CN85100788B - Automatic control of filter IC frequency characteristics - Google Patents

Abstract

A filter integrated circuit includes: a reference circuit for attenuating an input signal and generating a reference level signal; a pseudo filter circuit including resistors and variable capacitors as filter elements; and an error amplifier unit for comparing The output signal level of the reference level generator and the output signal level of the pseudo filter are used to generate an automatic adjustment control signal according to the level difference of the output signals. The automatic adjustment control signal is connected to the pseudo filter circuit to change the capacitance value of the variable capacitor in the pseudo filter circuit, so that the output signal level of the pseudo filter circuit can be equal to the reference signal level. The automatic adjustment control signal is connected to at least one filter circuit to change the capacitance value of a variable capacitor in the circuit.

Description

The present invention relates to a filter integrated circuit, which is manufactured by integrating a filter into a monolithic integrated circuit on a chip such as silicon.

In commonly used electronic circuits, when low-pass, high-pass and band-pass filters or phase equalizers are used to generate the desired signal, the filter unit with separate components inductor L, capacitor C and resistor R is widely used. As more and more electronic circuits are made in the form of integrated circuits (monolithic integrated circuits), these filters have become an obstacle to reducing the cost, size and weight of electronic circuits, especially in portable devices, which can be moved Sexuality is an important factor, so it is very important to reduce the size and weight, so that the filter needs to be realized in the form of an integrated circuit.

Since it is difficult to realize the inductor L with an integrated circuit, the filter suitable for integration is an active filter, and the active filter can only be composed of a capacitor C and a resistor R. For example, a double T-trap can be formed using only capacitors and resistors. Its notch frequency f _r is expressed as:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}}$

Where C _a is the capacitance value C of the capacitor used in the double T-shaped notch filter, and R _a is the resistance value R of the resistor used in the notch filter.

When this notch filter is integrated, the deviation of the capacitance value and the resistance value becomes a problem. That is to say, the capacitance value and resistance value in the integrated circuit will be affected by deviations such as impurity concentration and mask alignment. For example, the absolute value of the capacitor will change by ±10 to 15%, and the absolute value of the resistor will change by ±10%. The notch frequency of the T-notch filter varies by ±20 to 25%, so practical use is extremely difficult. According to a countermeasure disclosed in Japanese Patent Publication No. 58083/82, a laser can be used to change the resistance value of a resistor located on an integrated circuit to correct the deviation. Although this countermeasure has been used, there are still many problems related to accuracy and productivity.

In the variable attenuation circuit disclosed in Japanese Pat-ent Publication No. 58083/82 and US Patent No. 3761741, the transistor emitter resistance is actually changed by changing the DC current. Using similar techniques, it is possible to correct for changes in filter characteristics caused by deviations in IC component values. However, this technique is difficult to apply to all filters including notch filters. Furthermore, correcting for deviations of integrated circuit components through external adjustments increases costs.

The purpose of the present invention is to provide a filter integrated circuit, which can automatically correct the deviation of the filter characteristics caused by the deviation of the value of the integrated capacitor and the integrated resistor, and does not need any external adjustment to ensure the expected effect, and there is no such prior art in the question.

The filter integrated circuit made according to the present invention includes: a reference level generating circuit to receive an input signal with a reference frequency and generate a reference level signal; a pseudo filter circuit to filter the input signal, This circuit includes an integrated resistor and an integrated capacitor with variable capacitance; an error amplifier unit to receive the output signal of the reference level generator and the output signal of the pseudo filter circuit, and based on these output signals The level difference between them generates an automatic adjustment control signal; the device used to connect the automatic adjustment control signal to the pseudo filter circuit, the device also changes the capacitance value of the variable capacitor, so that the output signal level of the pseudo filter circuit can be adjusted Equal to the level of the reference level signal; a filter circuit, which contains an integrated resistor and an integrated variable capacitor, which correspond to the respective corresponding elements of the pseudo-filter, the ratio accuracy of the resistor and capacitor of the pseudo-filter is very high. The automatic adjustment control signal is connected to the filter circuit to change the capacitance value of the variable capacitor so as to correct the deviation of the filter characteristic curve.

