JPH09130206A - Characteristic frequency adjustment device for active filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correctly operate a phase comparator circuit at all times and to sufficiently secure the adjustment precision of the characteristic frequency of an active filer even in the case of raising the definition Q of a band-pass filter. SOLUTION: Reference frequency signals Sr from a reference frequency signal generator 11 are supplied through a gain control circuit 14 to the band- pass filter 12. The output signal level of the band-pass filter 12 is detected in a level detection circuit 15 and the gain control circuit 14 is controlled so as to fix the output signal level of the band-pass filter 12 by the detected result output Sd. In the phase comparator circuit 13, the phases of the reference frequency signals Sr and the output signals Sb of the band-pass filter 12 are compared. By the compared result output Sc, the characteristic frequency of the band-pass filter 12 are controlled and also the characteristic frequency of the active filter 1 is adjusted.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an active filter,
In particular, the present invention relates to a device used for a mobile communication terminal or the like, and adjusting the characteristic frequency of an active filter integrated into a circuit for downsizing the terminal or the like.

[0002]

2. Description of the Related Art The frequency characteristic of a filter can be considered as being divided into a relative frequency characteristic and a characteristic frequency.

The relative frequency characteristic is, for example, the shape of the filter frequency characteristic, which is determined by the topology of the filter, its parameters, and the type of the pass band, such as a fifth-order Butterworth type low-pass filter. Point to.

On the other hand, the characteristic frequency is a frequency of a singular point which can determine at which position in the frequency domain the filter having such a fixed frequency characteristic is arranged. It means that. For example, in a Butterworth filter, the frequency of the 3 dB attenuation pole can be said to be the characteristic frequency described here.

Generally, variations in the manufacturing process of an integrated circuit are ± 10% for resistors and ± 10% for capacitors, and the characteristic frequency of a filter formed by resistors and capacitors varies by ± 20% or more. .

Therefore, conventionally, when an active filter is integrated into a circuit, in order to improve the accuracy of the characteristic frequency of the integrated circuit active filter, the characteristic frequency as shown in FIG. Adjustment devices were sometimes provided.

The characteristic frequency adjusting device 20 comprises a reference frequency signal generator 21, a band pass filter 22 and a phase comparison circuit 23. The reference frequency signal Sr from the reference frequency signal generator 21 is supplied to the band pass filter 22. The output signal Sb of the band-pass filter 22 and the reference frequency signal Sr from the reference frequency signal generator 21 are supplied to the input terminal 22i, and are supplied to the phase comparison circuit 23. The output signal Sc as a result of the comparison is supplied to the control terminal 22c of the bandpass filter 22 for changing the characteristic frequency.

The output signal Sc of the phase comparison circuit 23 is used as an adjustment signal for the output of the characteristic frequency adjusting device 20 of the active filter 2 whose characteristic frequency is to be adjusted.
It is supplied to the control terminal 2c for changing the characteristic frequency.

Here, the bandpass filter 22 shows its amplitude characteristic in FIG. 4 (A) and its phase characteristic in FIG. 4 (B).
As shown in (1), the filter has a relatively low sharpness Q and a characteristic frequency of Fo.

Therefore, the reference frequency signal Sr from the reference frequency signal generator 21 passes through the band pass filter 22 to obtain the reference frequency fr and the characteristic frequency Fo of the band pass filter 22 according to the characteristic of FIG. It undergoes phase rotation according to the difference Δf.

The characteristic frequency Fo of the bandpass filter 22 is changed by the output signal Sc of the phase comparison circuit 23 so that the phase rotation amount becomes zero.
Therefore, the characteristic frequency Fo of the bandpass filter 22 is
Will be linked to the reference frequency fr.

In this case, by configuring the respective circuits such that the characteristic frequency Fo of the bandpass filter 22 and the characteristic frequency Fa of the active filter 2 are strongly correlated with each other, the phase is increased as described above. The characteristic frequency Fo of the bandpass filter 22 is determined by the output signal Sc of the comparison circuit 23.
Is controlled and the output signal Sc of the phase comparison circuit 23 is controlled.
By adjusting the characteristic frequency Fa of the active filter 2 by the above, the characteristic frequency Fa of the active filter 2 also interlocks with the reference frequency fr, and the characteristic frequency Fa of the active filter 2 can be adjusted.

