JPH0478048B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to a filter circuit device, and particularly to a printed circuit device equipped with a filter circuit for selecting a high frequency signal. The present invention relates to a filter circuit device that obtains attenuation characteristics) and impedance characteristics.

[Technical background of the invention]

In general, the tuner of a television receiver or
In a high frequency receiving device such as a tuner for CATV (CABLE TELEVISION), a filter circuit is arranged at the high frequency input section in order to cope with interference including cross modulation interference and beat interference. Since this filter circuit is located at the input stage of the receiving device, it has a large influence on the interference characteristics, noise figure, impedance characteristics, etc. Therefore, for such a filter circuit, it is desirable that the pass frequency band has as little loss as possible and the impedance characteristics are constant, and that the attenuation frequency band has as much attenuation as possible. For this reason, for example, in a tuner for a television receiver, as shown in FIG. 1, an adjustment signal generator 3 is connected to the input terminal 2 of the tuner 1, and the adjustment signal is passed through the filter circuit 4 of the tuner 1. Furthermore, after passing through a filter load 5 such as an amplifier or a mixer, the output is measured using a detection device 6 and adjusted so that the pass loss characteristics and impedance characteristics of the filter circuit portion are optimized.

[Problems with background technology]

By the way, in a high frequency receiving device such as a television receiver tuner or a CATV converter tuner using a super-heterodyne method or a double super-heterodyne method, when the above-mentioned filter circuit is composed of a wideband filter, the back - There is a problem that an interference signal called a talk component is generated in the mixer, and this back talk component leaks to the input terminal. Next, back
The generation process of the talk component will be explained with regard to a tuner for a CATV converter.

FIG. 2 is a block diagram showing the main parts of a tuner for a CATV converter, and a characteristic diagram showing the pass frequency band of its input filter circuit portion.

As shown in Figure 2a, the received signal is input to input terminal 7.
The signal that has passed through the input filter circuit 8 is mixed with the local oscillation signal from the local oscillation circuit 10 in the mixer 9 and converted into an intermediate frequency signal. After this intermediate frequency signal further passes through an intermediate frequency band filter circuit 11,
Output to the next stage circuit.

In such a configuration, the signal frequency received in CATV broadcasting is 55 to 440 MHz (reception channel 58CH transmission), and the signal is introduced into the input terminal 7 of the converter/tuner. As shown in FIG. 2b, the input filter circuit 8 has all of the above transmission frequency bands as a pass band.
All of the above transmission signals are input to the mixer 9 at the next stage. On the other hand, a prescribed local oscillation frequency signal is injected from the local oscillation circuit 10 into the mixer 9, and a predetermined intermediate frequency signal is obtained, but in reality, the intermediate frequency band filter circuit 1, which is the load of the mixer
1, an intermediate frequency signal is obtained. Therefore, inside the mixer, all the frequencies mentioned above are frequency-converted by the local oscillation frequency signal. In this case, the frequency characteristics of the intermediate frequency band filter circuit 11 are usually designed to be steep, so that almost no frequency components other than the desired signal are output to the next stage circuit.

However, the intermediate frequency band filter circuit 11
Frequency components reflected by (back/talk components)
travels in the reverse direction through the mixer 9 and reaches the input filter circuit 8. If the reflected frequency component is in the passband of the input filter circuit 8, that signal component appears at the input terminal 7 of the converter tuner and is further transmitted to the signal supply cable, causing interference to many receiving equipment. Become.

Next, the above matters will be explained using mathematical formulas.

The input frequency range reaching the mixer 9 is defined as fd ₁ ~
fd ₂ , corresponding to this frequency, the local oscillation frequency range for converting to an intermediate frequency in the mixer 9 is fo ₁
~ fo ₂ , a certain reception frequency is fdx, the local oscillation frequency at that time is fox, the intermediate frequency is f _IF , and the backtalk frequency is fu, then the frequency conversion of the desired signal is expressed as f _IF = fox − fdx , the frequency conversion of the interference signal (back-talk component) is expressed as fu=fox±fd ₁ to fox±fd ₂ . Therefore, when receiving a certain channel, frequency-converted components are generated between fox−fd ₁ and fox+fd ₂ , and among these, a signal component that passes within the passband of the input filter circuit 8 is generated. However, interference actually occurs as a back-talk component. Furthermore, since the local oscillation frequency changes from fo ₁ to _{fo 2} depending on the receiving frequency, countless interference signals are generated.

Figure 3 shows the reception frequency range 55~440MHz, local oscillation frequency range 667~1052MHz, and intermediate frequency 612M.
FIG. 3 is an explanatory diagram illustrating the generation of back-talk components in the case of Hz.

