JPH07109972B2 - Filter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in the following order.

A. Industrial field of use B. Outline of invention C. Prior art D. Problems to be solved by the invention E. Means for solving the problem F. Action G. Example G-1. Example (Figs. 1 and 2) G-2. Concrete example of integrator (Fig. 3) G-3. Application example to analog IC (Fig. 4) G-4. Concrete example of filter adjustment (Fig. Fig. 5 and Fig. 6) G-5. Other Embodiments (Figs. 7 and 8) H. Effects of the Invention A. Field of Industrial Application The present invention relates to a filter circuit, and more particularly to two integral circuits. The present invention relates to a so-called double-integral type filter circuit in which resistors are connected in series.

B. SUMMARY OF THE INVENTION The present invention is a double integration type filter circuit in which two integrators are connected in series, and one input terminal of the first integrator and an integration capacitance of the first integrator. , At least one of the integration capacitance of the second integrator can be appropriately switched and selected by the changeover switch between the input signal supply and the ground, and at the time of filter adjustment, the filter circuit has a trap filter characteristic. By connecting the changeover switch so that it has a structure, the time for filter adjustment is shortened,
It is possible to realize high precision with a simple configuration and to automatically adjust filter characteristics.

C. Conventional Technology Generally, it is necessary to adjust a peak frequency, a dip frequency, a cutoff frequency, etc. of a filter circuit to a predetermined target value in an electronic circuit check process or the like. In particular, in circuits formed in analog integrated circuits (ICs), the relative ratio of rated values of transistors, resistors, capacitors, etc. can take relatively high accuracy, but the absolute value varies from IC to IC, so accuracy is required. The above adjustment is indispensable in the filter circuit to be used.

Here, as a filter circuit that can be configured in the IC, for example, a double integration type filter circuit as disclosed in Japanese Patent Publication No. 61-17370 is conventionally known. This double integration type filter circuit has a feature that it is suitable for being constructed in an IC and can obtain high accuracy, and that each filter characteristic including a so-called Q value is easily adjusted. .

D. Problems to be Solved by the Invention By the way, when the conventional filter circuit is adjusted, characteristic portions of the characteristic curve such as so-called cutoff points and peak portions are detected, and the characteristic portions of these characteristic portions are detected. It is necessary to adjust the frequency, so-called cutoff frequency, peak frequency, etc. to a predetermined target value, but finding the cutoff point, peak point, etc. from the characteristic curve is an LPF (low pass filter). In the case of an HPF (high-pass filter) or a so-called BPF (band-pass filter) having a low Q value, it is troublesome or difficult.

In particular, when attempting to automate such filter adjustment, it is difficult to mechanically distinguish and read the characteristic portion such as the cutoff point from the frequency characteristic curve, and the adjustment accuracy may be reduced. There is.

The present invention has been made in view of the above circumstances, and it is possible to easily and accurately detect the characteristic portion of the filter characteristic during the filter adjustment, the filter adjustment accuracy is high, and the adjustment time is short. The purpose is to provide such a filter circuit.

E. Means for Solving the Problems In order to solve the above-mentioned problems, a filter circuit according to the present invention has a differential amplifier and an integrating capacitor whose one end is connected to the output of the differential amplifier. An output terminal is connected to the output terminal of the amplifier, and the first and second integrators have the two input terminals of the differential amplifier as one input terminal and the other input terminal, respectively.
The output terminal of the first integrator is connected to one input terminal of the second integrator, and the output terminal of the second integrator is connected to the other input terminal of the first integrator and the second integrator. In the filter circuit configured to be connected to the other input terminal of the integrator, the other end of the integration capacitance of the first integrator, one input terminal of the first integrator, and the second integrator of the second integrator. At least one of the other ends of the integration capacitors is provided with a changeover switch for properly changing the connection to the input signal or the ground, and at the time of filter adjustment, the changeover switch is supplied with a changeover signal from outside the filter circuit to perform the first integration. The changeover switch so as to connect one input terminal of the capacitor and the other end of the integrating capacitor of the second integrator to the input signal, and to connect the other end of the first integrating capacitor to the ground. Switch, fill above Circuit is characterized in that as confirmation of a characteristic portion of the filter characteristic so as to have a trap filter characteristics can be easily filter configuration.