Here, ratio precision (ratio precision) refers to the accuracy of the ratio between the circuit element values. For example, high ratiometric accuracy is the same for capacitance values if the ratio of the resistance values of the two resistors remains constant even though individual resistance values may deviate from their normal values. In a semiconductor integrated circuit, high ratiometric accuracy among circuit elements on a chip can be obtained although the values of individual circuit elements may deviate from their normal values.

In the filter integrated circuit made according to the present invention, the automatic adjustment control signal generated is to make the output signal level of the pseudo filter circuit equal to the reference level, that is to say, the automatic adjustment control signal corrects the difference caused by the deviation of the component value. The deviation of the filter characteristics of the pseudo filter circuit is caused, and the automatic adjustment control signal is simultaneously input to the filter circuit. The ratio accuracy between the element values of the filter circuit and the corresponding element values of the dummy circuit is high. When the filter characteristics of the pseudo filter circuit are changed due to the deviation of the component values, the filter characteristics of the filter circuit will be changed like the pseudo filter circuit. Thereby, the characteristic deviation of the filter circuit can be corrected by using an automatic adjustment control signal for correcting the characteristic deviation of the false filter circuit.

In the filter integrated circuit, the reference level generating circuit is composed of a plurality of integrated resistors or external resistors with high ratio accuracy, so as to use the resistors to attenuate the reference input signal and generate a reference level signal. The error amplifier unit can be composed of a detection circuit used to detect the output signal of the reference level generation circuit, another detection circuit used to detect the output signal of the pseudo filter circuit, and a detection circuit used to receive the output signals of the two detection circuits and amplify the two output signals. It is composed of level difference and an amplifier that generates an automatic adjustment control signal. The frequency of the input signal input to the filter integrated circuit may be variable. In this case, the automatic adjustment control is performed for each frequency of the signal.

According to the present invention, characteristic deviations of a filter including an integrated resistor and an integrated variable capacitor included as elements in a semiconductor integrated circuit are automatically adjusted. Thus, it is possible to improve the accuracy of the filters and to dispense with the filter-by-filter tuning previously employed. Thus, with the present invention, bulky filter filters, previously used as external components, can be integrated without adjustment. As a result, the circuit cost, unit volume, weight and number of components in the filter unit can be reduced.

Advantages and other objects of the present invention will become apparent with reference to the following description and accompanying drawings. in:

Fig. 1 is a circuit diagram of a double T-shaped notch filter;

Fig. 2 is a characteristic curve diagram, is used for representing the frequency characteristic deviation of filter shown in Fig. 1;

Fig. 3 is a block diagram, is used for representing the embodiment structure of the filter integrated circuit that forms according to the present invention;

Figure 4 shows a specific example of a reference level generating circuit used in the filter integrated circuit in Figure 3;

Fig. 5 shows a concrete example of the pseudo-filter circuit used in the filter integrated circuit;

Figure 6 shows the waveform signals appearing at each point of the filter integrated circuit;

Fig. 7 is a characteristic curve diagram of a variable capacitor used in the pseudo filter circuit;

Fig. 8 shows the frequency characteristic shift of pseudo filter circuit;

FIG. 9 shows a specific example of a filter circuit used in an integrated circuit;

Fig. 10 represents another specific example of using a filter circuit in a filter integrated circuit;

Fig. 11 is another implementation circuit diagram of the filter circuit in the filter integrated circuit made according to the present invention;

Figure 12 is a resistor of another embodiment of a filter integrated circuit made in accordance with the present invention.

Before describing the embodiments of the present invention, a double T-shaped filter structure suitable for integration will be described. Figure 1 shows a well-known double T-shaped notch filter structure, if the resistor and capacitor values in Figure 1 are selected as:

R ₁ =R ₂ =2R ₃ =R _a

C ₁ =C ₂ =C ₃ /2=C _a

but In Fig. 1, V _i is the input signal, and V ₀ is the output signal.

