JP2008098730A - Adjustable filter circuit and center frequency adjusting method for filter circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a band-pass or band-stop filter which has its constitution simplified and is adjustable in center frequency, and a center frequency adjusting method for a filter circuit. <P>SOLUTION: A band-pass (or band-stop) filter circuit section 100 is supplied with input signals of two different kind of frequencies f<SB>1</SB>and f<SB>2</SB>, which are equal in level and a predetermined value α larger and smaller than a rated value f<SB>0</SB>of the center frequency, in sequence, and a comparing means compares levels V<SB>1</SB>and V<SB>2</SB>of output signals output in sequence corresponding to the two kind of input signals which are supplied in sequence. A control signal generating means 120 generates a control signal for switching a constant of the band-pass (or band-stop) filter circuit section 100 according to the comparison result to adjust the center frequency so that the difference value D between the levels V<SB>1</SB>and V<SB>2</SB>is zero. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an adjustable filter circuit and a filter circuit that perform control for switching the constant of a passive element among elements constituting an active filter, for example, and change the frequency of the pass (or stop) band of the filter. In particular, the present invention relates to an adjustable filter circuit having a simple configuration and a center frequency adjustment method of the filter circuit.

In general, fluctuations in characteristic values due to variations in the quality of circuit elements of an integrated circuit cannot be avoided. For this reason, in a filter circuit or the like built in an integrated circuit, a circuit for adjusting a cutoff frequency (adjusting a center frequency in the case of a band-pass filter) is necessary.
Several proposals have already been made regarding a low-pass filter circuit and a high-pass filter circuit that can easily adjust the cut-off frequency. In one proposal, a filter circuit is configured to exhibit a cut-off frequency that depends on the selected impedance element among a plurality of selectable impedance elements, and a variable frequency in which the frequency is continuously increased or decreased from the pulse generation circuit. A pulse is supplied to the input of the filter circuit, the state of attenuation of the output pulse of the filter circuit corresponding to the increase or decrease of the input frequency is monitored, and based on the position (repetition frequency) of the pulse that attenuates below the reference value The technique of selecting the impedance element of the constant which is suitable is shown (refer patent document 1).

In another proposal, a control voltage generated in a PLL (Phase Locked Loop) circuit is supplied to a low-pass filter, thereby controlling a gate voltage of a transistor constituting the low-pass filter to control a cutoff frequency. Such a technique is presented (see Patent Document 2).
Japanese Patent Laying-Open No. 2006-74435 (paragraphs 0003 to 0007, paragraphs 0023 to 0030, FIG. 4) JP 2002-271173 A (paragraphs 0002 to 0005, paragraphs 0016 to 0026, FIG. 4)

However, in the technique proposed in Patent Document 1 described above, when the input frequency to the filter is increased or decreased, the frequency value is strictly detected one by one, and the value of the attenuation amount of the output pulse of the filter circuit corresponding to the frequency is detected. In order to adjust the characteristics of the filter by actually applying this technique, a highly accurate detection technique is required.
Further, in the technique proposed in Patent Document 2, the configuration of the filter circuit itself and the configuration relating to the adjustment of characteristics become complicated.
The present invention has been made in view of the above situation, and an object thereof is to provide an adjustable filter circuit and a center frequency adjusting method of the filter circuit which are further simplified.

In order to solve the above problems, the present application proposes the following technologies.
(1) A bandpass or bandstop filter circuit unit configured to be able to change a passband or stopband frequency according to a control signal supplied to itself, and input to the bandpass or bandstop filter circuit unit The level of the first output signal output corresponding to the input signal of the first frequency which is a predetermined level and is lower than the rated value of the center frequency by a predetermined value, and the band pass or band rejection filter circuit A level of a second output signal output corresponding to an input signal of a second frequency that is input to the unit and is higher than the rated value of the center frequency by a predetermined value. And a control signal generating means for generating the control signal in accordance with a comparison result by the comparing means. Filter circuit.

