JPH0846480A - Bandpass filter - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a bandpass filter capable of increasing Q without deteriorating accuracy and less affected by fluctuations in temperature and the like. According to the present invention, an input signal is input to a coupling point of voltage-current converters 8 and 9 via a capacitor 12, and an output signal of a BPF which is an output of an amplifier 7 is positively applied to the coupling point via a capacitor 13. Returned and configured. The capacitance values of the capacitors 12 and 13 have the values of C3 and C4, respectively. And illustration B
In order to set the value of Q to N without changing the center frequency W0 of PF, for example, if the conversion conductances of the voltage-current converters 8 and 9 are equal, C3: (C3 + C4)
= 1: N. As a result, even when the value N of Q is increased, it is easy to increase the ratio accuracy of the capacitance values C3 and (C3 + C4), and Q of a desired value is obtained.
The value of can be easily obtained. Voltage-current converter 8, 9
Can be controlled by providing a phase detector so that the phase difference between the input and output signals of the BPF becomes 0 °.
The center frequency of the PF can be matched with the center frequency of the passing signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bandpass filter, and more particularly to a bandpass filter which can obtain a sharp frequency characteristic with high accuracy when integrated into an IC.

[0002]

2. Description of the Related Art As a conventional technique for integrating a bandpass filter (hereinafter referred to as BPF) in an IC, a technique described in, for example, "Design of active filter" (published by Kobo) is known. .

FIG. 10 is a diagram showing a circuit of a BPF according to this conventional technique. In FIG. 10, 1 is an input terminal, 2 is an output terminal, 3 and 6 are capacitors, 4, 5 and 32 are resistors, and 7 is an amplifier.

The BPF according to the prior art shown in FIG. 10 is constructed by using an active filter circuit composed of an amplifier 7 acting as a buffer, capacitors 3 and 6, and transfers 4, 5 and 32. However, even if this circuit is integrated in the IC as it is, the variation in the frequency characteristic is large due to the variation in the resistance value and the capacitance value formed in the IC.

FIG. 11 is a diagram showing a conventional second-order BPF circuit suitable for integration in which variations in frequency characteristics are improved. In FIG. 11, 8 and 9 are voltage-current converters,
Reference numerals 10 and 11 are capacities, and other reference numerals are the same as in the case of FIG.

The conventional BPF shown in FIG. 11 has a voltage-current converter 8 having a voltage-current conversion conductance gm1, a voltage-current converter 9 having a voltage-current conversion conductance gm2, and a capacitance 1 having capacitance values C1 and C2.
0 and 11 and an amplifier 7 that operates as a buffer, the first voltage-current converter 8, the second voltage-current converter 9 and the amplifier 7 are connected in series, and first and second voltages are provided. A first capacitance is connected between a connection point of the current converter and the input terminal, and a second capacitance is connected between a connection point of the second voltage-current converter and the amplifier and ground. A desired frequency characteristic can be obtained by controlling the conversion conductances gm1 and gm2 of the voltage-current converters 8 and 9.

The transfer function V0 of the BPF circuit shown in FIG.
/ V1, Q, and the center frequency W0 through which the signal passes without being attenuated, can be respectively expressed by the following equations.

[0008]

[Equation 1]

[0009]

[Equation 2]

[0010]

(Equation 3)

This prior art is based on the above formula (1),
According to (2), in order to set the value of Q to N without changing the center frequency W0, assuming that gm1 = gm2, for example, the ratio of the capacitance values C1 and C2 of C1: C2 = 1: N ² becomes It is necessary.

[0012]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
As the Q value N increases, it becomes difficult to obtain the ratio accuracy of the capacitance values C1 and C2, and it becomes difficult to obtain a desired Q value.

