JPH0728164B2 - Tunable bandpass filter - Google Patents

Description

TECHNICAL FIELD The present invention relates to an image filter of a satellite broadcast receiver.

Conventional technology For satellite receivers, input 1 GHz band 1st-I
The F signal is selected and converted into a 400 MHz band 2nd-IF signal. Since the upper side local is generally adopted as the frequency at this time, the image frequency is the 1st IF frequency plus twice the 2nd IF frequency (Reference: Yukihiko Miyamoto "Design and Implementation of High Frequency Circuits", Showa 62) Published by Japan Broadcast Publishing Association). A signal or noise that depends on the image frequency is converted into a 2n dIF signal in the same manner as the 1st IF signal, and thus becomes an interference signal. Therefore, an image filter that suppresses the image frequency is required. Conventionally, LPF (low pass filter) has been adopted as a filter for this purpose.

Hereinafter, the conventional image filter as described above will be described with reference to the drawings. FIG. 3 shows a conventional image filter. In FIG. 3, 50 is an input terminal. 51,53,57,61,63 are chip capacitors,
2,54,55,56,58,59,60,62 are microstrip lines. 64 is an output terminal. 52, 53 and 54 form a resonance circuit. Similarly, 56, 57, 58 and 60, 61, 62 also constitute a resonance circuit. A low pass filter is configured by coupling these resonant circuits with microstrip lines 55 and 59. An example of the frequency characteristic is shown in FIG.

The frequency band of the 1st IF signal for satellite broadcasting in Japan is 1049.48
MHz to 1318 MHz. The second intermediate frequency is 402.78MHz, so the image frequency band is 1720.78MHz to 2123.56MHz
Is. The LPF shown in FIG. 4 has an attenuation characteristic of 40 dB or more in the above-mentioned image frequency band, which is a sufficient characteristic for removing image interference.

However, the above-described configuration has a problem that the 1st IF signal band is widened, and when the number of channels is large, interference of third-order distortion easily occurs in an amplifier or the like in the subsequent stage.

In view of the above problems, the present invention provides a tunable bandpass filter for preventing interference of third-order distortion.

Means for Solving the Problems A tunable bandpass filter of the present invention for solving the above problems uses one terminal of a first micro trip line as an input terminal and the other terminal of a first variable capacitance diode. An anode terminal is connected, a cathode terminal thereof is grounded by a first capacitance, and a control terminal is connected through a first resistor, and a second microstrip line is connected to the other terminal of the first microstrip line. One terminal of a coupled line, which is formed by grounding one terminal of each of the two microstrip lines connected to each other and connected to the other terminal, and connects the other terminal of the coupled line to a third microstrip line. One terminal is connected, and the anode terminal of the second variable capacitance diode is connected to the other terminal. The sword terminal is grounded at a second capacitance and connected to the control terminal through a second resistor, and the other terminal of the third microstrip line is connected to one terminal of a fourth microstrip line. , The other terminal is used as the output terminal.

The present invention has the above-mentioned configuration, and has a first resonant circuit including a first variable capacitance diode, a second microstrip line and a coupled line, a second variable capacitance diode, a third microstrip line and a coupled line. The second resonance circuit consisting of is coupled by a coupled line, and the connection to the input / output terminals is realized by the first and fourth microstrip lines, respectively.

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 depending on the terminal voltage. As a result, the capacitance value of the variable capacitance diode changes. As a result, the tuning frequency of each resonant circuit changes, so that a tunable bandpass filter can be realized. The first and fourth microstrip lines have an input / output impedance of 50Ω.
This is to match with. 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.
If the frequency is selected so as to match the image frequency of the 1st IF signal, a large image suppression of about 50 dB can be realized despite the bandpass filter using two resonant circuits.