  In the filter circuit of the above (1), constants such as resistors and capacitors as circuit elements are switched in accordance with a control signal supplied to the band-pass (or band-stop) filter circuit unit, and the filter is passed (or The bandpass (or bandstop) filter circuit section has the same level, the frequency is equal to the lower and upper frequencies from the rated value of the center frequency, and is different from each other by a predetermined value. The types of input signals are sequentially supplied, the levels of the output signals sequentially output corresponding to the two types of sequentially supplied input signals are compared by the comparison means, and the control signal generation means is determined according to the comparison result. A control signal for switching the above constant is generated. An adjustable filter circuit with a simple configuration is realized.

(2) The adjustable band according to (1), wherein the band-pass or band-stop filter circuit unit is an active filter configured to switch a constant of a predetermined passive element in accordance with the control signal. Filter circuit.
In the adjustable filter circuit of (2) above, the bandpass (or band rejection) filter circuit unit is configured as an active filter, particularly in the adjustable filter circuit of (1), according to the control signal supplied to itself. Thus, the frequency of the pass (or stop) band is changed by switching the constant of a predetermined passive element. The advantage of the active filter that it is easy to design and adjust is utilized, and the circuit configuration is simplified.

(3) The comparing means detects a level of the first output signal and a level of the second output signal, and detects the levels of the two output signals detected by the level detector. Each holding data holding unit, and a comparison operation unit that compares the levels of the two output signals held in the data holding unit and calculates a level difference as a result of the comparison. (1) Adjustable filter circuit characterized by the above.
In the adjustable filter circuit of (3) above, in particular in the adjustable filter circuit of (1), the level of the first output signal and the level of the second output signal detected by the level detector are retained. And the level difference between the held output signals is calculated by the comparison operation unit.

  (4) A center frequency adjustment method for a bandpass or bandstop filter circuit configured to be able to change the frequency of a passband or stopband according to a control signal supplied thereto, wherein the bandpass or bandstop filter A first frequency input signal having a predetermined frequency lower than a rated value of the center frequency is supplied to the circuit to detect a level of the first output signal corresponding to the first frequency input signal. Level detection processing, and supplying a second frequency input signal that is higher than a rated value of the center frequency to the band pass or band rejection filter circuit by a predetermined value and corresponding to the input signal of the second frequency. A second level detection process for detecting a level of the second output signal; a level of the first output signal obtained by the first level detection process; and a second output obtained by the second level detection process. A comparison process for obtaining a difference value between the two levels by comparing with the level of the signal, and generating the control signal based on the difference value obtained by the comparison process and supplying the control signal to the band pass or band rejection filter circuit And a control signal generating process for adjusting the center frequency of the filter circuit.

  In the center frequency adjustment method for the filter circuit of (4) above, the first frequency that is lower than the rated value of the center frequency by a predetermined value is input to the band pass (or band rejection) filter circuit in the first level detection process. A signal is supplied to detect the level of the first output signal corresponding to the input signal of the first frequency, and the rated value of the center frequency is applied to the band pass (or band rejection) filter circuit in the second level detection process. A second frequency input signal that is higher than the second frequency by a predetermined value to detect the level of the second output signal corresponding to the second frequency input signal, and the first level detection process in the comparison process The level of the first output signal obtained in step 2 is compared with the level of the second output signal obtained in the second level detection process to obtain a difference value between the two levels. processing Based on the difference value obtained by generating the control signal supplied to the band pass (or band-stop) filter circuit. The center frequency of the filter circuit is adjusted by a simple method.

(5) The comparison process includes a process of temporarily holding the level of the first output signal obtained by the first level detection process and the level of the second output signal obtained by the second level detection process. (4) The center frequency adjusting method of the filter circuit according to (4),
In the center frequency adjustment method of the filter circuit of (5) above, the level of the first output signal obtained by the first level detection process and the second level are particularly high in the center frequency adjustment method of the filter circuit of (4). Since the level of the second output signal obtained by the detection process is temporarily held, both the first and second levels can be easily compared.

