JP2002171149A - Variable attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resonance type variable attenuator which can be secured for a good low insertion loss characteristic and a high attenuation characteristic over a wider band. SOLUTION: The variable attenuator 40 uses two stages of LC parallel resonance circuits. A serial connection between the LC parallel resonance circuits 3 and 4 of the two-stage structure is connected to a reference potential point via a capacitor 41, while it is also connected to the reference potential point via a high frequency variable resistor element 9. When the control voltage Vc=0, high frequency variable resistor elements 5, 6, 7a, 7b, and 9 are turned into an off-state and therefore serve as a LC low-pass filter, and a pass-band characteristic of a high frequency signal has a low loss and becomes nearly constant over a wide band up to the cutoff frequency of the LC low-pass filter. When the control voltage Vc is large, the high frequency variable resistor elements 5, 6, 7a, 7b, and 9 are turned into an on-state and therefore serve a LC parallel resonance preventive filter, providing the maximum attenuation to a high frequency signal near the parallel resonance frequency of the filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-frequency variable attenuator suitable for a wired or wireless high-frequency, microwave or wideband telecommunication device.

[0002]

2. Description of the Related Art In general, mobile communication devices such as mobile phones and other high-frequency and microwave radio devices,
High frequency variable attenuators are frequently used in wired network systems such as ANs and WANs. The main applications are input / output signal power control, AGC, and the like. The general characteristics required for these variable attenuators are listed below.

(1) High attenuation can be obtained (2) Low insertion loss (3) Good input / output consistency (4) Frequency characteristics of attenuation are flat in required frequency band (5) Low distortion (6) Low noise (7) Good temperature characteristics (8) Smooth attenuation characteristics for control signal magnitude (preferably linear) An example of this type of conventional variable attenuator (hereinafter referred to as Conventional Example 1) is an LC resonance type RF variable attenuator 1 shown in FIG. This variable attenuator is described in JP-A-11-195949.

The LC resonance type RF variable attenuator 1 has, for example, left and right symmetrically connected LC parallel resonance circuits 3 and 4 including a pair of left and right LCs on both sides of a coupling capacitor 2.
PIN diodes 5 and 6 are respectively inserted in series with a capacitor C.

The input terminal 11 of the high-frequency input signal RF IN is connected to a reference potential point (ground) via a DC blocking capacitor Ca and a series circuit of a PIN diode 7a and a resistor 8a, while the output terminal 12 is connected to the reference terminal. It is connected to a reference potential point via a DC blocking capacitor Ca and further via a series circuit of a PIN diode 7b and a resistor 8b. Further, a pair of right and left PIs in the figure
They are connected to reference potential points via N diodes 9a and 9b, respectively. In FIG. 10, Cb is a bypass capacitor for grounding a high-frequency component, and 10a and 10b supply a control current Ic to the LC parallel resonance circuits 3 and 4 on the left and right in the figure.
This is a high-frequency choke coil for blocking high frequencies from the LC parallel resonance circuits 3 and 4 to the control terminal 13.

The resistor 8a on the input side 11 of the high-frequency signal
Is a resistor for input impedance matching, and the output side 12
Is a resistor for output impedance matching,
The impedance viewed from the input side 11 and the output side 12 must be matched to the input / output impedance of the external device or the characteristic impedance of the external transmission line (for example, 50Ω), and the resistance value R of the resistors 8a and 8b is set to approximately 50Ω. You. The capacitance value of the coupling capacitor 2 is half (C
/ 2).

When the control voltage Vc is applied to the control terminal 13 which is a control voltage applying unit to reduce the high-frequency resistance of each of the PIN diodes 5 and 6 of each of the LC parallel resonance circuits 3 and 4, each of the LC parallel resonance circuits is reduced. 3 and 4 are respectively configured as parallel resonance circuits. FIG. 11 shows an ideal equivalent circuit for a high-frequency signal when the control voltage Vc is increased. The high frequency input signal of, for example, several GHz input from the input terminal 11 is blocked by the LC parallel resonance circuits 3 and 4 and connected to the reference potential points via the PIN diodes 9a and 9b. The output RF OUT of the high-frequency signal is greatly attenuated.

On the other hand, an ideal equivalent circuit for a high-frequency signal when the control voltage Vc is zero is shown in FIG. At this time, the high-frequency resistance of each of the PIN diodes 5 and 6 of each of the LC parallel resonance circuits 3 and 4 becomes almost maximum. As an ideal case, if this is set to infinity, these LC parallel resonance circuits 3 and 4 are configured as an LC series resonance circuit by the inductor L and the capacitor 2 (capacitance value C / 2). For this reason, the high-frequency input signal input from the input side 11 is output to the output side 12 through the LC series resonance circuits 3 and 4, so that the attenuation of the high-frequency signal is extremely small, and ideally at a specific frequency. Becomes zero.

