JP3553871B2 - Automatic adjustment filter circuit - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic adjustment filter circuit composed of a semiconductor integrated circuit and having filter characteristics (frequency characteristics and the like) automatically adjusted to design values by a reference signal.
[0002]
[Prior art]
FIG. 3 and FIG. 4 are block diagrams showing a configuration example of a conventional automatic adjustment filter circuit that is often used.
[0003]
2. Description of the Related Art Conventionally, in a filter circuit composed of a semiconductor integrated circuit, a cutoff frequency is variably configured by using a variable conductance circuit for a circuit for producing a desired filter characteristic (frequency characteristic and the like), and a circuit element related to the semiconductor integrated circuit is formed. When the cutoff frequency of the filter circuit fluctuates due to the value fluctuation, the characteristic fluctuation of the filter circuit is detected and the fluctuation of the cutoff frequency of the filter circuit is automatically corrected as in the conventional configuration of FIG. There are cases.
[0004]
FIG. 3 is a block diagram showing a first configuration example of a conventional automatic adjustment filter circuit. In FIG. 3, reference numeral 1 denotes an input terminal of a reference signal, reference numeral 2 denotes a reference variable filter designed to have a predetermined input / output phase difference (90 degrees at a reference frequency), and reference numeral 3 denotes a phase difference by detecting a phase difference between two signals. , 4 is a low-pass filter (hereinafter referred to as LPF) for smoothing the output signal of the phase comparator 3, and 5 is the upper and lower limits of the output signal of the LPF 4. A fixed limiter for limiting the amplitude, 6 is a control circuit for converting an output signal from the phase comparator 3 passing through the LPF 4 and the fixed limiter 5 into a predetermined variable filter control signal, and 7 is a reference variable filter 2. This is another variable filter for signal processing.
[0005]
In the automatic adjustment filter circuit having the above configuration, the reference signal is input from the input terminal 1 to the reference variable filter 2 and the phase comparator 3, and the output signal of the reference variable filter 2 is input to the phase comparator 3. The output of the phase comparator 3 is input to the LPF 4, the output of the LPF 4 is input to the fixed limiter 5, the output of the fixed limiter 5 is input to the control circuit 6, and the output signal of the control circuit 6 is a variable filter control signal. Is input to the reference filter 2 and the signal processing variable filter 7. With the above signal paths, the reference variable filter 2, the phase comparator 3, the LPF 4, the fixed limiter 5, and the control circuit 6 form a looped feedback control loop.
[0006]
Here, when the frequency of the reference signal is fixed, when the phase difference between the input and output of the reference variable filter 2 fluctuates from the initial design value due to the fluctuation of the semiconductor process parameters constituting the integrated circuit, the output of the phase comparator 3 changes. As a result, the output of the control circuit 6 also changes.
[0007]
At this time, the variable filter control signal, which is the output signal of the control circuit 6, is input not only to the signal processing variable filter 7 but also to the reference variable filter 2 in order to adjust the cutoff frequency of the reference variable filter 2. . Therefore, the polarity of the signal is designed so that the variation of the cut-off frequency of the reference variable filter 2 is corrected by the output signal (variable filter control signal) of the control circuit 6 changed by the input / output phase difference variation of the reference variable filter 2. By doing so, the cutoff frequency of the reference variable filter 2 is automatically adjusted, and the cutoff frequency fluctuation of the signal processing variable filter 7 controlled by the output signal of the control circuit 6 is also corrected by the automatic adjustment.
[0008]
FIG. 4 is a block diagram showing a second configuration example of the conventional automatic adjustment filter circuit. FIG. 4 shows a first configuration example in which a variable filter control signal that is an output signal of the control circuit 6 is input to a control signal adjustment circuit 12 controlled by an external signal, and the output of the control signal adjustment circuit 12 is used as a reference variable filter 2. Is input. The control signal adjusting circuit 12 changes a control amount of an input signal by giving a certain amount of change (offset) to another control amount.