(6) When the difference value obtained in the comparison process is not zero, the first level detection process, the second level detection process, the comparison process, and the control signal generation process are repeatedly executed. The center frequency adjusting method for a filter circuit according to (4), which is characterized.
In the center frequency adjustment method of the filter circuit of (6) above, in the center frequency adjustment method of the filter circuit of (4), when the difference value obtained by the comparison processing is not zero, the first level detection processing, Since the level detection process, the comparison process, and the control signal generation process are repeatedly executed, the difference value is converged to zero, and the center frequency of the filter circuit is adjusted to an appropriate value.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings to be referred to below, for the sake of convenience, the main part that is the subject of the description is exaggerated as appropriate, and other than the main part is appropriately simplified or omitted.
FIG. 1 is a block diagram showing the configuration of an adjustable filter circuit as an embodiment of the present invention.

In FIG. 1, the band-pass filter circuit unit 100 can change the frequency of the pass band by switching constants such as resistors and capacitors as circuit elements that are, for example, the variable elements 101 in accordance with a control signal supplied to itself. It is configured. The comparison means 110 shown in the figure is connected to the output side of the band pass filter circuit section 100 by a one-dot chain line.
The comparison means 110 sequentially supplied two different types of input signals having the same level and the frequency equal to the lower and upper directions from the rated value of the center frequency and separated from each other by a predetermined value as inputs to the bandpass filter circuit unit 100. In some cases, the levels of the output signals sequentially output corresponding to the two types of input signals sequentially supplied are compared and the comparison is output as follows.

That is, the comparing means 110 outputs a first level input signal corresponding to a first level input signal to the band pass filter circuit unit 100 that is lower than the rated value of the center frequency by a predetermined value. The level of one output signal and the level of the second output signal that is output corresponding to the input signal of the second frequency that is the same level as the input signal and is higher than the rated value of the center frequency by a predetermined value The level detection unit 111 for detecting each of the levels, the data holding unit 112 for holding the levels of the output signals detected by the level detection unit 111, and the levels of the output signals held by the data holding unit 112, respectively. And a comparison operation unit 113 for calculating a level difference as a result of the comparison.
A control signal generation unit 120 that generates a control signal according to the comparison result by the comparison unit 110 is connected to the subsequent stage of the comparison unit 110. The control signal generated by the comparison unit 110 is supplied to the bandpass filter circuit unit 100, and the passband frequency can be changed as described above.

FIG. 2 is a diagram for explaining how the passband frequency deviates from the rated value (design value) due to a change in environmental temperature or a variation in the quality of circuit elements in the bandpass filter circuit unit 100 of FIG.
In FIG. 2, the characteristic indicated by the solid line is the characteristic of the pass band as rated in the band-pass filter circuit unit 100, and this characteristic is indicated by the broken line due to the change in the environmental temperature or the variation in the quality of the circuit elements. In this way, it shifts to the high frequency side (right side) or the low frequency side (left side).

FIG. 3 is a characteristic diagram illustrating a case where the bandpass filter circuit unit 100 exhibits a bandpass characteristic as rated.
In the case of FIG. 3, it corresponds to the input signal of the first frequency f ₁ lower by a predetermined value α than the center frequency f ₀ of the pass band of the band pass filter circuit section 100 (its rated value; the same applies hereinafter). first output signal output Te of a level V _1, the second output signal is output corresponding to the predetermined value α higher by a second input signal of frequency f ₂ than the center frequency f ₀ There is substantially no difference between the level V ₂ and the two levels V ₁ and V ₂ .

In the above description, it is assumed that the condition is set so that the level of the input signal is constant regardless of the frequency.
The phenomenon described with reference to FIG. 3 is, in other words, the level V ₁ of the first output signal output corresponding to the first frequency f ₁ that is lower than the center frequency f ₀ by the value α and the center frequency. when the both levels V _1, V ₂ of the level V ₂ of the second output signal is output corresponding to the predetermined value α higher by a second input signal of frequency f ₂ is equal than f ₀ is This means that the center frequency of the pass band of the band pass filter circuit unit 100 matches the rated value (design value) f ₀ .