FIG. 13 shows the relationship between the frequency f (GHz) and the attenuation (dB) when Vc = 0. The ideal equivalent circuit of FIG. 12 has a bandpass filter characteristic in which the amount of attenuation is the smallest when the frequency f is f0 below and becomes zero.

[0010]

(Equation 1) Then, by continuously changing the control voltage Vc within the range from the minimum to the maximum of the attenuation of the high-frequency input signal, the attenuation of the high-frequency input signal can be continuously controlled from the minimum to the maximum.

As can be seen from the above description, the circuit of FIG. 10 is ideally used as an LC parallel resonance rejection filter as shown in FIG. 11 in a high attenuation state, and as shown in FIG. 12 in a low attenuation state. Acts as a series resonance pass filter.

In the circuit configuration of FIG. 10, a higher attenuation can be obtained by increasing the value of the inductance L of the resonance inductor. However, the larger the value, the narrower the frequency bandwidth at the maximum signal passage. (That is, the characteristic shown by the solid line in FIG. 13 has a narrow width). Also, in an actual PIN diode, the high-frequency resistance when the control current is zero has a finite value, so that the insertion loss increases. That is, obtaining a high attenuation and obtaining a low insertion loss in a relatively wide band are incompatible.

FIG. 14 shows a second conventional example. FIG. 14 shows a three-stage LC parallel resonance circuit configuration in contrast to the two-stage LC parallel resonance circuit shown in FIG. In FIG.
Since the LC parallel resonance circuit is increased by one more and the number of PIN diodes shunting to the reference potential point side is increased, in a high attenuation state in which the control voltage Vc is increased, the cutoff effect for a high frequency signal is further increased. Signal output RF OUT
Will experience greater attenuation. When Vc is set to zero, the PIN diode is cut off. Ideally, when this resistance Rs is set to infinity, an LC series resonance circuit composed of the inductor 3L and the capacitor C / 3 is formed at a high frequency. In this case, the relationship between the frequency f and the amount of attenuation (attenuation characteristic) is a characteristic shown by a dotted line in FIG.

FIG. 15 shows a conventional example 3, in which two LC parallel resonance circuits shown in FIG. 10 and a coupling capacitor C /
Instead of the two series connection, two inductors L are connected in series, the connection point is connected to a reference potential point by a capacitor C, and a PIN diode 9 is connected in parallel to the capacitor C.

In the circuit of FIG. 15, when the control voltage Vc is increased, the shunt PIN diode 9 at the center of the circuit (the connection point between the inductors L and L) connects the two inductors L and L to each other. Is short-circuited, and two parallel resonance blocking circuits of inductor L and capacitor C / 2 are formed at high frequency.
Compared with the conventional example, the output RF OUT of the high-frequency signal has a drawback that large attenuation cannot be obtained. When Vc = 0,
The low pass filter of the LC is formed, and good low attenuation characteristics can be obtained in a relatively wide band.

FIG. 16 shows a non-resonant π-type variable attenuator 20 of the fourth conventional example. This variable attenuator is described in JP-A-2000-286659.

The non-resonant π-type variable attenuator 20 has an input terminal 21 to which a high frequency signal RF such as a microwave is input.
In the middle of a signal line 23 connecting the output terminal 22 and the first and second Ps, which are a pair of high-frequency variable resistance elements on the left and right in FIG.
While the IN diodes 24 and 25 are inserted in series, the cathodes of a pair of left and right third and fourth PIN diodes 26 and 27 are connected in a shunt manner on the cathode side of the PIN diodes 24 and 25, and these third and fourth PIN diodes 26 and 27 are connected. Of the PIN diodes 26 and 27 are connected to reference potential points via respective bypass capacitors 28 and 29.

The third and fourth PIN diodes 2
A bias terminal 32 of a bias power supply is connected to the anode side of each
While applying a constant DC voltage (BIAS) to the IN diodes 26 and 27 to drive them, the first and second PIN diodes 2 and 27
A control terminal 34 of a control power supply unit is connected to the anode side of each of the power supply devices 4 and 25 via a choke coil 33 for blocking high-frequency components.
To control the internal resistance of the PIN diodes 24 to 27 to attenuate the signal passing through the signal line 23 as appropriate by changing the control DC voltage (CONT) applied to the PIN diodes 24 and 25. It has become.

