JPH0120806B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention selects a target frequency from an input signal,
Or, it relates to a tuning circuit used for inter-stage coupling, and is particularly suitable for use in a receiving circuit that has a wide reception band and selects a desired signal by tuning at each predetermined frequency, and each tuning characteristic is uniform. At the same time, it provides large attenuation to signals outside the band, and more importantly, it satisfies the above characteristics when the tuning frequency is changed, while also suppressing fluctuations in bandwidth between high and low frequency bands. Concerning a double-tuned circuit that does not occur.

[Background technology of the invention]

Generally, in order to extract a signal at a desired frequency from radio waves received by an antenna, a tuned circuit that tunes to that frequency is used, but due to its selectivity characteristics, a double tuned circuit with two tuned frequencies has a wider range. It is suitable for television receivers and the like that require bandwidth. Although this double-tuned circuit has various configurations, it usually has two tuned circuits, and the coupling between these circuits is basically performed by mutual inductance or capacitance, and the method is used depending on the situation. In addition, in order to vary the tuning frequency, each inductance or each capacitance of the above two tuning circuits is varied to obtain resonance of the circuit, but in recent so-called electronic tuning tuners, each capacitance of the double tuning circuit is changed to a variable capacitance. Since it is composed of diodes, tuning is achieved by varying the capacitance.

By the way, if the frequency tuned and transmitted by this type of double-tuned circuit is set to a band such as the UHF band and made variable, problems will arise as will be explained in the section below. An example of the setting will be explained using FIG. 1 and FIG. 2. FIG. 1 is a block diagram showing the circuit configuration when the intermediate frequency of the SHF satellite broadcasting receiver is set to the above-mentioned UHF band. This SHF satellite broadcast receiver is installed on the center axis of an outdoor parabolic antenna.
A SHF converter (hereinafter referred to as the first converter) is provided,
In combination with this, an AM converter (hereinafter referred to as the second converter) is installed indoors, and the first converter is SHF
Each broadcast signal in the UHF band (for example, 1000 to 1400 [M
Hz] to a signal of the first intermediate frequency),
The second converter further frequency converts the first intermediate frequency to generate a second intermediate frequency, which is then applied to the high frequency amplifier of the VHF tuner or UHF tuner. This configuration basically has something in common with a double superheterodyne receiving circuit often used in mobile radio equipment, all-channel tuners, and the like.

The configuration shown in FIG. 1 will be explained below. Figure 1 is above
The second converter of the SHF satellite broadcasting receiver is shown, and reference numeral 1 is a terminal to which the first intermediate frequency from the first converter is input. This incoming terminal 1 is the first
It is coupled to a preselector 3 via a high frequency amplifier 2. This preselector 3 is normally provided before the mixer 4 when the frequency before conversion is high, and is a resonant circuit in which a predetermined number of stages of the above-mentioned double-tuned circuits are stacked. This double-tuned circuit is a UHF
A double-tuned circuit is created by inductively coupling tuned circuits composed of a quarter-wavelength or half-wavelength resonant line and variable capacitance diodes (hereinafter referred to as varicaps) that are inductive to the band frequency. form,
These are stacked in a predetermined number of stages depending on the situation.
The signal selected by this preselector 3 is sent to the mixer 4
injected into. A local oscillation frequency from a local oscillator 5 is also injected into this mixer 4, and the first
A heterodyne frequency of the intermediate frequency and the local oscillation frequency, ie a second intermediate frequency, is formed at the output of this mixer 4. This mixer 4
The output terminal of is connected to the output terminal 1 through a series of a second high frequency amplifier 6, a bandpass filter 7, and a third high frequency amplifier 8, so that the second intermediate frequency is guided to this output terminal 9. . This output terminal 9 is coupled to a UHF or VHF tuner as described above. However, if the arriving radio waves are FM signals such as SHF satellite broadcasting, the first and second
Since the intermediate frequency signal is also an FM signal, an FM-AM converter may be provided between the third high frequency amplifier 8 and the output terminal 9 to enable coupling with a television receiver tuner.

