JP2012028989A - Filter device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a filter device realizing an adjustment function of passband characteristics in small circuit scale.SOLUTION: A filter device according to the present invention comprises: a filter circuit 2 performing band limitation to an input signal; and an adjustment circuit adjusting frequency characteristics of the filter circuit 2. The adjustment circuit adjusts, in the state that a reference signal is input to the filter circuit 2, the frequency characteristics of the filter circuit 2 based on a comparison result of a first signal as an input signal to the filter circuit 2 and a second signal as an output signal from the filter circuit 2.

Description

  The present invention relates to a filter device made of a semiconductor and having a passband characteristic adjustment function.

  A filter made of a semiconductor is affected by variations in passive elements such as resistors and capacitive elements (capacitors), and thus has a problem that a pass band characteristic (sometimes expressed as a frequency characteristic) changes. For this reason, it is necessary to add an adjustment circuit and sequentially adjust the passband characteristics (for example, see Patent Documents 1 and 2 below).

  In Patent Document 1, phase comparison means for comparing a phase relationship between a reference signal (reference signal) input to a filter and a signal output from the filter, and a comparison signal output from the phase comparison means are disclosed. An up / down counter that counts up or down in response, and a control signal generation that supplies a control signal for controlling the filter to a control terminal for controlling a filter constant based on the count output of the up / down counter A filter comprising means is described.

  The control signal generating means includes a current source for generating a reference bias current for controlling the filter, a current source for generating a bias current for fine adjustment corresponding to the count output of the up / down counter, and the reference It is necessary to provide an adder for adding the bias current and the fine adjustment bias current.

  Patent Document 2 discloses an oscillator that outputs a first reference frequency signal and a second reference frequency signal having a frequency higher than the first reference frequency signal, and a normal input signal and the oscillator output at the input terminal of a filter. A switch for switching and supplying, a filter output peak value P1 based on the first reference frequency signal, and a filter output peak value P2 based on the second reference signal, and comparing means for comparing them; A filter adjustment circuit is provided that includes control means for controlling to lower the passband of the filter when the comparison result is P2> P1 + α (α is a dead zone) and to increase the passband when P1> P2 + α. ing.

Japanese Patent Laid-Open No. 9-83294 Japanese Patent Laid-Open No. 5-22076

  However, the conventional filter described in Patent Document 1 and the filter adjustment circuit described in Patent Document 2 require many circuits as adjustment circuits for the passband characteristics, and in particular, have high-accuracy performance as the passband characteristics. Even when it is not necessary, many circuits are required as the adjustment circuit, which has a problem that it has hindered the miniaturization of the circuit.

  The present invention has been made in view of the above, and an object of the present invention is to obtain a filter device that realizes a passband characteristic adjustment function with a smaller circuit scale than conventional ones.

  In order to solve the above-described problems and achieve the object, the present invention is a filter device including a filter circuit that performs band limitation on an input signal, and an adjustment circuit that adjusts the frequency characteristics of the filter circuit, The adjustment circuit compares a first signal that is an input signal to the filter circuit and a second signal that is an output signal from the filter circuit in a state where a reference signal is input to the filter circuit. The frequency characteristics of the filter circuit are adjusted based on the above.

  According to the present invention, in a filter device composed of passive elements such as resistors and capacitors, there is an effect that a function of adjusting variation in passband characteristics can be realized with a small circuit scale.

FIG. 1 is a diagram illustrating a configuration example of a filter device. FIG. 2 is a diagram illustrating a configuration example of the control circuit. FIG. 3 is a diagram illustrating an example of passband characteristics of the filter circuit.

  Embodiments of a filter device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment.
FIG. 1 is a diagram illustrating a configuration example of a filter device according to the present embodiment. As illustrated, the filter device includes a control circuit 1, a filter circuit 2, a comparator 3, a switch 4 and an oscillator 5.

  The control circuit 1 adjusts the passband characteristic of the filter circuit 2 based on the input signal from the comparator 3 and the input signal from the oscillator 5. The filter circuit 2 performs frequency passband restriction on the input signal from the switch 4 according to the input signal from the control circuit 1. The frequency pass band of the signal input from the switch 4 is limited. The comparator 3 compares the input signal to the filter circuit 2 with the output signal from the filter circuit 2 and outputs the comparison result to the control circuit 1. The switch 4 selects either an external input signal or an input signal from the oscillator 5 and outputs the selected signal to the filter circuit 2. The oscillator 5 generates a reference signal for adjusting the passband characteristic of the filter circuit 2 and outputs the reference signal to the control circuit 1 and the switch 4.