When a notch filter is integrated, the notch frequency is deviated due to the deviation of the capacitance and resistance of the integrated circuit. In integrated circuits, the absolute value of capacitors varies by up to ±10% to 15%, and the absolute value of resistors varies by up to ±10%. In this case, the notch frequency of the notch filter shown in Fig. 1 varies by about ±20 to 25% in the worst case. Figure 2 shows the variation of the notch frequency due to the variation of the resistance and capacitance values of the filter circuit shown in Figure 1, and its range extends from a to b. The present invention provides a filter integrated circuit, which has the function of automatically correcting the notch frequency variation of the notch filter.

Embodiments of the present invention are described below with reference to the accompanying drawings. Fig. 3 is a block diagram showing the structure of an embodiment of a filter circuit made in accordance with the present invention. In Fig. 3, an input signal 3 having a constant _reference frequency fin is applied to IC pin 2 of the IC. Thus, the input signal 3 is supplied to a reference level generating circuit 4 and a pseudo-filtering circuit 5 . The reference level generating circuit 4 attenuates the input signal to generate a reference level signal. The reference level signal is detected by the detection circuit 6 , and the detection output is connected to an input terminal of the error amplifier 7 . The dummy filter circuit 5 is a filter circuit including resistors and variable capacitor elements. The pseudo filter circuit 5 filters the input signal and outputs the filtered signal to the detection circuit 8 . The detection circuit 8 detects the signal, and outputs the detected signal to the other input terminal of the error amplifier 7 . The error amplifier 7 amplifies the level difference between the output of the detection circuit 6 and the output of the detection circuit 8 . This synthesized voltage signal is output to the variable capacitor of the dummy filter circuit 5 through the wire 9. In the pseudo filter circuit 5, the voltage signal provided by the error amplifier 7 changes the filter characteristics by changing the capacitance value of the variable capacitor, so that the level of the filtered signal can be equal to the level of the reference signal provided by the reference level generation circuit 4 . The reference signal level generation circuit 4 , the false filter circuit 5 , the detection circuits 6 and 8 , and the error amplifier 7 constitute an automatic adjustment control signal generation circuit 10 . The automatic adjustment control voltage signal output by the error amplifier 7 corrects the filter characteristic deviation, which is caused by the deviation of the resistance value and the capacitance value in the pseudo filter circuit 5 . The automatic adjustment control voltage signal is input to the filter circuits 11 and 12 in IC _t . Each of circuits 11 and 12 includes a resistor and variable capacitor element. The resistors and capacitors in each filter circuit 11 and 12 have a very high ratio accuracy with the corresponding resistors and capacitors of the pseudo filter circuit respectively. In each filter circuit 11 and 12, the automatic adjustment control voltage signal changes the variable capacitor. The capacitor value is used to adjust the deviation of the filter characteristics due to the deviation of the resistance value and the capacitance value. The input end and output end of the filter circuit 11 are respectively connected to the integrated circuit pins 13 and 14 for external use of the integrated circuit 1 . Filter 12 is used by circuits within integrated circuit 1 .

The embodiment of Figure 3 will now be described in more detail. FIG. 4 is a specific circuit example of a reference level generating circuit 4 , and FIG. 5 shows a specific circuit example of a pseudo filter circuit 5 . FIG. 6( a ) shows the waveform of the input signal 2 . FIG. 6( b ) shows the waveforms at the output end of the reference level generating circuit 4 and the output end of the detection circuit 6 , which are represented by solid lines and dotted lines, respectively. Fig. 6(c) is the waveform of the output terminal of the pseudo filter circuit 5 and the waveform of the output terminal of the detection circuit 8, which are represented by solid lines and dashed lines respectively. FIG. 7 shows a characteristic graph of the variable capacitor included in the dummy filter circuit 5. As shown in FIG. FIG. 8 shows a frequency characteristic curve of the dummy filter circuit 5. As shown in FIG. FIG. 9 is a circuit diagram of a specific example of the filter circuit 11. As shown in FIG. FIG. 10 shows a circuit diagram of a specific example of the filter circuit 12. As shown in FIG.