Next, FIG. 2 shows a specific circuit diagram of a double tuning circuit used in the preselector 3. In FIG. In this figure, the resonant line 10 is the input side, and the resonant line 11
When the output side is set to the output side, each of the resonant lines 10 and 11 is configured to be inductively coupled, and one end thereof is connected to the ground line, and the other end is connected to the anode of the varicap 12. This variable cap 12,1
Each cathode of 2 is connected to a voltage holding capacitor 1.
3 and 13, and a tuning voltage Vt for varying the tuning frequency is applied thereto. Incidentally, portions 10a and 11a slightly drawn out from each of the resonant lines 10 and 11 correspond to input/output lines. In addition, the structure of each resonant line 10, 11 is such that a metal plate material is three-dimensionally mounted on a printed circuit board, and this plate material and a chassis plate as an external conductor combine to couple the two resonant lines 10, 11, respectively. or each resonant line 10,1
1 is formed as a flat line on a dielectric substrate, and a so-called microstrip is formed between it and a ground line sandwiching the dielectric substrate for coupling.

In the above configuration, when the tuning frequency is varied, the first intermediate frequency input from the first converter is
The following explanation assumes that the signal is in the range of 1000 to 1400 [MHz], and the second intermediate frequency is set to, for example, a signal of 130 [MHz] in the VHF band.

The SHF converter (not shown) fixes the local oscillation frequency of the 12 [GHz] band signal input to the parabolic antenna and converts it to the first intermediate frequency in the above range. This first intermediate frequency signal (hereinafter referred to as the first IF signal) is amplified to a predetermined level by the first high frequency amplifier 2 and guided to the preselector 3. The varicaps 12, 12 of the preselector 3 are changed in conjunction with the varicap of the local oscillator 5 and the tuning voltage Vt, so that the frequency tuned to the preselector 3 and the oscillation frequency of the local oscillator 5 are tracked. That is, the local oscillator 5 is
For example, in the case of the lower heterodyne system, an oscillation frequency varying from 870 to 1270 [MHz] is injected into the mixer 4 for the first IF signal of 1000 to 485 [MHz], and the double tuning circuit of the preselector 3 is injected into the mixer 4. is 1000~1400
The capacitances of the varicaps 12, 12 are varied so as to tune to the tuning center frequency of [MHz]. In other words, the double-tuned circuit is tuned to approximately the tuning center frequency by the input side resonant line 10 and the varicap 12, approximately tuned to the tuning center frequency by the output side resonant line 11 and the varicap 12, and is tuned by these tuning and each resonant line 10. , 11, it exhibits a wider band tuning characteristic than single tuning, and selects the frequency of the first IF signal. The tuning curve group is shown in FIG.
FIG. 3 is a receiving frequency characteristic diagram in which the vertical axis represents the response level of the tuning circuit and the horizontal axis represents the frequency. The curve F ₁ has a tuning center frequency f ₀₁ and a bandwidth
B ₁ , the curve F ₂ has a tuning center frequency f ₀₂ and a bandwidth B ₂ , and the curve F ₃ has a tuning center frequency f ₀₃ and a bandwidth B ₃ . Regarding each tuning center frequency, f ₀₁ is the lowest and corresponds to the above-mentioned 1000 [MHz] if specified specifically. Therefore, it can be seen that as the tuning center frequency becomes higher, the bandwidth increases.

This can also be understood from the following equation (1).

This equation (1) shows the bandwidth BW when the double-tuned circuit is in so-called critical coupling, which is √
f ₀ is the tuning center frequency, and Q is the load Q of the resonator consisting of the input or output side resonant line and the varicap 12. That is, in equation (1), as the tuning center frequency f ₀ increases, the bandwidth BW increases.

In this way, the double tuning circuit can be tuned for each tuning center frequency equivalent to each broadcast frequency,
As a result, the local oscillation frequency and the tuning center frequency are mixed in the mixer 4 to form a second IF signal. The frequency of this second IF signal is a fixed frequency, and the frequency deviation is ±
It is a 13.5 [MHz] FM signal. This signal passes through the second high-frequency amplifier 6, the bandpass filter 7, and the third high-frequency amplifier 8, and is outputted to the output terminal 9 as a signal with unnecessary signals further removed. still,
In the above SHF satellite broadcast receiver, the local oscillation frequency of the first converter is fixed and the local oscillation frequency of the second converter is variable. The local oscillation frequency may be made variable; in this case, the tuning frequency of the preselector 3 is also fixed.

