JP3334307B2 - Filter device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter device in which a circuit constant can be easily set and which has a sharp cutoff characteristic.

[0002]

2. Description of the Related Art Butterworth filters and Chebyshev filters are known as typical ones of active filters including an operational amplifier (op-amp). Butterworth filters have a flat characteristic in the pass band,
The cutoff characteristic is relatively gentle, and the characteristic of the second-order Butterworth low-pass filter is shown by a broken line in FIG. Chebyshev filters lack flat characteristics in the passband,
The filter has a relatively sharp cutoff characteristic, and when the filter order is increased to about the seventh order, as shown by a chain line in FIG. 6, a steep cutoff characteristic in which the output becomes almost zero at a frequency of about twice the cutoff frequency. Is obtained.

[0003]

However, as shown in FIG. 7, a seventh-order filter circuit uses four operational amplifiers A3-A6 to form one first-order filter circuit and a third-order filter circuit.
Two secondary filter circuits are connected in series, and in order to obtain a desired cutoff characteristic, it is necessary to determine the circuit time constant of the eleven resistors R8 to R18 and the seven capacitors C3 to C9. Therefore, the degree of freedom in circuit design is limited due to restrictions on the resistance value and capacitance of the element used, and
The determination of the circuit time constant was complicated.

Japanese Unexamined Patent Publication No. Hei 2-183612 proposes a simple and inexpensive filter device having a combined characteristic of a secondary filter and a tertiary filter, using only one operational amplifier. However, even with this, the number of resistors and capacitors that determine the filter characteristics is still large, and the difficulty in determining the circuit time constant and the restrictions on the circuit design have not been improved so much.

The present invention solves the above problem in consideration of the fact that an analog signal input to a vehicle or the like is often logarithmically converted in view of the range of the range. It is an object of the present invention to provide a filter device which is easy and can obtain a sharp cutoff characteristic.

[0006]

To explain the construction of the present invention, an analog signal input is obtained by combining a logarithmic signal part a above or below a predetermined reference voltage with a raised signal part b below or above a reference voltage. A logarithmic conversion circuit 1 composed of an operational amplifier and outputting the signal 1a;
a, a low-order active filter circuit 2 composed of an operational amplifier A1 and an active filter circuit 2
A subtraction circuit 3 composed of an operational amplifier A2, which cuts and outputs the output equal to or lower than the reference voltage from the output 2a of the output 3a.
Is provided.

[0007]

In the above configuration, when the ratio of the logarithmic signal portion a to the raised signal portion b of the composite signal 1a output from the logarithmic conversion circuit 1 is set to a predetermined value and the reference voltage is appropriately selected, the secondary active filter 2 When the output 2a is attenuated by a predetermined amount, the output 3a of the subtraction circuit 3 becomes 0, and a steep cutoff characteristic is exhibited. Since the active filter circuit 2 having a small order is used, the selection of the circuit time constant is easy.
High degree of freedom in design.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, a filter device comprises a logarithmic conversion circuit 1, a low-pass filter circuit 2, and a subtraction circuit 3.
Are connected in series. Hereinafter, the details of each circuit and the operation thereof will be described together.

The logarithmic conversion circuit 2 is a known one including operational amplifiers A7 and A8 as shown in FIG. 2, and has a reference voltage of 6 V and a saturation voltage slightly lower than the power supply voltage of 12 V. An AC coupling circuit is constituted by the capacitor C10 and the resistors R20 and R21. A connection point between the resistors R20 and R21 is connected to an inverting input terminal of an operational amplifier A7 constituting a logarithmic converter via a resistor R22, and the inverting input terminal is connected to a 12V power supply via a resistor R23. A transistor Tr1 in which a diode D1 is connected in parallel
Is connected to a resistor R24 provided at the output terminal.
The 6 V power supply is input to the non-inverting input terminal of the operational amplifier A7 via the resistor R28.

A transistor Tr2 whose collector and base are short-circuited is connected to the resistor R24, the collector of which is connected to a 12V power supply via a resistor R25, and the base of which is an operational amplifier A.
8 non-inverting input terminals. The operational amplifier A8 forms an amplifier circuit, and its inverting input terminal is connected to a 6V power supply via a resistor R26 and to an output terminal via a resistor R27. In this logarithmic conversion circuit 2, 6
When a sine wave signal SW that changes up and down with respect to V is input, its output signal 1a is negative (see FIG. 4A) with respect to a positive portion (a portion larger than 6 V) of the sine wave input SW. The logarithmic signal portion a (less than 6 V) becomes a combined signal corresponding to the positive raised signal portion b saturated at 12 V for the negative portion of the sine wave input SW.

The low-pass filter circuit 2 is a known secondary active filter including an operational amplifier A1 as shown in FIG. That is, the resistors R1 and R2 are connected in series to the inverting input terminal of the operational amplifier A1.
, R2, a capacitor C1 is arranged between the reference power source of 6V and a resistor R3 is connected to an operational amplifier A.
1 output terminal. The inverting input terminal and the output terminal are connected via a capacitor C2.
A reference voltage of 6 V is input to the non-inverting input terminal of the operational amplifier A1. The output signal 1a of the logarithmic conversion circuit 1 is input to a resistor R1. In this filter circuit 2, the resistance values of the resistors R1 to R3 and the capacitances of the capacitors C1 and C2 are determined so as to constitute a Butterworth filter. For example, R1 = R2 = R3 = 6.4 KΩ and C1 = 560.
0pF, C2 = 1200pF, cutoff frequency is 1
0 KHz. The power supply voltage is 12V.

