JPH05299971A - Variation correction system for ic filter - Google Patents

Abstract

PURPOSE:To correct not only the variation of a resistance but also the variation of a capacitor, to reduce the number of elements, and to widen a draw range. CONSTITUTION:This system is equipped with a reference signal generating means 2 which generates a reference signal of a prescribed frequency, all pass filter 4 having a variable time constant to be controlled, which receives the reference signal, phase comparing means 6 which compares the phase of the output signal of the all pass filter with the phase of the reference signal, and low pass filter 8 which interrupts the passage of the high frequency components of a control signal outputted from the phase comparing means. Then, the all pass filter is formed on the same chip as an IC filter having the variable time constant to be controlled, and the variable time constants of the all pass filter and the IC filter are controlled by the control signal obtained through the low pass filter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an IC filter variation correction system for correcting variations in filters formed on an IC chip, and more particularly to a TV and VTR.
It is used for a filter-equipped analog IC for AV equipment such as, or for OA equipment such as a floppy disk drive or hard disk drive, or an analog / digital mixed IC.

[0002]

2. Description of the Related Art FIG. 8 shows the configuration of a conventional IC filter variation correction system. In this correction system, a resistance ratio detection circuit 83 is formed in an IC chip in which a filter 81 and a resistor 82 are formed, and a resistor 100 is externally attached. This detects the ratio of the resistance value of the resistor 82 inside the IC chip and the resistance value of the resistor 100 outside the IC chip by the resistance ratio detection circuit 83, controls the transconductance gm of the filter based on this ratio, and determines the frequency characteristic of the filter 81. Among the determined time constants, only the variation of the resistor 82 is corrected.

Another example of the variation correction system is shown in FIG.
Shown in. The correction system shown in FIG. 9 includes a reference signal source 92,
Biquad bandpass filter 94 and phase comparator 96
And a low-pass filter 98 including a capacitor and a DC amplifier 99. The reference signal source 92 generates two types of signals having a constant frequency and a phase difference of 90 degrees. The bandpass filter 94 has a controllable variable center frequency, and is formed on the same chip together with the filter to be corrected,
Of the two types of signals output from the reference signal source 92, the signal whose phase is advanced by 90 degrees is input. The center frequency of the bandpass filter 94 is designed to be the frequency of the signal generated from the reference signal source 92. The phase comparator 96 detects the phase ψ ₂ of the output signal of the bandpass filter 94.
And the phase ψ ₁ of the signal having no phase delay generated from the reference signal source 92 are compared, and a control signal such that the phase difference ψ ₁ −ψ ₂ becomes 90 degrees is output. The output of the phase comparator 96 is sent to the DC amplifier 99 via the low pass filter 98, amplified, and sent to the band pass filter 94 to control the center frequency of the band pass filter 94. The output of the amplifier 99 is sent to the filter to be corrected (IC filter).

[0004]

In the conventional variation correction system shown in FIG. 8, resistance variation can be corrected among the time constants that determine the frequency characteristic of the filter 81 to be corrected, but the capacitance in the IC is reduced. Is not corrected at all.

Further, in the conventional variation correction system shown in FIG. 9, the sensitivity of phase change to frequency is high,
Although the effect of correcting variations is large, it is necessary to provide at least a second-order bandpass filter, which increases the number of elements, and the amplitude response decreases as the frequency shifts, resulting in a wide pull-in range. There was a problem of not having.

The present invention has been made in consideration of the above circumstances. It is possible to correct not only resistance but also variation in capacitance, and it is possible to reduce the number of elements.
Another object of the present invention is to provide an IC filter variation correction system having a wide pull-in range.

[0007]

An IC filter variation correction system according to the present invention comprises a reference signal generating means for generating a reference signal having a predetermined frequency, and an all-pass filter having a controllable variable time constant for receiving the reference signal. A filter, a phase comparison means for comparing the phase of the output signal of the all-pass filter with the phase of the reference signal, and outputting a control signal such that the phase difference is 90 degrees, and a control signal output from the phase comparison means. And a low-pass filter that blocks passage of high-frequency components of, and the all-pass filter is formed on the same chip as the IC filter having a controllable variable time constant, and the variable time constants of the all-pass filter and the IC filter pass through the low-pass filter. It is controlled by a control signal obtained by

[0008]

According to the IC filter variation correction system of the present invention thus configured, the all-pass filter and the IC filter are formed on the same chip. As a result, the deviations of the respective time constants due to the variations of the elements are in the same direction with respect to the respective design values. If the design value of the time constant of the all-pass filter is designed so that the phase shift amount is −90 degrees with respect to the frequency of the reference signal generated from the reference signal generating means, the phase obtained through the low-pass filter The time constant of the all-pass filter is controlled by the control signal output from the comparison means so as to be the designed value, and the time constant of the IC filter is also controlled by the control signal so as to be the designed value. This makes it possible to correct not only the variation in resistance but also the variation in capacitance. Again 1
By using the next all-pass filter, the number of elements can be reduced and the pull-in range can be widened as compared with the conventional case.