Next, the specific structure of the above double-tuned circuit is shown in Figure 4.
This will be explained with reference to FIG. First, in FIG. 4, each of the resonant lines 10 and 11 is a coaxial resonant line with a main structure, one end of each is bent into a respective shape, and the other end is connected to the conducting wire of the varicap 12. The material is a metal plate formed into a predetermined shape. Then stand this on the board. In addition, the input line 10a and the output line 11a are made by bending rod-shaped conductors at a predetermined length, and the legs of the bent portions are mounted on a board and are connected to the resonant lines 10 and 11, respectively. It is configured as follows. In addition, each resonant line 1
0, 11 and each of the input/output lines 10a, 11a can adjust the spacing between their effective parts to correct the coupling.

In FIG. 5, each line is constructed of microstrips. That is, one side of the dielectric substrate 14 is used as the ground plane 15, and microstrip lines are formed on the other side, and signals flowing into the lines forming the input terminal _P1 are routed between each line and the ground plane 15. The signal is then transmitted to the line forming the output terminal _P2 .
Specifically, the line of the input terminal _P1 is connected to one end of a line forming the input line 10a, and the other end of this line is connected to the ground plane 15a on the other side. Similarly, two lines longer than the input line 10a are formed extending from the ground plane 15a to constitute the input side and output side resonant lines 10, 11, and a line having approximately the same length as the input line 10a is the output terminal P. ₂
It is formed together with the railroad tracks. Further, the varicap 12, the capacitor, etc. are fixed at predetermined positions by vapor deposition or the like.

As mentioned above, there are two types of double-tuned circuits: a three-dimensional structure and a planar structure.The latter structure allows for a smaller overall volume than the former structure, and is particularly advantageous for transmitting frequencies in the UHF band.

[Problems with background technology]

By the way, in such a double-tuned circuit, as explained in equation (1), the higher the tuning frequency, the wider the bandwidth, and conversely, the lower the tuning frequency, the narrower the bandwidth. As the bandwidth of the double-tuned circuit becomes wider, the ability to distinguish between unnecessary signals and selected signals decreases, and there is a possibility that cross-modulation interference with adjacent channels or interference due to image frequencies may occur. Furthermore, if the tuning frequency is low, it causes the inconvenience that the required band is not transmitted.

In order to correct this, it is possible to use several stages of double-tuned circuits and slightly shift the tuning frequency of each circuit to obtain uniform tuning characteristics over the entire receiving frequency band. This is not easy as there are cases where the constants have to be different on each side.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a double-tuned circuit whose tuning curve has a constant bandwidth for each frequency of the receiving band and which has flat characteristics in terms of output level.

[Summary of the invention]

The present invention is an input side variable resonator (primary side resonator)
A sub-resonator composed of a microstrip is interposed between the output-side variable resonator (secondary-side resonator) and the resonator that constitutes each variable resonator and the sub-resonator. This double-tuned circuit is configured to sequentially couple the lines to each other, and the tuning frequency of the sub-resonator is set near the lower limit frequency of the input frequency range.