However, in such a conventional π-type variable attenuator 20, the maximum attenuation is limited by the maximum internal equivalent resistance and the equivalent interelectrode capacitance of the high-frequency variable resistance elements such as the PIN diodes 24-27. For this reason, in order to increase the maximum attenuation, a plurality of π-type variable attenuators 20 having the same configuration are connected in cascade, or at least the number of PIN diodes 24 and 25 inserted in series in the signal line 23 is connected in series. There is a way to increase. However, in this method, the insertion loss at the time of minimum attenuation increases almost in proportion to the number of cascades. Therefore, it is difficult to achieve both high attenuation and low insertion loss particularly in a high frequency region where the attenuation is small. Also, the insertion loss is relatively large due to the residual resistance of the PIN diode, and the signal distortion and noise at the time of the minimum attenuation are also affected by the characteristics peculiar to the PIN diode used, so that a drastic improvement cannot be expected.

[0020]

As described above, in the conventional example 1 shown in FIG. 10, a higher attenuation can be obtained by increasing the value of the inductance L of the resonance inductor. The frequency bandwidth at the time of maximum signal transmission becomes narrow, and the insertion loss increases. That is, there is a problem that obtaining a high attenuation and obtaining a low insertion loss in a relatively wide band cannot be achieved at the same time.

Further, in the conventional example 2 shown in FIG. 14, in the high attenuation state, the cutoff effect for the high frequency signal becomes larger.
Since the number of stages in FIG. 10 is increased, there is a problem similar to the conventional example 1 in FIG.

Further, the conventional example 3 shown in FIG. 15 has a problem that a large attenuation cannot be obtained in a high attenuation state.

Further, in the conventional example 4 of FIG. 16, the maximum attenuation is increased by the maximum internal equivalent resistance and the equivalent interelectrode capacitance of the high-frequency variable resistance element such as the PIN diode, so that the maximum attenuation is increased. When the method is adopted, there is a problem that the insertion loss at the time of minimum attenuation increases. Further, when the frequency becomes high, there is a problem that a high attenuation cannot be obtained, and there is a physical limit to the reduction of insertion loss, signal distortion, noise, and the like at the time of minimum attenuation.

Accordingly, the present invention has been made in view of the above problems, and has good attenuation characteristics, low insertion loss, low distortion, and
It is an object of the present invention to provide a variable attenuator having low noise and the like.

[0025]

A variable attenuator according to the present invention includes an LC parallel resonance circuit connected in series to a high-frequency signal path; and a first high-frequency variable resistor inserted in the LC parallel resonance circuit. A series circuit of a second high-frequency variable resistance element and a first impedance matching resistor connected in a shunt manner to an input terminal of the LC parallel resonance circuit; and a shunt at an output terminal of the LC parallel resonance circuit. A series circuit of a third high-frequency variable resistance element and a second impedance matching resistor connected in a shunt manner; a first capacitor connected in a shunt manner to an input terminal of the LC parallel resonance circuit; A second capacitor shunt-connected to an output terminal of the LC parallel resonance circuit; and control voltage applying means for applying a control voltage to the first, second, and third high-frequency variable resistance elements. Characterized in that it was.

In the above configuration, the internal resistance of the high-frequency variable resistance element is varied according to the value of the control voltage Vc. As high frequency variable resistance elements, PIN diodes, MESFETs (Metal Semiconductor FETs), MOs
SFET (Metal Oxide Semiconductor FET), JFE
T (Junction FET) or the like can be used. Also,
A series circuit of a high-frequency variable resistance element connected in a shunt and an impedance matching resistor, or a capacitor connected in a shunt, has one end of the series circuit or the capacitor connected to a signal path, and This means that one end is connected to a reference potential point (ground). These are the same in the following inventions.

According to a second aspect of the present invention, in the variable attenuator according to the first aspect, the capacitance value of the capacitor in the LC parallel resonance circuit is C, and each capacitance value of the first and second capacitors is C '/. 2, the capacitance value is determined so that C> C ′ / 2.

According to a third aspect of the present invention, there is provided a variable attenuator having a first and a second L connected in series with a high-frequency signal path.
A C parallel resonance circuit; first and second high-frequency variable resistance elements respectively inserted into the first and second LC parallel resonance circuits; and a shunt connection to an input terminal of the first LC parallel resonance circuit. A series circuit of a third high-frequency variable resistance element and a first impedance-matching resistor;
A fourth shunt-connected output terminal of the parallel resonance circuit;
A series circuit of a high-frequency variable resistor element and a second impedance matching resistor; a first capacitor shunt-connected to an input terminal of the LC parallel resonance circuit;
A second shunt-shaped output terminal of the parallel resonance circuit;
A fourth high-frequency variable resistance element shunt-connected to a connection point of the first and second LC parallel resonance circuits; and a connection point of the first and second LC parallel resonance circuits. A third capacitor connected in a shunt form; and control voltage applying means for applying a control voltage to the first, second, third, fourth, and fifth high-frequency variable resistance elements. Features.