F. Action When adjusting the filter, it is easy to detect the characteristic part of the frequency characteristic curve, such as a trap filter, and the filter circuit is switched to the circuit configuration of the characteristic suitable for adjustment, and the filter adjustment is performed. Easy and highly accurate adjustment.

G. Embodiment G-1. One embodiment (FIGS. 1 and 2) FIG. 1 is a block circuit diagram showing a filter circuit according to one embodiment of the present invention. 1 shows a so-called biquad filter of the type. In this embodiment, for example, in an LPF (low-pass filter) circuit, the connection relation of the circuit is switched and changed only when the filter adjustment is required, and the characteristic part of the filter characteristic is easily detected and the characteristic suitable for the filter adjustment is obtained. An example is shown in which a trap filter circuit having the above is configured and is returned to the original LPF circuit after the adjustment is completed.

The biquad filter shown in FIG. 1 comprises a first integrator composed of a first operational amplifier (opamp) 11 and a first integrating capacitance (capacitor) 12, and a second operational amplifier 13
Is a so-called active filter configured by connecting in series with a second integrator composed of the second capacitor 14 and
The output of the operational amplifier 11 is supplied to the non-inverting input terminal of the operational amplifier 13, the output of the operational amplifier 13 is fed back to the inverting input terminal of the operational amplifier 11, and the output of the operational amplifier 13 is the feedback ratio β.
It is fed back to the inverting input terminal of the operational amplifier 13 via the feedback circuit 15. This feedback circuit 15 is composed of a voltage divider circuit composed of resistors R ₁ and R ₂ , and the resistor R ₂ is an operational amplifier.
It is connected to the 13 output terminal side. 2 of such a configuration
In the multiple integration type biquad filter, the non-inverting input terminal Ta of the operational amplifier 11 and the capacitor 12 which is the integration capacitance
Other end (connection point with resistor R ₁ ) Tb and the other end of capacitor 14
The characteristics of BPF, LPF, HPF, trap, phase shifter, etc. can be realized by appropriately selecting whether to supply an input signal or ground to Tc. Each of these terminals Ta,
The filter type according to the selection of signal input to Tb and Tc and ground is become that way.

Here, in the example of FIG. 1, the input signal from the signal source SG is supplied to the non-inverting input terminal Ta of the operational amplifier 11 via the input terminal 16, and the other end of the capacitor 12 (connection with the resistor R ₁ is connected). Point) and the other end Tc of the capacitor 14 is selectively switched between the input signal supply and the grounding through the changeover switch 18, as described above, for the characteristic adjustment trap filter. And an LPF which is an original filter circuit can be selected. That is, the input terminal 16 is supplied to the selected terminal a of the changeover switch 18, and the selected terminal b is grounded. Also,
The output signal is taken out from the output terminal 17 of the operational amplifier 13. In this case, the frequency characteristic of the trap filter constituted by switching the changeover switch 18 to the selected terminal a is , And the frequency characteristic of the LPF configured by switching the changeover switch 18 to the selected terminal b is It is expressed by the transfer function of.

Therefore, in normal use, the changeover switch 18 is changeably connected to the terminal b side to realize an LPF having a frequency characteristic as shown by the solid line in FIG. Switch 18 to the terminal a side and connect the second
While switching to the trap filter characteristics as shown by the phantom lines in the figure, the output level of the output signal from the output terminal 17 is measured by a level measuring device LM such as a level meter. A certain dip portion is detected, and filter adjustment is performed so that this dip frequency f ₀ matches a predetermined target frequency. As a result, the filter characteristics can be adjusted easily by using filter characteristics that are easy to detect, such as the dip section of the trap characteristics, compared to filter characteristics that are difficult to detect, such as the LPF cutoff point. It can be done accurately.