FIG. 4 is a characteristic diagram illustrating a case where the bandpass characteristic of the bandpass filter circuit unit 100 deviates from a rated value due to a change in environmental temperature or the like.
In the case of FIG. 4, since the center frequency in the actual band pass characteristic of the band pass filter circuit unit 100 is deviated from the above center frequency f ₀ (rated value), the first frequency f _{1 described} above is input. The level V ₁ of the first output signal output corresponding to the signal and the level V ₂ of the second output signal output corresponding to the input signal of the second frequency f _{2 described} above A level difference D as shown in the figure occurs between the two levels V ₁ and V ₂ .

As described above with reference to FIGS. 3 and 4, if the constants of the circuit elements constituting the band-pass filter circuit unit 100 are adjusted so that the level difference D is zero, the band-pass The center frequency of the pass band of the filter circuit unit 100 matches the rated value (design value) f ₀ .
That is, in FIG. 4, when V ₁ > V ₂ , the center frequency is shifted to a lower frequency side than f _0, so that the center frequency is adjusted to be higher, and when V ₁ <V ₂ , the center frequency is Since it is shifted to a higher frequency side than f _0, it is possible to match the design value by adjusting the center frequency to be lower.
The fact that the center frequency of the pass band matches f ₀ substantially corresponds to the fact that the cut-off frequency of the band pass filter circuit unit 100 is adjusted to an appropriate value.

FIG. 5 is a circuit diagram showing a configuration of a biquad BPF applied as the bandpass filter circuit unit 100 of FIG. The inverting input terminal and the non-inverting input terminal of the operational amplifier OP1 are respectively provided with an input resistor R ₁₁ and an input resistor R ₁₂ , and the non-inverting output terminal and the inverting input terminal are connected in parallel by a resistor R ₁₃ and a capacitor C _11. Similarly, the inverting output terminal and the non-inverting input terminal are connected by a feedback circuit in which a resistor R ₁₄ and a capacitor C ₁₂ are connected in parallel.

The subsequent stage of the operational amplifier OP1, an operational amplifier OP2 are connected in cascade via a resistor R ₃₁ and resistor R ₃₂ as shown. The non-inverted output terminal of the operational amplifier OP2 and the inverting input terminal connected by a feedback circuit by the capacitor C _21, similarly, are connected by a feedback circuit by the capacitor C ₂₂ and the inverted output terminal and the non-inverting input.
On the other hand, the inverting input terminal of the inverting output terminal and an operational amplifier OP1 of the operational amplifier OP2 are connected by a feedback circuit by the resistor R _41, similarly, the non-inverting input terminal of the non-inverting output terminal and an operational amplifier OP1 resistance of the operational amplifier OP2 R ₄₂ A fully differential operational amplifier with differential input and differential output is applied to the operational amplifier OP1 and the operational amplifier OP2.

The characteristics of the filter in the circuit of FIG. 5 are as follows.
That is, if R ₄₁ = R ₄₂ = R, C ₁₁ = C ₁₂ = C ₂₁ = C ₂₂ = C:
Center frequency f ₀ = 1 / 2πRC
Therefore, the shift of the center frequency can be corrected by adjusting the values of the resistors R ₄₁ and R ₄₂ or the values of the capacitors C ₁₁ , C ₁₂ , C ₂₁ and C ₂₂ .

FIG. 6 is a circuit diagram showing a configuration in which a plurality of types of resistors R _{1 to} R ₄ are selectively applied by switching them with a switch circuit in order to make the resistance value R variable.
FIG. 7 is a circuit diagram showing a configuration of a capacitive element in which a plurality of types of capacitors C _{1 to} C ₄ are selectively switched by a switch circuit in order to make the capacitance value C variable.
The configuration for making the resistance value R and the capacitance value C variable is simple, as shown in FIG. 6 and FIG. In addition to these, a voltage variable element whose constant changes in accordance with a control voltage applied to itself, or a circuit and voltage variable which selectively switches the constant by a switch as shown in FIGS. It is possible to adopt an appropriate configuration in which an element is used in combination.