  FIG. 2 is a diagram illustrating a configuration example of the control circuit 1. As illustrated, the control circuit 1 includes a counter 11, a logical product circuit (AND circuit) 12, an n-bit shift register 13, a plurality of switches S1 to Sn, and a plurality of capacitors C1 to Cn.

  In the control circuit 1, the counter 11 divides the reference signal generated by the oscillator 5 to generate a switching clock having a predetermined period. The logical product circuit 12 outputs a logical product of the switching clock generated by the counter 11 and the output signal from the comparator 3. The n-bit shift register 13 controls ON / OFF of the switches S1 to Sn according to the output signal (logical product) from the logical product circuit 12. The switches S1 to Sn switch use / nonuse of the capacitors C1 to Cn. That is, each switch is turned ON when using a connected capacitor. Capacitors C1 to Cn are capacitors having the same capacitance, and constitute a capacitance circuit used by the filter circuit 2.

  Next, an operation example of the filter device of the present embodiment will be described with reference to FIG. FIG. 3 is a diagram illustrating an example of the passband characteristics of the filter circuit 2. Note that the broken line (vertical broken line) shown in FIG. 3 indicates the frequency of the reference signal generated by the oscillator 5.

  In the filter device of the present embodiment, when the pass band characteristic is adjusted immediately after the power is turned on, the setting of the switch 4 is switched to select and output the input signal from the oscillator 5. As a result, the reference signal generated by the oscillator 5 is input to the filter circuit 2. The filter circuit 2 performs frequency passband limitation corresponding to the set value of the control circuit 1 on the reference signal that is an input signal, and outputs a signal obtained as a result (reference signal after band limitation is performed). At this time, the comparator 3 compares the input signal to the filter circuit 2 (reference signal before band limitation) and the output signal of the filter circuit 2 (reference signal after band limitation) and outputs the comparison result to the control circuit 1. To do. Specifically, the comparator 3 compares the amplitude width of the reference signal before the band limitation is performed in the filter circuit 2 with the amplitude width of the reference signal after the band limitation is performed, and the difference is determined in advance. When the threshold value is smaller than the set threshold value, “H” logic is output, and when not (when the threshold value is equal to or greater than the threshold value), “L” logic is output.

  In the control circuit 1, first, as an initial value of the n-bit shift register 13, a value for setting all the switches S1 to Sn to OFF and setting the filter circuit 2 to an initial state (a state before adjusting the passband characteristics), for example, All bit “L” signals are contained. The passband characteristic of the filter circuit 2 in this initial state is the state (3) shown in FIG. In this case, the difference between the reference signals before and after the band limitation (difference in amplitude width) is smaller than a preset value (here, 10 dB) (5 dB in the case where (3) in FIG. 3 is set to the initial state). Therefore, the comparator 3 outputs “H”, and this signal becomes an input to the logical product circuit 12.

  Since the “H” signal is input from the comparator 3 to one input terminal of the AND circuit 12, the output of the counter 11 (switching clock) that is an input signal to the other input terminal of the AND circuit 12 is n bits. Input to the shift register 13. When the switching clock is input, the n-bit shift register 13 shifts and outputs the register value bit by bit. In addition, an “H” signal is input to the first bit (when operating so as to shift the value of the first bit to the second bit).

  As a result, only the switch S1 is turned on, and the capacitor C1 is used. The passband characteristic of the filter circuit 2 when the capacitor C1 is used is, for example, in the state of (4) in FIG. 3, and a signal (reference signal after band limitation) corresponding to the adjusted passband characteristic (4) is obtained. It is output from the filter circuit 2. However, even in this state, the difference between the amplitudes of the two input signals to the comparator 3 (6 dB) is smaller than a preset value (10 dB), so the output of the comparator 3 remains “H” and does not change. Therefore, the switching clock is further input to the n-bit shift register 13 and the capacitor C2 is also used (the capacitor C1 and C2 are used). Along with this, the passband characteristic of the filter circuit 2 further changes (approaches the state of (2) in FIG. 3).

  The above adjustment operation is repeated until the difference between the amplitudes of the two input signals to the comparator 3 reaches a set value (in this example, 10 dB corresponding to the state (2) in FIG. 3).

