JPH0129323B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to frequency dependent negative resistance or FDNR.
(Frequency Dependent Negative Resistance)
The present invention relates to a low-pass filter using a circuit, and more specifically to an active filter that can efficiently block noise in a specific frequency band generated during conversion in a digital-to-analog converter.

Conventional technology In order to efficiently block noise in specific frequency bands, GIC (Generalized Impedance Converter)
A conventional active filter including an FDNR circuit, which is one of the circuits, is configured as shown in FIG. That is, a first resistor _R1 , a first capacitor _C1 , and a second resistor are connected in sequence between the signal transmission line 1 and a grounded common line (ground).
R ₂ , a third resistor R ₃ , a fourth resistor R ₄ , and _a second capacitor C ₂ . The other input terminal ( _inverting input terminal) is connected between the second resistor _R2 and the third resistor _R3.
a first operational amplifier _A1 whose output terminal is connected between the third resistor _R3 and the fourth resistor _R4 , and one input terminal (non-inverting input terminal).
is connected between the fourth resistor _R4 and the second capacitor _C2 , and the other input terminal (inverting input terminal) is connected between the second resistor _R2 and the third resistor _R3 . ,
The output terminals are the first capacitor C ₁ and the second resistor R ₂
A frequency dependent negative resistance circuit, that is, a FDNR circuit _2a , consisting of a second operational amplifier A2 connected between
Further, between the input terminal 3 and the output terminal 4, fifth, sixth, seventh, eighth, ninth, and tenth resistors R ₅ , R ₆ , R ₇ , R ₈ , R ₉ , R ₁₀ , 3rd and 4th
capacitors C ₃ and C ₄ and a third operational amplifier A ₃ . In addition, the third capacitor connected in parallel with each other
C ₃ and the fifth resistor R ₅ are connected in series to line 1, and the sixth and seventh resistors R ₆ and R ₇ are also connected to line 1.
connected in series. On the other hand, a fourth capacitor C ₄ and an eighth resistor R ₈ are connected in parallel between line 1 and ground. The non-inverting input terminal of the third operational amplifier A ₃ is connected to the signal transmission line 1, the inverting input terminal is grounded via the tenth resistor R ₁₀ , and the ninth A resistor R ₉ is connected.

Examples of the values of each element are as follows. R ₁
is 419Ω, R ₂ and R ₃ are 4000Ω, R ₄ is 3434Ω, R ₅
is 200kΩ, R ₆ and R ₇ are 2302Ω, R ₈ is 204.6kΩ,
_R9 is 5000Ω, _R10 is 10kΩ, and _C1 to _C4 are 2200pF. Further, the positive power supply line +V of the first to third operational amplifiers A ₁ to A ₃ is connected to the +15V power supply, and the negative power supply line -V is connected to the -15V power supply, and the +V
A 100μF capacitor (not shown) is connected between and ground, and also between -V and ground.
A 100μF capacitor (not shown) is connected.

The filter configured as shown in FIG.
It functions as a low-pass filter with a third-order simultaneous Tiebishiev characteristic with a cutoff frequency of 18 kHz, and a frequency-amplitude characteristic as shown in FIG. 2 is obtained.

However, the capacitor C ₁ of the FDNR circuit 2a,
When the Q of C ₂ decreases, the level of the input signal amplitude decreases in the pass frequency band near the cutoff frequency,
There was no suitable way to correct this.