Further, in the above-mentioned conventional technique, the voltage-current converter which constitutes the BPF is composed of the transistor and the resistor with the built-in IC, and the characteristics and values of these constituent elements vary from IC to IC, so that a desired frequency characteristic is obtained. In order to obtain it, it is necessary to externally adjust the voltage-current conversion conductance of the voltage-current converter for each IC, and even if the adjustment can be easily performed for each IC, the temperature change of the elements in the IC is large. Therefore, there is also a problem that the frequency characteristic fluctuates.

The object of the present invention is to solve the above-mentioned problems of the prior art, to increase the Q without degrading the accuracy, and to be affected by fluctuations due to variations in temperature, element characteristics, values and the like. It is to provide a second-order bandpass filter with less noise.

[0015]

According to the present invention, the above object is further provided with means for applying positive feedback from the output of the secondary BPF to the input side, thereby further detecting the phase difference between the input signal and the output signal. It is achieved by providing a means for controlling the frequency characteristic of the secondary BPF based on the result.

Further, the above-mentioned object is the secondary BP of the above construction.
N in series connection BP by connecting F in multiple stages as a unit BPF
And means for detecting the phase difference between the input signal and the output signal of the N-stage series-connected BPF and controlling the frequency characteristic of each unit BPF constituting the N-stage series-connected BPF based on the result. It is achieved by

[0017]

The present invention provides the following two means by providing the above-mentioned means.
It becomes possible to increase the amount of positive feedback from the output to the input in the vicinity of the center frequency of the second-order BPF to perform peaking, and the second-order BPF
The Q of PF can be increased. Further, since the secondary BPF is controlled by the phase detector so that the phase difference between the input and output signals of the secondary BPF becomes 0 °, the center frequency of the pass signal of the secondary BPF and the center frequency of the secondary BPF are Can be made to coincide with each other, and a second-order BPF having an optimum frequency characteristic for the passing signal can be obtained.

[0018]

Embodiments of the bandpass filter according to the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a secondary B according to the first embodiment of the present invention.
It is a circuit diagram which shows PF structure. In FIG. 1, 12, 1
Reference numeral 3 is a capacity, and other symbols are the same as those in FIG. The first embodiment of the present invention is a configuration example of a secondary BPF in which positive feedback is applied from the output to the input.

The first embodiment of the present invention shown in FIG.
For the secondary BPF according to the conventional technique described in 1,
The input signal is input to the connection point of the voltage-current converters 8 and 9 via the capacitor 12, and the output signal is positively fed back to the connection point via the capacitor 13. The capacitance values of the capacitors 12 and 13 have the values of C3 and C4, respectively.
In the BPF according to the first embodiment of the present invention, in order to make its center frequency the same as that of the conventional BPF shown in FIG.
The capacitance value C1 and the capacitance 1 of the capacitance 10 in the circuit of the related art
The relationship between the capacitance values C3 and C4 of 2 and 13 is set so that C1 = C3 + C4.

In this case, the transfer functions V0 / V1 and Q of the BPF and the center frequency W0 of the BPF according to the first embodiment of the present invention shown in FIG. 1 can be respectively expressed by the following equations.

[0022]

[Equation 4]

[0023]

(Equation 5)

[0024]

(Equation 6)

In this embodiment, in order to set the value of Q to N without changing the center frequency W0 by the above equation, for example, assuming that gm1 = gm2, C3: (C3 + C4) = 1: It turns out that N is enough.

According to the first embodiment of the present invention described above,
It is easy to increase the ratio accuracy of the capacitance values C3 and (C3 + C4). Therefore, even when the Q value N is to be increased, a desired Q value can be easily obtained.

FIG. 2 is a secondary B according to the second embodiment of the present invention.
FIG. 3 is a circuit diagram showing a configuration of a PF, FIG. 3 is a diagram for explaining input / output phase difference frequency characteristics of a secondary BPF, and FIG. 4 is a diagram for explaining input / output characteristics of a phase detector. In FIG. 2, 14 is a phase detector, 16 is a second-order BPF, and other reference numerals are the same as in FIG. In the second embodiment of the present invention, the phase difference between the input signal and the output signal of the secondary BPF 16 described with reference to FIG. 1 is detected and the frequency characteristic of the secondary BPF 16 is changed.