The reference level generating circuit 4 attenuates the input signal 3 at a fixed precise ratio. In the example shown in FIG. 4 , the reference level generating circuit 4 can be composed of integrated resistors 15 and 16 . Since the ratio of component values in the IC can be realized with very high precision, the reference level generating circuit

The above equation can be realized with extremely high precision. Pseudo-filter 5 is a filter circuit containing integrated resistors and integrated variable capacitors, wherein the capacitance of the integrated capacitors is changed by the applied voltage. For example, the pseudo filter circuit 5 shown in FIG. 5 can be composed of integrated resistors 17 and 18 , a variable capacitor 19 and a fixed voltage source 20 . The resistance value R ₁₈ of the integrated resistor 18 and the capacitance value C ₁₉ of the variable capacitor 19 form a first-order capacitor-resistor filter. Its cut-off frequency f _c is:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 18</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="19"\; label="C"\; filenames="H85100788E0000083.png,H85100788E0000121.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 19</span>}}}$

The fixed voltage source 20 applies the DC voltage to the anode of the variable capacitor 19 through the integrated resistors 17 and 18. On the other hand, the output voltage of the error amplifier 7 is applied to the cathode of the variable capacitor 19 in a negative feedback manner.

The capacitance value of the variable capacitor 19 is changed by the voltage applied across it. When the capacitance of the base-emitter junction is used as the variable capacitor 19, the capacitance value can be expressed as:

$\mathrm{<span\; class="notranslate">\; C\; j</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; C\; j</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>}{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; Vj</span>}}{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}}$

$<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; C\; j</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; \&phi;</span>}}^{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; Vj</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}}$

or:

logC _j ＝K-αlog(φ+V _j )

in:

C _j = base-emitter junction capacitance value

C _j (0) = Capacitance of the base-emitter junction at zero bias

V _j = base-emitter voltage (reverse-biased diode voltage)

φ = internal voltage

α = voltage correlation coefficient

K＝log〔C _j (0)φ ^a 〕

Figure 7 shows an example of the base-emitter junction capacitance characteristic curve. With a supply voltage of 5V, V _j can be 0-3V, while C _j can be offset by ±20% to 25% around its typical value.

The output of reference level generation circuit 4 and pseudo-filter circuit 5 is represented with solid line in Fig. 6 (b) and 6 (c) respectively, they are respectively connected to above-mentioned detection circuit and do peak detection, become respectively Fig. 6 (b) ) and the signal indicated by the dotted line in 6(c). Output signals of these detection circuits are connected to an error amplifier 7 . When the automatic filter adjustment control voltage signal 9 is used as negative feedback, the output of the error amplifier 7 is connected to one end of the variable capacitor 19 to fix the filter characteristics of the false filter circuit 5, so that the outputs of the detection circuits 6 and 8 can be equal to each other , that is, the levels 21 and 22 shown in FIGS. 6(b) and 6(c), respectively, may be equal to each other. By automatically adjusting the control voltage signal, the capacitance value of the variable capacitor 19 is automatically changed to absorb the deviation of the pseudo filter circuit 5 .

Each filter circuit 11 and 12 includes an integrated resistor and variable capacitor to obtain ideal filter characteristics, and is connected with an automatic adjustment control signal 9 .

Since elements integrated on a chip can be formed with high comparative accuracy, the frequency characteristic deviation of the dummy filter circuit 5 can be made almost equal to that of the filter circuits 11 and 12 . Thus, the frequency characteristic deviation of the filter circuits 11 and 12 can be automatically absorbed by the automatic control signal 9 .

The operation of the circuit of Figure 3 will now be described in more detail. Assume now that the integrated resistors 15 and 16 are fixed so that the reference level generating circuit 4 has an attenuation loss of 3dB. If the total deviation of the integrated resistor and variable capacitor is -20%, the pseudo filter circuit 5 has the characteristic represented by 23 in FIG. 8 . If an input signal having a frequency f _in is connected to the circuit 3 at this time, the output of the pseudo filter circuit 5 will be larger than the output of the reference level generating circuit 4 . The outputs of the dummy circuit 5 and the reference level generating circuit 4 are fed back to the dummy circuit 5 through the detection circuits 6 and 8 and the error amplifier 7 to reduce the voltage connected to the variable capacitor of the dummy circuit 5 . As the voltage to the variable capacitor decreases, as shown in FIG. 7, its capacitance value increases, and the frequency characteristic 23 of FIG. 8 is shifted to the left to produce the frequency characteristic 24 of FIG. That is, the capacitance value of the variable capacitor is changed so that the outputs of the reference level generating circuit 4 and the dummy filter circuit 5 are equal to each other at the frequency _fin . Since the capacitance of the variable capacitor varies by ±20 to 25% according to _Vj , the maximum deviation can be absorbed. If the total deviation of the integrated resistors and variable capacitors is +20% maximum, the pseudo-filter circuit 5 assumes the characteristic 25 shown in FIG. 8 . In this case, when the dummy filter circuit 5 receives the fin _input , there will of course be the frequency characteristic of FIG. 8 at the end.