According to a fourth aspect of the present invention, in the variable attenuator according to the third aspect, the capacitance value of each capacitor in the first and second LC parallel resonance circuits is C, and
The capacitance value is determined so that C> C ′ / 2, where C ′ / 2 is the capacitance value of the third capacitor and C ′ is the capacitance value of the third capacitor.

According to a fifth aspect of the present invention, in the variable attenuator of the first aspect, the LC attenuator is connected to the high-frequency signal path.
A plurality of parallel resonance circuits were connected in series, and a capacitor was connected in a shunt shape to each connection point between these series-connected LC parallel resonance circuits, while a high-frequency variable resistance element was connected in a shunt shape to each of the connection points. It is characterized by the following.

According to a sixth aspect of the present invention, in the variable attenuator according to the fifth aspect, the capacitance value of each capacitor in the LC parallel resonance circuit is C, and each capacitance value of the first and second capacitors is C ′. / 2, when each capacitance value of the shunt capacitor between the LC parallel resonance circuits is C ′,
It is characterized in that the capacitance value is determined so that> C ′ / 2.

According to the first aspect of the present invention, one stage of the LC parallel resonance circuit is used, and when the control voltage Vc = 0,
The high-frequency variable resistance element is turned off, acts as an LC low-pass filter, the pass characteristic of the high-frequency signal becomes almost constant over a wide band up to the cutoff frequency, and the distortion is low and the amount of attenuation when passing the signal is the least. Low insertion loss. When the control voltage Vc is large, the high-frequency variable resistance element is turned on and functions as an LC parallel resonance blocking filter, and the maximum attenuation can be given to the high-frequency signal near the parallel resonance frequency. Therefore, the control voltage V
By increasing c from 0, it is possible to change the frequency characteristic of the low-pass filter from the lower right to the concave frequency characteristic of the band rejection filter, and to increase the attenuation over the entire required frequency band. .

In the second aspect of the present invention, conditions for obtaining a wider band signal passing characteristic in a low attenuation state near Vc = 0 in the first aspect of the invention are presented. In order to obtain pass characteristics in a wider band, the parallel resonance frequency when Vc is large and the filter operates as an LC parallel resonance rejection filter is larger than the cutoff frequency f1 'when Vc = 0 and the filter operates as an LC low-pass filter. It is sufficient that f1 is set to satisfy the relationship of f1 <f1 ', and this relationship (f1 <f1')
From the above, it suffices that the relationship of C> C ′ / 2 is established between the capacitor C in the LC parallel resonance circuit and the first and second capacitors C ′ / 2.

According to the third aspect of the present invention, the two-stage LC parallel resonance circuit is used.
The series connection point of the parallel resonance circuit is connected to a reference potential point via a capacitor, while the series connection point of two LC parallel resonance circuits is connected to the reference potential point via a high frequency variable resistance element. Thereby, the maximum attenuation as a band rejection filter when the control voltage Vc is increased can be increased, and the cutoff frequency when the LC low-pass filter is operated when Vc = 0 is a one-stage configuration according to claim 1. Therefore, there is an advantage that a broad band pass characteristic can be maintained.

According to the fourth aspect of the present invention, conditions for obtaining a wider band signal passing characteristic in a low attenuation state near Vc = 0 in the third aspect of the present invention are presented. Unlike the case of claim 2, a shunt-shaped capacitor (third capacitor) is added, but the capacitance of the shunt-shaped capacitor is C ′, and each capacitance of the first and second capacitors is C ′ / If it is set to 2, the relationship of C> C ′ / 2 should be satisfied.

According to the fifth aspect of the present invention, a configuration in which a plurality of LC parallel resonance circuits are generally used is shown.
The series connection point of each of the multi-stage LC parallel resonance circuits is connected to a reference potential point via a capacitor, while the series connection point of the multi-stage LC parallel resonance circuit is connected to a reference potential point via a high-frequency variable resistance element. Connected to. Thereby, the maximum attenuation as a band rejection filter when the control voltage Vc is increased can be further increased, and the cut-off frequency when the LC low-pass filter works when Vc = 0 is one of the first and second embodiments. , Two-stage configuration, so that there is an advantage that a wide band pass characteristic can be maintained. The insertion loss does not increase as much as the conventional type.

In the sixth aspect of the present invention, conditions for obtaining a wider band signal passing characteristic in a low attenuation state near Vc = 0 in the fifth aspect of the present invention are presented. Unlike the case of claim 4, a plurality of shunt-shaped capacitors are added. If each capacitance of the shunt-shaped capacitor is C ′ and each capacitance of the first and second capacitors is C ′ / 2,
It suffices that the relationship of C> C ′ / 2 holds.