G-2. Specific example of integrator (Fig. 3) Next, Fig. 3 shows a specific example of one integrator used in the above biquad filter. In this FIG.
The non-inverting input terminal 21 and the inverting input terminal 22 of the operational amplifier (the operational amplifiers 11 and 13 in FIG. 2) are connected to the bases of the transistors 23 and 24 forming the differential amplifier,
A current corresponding to the input voltage between the terminals 21 and 22 flows through the resistor R _E connected between the emitters of the transistors 23 and 24. The sum and difference of this current and the respective currents I ₁ and I _{1 of} the constant current sources connected to the respective emitters of the transistors 23 and 24 are the diode 25 connected to the respective collectors of the transistors 23 and 24. , 26, and the terminal voltages of the diodes 25, 26 appearing in accordance with the respective currents are supplied to the bases of the transistors 27, 28 constituting the common emitter differential transistor pair. The common emitter of these transistors 27 and 28 has a current value of 2
It is grounded via a constant current source 29 of I _2, the signal current flowing through the collector of the differential transistor pair will be amplified in the I ₂ / I ₁ times. The collector output of the transistor 28 is taken out through the current mirror circuit 30 and charges the capacitor 32 that serves as the integral capacitance. The voltage at one end of the capacitor 32 is received by the transistor 34 and taken out from the output terminal 35. The other end 33 of the capacitor 32 is input or grounded as described above.

In the integrating circuit configuration of FIG. 3, the current value I _{2 of the} constant current source 29 and the current source 31 on the output side of the current mirror circuit 30 is
By changing the, the frequency characteristics of the biquad filter in FIG. 1, specifically the low-pass filter characteristic curve and the trap filter characteristic curve in FIG. 2, move in parallel in the frequency axis direction, and the cutoff frequency and dip Frequency adjustments are made. However, at the time of actual filter adjustment, the changeover switch 18 is switched to the terminal a side to change the trap characteristic so that the characteristic portion of the filter characteristic curve can be easily detected.

G-3. Application example to analog IC (FIG. 4) Next, a specific example in which the filter circuit of the above-described embodiment is provided inside an analog integrated circuit such as an audio multiplex demodulation IC used in a television receiver. Will be described with reference to FIG. That is, the filter 2 provided inside the analog integrated circuit 1 of FIG. 4 corresponds to the biquad filter circuit shown in FIG.
The frequency characteristics of shall be adjusted.

In FIG. 4, the filter 2 to be adjusted is
A signal having a constant frequency f ₀ is supplied from the signal source SG via an external connection terminal (so-called IC pin) 3 of the analog integrated circuit (IC) 1. Here, the filter 2 has a circuit constant (the constant current source 29 described above) according to the filter adjustment data as described above.
And the current value I _{2 of} 31) are changed, and the filter characteristics are changed. As described above, this filter 2 has the LPF (low-pass filter) characteristic as shown by the solid line in FIG. 2 during normal use, and the trap characteristic as shown by the phantom line in FIG. 2 only when the filter is adjusted. Present. Here, it is assumed that the frequency f ₀ of the signal from the signal source SG is set to the adjustment target value of the cutoff frequency of the LPF and the dip frequency of the trap filter. That is, in the present embodiment, the cutoff frequency (the dip frequency when adjusting the filter) is finally the constant frequency f _0.
The filter is adjusted so that

The output from the filter 2 is sent to a level detector 5 such as a so-called AM detector provided in advance in the analog IC 1 to detect the level (amplitude) of the signal, and this level detection output is used for level discrimination. One input terminal of the comparator 6 for
For example, it is sent to the non-inverting input terminal. A predetermined reference level V _ref is supplied to the other input terminal (inverting input terminal) of the comparator 6 via the reference input terminal 7. The comparator 6 discriminates whether the level detection output is high or low with respect to the reference level V _ref . Comparator 6
The comparison output (or level discrimination output) from is sent to the internal bus 40 in IC1. The bus decoder 41 connected to the IC internal bus 40 is connected to the external bus 50 via the external connection terminal 42.
It is also used as an interface circuit for mutually converting the data on the external bus 50 and the data on the internal bus 40. The data transferred from the external bus 50 to the internal bus 40 via the bus decoder 41 is temporarily stored in the latch circuit 43, and this data is converted into an analog signal by the DA converter 44 to adjust the circuit constant control signal or the filter characteristic. It is sent to the filter 2 as a signal. The external bus 50 has a CPU 5 such as a so-called microprocessor.
1. A ROM (Read Only Memory) 52 in which programs and data etc. are written in advance, a RAM (Random Access Memory) 53 in which data etc. are temporarily written, and filter adjustment data etc. which will be described later are used as a power source. A non-volatile memory 54 is connected for storing regardless of whether it is on or off. The computer system including the CPU 51, the ROM 52, the RAM 53, the non-volatile memory 54, and the like controls the changeover of the changeover switch 18 in the filter 2 and adjusts the characteristics to change the filter adjustment data. Perform a series of control operations to determine the optimum filter adjustment data based on the output from

G-4. Specific Example of Filter Adjustment (FIGS. 5 and 6) Next, an operation for obtaining such optimum filter adjustment data will be described below.

