JPS643090B2 - - Google Patents

Description

Detailed Description of the Invention This invention integrates an input signal with a first integrator, feeds back the integrated output to the input side, integrates it with a second integrator, feeds back the integrated output to the input side, and The present invention relates to a variable active filter in which the characteristics of the filter are changed by switchingly connecting a capacitor connected to a negative feedback path of at least one integrator.

<Prior art> This type of conventional variable active filter is
As shown in the figure, the input signal from the filter input terminal 11 is input to the analog adder 12, the added output is integrated by the integrator 13, and the integrated output is fed back to the analog adder 12 through the inverting amplifier 14 and then input to the filter. It is added to the input signal of input terminal 11. Further, the output of the integrator 13 is integrated by an integrator 15, and is supplied to an analog adder 12, and is sent to a terminal 11.
is added to the input signal of

For example, the analog adder 12 has an input terminal 11 connected to an inverting input side of an operational amplifier 17 through an input resistor 16, a feedback resistor 18 connected between the output side and the inverting input side of the operational amplifier 17, and a non-inverting The input side is connected to a common potential point, and the output sides of the inverting amplifier 14 and the integrator 15 are commonly connected to the inverting input side.

In the integrator 13, the output side of the adder 12 is connected to the inverting input side of the operational amplifier 21 through the voltage-current converter 19, and the non-inverting input side of the operational amplifier 21 is connected to a common potential point, so that the inverting input side and the output side Between them, capacitors 1 ₁ to 1 _n are connected in parallel through switches 2 ₁ to 2 _n , respectively. The voltage-current converter 19 is constructed by, for example, resistors 3 ₁ to 3 _n connected in series with switches 4 ₁ to 4 _n , respectively, which are connected in parallel.

Similarly, the output side of the integrator 13 is connected to the inverting input side of the operational amplifier 23 through the voltage-current resistor 22, and the non-inverting input side of the operational amplifier 23 is connected to the common potential point, and the inverting side of the integrator 13 is connected to the inverting input side of the operational amplifier 23. Capacitors 5 ₁ to 5 _n are connected between the input side and the output side through switches 6 ₁ to 6 _n , respectively. The voltage-current converter 22 is also constructed by connecting resistors 7 ₁ to 7 _n in series with switches 8 ₁ to 8 _n , respectively, and connecting these in parallel.

The output terminal 24 is derived from the output side of the adder 12, and when the output is extracted from this, a high-pass output is extracted, and when the output terminal 25 is derived from the output side of the integrator 13, the band-pass output is extracted from this. When the output terminal 26 is led out from the output side of the integrator 15, a low-pass output is taken out from this. To change the characteristics of these filters, switch 2 ₁ to 2 _n and switch 4 in integrator 13 are used.
This can be done by changing the integral constant of the integrator 13 by switching and connecting the switches ₁ to 4 _n , and similarly, the integral constant of the integrator 15 can be changed by controlling the switches 5 ₁ to 5 _n and 7 ₁ to 7 _n . By changing the filter characteristics, the filter characteristics can be changed. Generally these integrators 13,1
The integral constants of 5 are approximately equal values. Note that the voltage-current converters 19 and 22 each constitute a multiplier type digital-to-analog converter, and the output can be changed by inputting the switches 4 ₁ to 4 _n and 7 ₁ to 7 _n according to the input digital value. An analog current corresponding to the digital value is output to the input side, and the reference voltage at that time is the input side, that is, the output of the adder 12 or the output of the integrator 13.

In such a variable active filter, the filter characteristics are determined by the switch 2 ₁ as described above.
Control is performed by switching and connecting 2 _n to 5 ₁ to 5 _n , but at this time, the inverting input side of the operational amplifier 23 is virtually connected to a common potential point, that is, to the same potential point as the non-inverting input side. For example, Switch 2
When the switch ₂₂ is turned on from the state where the switch ₃ is on, a charging current flows from the output side of the operational amplifier 21 to the capacitor ₁₂ , and as a result, current flows into the inverting input side of the operational amplifier 21, and therefore the operational amplifier 21 The output side of changes. A similar phenomenon occurs when the capacitors 5 ₁ to 5 _n are switched and connected in the integrator 15.

