JPH05110380A - Time constant automatic adjustment circuit for filter - Google Patents

Abstract

PURPOSE:To provide the time constant automatic adjustment circuit for a filter able to adjust the time constant of the filter circuit built in an IC automatically without increasing the circuit scale. CONSTITUTION:A signal whose frequency is around the center frequency of a trap filter 11 in an input signal 50 is trapped by a trap filter 11 and the result is inputted to a phase shifter 12. Moreover, a signal 70 whose phase is shifted by the phase shifter 12 is inputted to a multiplier 13. The multiplier 13 multiplies a signal 70 with the input signal 50 to use a signal 80 being the result of multiplication for a control signal 90 via a smoothing filter 14. As soon as the center frequency of the trap filter 11 is subject to feedback control to a prescribed value by the control signal 90, the time constant of the filter 15 is controlled to be a prescribed value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to, for example, an integrated circuit (IC).
The present invention relates to a time constant automatic adjustment circuit for a filter, which automatically adjusts the time constant of a filter circuit incorporated in a computer to a predetermined value.

[0002]

2. Description of the Related Art Generally, when a filter circuit is formed inside an IC, a time constant circuit of the filter circuit is constructed by using a resistor and a capacitor. However, the absolute value of the element having the resistance and the capacitance is poor in accuracy, and it is necessary to make some adjustment later so that the time constant becomes a predetermined value particularly when designing a high-order filter circuit. Occurs. Therefore, as described above, the time constant is adjusted to a predetermined value by using, for example, a variable capacitor or a variable current conversion amplifier so that the time constant can be changed in the filter circuit formed inside the IC. Is being done.

FIG. 3 is a block diagram showing an example of a filter time constant automatic adjustment circuit for automatically adjusting the time constant of a filter circuit of this type. Reference signal 100 of frequency f _s
Is input to the low-pass filter (LPF) 1 and the phase detection circuit 2. Here, it is assumed that the low-pass filter 1 has a second-order characteristic whose time constant is variable and has a phase delay of 90 degrees at a frequency f _c as shown in the graph of FIG. 4, for example. .. Therefore, the frequency f _s of the reference signal 100 is set so that f _s = f _c . The output signal 200 of the low-pass filter 1 and the reference signal 100 are phase-compared (phase-detected) by the phase detection circuit 2, and the comparison result 300 is filtered by the smoothing filter 3 to remove the ripple component, Control signal 40 to pass filter 1
It is fed back as 0 and is output to the built-in filter 4. Thereby, the time constant of the low-pass filter 1 is adjusted so that the frequency of the output signal 200 of the low-pass filter 1 maintains a phase difference of 90 degrees with respect to the frequency f _c of the reference signal 100, As a result, the time constant of the built-in filter 4 is controlled. The phase detection circuit 2 settles so that the phase difference between the two input signals becomes 90 degrees.
In this way, the frequency f _c of the low-pass filter 1 is automatically adjusted to the reference frequency f _s , so that the time constants of the other built-in filters are also automatically adjusted.

However, in the circuit as described above, the reference signal 100 is converted into a signal having a phase delay of 90 degrees by the low-pass filter 1 and then this signal is used for adjustment.
There is a drawback that the frequency of this signal _tends to deviate from the preset frequency f _{c due} to parasitic capacitance, parasitic resistance, and the like. Furthermore,
Since the offset of the phase detection circuit 2 is settled where the phase difference between the input signals deviates from 90 degrees, there is a drawback that the adjustment accuracy deteriorates. Moreover,
To adjust the time constant, the reference signal 100 and this signal 1
Low-pass filter 1 for phase-shifting 00 by 90 degrees
However, there is a drawback that the circuit scale of the filter circuit is increased.

[0005]

As described above, in the time constant automatic adjustment circuit of the filter for automatically adjusting the time constant of the filter circuit formed inside the IC to a predetermined value, the time constant of the filter is adjusted. There are drawbacks that the accuracy is not so good, a reference signal for the adjustment is required, and an additional circuit such as a low-pass filter is required, which increases the circuit scale of the filter circuit.