When this sub-resonator is set lower than the tuning center frequency, the double-tuning characteristic due to the coupling between the variable resonators will expand into a characteristic in which single tuning is added to double tuning in the low range, and In this case, since the tuning performance (Q) of the sub-resonator is low, the influence of this sub-resonator does not appear in the double tuning between the variable resonators; rather, its low impedance suppresses the bandwidth and suppresses the bimodality. It exhibits a flat tuning curve, resulting in a set of pass double tuning curves that are uniform over the entire receiving band.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to FIGS. 6 and 7. Here, FIG. 6 shows a double-tuned circuit according to an embodiment of the present invention, and FIG. 7 shows a receiving band pass characteristic passing through the same circuit. First, the configuration of FIG. 6 will be explained, and the same elements as those in FIG. 1 are given the same reference numerals. On the input side of the circuit, there is an input line 10a having one end connected to the input terminal _P1 and the other end grounded, and the input side resonant line 10 is coupled to this input line 10a. This input side resonant line 10 has one end grounded,
The anode of the variable cap 12 is connected to the other end. Further, the cathode of the varicap 12 is configured to be applied with a tuning voltage Vt and is grounded via a capacitor 13.
These input side resonant line 10, variable cap 12,
A series circuit including the capacitor 13 constitutes a primary resonator RS ₁ , and the primary resonator RS ₁ is inductively coupled to a secondary resonator RS ₂ having a similar configuration. That is, this secondary resonator RS ₂ has one end of the output side resonant line 11 grounded, and the other end grounded via the varicap 12 and the capacitor 13. Further, an output line 11a having one end open and the other end grounded is provided so as to be coupled to this output side resonant line 11, and this output line 11
One end of a is connected to output terminal _P2 . The output line 11a and the input line 10a have a predetermined length so as to function as a simple impedance converter, and set the input impedance and output impedance of the double-tuned circuit consisting of the primary and secondary resonators. At the same time, each input side and output side resonant line 10, 11 and each input/output line 1
The degree of coupling with 0a and 11a can be adjusted.

Furthermore, in the double-tuned circuit of the present invention, an intermediate resonant line 16 is provided between the input side resonant line 10 and the output side resonant line 11, and is side-coupled with each of the input and output side resonant lines 10 and 11. This intermediate resonant line 16 becomes a sub-resonator RS ₃ with a capacitance 17 existing between it and the ground line, and each primary side and secondary side resonator
It is configured to be coupled to RS ₁ and RS ₂ , respectively. However, this capacitor 17 may be formed by providing a normal capacitor. Further, the intermediate resonant line 16 is tuned to a frequency slightly lower than the lowest frequency of the reception band, and is formed to have a length of, for example, one-fourth or less of the wavelength of this frequency. Further, the conductor used for the intermediate resonant line 16 is configured such that its impedance during resonance is smaller than the impedance of the input side and output side resonant lines 10 and 11.
Further, the shape has a very low tuning performance (Q), and uses, for example, a microstrip line as described later. With this configuration, the tuning curve of the intermediate resonant line 16 has sufficient passing characteristics in the low range of the reception band, but attenuation gradually increases in the high range.

Each line (input/output side resonant lines 10, 11, input/output lines 10a, 11a, intermediate resonant line 16) may be a combination of coaxial lines as shown in Fig. 4, or a combination of coaxial lines as shown in Fig. 5. A combination of microstrip lines is used. Also, the tuning voltage Vt
is applied to each variable cap 12, 12, so that the tuning frequencies of the primary and secondary resonators can be varied. Furthermore, output line 1
1a is connected to output terminal _P2 .

Next, the operation of the double-tuned circuit having the above configuration will be explained below.

In this double-tuned circuit, an intermediate resonant line 16 made of a microstrip line is inserted between two conventional inductance members. When the resonant frequency is set near the lower limit frequency of the input frequency, the microstrip line originally has low Q and low impedance, so it acts as a resonator near the resonant frequency, but if the frequency shifts, it will resonate. This effect becomes stronger as the input frequency increases. Intermediate resonant line 11
When the resonator no longer acts as a resonator, the original coupling between the input side and output side resonant lines 10 and 11 becomes effective.

Therefore, at low frequencies, the intermediate resonant line 1
6 is an input side resonant line 10 and an output side resonant line 1
1 and form a three-tuned circuit with two M connections. The three-tuned circuit has double-tuned characteristics exhibiting three pole frequencies, and can widen the characteristics at low input frequencies.

On the other hand, when the input frequency becomes high, the intermediate resonant line 16 no longer has resonance characteristics, and becomes a double-tuned circuit with one M coupling between the input side resonant line 10 and the output side resonant line 11. Moreover, in this case, since the input-side and output-side resonant lines 10 and 11 have the intermediate resonant line 16 interposed between them, the coupling is weaker than in the conventional configuration in which no intervening line is present. In a double-tuned circuit, when the coupling is weakened, the width of the selection characteristic becomes narrower, so this double-tuned circuit narrows the bandwidth at high frequencies.