  In the filter circuit described above with reference to FIGS. 5 to 7, the band-pass filter circuit unit is configured as an active filter (biquad type BPF), and a constant of a predetermined passive element is set according to a control signal supplied to itself. By switching, the frequency of the pass (or stop) band is changed. The advantage of the active filter that it is easy to design and adjust is utilized, and the circuit configuration is simplified.

FIG. 8 is a flowchart showing a center frequency adjustment method in the adjustable filter circuit of FIG. First, the data held in the data holding unit is once reset (step S801). Next, it is output corresponding to the input signal f ₂ of the second frequency that is higher than the center frequency f ₀ (its rated value; the same applies hereinafter) described with reference to FIGS. 3 and 4 by a predetermined value α. It is determined whether or not the level V ₂ of the second output signal has already been measured (step S802). When it is determined that the level V ₂ has already been measured (step S802: Yes), the center frequency f An input signal having the first frequency f ₁ lower than ₀ by the predetermined value α is supplied to the input terminal of the bandpass filter circuit unit 100 (step S803).

In step S 803, the band pass filter circuit unit 100 corresponds to the fact that the input signal of the first frequency f ₁ lower than the center frequency f ₀ by the predetermined value α is supplied to the input end of the band pass filter circuit unit 100. The level V _{1 of the} output signal output from the output terminal is measured (step S804... First level detection process), and this measured value is held in the data holding unit 112 (step S805).

On the other hand, in step S802, the level V ₂ is when it is determined that has not yet been determined (step S802: No), the second input signal of frequency f ₂ higher by the predetermined value α than the center frequency f ₀ The signal is supplied to the input terminal of the band pass filter circuit unit 100 (step S806).
In step S806, the band pass filter circuit unit 100 corresponds to the fact that the input signal of the second frequency f ₂ higher than the center frequency f ₀ by the predetermined value α is supplied to the input end of the band pass filter circuit unit 100. The level V _{2 of the} output signal output from the output terminal is measured (step S807... Second level detection process), and this measured value is held in the data holding unit 112 (step S805).

In the above description, the level of the input signal of the frequency f _{1 and} the input signal of the frequency f ₂ supplied to the input terminal of the bandpass filter circuit unit 100 are equal.
When the data is held in step S805, the measurement data of the level V ₁ of the output signal corresponding to the input signal of the frequency f ₁ and the measurement data of the level V ₂ of the output signal corresponding to the input signal of the frequency f ₂ are then stored. It is determined whether both data are prepared (step S808),
When it is determined that the aligned (step S808: Yes), on the basis of both the data held aligned such, calculates the level difference D between the levels V ₁ and level V ₂ (step S809 ... comparison) .

It is determined whether or not D = 0 for the level difference D calculated in step S809 (step S810). If it is determined that D = 0 (step S810: Yes), the adjustment process is terminated.
On the other hand, for the level difference D calculated in step S809, when it is determined that D = 0 is not satisfied (step S810: No), it is determined whether or not the relationship of level V ₁ > level V ₂ is satisfied (step S811). ).

When it is determined in step S811 that level V ₁ > level V ₂ (step S811: Yes), the constant of the variable element 101 (FIG. 1) is adjusted so that the center frequency of the pass band is shifted to the low frequency side. The control signal generation unit 120 (FIG. 1) generates a control signal (control signal generation process), supplies the generated control signal to the variable element 101, and executes the adjustment (step S812).

If it is determined in step SS811 that level V ₁ <level V ₂ (step S811: No), the center frequency of the pass band shifts the constant of the variable element 101 (FIG. 1) to the high frequency side. A control signal for adjusting the signal is generated by the control signal generator 120 (FIG. 1) (control signal generation process), and the generated control signal is supplied to the variable element 101 to execute the adjustment (step S813).