Therefore, the present invention eliminates the above-mentioned drawbacks, and provides an automatic time constant adjustment circuit for a filter, which is capable of automatically adjusting the time constant of a filter circuit incorporated in an IC with high accuracy and without increasing the circuit scale. The purpose is to do.

[0007]

According to the present invention, in a filter time constant automatic adjusting circuit for adjusting a time constant of a filter to a predetermined value, a trap having a variable center frequency having a carrier frequency of an input carrier signal as a substantially center frequency. A filter, a phase adjusting unit that adjusts a phase between the input carrier signal that has passed through the trap filter and the original input carrier signal, and the input carrier signal that has passed through the trap filter whose phase has been adjusted by the phase adjusting unit. And a multiplier that multiplies the original input carrier signal, and a smoothing filter that feeds back to the trap filter as a control signal that smoothes the multiplication result signal of the multiplier and changes the center frequency of the trap filter. Adjusting the time constant of the filter by the control signal Having formed.

[0008]

In the circuit for automatically adjusting the time constant of the filter of the present invention, the trap filter having the variable center frequency has the carrier frequency of the input carrier frequency as the center frequency. The phase adjusting means adjusts the phase between the input carrier signal that has passed through the trap filter and the original input carrier signal. The multiplier multiplies the input carrier signal that has passed through the phase-adjusted trap filter and the original input carrier signal. The smoothing filter smoothes the multiplication result signal of the multiplier and feeds it back to the trap filter as a control signal for changing the center frequency of the trap filter. At the same time, the control signal automatically adjusts the time constant of the filter.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an automatic time constant adjusting circuit for a filter according to the present invention. 11 is a trap filter having a frequency f _SC as a center frequency in a normal state, 12 is a phase shifter for shifting the phase of an input signal, 1
Reference numeral 3 is a multiplier for multiplying two input signals, 14 is a smoothing filter for smoothing the input signals, and 15 is a filter built in the IC, and the time constant of this filter is adjusted.

Next, the operation of this embodiment will be described. For example, a signal 50 having a waveform as shown in FIG. 2A such as a modulated color signal is input to the trap filter 11 and the multiplier 13. The trap filter 11 outputs to the phase shifter 12 a signal 60 as shown in FIG. The phase shifter 12 shifts the phase of the input signal 60 as shown in FIG. 2C, and then outputs this shift signal 70 to the multiplier 13. The multiplier 13 multiplies the input signal 50 by the signal 70 input from the phase shifter 12, and outputs a multiplication result signal 80 as shown in FIG. The smoothing filter 4 removes the ripple component from the input multiplication result signal 80 to smooth it, and then supplies this to the control signal 9
It is fed back to the trap filter 11 as 0 and output to the filter 15. As a result, the bottom frequency of the trap filter 11 is controlled to the center frequency by the control signal 90.

Consider a burst color signal of carrier frequency f _sc as the input signal. The input signal 50 is input to the trap filter 11 as described above, and the transmission characteristic G _T of the trap filter 11 is expressed by the following equation. G _T = (S ² + ω ₀ ² ) / (S ² + δω ₀ S + ω ₀ ² ) ... (1) where δ is the damping coefficient, ω ₀ is the resonance angular frequency, and this ω ₀ depends on the control voltage 90. However, the center frequency of the trap filter 11 is set to be equal to 2πf _sc in the normal state (control center). When the trap filter 11 is in the normal state, in the output signal 60, the in-phase component mainly remains at the rising portion of the input signal 50, the anti-phase component mainly remains at the falling portion, and the other portions have zero output. Therefore, a signal 70 obtained by phase-shifting such an output signal 60 by the phase shifter 12 is provided.
Will be input to the multiplier 13. Therefore, the signal 80 obtained by multiplying the signal 70 and the input signal 50 by the multiplier 13 is at the rising and falling portions if the phase shift in the phase shifter 12 is +90 degrees and is a complete differentiation. Both are zero in terms of direct current. Further, when the phase shift does not reach 90 degrees, the in-phase component increases at the rising portion of the signal 80, but the anti-phase component increases at the falling portion. Therefore, if these are averaged, the signal 80 is zero. Output. Therefore, the trap filter 1
When 1 is in the normal state, the control signal 90 output from the smoothing filter 14 and fed back to the trap filter 11 becomes substantially zero.