[0038]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a variable attenuator according to a first embodiment of the present invention.

In FIG. 1, the LC resonance type high frequency variable attenuator 40 has, for example, LC parallel resonance circuits 3 and 4 including a pair of left and right LCs symmetrically connected to both sides of a center coupling point H. PIN diodes 5, 6 as resistance elements are inserted in series with the respective capacitors C in the LC parallel resonance circuits 3, 4, respectively.

The input terminal 11 of the high-frequency input signal RF IN is connected to a reference potential point (ground) via a series circuit of a PIN diode 7a and an input impedance matching resistor 8a after a DC blocking capacitor Ca is interposed. On the other hand, the output terminal 12 of the high-frequency input signal RF OUT is connected to a reference potential point via a series circuit of a PIN diode 7b and a resistor 8b for output impedance matching after interposing a DC blocking capacitor Ca. Further, the connection point H
Are connected to a reference potential point via a PIN diode 9.

A control voltage Vc is applied to the control terminal 13 from control voltage applying means (not shown). The capacitor Cb between the control terminal 13 and the reference potential point is a bypass capacitor for short-circuiting (shunting) a high-frequency component to the reference potential point (ground), and the coils 10a and 10b are controlled based on the control voltage Vc from the control terminal 13. The current Ic is supplied to the PIN diodes 5 and 6 of the left and right LC parallel resonance circuits 3 and 4 in FIG.
3 is a high-frequency choke inductor for preventing high-frequency components from entering the circuit 3. In order to make the change of the control current Ic flowing through each PIN diode substantially linear with respect to the change of the control voltage Vc, the choke coils 10a and 10a
Ob may be changed to an appropriate resistance, or an appropriate resistance may be inserted between the control voltage source and the control terminal 13. At this time, the relationship between the attenuation in dB (decibel) display and the control applied voltage becomes almost linear.

Further, the connection point H is connected to a reference potential point via a capacitor 41, and a connection point I between one end of the LC parallel resonance circuit 3 and the anode of the PIN diode 7a is connected to a reference point via a capacitor 42. Connect to the potential point and
A connection point J between one end of the C parallel resonance circuit 5 and the anode of the PIN diode 7b is connected to a reference potential point via a capacitor 43. Assuming that the capacitance value of the capacitor 41 is C ′, the capacitance values of the capacitors 42 and 43 are respectively C ′ /
2, C '/ 2. Although the capacitance value C 'may be different from the capacitance value C of the capacitor C in the LC parallel resonance circuits 3 and 5, even if both capacitance values C' and C are the same, the variable attenuator according to the embodiment of the present invention is used. It is suitable as a circuit element constituting 40.

The resistor 8a on the input side 11 of the high-frequency signal
Is a resistor for input impedance matching, and the output side 12
Is a resistor for output impedance matching.
The impedance viewed from the input side 11 and the output side 12 needs to be matched to the input / output impedance of the external device or the characteristic impedance (for example, 50Ω) of the external transmission line, and the resistance value R of the resistors 8a and 8b is approximately 50Ω.
, These conditions are satisfied in a wide attenuation range.

Next, the operation of the variable attenuator according to the first embodiment of the present invention will be described. When the control voltage Vc applied to the control terminal 13 which is a control voltage applying unit is zero, an ideal equivalent circuit for a high-frequency signal input is as shown in FIG. In this case, each LC parallel resonance circuit 3,
4 each PIN diode 5, 6 and shunt PIN
Since the high-frequency resistance Rs of the diodes 7a, 7b, 9 is processed as ∞, the series circuit of each inductor L and the connection points H,
A capacitor 41 connected between I, J and the reference potential point,
A low-pass filter is formed by 42 and 43 (capacitance values C ′, C ′ / 2, C ′ / 2).

The signal frequency f (G
FIG. 3 shows the relationship of the attenuation (dB) with respect to (Hz). The cutoff frequency f1 'in FIG. 3 is as follows.

[0046]

(Equation 2) On the other hand, a control voltage Vc is applied to the control terminal 13 which is a control voltage application unit, and each P of each LC parallel resonance circuit 3, 4 is controlled.
When the high-frequency resistance Rs of the IN diodes 5 and 6 is reduced, the resonance Q of each of the LC parallel resonance circuits 3 and 4 increases,
The capability as a rejection filter increases. FIG. 4 shows an ideal equivalent circuit for a high-frequency signal when the control voltage Vc is increased. The high-frequency input signal of, for example, several GHz input from the input terminal 11 is blocked by the LC parallel resonance circuits 3 and 4 and connected to the reference potential point via the PIN diode 9. The output RF OUT experiences a large (infinite in this example) attenuation.

