal is connected to the control terminal 10 via a second resistor 17. Wherein the third other terminal of the microstrip line 12 is connected to one terminal of the fourth microstrip line, it outputs the other terminal pin 1
It is set to 3.

The width of the first and third microstrip lines 2 and 11 was selected to be 0.3 mm and the length thereof was selected to be 16 mm. Second and fourth microstrip lines 3, 1
2, the width was selected to be 0.3 mm and the length was selected to be 7 mm. The widths of the fifth and sixth microstrip lines 8 and 15 were selected to be 0.3 mm and the length was set to 5 mm. Therefore, the chip capacitors 7 and 16 connected between the variable capacitance diodes 6 and 14 and the ground,
Even if the impedance becomes dominant, the L component of the fifth and sixth microstrip lines 8 and 15 is small, so that the high frequency band can be sufficiently attenuated, and the low-pass As shown in FIG. 4, the high frequency band could be sufficiently attenuated together with the filter.

As described above, according to the present invention, a tunable band-pass filter having a trap is constituted by a microstrip line, and even at a high frequency band,
A sufficient image frequency band can be attenuated with a simple configuration without lifting.

[0021]

As described above, according to the present invention, if the variable capacitance diode and the resistor connected to the control terminal are connected by a microstrip line of an appropriate length, a high frequency band can be obtained. Even at the above frequency, a sufficient image frequency band can be attenuated with a simple configuration without raising.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of a tunable bandpass filter according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a frequency characteristic of the tunable bandpass filter.

FIG. 3 is a diagram showing an example of a simulation characteristic of the tunable bandpass filter.

FIG. 4 is a diagram showing an example of frequency characteristics when a control voltage of the tunable bandpass filter is varied.

FIG. 5 is a circuit configuration diagram of a conventional tunable bandpass filter.

FIG. 6 is a diagram showing frequency characteristics of a conventional tunable bandpass filter.

FIG. 7 is a diagram showing simulation characteristics of a conventional tunable bandpass filter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Input terminal 2, 3, 4, 5, 11, 12, 8, 15 Microstrip line 6, 14 Variable capacitance diode 7, 16 Capacitor 9, 17 Resistance 10 Control terminal 13 Output terminal

Continuation of the front page (72) Inventor Hiroshi Ito 1006 Kazuma Kadoma, Kazuma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-1-135211 (JP, A) (58) Fields investigated (Int .Cl. ⁷ , DB name) H04B 1/10-1/26 H03H ⁷ /00-7/12 H03J 3/24-3/28

Claims (1)

(57) [Claims] A first terminal having one terminal connected to an input terminal and the other terminal connected to an anode terminal of a first variable capacitance diode;
And a cathode terminal of the first variable capacitance diode and the other end of the first capacitance having one end grounded to one terminal, and the other terminal connected to a control terminal via a first resistor. And a fifth microstrip line connected to the first microstrip line and one terminal connected to the first microstrip line, and the other terminal grounded to one terminal of each of the two microstrip lines coupled to each other. A second microstrip line connected to one terminal of the coupled line, and a second microstrip line connected to one terminal of the coupled line and the other terminal connected to the anode terminal of the second variable capacitance diode. 3 microstrip lines and the second
A sixth microstrip in which the cathode terminal of the variable capacitance diode and the other end of the second capacitance, one end of which is grounded, are connected to one terminal, and the other terminal is connected to the control terminal via a second resistor. A tunable bandpass filter, comprising: a line; and a fourth microstrip line having one terminal connected to the terminal of the third microstrip line and the other terminal serving as an output terminal.