Next, ω ₀ deviates from the above center state by 2π
When it is smaller than f _sc , if ω ₀ = 2πf _sc −Δω, the gain G _T of the trap filter 11 at the carrier frequency f _sc can be expressed as follows. ^{_{G T = (Δω 2 -2Δω ·}} 2πf sc) / {Δω 2 -2Δω2πf sc + δj2πf sc (2πf sc -Δω)} ... (2) However, the S = j2πf _sc. Where Δω << 2πf
_{If the} equation (2) is approximated as _sc , G _T = j2Δω / δ
2πf _sc (3), and the output signal 60 phase-shifted by approximately +90 degrees with respect to the input signal 50 is obtained. The signal 60 at this time has a waveform as shown by the dotted line in FIG. When such a signal is further differentiated by the phase shifter 12, its output signal (dotted line portion) 70 is the input signal 50.
However, the signal has a phase inverted by 180 degrees. Therefore, the signal 80, which is the result of multiplying the input signal 50 and the output signal 70 by the multiplier 13, has a DC negative polarity as shown by the dotted line in FIG. However, in contrast to the above, when ω ₀ deviates from the normal state and is larger than 2πf _sc , Δω may be regarded as negative, and the output signal 80 of the multiplier 13 has a DC positive polarity.

According to this embodiment, the multiplication result (detection output) signal 80 of the multiplier 13 reflects the deviation from the center frequency of the trap filter 11 as it is, so that the control signal 90 of the center frequency is supplied to the trap filter 11. The trap frequency of the trap filter 11 is automatically adjusted to the original center frequency and the filter 15
Since the time constant is automatically adjusted, the time constant adjustment accuracy can be improved as compared with the conventional method in which the phase is once changed and then the phase is detected. Moreover, the ripple component due to the detection of the multiplier 13 is very small,
Since smoothing in the smoothing filter 14 is easy, the capacity of the capacitor of the smoothing filter 14 can be reduced and the circuit can be downsized. Further, since the phase shifter 12 is a simple circuit composed of a capacitor and a resistor, this also does not lead to an increase in circuit scale.

Even when the phase shifter 12 does not reach +90 degrees, there is no problem because the detection sensitivity is slightly lowered. In particular, when the center frequency of the trap filter 11 is emphasized, it can be said to be ideal because it is adjusted to be the carrier frequency. Moreover, for example, when performing color side-band emphasis, etc., a trap filter having a carrier frequency to which a color signal is input at the bottom is originally required. Can be significantly reduced, and the circuit scale can be reduced.

[0015]

As described above, according to the automatic time constant adjusting circuit for a filter of the present invention, the time constant of the filter circuit with a built-in IC can be automatically adjusted accurately and without increasing the circuit scale.

[Brief description of drawings]

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

FIG. 2 is a waveform diagram illustrating the operation of each unit of the circuit shown in FIG.

FIG. 3 is a block diagram showing an example of a conventional time constant automatic adjustment circuit for a filter.

4 is a characteristic diagram showing characteristics of the low-pass filter shown in FIG.

[Explanation of symbols]

 11 ... Trap filter 12 ... Phase shifter 13 ... Multiplier 14 ... Smoothing filter 15 ... Filter

Claims (1)

[Claims] 1. A time constant automatic adjusting circuit for adjusting a time constant of a filter to a predetermined value, wherein a trap filter having a variable center frequency having a carrier frequency of an input carrier signal as a substantially center frequency, and this trap filter are provided. Phase adjusting means for adjusting the phase between the passed input carrier signal and the original input carrier signal, and the input carrier signal and the original input carrier signal passed through the trap filter whose phase is adjusted by the phase adjusting means. And a smoothing filter that feeds back to the trap filter as a control signal that smoothes the multiplication result signal of the multiplier and changes the center frequency of the trap filter, and the filter is controlled by the control signal. A time constant automatic adjustment circuit for a filter, which is characterized by adjusting the time constant of.