When Rs of the actual circuit, that is, the PIN diode is not zero, the amount of attenuation becomes maximum at the frequency f1 shown below.

[0048]

(Equation 3) Here, if C ′ = C, it can be seen that the cutoff frequency f1 ′ shown in the equation (2) is about 1.4 times higher than the parallel resonance stop frequency f1 shown in the equation (3).

By controlling the control voltage Vc to be changed continuously, the attenuation of the high frequency input signal can be continuously controlled from the minimum to the maximum while maintaining good input / output matching. Shows the relationship (attenuation characteristic) of the attenuation (dB) with respect to the signal frequency f (GHz) when the control voltage Vc is sequentially increased. Vc = 0
Assuming that Rs of the PIN diode is ∞, FIG.
And shows the pass characteristic of the low-pass filter. Up to the cutoff frequency f1 ', the attenuation has a flat frequency characteristic close to zero. As Vc increases, the amount of attenuation increases in accordance with each Vc value and the right shoulder slightly tilts. However, when Vc exceeds a certain value, FIG.
And the maximum attenuation is obtained at the LC parallel resonance frequency f1, and before and after the frequency f1, the characteristic of the band-shaped rejection with a smaller attenuation is obtained.

As can be seen from the above description, the circuit shown in FIG. 1 functions as an LC parallel resonance rejection filter in the high attenuation state as in the conventional example 1 in FIG. Works as an LC low-pass filter, not as an LC series resonance pass filter described in Related Art Example 1.

In the case of the optimal design, by setting the cutoff frequency f1 'of this LC low-pass filter to be appropriately higher than the LC parallel resonance rejection frequency f1, a low insertion loss can be obtained in a wide frequency band.

Therefore, in the circuit configuration of FIG. 1, at least the condition necessary for the optimum design is f1 <f1 '(4). By substituting equations (2) and (3) into equation (4), L
In the C parallel resonance circuit, the capacitor C (capacitance C) and the shunt capacitors 41, 42, 43 (capacity C ',
C ′ / 2, C ′ / 2), the relationship C> C ′ / 2 (5) holds. Here, when C = C ′, equation (5)
Holds, C = C ′ can be used as an example of the optimal design.

The optimum LC constant can be determined as follows. The stop center angular frequency due to LC parallel resonance is ω1
If the specific impedance when viewed as a transmission line in a low attenuation state is Z0 and C = C ', at the time of minimum attenuation, that is, at the time of operation of a low-pass filter,

(Equation 4) In the damping operation in which the LC parallel resonance rejection filter works effectively,

(Equation 5) Thus, L and C can be uniquely expressed as follows using ω1 and Z0.

[0054]

(Equation 6) The cutoff angular frequency ω1 'of the low-pass filter is

(Equation 7) That is, ω1 ′ is about 1.4 times higher than ω1, and this is a factor for obtaining a low insertion loss in a wide frequency band centered on ω1.

FIG. 6 is a circuit diagram showing a variable attenuator 40A according to a second embodiment of the present invention. FIG. 6 shows a three-stage configuration of LC parallel resonance circuits 3, 14, and 4 in which another stage of LC parallel resonance circuit 14 is connected in series to the circuit configuration of FIG. Each connection point on the signal line between the three LC parallel resonance circuits 3, 14, and 4 (ie, both ends of the LC parallel resonance circuit 14) is connected to the PIN diodes 9g and 9h, the capacitors 41a and 41b (both having a capacitance value of Are connected to a reference potential point (ground) via C ′). When the control voltage Vc is set to be large due to the large number of stages, LC
Since the number of parallel resonance rejection filters increases and the number of connection points on the signal line between the stages is shunted, a larger attenuation is obtained. Control voltage Vc = 0
In the case of, the amount of attenuation up to the cutoff frequency f1 'is small and slightly larger than in the case of FIG. 1, but since the number of low-pass elements is increased, a characteristic that attenuates rapidly when the frequency exceeds the cutoff frequency is obtained. (See FIG. 8).

By configuring the LC parallel resonance circuit in multiple stages as in the circuit of FIG. 6, the capacitance value of the capacitor C of the central LC parallel resonance circuit 14 in the circuit of FIG. By making the capacitance values different from the capacitance values of the capacitors C of the third and fourth, it is possible to obtain a wider band attenuation characteristic. Alternatively, by making the inductance value of the inductor L of the central LC parallel resonance circuit 14 in the circuit of FIG. 6 different from the inductance value of each inductor L of the adjacent LC parallel resonance circuits 3 and 4,
A wider band attenuation characteristic can be obtained.

