JPS62224107A - Cut-off frequency variable filter circuit - Google Patents

Abstract

PURPOSE:To adjust the cut-off frequency to a desired frequency by comparing an output signal obtained from a filter circuit with an output signal obtained from a voltage control circuit and supplying a control signal corresponding to the level difference between the both to a voltage-capacitance variable element. CONSTITUTION:A resistor R1 and a capacitive element C1 of a filter circuit A constitute a low-pass filter, and an output signal Va of the filter circuit A is rectified by a rectifier circuit 1 into a rectified output Va' whose level is changed corresponding to the cut-off frequency. An output signal Vb is obtained by a voltage control circuit B by dividing a reference frequency signal Fa and a rectified output Vb' is obtained by a rectifier circuit 2. The output signals Va', Vb' are compared by a comparator 3 and a control signal Vc controls the capacitance of the capacitive element C1. In setting an output signal Vb to a desired value by using a control signal Vx, the cut-off frequency of the filter circuit A is varied into an optional frequency. In supplying the control signal Vc to the capacitive element C1', the cut-off frequency is set to a desired frequency in the filter circuit A'.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a filter circuit that can change the cutoff frequency to a desired frequency, and particularly relates to a circuit technique that is effective when formed in a semiconductor integrated circuit.

[Conventional technology]

Switched capacitors and
One method is to use a filter (hereinafter referred to as SCF) or a voltage-capacitance variable element known to those skilled in the art as a varicap.

The above-mentioned SCF is described in "Electronic Technology" (Volume 22, No. 10, Published by Industrial Research Association, pp. 91-97). The general idea is that charge is accumulated and discharged through a switch circuit consisting of a MOS transistor in a capacitive component formed on a semiconductor substrate. The cutoff frequency is adjusted by the number of connection stages between each of the above components and the switch circuit, the switch speed, etc.

The present inventor studied a filter circuit that can be formed in a semiconductor integrated circuit and whose cut-off frequency can be varied.

Although the following is not a publicly known technique, it is a technique studied by the present inventor, and its outline is as follows.

[Problem that the invention seeks to solve]

2. Description of the Related Art Linear ICs that process low frequency signals in audio equipment and the like are often constructed of bipolar transistors. The above-mentioned SCF is constructed using a MOS process, and a configuration using a bipolar process is less likely to be practical.

Furthermore, although it is possible to configure a filter circuit consisting of a resistor and a capacitor within an IC, the resistance value and capacitance value of the capacitor vary, making it difficult to set the cutoff frequency to a desired frequency. This was clarified through a study by a number of people.

An object of the present invention is to configure a filter circuit in an IC that can adjust the cutoff frequency to a desired frequency.

The above and other objects and objectives of the present invention [parent features include:
It will become clear from the description of this specification and the accompanying drawings.

[Means for solving problems]

A brief summary of typical inventions disclosed in this subject is as follows.

That is, a first filter circuit consisting of a resistor and a voltage-capacitance variable element formed on a semiconductor substrate by a bipolar process;
a voltage control circuit that performs voltage division operation using resistors;
a comparator for comparing a first output signal obtained from the filter circuit and a second output signal obtained from the voltage control circuit to obtain a control signal corresponding to a level difference between the two;
The control signal is supplied as a negative feedback signal to a voltage-capacitance variable element constituting the first filter circuit to vary the cutoff frequency of the first filter circuit.

[Effect]

According to the above means, the level of the control signal is controlled by controlling the resistance ratio of the voltage control circuit to control the voltage level of the second output signal to a desired level, and furthermore, the level of the control signal is controlled to a desired level.
The capacitance value of the voltage-capacitance conversion element of the filter circuit is controlled, and the cutoff frequency of the first filter circuit can be set to a desired frequency, and at the same time, the cutoff frequency can be set by supplying the control signal to the other filter circuits. This achieves the object of the present invention, which is to obtain another frequency signal whose cut-off frequency is adjusted, and to configure a filter circuit with a variable cut-off frequency in an IC.

〔Example〕

Hereinafter, one embodiment of a variable cut-off frequency filter circuit to which the present invention is applied will be described with reference to FIGS. 1 to 3. In addition,
Figure 1 is a circuit diagram of a filter circuit showing the basic concept;
The figure is a circuit diagram showing a specific example of the filter circuit, and FIG. 3 is a circuit diagram showing each side of the voltage control circuit.

The feature of this embodiment is that the reference frequency signal is supplied to the first filter circuit to obtain an output signal corresponding to the cut-off frequency, and at the same time, the voltage of the reference frequency signal, in other words, the amplitude is controlled by the voltage control circuit as desired. The present invention is configured to obtain an output signal controlled to a voltage level and to control the cutoff frequency of the first filter circuit by comparing the two.