[0009]

1 shows the configuration of a first embodiment of an IC filter variation correction system according to the present invention. The variation correction system of this embodiment includes a reference signal source 2, an all-pass filter 4, a phase comparator 6, and a low-pass filter 8.
And an amplifier 9. The reference signal source 2 generates a reference signal having a predetermined frequency. Allpass filter 4
A time constant circuit including a voltage-controlled current source (hereinafter also referred to as Gm amplifier) 4a for current control, a capacitor 4b, and a buffer 4c, and resistors 4d and 4e are provided, and the outputs of the reference signal source are provided to the resistors 4d and 4e. The difference between the voltage divided by 1/2 and the output of the time constant circuit is sent to the phase comparator 6. Here, the characteristics of the all-pass filter 4 will be described by taking the most primitive first-order all-pass filter shown in FIG. 2 as an example. The voltage V _A of the output voltages V ₁ divided by the resistance R ₁ of the reference signal source _{22 (= 1 / 2V 1)} , the differential voltage V ₂ and the voltage V _B which is divided by a capacitor C and a resistor R (= V _A
-V _B ) is

[0010]

[Equation 1] Therefore, the transfer function T (s) is

[0011]

[Equation 2] Becomes When s = jω is set here, the amplitude of the transfer function T (s) is

[0012]

[Equation 3] And becomes constant regardless of the frequency w.

On the other hand, the phase θ is θ = tan ⁻¹ (−ω / ω ₀ ) −tan ⁻¹ (ω / ω ₀ ) = − 2 tan ⁻¹ (ω / ω ₀ ). That is, it indicates that a phase rotation of 90 degrees occurs at ω = ω ₀ . The characteristics of this all-pass filter are shown in FIG. In FIG. 3, a graph h ₁ shows an amplitude characteristic and a graph h ₂ shows a phase characteristic. In the all-pass filter 4 of the embodiment shown in FIG. 1, when the transconductance of the Gm amplifier 4a is gm and the capacitance of the capacitor 4b is C _T , the frequency (angular frequency) of ω ₀ = gm / C _T is 90 degrees. Phase rotation occurs. The capacitance C _T and the transconductance gm of the all-pass filter 4 are designed so that the phase shifts by 90 degrees at the frequency (predetermined value) of the signal output from the reference signal source 2.

The phase comparator 6 is realized by, for example, a double balance circuit, compares the output phase of the reference signal source 2 with the output phase of the all-pass filter 4, and outputs a control current proportional to the phase difference ( (See FIG. 4). The output of the phase comparator 6 is amplified by an amplifier 9 via a low-pass filter 8 and a filter to be corrected (IC filter) not shown is shown.
And to the Gm amplifier 4a for controlling the mutual conductance gm of the Gm amplifier 4a.

Next, the operation of this embodiment will be described. For example, if the natural angular frequency ω _{0 of the} filter to be corrected is shifted to the lower side due to the variation of the element, the design angular frequency ω ₀ (= gm of the all-pass filter 4 formed on the same chip and the same process as the above filter to be corrected. Similarly, / C _T ) also shifts to the lower side due to element variations. Then, the signal sent from the all-pass filter 4 to the phase comparator 6 has a delay of a phase shift amount larger than 90 degrees with respect to the reference signal output from the reference signal source 2. This is because the characteristic curve h ₂ in the phase characteristic shown in FIG.
Corresponding to ₀ being shifted to the lower side. Then, Fig. 4
From the characteristics of the phase comparator 6 shown in (1), the output of the phase comparator 6 increases and the mutual conductance gm of the Gm amplifier 4a increases. As a result, ω ₀ becomes high, the phase of the output of the all-pass filter approaches 90 degrees with respect to the reference signal, and the difference is finally 1 / loop gain of the initial error.
Stabilizes when it becomes. Therefore, the output of the phase comparator 6 is also sent to the Gm amplifier of the filter to be corrected via the amplifier 9, so that the variation is similarly corrected and the corrected natural angular frequency becomes the design value.

The amount of phase shift (the amount of phase delay) of the all-pass filter 4 is that when the input signal is a sine wave, so when the input signal is a square wave or a triangular wave, the zero-cross point is affected by the high frequency contained in this input signal. Phase error occurs at. That is, when the input signal is a square wave, the phase tends to advance more than 90 degrees, and when the input signal is a triangle wave, the phase tends to slightly lag.
Further, when the input signal is a trapezoidal wave having a specific shape, almost no error occurs, but setting is difficult. Therefore, the reference signal source 2
As the reference signal output from the pseudo-sine wave obtained by passing the trapezoidal wave through the low-pass filter, the phase error will not occur. This pseudo sine wave generation circuit is shown in FIG. This generation circuit includes transistors T ₁ and T
_A buffer consisting of _2, a time constant circuit consisting of resistors R ₁ and R ₂ and a capacitor C ₁ , and a limiter circuit consisting of diodes d ₁ and d ₂ , transistors Q ₁ and Q ₂ , resistors R ₃ and R ₄ and a current source I. And a capacitor C ₂ and a resistor R ₅ ,
It consists of a low pass filter consisting of R ₆ . When a square wave is input to the above buffer, a triangular wave is output from the time constant circuit, this triangular wave is made into a trapezoidal wave by a limiter circuit, this trapezoidal wave is made into a pseudo triangular wave by a low-pass filter, and this triangular wave is low-passed. It is output from the output terminals OUT ₁ to OUT _{2 of the} filter.