In the second embodiment of the present invention shown in FIG. 2, the phase detector 14 receives the input signal from the input terminal 1 and the output signal from the output terminal 2 of the secondary BPF 16 and outputs the phase difference between them. Is output as the detection output signal 15. The detection output signal 15 is input to the voltage-current converters 8 and 9 for controlling the voltage-current conversion conductances gm1 and gm2 of the voltage-current converters 8 and 9.

Next, the operation of the second embodiment of the present invention configured as described above will be described with reference to FIG. 3 showing the frequency-input / output phase difference characteristic of the secondary BPF 16 and the input signal level of the phase detector 14. The phase difference-phase detection output (voltage-current conversion conductance gm of 8 and 9) characteristics will be described with reference to FIG.

The phase difference between the input and output of the secondary BPF 16 is shown in FIG.
As shown in (1), it has a property of becoming 0 ° at the center frequency W0 of the BPF. When a signal having an average frequency W1 (assuming W1> W0) is input to the BPF 16, the input / output phase difference of the BPF 16 becomes θ1. When this phase difference θ1 is input to the phase detector 14 having the characteristics shown in FIG. 4, the magnitude of the detection output signal 15 of the phase detector 14 increases.

As a result, the voltage-current conversion conductances gm1 and gm2 of the voltage-current converters 8 and 9 become large, and the center frequency W0 of the BPF 16 rises and the center frequency W0 approaches the input frequency W1 as shown in the equation (6). It will be. That is, the circuit including the phase detector 14 is
The center frequency W0 of 16 acts as a negative feedback loop that tries to approach the input frequency W1.

In the second embodiment of the present invention described above, the center frequency W0 of the BPF 16 and the input frequency W1 can be matched by setting the gain of the phase difference -gm of the phase detector 14 sufficiently high. it can. Then, the second embodiment of the present invention can pass the input signal W1 without attenuating it, and can attenuate the frequency W0, that is, the unnecessary frequency signal distant from W1.

FIG. 5 shows a secondary B according to the third embodiment of the present invention.
It is a block diagram which shows the structure of PF. In FIG. 5, 1
Reference numerals 7 and 18 are phase shifters, 19 is a phase detector, and other reference numerals are the same as those in FIG.

The third embodiment of the present invention shown in FIG. 5 is shown in FIG.
The phase detector 14 according to the second embodiment of the present invention described above is composed of a phase shifter 17, a phase shifter 18 that changes the phase by 90 ° with respect to the phase shifter 17, and a phase difference between input signals. It is composed of a phase detector 19 whose output becomes 0 at 90 °.

The third embodiment of the present invention thus constructed has the phase shifter 17 when the phase difference between the output signal of the BPF 16 input to the phase shifters 17 and 18 and the input signal is 0 °. ,
The phase difference between the outputs of 18 becomes 90 °, and the output of the phase shifter 19 becomes 0. The third embodiment of the present invention is the phase detector 14 including the phase shifters 17, 18 and the phase shifter 19.
Is equivalent to the phase detector 14 shown in FIG. 2, and the same effects as those of the second embodiment of the present invention can be obtained.

FIG. 6 is a secondary B according to the fourth embodiment of the present invention.
It is a block diagram which shows the structure of PF. In FIG. 6, 2
Reference numerals 0 and 21 are amplitude limiters, and other symbols are the same as those in FIG.

The fourth embodiment of the present invention shown in FIG. 6 is shown in FIG.
Phase shifter 1 in the third embodiment of the present invention described by
Amplitude limiters 21, 2 are provided between 7, 18 and the phase detector 19.
It is configured by inserting 0.