In Figure 3, part A is the signal frequency range generated by the sum of the local oscillation frequency and the input frequency, part B is the signal frequency range generated by the difference between the local oscillation frequency and the input frequency, and the diagonally shaded range in part C. is the range within the portion B that falls within the band of the received signal, ie, the range where the backtalk component appears at the input terminal 7. Further, the straight line indicated by symbol D indicates the frequency at which at least the input filter circuit 8 must be in the passband, and the symbol E indicates the receiving frequency range,
This shows that frequencies below the straight line D can be set as a pass band, and frequencies above the straight line D can be set as an attenuation band. Therefore, when the reception frequency is 270 MHz or less, the shaded area is the range that passes through the passband of the input filter circuit 8 as a backtalk component, and is the area where it appears at the input terminal 7 and causes interference.

However, for example, if the reception frequency is 55MHz, the interference generation frequency will be 230MHz or higher, and there is a certain frequency difference between the reception frequency and the interference signal frequency, so using a variable attenuation filter will counteract the interference. will be possible.

FIG. 4 is a circuit diagram showing an example of the input filter circuit 8 using a variable attenuation filter.

In FIG. 4, the filter circuit is constructed by combining an input band pass filter whose passband is the entire reception frequency band and a variable attenuation filter. The received signal is input to the input terminal 12 of the converter/tuner, and the capacitors C ₁ to _{C 3} ,
A high pass filter consisting of coils _L1 to _L2 ,
After passing through a low pass filter consisting of capacitors C ₄ to C ₁₀ and coils L ₃ to _{L 5} , the coils L ₆ to
L ₈ , a resistor R ₁ , a capacitor C ₁₁ and a variable capacitance element 13 such as a varactor diode. A control voltage is applied to a terminal 15 of the varactor diode 13 of the variable attenuation filter to vary the capacitance.

In this way, by configuring the input filter circuit using a variable attenuation filter, it is possible to attenuate the interference component frequency. However, since this circuit includes a varactor diode whose impedance varies depending on the magnitude of the control voltage, even if the loss characteristics or impedance characteristics within the pass frequency range are adjusted at a certain control voltage, the entire control voltage range In this case, the impedance values of the varactor diodes are different, and the above-mentioned loss characteristics and impedance characteristics fluctuate greatly, making it difficult to obtain a good receiving device.

[Purpose of the invention]

In view of the above-mentioned points, an object of the present invention is to provide a receiving device such as a tuner for a television receiver or a tuner for a CATV converter, in which an element whose impedance fluctuates, such as a varactor diode, is present in a filter circuit at the input stage. It is an object of the present invention to provide a filter circuit device in which loss characteristics, impedance characteristics, etc., fluctuate little over the entire pass frequency band.

[Summary of the invention]

In the filter circuit device of the present invention, a center conductor is arranged on one side of a wiring board, ground conductors are arranged on both sides to form a coplanar line, and all filter elements or one of them are arranged on this coplanar line. arranging a filter circuit, and using at least one variable capacitance element as an element of the filter circuit,
The filter characteristics (passage loss characteristics and impedance characteristics) are made variable by applying a control voltage to this variable capacitance element. In practice, the center conductor of the coplanar line is divided into a plurality of pieces, and the filter element is connected between the center conductor and the ground conductors on both sides thereof, or between each center conductor.

[Embodiments of the invention]

Embodiments of the present invention will be described below based on the drawings.

FIG. 5 is a plan view showing the substrate pattern surface of the filter circuit device according to the present invention, FIG. 6 is a sectional view taken along line A-A shown in FIG. 5, and FIG. 7 is the opposite side of the substrate shown in FIG. 5. FIG. 3 is a plan view showing a state in which filter circuit components are attached to the filter circuit component.

As shown in FIGS. 5 and 6, a center conductor (conductive pattern) 1 is placed on the same plane of the dielectric substrate 16.
7 and ground conductors (earth patterns) 18 are printed at predetermined intervals on both sides of this center conductor 17 to form a coplanar transmission line. In this case, the center conductor 17 is divided into a plurality of pieces. Component mounting holes 19 penetrating the board 16 are formed on the conductive pattern 17 surface and the ground pattern 18 surface.
A plurality of filter elements (capacitors C ₁ to C ₁₁ , coils L ₁ to _{L 8} , resistor R ₁ , and varactor) shown in FIG.・A circuit is constructed by connecting a diode 13). However, numeral 12 is an input terminal, 14 is an output terminal, and 15 is a control voltage application terminal.

In the above-mentioned printed circuit board having a coplanar line structure, ground conductors 18 are printed at a predetermined interval on both sides of the center conductor 17 which is arranged separately, and the ground conductors 18 are printed between the center conductors and between the center conductor and the ground conductor. The filter element is connected between them. In this way, when the ground conductor 18 and the center conductor 17 are on the same plane, if the effective dielectric constant including the substrate 16 is constant, the characteristic impedance is the center conductor width a ₁ shown in FIG. 17 and the distance b ₁ from the ground conductor 18. Therefore, the characteristic impedance of the transmission line can be freely selected by changing the ratio of a ₁ and b ₁ mentioned above.

When a filter circuit is formed using a coplanar transmission line in this way, the transmission line itself has a constant impedance, so even if the impedance of the elements that make up the filter circuit slightly fluctuates, it is hardly affected by it and is good. characteristics can be obtained.