As described above, according to this embodiment, not only the resistance but also the variation in capacitance can be corrected. Further, by using the first-order all-pass filter, the number of elements can be reduced at least as compared with the conventional case using the second-order bandpass, the amplitude characteristic is flat, and the phase rotation is gentle over a wide frequency range. Therefore, the pull-in range can be made wider than that of the conventional one.

Next, FIG. 6 shows the configuration of the second embodiment of the IC filter correction system according to the present invention. The correction system of this embodiment is an example specifically incorporated in an IC, and includes a bias circuit 60, a pseudo sine wave generation circuit 62, and a variable G.
All-pass filter 6 having m amplifier and capacitor
4 and a phase comparator 66 using a double balance circuit
And a low-pass filter 68 formed of a capacitor and an output circuit 69, and the transistors T ₅ and T connected to the output of the double balance circuit (phase comparator 66).
It largely depends on the pairing property of the current mirror composed of ₆ and T ₇ and the pairing property of the constant current source used for the Gm amplifier. For this reason, it is necessary to pay sufficient attention to the manufacture of both the circuit and the IC pattern.

In the correction system shown in FIG. 6, the output of the phase comparator 66 is obtained as a differential current, and one output is folded back by the current mirror composed of the transistors T ₅ , T ₆ and T ₇ of the output circuit 69. Combined with the other output, it becomes a single-ended output. The capacitor C ₁ of the low pass filter 68 is connected to this output point. Further, the control current is supplied to the Gm amplifier via the emitter follower T ₃ and the resistor R ₁ .

Further, this correction system is provided with a protection circuit so as not to deviate from the operating range when the power source is started. That is, it is a circuit that limits the maximum and minimum values of the control current. The minimum value is determined by the current supplied to the transistor T ₄ from the current mirror of the transistors T ₁ and T ₂ . The maximum value is determined by the sum of the minimum value setting current and the current determined by V _F / R ₁ . Because the base potential of the transistor is 3V from ground due to the action of the voltage sine diode D _1.
Limited to the potential of _F (the sum of V _F of transistors T ₆ , T ₅ and diode D ₁ ). The current of the transistor T ₃ is 3V _F to 2V _F (V of the transistors T ₃ and T ₄
It is determined by the ratio of the resistance R ₁ to one V _{F obtained} by subtracting ( _F ). In this way, stable operation is guaranteed by the minimum and maximum control current limiting circuits. Since the system originally has a wider pull-in range than the conventional circuit, the minimum and maximum widths can be large.

It goes without saying that the correction system of the second embodiment also has the same effect as that of the first embodiment.

Incidentally, the frequency characteristic of the filter to be corrected (IC filter) increases as the control current from the correction system of the present invention sent to the filter to be corrected increases.
As shown in FIG. 7, the characteristics shift from the graph k ₁ to the graph k ₂ and from the graph k ₂ to the graph k ₃ .

[0023]

According to the present invention, not only the resistance but also the variation in capacitance can be corrected, the number of elements can be reduced, and the pull-in range can be widened.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating the principle of an all-pulse filter according to the present invention.

FIG. 3 is a graph showing frequency characteristics of an all-pass filter.

FIG. 4 is a graph showing characteristics of the phase comparator according to the present invention.

FIG. 5 is a circuit diagram showing a configuration of a generation circuit that generates a pseudo sine wave according to the present invention.

FIG. 6 is a circuit diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 7 is a graph showing changes in the frequency response of the corrected filter.

FIG. 8 is a block diagram showing an example of a conventional correction system.

FIG. 9 is a block diagram showing another example of a conventional correction system.

[Explanation of symbols]

 2 Reference signal source 4 All pass filter 6 Phase comparator 8 Low pass filter 9 Amplifier

Claims (2)

[Claims] 1. A reference signal generating means for generating a reference signal of a predetermined frequency, an all-pass filter having a controllable variable time constant and receiving the reference signal, a phase of an output signal of the all-pass filter and the reference signal. And a low-pass filter that blocks passage of high-frequency components of the control signal output from the phase comparison means, the all-pass filter having an adjustable time constant A variation correction system for an IC filter, which is formed on the same chip as above, wherein variable time constants of the all-pass filter and the IC filter are controlled by a control signal obtained through the low-pass filter. 2. The I according to claim 1, wherein said reference signal generating means generates a pseudo sine wave as a reference signal.
C filter variation correction system.