[0009]
With this configuration, the output signal (variable filter adjustment signal) of the control circuit 6 that is fed back to the reference variable filter 2 is given an offset by the control signal adjustment circuit 12, and the feedback control is corrected including the offset amount. Therefore, the cutoff frequency of the reference variable filter 2 is corrected to a frequency changed by a frequency corresponding to the offset amount from a frequency when only the original variation of the process parameter is corrected. As a result, the cutoff frequency of the signal processing variable filter 7, which is simultaneously corrected by the control circuit 6, changes from the initial design value by a frequency corresponding to the offset amount. That is, the cutoff frequency of the signal processing variable filter 7 can be controlled by the control signal adjustment circuit 12.
[0010]
[Problems to be solved by the invention]
The reference variable filter 2 and the signal processing variable filter 7 shown in FIGS. 3 and 4 are generally composed of a variable conductance circuit as shown in FIGS. 5A and 5B and some capacitors. In FIG.₁  , IN₂  Is a signal input terminal, OUT is a signal output terminal, I₁  , I₂Is the current source, V_R  Is a reference voltage.
[0011]
The phase comparator 3 is a balanced modulator as shown in FIG.GeneralIt is structured. FIG. 6B shows the relationship between the phase difference between two input signals in the phase comparator 3 and the output current average value.
[0012]
Further, the LPF 4 is constituted by a capacitor as shown in FIG.
[0013]
The fixed limiter 5 is configured using a diode or a transistor as shown in FIGS.
[0014]
Further, the control circuit 6 is often constituted by a V / I (voltage / current) conversion circuit as shown in FIG. I₁₁, I₁₂Is the output current.
[0015]
Next, a description will be given of the transient response of the automatic adjustment filter circuit at the time of transient power supply voltage fluctuation such as power supply start-up, power-down, and instantaneous voltage drop in the conventional configuration.
[0016]
In the operation of the automatic adjustment filter circuit shown in FIG. 3 during a transition period in which the power supply voltage is disturbed such as when the power supply rises, the control circuit 6 may be operated due to the instability of the reference signal input from the input terminal 1 or the fluctuation of each circuit during the transition period. Is disturbed, and the frequency control of the reference variable filter 2 and the signal processing variable filter 7 is also disturbed.
[0017]
The fixed limiter 5 is a limiter incorporated for the purpose of limiting abnormal vibration of a signal input to the control circuit 6 in order to reduce such disturbance as much as possible. Therefore, it is difficult to limit the abnormal vibration of the signal because the adjustment range by the feedback becomes narrow. Further, even if the input signal of the control circuit 6 can be slightly stabilized, the fluctuation of the output signal of the control circuit 6 increases again due to the transient fluctuation of the control circuit 6 itself. For example, even if a circuit similar to the fixed limiter 5 is also provided on the output side of the control circuit 6, it is impossible to completely eliminate the fluctuation due to the above-mentioned adjustment amount.
[0018]
Due to the configuration of the feedback control loop, it is generally most effective and efficient to install the fixed limiter 5 before the control circuit 6, and the effect of the fixed limiter 5 when installed after the control circuit 6 is as follows. In many cases, the control circuit 6, the reference variable filter 2, and the signal processing variable filter 7 need to be improved in order to exhibit the effect. The circuit scale required for the improvement is generally minimum. It is much larger than it would be if it were configured to provide the required functionality.
[0019]
Further, in the conventional configuration shown in FIG. 4, even if signal stabilization by the control circuit 5 and further stabilization by providing a fixed limiter after the control circuit 6 are performed, the control Of the variable filter control signal of FIG.
[0020]
Further, a circuit as shown in FIG. 6A is used for the phase comparator 3 used to form the feedback control loop, and between the input terminals 1a and 1b and between the input terminals 2a and 2b of this circuit. Two signals N added₁  , N₂  The current average value output from the output terminals 3a and 3b with respect to the phase difference has characteristics as shown in FIG. 6B, and the average value of the output current becomes 0 every 90 degrees of the phase difference.