[0013]

However, in the characteristic frequency adjusting device 20 described above, the characteristic frequency F of the active filter 2 is reduced.
In order to sufficiently secure the adjustment accuracy of a, it is necessary to sufficiently increase the ratio of the phase change to the frequency change in the vicinity of the characteristic frequency Fo of the bandpass filter 22.

Therefore, as the bandpass filter 22,
Its amplitude characteristic is shown in FIG. 2 (A), and its phase characteristic is shown in FIG.
As shown in (B), it is possible to use a material having a high sharpness Q.

However, if the sharpness Q of the bandpass filter 22 is increased in this way, firstly, as the sharpness Q is increased, many parameters are involved in the value of the sharpness Q, and thus the sharpness is increased. It becomes difficult to control the absolute value of the degree Q, and as a result, the output signal S of the bandpass filter 22 is
The amplitude of b varies and the phase comparison circuit 23 does not operate correctly.

Second, the sharpness Q of the bandpass filter 22.
Is increased, the reference frequency fr and the band pass filter 22
When the difference Δf from the characteristic frequency Fo is large, the amplitude of the output signal Sb of the bandpass filter 22 is extremely small. Conversely, when the difference Δf is small, the amplitude of the output signal Sb is extremely large and the phase comparison is performed. Since the amplitude of the signal Sb supplied to the circuit 23 largely fluctuates, the phase comparison circuit 23 also does not operate properly.

Therefore, in the prior art, as described above, the bandpass filter 22 having a low sharpness Q must be used, and therefore the characteristic frequency F of the active filter 2 is required.
It was not possible to sufficiently secure the adjustment accuracy of a.

According to the present invention, the reference frequency signal that has passed through the band pass filter and the reference frequency signal that does not pass are compared in phase, and the characteristic frequency of the band pass filter is controlled by the output of the comparison result. In a characteristic frequency adjusting device for an active filter for adjusting a characteristic frequency of an active filter to be adjusted, a phase comparison circuit is always operated correctly even when the sharpness Q of the bandpass filter is increased, By increasing the sharpness Q of, the adjustment accuracy of the characteristic frequency of the active filter can be sufficiently ensured.

[0019]

According to the present invention, between the reference frequency signal generator and the input terminal of the band pass filter,
A gain control circuit that controls the level of the reference frequency signal input to the bandpass filter is inserted and connected, and a level detection circuit that detects the output signal level of the bandpass filter is provided on the output side of the bandpass filter. The gain control circuit is controlled by the output signal of the detection circuit so that the output signal level of the bandpass filter becomes constant.

In the characteristic frequency adjusting device of the present invention configured as described above, the sharpness Q of the band pass filter is obtained.
Even when the value is increased, variations or fluctuations in the output amplitude of the bandpass filter are suppressed due to variations in the sharpness Q or the amplitude characteristics of the bandpass filter.

Therefore, the amplitude of the signal supplied to the phase comparison circuit is always constant, and the phase comparison circuit always operates correctly. Therefore, by increasing the sharpness Q of the bandpass filter, it is possible to sufficiently secure the adjustment accuracy of the characteristic frequency of the active filter.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a characteristic frequency adjusting device of the present invention.
0 is the reference frequency signal generator 11 and the band pass filter 1
2, a phase comparison circuit 13, a gain control circuit 14, and a level detection circuit 15.

The reference frequency signal Sr from the reference frequency signal generator 11 is directly supplied to one input terminal of the phase comparison circuit 13 on the one hand and to the input terminal of the gain control circuit 14 on the other hand, The level-controlled reference frequency signal Sg, which is the output signal of the gain control circuit 14.
Is supplied to the input terminal 12i of the bandpass filter 12, and the output signal Sb of the bandpass filter 12 is supplied to the other input terminal of the phase comparison circuit 13.

Further, the output signal Sb of the bandpass filter 12 is supplied to the level detection circuit 15, and the level detection circuit 15 detects the level of the output signal Sb.
The output signal Sd of the level detection circuit 15 is supplied to the control terminal of the gain control circuit 14, and the band pass filter 12 is supplied.
The gain of the gain control circuit 14, that is, the level of the output signal Sg of the gain control circuit 14 is negatively feedback-controlled so that the level of the output signal Sb of is constant.

Then, the phase comparison circuit 13 compares the phase of the reference frequency signal Sr from the reference frequency signal generator 11 with the phase of the output signal Sb of the band pass filter 12,
The output signal Sc of the phase comparison circuit 13 is supplied to the control terminal 12c of the bandpass filter 12 that changes the characteristic frequency.