A 9-order simultaneous Tievishev-type low-pass filter including a conventional FDNR circuit is as shown in Figure 3.
In the circuit of Fig. 1, three FDNR circuits 2b, 2c,
2d and three resistors R ₁₅ , R ₁₆ , and R ₁₇ are added. In the circuit of FIG. 3, the symbols 1, 2a, 3, 4, R ₁ to R ₁₀ , C ₁ to _{C 4} ,
The parts indicated by A ₁ to _{A 3} are substantially the same as the parts indicated by the same numbers in Fig. 1, so their explanation will be omitted, and the added parts will be further explained.
R ₁₅ , R ₁₆ , and R ₁₇ are sequentially connected in series to the signal line 1, and the second, third, and fourth FDNR circuits 2b,
2c and 2d are connected in parallel to line 1 after the resistors R ₁₅ , R ₁₆ and R ₁₇ , respectively. 2nd, 3rd,
and the fourth FDNR circuits 2b, 2c, 2d are the first
It has the same circuit configuration as the FDNR circuit 2a. Third
In each FDNR circuit 2a, 2b, 2c, 2d shown in the figure, capacitors C ₁ , C ₂ , C ₁₁ , C ₁₂ , C ₂₁ , C ₂₂ ,
C ₃₁ and C ₃₂ are set to the same value of 2200 pF, and the resistors
R ₂ , R ₁₂ , R ₂₂ , R ₃₂ , R ₃ , R ₁₃ , R ₂₃ , and R ₃₃ are each set to the same value of 4500Ω, and the resistor R ₁ is set to 445Ω,
Resistor R ₁₁ is 2700Ω, resistor R ₂₁ is 3610Ω, resistor R ₃₁ is
The resistance R ₄ is set to 1655Ω, the resistance R 4 is set to 5880Ω, the resistance R ₁₄ is set to 4405Ω, the resistance R ₂₄ is set to 3830Ω, and the resistance R ₃₄ is set to 4710Ω. FDNR circuit 2a, 2b, 2c, 2d
Other capacitors C ₃ and C ₄ are 2200pF and resistance
R ₅ is 300 kΩ, resistor R ₆ is 5720 Ω, resistor R ₇ is 4740
Ω, resistor R ₈ is 325.6kΩ, R ₉ is 5kΩ, R ₁₀ is 10kΩ,
_R15 is set to 7755Ω, _R16 is set to 6280Ω, and _R17 is set to 6790Ω. Further, the +V power supply line in FIG. 3 is connected to the +15V power supply without using a parallel capacitor, and the -V power supply line is connected to the -15V power supply without using a parallel capacitor.

The circuit configured as shown in Figure 3 is 15.84kHz.
It functions as a 9th-order simultaneous Thievishev-type low-pass filter with a cutoff frequency of , and a frequency-amplitude characteristic as shown in FIG. 4 is obtained. However, a level drop of 0.41 dB occurred at the cut-off frequency, and there was no appropriate means to correct this.

OBJECTS OF THE INVENTION Therefore, an object of the present invention is to provide a low-pass filter including an FDNR circuit that can freely increase the level of a pass frequency band near the cutoff frequency.

Structure of the Invention The present invention to achieve the above object is as shown in FIGS. 5 and 7 which show a first embodiment for easy understanding.
To explain with reference to _the symbols in FIG _{. 2} , a third resistor _R3 , a fourth resistor _R4 , and a second capacitor _C2 ; one input terminal is connected to the first resistor _R1 and the first capacitor _C1 ; The other input terminal is connected between the second resistor _R2 and the third resistor R3, and _the output terminal is connected between the third resistor _R3 and the fourth resistor R3. A first operational amplifier _A1 connected between _{the fourth} resistor and the fourth resistor
connected between R ₄ and the second capacitor C ₂ ;
The other input terminal is connected to the second resistor R2 and the third resistor _R2 .
_a second operational amplifier A ₂ whose output terminal is connected between the first capacitor C ₁ and the second resistor R ₂ ; A low-pass filter including a frequency-dependent negative resistance circuit comprising a level adjustment capacitor C connected between the other input terminal of the amplifier _A2 and the output terminal of the first operational amplifier _A1 . It is.

Effects According to the present invention, the newly added level adjustment capacitor C contributes to raising the level of the pass frequency band near the cutoff frequency, making it possible to obtain the desired level. Therefore, it becomes possible to easily obtain a filter with good characteristics.

Embodiments Next, embodiments of the present invention will be described with reference to FIGS. 5 to 16. However, in FIGS. 5 to 16, parts common to those in FIGS. 1 to 4 are designated by the same reference numerals, and their explanations will be omitted.

First Embodiment (Figures 5 and 6) The third-order low-pass filter of the first embodiment shown in Figure 5 has a configuration in which a level adjustment capacitor C is added to the circuit in Figure 1. There is. That is, the circuit configuration is such that a capacitor C is connected between the output terminal of the first operational amplifier _A1 and the inverting input terminal of the second operational amplifier _A2 , that is, the lower end of the second resistor _R2 .
The constants of each circuit element other than this capacitor C are
As shown in the figure, the value of capacitor C is 12pF, 24pF,
When set to 47 pF, frequency-amplitude characteristics as shown in FIGS. 6(A), (B), and (C) can be obtained. That is, it is possible to raise the level of the passband near the cutoff frequency (18kHz). Note that as the capacitance of capacitor C increases, the level increases.

Second Embodiment (FIGS. 7 and 8) The low-pass filter of the second embodiment shown in FIG. 7 connects the inverting input terminal of the second operational amplifier _A2 of the circuit of FIG. Level adjustment resistor R between
and a level adjustment capacitor C is connected between the output terminal of the first operational amplifier _A1 and the inverting input terminal of the _second operational amplifier A2.