[0021]
Generally, a second-order LPF or the like is used for the reference variable filter 2, and the phase of the reference signal input to the reference variable filter 2 changes in the delay direction. Therefore, utilizing the fact that the phase difference between the input and output of the reference variable filter becomes −90 degrees at the cutoff frequency, the phase comparator 3 sets the feedback control loop so that the point a in FIG. Designed.
[0022]
Here, when the cutoff frequency of the reference variable filter 2 fluctuates due to the transient fluctuation of the variable filter control signal in the transient fluctuation due to the power supply voltage fluctuation or the like, the phase difference between the input and output of the reference variable filter 2 also fluctuates. The operating point of the detector 3 deviates from the point a, the output of the phase comparator 3 further fluctuates, and the transient fluctuation itself goes through the feedback control loop. Therefore, even if the feedback operation during normal operation is stable, during transient, each circuit is in a transient response with characteristics different from the transfer characteristics during normal operation, so the loop abnormally oscillates and the loop reaches normal feedback control. Prolong the time.
[0023]
Further, the reference variable filter 2 is configured by a variable conductance circuit as shown in FIG. Generally, the frequency characteristic of such a signal transmission circuit itself is represented by a pass frequency fc (in this conventional configuration example) as shown in a curve group (solid line is gain, broken line is phase) (1) in FIG. Is the frequency of the reference signal, which is the cutoff frequency of the reference variable filter 6, and this frequency is set to fc).
[0024]
The frequency characteristic of the reference variable filter 2, including the characteristic of the curve group (1) in FIG. 10B, shows that the phase difference between the input and the output at the frequency fc of the reference signal is just as shown in FIG. Designed to be -90 degrees.
[0025]
However, at the time of transient fluctuation, the abnormal operation of the circuit is superimposed on a phenomenon such as a drastic decrease in the current for driving the variable conductance circuit, and the frequency characteristics thereof are represented by a curve group in FIG. 10B (solid line is gain, broken line is phase) ( As shown in 2), the frequency characteristics may be greatly deteriorated, and in some cases, the cutoff frequency of the reference variable filter 2 may be significantly reduced. Accordingly, when a state occurs where the phase of the reference variable filter 2 becomes the point b in FIG. 6B at the reference frequency, that is, −450 degrees, the operating condition as the feedback control is the same as the point a in FIG. In some cases, the feedback control is continued without returning the feedback control loop to an appropriate control state.
[0026]
As described above, when the feedback control of the automatic adjustment filter circuit is disturbed due to a transient power supply voltage fluctuation such as at the time of power supply rising,
(1) The output signal of the signal processing variable filter 7 fluctuates according to the disturbance of the feedback control, hinders the stable operation of other signal processing circuits, and the time required for appropriate signal processing to be performed is prolonged. The time required for the whole to reach a stable operation becomes longer.
(2) If the cutoff frequency of the reference variable filter 2 is shifted more than the variation related to the process parameter variation of the semiconductor constituting the integrated circuit, the time required for the feedback control to control the filter cutoff frequency to an appropriate position. And the time required for the entire system to reach stable operation becomes longer.
(3) If the loop is stably maintained with the reference variable filter 2 being controlled so that the operating point of the phase comparator 3 becomes the point b in FIG. 6B, the signal processing being controlled at the same time The frequency characteristic of the variable filter 7 becomes completely different from the design characteristic, and correct signal processing cannot be performed.
[0027]
The above-mentioned problems (1), (2), and (3) have been problems.
[0028]
SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned conventional problems, and an object of the present invention is to provide an automatic adjustment filter circuit in which an intended automatic adjustment is performed stably and at high speed.