FIG. 7 is a circuit diagram showing a variable attenuator 40B according to a third embodiment of the present invention. FIG. 7 shows an example in which one stage of the LC parallel resonance circuit is deleted from the circuit configuration of FIG.
This is a configuration having only stages. When the control voltage Vc is set to be large as the number of stages is small, the number of LC parallel resonance suppression filters is reduced, and there is no point where the connection point on the signal line between the stages is shunted, so that large attenuation is hardly obtained. Become. However, when the control voltage Vc = 0,
The amount of attenuation up to the cut-off frequency f1 'is even smaller than in the case of FIG. 1, and the number of low-pass elements is reduced, so that the attenuation characteristics are gradual even beyond the cut-off frequency (see FIG. 8).

FIG. 8 is a graph showing LC values when the control voltage Vc = 0.
It shows a difference in low-pass filter characteristics depending on the number of parallel resonance circuit stages.

FIG. 9 is a circuit diagram showing a variable attenuator 40C according to a fourth embodiment of the present invention. In FIG. 9, FIG.
The PIN diode 5 of the variable attenuator 40 shown in FIG.
6, 7a, 7b and 9 are replaced by MOSFETs (MOS field effect transistors) 5 ', 6', 7a ', 7b' and 9 'as high-frequency variable resistance elements. The LC parallel resonance circuits into which the MOSFETs 5 'and 6' are inserted are indicated by reference numerals 3 'and 4', respectively. Accordingly, these MOSFETs 5 ′, 6 ′, 7
The control voltage Vc is applied to the gates of a ', 7b', and 9 'from the control terminal 13 by resistors 44a, 44b, 44c, 44d, 4
It is configured to be additionally driven by a voltage via the terminal 4e.
The resistors 44a, 44b, 44c, 44d, and 44e are collectively formed as one resistor, and the control voltage Vc is
MOSFETs 5 ', 6', 7a ', 7
The signals may be supplied to the gates b ′ and 9 ′. Other configurations are the same as those in FIG.

As described above, according to the embodiment of the present invention, (1) a high attenuation can be obtained in a frequency band centered on the LC parallel resonance frequency. (2) L for low attenuation
Since it functions as a C low-pass filter, insertion loss at the time of maximum signal transmission is small over a wide band up to the cutoff frequency. (3) The maximum attenuation increases according to the number of connection stages of the LC parallel resonance circuit. Although the insertion loss also increases, the wideband property is maintained unless the cutoff frequency of the low-pass filter is changed, so that the insertion loss does not increase as much as in the conventional case.

[0061]

According to the first aspect of the present invention, the configuration is such that one stage of the LC parallel resonance circuit is used.
Acting as a low-pass filter, the pass characteristic of the high-frequency signal becomes almost constant over a wide band up to its cutoff frequency, and the distortion is low and the insertion loss at the time of signal passage is small.
That is, the usable frequency band can be expanded as compared with the related art. When the control voltage Vc is large, the filter functions as an LC parallel resonance rejection filter as in the conventional case, and the maximum attenuation can be given to the high frequency signal near the parallel resonance frequency.

According to the second aspect of the present invention, it is possible to obtain a wider band signal passing characteristic in the low attenuation state near the control voltage of zero according to the first aspect of the present invention.

According to the third aspect of the present invention, the configuration is such that two stages of the LC parallel resonance circuit are used, and the series connection point of the two-stage LC parallel resonance circuit is connected via the capacitor and the high frequency variable resistance element, respectively. It is connected to the reference potential point. As a result, the attenuation capability as a band rejection filter when the control voltage is increased can be further increased, and
The cut-off frequency when operating as an LC low-pass filter at a control voltage of zero can be made equal to that of the configuration of the first aspect, so that there is an advantage that a low-loss pass characteristic can be maintained over a wide band.

According to the fourth aspect of the present invention, it is possible to obtain a wider band signal passing characteristic in the low attenuation state near the control voltage of zero according to the third aspect of the present invention.

According to the fifth aspect of the present invention, a configuration in which an LC parallel resonance circuit is generally used in a plurality of stages is shown. It is connected to a reference potential point via each high-frequency variable resistance element. As a result, the maximum attenuation as a band rejection filter when the control voltage is increased can be further increased.
The cut-off frequency when operated as a low-pass filter can be made equal to that of the one-stage or two-stage configuration of claims 1 and 2,
Therefore, there is an advantage that a broad band pass characteristic can be maintained. The insertion loss does not increase as much as the conventional type.

According to the sixth aspect of the invention, it is possible to obtain a wider band signal passing characteristic in the low attenuation state near the control voltage of zero according to the fifth aspect of the invention.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a variable attenuator according to a first embodiment of the present invention.