Figure 8 shows the insertion loss characteristics and impedance characteristics (when the same load is connected to the output terminal 14) of a device in which the filter circuit shown in Figure 4 is configured with a coplanar transmission line and a filter device with a conventional printed pattern configuration. (expressed in terms of reflection loss).

In Fig. 8, the characteristic impedance of the coplanar transmission line is designed to be 75Ω, and the
This shows the insertion loss N (dB) and reflection loss T (dB) of the filter when viewed at an impedance of 75Ω. However, the solid line is for the filter circuit device of the present invention, and the chain line is for the conventional filter circuit device.

In this way, a filter circuit constructed using a coplanar transmission line has a smaller insertion loss N and a larger reflection loss T than a filter circuit constructed using a conventional printed pattern. It is possible to effectively reduce the back-talk components that occur in the latter stages of the process. In addition, as mentioned above, coplanar transmission lines have conductors on the same plane, so there is no need to use a substrate with conductors on both sides like micro-strip lines, which simplifies the manufacturing process and reduces costs. It is also advantageous.

For example, as a frequency conversion circuit used in a tuner for a CATV converter, a balanced mixer or a double balanced mixer using diodes or transistors is usually used, which prevents leakage of local oscillation frequency components and generation of unnecessary frequency components. However, it will be more effective if a filter circuit device as shown in FIG. 7 is used before these balance mixers or double balance mixers.

In addition, super-heterodyne tuners always have a local oscillator, and a varactor is used to control the oscillation frequency of this local oscillator.
A diode is used. Therefore, if the control voltage of the varactor diode used in this local oscillator and the control voltage of the varactor diode used in the variable attenuation filter shown in FIG. No special equipment is required, and the receiving frequency and attenuation frequency can be tracked.

〔Effect of the invention〕

As described above, according to the present invention, all or a portion of the elements constituting the filter circuit are constructed using coplanar lines, and at least one
Since two variable capacitance elements are used as elements of the filter circuit and the variable capacitance elements are controlled by a control voltage to make the filter characteristics variable, backtalk components generated in the latter stage of the filter circuit can be effectively reduced. It can reduce the passing loss characteristics,
A filter circuit device with significantly improved impedance characteristics can be realized. In addition, in a coplanar line structure, there is a ground connection on both sides of the conductive pattern.
Since the pattern is arranged, it is easy to connect one end of the filter element to ground, and the arrangement of components is facilitated. Therefore, if this filter circuit device is applied to the input filter portion of a television receiver tuner or a CATV converter tuner, a receiving device that is advantageous in manufacturing and has excellent performance can be realized.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing a conventional adjustment method for a filter circuit, Figure 2 is a block diagram of the main part of a tuner for a CATV converter, and a frequency characteristic diagram showing the pass band of the input filter circuit.
4 is a circuit diagram showing an example of an input filter circuit using a variable attenuation filter; FIG. 5 is a plan view showing a substrate pattern surface of a filter circuit device according to the present invention; 6 is a sectional view taken along the line A-A shown in FIG. 5, FIG. 7 is a plan view showing a state in which a filter circuit component is attached to the opposite side of the board shown in FIG. 5, and FIG. FIG. 3 is a frequency characteristic diagram showing insertion loss characteristics and impedance characteristics of the device and a conventional device. _C1 to _C11 ...Capacitor, _L1 to _L8 ...Coil,
R ₁ ...Resistor, 12...Input terminal, 13...Variable capacitance element (varactor diode), 14...Output terminal, 15...Control voltage application terminal, 16...Dielectric substrate, 17...Center conductor (conductive pattern),
18...Grounding conductor (earth pattern), 19...
...Hole for mounting parts.

Claims (1)

[Scope of Claims] 1. A plurality of central conductors having a constant width a are extended and arranged on a substrate, and grounding is provided at regular intervals on both sides along the extending direction of the central conductors on the substrate. A conductor is arranged, and the width a of the center conductor is
and a pitch b between the ground conductors located on both sides of the center conductor. A circuit element in which a plurality of elements constituting a filter circuit are connected to the filter circuit, at least one of these elements is a variable capacitance element, and a control voltage is applied to the variable capacitance element to vary the filter characteristics. A filter circuit device characterized by: 2. Claims characterized in that the filter circuit is comprised of a band pass filter consisting of a high pass filter section and a low pass filter section, and a variable filter using a variable capacitance element. The filter circuit device according to item 1. 3. The filter circuit device according to claim 1, wherein the filter circuit is disposed between a high frequency input terminal of a tuner and a mixer. 4. The filter circuit according to claim 3, wherein the filter circuit passes at least the desired signal frequency, and the variable attenuation frequency by the variable capacitance element is set to the backtalk component frequency generated by the mixer. Filter circuit device. 5. A frequency limiting voltage of a local oscillator to be injected into the mixer is used as a control voltage of a variable capacitance element of the filter circuit, and the input frequency and the variable attenuation frequency are tracked. A filter circuit device according to claim 4 characterized by: