JPH1013188A - Automatic-adjusting circuit for filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatically execute adjustment with precision by constituting an automatic adjusting filter by means of a low-pass filter and a high-pass filter and executing feedback by means of the output amplitudes of the respec tive filters. SOLUTION: The output signals of the low-pass filter 1a and the high-pass filter 2a are amplitude and wave-detected in amplitude detection circuits 3a and 3b, and the amplitude values of the respective signals are converted into a D.C. voltage or a D.C. current. A feedback loop which is constituted, so as to permit the amplitudes of output signals in cut-off frequencies to be equal when the cut-off frequencies of the low-pass filter 1a and the high-pass filter 2a are deviated by the various kinds of fluctuation main cause is provided, and feedback is executed, so as to permit the output signals of the amplitude detection circuit 3a and 3b to be normally equal. Therefore, a sensitive characteristic filter such as a trap filter, etc., is automatically adjusted with precision, when compared with conventional automatic adjustment where feedback is executed by a 90 deg phase shift of a reference filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter automatic adjustment circuit for maintaining a frequency characteristic of a filter in a predetermined state.

[0002]

2. Description of the Related Art In order to discriminate only a target signal from various signals, a circuit such as a low-pass filter or a band-pass filter is often used. Also in a semiconductor integrated circuit, a filter including a resistor and a capacitor, an active filter using the conductance of a capacitor and a transistor, and the like are widely used. However, in active filters used in semiconductor integrated circuits, the absolute values of resistors and capacitors fluctuate or vary greatly in temperature during manufacturing. Is provided with an automatic adjustment circuit so that can be kept constant.

FIG. 5 shows a conventional filter automatic adjustment circuit. The automatic adjustment circuit of the filter has a circuit configuration similar to that of the filter 12 and has a frequency of 90 degrees at the frequency of the reference signal.
It comprises a reference filter 11 designed to be phase-shifted, a phase detection circuit 13, and a conductance control circuit 5.

First, the reference signal is shifted by 90 degrees in the reference filter 11. Next, the phase detection circuit 13 detects the reference signal and the phase-shifted signal. Phase detection circuit 1
The output of No. 3 is 0 when the phase difference between the reference signal and the signal shifted in the reference filter 11 is exactly 90 deg, and an error component is output when the phase difference deviates from 90 deg due to a variation factor or the like. become. By adding the error component to the conductance control circuit 5 and controlling the characteristics of the reference filter, in the closed loop of the automatic adjustment circuit,
Feedback is always applied so that the phase difference between the reference signal and the signal shifted by the reference filter 11 is always exactly 90 deg.

On the other hand, the filter 12 is a reference filter 11
Since the relative variation and fluctuation of the elements in the semiconductor integrated circuit chip are small, the feedback of the above automatic adjustment is similarly applied to the filter 12, and the characteristic of the filter 12 is always obtained. Can be kept.

As described above, in the conventional filter automatic adjustment circuit, since feedback is applied to the 90-degree phase shift of the reference filter, for example, if the circuit is sensitive to the phase relation such as the dip frequency of the trap filter, The automatic adjustment point may be shifted from the dip frequency. For this reason, it is necessary to apply feedback to the dip frequency itself of the trap filter instead of applying feedback to the 90-deg phase shift of the reference filter, and manage the feedback.

[0007]

According to a first aspect of the present invention, a filter for automatic adjustment is constituted by a low-pass filter and a high-pass filter, and a feedback is performed by the output amplitude of each filter to automatically adjust the filter automatically. Try to get a circuit.

According to a second aspect of the present invention, the reference filter for automatic adjustment comprises a low-pass filter and a high-pass filter,
Apply feedback by the output amplitude of each filter,
An object of the present invention is to provide an automatic adjustment circuit for a filter that can be automatically adjusted with high accuracy.

According to a third aspect of the present invention, a reference filter and a trap filter for automatic adjustment are composed of a low-pass filter and a high-pass filter, respectively, and feedback is performed according to the output amplitude of each filter to automatically adjust the filter with high accuracy. Try to get a circuit.

A fourth aspect of the present invention is to provide a filter for automatic adjustment in which the filter main body constitutes an automatic adjustment filter, and which has a simple configuration and can be automatically adjusted with high accuracy.

According to a fifth aspect of the present invention, there is provided a filter automatic adjustment circuit capable of performing automatic adjustment with a simple configuration with high accuracy by using a signal processed by a filter body as a reference signal and automatically adjusting the reference signal during a predetermined period. It is trying to get.

A sixth aspect of the present invention is to provide an automatic adjustment circuit for a filter that can be automatically adjusted with a simple configuration by using a sample-and-hold circuit to maintain a state other than a predetermined period.

A seventh aspect of the present invention is to provide an automatic adjustment circuit of a filter which can use a burst signal portion of a video signal as a reference signal and which can be automatically adjusted with a simple configuration and with high accuracy.

[0014]

According to a first aspect of the present invention, there is provided an automatic filter adjusting circuit for maintaining a frequency characteristic of a filter in a predetermined state, wherein the automatic adjusting filter includes a low-pass filter and a high-pass filter. , And the feedback is applied by the output amplitude of each filter.

In the filter automatic adjustment circuit according to a second aspect of the present invention, in the filter automatic adjustment circuit for maintaining the frequency characteristic of the filter in a predetermined state, the reference filter for automatic adjustment is composed of a low-pass filter and a high-pass filter. It is characterized in that feedback is applied by the output amplitude of each filter.

According to a third aspect of the present invention, there is provided an automatic filter adjusting circuit for maintaining a frequency characteristic of the filter in a predetermined state, wherein the reference filter includes a low-pass filter and a high-pass filter; A trap filter constituted by a high-pass filter, wherein feedback is applied by the output amplitude of each of the reference filter and the trap filter.

The filter automatic adjusting circuit according to a fourth aspect of the present invention is characterized in that the filter main body constitutes an automatic adjusting filter of the filter automatic adjusting circuit.

The filter automatic adjusting circuit according to a fifth aspect of the present invention includes a filter main body composed of a low-pass filter and a high-pass filter, wherein a signal processed by the filter main body is used as a reference signal, and the reference signal is used for a predetermined period. Automatic adjustment is performed.

The filter automatic adjusting circuit according to a sixth aspect of the present invention is characterized in that a sample and hold circuit for holding a state other than a predetermined period is provided.

In the filter automatic adjustment circuit according to a seventh aspect of the present invention, the burst signal portion of the video signal is used as a reference signal, the filter is automatically adjusted only during the burst period, and the hold operation is performed during periods other than the burst period. And

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. FIG. 1 shows an embodiment of the present invention.
In this embodiment as well, the automatic adjustment of the filter is basically the same as in the prior art, and is composed of a filter portion and a reference filter portion. However, the difference from the conventional technology is that the reference filter has a 90 ° phase shift. Is to apply feedback to the amplitude of the output signal of the reference filter instead of applying feedback to

In the automatic adjustment circuit of the filter shown in FIG.
The reference filter has a cutoff frequency of the frequency f0 of the reference signal.
Secondary low-pass filter 1a
A secondary high-pass filter 2a, amplitude detectors 3a and 3b, a differential amplifier 4, designed so that the cut-off frequency and Q are exactly equal to those of the low-pass filter 1a.
And a conductance control circuit 5.

Further, the low-pass filter 1b and the high-pass filter 2b in the trap filter portion are constituted by exactly the same circuits as the low-pass filter 1a and the high-pass filter 2a, respectively. Here, the transfer functions of the low-pass filter 1a and the high-pass filter 2a are respectively represented by the following equations.

[0024]

(Equation 1)

First, the reference signal is input to the low-pass filter 1a and the high-pass filter 2a. The cutoff frequencies of the low-pass filter 1a and the high-pass filter 2a are equal to the frequency f0 of the reference signal. Required by

[0026]

(Equation 2)

Here, since the cut-off frequency and Q of the low-pass filter 1a and the high-pass filter 2a are designed to be equal, the amplitudes of the respective output signals are equal, and the phase difference Is 1
It turns out that it is 80 deg.

Next, the output signals of the low-pass filter 1a and the high-pass filter 2a are output to the amplitude detectors 3a and 3a.
At b, amplitude detection is performed, and the amplitude value of each signal is converted into a DC voltage or DC current. From the results of Equations 3 and 4, the output signals of the amplitude detection circuits 3a and 3b have exactly the same value. Here, the low-pass filter 1 depends on various fluctuation factors.
When the cut-off frequencies of “a” and the high-pass filter 2a deviate, the amplitudes of their output signals become equal. That is, the output signals of the amplitude detection circuits 3a and 3b do not have the same value. The difference between the output signals 3a and 3b is detected by the differential amplifier 4, and the result is applied to the conductance control circuit 5 to control the characteristics of the low-pass filter 1a and the high-pass filter 2a.

In the feedback loop configured as described above, feedback is always applied so that the output signals of the amplitude detection circuits 3a and 3b become equal.

On the other hand, also in the trap filter portion, the low-pass filter 1b and the high-pass filter 2b correspond to the low-pass filter 1a of the above-described reference filter.
Control is performed in exactly the same way as the high-pass filter 2a. That is, the cutoff frequency of the low-pass filter 1b and the high-pass filter 2b is always kept at f0 for various fluctuation factors.

When the output signals of the low-pass filter 1b and the high-pass filter 2b are added in the adding circuit 6,

[0032]

(Equation 3)

This is equal to the transfer characteristic of the trap filter. That is, a trap filter is formed by the low-pass filter 1b, the high-pass filter 2b, and the adding circuit 6, and the dip frequency is f0.

As described above, the configuration as shown in FIG. 1 allows a sensitive characteristic filter such as a trap filter to be more accurately formed than a conventional automatic adjustment in which feedback is performed at a phase shift of 90 degrees of the reference filter. Can be adjusted automatically.

FIGS. 2 and 3 illustrate a method for realizing a low-pass filter 1a and a high-pass filter 2a having exactly the same cutoff frequency and Q. The cutoff frequency and Q of these filters are represented by the following equations.

[0036]

(Equation 4)

As can be seen from a comparison between FIG. 2 and FIG. 3, the low-pass filter and the high-pass filter having exactly the same cut-off frequency and Q can be easily obtained simply by changing the input portion of the filter composed of exactly the same elements. Can be configured. This is a known fact.

Embodiment 2 FIG. 4 shows another embodiment of the present invention. In this embodiment, the reference filter is not adjusted automatically by using a reference signal and a reference signal that are different from the signal processed by the filter, but by using the signal itself that is processed by the filter as the reference signal. Automatic adjustment of the filter can be realized without providing.

For example, by using a burst signal portion of a video signal as a reference signal and automatically adjusting a filter only during a burst period and holding the other portions, automatic adjustment of a filter can be realized without providing a reference filter. it can.

The operation principle is the same as that of the first embodiment described above. The difference from the first embodiment is that the difference between the output signals of the amplitude detection circuits 3a and 3b is detected. This is performed only during the period of the reference signal included in the signal. Further, since the filter is not automatically adjusted during periods other than the period of the reference signal, a sample-and-hold circuit 10 is provided to hold the state during that period.

With this configuration, the automatic adjustment can be performed by the filter itself, so that a more accurate filter automatic adjustment circuit can be realized.

[0042]

According to the first aspect of the present invention, the automatic adjustment filter is composed of a low-pass filter and a high-pass filter, and a feedback circuit is provided based on the output amplitude of each filter to provide an automatic adjustment circuit for the filter that can be automatically adjusted with high accuracy. Obtainable.

According to the second aspect of the present invention, a reference filter for automatic adjustment is composed of a low-pass filter and a high-pass filter, and a feedback circuit is provided by the output amplitude of each filter to provide an automatic adjustment circuit for a filter that can be automatically adjusted with high accuracy. Obtainable.