If the circuit constants other than C and R of the circuit in Fig. 7 are the same as in Fig. 1, and C is set to 27 pF and R to 330 kΩ, the frequency-amplitude characteristic shown in Fig. 8 A is obtained, and C is set to 47 pF, and R to 330 kΩ. When R is set to 200kΩ, Figure 8B
A frequency-amplitude characteristic of is obtained. That is, it becomes possible to increase the level near the cutoff frequency.

Third Embodiment (FIGS. 9 and 10) The low-pass filter of the third embodiment shown in FIG. 9 is the second stage FDNR circuit 2b of the circuit in FIG.
A capacitor C is connected between the output terminal of the first operational amplifier _A11 and the inverting input terminal of the second operational amplifier _A12 .
It has a configuration in which it is additionally connected. In the circuit of FIG. 9, if C is set to 17 pF and other circuit constants are set to be the same as in FIG. 3, the frequency-amplitude characteristics shown in FIG. 10 can be obtained. In other words, the level reduction at the cut-off frequency can be suppressed to 0.15 dB.

Fourth embodiment (FIGS. 11 and 12) The low-pass filter of the fourth embodiment shown in FIG. 11 is the second stage FDNR circuit 2 of the circuit in FIG.
A resistor R is additionally connected between the inverting input terminal of the second operational amplifier A ₁₂ in b and the ground, and the output terminal of the first operational amplifier A ₁₁ and the second operational amplifier A _{12 are} further connected.
The configuration is such that a capacitor C is additionally connected between the inverting input terminal and the inverting input terminal of the inverter. In the circuit shown in Figure 11, the resistor R is set to 530kΩ, the capacitor C is set to 12pF,
If the other circuit constants are set the same as in Figure 3, the first
The frequency-amplitude characteristics shown in Figure 2 are obtained, and the level down at the cut-off frequency can be reduced to 0.15 dB.

Modifications Although the present invention has been described in terms of embodiments, the present invention is not limited to these examples and can be further modified. For example, it is of course applicable to low-pass filters other than 3rd order or 9th order. plural
In the case of having FDNR circuits 2a, 2b, 2c, and 2d, a capacitor C may be additionally connected to a circuit other than the second stage circuit 2b, or a plurality of FDNR circuits may be connected.
A capacitor C may be connected to the circuit.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a conventional low-pass filter including an FDNR circuit, Fig. 2 is a frequency-amplitude characteristic diagram of the circuit in Fig. 1, Fig. 3 is a circuit diagram showing a conventional 9th-order low-pass filter, 4 is a frequency-amplitude characteristic diagram of the circuit in FIG. 3, FIG. 5 is a circuit diagram showing the low-pass filter of the first embodiment of the present invention, and FIG. 6 is a frequency-amplitude characteristic diagram of the circuit in FIG. 5. , FIG. 7 is a circuit diagram showing a low-pass filter according to a second embodiment of the present invention, FIG. 8 is a frequency-amplitude characteristic diagram of the circuit of FIG. 7, and FIG. 9 is a diagram showing a low-pass filter according to a third embodiment of the present invention. A circuit diagram showing a low-pass filter, FIG. 10 is a frequency-amplitude characteristic diagram of the circuit of FIG. 9, and FIG. 11 is a diagram of the fourth circuit of the present invention.
A circuit diagram showing a low-pass filter according to an embodiment of
FIG. 2 is a frequency-amplitude characteristic diagram of the circuit of FIG. 11. 1... line, 2a, 2b, 2c, 2d...
FDNR circuit, _R1 to _R10 ...resistor, _C1 to _C4 ...capacitor, _A1 to _A3 ...operational amplifier, R...level adjustment resistor, C...level adjustment capacitor.

Claims (1)

[Claims] 1. A first resistor R ₁ and a first capacitor connected in sequence between the signal transmission line 1 and the common line.
C ₁ and the second resistor R ₂ and the third resistor R ₃ and the fourth resistor
_R4 and a second capacitor _C2 , one input terminal is connected between the first resistor _R1 and the first capacitor _C1 , and the other input terminal is connected to the second capacitor C2. resistance R ₂ and said third resistance R ₃
is connected between the third resistor and the output terminal is connected between the third resistor and
A first resistor connected between R ₃ and the fourth resistor R ₄
operational amplifier _A1 , one input terminal connected between the fourth resistor _R4 and the second capacitor _C2 , and the other input terminal connected between the second resistor R2 and the third capacitor _C2 ; Resistance of R ₃
a second operational amplifier _A2 whose output terminal is connected between the first capacitor _C1 and the second resistor _{R2 ;} a level adjustment capacitor C connected between the other input terminal and the output terminal of the first operational amplifier _A1 ; and a low-pass filter including a frequency-dependent negative resistance circuit.