As described above, the automatic adjustment of the control signal 9 for automatically absorbing the deviation between the integrated resistor 18 and the variable capacitor 19 can be achieved. Due to the relationship that exists on the same chip, the integrated resistor 18 and the variable capacitor 19 can respectively correspond to the integrated resistor 30 and the variable capacitor 32 shown in Figure 9, and respectively correspond to the integrated resistor 31 and the variable capacitor 33 shown in Figure 10 Achieve high ratio accuracy. The cut-off frequencies f _c (11) and f _c (12) of the filtering circuits 11 and 12 illustrated in FIGS. 9 and 10 can be expressed as:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 11</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 30</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="32"\; label="C"\; filenames="H85100788E0000102.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 32</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 18</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="19"\; label="C"\; filenames="H85100788E0000083.png,H85100788E0000121.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 19</span>}}}$

as well as

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 12</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 31</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="33"\; label="C"\; filenames="H85100788E0000103.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 33</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 18</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="19"\; label="C"\; filenames="H85100788E0000083.png,H85100788E0000121.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 19</span>}}}$

where n ₁ to n ₄ are constants.

In the filter circuits 11 and 12, deviations can therefore be automatically absorbed in the integrated filter without adjustment requirements.

滤波特性并不取决于温度、电源电压或输入信号3的电平变化。结果，理想的稳定滤波特性便可经常达到。Furthermore, as mentioned above, since the reference level of the negative feedback is the attenuation of the above-mentioned reference level generating circuit 4

The filter characteristics do not depend on temperature, supply voltage or level changes of the input signal 3 . As a result, ideal stable filter characteristics can often be achieved.

In the above embodiments, the input signal 3 is input from the outside of the integrated circuit 1 . However, even if the input signal 3 is supplied by a circuit included in the same integrated circuit, a similar effect can of course be achieved. In a video signal (tape) recorder, for example, a specific signal source generated in a color signal processing circuit using a crystal oscillator is a chrominance subcarrier (3.58 MHz in NTSC system).

In the above description, the detection circuits 6 and 7 send out half-wave rectified waveforms. If the full-wave rectified waveform is used for peak detection, a more stable filter circuit can be achieved.

Figure 11 shows another embodiment of the present invention. The same reference numerals as in Fig. 3 are assigned to the same parts. The same reference numerals as in Fig. 3 are assigned to similar parts. The fixed voltage sources 40, 41 and 42 supply the same voltage. This voltage minus the transistor's base-emitter voltage of about 0.7V is connected to the anode sides of the variable capacitors 19, 43, 44 and 45, and also to the detection circuits 6 and 8. Reference numerals 46 to 48 denote integrated capacitors. Reference numerals 471 to 521 and 53 to 61 denote npn transistors and integrated resistors, respectively.

The filter circuit 11 is the aforementioned double T-shaped notch filter. If the component is selected as:

R ₅₈ =R ₅₉ =2R ₆₀ =R _b

${\mathrm{<span\; class="notranslate">\; <figure-callout\; id="44"\; label="C"\; filenames="H85100788E0000111.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 44</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="45"\; label="C"\; filenames="H85100788E0000111.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 45</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="43"\; label="C"\; filenames="H85100788E0000111.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 43</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}$

Then the notch frequency f _r can be expressed as:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}}$

Since elements on a crystal can be achieved with high ratiometric accuracy,

R _b and C _b can be expressed as: R _b = n ₅ R ₁₈ and C _b = n ₆ C ₁₉

Where n ₅ and n ₆ are constants. Therefore, following:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 5</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 6</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 18</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="19"\; label="C"\; filenames="H85100788E0000083.png,H85100788E0000121.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 19</span>}}}$

Described constant generally depends on R ₁₈ and C ₁₉ , and in the present invention, the value of n ₅ is preferably around 3, i.e. (n ₅ =3), and n ₆ is preferably also valued at about 3 i.e. (n ₆ = 3).