FIG. 2 is an ideal equivalent circuit diagram when a control voltage Vc = 0 in FIG. 1;

FIG. 3 is a characteristic diagram showing a relationship between frequency and attenuation when Vc = 0 in the first embodiment of FIG. 1;

FIG. 4 is an ideal equivalent circuit diagram when Vc is large in FIG. 1;

FIG. 5 is a characteristic diagram showing a relationship between frequency and attenuation when Vc is changed in the first embodiment.

FIG. 6 is a circuit diagram showing a variable attenuator according to a second embodiment of the present invention.

FIG. 7 is a circuit diagram showing a variable attenuator according to a third embodiment of the present invention.

FIG. 8 shows Vc in the first, second, and third embodiments.
The figure which shows the low-pass filter characteristic at the time of = 0.

FIG. 9 is a circuit diagram showing a variable attenuator according to a fourth embodiment of the present invention.

FIG. 10 is a circuit diagram showing a variable attenuator of Conventional Example 1.

FIG. 11 is an ideal equivalent circuit diagram when the control voltage Vc is large in Conventional Example 1.

FIG. 12 is an ideal equivalent circuit diagram when Vc = 0 in Conventional Example 1.

FIG. 13 is a characteristic diagram showing a relationship between frequency and attenuation when Vc = 0 in Conventional Example 1.

FIG. 14 is a circuit diagram showing a variable attenuator of Conventional Example 2.

FIG. 15 is a circuit diagram showing a variable attenuator of Conventional Example 3.

FIG. 16 is a circuit diagram showing a variable attenuator of Conventional Example 4.

[Explanation of symbols]

 3, 4 ... LC parallel resonance circuit 5, 6 ... High frequency variable resistance element 7a, 7b, 9 ... High frequency variable resistance element 8a, 8b ... Impedance matching resistor 11 ... High frequency signal input terminal 12 ... High frequency signal output terminal 13 ... Control voltage Applying terminals 40, 40A, 40B, 40C ... variable attenuators 41, 42, 43 ... capacitors

Claims (6)

[Claims] An LC parallel resonance circuit connected in series to a high-frequency signal path; a first high-frequency variable resistance element inserted into the LC parallel resonance circuit; and a shunt-shaped input terminal of the LC parallel resonance circuit. A series circuit of a second high-frequency variable resistance element and a first impedance matching resistance connected; a third high-frequency variable resistance element shunt-connected to an output terminal of the LC parallel resonance circuit; A first capacitor connected in a shunt manner to an input terminal of the LC parallel resonance circuit;
A second capacitor connected in a shunt manner to an output terminal of the LC parallel resonance circuit; and a control voltage applying unit for applying a control voltage to the first, second, and third high-frequency variable resistance elements. A variable attenuator characterized by: 2. When the capacitance value of a capacitor in the LC parallel resonance circuit is C and the capacitance values of the first and second capacitors are C '/ 2, C>C' / 2. 2. The variable attenuator according to claim 1, wherein a capacitance value is determined. 3. A first and a second LC parallel resonance circuit connected in series to a high-frequency signal path; and a first and a second high-frequency variable inserted into the first and the second LC parallel resonance circuits, respectively. A third high-frequency variable resistor connected in a shunt manner to an input terminal of the first LC parallel resonance circuit;
A series circuit with an impedance matching resistor;
A series circuit of a fourth high-frequency variable resistance element and a second impedance matching resistor connected in a shunt manner to the output end of the LC parallel resonance circuit; and a shunt connection to the input end of the LC parallel resonance circuit A first capacitor;
A second capacitor connected in a shunt manner to an output terminal of the LC parallel resonance circuit; and a fourth high-frequency variable resistor connected in a shunt manner to a connection point between the first and second LC parallel resonance circuits. An element; a third capacitor shunt-connected to a connection point between the first and second LC parallel resonance circuits; and the first, second, third, fourth, and fifth high-frequency variable resistors And a control voltage applying means for applying a control voltage to the element. 4. The capacitance value of each capacitor in the first and second LC parallel resonance circuits is C, the capacitance value of each of the first and second capacitors is C '/ 2, and the capacitance value of the third capacitor is 4. The variable attenuator according to claim 3, wherein the capacitance value is determined so that C> C '/ 2 when the capacitance value is C'. 5. A plurality of LC parallel resonance circuits are connected in series to the high-frequency signal path, and a capacitor is connected in a shunt shape to a connection point between the LC parallel resonance circuits connected in series. 2. A high-frequency variable resistance element is connected in a shunt manner to each of the first and second embodiments.
Variable attenuator as described. 6. The capacitance value of each capacitor in the LC parallel resonance circuit is C, the capacitance value of each of the first and second capacitors is C ′ / 2, and each of the shunt capacitors between the LC parallel resonance circuits is When the capacitance value is C ′, C> C ′ / 2
6. The capacitance value is determined so that
Variable attenuator as described.