[0034]
[Means for Solving the Problems]
To achieve this object, the present inventionThe automatic adjustment filter circuit includes a reference variable filter in which a phase difference between input and output is 90 degrees at a reference frequency, a phase comparator that detects a phase difference between two input signals and outputs a signal proportional to the phase difference. A low-pass filter, a variable filter for signal processing different from the reference variable filter, a control circuit for converting an input signal into a predetermined variable filter control signal for varying a filter characteristic, and an external control signal. A variable limiter that narrows the change width of the variable filter control signal supplied to the reference variable filter, and a power supply voltage fluctuation detection circuit that detects fluctuations in the power supply voltage.
[0035]
Then, the reference signal of the reference frequency is input to the reference variable filter, the output signal of the reference variable filter and the reference signal are input to the phase comparator, and the output of the phase comparator is input to the low-pass filter. The output of the pass filter is input to the control circuit, and the variable filter control signal output from the control circuit isVariable limiterTo form a one-cycle feedback control loop to perform feedback control.
[0036]
In addition, by inputting the power supply voltage fluctuation detection signal output from the power supply voltage fluctuation detection circuit to the variable limiter as an external control signal, when the power supply voltage fluctuates, the variable limiter is input to the reference variable filter under the control of the power supply voltage fluctuation detection circuit. The change width of the variable filter control signal is narrowed.
[0037]
Further, the variable filter control signal is input to the signal processing variable filter.
[0038]
According to this configuration, immediately before the reference variable filter in the feedback control loop, the variable limiter that narrows the change width of the variable filter control signal when the power supply voltage changes transiently is provided, so that abnormalities at the time of power supply voltage fluctuation are provided. A high-performance automatic adjustment filter circuit capable of extremely fast transition from operation to normal feedback operation is obtained.
[0039]
In addition,The above inventionIn the above configuration, a fixed limiter may be inserted between the low-pass filter and the control circuit to limit the upper and lower amplitudes of the output of the low-pass filter before input to the control circuit. When the fixed limiter is inserted as described above, abnormal vibration of a signal input to the control circuit can be limited.
[0040]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, a reference example of the automatic adjustment filter circuit which will be referred to in the description of the embodiment of the present invention will be described.
[0041]
[Reference example]
Figure 1Reference example of automatic adjustment filter circuitFIG. 3 is a block diagram showing the configuration of FIG. In FIG. 1, reference numeral 1 denotes an input terminal of a reference signal, reference numeral 2 denotes a reference variable filter that delays the phase of the reference signal by 90 degrees in the reference signal frequency, and a frequency characteristic can be changed by a variable filter control signal, and reference numeral 3 denotes two input signals. , 4 is an LPF, 5 is a fixed limiter having a fixed limit width, 7 is a signal processing variable filter that can be changed by a variable filter control signal (frequency characteristic control signal), and 6 is a predetermined input signal. A control circuit for converting the reference variable filter 2 into a variable filter control signal and controlling the reference variable filter 2 and the signal processing variable filter 7; a fixed circuit 8 for fixing a variable filter control signal for controlling the reference variable filter 2; Is a variable filter control signal for controlling the variable filter.
[0042]
A reference signal is input from an input terminal 1 to a reference variable filter 2, a reference signal and an output signal of the reference variable filter 2 are input to an input of a phase comparator 3, and an output of the phase comparator 3 is input to an LPF 4 and an LPF 4 Is input to the fixed limiter 5, the output of the fixed limiter 5 is input to the control circuit 6, and the variable filter control signal output from the control circuit 6 is input to the variable filter 7 for signal processing and the fixed circuit 8, The control signal from the power supply voltage fluctuation detection circuit 9 is also input to the fixed circuit 8, and the output of the fixed circuit 8 is input to the reference variable filter 8. The reference variable filter 2, the phase comparator 3, the LPF 4, the fixed limiter 5, the control circuit 6, and the fixed circuit 8 form a feedback control loop.