  When the difference between the amplitudes of the two input signals to the comparator 3 is equal to or larger than a preset value (10 dB), the output of the comparator 3 is “L”. As a result, the output of the logical product circuit 12 of the control circuit 1 is fixed to “L” and the switching clock is not transmitted to the n-bit shift register 13, so that the capacitance value (number of capacitors) to be used is determined. When the capacitance value is determined, the switch 4 is switched to a setting for selecting and outputting an external input signal, and the adjustment of the passband characteristic of the filter circuit 2 is completed.

  As described above, the filter device according to the present embodiment generates a reference signal for adjusting the passband characteristics of a filter circuit that limits the band of the input signal (passes only a specific frequency component) and the filter circuit. The control circuit adjusts the passband characteristics of the filter circuit and the control circuit, and the control circuit performs the band limitation with the reference signal (the input signal to the filter circuit) before the band limitation is performed in the filter circuit. Based on the difference of the later reference signal (output signal from the filter circuit), the passband characteristic of the filter circuit is adjusted. Specifically, when the difference is smaller than a preset value, the operation of changing the capacity value to be used with a certain adjustment width is repeatedly executed until the difference becomes equal to or greater than the set value. This makes it possible to adjust the influence of variations in passive elements such as resistors and capacitors generated from semiconductors with a simple circuit, and to reduce the size of the apparatus.

  Note that the initial state of the filter circuit 2 may be set to the opposite of the above, that is, the initial passband characteristic (before adjustment) of the filter circuit 2 may be (1) in FIG. In this case, for example, the output of the comparator 3 is inverted and then input to the control circuit 1 so that the amplitude of the input signal to the filter circuit 2 and the amplitude of the output signal from the filter circuit 2 are larger than the threshold value. In this case, “H” is input to the logical product circuit 12. Further, in the control circuit 1, all bits “H” are set as the initial value of the n-bit shift register 13, so that when the switching clock is input to the n-bit shift register 13 and the bit shift is performed, 1 bit Set "L" signal to the eye. By doing so, the capacitance value used by the filter circuit 2 gradually decreases, and the output value of the n-bit shift register 13 is fixed when the desired passband characteristic is obtained.

  As described above, the filter device according to the present invention is useful as a device that performs band limitation on an input signal and allows a specific frequency component to pass, and particularly, a component (passive element such as a resistor or a capacitor). This is suitable for a filter device that realizes a desired passband characteristic while suppressing the influence of variations in the above.

DESCRIPTION OF SYMBOLS 1 Control circuit 2 Filter circuit 3 Comparator 4 Switch 5 Oscillator 11 Counter 12 AND circuit 13 n bit shift register

Claims (4)

A filter device comprising a filter circuit that performs band limitation on an input signal, and an adjustment circuit that adjusts frequency characteristics of the filter circuit,
The adjustment circuit includes:
Based on a comparison result between a first signal that is an input signal to the filter circuit and a second signal that is an output signal from the filter circuit in a state where a reference signal is input to the filter circuit, A filter device that adjusts frequency characteristics of a filter circuit. The adjustment circuit includes:
An oscillator that generates the reference signal for use in adjusting the frequency characteristics of the filter circuit;
The difference between the amplitude of the first signal and the amplitude of the second signal is calculated. Further, it is confirmed whether the calculated difference is smaller than a predetermined threshold value, and the confirmation result is output as the comparison result. A comparator,
A control circuit for adjusting a capacitance value used by the filter circuit based on the comparison result;
The filter device according to claim 1, comprising: The control circuit includes:
A counter that generates a switching clock of a predetermined period based on the reference signal;
An n-bit shift register that shifts one bit at a time each time the switching clock is input, with all bits set to 0 or all bits set to 1 as an initial state;
An AND circuit for inputting the switching clock to the n-bit shift register in accordance with an output value from the comparator;
A capacitance circuit that is used by the filter circuit and changes a capacitance value according to an output value of the n-bit shift register;
With
When the n-bit shift register is configured to shift the i-th (i = 1, 2,..., N−1) bits to the (i + 1) th bit shift operation, the AND circuit uses the switching clock as the switching clock. The first bit value is set to a value different from the bit value set in each register in the initial state of the n-bit shift register when input to the n-bit shift register. 2. The filter device according to 2. The reference signal and an external signal are used as input signals. When the frequency characteristic of the filter circuit is not fixed, the reference signal is selected and output to the filter circuit, and the frequency characteristic of the filter circuit is A switch that selects a signal from the outside and outputs it to the filter circuit in a confirmed state,
The filter device according to claim 2, further comprising:

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