Embodiment A tunable bandpass filter according to an 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, and 2,3,4,5,7,16 are microstrip lines. The microstrip lines 4 and 5 are electrically coupled to each other to form a so-called coupled line. The degree of connection is determined by the width, length and spacing of the strip lines. 9 and 10 are variable capacitance diodes whose capacitance is about 4.5PF (at V _T = 1V), 0.9PF
(At V _T = 15V). 11,12 is capacitance,
The value should be sufficiently large (for example, 0.01 μF) to short-circuit at high frequencies. Reference numerals 13 and 14 are resistors for applying the control voltage V _T, and are selected to be 22 KΩ, for example, and are sufficiently large. To explain the configuration of FIG. 1 in more detail, one terminal of the first microstrip line 2 is used as the input terminal 1,
The other terminal is connected to the anode terminal of the first variable capacitance diode 9, and its cathode terminal is connected to the first capacitance.
Control terminal 15 through the first resistor 13 and grounded at 11
Connected to the second microstrip line 3 to the other terminal of the first microstrip line 2, and one terminal of each of the two microstrip lines 4 and 5 connected to the other terminal. Is connected to one terminal of a coupled line, and the other terminal of the coupled line is connected to the third microstrip line 7
One terminal is connected, the other terminal is connected to the anode terminal of the second variable capacitance diode 10, and the cathode terminal is grounded by the second capacitance 12 and the second terminal is connected to the second terminal.
Connected to the control terminal via the resistor 14, the other terminal of the third microstrip line 7 is connected to one terminal of a fourth microstrip line 16, and the other terminal is used as an output terminal 17. There is.

The printed circuit board of the microstrip line is 1mm thick
The case where the glass epoxy base material is double-sided steel is described below as an example of the configuration. The microstrip lines 2 and 16 are equivalent and have a width of 0.3 mm and a length of about 30 mm. The microstrip lines 3 and 7 are also equivalent, with a width of 0.3 mm and a length of about 7 mm. The microstrip lines 4 and 5 that make up the coupled line are both 1.2 mm wide and 7 mm long, and the distance between them is approximately 0.3 mm. In this case, FIG. 2 shows an example of measurement of the tuning characteristic of the variable tuning bandpass filter of this configuration. According to FIG. 2, the bandwidth of the bandpass filter can be set to about 150 MHz and the tuning frequency can be changed by the control voltage, so that the desired characteristics can be obtained. Also, by forming a trap for image suppression in the high frequency range, it is possible to obtain an attenuation of 40 dB or more. However, when this trap is formed, the frequency characteristic rises and the characteristic deteriorates in the frequency band higher than the trap. Outside the band of the first intermediate frequency of satellite broadcasting, the deterioration of the frequency characteristics due to these traps can be improved by adding a low-pass filter, so there is no problem. Rather, it is desirable to positively use traps to enable high image suppression.

EFFECTS OF THE INVENTION As described above, according to the present invention, a tunable bandpass filter having a trap can be configured by a microstrip line, and a large image suppression can be obtained with a simple configuration.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a tunable bandpass filter according to an embodiment of the present invention, FIG. 2 is a frequency characteristic diagram showing an example of measurement results of the embodiment, and FIG. 3 is used for a conventional image filter. FIG. 4 is a circuit diagram of a low-pass filter according to the present invention, and FIG. 4 is a frequency characteristic diagram showing an example of the measurement result. 2,3,7,16 …… Microstrip line, 4,5 …… Coupled line, 9,10 …… Variable capacitance diode, 11,12…
… Capacitance, 13,14… Resistance.

Claims (1)

[Claims] Claim: What is claimed is: 1. One terminal of a first micro stop line is used as an input terminal, the other terminal is connected to the anode terminal of a first variable capacitance diode, and the cathode terminal is connected to the first terminal.
2 connected to the control terminal through the first resistor, connected to the control terminal through the first resistor, connected to the other terminal of the first microstrip line with the second microstrip line, and connected to the other terminal. One microstrip line is connected to one terminal of a coupled line formed by grounding each one terminal, the other terminal of the coupled line is connected to one terminal of a third microstrip line, and the other terminal is connected. Is connected to the anode terminal of the second variable capacitance diode, the cathode terminal thereof is grounded by the second capacitance, and is connected to the control terminal via the second resistor, and the other of the third microstrip lines is connected. One terminal of the fourth microstrip line is connected to the terminal, and the other terminal is connected to the output terminal. A first resonant circuit including the coupled line, the second microstrip line, and the first variable capacitance diode, the coupled line, the third microstrip line, and the second resonant circuit. A second resonance circuit composed of a variable capacitance diode constitutes a double tuning circuit which is coupled by the coupled line and resonates at an input signal frequency, and the second variable circuit is connected from the second microstrip line to the first variable circuit. A path from the capacitance diode and the first capacitance to the ground, and a path from the third microstrip line to the ground via the second variable capacitance diode and the second capacitance Having traps at frequencies corresponding to image frequencies Tunable bandpass filter, wherein.