Generally, for filter adjustment, the dip frequency is detected based on the filter output characteristic curve (frequency characteristic curve) when the input signal frequency is changed (so-called sweep), and the dip frequency is the target, as in the conventional case. The frequency sweep may be repeated while adjusting the filter characteristics until the value becomes f _0. However, in the embodiment of the present invention, as shown in FIG. 4, the filter adjustment can be performed accurately with a simple configuration. , Proposes a system that enables automated adjustment.

The outline of this filter adjustment system is based on the filter adjustment data when the level detection output of the filter output crosses a predetermined reference level while changing the characteristics of the filter with respect to the input signal fixed at the constant frequency f _0. This is for obtaining the optimum filter adjustment data.

First, at the time of filter adjustment, as described above, the changeover switch 18 in the filter 2 is controlled to be switched to the selected terminal a by the switch changeover data from the control means including the CPU 21 and the characteristics of the filter 2 are described above. Switch to such trap characteristics. FIG. 5 shows the trap characteristic curve when the filter is adjusted. The filter characteristic is adjusted so that the dip frequency of the trap characteristic curve becomes the predetermined target frequency f ₀ .

Here, the filter 2 has a constant frequency from the signal source SG.
The signal of f ₀ is supplied, and at this time, the control means consisting of a computer system such as the CPU 51
Latch circuits 43A, 4 via decoder 41 and IC internal bus 40
The filter adjustment data is sent to the current control terminals of the constant current sources 29 and 31 of the integrator in the filter 2 via the 3B and AD converter 44. The adjustment data is a series of data for gradually moving the characteristic curve of the filter 2 in one direction (for example, in the arrow direction in the drawing) on the frequency axis, as schematically shown by the broken line in FIG. Such a substantially continuous change of the filter characteristic corresponds to a conventional change (sweep) of the frequency of the input signal.

On the other hand, since the input signal frequency is fixed to a constant value f ₀ , the output obtained from the level detection of the output signal from the filter 2 by the level detector 5 is, for example, the detection shown in FIG. It looks like the output. That is, this detection output is
The level changes in accordance with the change of the filter adjustment data shown on the horizontal axis of FIG. 6, and a curve obtained by horizontally inverting the filter characteristic curve of FIG. 5 about the frequency f ₀ is obtained. This detection output is sent to the non-inverting input terminal of the comparator 6 and compared with the reference level V _ref to obtain a comparison output (discrimination output) as shown in FIG. When the filter adjustment data at the inverting switching position of the comparison output, that is, when the detection output crosses the reference level V _ref is sequentially set to D _a and D _b , when the dip frequency of the trap characteristic matches the frequency f ₀ the optimum adjustment data, each data D _a, the average value of D _b (D _a +
It is calculated by D _b ) / 2. The optimum adjustment data is written in the non-volatile memory 54 shown in FIG. 4 and stored even when the power is turned off. Then, as one of the initial setting operations accompanying power-on during normal use, the nonvolatile memory 54
The optimum adjustment data stored in (1) is sent to the latch circuit 43 via the buses 50, 40, etc. to set the filter 2 in the optimum adjustment state.

With such a configuration and operation, the conventional configuration for frequency sweep is not required, the configuration is simplified and the adjustment time is shortened, the characteristic curve is not required to be monitored, and the filter output crosses the reference level. The optimum filter adjustment data can be accurately obtained by a simple configuration in which the comparator 6 is detected, and the application to automatic adjustment using a bus can be easily realized.

G-5. Other Embodiments (FIGS. 7 and 8) The present invention is not limited to the above-mentioned embodiments, and the same applies to, for example, cutoff frequency adjustment of a BPF (bandpass filter). You can do it. In this case, for example, as shown in FIG. 7, two changeover switches interlocked with each other.
18A and 18B are used, the non-inverting input terminal Ta of the operational amplifier 11 and the other end Tc of the capacitor 14 which is the second integral capacitance are commonly connected to the common terminal of the changeover switch 18A, and the capacitor 12 which is the second integral capacitance is used. The other end Ta of the selector switch 18B is connected to the common terminal of the changeover switch 18B, and the selected terminal b of the changeover switch 18A and the selected terminal a of the changeover switch 18B are commonly connected to the signal input terminal 16 and connected. The selected terminal a and the selected terminal b of the changeover switch 18B are commonly connected and grounded. Since other configurations are similar to those in FIG. 1,
Corresponding parts are designated by the same reference numerals and description thereof is omitted.