[0043]
It was configured as aboveReference example of automatic adjustment filter circuitIn FIG. 11, the power supply voltage fluctuation detecting circuit 9 can be realized by, for example, a circuit as shown in FIG. In FIG. 11A, R₁ , R_Two , R_Three Is a resistor, I is a current source, OUT₁ , OUT_Two Is the output terminal, D₁ ~ D_nIs a diode connected in series, V₁ Is the resistance R₁And resistance R_Two Voltage at the connection point, V_Two Is the resistance R_Three And diode D₁ ~ D_nIs the voltage at the connection point.
[0044]
In FIG. 11A, the voltage V₂Is a diode D having a relatively low power supply voltage Vcc.₁~ D_n  And the base-emitter voltage V_BEWhen the power supply voltage Vcc drops by a predetermined value, the power supply voltage Vcc₁  , R₂  V divided by₁  Is the voltage V₂  So that the resistance R₁  And resistance R₂  Is set, a voltage V proportional to the power supply voltage Vcc is obtained.₁  And output terminal OUT₁  And output terminal OUT₂Output current I_OUT1, I_OUT211B is as shown in FIG. 11B corresponding to the fluctuation of the voltage Vcc. Therefore, the output current I_OUT1, I_OUT2Is used, power supply voltage fluctuation detection can be performed by determining the magnitude relationship.
[0045]
Further, the fixed circuit 8 can be realized by a circuit as shown in FIG. 12A or 12B, for example. If a circuit as shown in FIG. 5A or 5B is used for a variable conductance circuit for forming a variable filter, the conductance can be varied by the current I shown in each circuit diagram of FIG.₁  And current I₂  Is controlled by controlling the ratio of the two currents I₁  , I₂  In the case where one of them is fixed originally and only the other current is used, that is, when there is only one kind of control current serving as the variable filter control signal 10, the circuit shown in FIG. The current which becomes I in FIG._FAnd the current I in FIG._constIs set to the same current value as the other fixed current with respect to the current used for the control, and the current I is controlled by controlling the voltage V in FIG._FOr current I_constTo switch the current I_F  ’, The current I_constIs output, the current I used to vary the conductance of FIG.₁Is fixed, and the control of the reference variable filter 2 is also fixed.
[0046]
In addition, the current I used for varying the conductance of FIG.₁And I₂  When the control current 10 shown in FIG. 1 is changed by changing both of them, a circuit such as that shown in FIG. 12B may be used as the fixed circuit 8._F1, I_F2Then, the output current I_F1′ Has the input current I_F1Is output as it is, and the control current I outputted by controlling the voltage V in FIG._F2’Is the current I_F2And current I_F1Are switched, the current I_F1, The current I used to vary the conductance of FIG.₁  And current I₂  Is fixed, and the control of the reference variable filter 2 is fixed.
[0047]
As mentioned above,Reference exampleAccording to the above, the fixed circuit 8 for fixing the variable filter control signal when the power supply voltage changes transiently is provided immediately before the reference variable filter 2 in the feedback control loop, so that the control of the reference variable filter 2 at the time of power supply fluctuation is performed. A high-performance automatic adjustment filter circuit that can transition from abnormal operation to normal feedback operation very quickly even when the power supply voltage fluctuates, such as when the power is turned on or when the power is momentarily stopped, is realized. can do.
[0048]
The variable filter control signal output from the control circuit 6 may be input to other circuits in addition to the reference variable filter 2 and the signal processing variable filter 7. In addition, the fixed limiter 5 may be omitted, and a circuit that performs some processing on the control signal, for example, a control signal adjustment circuit 12 may be inserted between the control circuit 6 and the fixed circuit 8.
[0049]
The effect of the fixed limiter 5 is the same as that of the conventional example.
[0050]
If the signal output from the control circuit 6 is a voltage, a voltage changeover switch may be used based on the same idea as the operation of the circuit shown in FIG. 12, and it is extremely easy to realize.