FIG. 7 shows the characteristics when applied to a preselector of a satellite broadcasting receiver. In this case, since the lower limit frequency of the input frequency range is 1000 [MHz], the resonant frequency of the intermediate resonant line 16 is set to 800 [MHz], which is lower than this lower limit frequency.

As shown in equation (1), the bandwidth BW of the double-tuned circuit becomes narrower as the input frequency f0 becomes smaller. In other words, the dotted line in FIG. 7 shows the double tuning characteristic when receiving each frequency when the intermediate resonant line 16 according to the present invention is not provided, and at low input frequencies, F1
As shown in
It becomes wider with F2 and F3.

However, the double tuning circuit of this invention has a frequency of 1000 [MHz]
It forms a three-tuned circuit at the low frequency of ω1, ω2,
By exhibiting a triple tuning characteristic with three polar frequencies called ω3, it has a wide characteristic called F1′,
You will be able to select the information you need. In addition, the input side and output side resonant line 1 at this time
The impedance of 0 and 11 was 100 [Ω], and the impedance of the intermediate resonant line 16 was 25 [Ω].

On the other hand, at high frequency F3 (1400MHz), the intermediate resonant line 16 acts to weaken the coupling between the input side resonant line 10 and the output side resonant line 11, so that the coupling between the input side resonant line 10 and the output side resonant line 11 is reduced. The frequency is smaller than that of a conventional circuit in which the intermediate resonant line 16 is not interposed, and the selection characteristic is narrower. This reduces cross-modulation interference or image frequency interference due to adjacent channels.

In the embodiment, at the intermediate frequency F2, the intermediate resonant line 16 operates so as not to have inductance, as in the case of F3, and functions as a double-tuned circuit using one M coupling. Therefore, the characteristic is F2′,
Since this is the same in terms of equivalent circuit, it has approximately the same width as F3'.

Thus, in this invention, by interposing the intermediate resonant line 16 made of a microstrip line between the input side and output side resonant lines 10 and 11 forming M-coupling, the intermediate resonant line 16 is selective at low input frequencies. It acts as a resonator that broadens the selection characteristics, and at high input frequencies it acts as a coupling window, narrowing the selection characteristics. This makes it possible to sufficiently secure the necessary band at low frequencies, and reduce cross-modulation interference and image interference due to adjacent channels at high frequencies.

The reason why the resonant frequency of the intermediate resonant line 16 is shifted to 800 [MHz] from the lower limit frequency of the input frequency (1000 MHz) is because the three lines 10, 11, and 16 all have the same resonant frequency around 1000 [MHz]. Then,
This is to suppress the characteristic of F2' that causes a response at 1000 [MHz].

Next, a structural example of this double tuning circuit will be described with reference to FIG. In the device shown in FIG. 8, a ground plane 15 is formed on one side (hereinafter referred to as the back side) of a dielectric substrate 14 made of ceramic, quartz, etc., and a strip line, coaxial line, etc. is mounted on the opposite side (hereinafter referred to as the front side). This is a circuit device in which a double-tuned circuit is formed.
Note that the same elements as in FIGS. 4 and 5 are designated by the same reference numerals. The ground surface 15 on the back surface and the ground line 15a on the front surface are electrically connected, and a microstrip line 16' is formed extending from this ground line 15. As a characteristic of this microstrip line 16', the unloaded Q is much smaller than that of the coaxial resonant line described next, and it can be adjusted to a predetermined value by changing the width W relative to the thickness of the dielectric substrate 14. The characteristic impedance can be configured. In addition, this microstrip line 16'
A capacitance 17 is parasitic between the open end of the ground plane 15 and the ground plane 15 .