Through the automatic adjustment of the pseudo filter circuit 5, the deviation of the components in the double T-shaped circuit can also be absorbed without any adjustment.

Figure 12 is yet another embodiment of the present invention. The same reference numerals as in Figs. 3 and 11 are assigned to similar parts. 12 shows variable capacitors 62 and 63, npn transistors 641 to 671, constant current sources 68 and 69, integrated resistors 70 to 77, constant voltage source 78, AC signal bypass capacitor 79, and integrated circuit pin 80. Resistor 74 with a high resistance value passes only a DC voltage signal for supplying bias voltage. Since the resistor 74 has a high resistance value, if an AC signal flowing through the resistor 74 is greatly attenuated compared with an AC signal flowing through the capacitor 79 to the variable capacitor 62, it can be ignored. The automatic adjustment control signal from the error amplifier circuit 7 is connected to the variable capacitor 62 through the resistor 74 . Reference numeral 81 denotes a differential amplifier. The differential amplifier 81 constitutes a secondary positive feedback low-pass filter, and is connected to the resistors 72 and 73 via the variable capacitors 62 and 63 . The effects of the present invention can also be obtained with this embodiment.

In the embodiment of Fig. 12, the filtering circuit constitutes a low-pass filter, alternatively, a high-pass filter or a band-pass filter may be used. In this case, the dummy filter circuit is configured in a manner similar to that shown in FIG. 12 . In a filter circuit containing a high-pass filter or a band-pass filter, the capacitance value of the variable capacitor is changed by an automatic adjustment control signal to correct the deviation of the filter characteristics.

In the embodiments described above, the input signal has a fixed frequency. However, an input signal with variable frequency can also be used.

While individual embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the broader aspects of the invention.

Claims (4)

1. one comprises containing the automatic control for frequency characteristics of filter integrated of resistor and capacitor, comprising:

One reference level produces circuit, to be used for an input signal and generation one reference level signal of decaying;

One pseudo-filter circuit, with by said input signal filtering, and said pseudo-filter circuit contains a resistance and a variable capacitance as filter element;

One error amplifying unit, produces the output signal of circuit and the output signal of said pseudo-filter circuit to receive said reference level, the relatively signal level of two output signals, and produce an automatic control signal of adjusting according to said signal level difference;

Be used to provide the device that said adjustment controls signal to said pseudo-filter circuit, to change the capacitance of said variable capacitance, thereby change the filtering characteristic of said tseudo circuit, to lower said signal level difference,

It is characterized in that,

One filter circuit that contains resistance and variable capacitance, these elements are to form as high ratio accuracy compared with the filter element of said pseudo-filter circuit, and the capacitance of said variable capacitance is to change with said automatic adjustment control signal, to change the filtering characteristic of said filter circuit.

2. automatic control for frequency characteristics of filter integrated claimed in claim 1, is characterized in that said error unit comprises:

One detecting circuit, to produce said reference level the output signal detection of circuit;

Another detecting circuit, is used for the output signal detection of said pseudo-filter circuit; And

One amplifier, to receive the output signal of said detecting circuit, amplifies the level difference of these output signals, and produces said automatic adjustment control signal;

3. automatic control for frequency characteristics of filter integrated claimed in claim 1, is characterized in that said reference level produces circuit and contains resistor, and said pseudo-filter circuit contains resistor and variable capacitor, and said filter circuit contains resistor and variable capacitor.

4. automatic control for frequency characteristics of filter integrated claimed in claim 1, is characterized in that said reference level produces circuit, and said pseudo-filter circuit and said filter circuit also include respectively transistor as active element.

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