[0051]
[No.1Embodiment]
FIG. 2 shows the second embodiment of the present invention.1FIG. 3 is a block diagram illustrating a configuration of an automatic adjustment filter circuit according to the embodiment. 2 differs from FIG. 1 in that a variable limiter 11 is used in place of the fixed circuit 2, and the other points in FIG.Reference exampleIs the same as
[0052]
The variable limiter 11 narrows the change width of the variable filter control signal given to the reference variable filter 2 according to the external control signal. The variable limiter 11 inputs a power supply voltage fluctuation detection signal output from the power supply voltage fluctuation detection circuit 9 to the variable limiter 11 as an external control signal. The limiter 11 narrows the change width of the variable filter control signal input to the reference variable filter 2.
[0053]
In the automatic adjustment filter circuit configured as described above, a reference signal is input from the input terminal 1 to the reference variable filter 2, a reference signal and an output signal of the reference variable filter 2 are input to the input of the phase comparator 3, The output of the phase comparator 3 is input to the LPF 4, the output of the LPF 4 is input to the fixed limiter 5, the output of the phase comparator 3 is input to the control circuit 6, and the output of the control circuit 6 is connected to the variable filter 7 for signal processing. The control signal from the power supply fluctuation detecting circuit 9 is also input to the variable limiter 11, and the output of the variable limiter 11 is input to the reference variable filter 2. The reference variable filter 2, the phase comparator 3, the LPF 4, the fixed limiter 5, the control circuit 6, and the variable limiter 11 form a feedback control loop.
[0054]
The above configured1In the automatic adjustment filter circuit of the embodiment, the power supply fluctuation detection circuit 9Reference exampleCan be realized in the same manner as described above. Further, the variable limiter 11 can be realized by a circuit as shown in FIG. 13, for example.Reference exampleIf a circuit as shown in FIGS. 5A and 5B is used for a variable conductance circuit for forming a filter in the same manner as in FIG. 5, the conductance can be varied by the current I shown in each circuit diagram of FIG.₁And I_TwoBy controlling the ratio of Therefore, these two currents I₁ , I_Two In the case where one of them is fixed originally and only the other current is used, that is, when only one type of control current is used as the control signal 10, the circuit shown in FIG. 13 may be used. In addition, the current I used for varying the conductance of FIG.₁ And current I_Two13 can be similarly used in the case where the control current 10 shown in FIG.
[0055]
More specifically, the control current serving as the control signal 10 is represented by the current I in FIG._F  And the current I_F  The resistance R_E  Voltage V_RIs set to an appropriate value, and the voltage V of the current mirror circuit is set._B  To the diode D as shown in FIG.₃₁, D₃₂Current I output by limiting the voltage_F  ′ Is also limited, and when there are two control currents, the respective currents may be limited by using two circuits in FIG. Then, the diode D of FIG.₃₁, D₃₂Bias voltage V₁  , V₂  Is controlled by the signal of the power supply voltage fluctuation detection circuit 9 to control the limit width of the variable limiter 11. As a result, the control amount of the variable filter control signal 10 is limited, and the control of the reference variable filter 2 is limited.
[0056]
As mentioned above,1According to the embodiment, the variable limiter 11 that fixes the variable filter control signal when the power supply voltage changes transiently is provided immediately before the reference variable filter 2 in the feedback control loop. It is possible to limit the control width of the filter 2, and even in the case of power supply voltage fluctuations such as when the power supply is turned on or when the power supply is momentarily stopped, a high-performance automatic operation that can very quickly shift from an abnormal operation to a normal feedback operation. An adjustment filter circuit can be realized.
[0057]
The variable filter control signal output from the control circuit 6 may be input to other circuits in addition to the reference variable filter 2 and the signal processing variable filter 7. Further, the fixed limiter 5 may be omitted, and a circuit for performing some processing on the control signal, for example, a control signal adjusting circuit 12 may be inserted between the control circuit 6 and the variable limiter 11.
