JPH0646695B2 - Filter circuit - Google Patents

Description

The present invention relates to a filter integrated circuit suitable for integrating a filtering circuit (hereinafter referred to as a filter) in a monolithic IC formed on a silicon wafer or the like. is there.

BACKGROUND OF THE INVENTION Conventionally, in order to obtain a desired signal in an electronic circuit, a reduction filter (hereinafter referred to as LPF), a high-pass filter (hereinafter referred to as HPF), a bandpass filter (hereinafter referred to as BPF), a phase, etc. As a rectifier (hereinafter referred to as an equalizer), a large number of external block filters composed of inductance, capacitance, and resistance are used. For this reason, as circuit integration progresses, these filters are factors that prevent cost reduction, size reduction, and weight reduction. Especially in portable VTRs, where miniaturization and weight reduction are desired, integration of filters is an essential technology.

Inductance is not suitable for integration, and capacitance,
A resistor-only configuration is suitable for integration. For example, the Twin-T circuit shown in FIG. 7 is known as a trap filter. In Fig. 7, the resistance and capacitance are Then the trap frequency _T is Becomes

When the trap filter of FIG. 7 is integrated, there arises a problem of variation. In other words, the capacitance value and resistance value in the IC are affected by variations in the impurity concentration in the semiconductor and mask shift, and as a example, large variations occur, such as the absolute value of capacitance ± 15% and the absolute value of resistance ± 10%. Therefore, the trap frequency of the trap filter of FIG. 7 is also from a to b as shown in FIG.
It fluctuates within the range of, and in the above example, it fluctuates up to ± 25%, which is extremely difficult to put into practical use. As one of the measures against this, it has been proposed to change the resistance value on the IC chip by laser trimming to absorb the variation as shown in Japanese Patent Publication No. 57-58083, but in terms of accuracy and yield. It still leaves many problems.

As another method, Japanese Patent Publication No. 52-36813, U.S. Pat. No. 37
As disclosed in Japanese Patent No. 61741, there is proposed a variable attenuation circuit that utilizes the fact that the emitter resistance of a transistor changes depending on the emitter current. According to this method, it is possible to adjust the variation of the filter characteristics due to the variation of the elements in the IC, as in the above method. However, this method is difficult to apply to a trap filter composed of resistors R ₁ , R ₂ and R ₃ , as shown in FIG. 7, and is difficult to apply to all filters. Furthermore, it is necessary to adjust from the outside to absorb variations in IC elements, which causes a cost increase.

[Object of the Invention]

It is an object of the present invention to provide a filter integrated circuit which can automatically absorb variations in filter characteristics caused by variations in IC internal capacitance and IC internal resistance, and can also ensure performance without adjustment.

[Outline of Invention]

In order to achieve this object, the present invention provides a reference level generating circuit composed of a plurality of IC internal resistors (or external resistors may be used) with a sufficient ratio accuracy for an input signal having a certain reference frequency, an IC internal resistor and an IC. It is supplied to a pseudo filter circuit composed of an internal junction capacitance, each output of each is input to a voltage comparison circuit through an amplitude detection circuit, and the comparison output is negatively fed back to the junction capacitance. This feedback changes the voltage across the junction capacitance and absorbs variations in the pseudo filter circuit due to variations in IC internal resistance.
Further, the comparison output is supplied to another filter circuit on the same chip, which is composed of the resistance and the junction capacitance which form the pseudo filter circuit, to automatically absorb the variation in the IC element and accurately perform adjustment-free adjustment. be able to.

A sinusoidal signal without distortion is used as the reference signal in order to accurately absorb the variation. When the signal to be used as the reference signal is a sine wave signal having a distortion component (for example, a rectangular wave), it is necessary to remove the harmonic component from the signal component, and therefore a harmonic elimination filter is provided.

When the above-mentioned filter circuit is used in a video magnetic recording / reproducing apparatus (hereinafter referred to as VTR), the most stable color subcarrier is used as a reference signal. The frequency of the color subcarrier is about 3.58MHz in the NTSC system, and the carrier of the frequency-modulated luminance signal (hereinafter referred to as the FM luminance signal) is 3.4 to 4.4MHz in the VHS system, so the color subcarrier leaks to the luminance signal. If you put it in, beat interference will occur. Therefore, a signal obtained by multiplying the color subcarrier is used as the reference signal so that it does not fall within the band of the FM luminance signal.