When the frequency of the signal 1a is in the pass band of the filter circuit 2, the signal 1a is only inverted and hardly attenuated and the signal 2a is output to the output of the filter circuit 2 due to the flat characteristic of the Butterworth filter. ((2) in FIG. 4). On the other hand, when the frequency of the signal 1a exceeds the cutoff frequency of the filter circuit 2, the signal 2a is attenuated and falls below the reference voltage of 6 V ((1) and (2) in FIG. 5).

The subtracting circuit 3 includes an operational amplifier A2, and an output signal 2a of the filter circuit 2 is input to a non-inverting input terminal of the operational amplifier A2 via a resistor R4. The non-inverting input terminal is connected to a 6 V reference power supply via a resistor R5.
The inverting input terminal of the operational amplifier A2 is connected to 12 through a resistor R6.
It is connected to a V power supply and to an output terminal via a resistor R7. Here, the voltage of the signal 2a is V2a, the voltage of the non-inverting input terminal is V1, and the voltage of the inverting input terminal is V2.
Assuming that the voltage of the output signal 3a of the operational amplifier A1 is V3a, since the negative feedback is applied with the large gain of the operational amplifier A2, V1 = V2. Since V1 is determined by the ratio of the resistors R4 and R5, V1 = R5 (V2a-6) / (R4 + R5) +6... Further, V3a assumes that the current flowing through the resistor R6 flows directly through the resistor R7. V3a = V2−R7 (12−V2) / R6. Here, R4 = R7 = 24 KΩ, R5 = R6 =
When it is set to 27 KΩ, V3a = V2a−5.3 from the above equation. As is clear from this, when the output signal 2a of the filter circuit 2 becomes 6V of the reference voltage, the output signal 3a of the subtraction circuit 3 becomes 0.7V which is the lower limit of the output voltage V3a of the operational amplifier A2. Therefore, the frequency of the output signal 1a of the logarithmic conversion circuit 1 (which is also the frequency of the sine wave input SW) exceeds the cut-off frequency of the filter circuit 2, and FIG.
As shown in (2), when the output signal 2a of the filter circuit 2 falls below the reference voltage, the output signal 3a of the subtraction circuit 3 becomes 0 level, and the signal is completely cut off.

When this cutoff characteristic is calculated, the voltage of the logarithmic signal part a of the logarithmically converted signal 1a is Va, the voltage of the raised signal part b is Vb, and the output signal 3a of the subtraction circuit 3 (this is the output of the filter device). Voltage of the portion c of 0 level or more of V
As c, Vc = (1 + α) Va / 2− (1−α) Vb / 2... Here, α (≦ 1) is the amplification factor of the filter circuit 2. Thus, since α = 1 in the pass band of the filter circuit 2, Vc = Va from the equation, and the logarithmic signal portion a is passed without any attenuation. V
From the equation, the amplification factor α at which c becomes 0 is α = (Vb−Va) /
(Vb + Va). For example, if Vb = 3.2 Va, α = 0.5. This indicates that the output of the filter device is completely cut off at a frequency at which the amplification factor α of the second-order Butterworth filter is 0.5, and the characteristic is shown by a solid line in FIG. As is known from the drawing, the cut-off characteristic of the filter device in this case is equivalent to that of the seventh-order Chebyshev filter (dashed line in the drawing), and is superior to the above-mentioned Chebyshev filter in consideration of the flat characteristic of the pass band.

Thus, according to the filter device of this embodiment, the logarithmic conversion circuit 1, the secondary filter circuit 2, the subtraction circuit 3
Can realize a cutoff characteristic equivalent to that of a conventional high-order filter circuit, and since the filter circuit only includes a second-order filter circuit, the circuit time constant can be easily set and the design can be simplified. The degree of freedom is high. In this embodiment, to obtain a steep cutoff characteristic at a position close to the cutoff frequency, the larger the value of Vb / Va, the better. However, it is necessary because the voltage Va of the logarithmic signal portion a becomes relatively small. It is determined in consideration of the resolution of the logarithmic signal.

In the above embodiment, the application to the low-pass filter has been described. However, the invention can be applied to a high-pass filter and a band-pass filter by changing the configuration of the filter circuit 2.

The order of the filter circuit 2 is not limited to the second order and may be the first to third order or higher. However, the first order does not provide sufficient cutoff characteristics, while the third or higher order sets the circuit time constant. Increases the difficulty.

In the above embodiment, the signal is inverted by the logarithmic conversion circuit 1 and the filter circuit 2. However, a circuit configuration that does not invert the signal may be used.

In the above embodiment, the input signal SW to the logarithmic conversion circuit 1 is a sine wave, but this is for ease of explanation, and it is needless to say that it can be applied to general analog signals.

[0020]

As described above, according to the filter device of the present invention, excellent cutoff characteristics can be obtained, and the circuit design is easy and the degree of freedom is large. In addition, logarithmic conversion processing conventionally performed by software in a computer is unnecessary.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a filter device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a logarithmic conversion circuit.

FIG. 3 is a circuit diagram of a filter circuit and a subtraction circuit.

FIG. 4 is a signal waveform diagram.

FIG. 5 is a signal waveform diagram.

FIG. 6 is a graph showing a cutoff characteristic of the filter device.

FIG. 7 is a circuit diagram of a conventional filter device.

[Explanation of symbols]

 1 logarithmic conversion circuit 2 filter circuit 3 subtraction circuit

Claims (1)

(57) [Claims] 1. A logarithmic conversion circuit composed of an operational amplifier for outputting an analog signal input as a composite signal in which a logarithmic signal part above or below a predetermined reference voltage and a raised signal part below or above the reference voltage are added to the logarithmic signal part. A low-order active filter circuit composed of an operational amplifier that inputs the synthesized signal, and an operational amplifier that cuts and outputs an output of the active filter circuit that is equal to or less than the reference voltage and outputs the cut-off voltage. A filter device comprising a subtraction circuit.