According to the third aspect of the present invention, the reference filter and the trap filter for automatic adjustment are each composed of a low-pass filter and a high-pass filter. An automatic adjustment circuit can be obtained.

According to the fourth aspect of the present invention, an automatic adjustment filter is constituted by the filter body, and an automatic adjustment circuit of the filter which can be adjusted automatically with a simple structure and with high accuracy can be obtained.

According to the fifth aspect of the present invention, a signal processed by the filter body is used as a reference signal, and automatic adjustment is performed by the reference signal in a predetermined period, so that the filter can be automatically adjusted with a simple configuration and accurately. An adjustment circuit can be obtained.

According to the sixth aspect, an automatic adjustment circuit of a filter that can be automatically adjusted accurately with a simple configuration can be obtained by maintaining the state other than the predetermined period by the sample and hold circuit.

According to the seventh aspect of the present invention, it is possible to obtain an automatic adjustment circuit of a filter that can be automatically adjusted accurately with a simple configuration by using a burst signal portion of a video signal as a reference signal.

[Brief description of the drawings]

FIG. 1 is a diagram showing an automatic filter adjustment circuit according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of a low-pass filter used in a semiconductor integrated circuit.

FIG. 3 is an example of a high-pass filter used in a semiconductor integrated circuit.

FIG. 4 is a diagram showing an automatic filter adjustment circuit according to another embodiment of the present invention.

FIG. 5 is a diagram illustrating a conventional filter automatic adjustment circuit.

[Explanation of symbols]

1a, 1b A second-order low-pass filter designed so that the cutoff frequency is equal to the frequency f0 of the reference signal.
2b A secondary high-pass filter designed so that the cutoff frequency becomes the frequency f0 of the reference signal, 3a and 3b
Amplitude detection circuit, 4 differential amplifier circuit, 5 filter 1a
A circuit for electrically controlling the frequency characteristics of 1b, 2a, 2b, a 6-addition circuit, 7a, 7b, and a conductance of gm1
Gm2 amplifier circuit, 8 buffer amplifier circuit, 9a / 9b
A capacitor having a capacitance value of C1 and C2, a circuit for sampling and holding an input signal, 11 a reference filter designed to shift the phase by 90 degrees at a frequency f0 of a reference signal, 12 a main filter to be automatically adjusted, 13 Phase detection circuit.

Claims (7)

[Claims] 1. An automatic adjustment circuit for a filter for maintaining a frequency characteristic of a filter in a predetermined state, wherein the automatic adjustment filter is constituted by a low-pass filter and a high-pass filter, and feedback is applied by the output amplitude of each filter. Automatic filter adjustment circuit. 2. A filter automatic adjustment circuit for maintaining a frequency characteristic of a filter in a predetermined state, wherein a reference filter for automatic adjustment is composed of a low-pass filter and a high-pass filter, and feedback is performed by the output amplitude of each filter. A filter automatic adjustment circuit characterized by the following. 3. A filter automatic adjustment circuit for maintaining a frequency characteristic of a filter in a predetermined state, comprising: a reference filter including a low-pass filter and a high-pass filter; and a trap filter including a low-pass filter and a high-pass filter. Wherein the feedback control is performed by the output amplitude of each of the reference filter and the trap filter. 4. The filter automatic adjustment circuit according to claim 1, wherein the filter body constitutes an automatic adjustment filter of the filter automatic adjustment circuit. 5. A filter body comprising a low-pass filter and a high-pass filter, wherein a signal processed by the filter body is used as a reference signal, and automatic adjustment is performed by the reference signal in a predetermined period. 2. The automatic adjustment circuit of the filter according to 1. 6. The automatic filter adjustment circuit according to claim 5, further comprising a sample and hold circuit for holding a state other than a predetermined period. 7. The automatic filter control according to claim 6, wherein a filter is automatically adjusted only during a burst period using a burst signal portion of the video signal as a reference signal, and a hold operation is performed during periods other than the burst period. Adjustment circuit.