First, the basic concept of the present invention will be explained with reference to FIG.

fa corresponds to a reference frequency signal as referred to in the present invention, and is supplied to the first filter circuit A and voltage control circuit B as referred to in the present invention. The first filter circuit A is constituted by a resistor and a voltage-capacitance variable element (hereinafter referred to as a capacitance element) rectifying element.

The first filter circuit A then obtains an output signal Va with a cutoff frequency determined by the time constant of the resistor and capacitive element.

1. The pressure control circuit B is basically a voltage dividing circuit composed of a fixed resistor and a variable resistor, and is connected to the reference frequency signal fa.
A voltage-divided output signal vb is obtained regardless of the frequency.

The voltages of the output signals Va and Vb are compared by a comparator C according to the present invention, and a control signal Vc corresponding to the level difference between the two is obtained. The control signal Vc is negatively fed back to the capacitive element of the first filter circuit A.

The capacitance value of the capacitive element is controlled in accordance with the voltage level of the control signal Vc, and the time constant is varied. Therefore, the cutoff frequency of the output signal Va is also controlled, and the time constant is controlled until Va=vb. The voltage level of the output signal Va changes by controlling the cutoff frequency, and the voltage level of the output signal Va changes so as to approach the voltage level of the output signal vb.

Along with this, the voltage level of the control (N number Vc) gradually changes, and when Va=Vb, the cutoff frequency of the output signal Va is set to a predetermined frequency.

As a result of the above circuit operation, if the voltage control circuit B is controlled to set the output signal vb to a desired voltage level, the above series of operations can be performed! The @ operation is performed, and the cutoff frequency of the output signal Va can be adjusted to a desired frequency.

Next, the above circuit operation will be explained in more detail with reference to FIGS. 2 and 3.

In addition, in the explanation, the same parts are given the same reference numerals to avoid duplication of explanation.

In filter circuit A, resistor R7 and capacitive element C1 constitute a low-pass filter. The cutoff frequency of the output No. 13 Va is determined by the time constant R3·C3, and 1 is a rectifier circuit, which obtains a rectified output Va' whose level changes in accordance with the cutoff frequency from the output signal Va.

The voltage control circuit B divides the reference frequency signal Fa by the ratio of the fixed resistor R1 and the variable resistor Ra to obtain the output signal vb.The voltage level of the output signal vh can be freely set to a desired voltage by the variable resistor Ra. Can be set to level. A specific example of the voltage control circuit B will be described later with reference to FIG.

2 is a rectifier circuit, which provides a rectified output vb'. The voltage level of this rectified output Vb' is not affected by the frequency of the reference frequency signal fa, but is varied by the resistance value of the variable resistor Ra. Note that the variable resistor Ra can be adjusted to a desired resistance value using the control signal Vx.

The output signals Va' and Vb' are compared by a comparator 3.

Initially, the capacitance of the capacitive element C1 is not controlled, so the voltage difference between the output signals Va' and Vb' is small and is also influenced by variations in the resistor R7 and the capacitive element CI.

Note that the capacitor Co is provided for voltage stabilization.

The control signal Vc controls the capacitance value of the capacitive element C0. Then, due to the change in the capacitance of CI, the time constant of filter circuit A changes, and correspondingly, the voltage level of the output voltage Va', which affects the frequency of the output signal V, gradually decreases. , the voltage difference between Va' and Vb' gradually decreases. The voltage level of control signal Vc also gradually decreases.

Control (capacitance control of capacitive element C3 is continuously performed by No. i Vc.
When it becomes so small that it can be considered as , the capacitance Vc also becomes 0, and the capacitance change of the capacitor C1 is stopped.

As is clear from the above circuit operation, the cutoff frequency of the filter circuit A is determined based on the voltage level of the output signal -qVb. Therefore, the system? If the output signal vb is set to a desired value by the flIl m-no. This means that the frequency can be varied to any desired frequency.

By the way, the control signal Vc has the property of correcting variations in the cut-off frequency of the filter circuit A. Therefore, the control signal Vc can also be used to control the cutoff frequency of other filter circuits.

A filter circuit A' is constructed on the same semiconductor substrate by the same note process as the filter circuit described above. Resistor R2' has the same variation as the above-mentioned resistor R8. Element 01' has the same variations as C1.

By supplying the control signal Vc to the filter circuits C, / in the same manner as described above, the cutoff frequency of the filter circuit A' can be set to a desired frequency in the same manner as described above. If the filter circuit A' is used for other signal transmission, the filter circuit A can be said to be a test filter for setting the cutoff frequency of the filter circuit A'.

In addition, at the end of the filter circuit A'kct, the above reference frequency 1
Instead of No. 6 fa, another frequency signal fv may be supplied,
4 is a signal processing circuit such as an amplifier, and a filter circuit A'
It is replaced according to the purpose of use.

Next, regarding No. 3 and 1, a specific example of the voltage control circuit B will be described.

First, the voltage control circuit B shown in the third internal cause will be explained.