In the fourth embodiment of the present invention constructed as above, the amplitude of the input signal to the phase detector 19 can be made a constant value by the action of the amplitude limiters 20 and 21, and it can be input to the BPF. The fluctuation of the phase detection output signal 15 due to the fluctuation of the amplitude of the input signal can be eliminated, and stable B
PF characteristics can be obtained.

FIG. 7 shows a secondary B according to the fifth embodiment of the present invention.
It is a block diagram which shows the structure of PF. In FIG. 7, 2
Reference numeral 2 is an N-stage series-connected secondary BPF, and other symbols are the same as those in FIG.

In the fifth embodiment of the present invention shown in FIG. 7, the secondary BPF 16 explained above is used as a unit secondary BPF,
These are connected in series in N stages to form an N-stage series-connected secondary BPF 22 having a BPF characteristic with a center frequency W0, and this N-stage series-connected secondary BPF 22 is controlled by the phase detector 14. . The phase detector 14 is shown in FIG.
Although the same ones as described with reference to FIGS. 5 and 6 can be used, it is assumed here that the offset is such that the detection output becomes 0 when the input phase difference is θ.

Now, when a signal having an average frequency W1 is inputted to the input terminal 1 of the BPF according to the fifth embodiment of the present invention shown in FIG. 7, the circuit shown in the drawing shows the center frequency W0 deviated by the varied phase difference θ, Convergence stabilizes. N-stage series connection secondary BPF
The frequency-input / output phase difference characteristic of 22 has a steep slope N times as compared with the case of the one-stage secondary BPF described with reference to FIG.

Therefore, in the fifth embodiment of the present invention, the difference W1-W0 between the center frequency W0 and the average frequency W1 of the input signal is described with reference to FIG. Compared with the circuit of 1), it can be reduced to 1 / N, and highly accurate BPF characteristics with less error can be obtained.

FIG. 8 shows a secondary B according to the sixth embodiment of the present invention.
It is a block diagram which shows the structure of PF. In FIG. 8, 2
Reference numeral 3 is an adder, 24 is a control signal source, and other symbols are those shown in FIG.
Is the same as the case of.

A sixth embodiment of the present invention shown in FIG. 8 is shown in FIG.
In the fifth embodiment of the present invention described above, the center frequency W0 of the N-stage series-connected secondary BPF 22 is set to the input terminal 1
A control signal for inputting to the pass signal W1 in the vicinity is provided. For this purpose, the control signal source 2
4 is provided, and the control signal having a fixed value from the control signal source 24 is used as an output signal from the phase detector 14 and the adder 2
It is added by 3 and given to the N-stage series-connected secondary BPF 22.

In the sixth embodiment of the present invention having such a configuration, it is assumed that the input terminal 1 receives not the signal W1 having the frequency to be passed but the unnecessary signal W2 having the frequency to be removed. In this case, the output of the phase detector 14 is the center frequency W0 = W of the N-stage series-connected secondary BPF 22.
The control signal is output so that the output becomes 2. However, by comparing the control signal from the control signal source 24 with the control range by the output of the phase detector 14, the output 15 from the adder 23 is almost eliminated. It can be kept constant. As a result, the unnecessary signal W2 is sufficiently attenuated even when it is input alone.

After that, when the signal W1 to be passed is input, the amplitude of this signal W1 is larger than the unnecessary signal W2. Therefore, in the above-described sixth embodiment of the present invention, the center frequency W0 of the BPF is the signal W1. Converge to the frequency of
It is not affected by the unnecessary signal W2.

FIG. 9 is a secondary B according to the seventh embodiment of the present invention.
It is a block diagram which shows the structure of PF. In FIG. 9, 2
6 is a reference signal source, 27 is an N-stage series-connected secondary BPF, 28
Is a phase detector, 29 is a secondary BPF, 30 is an input terminal, 3
Reference numeral 1 is an output terminal, and other symbols are the same as in the case of FIG.