An L-shaped input coaxial line 10' and an output coaxial line 11' are arranged in a three-dimensional structure on both sides of the microstrip line 16' to form a coaxial line. This input side coaxial line 10' corresponds to the input side resonant line 10 in FIG. 6, and the output side coaxial line 11' corresponds to the output side resonant line 11. In addition, each input side and output side coaxial line 10',
11' has an L-shaped end fixed to the dielectric substrate 14, and the other end supported by the height of the varicaps 12, 12; It is fixed at 14. Further, bar-shaped conductors 10a' and 11a' are provided on the input and output coaxial lines 10' and 11', respectively, so that the distance between the effective parts of each line can be adjusted. In other words, this rod-shaped conductor 10a',
11a' is bent into a rectangular shape and attached so that its effective part is slightly inclined to the effective part of the input and output coaxial lines 10' and 11', and one end of each is provided with an input terminal formed of a microstrip. P ₁ and output terminal P ₂ , and each other end is electrically connected to the ground plane 15 . Further, a bare capacitor 13' corresponding to the capacitor 13 is connected to one end of the varicap 12 using the ground of this connection point. In addition, each input side and output side coaxial line 1
One ends of 0' and 11' are connected to the ground line 15a via patterns 10b and 11b.

The above structure constitutes the double-tuned circuit shown in FIG. That is, the microstrip line 16' with a low no-load Q is coupled to the input side and output side coaxial lines 10' and 11', respectively, and the degree of coupling is determined by adjusting the rod-shaped conductors 10a' and 11a'. Therefore, it can be adjusted relatively. Incidentally, the input side resonant line 10 and the output side resonant line 11 may have the same structure as the microstrip line 16', that is, the structure shown in FIG. 5 may be used. In this case, the strip line corresponding to the resonant line on the input side and the output side is formed so that the no-load Q thereof is high.

〔Effect of the invention〕

As explained above, according to the present invention, an intermediate resonant line that is tuned near the lower limit frequency of the tuning center frequency to which these resonant lines are tuned is interposed between two resonant lines that have double tuning characteristics. This has the effect that the double tuning characteristics formed by these resonant lines can be made to have a constant bandwidth between a lower tuning center frequency and a higher tuning center frequency. Further, the double-tuned circuit has flat characteristics, and provides a tuned circuit with good tuning performance and excellent ability to eliminate interference waves.

[Brief explanation of drawings]

Fig. 1 is a circuit block diagram showing the structure of the second converter of an SHF satellite broadcasting receiver, Fig. 2 is a circuit diagram showing a conventional double-tuned circuit used in a preselector, and Fig. 3 is a circuit diagram showing the above-mentioned double-tuned circuit. FIG. 4 is a perspective view of a circuit device showing an example of the structure of the double-tuned circuit; FIG. 5 is a perspective view showing another example of the structure of the same circuit; Figure 6 is a circuit diagram showing a double tuning circuit according to the present invention, Figure 7 is a characteristic diagram showing a tuning curve for each tuning frequency passing through the double tuning circuit, and Figure 8 is an example of the structure of the circuit shown in Figure 6. FIG. 10... Input side resonant line (10'... Input side coaxial line), 11... Output side resonant line (11'... Output side coaxial line), 10a... Input line, 11a...
...Output line, 12... Varicap, 13... Capacitor, 16... Intermediate resonant line (16'... Microstrip line), 17... Capacitance, P ₁ , P ₂
...Terminal, RS ₁ ...Primary side resonator, RS ₂ ...Secondary side resonator, RS ₃ ...Sub-resonator.

Claims (1)

[Claims] 1. A primary resonator whose one end of the input side resonant line is grounded and the other end is grounded via a capacitor whose capacity can be varied, and one end of the output side resonant line that is inductively coupled to the input side resonant line is grounded. a secondary resonator whose other end is grounded via a capacitor similar to the above, and an intermediate resonant line comprising a microstrip line inserted between the input and output side resonant lines. said intermediate resonant line is said 1
Reduces the degree of coupling between the next-side resonant line and the secondary-side resonant line, has a resonant frequency near the lower limit frequency of the input frequency range, and has a three-tuning characteristic with the primary side and secondary side resonators near the lower limit frequency. A double-tuned circuit characterized by exhibiting the following characteristics.