Voltage controlled amplification? S11 is known in the art as a VCA, and is gain controlled by a control signal Vx, and obtains an output (q-number vb) whose level is controlled by the control signal Vx.

Next, referring to the voltage control circuit B shown in FIG. 3), the switch Sa is activated by the control signal Vx.

Resistors R1 with different resistance values are connected by switching Sb and Sc.
1 to cj, , are connected to one end of the resistor 1 ( ). By switching the resistors Rl+ to R1, the voltage division ratio of the voltage control circuit B is controlled. Then, the reference frequency signal Fa is set to a desired level. An output signal vb divided into voltages can be obtained.

(1) A reference frequency signal is supplied to the first filter circuit and the voltage control circuit, and the output signal of the first filter circuit and the output signal of the voltage control circuit are compared to perform control corresponding to the level difference between the two. signal, and apply this control signal to the first control signal described above.
By providing negative feedback to the filter circuit and controlling the cut-off frequency of the first filter path until the output signal is balanced, the effect of stabilizing the cut-off frequency of the first filter circuit can be obtained.

(2) According to (1) above, the output signal of the voltage control circuit is set to a desired voltage level, and the output signal of the filter circuit is balanced to this voltage level, thereby arbitrarily setting the cutoff frequency of the filter circuit. This effect can be obtained.

(3) Since the filter circuit, voltage control circuit, etc. described above are formed without using particularly large capacitors, it is possible to easily integrate the filter into an IC.

(4) With the above (11 to (3)), it is possible to arbitrarily set the cut-off frequency of the IC filter circuit.

(5) With the above (1) to (3), it is possible to obtain the effect that the setting accuracy of the cut-off frequency of the IC filter circuit is improved.

(6) With the above (1) to (4), it is possible to obtain the effect of improving the quality of the filter circuit formed into an IC.

(7) Due to the above (11(2+), the filter circuit with a variable cut-off frequency can be integrated into an IC, so that a variable cut-off frequency filter can be produced at low cost.

Above, the invention made by the present inventor has been specifically explained based on examples, but the present invention is not limited to the above examples, and can be modified to a large extent without departing from the gist of the invention. Needless to say. For example, the number of second filter circuits shown in FIG. 2 may be greater.

Above, we have explained the case in which the invention made by the present inventors is applied to a variable frequency filter circuit, which is the field of application that formed the background of the invention. It can be widely used in electronic devices in general.

[Effects of the Invention] The effects obtained by typical inventions disclosed in this application are briefly explained below.

That is, a reference frequency signal is supplied to a first filter circuit and a voltage control circuit, the output signal of the first filter circuit and the output signal of the voltage control circuit are compared, and control is performed according to the difference voltage between the two. By obtaining No. 16 and setting the cutoff frequency of the filter circuit to a desired frequency, and simultaneously controlling the voltage level of the output signal of the voltage control circuit,
Since the BWr frequency can be arbitrarily varied, it is possible to obtain an IC-based filter circuit whose cut-off frequency can be varied.

[Brief explanation of drawings]

1 to 3 show an embodiment of a variable cut-off frequency filter circuit to which the present invention is applied. FIG. 1 is a circuit diagram showing the basic concept of the filter circuit, and FIG. FIG. 6 shows a circuit diagram showing each side of a voltage ft1llH circuit implementing a circuit illustrating a specific example of a filter circuit. A: first filter circuit, B: voltage control circuit,
C... Comparison circuit, fa... Reference frequency signal, Va,
Vb...output signal, Vc...control signal, 1, 2...
- Rectifier circuit, 3... Comparator, It, -wR,...
Resistance, C0...voltage-capacitance variable element. Figure 1 c, -Ichijo Pressure Ichimachiza I8 Figure 2 Figure 3 L -J

Claims (1)

[Claims] 1. (1) a first filter circuit to which a reference frequency signal is supplied and obtains a first output signal at a voltage level corresponding to the cutoff frequency; (2) a voltage level of the reference frequency signal; a voltage control circuit that obtains a second output signal obtained by dividing the voltage to a desired voltage level; (3) comparing the first and second output signals and controlling the first filter circuit according to the level difference between the two; a comparator that obtains a control signal for varying the cutoff frequency of the first filter circuit, and a cutoff frequency of the first filter circuit is varied by controlling the voltage division ratio of the voltage control circuit. Frequency variable filter circuit. 2. The variable cut-off frequency filter circuit according to claim 1, wherein the voltage control circuit is a voltage-resistance control circuit whose resistance value is controlled by a control voltage. 3. The cutoff according to claim 1, wherein the voltage control circuit is constituted by a voltage divider circuit consisting of a resistor and a plurality of resistors whose resistance values are selected by a switch circuit. Frequency variable filter circuit. 4. The invention according to claim 1, characterized in that a second filter circuit that obtains a second frequency signal different from the first filter circuit by supplying the control signal is provided. Variable cut-off frequency filter circuit.