As described above, in a conventional variable active filter, when a capacitor is switched and connected in order to change the filter characteristics, the output changes instantaneously at that moment, generating noise and phase jitter. Therefore, for example, when an input signal is taken in through such an active filter and its output is analyzed, the analysis is mistakenly performed assuming that noise and phase jitter at the time of switching are present in the input signal.

<Summary of the Invention> An object of the present invention is to provide a variable active filter that does not generate noise or phase jitter on the output side when changing filter characteristics.

According to this invention, in at least one integrator in a variable active filter, a switch for switching and connecting a capacitor for changing the filter characteristics is inserted into the inverting input side of an operational amplifier, and each connection point between the switch and the capacitor is inserted. are connected to a common potential point through an auxiliary switch. With this configuration, the auxiliary switch for the capacitor that is expected to be connected next is turned on before switching the filter characteristics, that is, the capacitor is charged in advance. Then, when the capacitor is switched and connected to the operational amplifier, it becomes possible to obtain a new filter characteristic without any change in the state of the output of the filter. Turning on the auxiliary switch in advance in this way is generally useful when controlling filter characteristics, for example in the case of a tracking filter, because the direction of the cut-off frequency of the filter that will follow can be predicted. It is easy to predict which capacitors will be switched and to keep the auxiliary switch on.

<Embodiment> FIG. 2 shows an example of a variable active filter according to the present invention, in which parts corresponding to those in FIG. 1 are given the same reference numerals. In this example, the integrator 1
3, switches 2 ₁ to 2 _n are inserted between the inverting input side of the operational amplifier 21 and the capacitors 1 ₁ to 1 _n . Further, each connection point between the switches 2 ₁ to 2 _n and the capacitors 1 ₁ to 1 _n is connected to an auxiliary switch 9 ₁
9 to _9n , respectively, to a common potential point. Similarly, in the integrator 15, switches ₆₁ to _6n are inserted between the inverting input side of the operational amplifier 23 and the capacitors ₆₁ to _6n , respectively, and the switches 61 to 6n are inserted between the capacitors ₅₁ to _5n and the switches ₆₁ to _6n. The respective connection points with the auxiliary switches 10 ₁ to 10 n are connected to a common potential point through auxiliary switches 10 1 to 10 _n , respectively.

Further, in this example, the voltage-current converters 19 and 22 are configured, for example, by multiplication type DA converters shown in FIG. 3. The input terminal 27 is connected to one end of a series connection of resistors 28 having a resistance value R, and the other end of the series connection is connected to a common potential point through a resistor 29 having a resistance value 2R. 28
and each connection point with the resistor 29 is connected to the movable contact of the switches S ₁ to _{S n} through the resistor 29 with a resistance value of 2R, and the fixed contact of each one of the switches S ₁ to S _n is common. The other fixed contacts are connected to the output terminal 31. Depending on the input digital signal, the switches S ₁ to S _n are switched and controlled by the corresponding bits to output the output terminal 3.
1, an output corresponding to the digital value can be obtained. In FIG. 2, digital inputs of voltage-current converters 19 and 22 are inputted from terminals 32 and 33, respectively.

In the configuration shown in FIG. 2, the conversion ratios of the voltage-current converters 19 and 22 are K ₁ and K ₂ , and the sums of the capacitances of the capacitors connected to the operational amplifiers 21 and 23 are C 1 and C ₁ , respectively. Assuming that C ₂ , the integral coefficients of the integrators 13 and 15 are K ₁ , C ₁ and K ₂ , C ₂
becomes. The filter characteristics can be changed by switching and connecting the switches 2 ₁ to 2 _n and 6 ₁ to 6 _n to change the total capacitance of the capacitors connected to the operational amplifiers 21 and 23, respectively.