According to the configuration of FIG. 7, during normal use, the interlocking changeover switches 18A and 18B are both switched and connected to the selected terminal a side, and the BPF having the frequency characteristic as shown by the solid line in FIG.
On the other hand, at the time of filter adjustment, both the changeover switches 18A and 18B are changed over and connected to the selected terminal b side, and trap / filter characteristics similar to those of the embodiment of FIG. 1 described above (see virtual line in FIG. 8). ), The characteristic portion of the characteristic becomes a dip, facilitating detection, and filter adjustment can be performed easily and accurately.

Besides, various modifications can be made without departing from the scope of the present invention.

H. Effect of the Invention According to the filter circuit of the present invention, during filter adjustment,
Since the characteristic part of the filter characteristic is easily detected and changed to the trap filter configuration suitable for the adjustment, the adjustment can be performed easily and accurately.

Further, according to the embodiment of the present invention, since the optimum data is obtained based on the filter adjustment data at the point where the filter output level crosses the reference level while changing the filter characteristic with respect to the signal input of the constant frequency, a simple configuration can be obtained. Nevertheless, the adjustment time can be shortened, the adjustment accuracy can be improved, and the automatic filter adjustment can be easily realized.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing a filter circuit according to an embodiment of the present invention, FIG. 2 is a graph for explaining a characteristic switching operation of the filter circuit of FIG. 1, and FIG.
FIG. 4 is a circuit diagram showing a specific example of an integrator used in the filter circuit shown in FIG. 4, FIG. 4 is a block circuit diagram showing an analog IC and a filter adjustment system to which the above embodiment is applied, and FIG. 5 is a frequency characteristic of a trap filter. FIG. 6 is a graph for explaining the adjustment, FIG. 6 is a graph for explaining a specific example of the adjustment operation of the trap filter, FIG. 7 is a block circuit diagram showing another embodiment of the present invention, and FIG. 8 is a graph for explaining the characteristic switching operation of the filter circuit of FIG. 7. 2 ... Filter 11 ... First operational amplifier 12 ... First integral capacitance (capacitor) 13 ... Second operational amplifier 14 ... Second integral capacitance (capacitor) 15 ... Feedback circuit 16 ... Signal input Terminal 17 …… Signal output terminal 18, 18A, 18B …… Changeover switch

Front page continued (72) Inventor Bunji Hashimoto 6-735 Kitashinagawa, Shinagawa-ku, Tokyo Sony Corporation (72) Inventor Koichi Otani 6-35 Kitashinagawa, Shinagawa-ku, Tokyo Sony Corporation Shares In-house (56) Reference JP-A-57-35410 (JP, A) JP-A-55-121732 (JP, A) JP-A-60-157316 (JP, A)

Claims (1)

[Claims] 1. A differential amplifier and an integrating capacitor whose one end is connected to the output thereof, wherein an output terminal is connected to the output end of the differential amplifier, and two input ends of the differential amplifier are respectively connected to one side, It has first and second integrators that serve as the other input terminal,
The output terminal of the first integrator is connected to one input terminal of the second integrator, and the output terminal of the second integrator is connected to the other input terminal of the first integrator and the second integrator. In the filter circuit configured to be connected to the other input terminal of the integrator, the other end of the integration capacitance of the first integrator, one input terminal of the first integrator, and the second integrator of the second integrator. A changeover switch is provided for switching at least one of the other ends of the integration capacitors to an input signal or ground, and at the time of filter adjustment, a changeover signal is supplied from the outside of the filter circuit to the changeover switch to perform the first integration. The changeover switch so as to connect one input terminal of the capacitor and the other end of the integrating capacitor of the second integrator to the input signal, and to connect the other end of the first integrating capacitor to the ground. Switch the above Filter circuit characterized in that capacitor circuit is as confirmation of a characteristic portion of the filter characteristic so as to have a trap filter characteristics can be easily filter configuration.