JPH08186471A - Band pass filter circuit - Google Patents

Abstract

PURPOSE: To avoid the quick gain change of an operational amplifier and to reduce the amplitude voltage of ringing by continuously turning on/off the circuit between the inverted input terminal and the output terminal of the operational amplifier in the refresh period. CONSTITUTION: When switches S1 and S2 are temporarily turned on by a clock ϕr at the time of switching from turning-on/off to turning-off/on of a set of switches X10, X10', X20, and X20', a set of switches X11, X11', X21, and X21'..., electric charge remaining in capacitors C10, C11... and C20, C21... is completely refreshed not to have a bad effect upon following band frequency determination. Switches S1 and S2 are turned-on/off by the high-speed clock in the period when they should be turned on. Since a switch takes action equivalent to a resistance component, the quick gain change of the circuit is suppressed even if switches S1 and S2 are continuously turned off by the end of the refresh period, and therefore, the voltage amplitude of ringing brought about at this time is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switched capacitor type bandpass filter circuit capable of obtaining a large number of pass frequencies (bands), and particularly when switching the capacitance of a capacitor that determines the pass frequency. The present invention relates to a bandpass filter circuit that reduces ringing that occurs.

[0002]

2. Description of the Related Art For example, there is a spectrum display device as a device for analyzing and displaying individual signal levels of a plurality of frequencies of an audio signal. As shown in FIG. 3, the input audio signals of the left and right channels are captured by band pass filter circuits 1L and 1R whose pass frequencies are periodically switched at high speed, and peak detectors 2L and 2R detect the peaks of the pass frequency signals. The value is detected and held, and this is A / D converter 3L, 3R
Digitized by and captured in the control unit (CPU) 4,
The display drive section 5 drives the LCDs 6L and 6R for displaying frequency spectra of a plurality of bands.

The above bandpass filter circuits 1L, 1
R is a switched capacitor type. FIG. 4 is a diagram showing the principle of this switched capacitor type bandpass filter circuit.

P1 and P2 are operational amplifiers, and capacitors C1 and C2 are connected between the output terminal and the inverting input terminal, respectively, and function as integrators. Capacitor C1
Of the switches X31 to X34 and the capacitor C3 connected in parallel to the output terminal of the operational amplifier P1 (OU
T) and an inverting input terminal of the operational amplifier P2, a circuit including switches X41 to X44 and a capacitor C4, a switch connected between the output terminal of the operational amplifier P2 and the inverting input terminal of the operational amplifier P1. A circuit including X51 to X54 and a capacitor C5, and an input terminal (I
N) and the inverting input terminal of the operational amplifier P1. The circuit composed of the switches X61 to X64 and the capacitor C6 is a switched capacitor circuit and functions equivalently to the resistance component.

Of the above switches, the switch X3
1, X34, X41, X44, X52, X54, X6
1, X63 are turned on / off by the clock φ ₁ , and switches X32, X33, X42, X43, X5
1, X53, X62, and X64 are on / off controlled by the clock φ ₂ .

These clocks φ ₁ and φ ₂ have the same frequency f whose phases are mutually inverted and are set to, for example, 50 KHz to 100 KHz which is more than twice the input voice signal. As shown in FIG. The duty is set so that when "H" (switch on), the other is completely "L" (switch off).

In this bandpass filter circuit, its pass frequency fo and attenuation characteristic Q are expressed by the following equations. fo = (f / 2π) ( C5 / C1) 1/2 (C4 / C2) 1/2 Q = 2π · fo / f (C5 / C1-C4 / C2)

Therefore, in the switched capacitor type bandpass filter circuit applied to the conventional spectrum display device, the integrating capacitors C1 and C2 are composed of a plurality of capacitors connected in parallel via respective switches, and the switches are sequentially arranged. By switching, the capacitor C
The capacities of C1 and C2 are periodically switched to sequentially switch the passing frequency fo, and the signal components of a plurality of bands are extracted per unit time.

However, as described above, the capacitors C1 and C
When the switching of the capacitance of 2 is realized by switching of a plurality of capacitors, after the capacitor connected until immediately before is turned off by the switch, residual charge remains in the capacitor until the switch is turned on again after one cycle, There is a problem of impairing the bandpass filter function corresponding to the input signal, and in order to solve this, capacitors C1 and C2 are temporarily provided before and after the switching timing.
The refresh operation was performed to short-circuit all of the above.

[0010]

However, at the time of this short circuit, the inverting input terminals and output terminals of the operational amplifiers P1 and P2 are also short-circuited.
When the gain of 1 and P2 becomes 1 (unit gain), the short circuit is released and the capacitor is connected again,
The operational amplifiers P1 and P2 momentarily enter a state of a predetermined gain (open loop gain) and amplify the offset of the operational amplifier and the like.

Therefore, until this time, the offset voltage becomes ringing that decays with time and appears on the output side until the gain stabilizes at the set value, and the peak detector in the latter stage holds this ringing amplitude voltage and outputs no signal. There is a problem that it is output as the offset voltage at the time.

