JPH0831771B2 - Automatic filter adjustment circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a filter automatic adjustment circuit for correcting a change in characteristics of a filter circuit.

(B) Conventional technology It is known that when the filter circuit is integrated into an IC, the value of the capacitor varies and the frequency characteristics of the filter circuit deviate from what is expected. Therefore, an automatic filter adjusting circuit as shown in FIG. 2 has been proposed which automatically adjusts the variation of the frequency characteristic of the filter circuit. In FIG. 2, the filter (1) is for correcting the deviation, and a reference filter (2) of the same type as the filter (1) is provided. Oscillator (3)
Generates a first oscillation output signal φ ₁ having a phase of 0 degree and a second oscillation output signal φ ₂ having a phase of 90 degrees. The first oscillation output signal φ ₁ is applied to the phase detector (4) via the reference filter (2). The second oscillation output signal φ ₂ is directly applied to the phase detector (4). here,
If the reference filter (2) is a bandpass filter,
When the frequency characteristic has a center frequency of f ₀ as shown in FIG. 3,
It is assumed that a signal having a phase change of 0 degrees is passed.
Further, it is assumed that the phase detector (4) has a detection output of zero when the phase difference between both input signals (φ ₁ and φ ₂ ) is 90 degrees. The detection output of the phase detector (4) is simultaneously applied to the reference filter (2) and the filter (1) to change its filter characteristic. Therefore, if the characteristic of the reference filter (2) in FIG. 2 does not have a phase delay and is as expected, the output of the phase detector (4) becomes zero and no control is performed.
Further, if the characteristic of the reference filter (2) has a phase delay and the center frequency deviates from the frequency f ₀ , a phase detection output corresponding to the deviation amount is generated and the characteristic of the reference filter (2) is changed. As a result, the characteristic of the reference filter (2) is corrected to the expected characteristic, that is, the phase delay becomes 0 degree and the center frequency f ₀ .

Therefore, the filter characteristic of the filter (1) is similarly corrected.

(C) Problems to be Solved by the Invention By the way, the filter (1) and the reference filter (2)
Both have a differential amplifier and a capacitor, and change the mutual conductance gm of the differential amplifier to change the frequency characteristic. The transconductance gm maintains a predetermined value when the level of the input signal (signal applied to the filter) is small, but when the level increases, the gm changes according to the signal level, and the value decreases. . This is shown in FIG. When the gm changes, the center frequency and the cutoff frequency of the filter change, and there is a problem that the entire filter characteristic changes.

(D) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and an oscillator for generating first and second oscillation output signals having a constant phase difference and a differential amplifier are provided. The frequency characteristic changes according to the conductance and the first oscillation output signal of the oscillator is applied, and the frequency characteristic changes according to the transconductance of the differential amplifier, and the input signal from the input terminal is applied. And a phase detection circuit that performs phase detection of the output signal of the reference filter and the second oscillation output signal of the oscillator, and controls the mutual conductance of the reference filter and the filter according to the detection output. In the automatic adjustment circuit of the filter, the level detection circuit for detecting the level change of the signal passing through the filter and the detection output of the level detection circuit are detected. Characterized in that a level adjusting means for adjusting the level of the first oscillation output signal applied to the reference filter Te.

(E) Operation According to the present invention, since the level of the signal applied to the reference filter is changed according to the level of the input signal applied to the filter, even if an excessive input signal is applied to the filter, the desired filter can be used. The characteristics can be maintained.

(F) Embodiment FIG. 1 shows an embodiment of the present invention, in which (5) detects the level of a signal passing through the filter (1) (in this case, a signal added to the filter (1)). The level detection circuit, and (6) are variable gain amplifiers whose gains change according to the detection output of the level detection circuit.

In FIG. 1, the same circuit elements as those in FIG. 2 are designated by the same reference numerals, and the description thereof will be omitted.

In FIG. 1, it is assumed that the level of the input signal applied to the input terminal (7) of the filter (1) is low and the transconductance of the filter (1) has an original value. In the case of an input signal of such a level, the level detection circuit (5) controls the gain of the variable gain amplifier (6) to 1. Therefore, the configuration of FIG. 1 becomes substantially the same as that of FIG. 2, and the frequency characteristic of the filter (1) is automatically adjusted. When the level of the signal applied to the reference filter (2) is V ₁ and the cutoff frequency at that time is f ₁ , the relationship is shown by the solid line in FIG. In this state,
If the input signal applied to the input terminal (7) increases and its level reaches V ₂ , the cutoff frequency of the filter (1) becomes f ₂ as shown in FIG. This is because the reference filter (2) and the filter (1) are controlled so as to have the same characteristic, and the filter (1) also has the characteristic indicated by the solid line in FIG.

Here, when the level applied to the input terminal (7) becomes V ₂ , the level detection circuit (5) detects it and the gain of the variable gain amplifier (6) changes according to the detection output. The gain of the variable gain amplifier (6) changes so that the level of the signal applied to the reference filter (2) changes from V ₁ to V ₂ .
Therefore, a signal of the same level V _{2 as} the signal applied to the filter (1) is applied to the reference filter (2), the transconductance of the reference filter (2) shows the same change, and the frequency characteristic becomes the same. In this state, the automatic adjustment loop composed of the oscillator (3), the reference filter (2) and the phase detector (4) works, so that the reference filter (2) is controlled to return to the original cutoff frequency f ₁ . As a result, the reference filter (2)
The characteristic of the cutoff frequency is as shown by the dotted line in FIG. 5, and at the same time, the frequency characteristic of the filter (1) is similar.

Therefore, according to the circuit of FIG. 1, even if the level of the input signal applied to the input terminal (7) is excessive, a constant cutoff frequency can be obtained.

In the above description, the operation of the variable gain amplifier (6) and the operation of the automatic adjustment loop are explained in time series for the sake of convenience of explanation, but they actually change at the same time.

Further, although the case where the cutoff frequency of the filter is adjusted has been described in the description of FIG. 1, it may be the center frequency or the like, which is the entire frequency characteristic.

(G) Effect of the Invention As described above, according to the present invention, it is possible to provide an automatic filter adjusting circuit which can always maintain a constant filter characteristic even when the level of the input signal becomes excessive.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a conventional automatic filter adjusting circuit, and FIGS. 3 and 4 are for explaining FIG. Characteristic diagram, and fifth
The figure is a characteristic diagram for the explanation of FIG. (1) ... filter, (2) ... reference filter, (3)
...... Oscillator, (4) …… Phase detector, (5) …… Level detection circuit, (6) …… Variable gain amplifier.

Claims (2)

[Claims] 1. An oscillator for generating first and second oscillating output signals having a constant phase difference, and a frequency characteristic thereof is changed according to a mutual conductance of a differential amplifier, and a first oscillating output signal of the oscillator is applied. A reference filter, a filter whose frequency characteristic changes according to the transconductance of a differential amplifier, and an input signal from an input terminal is applied; an output signal of the reference filter; and a second oscillation output signal of the oscillator. Phase detection circuit that controls the mutual conductance of the reference filter and the filter according to the detection output of the reference filter, and an automatic adjustment circuit of the filter, which detects the level change of the signal passing through the filter. The level detection circuit adjusts the level of the first oscillation output signal applied to the reference filter according to the detection output of the level detection circuit. Automatic adjustment circuit of the filter, characterized in that a level adjusting means for. 2. The automatic filter adjusting circuit according to claim 1, wherein the level adjusting means is a variable gain amplifier arranged between the oscillator and the reference filter.