[0058]
If the signal output from the control circuit 6 is a voltage, a voltage limiter as shown in FIG. 8A may be used instead of the circuit shown in FIG. 12, and it is extremely easy to realize. .
[0060]
【The invention's effect】
As described above, the present inventionAccording to the automatic adjustment filter circuit, immediately before the reference variable filter in the feedback control loop, a variable limiter that narrows the change width of the variable filter control signal when the power supply voltage changes transiently is provided, so Thus, a high-performance automatic adjustment filter circuit capable of extremely quickly shifting from an abnormal operation to a normal feedback operation can be obtained.
[Brief description of the drawings]
FIG.Reference example of automatic adjustment filter circuitFIG. 3 is a block diagram showing a configuration.
FIG. 2 of the present invention.1FIG. 3 is a block diagram illustrating a configuration of an automatic adjustment filter circuit according to the embodiment.
FIG. 3 is a block diagram illustrating a configuration example of a first conventional automatic adjustment filter circuit.
FIG. 4 is a block diagram illustrating a configuration example of a second conventional automatic adjustment filter circuit.
FIG. 5 is a circuit diagram showing a specific example of a variable conductance circuit used when forming a variable filter.
6A is a circuit diagram illustrating a configuration example of a phase comparator, and FIG. 6B is a characteristic diagram illustrating operation characteristics of the phase comparator.
FIG. 7 is a circuit diagram showing a configuration of an LPF.
FIGS. 8A and 8B are circuit diagrams showing a configuration of a fixed limiter.
FIG. 9 is a circuit diagram showing a configuration of a control circuit.
FIG. 10 is a characteristic diagram for explaining the operation of the reference variable filter.
11A is a circuit diagram showing a configuration of a power supply voltage fluctuation detection circuit, and FIG. 11B is a characteristic diagram for explaining an operation thereof.
FIGS. 12A and 12B are circuit diagrams showing a configuration of a fixed circuit.
FIG. 13 is a circuit diagram showing a configuration of a variable limiter.
[Explanation of symbols]
1 Reference signal input terminal
2 Reference variable filter
3 Phase comparator
4 Low-pass filter (LPF)
5 Fixed limiter
6 control circuit
7 Variable filter for signal processing
8 Fixed circuit
9 Power supply fluctuation detection circuit
10 Variable filter control signal
11 Variable limiter

Claims (2)

A reference variable filter having a phase difference between input and output of 90 degrees at a reference frequency, a phase comparator for detecting a phase difference between two input signals and outputting a signal proportional to the phase difference, and a low-pass filter A variable filter for signal processing different from the reference variable filter, a control circuit for converting an input signal into a predetermined variable filter control signal for varying a filter characteristic, and the reference variable filter according to an external control signal A variable limiter that narrows the change width of the variable filter control signal given to the power supply voltage fluctuation detection circuit that detects fluctuations in the power supply voltage,
The reference signal of the reference frequency is input to the reference variable filter, the output signal of the reference variable filter and the reference signal are input to the phase comparator, and the output of the phase comparator is output to the low-pass filter. Input, input the output of the low-pass filter to the control circuit, and apply the variable filter control signal output from the control circuit to the reference variable filter through the variable limiter , thereby providing a feedback control loop. And perform feedback control,
By inputting a power supply voltage fluctuation detection signal output from the power supply voltage fluctuation detection circuit to the variable limiter as the external control signal, the power supply voltage fluctuation detection circuit controls the variable limiter when the power supply voltage fluctuates. Narrowing the change width of the variable filter control signal input to the filter,
An automatic adjustment filter circuit, wherein the variable filter control signal is input to the signal processing variable filter. 2. The automatic adjustment filter circuit according to claim 1, wherein a fixed limiter is inserted between the low-pass filter and the control circuit so as to limit the upper and lower amplitudes of the output of the low-pass filter and input the same to the control circuit. .

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