In this invention, the auxiliary switches 9 ₁ to 9 _n ,
For example, when the capacitor _{1 1} is connected to the operational amplifier 21 with the switch 2 ₂ turned on in the integrator 13 and the capacitor _{1 2 connected} to the operational amplifier 21 _, , prior to the connection, turn on the auxiliary switch 9 ₁ for the capacitor 1 ₁ to be newly connected. Therefore, the capacitor ₁₁ is charged according to the potential difference between the output of the operational amplifier 21 and the common potential point, and in this state, the switch 2
₁ is turned on and the auxiliary switch ₉₁ is turned off, the inverting input side of the operational amplifier 21 is virtually a common potential point. Therefore, at the moment when the capacitor ₁₁ is connected to the operational amplifier 21, the
₁ is not charged or discharged at all, and the operational amplifier 2
1 does not change, and therefore the output of operational amplifier 21 also does not change. In other words, the filter output does not suddenly change due to switching of filter characteristics.
No noise or phase jitter occurs.

For example, a variable active filter like this,
When used as a so-called tracking filter that changes cutoff characteristics in accordance with the frequency of an input signal, the input signal frequency is counted and measured with a counter, and the switch 2 is set according to the measured frequency.
A tracking filter can be configured by automatically switching and connecting the digital inputs of ₁ to 2 _n , 5 ₁ to 5 _n , and further terminals 32 and 33; The auxiliary switch corresponding to the capacitor on the other side should always be turned on. When the capacitor is disconnected from the operational amplifier, the output does not change. When increasing the sum of capacitances of connected capacitors, it is easy to configure the auxiliary switch for the next connected capacitor to be turned on in advance.

It is also possible to turn on all the auxiliary switches in advance and disconnect the corresponding auxiliary switches as the capacitor is connected to the operational amplifier.
The ability to charge the capacitor with the output of the operational amplifier must be sufficiently large, which is undesirable. Of course, these switches are switched at a sufficiently high speed compared to the input signal frequency, and generally, such switches
MOSFET can be used.

The present invention can also be applied to the case where a plurality of such variable active filters are connected in series, and in that case, for example, the low-pass output and high-pass output of one variable active filter are added by an adder circuit. The present invention can also be applied to the case where the signal is input to another variable active filter. Further, in FIG. 2, the output of the integrator 13 may be supplied to the non-inverting input side of the adder 12 through a voltage dividing circuit without passing through the inverting amplifier 14. Generally, as shown in FIG. 4, the input from the filter input terminal 11 is added to the output of the integrator 15, and the added output is added to the output of the feedback circuit 14, and the added output is supplied to the output of the integrator 13. 1
The present invention can be applied to an active filter configured to supply the output of 3 to the feedback circuit 14 and the integrator 15. Further, as for the change in the filter, at least one of the integrators 13 and 15 may have the configuration shown in FIG. 2, and the other may have another configuration, for example, the integral constant may not be changed.

[Brief explanation of drawings]

FIG. 1 is a connection diagram showing an example of a conventional variable active filter, FIG. 2 is a connection diagram showing an example of a variable active filter according to the present invention, FIG. 3 is a connection diagram showing an example of the voltage-current converter, and FIG. FIG. 4 is a block diagram showing the general configuration of a variable active filter. 11: Filter input terminal, 12: Analog adder, 13, 15: Integrator, 14: Inverting amplifier for feedback, 19, 22: Voltage-current converter, 2 ₁ to 2 _n ,
6 ₁ to 6 _n : Capacitor switching connection switch, 9
₁ to 9 _n , 10 ₁ to 10 _n : Auxiliary switches.

Claims (1)

[Claims] 1 The input signal given to the filter input side is
Integration is performed by an integrator, the integral output is fed back to the filter input side, and a second integrator is used to integrate, and the integral output is fed back to the filter input side, and the first,
At least one of the second integrators consists of an operational amplifier and a capacitor connected to its negative feedback path, and the input side of the operational amplifier to which the capacitor is connected becomes a virtual common potential point during operation, and the capacitor In a variable active filter in which filter characteristics are changed by switching and connecting a plurality of filters, a switch for switching and connecting the capacitor is inserted on the input side of the operational amplifier, and a switch between the switch and the capacitor is inserted into the input side of the operational amplifier. A variable active filter characterized in that an auxiliary switch is connected between a connection point and a common potential point.