Further, the output signal Sc of the phase comparison circuit 13 serves as an adjustment signal for the output of the characteristic frequency adjusting device 10 of the active filter 1 whose characteristic frequency is to be adjusted.
It is supplied to the control terminal 1c for changing the characteristic frequency.

As shown in FIG. 2 (A) for its amplitude characteristic and for its phase characteristic in FIG. 2 (B), the bandpass filter 12 is a filter having a sharpness Q of sufficiently high and a characteristic frequency of Fo. To be done.

The level-controlled reference frequency signal Sg, which is the output signal of the gain control circuit 14, passes through the band-pass filter 12 to obtain the reference frequency fr and the characteristic of the band-pass filter 12 according to the characteristic of FIG. Frequency F
It undergoes phase rotation according to the difference Δf from o.

The characteristic frequency Fo of the bandpass filter 12 is changed by the output signal Sc of the phase comparison circuit 13 so that the phase rotation amount becomes zero.
Therefore, the characteristic frequency Fo of the bandpass filter 12
Will be linked to the reference frequency fr.

In this case, by configuring the respective circuits such that the characteristic frequency Fo of the bandpass filter 12 and the characteristic frequency Fa of the active filter 1 are strongly correlated with each other, the phase is changed as described above. The characteristic frequency Fo of the band pass filter 12 is determined by the output signal Sc of the comparison circuit 13.
Is controlled and the output signal Sc of the phase comparison circuit 13 is controlled.
By adjusting the characteristic frequency Fa of the active filter 1 by the above, the characteristic frequency Fa of the active filter 1 also interlocks with the reference frequency fr, and the characteristic frequency Fa of the active filter 1 can be adjusted.

Then, the output signal S of the level detection circuit 15
Since the gain control circuit 14 is controlled by d so that the level of the output signal Sb of the bandpass filter 12 becomes constant, the sharpness Q of the bandpass filter 12 is as described above.
Is sufficiently high, the bandpass filter 1 may be affected by variations in the sharpness Q and the amplitude characteristics of the bandpass filter 12.
Variation or fluctuation of the amplitude of the second output signal Sb is suppressed.

Therefore, the amplitudes of the signals Sr and Sb supplied to the phase comparison circuit 13 are always constant, and the phase comparison circuit 13 always operates correctly. Therefore, by sufficiently increasing the sharpness Q of the bandpass filter 12, the adjustment accuracy of the characteristic frequency Fa of the active filter 1 can be sufficiently ensured.

[0033]

As described above, according to the present invention, even when the sharpness Q of the bandpass filter is increased,
The phase comparator circuit always works correctly. Therefore, by increasing the sharpness Q of the bandpass filter, it is possible to increase the rate of change in the phase with respect to the change in the frequency in the vicinity of the characteristic frequency of the bandpass filter, and to sufficiently adjust the characteristic frequency of the active filter. Can be secured.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a characteristic frequency adjusting device of the present invention.

FIG. 2 is a diagram showing an example of characteristics of a bandpass filter used in the apparatus of FIG.

FIG. 3 is a block diagram showing a conventional characteristic frequency adjusting device.

FIG. 4 is a diagram showing an example of characteristics of a bandpass filter used in the apparatus of FIG.

[Explanation of symbols]

 1 Active Filter 1c Control Terminal 10 Characteristic Frequency Adjustment Device 11 Reference Frequency Signal Generator 12 Band Pass Filter 12i Input Terminal 12c Control Terminal 13 Phase Comparison Circuit 14 Gain Control Circuit 15 Level Detection Circuit

Claims (1)

[Claims] 1. A band pass filter having a reference frequency signal generator, an input terminal, and a control terminal for changing a characteristic frequency, and the reference frequency signal from the reference frequency signal generator is supplied to the input terminal. , Comparing the phase of the reference frequency signal from the reference frequency signal generator and the output signal of the band pass filter, and supplying the comparison result output to the control terminal of the band pass filter, the characteristic frequency of the band pass filter , A gain control circuit inserted between the reference frequency signal generator and the input terminal of the band pass filter, and detecting the output signal level of the band pass filter, the detection result A level detection circuit that controls the gain control circuit so that the output signal level of the bandpass filter becomes constant depending on the output, and A characteristic frequency adjusting device for an active filter, wherein an output signal of the phase comparison circuit is supplied to a control terminal for changing the characteristic frequency of an active filter whose characteristic frequency is to be adjusted.