Example of Invention

An embodiment of the present invention will be described below with reference to FIG. First
In the figure, 1 is a reference signal source, 2 is a signal source, and 3 to 6 are ICs.
Pin, 7 is a reference level generation circuit by resistance, 8 is resistance,
Pseudo filter circuit based on junction capacitance, 9 and 10 amplitude detection circuit, 11 voltage comparison circuit, 12 filter circuit, 13 capacitor, 14 multiplication circuit, 23 and 24 IC pins, 25 video signal source,
Reference numeral 26 is a video signal processing circuit, 27 is a frequency modulation circuit, 28 is a harmonic elimination filter, and 35 is a ground pin. The reference signal input from the IC pin 3 is multiplied by the multiplication circuit 14. The multiplied reference signal is converted into a sine wave signal having no distortion component by the harmonic removal filter 28 and supplied to the reference level generation circuit 7 and the pseudo filter circuit 8. A signal having a certain level is extracted from the reference level generating circuit 7,
It is supplied to the amplitude detection circuit 9. On the other hand, a signal attenuated according to its filter characteristic is taken out from the pseudo filter circuit 8 and supplied to the amplitude detection circuit 10. Each detection output is input to the voltage comparison circuit 11, and a comparison output is obtained. The pseudo filter circuit 8 uses this comparison output as a control signal.
To control the voltage across the junction capacitance. By this control, the capacitance value of the junction capacitance changes, and the output of the pseudo filter circuit 8 is controlled to be at the same level as the output of the reference level generation circuit 7. That is, if the relative accuracy of the resistors forming the reference level generating circuit 7 and the pseudo filter circuit 8 is set sufficiently large, even if the insulation value of the resistors varies, the capacitance value of the junction capacitance changes and the resistance variations occur. Will be absorbed.

The resistance and junction capacitance of the filter circuit 12 are the reference level generation circuit 7, the resistance and junction capacitance of the pseudo filter circuit 8 and the capacitance of the junction capacitance are the same as described above, provided that the relative accuracy is sufficient. The value changes and the filter circuit 12 can have a filter characteristic without variation.

Next, the reason for providing the harmonic elimination filter 28 will be described. Second
The figure shows the waveform of each part when a rectangular wave signal is input to the reference level generation circuit 7 and the pseudo filter circuit 8 as a reference signal. In FIG. 2, 29 is an input signal, 30 is an output signal of the reference level generating circuit 7, and 31 is an output signal of the pseudo filter circuit 8. Reference signal 29 input to the reference level generation circuit 7
Is attenuated by resistance division and becomes like the output signal 30. On the other hand, the reference signal 29 input to the pseudo filter (here, an LPF as an example) circuit 8 becomes an output signal 31. On the other hand, FIG. 3 shows waveforms at various parts when a sine wave having no distortion component is used as a reference signal. In FIG. 3, 32 is an input signal, 33 is an output signal of the reference level generating circuit 7, and 34 is an output signal of the pseudo filter circuit 8. When a sine wave is used as the reference signal, the output signal 33 of the reference level generating circuit 7 is a signal obtained by attenuating the input signal 32 by resistance division, as shown in FIG. The phase of the waveform is similar to that of the input signal 32, although the phase is rotated by its filter characteristic but only the amplitude is reduced. As described above, when a sine wave signal is used as the reference signal, if the circuit is in a balanced state, the output signal 33 of the reference level generating circuit 7
And the amplitude of the output signal 34 of the pseudo filter circuit 8 becomes equal. However, when a rectangular wave signal is used as the reference signal, the amplitudes of the output signal 30 of the reference level generating circuit 7 and the output signal 31 of the pseudo filter circuit 8 are not equal to each other even if the frequency is the same as that of the sine wave signal described above, and the adjustment accuracy is reduced. It causes a big error. For this reason, the reference signal used for the unadjusted filter circuit must be a sine wave signal having no distortion component, and the harmonic elimination filter 28 is essential.

Next, the reason for multiplying the reference signal will be described. When the filter circuit described above is used for the VTR, the most stable color subcarrier is used for the reference signal. In the VTR, the frequency modulation circuit 27 for the luminance signal is provided on the same chip, and the spectrum of the output signal is as shown in FIG. 4 in the VHS system. If the color subcarrier is used as the reference signal as it is, in the case of the NTSC system, it overlaps with the band of the FM luminance signal as shown in FIG. On the IC, the unadjusted filter circuit and the frequency modulation circuit 27 have a common ground impedance as shown in FIG. 1, and the ground of the frequency modulation circuit 27 is swung by the color subcarrier. Therefore, the FM luminance signal interferes with the chrominance subcarrier and causes beat interference. In order to prevent this beat interference, the chrominance subcarrier should not be in the band of the FM luminance signal. The multiplication circuit 14 is provided for the above reasons.

Further, when one horizontal period (hereinafter referred to as 1H) delay signal required for dropout compensation, line correlation, etc. is obtained by the CCD, this multiplied color subcarrier can be used as the CCD clock signal.