A seventh embodiment of the present invention shown in FIG. 9 is shown in FIG.
The control signal source 24 according to the sixth embodiment of the present invention described by the
s) is a highly accurate reference signal source 26, an N-stage series-connected secondary BPF 27 equivalent to the N-stage series-connected secondary BPF 22 described in the above-described embodiment, and the phase detector 14 described in the above-described embodiment. It is composed of the equivalent phase detector 28, and further has an input terminal 30 and an output terminal 31, and is equivalent to the one-stage secondary BPF 16 composed of the capacity and voltage-current converter described in the above-mentioned embodiment. The next BPF 29 is added and configured. The secondary BPF 29 is an output signal 1 of the adder 23 that controls the N-stage series-connected secondary BPF 22.
Controlled by 5. The secondary BPF 29 is L
It may be a two-stage series-connected secondary BPF.

In the seventh embodiment of the present invention shown in FIG. 9, the phase detector 28 is arranged so that the center frequency W0 of the N-stage series-connected secondary BPF 27 matches the reference signal frequency Ws from the reference signal source 26. Controlled by the output of. Then, one-stage secondary BF that constitutes the N-stage series-connected secondary BPFs 22 and 27.
When the ratio of the numbers of P16 is set to W1: Ws, the output signal of the phase detector 28 is the N-stage series-connected secondary BPF22.
It is possible to control the center frequency of W1 to be near W1. That is, the output signal of the phase detector 28 is equivalent to the control signal having the fixed value from the control signal source 24 described with reference to FIG.

When the secondary BPF 29 controlled by the output of the adder 23 is configured as an L-stage series-connected secondary BPF, the secondary BPF 29 is an N-stage series-connected secondary BPF 22.
It becomes possible to operate as a BPF having a desired center frequency determined by the ratio of the components. That is, according to the seventh embodiment of the present invention shown in FIG. 9, N-stage series-connected secondary B
The center frequency of the PF is set to the number of constituent elements of the multi-stage series-connected secondary BPF included in the control signal source and the N-stage series-connected secondary BP.
It can be determined by the ratio with the number N of stages of the constituent elements of F, and a BPF having a high Q can be easily constructed.

[0051]

As described above, according to the present invention, it is necessary to make the ratio of the values of the elements constituting the BPF M ² in order to increase the Q of the BPF M times by using the positive feedback. Instead, simply multiply by M, and
It is possible to improve accuracy by eliminating the need for a large element value ratio.

Further, according to the present invention, since the feedback can be performed so that the center frequency of the BPF coincides with the average frequency of the passing signal, the BPF which is always optimum for the passing signal is obtained.
It can have characteristics. Further, by narrowing the control range of the feedback loop, it is possible to configure the BPF while being influenced by unnecessary signals.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a secondary BPF according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a configuration of a secondary BPF according to a second embodiment of the present invention.

FIG. 3 is a diagram illustrating input / output phase difference frequency characteristics of a secondary BPF.

FIG. 4 is a diagram illustrating input / output characteristics of a phase detector.

FIG. 5 is a block diagram showing a configuration of a secondary BPF according to a third exemplary embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration of a secondary BPF according to a fourth exemplary embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of a secondary BPF according to a fifth embodiment of the present invention.

FIG. 8 is a block diagram showing a configuration of a secondary BPF according to a sixth embodiment of the present invention.

FIG. 9 is a block diagram showing a configuration of a secondary BPF according to a seventh embodiment of the present invention.

FIG. 10 is a diagram showing a circuit of a secondary BPF according to a conventional technique.

FIG. 11 is a diagram showing a circuit of a secondary BPF according to the related art, which is suitable for integration in which variations in frequency characteristics are improved.