After executing step S812 and step S813 once, the process returns to step S801.
Therefore, as long as the difference value D between the level V ₁ and the level V ₂ does not become 0 in step S810, the above-described first level detection process (step S804), second level detection process (step S807), comparison The processing (step S809) and the control signal generation processing (step S812, step S813) are repeatedly executed, the difference value D is converged to 0, and the center frequency wave in the pass band of the filter circuit is the rated value f ₀ . Is adjusted appropriately.

  The embodiment in the case where the filter circuit unit is a band-pass filter circuit has been described in detail above, but the present invention is also a technical idea that can be applied to the case where the filter circuit unit is a band rejection filter circuit. There is no question about.

It is a block diagram showing the structure of the adjustable filter circuit as embodiment of this invention. It is a figure explaining a mode that the frequency of the pass band of the band pass filter circuit part of FIG. 1 deviates from a rated value (design value). It is a characteristic view showing the case where the bandpass filter circuit part of FIG. 1 shows the bandpass characteristic as rated. It is a characteristic view showing the case where the bandpass characteristic of the bandpass filter circuit part of FIG. 1 has shifted | deviated from the value as rated. It is a figure showing the circuit structure of the band pass filter circuit part of FIG. It is a circuit diagram showing the structure of the resistance element which made resistance value variable. It is a circuit diagram showing the structure of the capacitive element which made the capacitance value variable. 2 is a flowchart illustrating a center frequency adjustment method in the adjustable filter circuit of FIG. 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 ... Band-pass filter circuit part 101 ... Variable element 110 ... Comparison means 111 ... Level detection part 112 ... Data holding part 113 ... Comparison calculating part 120 ... Control signal production | generation means

Claims (6)

  A band-pass or band-stop filter circuit configured to be able to change a pass-band or stop-band frequency according to a control signal supplied thereto; and a predetermined level input to the band-pass or band-stop filter circuit The level of the first output signal output corresponding to the input signal of the first frequency that is lower than the rated value of the center frequency by a predetermined value, and input to the band pass or band rejection filter circuit unit Both of the input signal having the predetermined level and the level of the second output signal output corresponding to the input signal having the second frequency higher than the rated value of the center frequency by the predetermined value. An adjustable filter comprising: comparing means for comparing levels; and control signal generating means for generating the control signal according to a comparison result by the comparing means Road.   2. The adjustable filter according to claim 1, wherein the band-pass or band-stop filter circuit unit is an active filter configured to switch a constant of a predetermined passive element in accordance with the control signal. circuit.   The comparison means holds a level detection unit that detects the level of the first output signal and the level of the second output signal, and holds the levels of both output signals detected by the level detection unit. A data holding unit and a comparison operation unit that compares the levels of the output signals held in the data holding unit and calculates a level difference as a result of the comparison are characterized. The adjustable filter circuit according to claim 1.   A center frequency adjustment method for a bandpass or bandstop filter circuit configured to change a passband or stopband frequency in accordance with a control signal supplied to the control signal, wherein the bandpass or bandstop filter circuit includes: First level detection processing for supplying an input signal having a first frequency lower than the rated value of the center frequency by a predetermined value and detecting the level of the first output signal corresponding to the input signal having the first frequency A second output corresponding to the input signal of the second frequency by supplying an input signal of a second frequency higher than a rated value of the center frequency to the band pass or band rejection filter circuit by a predetermined value A second level detection process for detecting a signal level; a level of the first output signal obtained by the first level detection process; and a second output signal obtained by the second level detection process. Comparison processing for obtaining a difference value between the two levels by comparing with a bell, and control for generating the control signal based on the difference value obtained by the comparison processing and supplying the control signal to the band pass or band rejection filter circuit A center frequency adjustment method for a filter circuit, characterized by performing signal generation processing.   The comparison process includes a process of temporarily holding the level of the first output signal obtained by the first level detection process and the level of the second output signal obtained by the second level detection process. 5. The center frequency adjusting method for a filter circuit according to claim 4, wherein the center frequency is adjusted.   When the difference value obtained by the comparison process is not zero, the first level detection process, the second level detection process, the comparison process, and the control signal generation process are repeatedly executed. The center frequency adjustment method of the filter circuit according to claim 4.

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