The present invention has been made in view of the above circumstances, and an object thereof is to avoid an abrupt gain change of an operational amplifier when refreshing a capacitor.
It is to significantly reduce the ringing amplitude voltage.

[0013]

To this end, the present invention is directed to a bandpass filter circuit which is composed of a switched capacitor circuit and which switches the pass frequency by switching the capacitance of a capacitor forming an integrating circuit in combination with an operational amplifier. In this case, a means for continuously turning on / off between the inverting input terminal and the output terminal of the operational amplifier is provided before and after the capacitance switching timing of the capacitor.

[0014]

In the present invention, since the inverting input terminal and the output terminal of the operational amplifier are continuously turned on / off during the refresh period, a resistance component having a predetermined value is equivalently connected thereto. As a result, the gain change of the operational amplifier at the completion of refresh becomes slow.

[0015]

Embodiments of the present invention will be described below. FIG. 1 is a diagram showing a bandpass filter circuit of the embodiment. The same parts as those described in FIG. 4 are designated by the same reference numerals. Capacitors C10, C11, ..., C1N
Is a capacitor functioning as the above-mentioned capacitor C1, and switches X10 and X10 ', X11 and X which are operated by switch signals φ _M0 , φ _M1 , ..., φ _MN , respectively.
, 11 ', ..., X1N and X1N', one of which is selectively connected between the inverting input terminal and the output terminal of the operational amplifier P1.

Capacitors C20, C21, ..., C2
N is a capacitor functioning as the above-mentioned capacitor C2, and the switch signals φ _M0 , φ _M1 , ..., φ _{MN, respectively.}
, X2N and X2N ', one of which is selectively connected between the inverting input terminal and the output terminal of the operational amplifier P2. It is like this.

S1 is capacitors C10, C11, ...
.., refresh switch connected in parallel with C1N,
S2 is a refresh switch connected in parallel with the capacitors C20, C21, ..., C2N and is controlled by the clock φ _r .

In this circuit, capacitors C10 and C20
Are selectively connected by a clock φ _M0 , then capacitors C11 and C21 are selectively connected by a clock φ _M1 , and then capacitors C1N and C2N are selectively connected by a clock φ _MN , Next, the capacitor C10
And C20 are selectively connected by the clock φ _M0 , the capacitors are sequentially and periodically switched to switch the capacities of the capacitors C1 and C2, and the pass frequency fo is switched. That is, a bandpass filter in which a plurality of band frequencies are set in a time division manner within a unit time is realized.

By the way, the switches X10, X10 ', X
20, X20 'set, X11, X11', X21, X2
1'set, ..., X1N, X1N ', X2N, X2
When the set of N ′ is switched from off to on and from on to off, the clocks φ _r temporarily turn on the switches S1 and S2, so that the capacitors C10, C11, ..., C1N, C20, C21 ...
.., the electric charge remaining in C2N is completely refreshed, and does not adversely affect the subsequent band frequency determination.

However, if the switches S1 and S2 are simply turned on continuously for a certain period of time as described above, when the switches change from on to off, the gains of the operational amplifiers P1 and P2 change as described above. The signal becomes violent, and the signal obtained at the output terminal largely rings as shown by the waveform of OUT 'in FIG.

Therefore, in this embodiment, the switch S is
During the period when 1, S2 should be turned on, the switches S1, S2
Is turned on / off with a high-speed clock. When the switch is turned on / off at a high speed in this way, the switch functions equivalently to the resistance component. Therefore, even if the switches S1 and S2 are continuously turned off by the end of the refresh period, a rapid change in the gain of the circuit occurs. Since the voltage is suppressed, the amplitude of the ringing voltage generated at that time can be reduced as shown by the waveform of OUT in FIG.

If the frequency for turning on / off at a high speed is too low, the ringing reduction effect is not obtained, and if it is too high, it becomes equivalent to the on state and the refreshing effect is lost. In the circuit, the range of several hundred KHz to 1 MHz is suitable. In the experiment, the ringing amplitude was the smallest at 400 KHz.

[0023]

As described above, according to the present invention, it is possible to prevent the ringing amplitude generated at the completion of the refresh operation from being reduced and prevent the bandpass filter circuit from malfunctioning.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switched capacitor type bandpass filter circuit according to an embodiment of the present invention.

FIG. 2 is a time chart of the operation of the circuit.

FIG. 3 is a block diagram of a conventional spectrum display device.

FIG. 4 is a circuit diagram showing the principle of a switched capacitor type bandpass filter circuit.

FIG. 5 is a waveform diagram of clock signals φ ₁ and φ ₂ .

[Explanation of symbols]

 S1, S2: Refresh switch

Claims (1)

[Claims] 1. A bandpass filter circuit which is composed of a switched capacitor circuit and which switches a pass frequency by switching the capacity of a capacitor which constitutes an integrating circuit in combination with an operational amplifier. A bandpass filter circuit comprising means for continuously turning on / off between an inverting input terminal and an output terminal of the operational amplifier.