FIG. 5 shows a specific embodiment of FIG. In FIG. 5, 15 is a coil, 16 and 17 are capacitors, 18 is a resistor, 19-
22 is a transistor. In FIG. 5, the video signal processing circuit 26 and the frequency modulation circuit 27 are omitted. Circuit operation is first
Similar to the figure, the reference level generation circuit 7 includes two resistors and a constant voltage source, and the pseudo filter circuit 8 includes two resistors and a junction capacitance 2.
It consists of individual pieces. In the case of FIG. 5, the multiplying circuit 14 is a multiplying circuit, and the reference signal input from the IC pin 3 is supplied to the multiplying circuit 14 composed of transistors 19 to 22 and the like.
After being multiplied, the harmonic removing filter 28 composed of the coil 15, the capacitors 16 and 17, and the resistor 18 extracts the multiplied reference signal. In case of using the color subcarrier for the reference signal in NTSC system, coil 15, capacitor
As an example, the values of 16 and resistor 18 are 15μH, 33pF, and 5K, respectively.
It should be Ω.

Another specific embodiment is shown in FIG. FIG. 6 also operates similarly to FIG. In the example of FIG. 6, the amplitude detection circuit is 1
Since it has the above gain and the level of the signal compared in the voltage comparison circuit 11 can be increased, it is possible to reduce the filter characteristic variation absorption accuracy due to the input DC offset of the voltage comparison circuit 11 and the like.

Although the NTSC system has been described above, even in the PAL system, the frequency of the color subcarrier is about 4.43 MHz, and beat interference is generated similarly to the NTSC system. Therefore, the present invention also has a great effect on the PAL system VTR. be able to.

〔The invention's effect〕

According to the present invention, a large block filter, which has been conventionally used as an external component, can be integrated without adjustment, and the cost of the circuit can be reduced, the size and weight can be reduced, and the number of components can be reduced. Since there is a means for removing harmonic components from the reference signal input to the adjustment filter circuit, even if the reference signal has distortion, it can be converted into a sine wave without distortion, and variations in filter characteristics can be accurately absorbed. . Further, since the input reference signal is multiplied, the filter circuit described above is used for the filter of the luminance signal system, and when the color subcarrier is used for the reference signal, the reference signal does not exist within the band of the FM luminance signal. Beat interference can be prevented. Furthermore, when the 1H delay signal is obtained by CCD, the above-mentioned multiplied signal can be used, so that the cost can be easily reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an unadjusted filter of the present invention, FIGS. 2 and 3 are diagrams showing waveforms of respective parts of the circuit shown in FIG. 1, and FIG. 4 is a recording signal in an NTSC VHS system VTR. FIG. 5, FIG. 5 and FIG. 6 are circuit diagrams showing a specific example of FIG. 1, and FIG. 7 is a diagram showing a Twin-T trap circuit, and FIG.
The figure is a frequency characteristic diagram of the trap circuit. 1 ... Reference signal source 2 ... Signal source 3 to 6 ... IC pin 7 ... Reference level generation circuit 8 ... Pseudo filter circuit 9, 10 ... Amplitude detection circuit 11 ... Voltage comparison circuit 12 ... Filter circuit 13 …… Capacitor 14 …… Multiplier circuit 23,24 …… IC pin 25 …… Video signal source 26 …… Video signal processing circuit 27 …… Frequency modulation circuit 28 …… Harmonic elimination filter 35 …… Ground pin

Claims (1)

[Claims] 1. A reference level generating circuit for inputting a reference signal and attenuating by a plurality of first resistors having a relative accuracy, and the first level of the reference level generating circuit for inputting the reference signal. Of the second resistor and the first junction capacitance, which have a specific accuracy with the resistor of FIG. 1, the amplitude detection of the outputs of the reference level generation circuit and the pseudo filtering circuit. Circuit, a voltage comparison circuit that compares the outputs of the two amplitude detection circuits and feeds the comparison output back to one end of the first junction capacitance, a capacitance that smoothes the comparison output of the voltage comparison circuit, and the reference It is composed of a level generating circuit, the first and second resistors forming the pseudo-filtering circuit, a third resistor having a relative accuracy with the first junction capacitance, and a second junction capacitance. And the second junction above A filtering circuit in which the voltage at one end of the quantity is controlled by the comparison output, a frequency modulating circuit for frequency-modulating the luminance signal, which is on the same IC chip as the reference level generating circuit, the pseudo filtering circuit, and the filtering circuit. In the luminance signal processing circuit having the above, the color subcarrier is used as the reference signal, the color subcarrier is set to a frequency of at least 2 or more, and a signal obtained by removing harmonic components from the frequency component is used as the reference level generation circuit. A filter circuit having means for inputting to the pseudo filter circuit.