[Explanation of symbols]

 1, 30 Input terminal 2, 31 Output terminal 3, 6, 10-13 Capacitance 4, 5, 32 Resistance 7 Amplifier 8, 9 Voltage / current converter 14, 19, 28 Phase detector 16, 29 Secondary BPF 17, 18 Phase shifter 20, 21 Amplitude limiter 22, 27 N-stage series connection secondary BPF 23 Adder 24 Control signal source 26 Reference signal source

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Ogushi 1410 Inada, Katsuta City, Ibaraki Hitachi, Ltd. AV Equipment Division (72) Inventor Keiichi Mizutani 1410 Inada, Katsuta City, Ibaraki Hitachi Corporation AV Equipment Business Department

Claims (10)

[Claims] 1. A first and second voltage-current converter connected in series, an amplifier connected to the output of the second voltage-current converter and having the output as an output terminal, and an amplifier as a buffer. A first capacitor connected between the connection point of the second voltage-current converter and the input terminal, and a second capacitor connected between the connection point of the second voltage-current converter and the amplifier and ground. A band pass filter comprising a capacitor, wherein a third capacitor is connected between the output terminal and a connection point of the first and second voltage-current converters. 2. A phase detector to which signals of the input terminal and the output terminal are input, and an output of the phase detector
The bandpass filter according to claim 1, wherein the voltage-current conversion conductance of the first and second voltage-current converters is controlled. 3. A phase detector is provided for inputting signals from the input terminal and the output terminal respectively via first and second phase shifters, and the output of the phase detector provides the first signal. The band-pass filter according to claim 1, wherein the voltage-current conversion conductance of the second voltage-current converter is controlled. 4. The first and second amplitude limiters are connected between the outputs of the first and second phase shifters and the inputs of the phase detector. Bandpass filter as described. 5. An amplifier as a buffer, which is connected to the outputs of the first and second voltage-current converters and the second voltage-current converters connected in series, and uses the output as an output terminal, the first and second amplifiers. A first capacitance connected between the connection point of the voltage-current converter and the input terminal, a second capacitance connected between the connection point of the second voltage-current converter and the amplifier and ground, and ,
An N-stage series-connected bandpass filter in which a plurality of unit bandpass filters each having a third capacitance connected between the output terminal and a connection point of the first and second voltage-current converters are connected in series; A phase detector to which the signals of the input terminal and the output terminal of the N-stage series-connected bandpass filter are input, and the output of the phase detector causes the first bandpass filter in each of the plurality of unit bandpass filters to be output. A band-pass filter which controls the voltage-current conversion conductance of the first and second voltage-current converters. 6. The bandpass according to claim 5, wherein each of the signals from the input terminal and the output terminal is input to the phase detector via first and second phase shifters. filter. 7. The first and second amplitude limiters are connected between the outputs of the first and second phase shifters and the inputs of the phase detector. Bandpass filter as described. 8. A control signal source is further provided, and a fixed value signal from said control signal source is added to an output of said phase detector to add said fixed value signal from each of said plurality of unit band pass filters. 8. The bandpass filter according to claim 5, wherein the control signal is used as a control signal for the voltage-current converter. 9. The control signal source is a first serially connected first control signal source.
And a second voltage-current converter, an amplifier connected to the outputs of the second voltage-current converter and having the output as an output terminal, a connection point of the first and second voltage-current converters, and an input terminal A first capacitance connected between the first and second capacitances, a second capacitance connected between the connection point of the second voltage-current converter and the amplifier and the ground, and the output terminal and the first and second capacitances. An N-stage series-connected bandpass filter having a plurality of unit bandpass filters connected in series connected to a connection point of the voltage-current converter, and an N-stage series-connected bandpass filter. A phase detector that receives signals from an input terminal and an output terminal, and controls the voltage-current conversion conductance of the first and second voltage-current converters in each of the plurality of unit bandpass filters by the output thereof. , Connect to the input terminal Is constituted by a reference signal source which,
9. The band pass filter according to claim 8, wherein the output of the phase detector is the fixed value signal. 10. A fixed value signal from the control signal source,
10. The band pass filter according to claim 8, wherein the addition signal with the output of the phase detector is used as a control signal for the voltage-current conversion conductance of the voltage-current converter of another filter.